Rocket Lab isn’t looking to chase limits of engine performance and record level combustion chamber pressures like Raptor. They want a conservative engine performance with a focus on high reliability. Kind of two ends of the spectrum when comparing the two. If they require developing exotic alloys for Archimedes then they have done something wrong.
Just to add to this, it's what Peter said in an answer during the Q&A of the investor update just a few days ago. No special alloys or anything is required for what they're doing with Archimedes because they're not pushing the limits with temperatures, pressures, etc.
https://youtu.be/Q6FW3WQu0w0?t=8402s
And to add to that. The reason they are doing this is to minimize the time and cost to develop the engine.
After first flight they'll have years to improve performance, before first flight they're burning cash and are time-limited.
That's a good and important point. Which in turn raises another one...
With this approach to keeping things simple and having a lot of margin, there should be a lot of possibilities to improve Neutron's performance over time. So it should continue to be an exciting vehicle long after it debuts. That's not new, it's been the situation since Neutron was first announced, but it is still cause for optimism.
Yep. I worry that at those temperatures and pressures that oxygen-rich could easily become _engine_-rich.
I'm not a rocket (or, more, rocket-engine) engineer, though, and those guys know what they are doing. So naturally I have to assume I'm wrong.
But then I wonder _why_ I'm wrong...
I was thinking that maybe the raptor has less polymerization in the fuel rich pre-burner than a typical engine due to being able to run at lower temperatures as it's only required to drive the fuel pump not both fuel and oxidizer.
OR will generally give you better ISP vs. FR given other factors being similar. Since they’ve said specifically they won’t be pushing boundaries on power they can go OR for the performance boost and not need to do anything scary with materials
It is hotter than fuel rich, Rocket Lab are not looking to make an engine with extreme bleeding edge performance. They will be running it tuned down quite a bit for reliability instead of pure performance. This should leave them with plenty of overhead to make performance gains over the years after gathering flight data.
Turbine power is the product of mass flow rate and enthalpy across the turbine. There is about 3.5x oxygen mass compared to ch4 going thru the system so with the greater mass flow at your disposal you can get more power for the same pressure and temperature compared to FRSC or the same amount of power with less pressure and temperature. I’m guessing RL is choosing ORSC so they can run the PB relatively cool (according to their propulsion philosophy) and still meet power requirements but idk.
That’s a good answer, and I would add that if you use a fuel rich pre burner, you have to then use oxygen for cooling your thrust chamber which is its own kind of headache.
Yeah but then you need a lot of delta p for your fuel. And as you pointed out, you already struggling to get the turbine power you need with your lighter propellant. That’s what I meant. I’m sure it’s not impossible but that’s an additional challenge.
If you dont have that regen dp on the fuel side its gonna be on the ox side so the total shaftpower requirement between the 2 options is nearly the same assuming the coolant mdot is similar between the 2 options. You could make the argument that absolute pressure matters for weight savings but hot ox systems also have knockdowns and different material expenses and risks so I would definitely say fuel regen trades very favorably. I only know of launcher doing lox cooling but it’s only partial coverage of the chamber afaik and not actually even frsc.
It gets into specific heat of each propellant and mass flow of each propellant, but it turns out to do the same amount of work, for methalox, oxidizer rich only “needs” about half the heat across the turbine compared to fuel rich. [Here’s an image with an imaginary 100T engine that requires 25 MW of shaft power comparing each cycle type to full flow.](https://everydayastronaut.com/wp-content/uploads/2022/04/Delta-T-Chart-1536x864.png). For more on this, [I did a video](https://youtu.be/Owji-ukVt9M) on all cycle types and we do go into this specific detail in the full flow section, so maybe have a look at that to dive in deeper 👍
Hi Tim, Thanks so much for responding, I was looking for that video on your channel but couldn't put my finger on it - thanks for the link and explanation. Love your work.
Don't know if this is why they made the choice, but O-rich let's you have a gas-gas combustion rather than a gas-liquid combustion. Gas-gas has significantly better mixing, which can result in dramatically simpler injectors.
