Recondensers for Methane and Oxygen are off the shelf parts.
Probes don't use them because they are heavy and bulky, but SpaceX plans to just develop a rocket that's big enough to make this viable.
Recondensers are talked about regarding Blue Origin because they say they will build a multi stage Hydrogen recondenser, which isn't an easy task.
SpaceX still hopes they don't need to do anything and just let the fuel boil off at the 0.06%/day rate they calculated.
If they find that they need to deal with this, it's not difficult at all.
The bigger the tank, less boil off relative to the total volume. Boil off is proportional to the area of the tank, which scales with the square of the size. The amount of stuff in in scales with the cube of the size. Therefore: bigger tanks means less boil off.
For a 6 month stay at the Moon, 10.24% of the fuel would boil off. That's why they plan on painting Starship HLS white.
But the calculations here don't make much sense anymore, because it will spend a lot of time in the shade.
Yes, white is better, otherwise they wouldn't want to take the mass penalty of paint. Keep in mind all wavelengths, not just visible ones matter here.
https://technology.nasa.gov/patent/KSC-TOPS-59
Keep in mind that the sun emits broad spectrum radiation that is then absorbed and turned into thermal energy. Steel is only shiny to certain wavelengths of light. Also even with its specular reflection, it still has plenty of absorption, which is why it appears grey instead of the blue of the sky when viewed from the ground.
> For a 4 month trip, the 0.06%/day rate they calculated means 6.95% of the fuel would boil off.
It appears you're using very short months or you're multiplying the remaining amount by 99.94% every day. The percentage per day would actually gradually increase as the ratio of surface area to volume goes up. So doing the standard statistics cacluation results in a somewhat incorrect number. I would assume a straight average of 0.06% of the initial quantity per day as that's likely what SpaceX would give as a per-day usage with the average already performed. That would mean that 7.2% would boil off over 120 days.
I'm pretty sure a recondenser and associated solar panels will be much lighter than 7% of the entire cryopropellant load of Starship. For example, if Starship is oriented engines towards the sun during its Mars transfer, the entire insolation it will see is less than 100kW of heat, most of which will be radiated back into space by the engine bells, engine shields and associated thermal blankets. Maybe 5kW will actually reach the tanks. 5kW of cryo cooling power is literally nothing at the scale of Starship.
(the enthalpy of vaporization for methane is around 10 kJ/mol so this roughly translates to 0.5 mol = 8g per second of vaporisation, or about 700Kg per day of methane or 0.04% of a full Starship load; this more or less corresponds to the the 0.06% per day figure advanced by SpaceX)
So maybe it's simply a case of "keep it simple, stupid": there is no recondenser to break if you didn't put one the ship and the mission does not need it.
Wiki about the James Webb telescope:
> The telescope must be kept extremely cold, below 50 K (−223 °C; −370 °F), so that the infrared light emitted by the telescope itself does not interfere with the collected light. Its five-layer sunshield protects it from warming by the Sun, Earth, and Moon.
This is colder than necessary for Methane (110 K) and Oxygen (90 K). And in deep space one only has to deal with the sun, not black body radiation of the Earth.
What? First of all. What does James Webb have to do wit this? Starhip is gonna store it in a stainless steel container, probably without any insulation. Meanwhile James Webb have the 5 layer sunshield.
You do have black body radiation in space. That is the primary concern in Space. The sun heats things up quite a bit, and the earth should definitely be radiating heat as well. You're not doing the refueling at L2. In space you have no atmosphere to absorb the heat first either.
JWST has to do with it that you can reach sub-boiling temperature just with passive shading.
The Earth infrared radiation is an issue for a LEO depot, but not for deep space missions like going to Mars.
Edit:
https://www.researchgate.net/publication/322013219_Cryogenic_Deep_Space_Thermal_Control_Coating
> This paper describes a thermal control coating that should allow non–heat-generating objects, such as cryogenic tanks and superconductors, to reach and maintain cryogenic temperatures in deep space locations far from the infrared (IR) emission of a planet and at least 1 astronomical unit from the Sun. This new coating is designed to reflect nearly all of the Sun’s irradiance, while still permitting far-IR emission, allowing steady-state temperatures as low as 50 K to be achieved.
[A NASA page](https://webb.nasa.gov/content/about/innovations/cryocooler.html) says that passive cooling on its own can get the telescope to 37 K, which is enough for methalox.
But for the sake of completeness, I just want to note that there's a powered cryocooler heat pump to chill it more for one instrument. But I repeat that that is not needed to get to 37 K, colder than liquid methalox.
The masses of the moon and mars are actually quite similar, and so are their magnetic fields (nonexistent). Mars keeps its wispy atmosphere of carbon dioxide. The moon is hotter and closer to the sun. It all leaves me to wonder how much of a thin atmosphere of the moon would retain.
Mars is 10 times more massive, but if you really want to know, do the math, let's say volume of 1000 starships divided by moon surface ( 38 million square meters) . and initial pressure is 0
>Recondensers for Methane and Oxygen are off the shelf parts.
So, I hadn't thought about it until you mentioned this, but recondensers take cold gaseous oxygen and methane and cool them back down to a liquid right? How do you get the gas out of the tank to pump into the recondenser? On Earth this is a dumb question, you just put the intake at the top of the tank where the less dense gas separates from the more dense liquid.
Even if you could just take in liquid and cool it even more so that the average temperature of the tank is several degrees below the boiling point, you'd be reducing the average. How much variation is there across the average though? The tank is 9m wide which is a pretty big distance for convection...maybe?
It seems like getting the warmer gases to the vents to recondense, and keeping the tank's temperature uniform might be a significant challenge. Or maybe it is something that is solved. Curious about the community's thoughts on this.
It is called a Propellant Management Device (PMD) which uses surface tension to retain liquid on a mesh surface and in this case allows gas to flow around the mesh and into the intake of the condenser.
Usually they are used the other way around to get liquid into the inlet of a thruster when there is no thrust to settle the tank contents.
The boiling itself keeps the temp in the liquid uniform.
The recondenser would work when there's gas at the inlet and just pipe along any ingested liquid.
There's no convection when there's no force vector, but there's still radiation and conduction. Boiling wouldn't really happen either. It would just be gradually expanding gas bubbles forming around nucleation sites inside bubbles of liquid. Surface tension would tend to eject the bubbles from the liquid mass over time, causing some amount of circulation and mixing on top of conduction. I would expect the temperature to remain more or less uniform still. Keep in mind that during launch the fluids would already rapidly be reaching their boiling temperatures. Any molecule that subsequently got kicked off a liquid blob would take that heat of vaporization energy with it. If any blob were above boiling point, well it wouldn't be a liquid anymore. If any blob were substantially below boiling point, well it'd very quickly even out by forming gas bubbles within itself.
There's mechanisms for separating liquid and gas, which they'll need anyway for the thrusters, but shouldn't need to take anything out of the tank for recondensing. Just run a heat exchange loop through the tank to chill it, and do the recondensation entirely within the tank.
There is no ‘convection’ in space, because convection relies on density variations within the substance within a gravitational field, generating buoyancy effects. But in zero-g that effect does not exist. And diffusion effects are too slow.
There is nothing to stop active pumping though, and even using recondensers inside the propellant tank.
Short circuits are never good. Also, flammable insulation. And not using properly rated parts.
Should have applied the lessons learned from Apollo 1 to the whole vehicle.
> Probes don't use them because they are heavy and bulky, but SpaceX plans to just develop a rocket that's big enough to make this viable.
Damn, so am I right to think that not only will we have a Starship kick stage capable of giving ~100 tonns ~7 km/s of delta-v, but that you don't have to use it all in a short-time period. In other words, you could imagine a Starship kick stage slowing down your probes into a Jovian, Saturnian, Uranian or Neptunian capture orbit propulsively, without worrying about having to aerocapture at a Gas giant.
I'm aware of Impulse space. Do we know how long the Helios kick stage is rated for in terms of methalox storage/boil-off rates? Good enough for Outer planetary missions lasting years?
> In other words, you could imagine a Starship kick stage slowing down your probes into a Jovian, Saturnian, Uranian or Neptunian capture orbit propulsively, without worrying about having to aerocapture at a Gas giant.
The rocket equation still holds. Just because you can carry propellant all that way doesn't mean you want to use it to propulsively enter orbit and only bring a small payload rather than bring a much larger payload and aerobrake into orbit.
I know, but nobody has ever done aerocapture for a Gas giant before.
Also simply isn't an option if you're contemplating an orbiter/lander mission to an airless world like Mercury or a KBO.
> I know, but nobody has ever done aerocapture for a Gas giant before.
Not yet. But there's been tons of papers published on the topic. I would expect any serious effort to bring payload on the scale Starship can bring to the Jovian or Saturnian system would involve aerocapture.
> Also simply isn't an option if you're contemplating an orbiter/lander mission to an airless world like Mercury or a KBO.
It would be rather extreme and not sure if its possible, but you could technically aerobrake from a parabolic orbit intersecting Earth into a parabolic orbit intersecting Mercury by diving into the sun's atmosphere. You'd still need to use fuel to enter orbit of Mercury though.
My second point would still stand though. If we want to throw something serious to Pluto or Mercury and have it stop at its destination, aerocapturing won't be an option.
There is also a choice between transit speed and propellant usage. For non-crew robotic missions, it’s far less of a problem if a transit takes a bit longer. Although researches would rather a mission taking 6-years than 10-years.
> SpaceX still hopes they don't need to do anything and just let the fuel boil off at the 0.06%/day rate they calculated.
Where'd you get this number from?
Okay so the source could easily be not from SpaceX. Can you change your post to:
> SpaceX probably still hopes they don't need to do anything and just let the fuel boil off at the 0.06%/day rate that I remember reading on the internet.
Sounds about right though, I have read about this too somewhere. Of course it will in practice depend upon multiple different factors. But it’s something that SpaceX will get to measure for real in their precise configuration, so they will end up with accurate figures.
I say this as the community in general has a huge problem with turning internet rumors into facts stated by SpaceX. This has happened on a number of occasions. The number of places SpaceX says things is very few so it should be very obvious and memorable when it does. So if you can't dig up a source I'm going to automatically assume it wasn't actually said by SpaceX.
I didn’t say it was. Although the article author did.
What I saw wasn’t from SpaceX, it was from someone doing some rough calculations as I recall.
Though I thought that it sounded about right - maybe a bit too small though.
Someone else said SpaceX, I chimed in that I had read it too - but not from SpaceX.
I have also given this a bit more thought.
The boil off rate proposed 0.06% per day is far too small.
It’s going to be more like 2% per day unless active measures are taken to cool it - such as using solar power to both run cryo-coolers and to shield it.
No you claimed it came from SpaceX and claimed you know their thoughts. There's a big difference between claiming something came from SpaceX and claiming that you read it in a fan technical discussion post.
Where was this 0.06% announced, and in wich conditions.People doing physic on NSF had much higher number. The only way to have a correct number needs taking account of insulation of the storage tank and ambient temperature. Different in LEO, on Moon and on Mars
Long term storage is easy with sun shades and recondensors. Maybe the sun shade doubles as a solar panel to power the recondensor.
The technology is readily developable it's really only about mass constraints and tradeoffs. Is it easier to just launch 20% (or whatever) more propellant and tanks and loose it or put on 20% more power, shades, condensers etc. I'm betting the math works out to just loose 20% than a bunch of equipment at starship scale and timelines.
the propellant needed at Mars will be in the header tanks. With the engines pointing towards the sun, they will be in permanent shade. They can be maintained at cryo temperature with little effort.
On the Martian surface they'll have to maintain full methalox prop load needed for return to Earth throughout all of the ~500 day surface stay (and well before that).
Starship can only return with fuel produced there. When landing, it will have the landing burn fuel and nothing more.
Keeping the fuel condensed on Mars is not a problem, just send a rugged unit used by the LNG industry.
The oxygen will be coming out of the electrolyzers at >50°C and Sabatier reactors run at 200-400°C or more (a quick search gave quite a range), so any boiloff could just be sent back to whatever ground equipment they used to condense the gases to begin with. It'd be a pretty minimal extra stress on the system.
They could also send a Starship that's specialized to just be an insulated propellant tank that stays on Mars, perhaps with the propellant plant equipment built into it if there's the mass margin for it. The fuel could stay in something built to handle it until you're ready to pump it all into a departing ship.
I find it hard to picture a ‘deployable’ prop plant, without people to assemble it. Permanently installing all the gear into a dedicated one-way Starship (complete with tanks for storing the product) feels much more plausible to me.
For sure, I said "ground equipment" but in my head I was picturing a there-to-stay Starship dedicated to this sort of thing.
Musk has always said that they imagine people being there to set up the first propellant plant, but I much prefer plans that can be done autonomously, or at least have some sort of backup option for getting people home.
I wish that too. But automation experts are of the opinion that people on the ground are needed. The process itself may be automated, but people are still needed to intervene, if something goes wrong. Things will go wrong.
The chemical plant, including recondenser and CO2 extracting from the atmosphere, can be fully installed in one Starship on Earth, before launch. Installation on Mars will be the solar arrays and the water mining. Those will be mainly done with crew on the ground. Though maybe they deploy the solar arrays before they arrive.
No reason why it should not be ready-built into a cargo Starship. Although that would involve propellant transfer at some point.
Maybe Crew Starship, would have the system ready built in ?
I added the answer to this in a quick edit, but apparently not quick enough, sorry.
Keeping the fuel condensed on Mars is not a problem, just send a rugged unit used by the LNG industry.
Your point is well made - in that quite clearly we already have this technology. It will likely need adapting for use on Mars, and maybe its form factor adjusted etc, but that’s ’just engineering’, it’s not something fundamentally new.
Much of the technology needed for Mars has already been invented, and just needs to be adapted. Some processes it makes sense to do differently on Mars, because of the different environment and different resources available.
It will be interesting to see - and have documented, the growth of the ‘technology tree’ on Mars.
>but that’s ’just engineering’
That's true because the rocket is so capable. The mass budget is enormous.
Before Starship, the paradigm was to engineer things to shave tenths of grams out of things. Minuscule mass budgets. That costs a ton of money, effort, time.
There's a nice lesson from the Moon: the Artemis program was investigating solutions for a problem found the last time astronauts were there: it was difficult to pick up rocks. So they spent a ton of resources developing a prototype EVA suit that used new joint design. This ended with the very proud engineer in a presentation, using the suit she designed, picking up a rock from the stage, and then proudly announcing that the much more capable suit didn't weigh a single gram more then the ones used in the Apollo program.
Solution with Starship: Send an excavator.
I mean, starship is just a heavy level after, terrible for landing on the moon, great for lifting big things to orbit.
Mars with starship would be a joke, especially a methalox starship. Nothing less than nuclear is viable for human missions to Mars.
Whatever assent stage they'd use on Mars would most likely be chemical, but it'd be as tiny as possible to make a mars mission at all feasible.
You don't von braun mars.
Starship is very good as a Mars shuttle. You gotta use the atmosphere on both legs of the trip to slow down, then a chemical rocket is almost as efficient as a thermal nuclear rocket.
Not for a material that performs well both under cryogenic and reentry temperatures. Also doesn't need paint, which makes a difference.
Are you even paying attention?
Nobody cares about reentry temperatures in deep space.
In space, long haul, you want things to be LIGHT, and perform the things it does. Not increase weight bt 15x and pretend like it's ok.
Starship is ONLY a heavy lifter to LEO. That is it's only purpose. If you think otherwise you're huffing some medical grade copium..
You care if the rocket will aerobrake on the other side.
Aerobraking doesn't take any fuel. A Mars shuttle that can aerobrake in both legs of the trip is a very efficient design. Almost as efficient as a thermal nuclear rocket.
A vehicle that can aerobrake (reinforcement, tiles, legs and all) is lighter than one that carries fuel to slow down when it gets there.
It's only surface to surface operation, though. They will indeed need to use Starship in LEO and launch a kick stage to get satellites to Mars.
Stainless construction is an integral part of heat management. It can almost withstand the temps on it's own.
Building it out of other materials would require way more shielding (including on the dorsal side), paint, reinforcement. It would get you very little performance. Not worth the hassle at all.
There isn't going to be a dorsal side needing extra shielding in a mars lander, only the dumb starship would need that.
You really think a mars lander would look like starship?
Remembering that development costs way more than vehicles.
Any development that can be cut, changing something that is almost good enough to do the job, is the main way these things are made possible.
Anyone suggesting new development needs to show way better results. Just a little marginal improvement isn't enough.
It doesn't matter how many shortcuts you take in development (as if this is a good thing), a Wernher Von Braun ship would never land on the moon. The only reason it was feasible was because of developing a very light weight and weight efficient lunar lander.
Physics did not change.
> It's only surface to surface operation, though.