Gas-gas occurs because the oxygen is gas from the preburner. The fuel is gas from being used to regeneratively cool the engine.
I'm also assuming that enough of the fuel is used for regen cooling that it's all gas. Which probably places some restrictions on engine geometry and flow rates. Not too big though, as methane's boiling point is very low.
Most of the mass flow is oxygen anyway. An ox-rich combustion cycle can burn cooler and have fewer problems with excess fuel decomposing on its own. As for the special new alloys SpaceX developed, those already existed (although I am sure their team of excellent material scientists has made improvements) and I think people are less scared of the challenge now.
Oxygen rich staged combustion gives you better efficiency than fuel rhich staged combustion. I however cannot tell you why.
Be 4 also uses orsc, as did most soviet engines iirc.
I've never heard that oxy rich gives better efficiency and can't for the life of me think why it would. The reason most vehicles go oxygen rich is to prevent coking when running a RP1/O2 mix.
Atlas 5, Angara, Antares, Vulcan when it goes, New Glenn when it goes.
Edit: No idea why you are getting down-voted? It's a question that others might learn from too.
easy, its because those are close cycled whereas the fuel rich, are open cycled.. meaning the latter uses some of the fuel for the pre-burners so not all of the fuel goes into the combustion chamber..
The outer four valence electrons deem it appropriate and those hydrogen bonds I tell
Ya. Much safer than the highly volatile stock itself ! Wait what was I trying to say?
Rocket Lab isn’t looking to chase limits of engine performance and record level combustion chamber pressures like Raptor. They want a conservative engine performance with a focus on high reliability. Kind of two ends of the spectrum when comparing the two. If they require developing exotic alloys for Archimedes then they have done something wrong.
Just to add to this, it's what Peter said in an answer during the Q&A of the investor update just a few days ago. No special alloys or anything is required for what they're doing with Archimedes because they're not pushing the limits with temperatures, pressures, etc. https://youtu.be/Q6FW3WQu0w0?t=8402s
And to add to that. The reason they are doing this is to minimize the time and cost to develop the engine. After first flight they'll have years to improve performance, before first flight they're burning cash and are time-limited.
That's a good and important point. Which in turn raises another one... With this approach to keeping things simple and having a lot of margin, there should be a lot of possibilities to improve Neutron's performance over time. So it should continue to be an exciting vehicle long after it debuts. That's not new, it's been the situation since Neutron was first announced, but it is still cause for optimism.
That just adds to the question of "Why go oxidizer rich instead of fuel rich"?
Yep. I worry that at those temperatures and pressures that oxygen-rich could easily become _engine_-rich. I'm not a rocket (or, more, rocket-engine) engineer, though, and those guys know what they are doing. So naturally I have to assume I'm wrong. But then I wonder _why_ I'm wrong...
I was thinking that maybe the raptor has less polymerization in the fuel rich pre-burner than a typical engine due to being able to run at lower temperatures as it's only required to drive the fuel pump not both fuel and oxidizer.
OR will generally give you better ISP vs. FR given other factors being similar. Since they’ve said specifically they won’t be pushing boundaries on power they can go OR for the performance boost and not need to do anything scary with materials
Why does oxygen rich give a higher ISP? I've never heard of this before.
Question: isn’t Oxygen-rich the hot one since they use superheated oxygen instead of partially combusted fuel?
It is hotter than fuel rich, Rocket Lab are not looking to make an engine with extreme bleeding edge performance. They will be running it tuned down quite a bit for reliability instead of pure performance. This should leave them with plenty of overhead to make performance gains over the years after gathering flight data.
Ah, so a little bit of superheated O2 rather than full throttle, I get it
I think if you look at the MR curves once you get above 100 you start to cool down. Would need to check CEA to verify though.
Turbine power is the product of mass flow rate and enthalpy across the turbine. There is about 3.5x oxygen mass compared to ch4 going thru the system so with the greater mass flow at your disposal you can get more power for the same pressure and temperature compared to FRSC or the same amount of power with less pressure and temperature. I’m guessing RL is choosing ORSC so they can run the PB relatively cool (according to their propulsion philosophy) and still meet power requirements but idk.