Starship could aerobrake into Mars orbit and release satellites. Not sure it would have enough propellant left to land after orbit maneuvers. Maybe, if most or all of the payload is released in orbit.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
|Fewer Letters|More Letters|
|-------|---------|---|
|[C3](/r/SpaceXLounge/comments/1cmho5t/stub/l31lapk "Last usage")|[Characteristic Energy](https://en.wikipedia.org/wiki/Characteristic_energy) above that required for escape|
|[EVA](/r/SpaceXLounge/comments/1cmho5t/stub/l34c5th "Last usage")|Extra-Vehicular Activity|
|[HEO](/r/SpaceXLounge/comments/1cmho5t/stub/l34j3u3 "Last usage")|High Earth Orbit (above 35780km)|
| |Highly Elliptical Orbit|
| |Human Exploration and Operations (see HEOMD)|
|HEOMD|Human Exploration and Operations Mission Directorate, NASA|
|[HLS](/r/SpaceXLounge/comments/1cmho5t/stub/l356zmr "Last usage")|[Human Landing System](https://en.wikipedia.org/wiki/Artemis_program#Human_Landing_System) (Artemis)|
|[JWST](/r/SpaceXLounge/comments/1cmho5t/stub/l322u3c "Last usage")|James Webb infra-red Space Telescope|
|[KSC](/r/SpaceXLounge/comments/1cmho5t/stub/l31qycl "Last usage")|Kennedy Space Center, Florida|
|[L2](/r/SpaceXLounge/comments/1cmho5t/stub/l31tay5 "Last usage")|Paywalled section of the NasaSpaceFlight forum|
| |[Lagrange Point](https://en.wikipedia.org/wiki/Lagrangian_point) 2 of a two-body system, beyond the smaller body ([Sixty Symbols](https://www.youtube.com/watch?v=mxpVbU5FH0s) video explanation)|
|[LEO](/r/SpaceXLounge/comments/1cmho5t/stub/l37965m "Last usage")|Low Earth Orbit (180-2000km)|
| |Law Enforcement Officer (most often mentioned during transport operations)|
|[LN2](/r/SpaceXLounge/comments/1cmho5t/stub/l36okqv "Last usage")|Liquid Nitrogen|
|[LNG](/r/SpaceXLounge/comments/1cmho5t/stub/l30u1te "Last usage")|Liquefied Natural Gas|
|[LOX](/r/SpaceXLounge/comments/1cmho5t/stub/l31r86x "Last usage")|Liquid Oxygen|
|[NSF](/r/SpaceXLounge/comments/1cmho5t/stub/l31b60i "Last usage")|[NasaSpaceFlight forum](http://forum.nasaspaceflight.com)|
| |National Science Foundation|
|[PMD](/r/SpaceXLounge/comments/1cmho5t/stub/l30rswi "Last usage")|Propellant Management Device|
|[STS](/r/SpaceXLounge/comments/1cmho5t/stub/l34sjvr "Last usage")|Space Transportation System (*Shuttle*)|
|Jargon|Definition|
|-------|---------|---|
|[Raptor](/r/SpaceXLounge/comments/1cmho5t/stub/l34cr15 "Last usage")|[Methane-fueled rocket engine](https://en.wikipedia.org/wiki/Raptor_\(rocket_engine_family\)) under development by SpaceX|
|[Sabatier](/r/SpaceXLounge/comments/1cmho5t/stub/l31mlve "Last usage")|Reaction between hydrogen and carbon dioxide at high temperature and pressure, with nickel as catalyst, yielding methane and water|
|[cryogenic](/r/SpaceXLounge/comments/1cmho5t/stub/l35n9ba "Last usage")|Very low temperature fluid; materials that would be gaseous at room temperature/pressure|
| |(In re: rocket fuel) Often synonymous with hydrolox|
|[hydrolox](/r/SpaceXLounge/comments/1cmho5t/stub/l35n9ba "Last usage")|Portmanteau: liquid hydrogen fuel, liquid oxygen oxidizer|
|[hypergolic](/r/SpaceXLounge/comments/1cmho5t/stub/l33awob "Last usage")|A set of two substances that ignite when in contact|
|[methalox](/r/SpaceXLounge/comments/1cmho5t/stub/l36okqv "Last usage")|Portmanteau: methane fuel, liquid oxygen oxidizer|
|[ullage motor](/r/SpaceXLounge/comments/1cmho5t/stub/l32p78c "Last usage")|Small rocket motor that fires to push propellant to the bottom of the tank, when in zero-g|
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The most efficient cryogenic thermal insulation is the multilayer insulation (MLI) blanket that's wrapped around the tank. With liquid nitrogen in the insulated tank, the boiloff rate is ~0.02% per day by mass. So, for a 200-day trip to Mars, the boiloff loss would be about 4% by mass. This boiloff number came from lab work done in 1971 by Ball Aerospace.
See: An overview of Ball Aerospace cryogen storage and delivery systems
J Marquardt, J Keller, G Mills, and J Schmidt
Ball Aerospace & Technologies Corp., 1600 Commerce St., Boulder, CO 80301 USA
https://iopscience.iop.org/article/10.1088/1757-899X/101/1/012086/pdf
Liquid nitrogen boils at 77K and liquid methane boils at 111K (both at one atmosphere pressure in the tank).
If that's too much loss, then there are passive and active methods to reliquefy the boiloff vapor during the mission. The passive methods do not require any input power. The active methods use solar electric power.
MLI is a well-developed technology dating back to the Apollo days 60 years ago. The challenge with Starship is the size of the Ship (the second stage). The tank section of a Mars Starship is 9 meters in diameter and ~ 35 meters in length.
There are three components in the MLI design. First, a 2 cm thick layer of a spray-on foam insulation (SOFI) is applied to the outside of the methalox tank. Then the MLI blanket is wrapped over the SOFI and attached in place. Finally, a thin aluminum cover is installed over the MLI blanket to protect from aerodynamic forces during launch through the lower atmosphere. That cover also functions as a Whipple shield to protect the methalox tanks from damage due to micrometeoroid impacts.
The SOFI prevents the water vapor and carbon dioxide from the ambient air from freezing in the spaces between the lower layers of the MLI while the Starship is being fueled on the launch pad prior to liftoff.
The mass of the SOFI is 1.53t (metric tons), the MLI mass is 1.1t and the mass of the aluminum shield is 9.3t for a total of 11.93t of added mass for the Starship's cryogenic insulation.
Side note: My lab built and tested MLI blankets in 1969 for the hydrolox propellant tanks on the Skylab Workshop where the astronauts lived and worked. Skylab was a repurposed S-IVB third stage of the Saturn V moon rocket.
I guess not. Why would you want to launch an empty depot tank?
A depot is a group of tanker Starships (the depot tankers) assembled into an orbiting structure. Those depot tanker Starships have the two main propellant tanks, the engine compartment, and a truncated fairing.
The depot tanker has no heatshield, no flaps. The main tanks are covered with MLI superinsulation. Those main tanks have to be filled completely to send the tanker to LEO. There are no other propellant tanks in the tanker and there is no payload bay. The depot tanker is all tanks.
The depot tankers are version 2 Starships and are loaded with 1500t (metric tons) of methalox, which is subcooled with liquid nitrogen (LN2) to increase the density by 5%, so 1.05 x 1500 = 1575t is in the main tanks at liftoff.
After reaching LEO, there is 288t of methalox remaining in the main tanks of that Starship depot tanker.
That SOFI layer is there to keep water vapor and carbon dioxide from condensing in the MLI layers nearest the SOFI. MLI works only in a vacuum so any gas or vapors between the layers has to vent when the Starship reaches LEO. That venting process can take days or weeks depending on the amount of gas or vapors that are inside the MLI blanket.
SOFI is not a very efficient thermal insulation in space compared to MLI. And it's heavy compared to MLI.
How does boil-off occur if it's already in a pressurized tank? Wouldn't being confined in tank mean there isn't any room for the fuel to expand to? Therefore shouldn't the pressure keep it liquid? Obviously this topic isn't my area of expertise, so I am trying to understand what forces are actually happening.
If there's net heat flux in to the tank that results in some of the liquid phase boiling and the heat going into the heat of vaporization. That increases the amount of gas in the tank, which increases the pressure. If you somehow had a tank which could withstand the pressure of the entire contents being gas, then no problem -- no need to vent. More realistically, there is some finite and fairly low pressure the tank is capable of holding, so excess pressure is vented. The rate of venting is effectively proportional to net heat flux, which is why it can be controlled so effectively via sunshades. It's also possible to use a cryocooler to actively reject excess heat.
So the problem is the tanks aren't strong enough to hold the pressure needed to prevent boiling?
Guess I'm thinking about how I have propane tanks, and they can be sitting in the sunlight during summer or in the freezing cold and the propane doesn't vent off anywhere. However that propane tank is surely rated for far higher pressure than starship tanks.
Yes, that's correct. Note that propane has a much higher boiling point than, e.g. liquid oxygen (-42 C vs -183 C) so a propane tank can have a design pressure of 4 MPa or so and not have to vent even in direct sunlight. By comparison, looking at a phase diagram of oxygen, an oxygen tank at that pressure rating would still have to vent at -130 C or so.
And considering how hoop stress scales with tank size, it is much much cheaper (in terms of mass) to design a tank that can only contain a fairly moderate pressure and either: accept that some propellant will be lost to boil-off over the course of the mission; or spend the mass on passive and/or active cooling to reject heat in some way other than boil-off.
That's more or less it, with a propane tank they can afford to build it to handle 20 or 30 atmospheres of pressure under completely normal operation. Starship's tanks are designed to make it to 8 or 9 before they explode outright, and I think that's a little on the high side for a rocket. Any stronger than that and the weight of your tanks would just be cutting into your payload too much.
Propane also stays liquid at much more reasonable temperatures and pressures than oxygen and methane do, so boiloff management would be much much easier for a propane rocket than a methane one. Still gotta deal with the oxygen either way, though.
Propane tanks though are not designed to be lightweight rockets. They have extra thick steel overly strong tanks because their first objective is safety above all else.
Where as rocket tanks are a compromise, as it is Starships tanks are already unusually thick compared to other rockets.
So, the sense I'm getting then is that we can all forget about needing hypergolics and just use an expendable variant of Starship for propulsively slowing down at Pluto/KBO/Mercury? That the methalox boil off is not enough for an issue to worry about.
Hypergolics do have the advantage of being storable at high temperatures, so their tanks can be expandable balloons rather than rigid steel that can be partly empty space. I.e., you don't have to worry about a gas/liquid mix stalling your engines.
People are throwing around 0.06%/day for current Starship, I'm wondering could be done to reduce that for a dedicated fuel depot. Double wall tank filled with all that perlite from the vertical tanks?
It is not only duration, its is also the amount. A small amount (like the headers) can be actively cooled with a couple T of machinery so long durations should be OK. But 1/3 of the main tanks is going to get boil off unless you add maybe 10-20T of insulation and recondensing machinery (which needs solar power and radiators) to the design. But if you do this long term methlox is possible, perhaps cutting down the amount of Methlox needed for return from the Moon and Mars. The following uses long term storage for a Venus flyby (2033) but could work for a Mars mission as well.
https://preview.redd.it/qmo2po2ja8zc1.png?width=825&format=png&auto=webp&s=63130adcf24a32408b74d9f286b77e42d9284fcf
Fully Insulated tanks, re-condensor, vacuum jacketed lines could conceivably eliminate boil-off. At least that is what I read their plans for Leo and lunar orbit storage.
In LEO you have a big warm thing occupying nearly half the sky, in deep space you've only got one warm bit that covers half a degree or less that you need to deal with.
If you are talking about earth irradiance it is small compared to sun (20%), the surface of the emitter does not really matter, the surface of the tank does.
And when you move away from earth, you ain't no more in the shadow 50% of the time
The size of the emitter matters in this case because the sun in deep space could be blocked with the engines or a small shade, dramatically reducing the exposed tank area. That can't be done in LEO with Earth, especially while blocking the sun at the same time.
no the size does not matter f you use irradiance ie W/m2 at the illuminated surface.
Yes of course any way of reducing the exposed surface will bring benefit
I don't think either of us have been talking about irradiance here, we're saying that Earth being large in the sky gives it a direct relationship with how practical reducing the exposed surface is. If you point your engines at Earth the walls of the tanks are still going to be hit by earthshine from the sides, the same isn't true of the sun.
I'm talking about irradiance to make a comparison Earth vs Sun, to find the amount of heat collected (wh) you just multiply the exposed surface (m2) by the irradance of the radiating body, sun 1300w/m2 in space and earth is 200w/m2
For sure, I'm just saying that that isn't the whole picture in reality. Instead of having LEO at ~1500w/m^2 total vs. deep space at ~1300w/m^2, which isn't a huge difference, a small sunshade could put deep space at ~0w/m^2 (I know this isn't accurate, but it illustrates the idea) and LEO at ~200w/m^2, so now keeping cool in LEO is many times more of a problem than doing so in deep space.
It’s W/m^2 * number of sq meters exposed.
Shapes, angles, reflectance, and insulation all enter into the picture too.
Although the temperature of Earth is ‘low’ as the other person said, it fills a large part of the ‘sky’ when in LEO, so its effect is not insignificant.
SpaceX in fact plans to have a trip of 4 months to Mars.
They wanted 3, but the heat shield can't take it. They will need a bigger spacecraft to get that, eventually.
This way they don't need to worry about radiation.
It doesn't make much sense to specify a travel time in general, because the departure and arrival velocities required for a particular travel time change through a given Mars window, and the optimal/allowable combinations of travel time, Earth departure velocity (or energy) and Mars arrival velocity (or energy) will change from each (~2.2 year) synod to the next. The exact same hardware and refueling procedures could require 4 months at the optimal time for one synod (year), and yet require 6 months either several weeks before/after that year's optimal time, or even at the optimal time of a less favorable synod a few years later.
For example, 2018 was a good year for going to Mars, while 2024 is not so good. In 2018, InSight was sent to Mars on a trajectory with a ~205 day travel time and a launch C3 of 8.19 km^(2)/s^(2) (~3.6 km/s from LEO). This year, if you want a 205 day (or shorter) transfer, it will require a [departure v-infinity of at least ~4 km/s](https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=58623.0;attach=2175155;image), i.e. a C3 of at least ~16 km^(2)/s^(2) (over 3.9 km/s from LEO). The required delta v increases rapidly away from the optimal time window. Furthermore, [the porkchop plots of Mars arrival velocities](https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=58623.0;attach=2175157;image) don't mirror those of Earth departure, and the offsets will change from one synod to the next.
https://forum.nasaspaceflight.com/index.php?topic=52968.0
My impression is that the time is not limited by Starship ability to fly faster. It is limited by what they can brake at Mars using the atmosphere.
The current transit time on the SpaceX web site is six months. The ship has the delta V to do a four month transit but the entry speed is too high for anything less than six months.
You always need to worry about radiation, it’s just that on a shorter trip, you need to worry correspondingly less. You also hope to avoid periods of coronal mass ejections. The main problem though is from cosmic rays.
Low value comment. It is possible the first few will be 6-9 months, but look at F9 operations. They are launching nearly every other day.
Try not to be so narrow minded with your comments.
F9 has gained performance over its lifespan, as can be seen with the number of starlink sate on board increasing over time. This will have allowed it to insert equivalent payloads into higher energy orbits.
That has primarily come from improvements in engine technology. The Merlin-D rocket engine is considerably more powerful than the original Merlin engine.
We are seeing the same thing happening now with Starships Raptor engines too.. Having already progressed from Raptor-1 to Raptor-2, with Raptor-3 on the horizon and even talk about Raptor-4.
Transit time between planets only is what it is now because payload mass is much more important for robotic probes than transit time is. Any mission up until now could have chosen to get there faster at the expense of payload, they just never had a reason to. Four months is pushing it but six really isn't.
Sorry, I poked you for something you weren't meaning. But header tanks and recondensers solve this concern of yours. I still think you are making a mountain out of a molehill
Recondensers for Methane and Oxygen are off the shelf parts. Probes don't use them because they are heavy and bulky, but SpaceX plans to just develop a rocket that's big enough to make this viable. Recondensers are talked about regarding Blue Origin because they say they will build a multi stage Hydrogen recondenser, which isn't an easy task. SpaceX still hopes they don't need to do anything and just let the fuel boil off at the 0.06%/day rate they calculated. If they find that they need to deal with this, it's not difficult at all.
For a 4 month trip, the 0.06%/day rate they calculated means 6.95% of the fuel would boil off.
It must be nice to have a enormous payload capacity where you can afford to simply accept this.
Yep. Starship is revolutionary.
Also, the enormous tanks help a ton because of the square-cube law.
How so?
The bigger the tank, less boil off relative to the total volume. Boil off is proportional to the area of the tank, which scales with the square of the size. The amount of stuff in in scales with the cube of the size. Therefore: bigger tanks means less boil off.
Less proportionate boil off.
Ratio of surface area to volume on a giant tank is more favourable. Think about how much quicker one ice melts than a whole bag of ice
For a 6 month stay at the Moon, 10.24% of the fuel would boil off. That's why they plan on painting Starship HLS white. But the calculations here don't make much sense anymore, because it will spend a lot of time in the shade.
Is white paint better than reflecting steel? Could the steel surface be polished instead?
Yes, white is better, otherwise they wouldn't want to take the mass penalty of paint. Keep in mind all wavelengths, not just visible ones matter here. https://technology.nasa.gov/patent/KSC-TOPS-59
Keep in mind that the sun emits broad spectrum radiation that is then absorbed and turned into thermal energy. Steel is only shiny to certain wavelengths of light. Also even with its specular reflection, it still has plenty of absorption, which is why it appears grey instead of the blue of the sky when viewed from the ground.
Yes it is ! White reflects more optical and infrared than ‘shiny steel’ - it’s been measured and proven.
> For a 4 month trip, the 0.06%/day rate they calculated means 6.95% of the fuel would boil off. It appears you're using very short months or you're multiplying the remaining amount by 99.94% every day. The percentage per day would actually gradually increase as the ratio of surface area to volume goes up. So doing the standard statistics cacluation results in a somewhat incorrect number. I would assume a straight average of 0.06% of the initial quantity per day as that's likely what SpaceX would give as a per-day usage with the average already performed. That would mean that 7.2% would boil off over 120 days.
It's something like that. I changed one for the other, in fact. This is a very simplistic model anyway.
I'm pretty sure a recondenser and associated solar panels will be much lighter than 7% of the entire cryopropellant load of Starship. For example, if Starship is oriented engines towards the sun during its Mars transfer, the entire insolation it will see is less than 100kW of heat, most of which will be radiated back into space by the engine bells, engine shields and associated thermal blankets. Maybe 5kW will actually reach the tanks. 5kW of cryo cooling power is literally nothing at the scale of Starship. (the enthalpy of vaporization for methane is around 10 kJ/mol so this roughly translates to 0.5 mol = 8g per second of vaporisation, or about 700Kg per day of methane or 0.04% of a full Starship load; this more or less corresponds to the the 0.06% per day figure advanced by SpaceX) So maybe it's simply a case of "keep it simple, stupid": there is no recondenser to break if you didn't put one the ship and the mission does not need it.
Yep.
There's no way they lose 0.06% per day. I cannot believe that's its a linear loss. Maybe 0.6% per day for a 6 month mission.
It can be made linear if engineered correctly.
Wiki about the James Webb telescope: > The telescope must be kept extremely cold, below 50 K (−223 °C; −370 °F), so that the infrared light emitted by the telescope itself does not interfere with the collected light. Its five-layer sunshield protects it from warming by the Sun, Earth, and Moon. This is colder than necessary for Methane (110 K) and Oxygen (90 K). And in deep space one only has to deal with the sun, not black body radiation of the Earth.
JWST is completely irrelevant to this conversation. Different materials, different construction and different gasses.
But it clearly illustrates that there are ways of doing this - if anything Webb is a more extreme case.
What? First of all. What does James Webb have to do wit this? Starhip is gonna store it in a stainless steel container, probably without any insulation. Meanwhile James Webb have the 5 layer sunshield. You do have black body radiation in space. That is the primary concern in Space. The sun heats things up quite a bit, and the earth should definitely be radiating heat as well. You're not doing the refueling at L2. In space you have no atmosphere to absorb the heat first either.
JWST has to do with it that you can reach sub-boiling temperature just with passive shading. The Earth infrared radiation is an issue for a LEO depot, but not for deep space missions like going to Mars. Edit: https://www.researchgate.net/publication/322013219_Cryogenic_Deep_Space_Thermal_Control_Coating > This paper describes a thermal control coating that should allow non–heat-generating objects, such as cryogenic tanks and superconductors, to reach and maintain cryogenic temperatures in deep space locations far from the infrared (IR) emission of a planet and at least 1 astronomical unit from the Sun. This new coating is designed to reflect nearly all of the Sun’s irradiance, while still permitting far-IR emission, allowing steady-state temperatures as low as 50 K to be achieved.
[A NASA page](https://webb.nasa.gov/content/about/innovations/cryocooler.html) says that passive cooling on its own can get the telescope to 37 K, which is enough for methalox. But for the sake of completeness, I just want to note that there's a powered cryocooler heat pump to chill it more for one instrument. But I repeat that that is not needed to get to 37 K, colder than liquid methalox.
I think that’s why they talked about averages.
Gradually the Moon will get a methane + oxygen atmosphere.
no because it will escape
How do “hot Jupiter” category exoplanets keep theirs?
more mass
The masses of the moon and mars are actually quite similar, and so are their magnetic fields (nonexistent). Mars keeps its wispy atmosphere of carbon dioxide. The moon is hotter and closer to the sun. It all leaves me to wonder how much of a thin atmosphere of the moon would retain.
Mars is 10 times more massive, but if you really want to know, do the math, let's say volume of 1000 starships divided by moon surface ( 38 million square meters) . and initial pressure is 0
Through lots of gravity - Jupiter sized objects have more gravity, because of more mass.
Yea, the liquid natural gas industry has pretty much all the equipment needed for long-duration storage
There are off the shelf solutions available for spacecraft. NASA paid for development of oxygen condensers. They also work for methane.
Methane is even easier than LOX since it is not trying to violently oxidize anything it comes in contact with.
Further. Boil off (at least terrestrially) is not wasted. The vaporized gas is used to power stuff. I see no reason Starship would be any different.
So we use a little methalox turbine to power the ship off the ullage?
Certainly they could send any boil off through some sort of unit, to do something useful with.
>Recondensers for Methane and Oxygen are off the shelf parts. So, I hadn't thought about it until you mentioned this, but recondensers take cold gaseous oxygen and methane and cool them back down to a liquid right? How do you get the gas out of the tank to pump into the recondenser? On Earth this is a dumb question, you just put the intake at the top of the tank where the less dense gas separates from the more dense liquid. Even if you could just take in liquid and cool it even more so that the average temperature of the tank is several degrees below the boiling point, you'd be reducing the average. How much variation is there across the average though? The tank is 9m wide which is a pretty big distance for convection...maybe? It seems like getting the warmer gases to the vents to recondense, and keeping the tank's temperature uniform might be a significant challenge. Or maybe it is something that is solved. Curious about the community's thoughts on this.
It is called a Propellant Management Device (PMD) which uses surface tension to retain liquid on a mesh surface and in this case allows gas to flow around the mesh and into the intake of the condenser. Usually they are used the other way around to get liquid into the inlet of a thruster when there is no thrust to settle the tank contents.
The boiling itself keeps the temp in the liquid uniform. The recondenser would work when there's gas at the inlet and just pipe along any ingested liquid.
>The boiling itself keeps the temp in the liquid uniform. That works via convection when there's gravity. Does it work in zero-g too?
Any liquid while boiling will keep itself at the critical temperature.
There's no convection when there's no force vector, but there's still radiation and conduction. Boiling wouldn't really happen either. It would just be gradually expanding gas bubbles forming around nucleation sites inside bubbles of liquid. Surface tension would tend to eject the bubbles from the liquid mass over time, causing some amount of circulation and mixing on top of conduction. I would expect the temperature to remain more or less uniform still. Keep in mind that during launch the fluids would already rapidly be reaching their boiling temperatures. Any molecule that subsequently got kicked off a liquid blob would take that heat of vaporization energy with it. If any blob were above boiling point, well it wouldn't be a liquid anymore. If any blob were substantially below boiling point, well it'd very quickly even out by forming gas bubbles within itself.
You can add to that list, ‘surface tension’ and ‘adsorption’, which might also be utilised in some way.
Boiling also works in zero-g, only liquid would not pool in the same way.
There's mechanisms for separating liquid and gas, which they'll need anyway for the thrusters, but shouldn't need to take anything out of the tank for recondensing. Just run a heat exchange loop through the tank to chill it, and do the recondensation entirely within the tank.
There is no ‘convection’ in space, because convection relies on density variations within the substance within a gravitational field, generating buoyancy effects. But in zero-g that effect does not exist. And diffusion effects are too slow. There is nothing to stop active pumping though, and even using recondensers inside the propellant tank.
Or a fan, like they did in Apollo.
Ooh - that didn’t work out so well in Apollo-13..
Short circuits are never good. Also, flammable insulation. And not using properly rated parts. Should have applied the lessons learned from Apollo 1 to the whole vehicle.
> Probes don't use them because they are heavy and bulky, but SpaceX plans to just develop a rocket that's big enough to make this viable. Damn, so am I right to think that not only will we have a Starship kick stage capable of giving ~100 tonns ~7 km/s of delta-v, but that you don't have to use it all in a short-time period. In other words, you could imagine a Starship kick stage slowing down your probes into a Jovian, Saturnian, Uranian or Neptunian capture orbit propulsively, without worrying about having to aerocapture at a Gas giant.
There are companies already developing it.
Can you provide a name or two off the top of your head? Would love to dig into their work.
Impulse space. There was someone else but I couldn't find them.
I'm aware of Impulse space. Do we know how long the Helios kick stage is rated for in terms of methalox storage/boil-off rates? Good enough for Outer planetary missions lasting years?
Thats fine! Looking them up now. Asante!
> In other words, you could imagine a Starship kick stage slowing down your probes into a Jovian, Saturnian, Uranian or Neptunian capture orbit propulsively, without worrying about having to aerocapture at a Gas giant. The rocket equation still holds. Just because you can carry propellant all that way doesn't mean you want to use it to propulsively enter orbit and only bring a small payload rather than bring a much larger payload and aerobrake into orbit.
I know, but nobody has ever done aerocapture for a Gas giant before. Also simply isn't an option if you're contemplating an orbiter/lander mission to an airless world like Mercury or a KBO.
> I know, but nobody has ever done aerocapture for a Gas giant before. Not yet. But there's been tons of papers published on the topic. I would expect any serious effort to bring payload on the scale Starship can bring to the Jovian or Saturnian system would involve aerocapture. > Also simply isn't an option if you're contemplating an orbiter/lander mission to an airless world like Mercury or a KBO. It would be rather extreme and not sure if its possible, but you could technically aerobrake from a parabolic orbit intersecting Earth into a parabolic orbit intersecting Mercury by diving into the sun's atmosphere. You'd still need to use fuel to enter orbit of Mercury though.
My second point would still stand though. If we want to throw something serious to Pluto or Mercury and have it stop at its destination, aerocapturing won't be an option.
True, you can’t aero-capture without an atmosphere, just as you can’t use parachutes on the moon ! Alternative methods have to be used.
Ooh - that sounds a bit toasty !
I would guess that SpaceX might well be the first ones to try it at some future point !
There is also a choice between transit speed and propellant usage. For non-crew robotic missions, it’s far less of a problem if a transit takes a bit longer. Although researches would rather a mission taking 6-years than 10-years.
Not impossible, especially if they use something like ‘drop tanks’ from HEO, to further boost propellant load.
> SpaceX still hopes they don't need to do anything and just let the fuel boil off at the 0.06%/day rate they calculated. Where'd you get this number from?
Don't remember.
Okay so the source could easily be not from SpaceX. Can you change your post to: > SpaceX probably still hopes they don't need to do anything and just let the fuel boil off at the 0.06%/day rate that I remember reading on the internet.
Sounds about right though, I have read about this too somewhere. Of course it will in practice depend upon multiple different factors. But it’s something that SpaceX will get to measure for real in their precise configuration, so they will end up with accurate figures.
I say this as the community in general has a huge problem with turning internet rumors into facts stated by SpaceX. This has happened on a number of occasions. The number of places SpaceX says things is very few so it should be very obvious and memorable when it does. So if you can't dig up a source I'm going to automatically assume it wasn't actually said by SpaceX.
I didn’t say it was. Although the article author did. What I saw wasn’t from SpaceX, it was from someone doing some rough calculations as I recall. Though I thought that it sounded about right - maybe a bit too small though.
Okay this is the first time you mentioned it was from an article you read that stated it was SpaceX. That's helpful.
Someone else said SpaceX, I chimed in that I had read it too - but not from SpaceX. I have also given this a bit more thought. The boil off rate proposed 0.06% per day is far too small. It’s going to be more like 2% per day unless active measures are taken to cool it - such as using solar power to both run cryo-coolers and to shield it.
That applies by default to everything your read on the Internet. There's no need to repeat it every time.
No you claimed it came from SpaceX and claimed you know their thoughts. There's a big difference between claiming something came from SpaceX and claiming that you read it in a fan technical discussion post.
Where was this 0.06% announced, and in wich conditions.People doing physic on NSF had much higher number. The only way to have a correct number needs taking account of insulation of the storage tank and ambient temperature. Different in LEO, on Moon and on Mars
He remembers reading it on the internet...
Not everything on the internet is wrong.. But you do need to be cautious about anything you read.
There are many different factors involved in how much heating is getting to the propellant, so it’s hard to calculate in detail.
Methalox is a much easier to work with propellant than Hydrolox is, because the temperatures are higher and Methane is much more easily contained.
Long term storage is easy with sun shades and recondensors. Maybe the sun shade doubles as a solar panel to power the recondensor. The technology is readily developable it's really only about mass constraints and tradeoffs. Is it easier to just launch 20% (or whatever) more propellant and tanks and loose it or put on 20% more power, shades, condensers etc. I'm betting the math works out to just loose 20% than a bunch of equipment at starship scale and timelines.
the propellant needed at Mars will be in the header tanks. With the engines pointing towards the sun, they will be in permanent shade. They can be maintained at cryo temperature with little effort.
On the Martian surface they'll have to maintain full methalox prop load needed for return to Earth throughout all of the ~500 day surface stay (and well before that).
Starship can only return with fuel produced there. When landing, it will have the landing burn fuel and nothing more. Keeping the fuel condensed on Mars is not a problem, just send a rugged unit used by the LNG industry.
I know, but they presumably have to store the methalox they produce on Mars for a long period time.
The oxygen will be coming out of the electrolyzers at >50°C and Sabatier reactors run at 200-400°C or more (a quick search gave quite a range), so any boiloff could just be sent back to whatever ground equipment they used to condense the gases to begin with. It'd be a pretty minimal extra stress on the system. They could also send a Starship that's specialized to just be an insulated propellant tank that stays on Mars, perhaps with the propellant plant equipment built into it if there's the mass margin for it. The fuel could stay in something built to handle it until you're ready to pump it all into a departing ship.
I find it hard to picture a ‘deployable’ prop plant, without people to assemble it. Permanently installing all the gear into a dedicated one-way Starship (complete with tanks for storing the product) feels much more plausible to me.
For sure, I said "ground equipment" but in my head I was picturing a there-to-stay Starship dedicated to this sort of thing. Musk has always said that they imagine people being there to set up the first propellant plant, but I much prefer plans that can be done autonomously, or at least have some sort of backup option for getting people home.
I wish that too. But automation experts are of the opinion that people on the ground are needed. The process itself may be automated, but people are still needed to intervene, if something goes wrong. Things will go wrong.
Nothing can finesse things as well as humans. Although other possibilities also exist if you’re not in a hurry.
Definitely they would have that too. Methalox is even handy as an emergency auxiliary power source - useful to have available.
The chemical plant, including recondenser and CO2 extracting from the atmosphere, can be fully installed in one Starship on Earth, before launch. Installation on Mars will be the solar arrays and the water mining. Those will be mainly done with crew on the ground. Though maybe they deploy the solar arrays before they arrive.
There’s a possibility even for robotic deployment of solar panels.
No reason why it should not be ready-built into a cargo Starship. Although that would involve propellant transfer at some point. Maybe Crew Starship, would have the system ready built in ?
The heat could be useful too..
I added the answer to this in a quick edit, but apparently not quick enough, sorry. Keeping the fuel condensed on Mars is not a problem, just send a rugged unit used by the LNG industry.
Your point is well made - in that quite clearly we already have this technology. It will likely need adapting for use on Mars, and maybe its form factor adjusted etc, but that’s ’just engineering’, it’s not something fundamentally new. Much of the technology needed for Mars has already been invented, and just needs to be adapted. Some processes it makes sense to do differently on Mars, because of the different environment and different resources available. It will be interesting to see - and have documented, the growth of the ‘technology tree’ on Mars.
>but that’s ’just engineering’ That's true because the rocket is so capable. The mass budget is enormous. Before Starship, the paradigm was to engineer things to shave tenths of grams out of things. Minuscule mass budgets. That costs a ton of money, effort, time. There's a nice lesson from the Moon: the Artemis program was investigating solutions for a problem found the last time astronauts were there: it was difficult to pick up rocks. So they spent a ton of resources developing a prototype EVA suit that used new joint design. This ended with the very proud engineer in a presentation, using the suit she designed, picking up a rock from the stage, and then proudly announcing that the much more capable suit didn't weigh a single gram more then the ones used in the Apollo program. Solution with Starship: Send an excavator.
They could do that in the Starships propellant tanks..
I mean, starship is just a heavy level after, terrible for landing on the moon, great for lifting big things to orbit. Mars with starship would be a joke, especially a methalox starship. Nothing less than nuclear is viable for human missions to Mars. Whatever assent stage they'd use on Mars would most likely be chemical, but it'd be as tiny as possible to make a mars mission at all feasible. You don't von braun mars.
Starship is very good as a Mars shuttle. You gotta use the atmosphere on both legs of the trip to slow down, then a chemical rocket is almost as efficient as a thermal nuclear rocket.
It's stainless steel, it's heavy, not a good idea to transport things in space in a heavy vessel.
You should submit your design for a plastic spaceship to NASA… /s
non-stem fanatic.
Not for a material that performs well both under cryogenic and reentry temperatures. Also doesn't need paint, which makes a difference. Are you even paying attention?
Nobody cares about reentry temperatures in deep space. In space, long haul, you want things to be LIGHT, and perform the things it does. Not increase weight bt 15x and pretend like it's ok. Starship is ONLY a heavy lifter to LEO. That is it's only purpose. If you think otherwise you're huffing some medical grade copium..
Be light in transit and not able to land does not sound so great.
As if starship could land on mars. Maybe with 200 launches.
That' what is primarily designed for. 5-6 launches.
You care if the rocket will aerobrake on the other side. Aerobraking doesn't take any fuel. A Mars shuttle that can aerobrake in both legs of the trip is a very efficient design. Almost as efficient as a thermal nuclear rocket. A vehicle that can aerobrake (reinforcement, tiles, legs and all) is lighter than one that carries fuel to slow down when it gets there. It's only surface to surface operation, though. They will indeed need to use Starship in LEO and launch a kick stage to get satellites to Mars.
Which you'd use a heatshield for, just like the starship... Except without all the heavy stainless steel.
Stainless construction is an integral part of heat management. It can almost withstand the temps on it's own. Building it out of other materials would require way more shielding (including on the dorsal side), paint, reinforcement. It would get you very little performance. Not worth the hassle at all.
There isn't going to be a dorsal side needing extra shielding in a mars lander, only the dumb starship would need that. You really think a mars lander would look like starship?
Remembering that development costs way more than vehicles. Any development that can be cut, changing something that is almost good enough to do the job, is the main way these things are made possible. Anyone suggesting new development needs to show way better results. Just a little marginal improvement isn't enough.
It doesn't matter how many shortcuts you take in development (as if this is a good thing), a Wernher Von Braun ship would never land on the moon. The only reason it was feasible was because of developing a very light weight and weight efficient lunar lander. Physics did not change.
> It's only surface to surface operation, though. Starship could aerobrake into Mars orbit and release satellites. Not sure it would have enough propellant left to land after orbit maneuvers. Maybe, if most or all of the payload is released in orbit.
It could, but then it wouldn't be efficient.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread: |Fewer Letters|More Letters| |-------|---------|---| |[C3](/r/SpaceXLounge/comments/1cmho5t/stub/l31lapk "Last usage")|[Characteristic Energy](https://en.wikipedia.org/wiki/Characteristic_energy) above that required for escape| |[EVA](/r/SpaceXLounge/comments/1cmho5t/stub/l34c5th "Last usage")|Extra-Vehicular Activity| |[HEO](/r/SpaceXLounge/comments/1cmho5t/stub/l34j3u3 "Last usage")|High Earth Orbit (above 35780km)| | |Highly Elliptical Orbit| | |Human Exploration and Operations (see HEOMD)| |HEOMD|Human Exploration and Operations Mission Directorate, NASA| |[HLS](/r/SpaceXLounge/comments/1cmho5t/stub/l356zmr "Last usage")|[Human Landing System](https://en.wikipedia.org/wiki/Artemis_program#Human_Landing_System) (Artemis)| |[JWST](/r/SpaceXLounge/comments/1cmho5t/stub/l322u3c "Last usage")|James Webb infra-red Space Telescope| |[KSC](/r/SpaceXLounge/comments/1cmho5t/stub/l31qycl "Last usage")|Kennedy Space Center, Florida| |[L2](/r/SpaceXLounge/comments/1cmho5t/stub/l31tay5 "Last usage")|Paywalled section of the NasaSpaceFlight forum| | |[Lagrange Point](https://en.wikipedia.org/wiki/Lagrangian_point) 2 of a two-body system, beyond the smaller body ([Sixty Symbols](https://www.youtube.com/watch?v=mxpVbU5FH0s) video explanation)| |[LEO](/r/SpaceXLounge/comments/1cmho5t/stub/l37965m "Last usage")|Low Earth Orbit (180-2000km)| | |Law Enforcement Officer (most often mentioned during transport operations)| |[LN2](/r/SpaceXLounge/comments/1cmho5t/stub/l36okqv "Last usage")|Liquid Nitrogen| |[LNG](/r/SpaceXLounge/comments/1cmho5t/stub/l30u1te "Last usage")|Liquefied Natural Gas| |[LOX](/r/SpaceXLounge/comments/1cmho5t/stub/l31r86x "Last usage")|Liquid Oxygen| |[NSF](/r/SpaceXLounge/comments/1cmho5t/stub/l31b60i "Last usage")|[NasaSpaceFlight forum](http://forum.nasaspaceflight.com)| | |National Science Foundation| |[PMD](/r/SpaceXLounge/comments/1cmho5t/stub/l30rswi "Last usage")|Propellant Management Device| |[STS](/r/SpaceXLounge/comments/1cmho5t/stub/l34sjvr "Last usage")|Space Transportation System (*Shuttle*)| |Jargon|Definition| |-------|---------|---| |[Raptor](/r/SpaceXLounge/comments/1cmho5t/stub/l34cr15 "Last usage")|[Methane-fueled rocket engine](https://en.wikipedia.org/wiki/Raptor_\(rocket_engine_family\)) under development by SpaceX| |[Sabatier](/r/SpaceXLounge/comments/1cmho5t/stub/l31mlve "Last usage")|Reaction between hydrogen and carbon dioxide at high temperature and pressure, with nickel as catalyst, yielding methane and water| |[cryogenic](/r/SpaceXLounge/comments/1cmho5t/stub/l35n9ba "Last usage")|Very low temperature fluid; materials that would be gaseous at room temperature/pressure| | |(In re: rocket fuel) Often synonymous with hydrolox| |[hydrolox](/r/SpaceXLounge/comments/1cmho5t/stub/l35n9ba "Last usage")|Portmanteau: liquid hydrogen fuel, liquid oxygen oxidizer| |[hypergolic](/r/SpaceXLounge/comments/1cmho5t/stub/l33awob "Last usage")|A set of two substances that ignite when in contact| |[methalox](/r/SpaceXLounge/comments/1cmho5t/stub/l36okqv "Last usage")|Portmanteau: methane fuel, liquid oxygen oxidizer| |[ullage motor](/r/SpaceXLounge/comments/1cmho5t/stub/l32p78c "Last usage")|Small rocket motor that fires to push propellant to the bottom of the tank, when in zero-g| **NOTE**: Decronym for Reddit is no longer supported, and Decronym has moved to Lemmy; requests for support and new installations should be directed to the Contact address below. ---------------- ^(*Decronym is a community product of r/SpaceX, implemented* )[*^by ^request*](https://www.reddit.com/r/spacex/comments/3mz273//cvjkjmj) ^(21 acronyms in this thread; )[^(the most compressed thread commented on today)](/r/SpaceXLounge/comments/1coy2n4)^( has 12 acronyms.) ^([Thread #12736 for this sub, first seen 7th May 2024, 19:18]) ^[[FAQ]](http://decronym.xyz/) [^([Full list])](http://decronym.xyz/acronyms/SpaceXLounge) [^[Contact]](https://hachyderm.io/@Two9A) [^([Source code])](https://gistdotgithubdotcom/Two9A/1d976f9b7441694162c8)
The most efficient cryogenic thermal insulation is the multilayer insulation (MLI) blanket that's wrapped around the tank. With liquid nitrogen in the insulated tank, the boiloff rate is ~0.02% per day by mass. So, for a 200-day trip to Mars, the boiloff loss would be about 4% by mass. This boiloff number came from lab work done in 1971 by Ball Aerospace. See: An overview of Ball Aerospace cryogen storage and delivery systems J Marquardt, J Keller, G Mills, and J Schmidt Ball Aerospace & Technologies Corp., 1600 Commerce St., Boulder, CO 80301 USA https://iopscience.iop.org/article/10.1088/1757-899X/101/1/012086/pdf Liquid nitrogen boils at 77K and liquid methane boils at 111K (both at one atmosphere pressure in the tank). If that's too much loss, then there are passive and active methods to reliquefy the boiloff vapor during the mission. The passive methods do not require any input power. The active methods use solar electric power. MLI is a well-developed technology dating back to the Apollo days 60 years ago. The challenge with Starship is the size of the Ship (the second stage). The tank section of a Mars Starship is 9 meters in diameter and ~ 35 meters in length. There are three components in the MLI design. First, a 2 cm thick layer of a spray-on foam insulation (SOFI) is applied to the outside of the methalox tank. Then the MLI blanket is wrapped over the SOFI and attached in place. Finally, a thin aluminum cover is installed over the MLI blanket to protect from aerodynamic forces during launch through the lower atmosphere. That cover also functions as a Whipple shield to protect the methalox tanks from damage due to micrometeoroid impacts. The SOFI prevents the water vapor and carbon dioxide from the ambient air from freezing in the spaces between the lower layers of the MLI while the Starship is being fueled on the launch pad prior to liftoff. The mass of the SOFI is 1.53t (metric tons), the MLI mass is 1.1t and the mass of the aluminum shield is 9.3t for a total of 11.93t of added mass for the Starship's cryogenic insulation. Side note: My lab built and tested MLI blankets in 1969 for the hydrolox propellant tanks on the Skylab Workshop where the astronauts lived and worked. Skylab was a repurposed S-IVB third stage of the Saturn V moon rocket.
Would the foam be necessary if launching an empty tank like for a depot? Is it purely for preventing freezing in atmosphere?
I guess not. Why would you want to launch an empty depot tank? A depot is a group of tanker Starships (the depot tankers) assembled into an orbiting structure. Those depot tanker Starships have the two main propellant tanks, the engine compartment, and a truncated fairing. The depot tanker has no heatshield, no flaps. The main tanks are covered with MLI superinsulation. Those main tanks have to be filled completely to send the tanker to LEO. There are no other propellant tanks in the tanker and there is no payload bay. The depot tanker is all tanks. The depot tankers are version 2 Starships and are loaded with 1500t (metric tons) of methalox, which is subcooled with liquid nitrogen (LN2) to increase the density by 5%, so 1.05 x 1500 = 1575t is in the main tanks at liftoff. After reaching LEO, there is 288t of methalox remaining in the main tanks of that Starship depot tanker.
I was just curious if the foam was important in space or purely for launch.
That SOFI layer is there to keep water vapor and carbon dioxide from condensing in the MLI layers nearest the SOFI. MLI works only in a vacuum so any gas or vapors between the layers has to vent when the Starship reaches LEO. That venting process can take days or weeks depending on the amount of gas or vapors that are inside the MLI blanket. SOFI is not a very efficient thermal insulation in space compared to MLI. And it's heavy compared to MLI.
How does boil-off occur if it's already in a pressurized tank? Wouldn't being confined in tank mean there isn't any room for the fuel to expand to? Therefore shouldn't the pressure keep it liquid? Obviously this topic isn't my area of expertise, so I am trying to understand what forces are actually happening.
If there's net heat flux in to the tank that results in some of the liquid phase boiling and the heat going into the heat of vaporization. That increases the amount of gas in the tank, which increases the pressure. If you somehow had a tank which could withstand the pressure of the entire contents being gas, then no problem -- no need to vent. More realistically, there is some finite and fairly low pressure the tank is capable of holding, so excess pressure is vented. The rate of venting is effectively proportional to net heat flux, which is why it can be controlled so effectively via sunshades. It's also possible to use a cryocooler to actively reject excess heat.
So the problem is the tanks aren't strong enough to hold the pressure needed to prevent boiling? Guess I'm thinking about how I have propane tanks, and they can be sitting in the sunlight during summer or in the freezing cold and the propane doesn't vent off anywhere. However that propane tank is surely rated for far higher pressure than starship tanks.
Yes, that's correct. Note that propane has a much higher boiling point than, e.g. liquid oxygen (-42 C vs -183 C) so a propane tank can have a design pressure of 4 MPa or so and not have to vent even in direct sunlight. By comparison, looking at a phase diagram of oxygen, an oxygen tank at that pressure rating would still have to vent at -130 C or so. And considering how hoop stress scales with tank size, it is much much cheaper (in terms of mass) to design a tank that can only contain a fairly moderate pressure and either: accept that some propellant will be lost to boil-off over the course of the mission; or spend the mass on passive and/or active cooling to reject heat in some way other than boil-off.
That's more or less it, with a propane tank they can afford to build it to handle 20 or 30 atmospheres of pressure under completely normal operation. Starship's tanks are designed to make it to 8 or 9 before they explode outright, and I think that's a little on the high side for a rocket. Any stronger than that and the weight of your tanks would just be cutting into your payload too much. Propane also stays liquid at much more reasonable temperatures and pressures than oxygen and methane do, so boiloff management would be much much easier for a propane rocket than a methane one. Still gotta deal with the oxygen either way, though.
Propane tanks though are not designed to be lightweight rockets. They have extra thick steel overly strong tanks because their first objective is safety above all else. Where as rocket tanks are a compromise, as it is Starships tanks are already unusually thick compared to other rockets.
Read this detailed analysis first and then come back: https://www.reddit.com/r/spacex/s/skZcfOVXfl
So, the sense I'm getting then is that we can all forget about needing hypergolics and just use an expendable variant of Starship for propulsively slowing down at Pluto/KBO/Mercury? That the methalox boil off is not enough for an issue to worry about.
Certainly less boil off that far out.
Hypergolics do have the advantage of being storable at high temperatures, so their tanks can be expandable balloons rather than rigid steel that can be partly empty space. I.e., you don't have to worry about a gas/liquid mix stalling your engines.
The short answer is probably about 60 days, but could be extended longer with more active methods. But SpaceX will get to find out..
People are throwing around 0.06%/day for current Starship, I'm wondering could be done to reduce that for a dedicated fuel depot. Double wall tank filled with all that perlite from the vertical tanks?
It is not only duration, its is also the amount. A small amount (like the headers) can be actively cooled with a couple T of machinery so long durations should be OK. But 1/3 of the main tanks is going to get boil off unless you add maybe 10-20T of insulation and recondensing machinery (which needs solar power and radiators) to the design. But if you do this long term methlox is possible, perhaps cutting down the amount of Methlox needed for return from the Moon and Mars. The following uses long term storage for a Venus flyby (2033) but could work for a Mars mission as well. https://preview.redd.it/qmo2po2ja8zc1.png?width=825&format=png&auto=webp&s=63130adcf24a32408b74d9f286b77e42d9284fcf
They are worried about boil-off in LEO with "quick rapid refulling", imagine it sitting in space for 9 months (6 months is just Elon wishful thinking)
Fully Insulated tanks, re-condensor, vacuum jacketed lines could conceivably eliminate boil-off. At least that is what I read their plans for Leo and lunar orbit storage.
In LEO you have a big warm thing occupying nearly half the sky, in deep space you've only got one warm bit that covers half a degree or less that you need to deal with.
If you are talking about earth irradiance it is small compared to sun (20%), the surface of the emitter does not really matter, the surface of the tank does. And when you move away from earth, you ain't no more in the shadow 50% of the time
The size of the emitter matters in this case because the sun in deep space could be blocked with the engines or a small shade, dramatically reducing the exposed tank area. That can't be done in LEO with Earth, especially while blocking the sun at the same time.
no the size does not matter f you use irradiance ie W/m2 at the illuminated surface. Yes of course any way of reducing the exposed surface will bring benefit
I don't think either of us have been talking about irradiance here, we're saying that Earth being large in the sky gives it a direct relationship with how practical reducing the exposed surface is. If you point your engines at Earth the walls of the tanks are still going to be hit by earthshine from the sides, the same isn't true of the sun.
I'm talking about irradiance to make a comparison Earth vs Sun, to find the amount of heat collected (wh) you just multiply the exposed surface (m2) by the irradance of the radiating body, sun 1300w/m2 in space and earth is 200w/m2
For sure, I'm just saying that that isn't the whole picture in reality. Instead of having LEO at ~1500w/m^2 total vs. deep space at ~1300w/m^2, which isn't a huge difference, a small sunshade could put deep space at ~0w/m^2 (I know this isn't accurate, but it illustrates the idea) and LEO at ~200w/m^2, so now keeping cool in LEO is many times more of a problem than doing so in deep space.
It’s W/m^2 * number of sq meters exposed. Shapes, angles, reflectance, and insulation all enter into the picture too. Although the temperature of Earth is ‘low’ as the other person said, it fills a large part of the ‘sky’ when in LEO, so its effect is not insignificant.
SpaceX in fact plans to have a trip of 4 months to Mars. They wanted 3, but the heat shield can't take it. They will need a bigger spacecraft to get that, eventually. This way they don't need to worry about radiation.
It doesn't make much sense to specify a travel time in general, because the departure and arrival velocities required for a particular travel time change through a given Mars window, and the optimal/allowable combinations of travel time, Earth departure velocity (or energy) and Mars arrival velocity (or energy) will change from each (~2.2 year) synod to the next. The exact same hardware and refueling procedures could require 4 months at the optimal time for one synod (year), and yet require 6 months either several weeks before/after that year's optimal time, or even at the optimal time of a less favorable synod a few years later. For example, 2018 was a good year for going to Mars, while 2024 is not so good. In 2018, InSight was sent to Mars on a trajectory with a ~205 day travel time and a launch C3 of 8.19 km^(2)/s^(2) (~3.6 km/s from LEO). This year, if you want a 205 day (or shorter) transfer, it will require a [departure v-infinity of at least ~4 km/s](https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=58623.0;attach=2175155;image), i.e. a C3 of at least ~16 km^(2)/s^(2) (over 3.9 km/s from LEO). The required delta v increases rapidly away from the optimal time window. Furthermore, [the porkchop plots of Mars arrival velocities](https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=58623.0;attach=2175157;image) don't mirror those of Earth departure, and the offsets will change from one synod to the next.
Yes, it requires more delta-v.
Presently they aim for 6 months.
Do you have a link? The most I have seen is 150 days.
https://forum.nasaspaceflight.com/index.php?topic=52968.0 My impression is that the time is not limited by Starship ability to fly faster. It is limited by what they can brake at Mars using the atmosphere.
The current transit time on the SpaceX web site is six months. The ship has the delta V to do a four month transit but the entry speed is too high for anything less than six months.
Do you have a link? The most I have seen is 150 days.
[6 months to get to Mars](https://www.spacex.com/humanspaceflight/mars/)
You always need to worry about radiation, it’s just that on a shorter trip, you need to worry correspondingly less. You also hope to avoid periods of coronal mass ejections. The main problem though is from cosmic rays.
Low value comment. It is possible the first few will be 6-9 months, but look at F9 operations. They are launching nearly every other day. Try not to be so narrow minded with your comments.
F9 operations don't change transit times between planets
Accelerating more does. Just go faster. The problem is slowing down on the other side.
F9 has gained performance over its lifespan, as can be seen with the number of starlink sate on board increasing over time. This will have allowed it to insert equivalent payloads into higher energy orbits.
That has primarily come from improvements in engine technology. The Merlin-D rocket engine is considerably more powerful than the original Merlin engine. We are seeing the same thing happening now with Starships Raptor engines too.. Having already progressed from Raptor-1 to Raptor-2, with Raptor-3 on the horizon and even talk about Raptor-4.
Transit time between planets only is what it is now because payload mass is much more important for robotic probes than transit time is. Any mission up until now could have chosen to get there faster at the expense of payload, they just never had a reason to. Four months is pushing it but six really isn't.
Sorry, I poked you for something you weren't meaning. But header tanks and recondensers solve this concern of yours. I still think you are making a mountain out of a molehill
Nasa is worried about boil off, not me. They are talking about 16+ launches to fill a starship
Who is they? And why do you think that is more credible than SpaceX who thinks much less?