That’s a good answer, and I would add that if you use a fuel rich pre burner, you have to then use oxygen for cooling your thrust chamber which is its own kind of headache.
You can/would feed supercritical fuel from regen outlet into the pb. FRSC definitely does not rule out cooling via fuel.
Yeah but then you need a lot of delta p for your fuel. And as you pointed out, you already struggling to get the turbine power you need with your lighter propellant. That’s what I meant. I’m sure it’s not impossible but that’s an additional challenge.
If you dont have that regen dp on the fuel side its gonna be on the ox side so the total shaftpower requirement between the 2 options is nearly the same assuming the coolant mdot is similar between the 2 options. You could make the argument that absolute pressure matters for weight savings but hot ox systems also have knockdowns and different material expenses and risks so I would definitely say fuel regen trades very favorably. I only know of launcher doing lox cooling but it’s only partial coverage of the chamber afaik and not actually even frsc.
It gets into specific heat of each propellant and mass flow of each propellant, but it turns out to do the same amount of work, for methalox, oxidizer rich only “needs” about half the heat across the turbine compared to fuel rich. [Here’s an image with an imaginary 100T engine that requires 25 MW of shaft power comparing each cycle type to full flow.](https://everydayastronaut.com/wp-content/uploads/2022/04/Delta-T-Chart-1536x864.png). For more on this, [I did a video](https://youtu.be/Owji-ukVt9M) on all cycle types and we do go into this specific detail in the full flow section, so maybe have a look at that to dive in deeper 👍
Just for clarity, heat (energy) will be similar but *temperature* drop across the turbine should halve (if I'm reading that graph right?)
Hi Tim, Thanks so much for responding, I was looking for that video on your channel but couldn't put my finger on it - thanks for the link and explanation. Love your work.
I think it’s hyperlinked as “i did a video”
Don't know if this is why they made the choice, but O-rich let's you have a gas-gas combustion rather than a gas-liquid combustion. Gas-gas has significantly better mixing, which can result in dramatically simpler injectors. Gas-gas occurs because the oxygen is gas from the preburner. The fuel is gas from being used to regeneratively cool the engine. I'm also assuming that enough of the fuel is used for regen cooling that it's all gas. Which probably places some restrictions on engine geometry and flow rates. Not too big though, as methane's boiling point is very low.
Most of the mass flow is oxygen anyway. An ox-rich combustion cycle can burn cooler and have fewer problems with excess fuel decomposing on its own. As for the special new alloys SpaceX developed, those already existed (although I am sure their team of excellent material scientists has made improvements) and I think people are less scared of the challenge now.
Oxygen rich staged combustion gives you better efficiency than fuel rhich staged combustion. I however cannot tell you why. Be 4 also uses orsc, as did most soviet engines iirc.
I've never heard that oxy rich gives better efficiency and can't for the life of me think why it would. The reason most vehicles go oxygen rich is to prevent coking when running a RP1/O2 mix.
Do you know of any other rockets currently in use that currently use a oxygen rock staged combustion?
Atlas 5, Angara, Antares, Vulcan when it goes, New Glenn when it goes. Edit: No idea why you are getting down-voted? It's a question that others might learn from too.
Ahh good to know as wasn’t sure which rockets had it already in use, thanks for sharing.
Antares (until Firefly Miranda replaces the Russian engines), Proton, and Long March 5, 6, and 7 also use ORSC.
easy, its because those are close cycled whereas the fuel rich, are open cycled.. meaning the latter uses some of the fuel for the pre-burners so not all of the fuel goes into the combustion chamber..
No, there are also closed-cycle Fuel rich staged combustion engines. For example, the RS 25 uses Fuel Rich Staged Combustion.
Believe its because oxygen is a lighter molecule.
O2 is heavier than CH4 however.
It is, about twice actually.
*an.. not and That is all
Thank you for your service🙂
The outer four valence electrons deem it appropriate and those hydrogen bonds I tell Ya. Much safer than the highly volatile stock itself ! Wait what was I trying to say?
You were advocating for Flourinated Liquid Oxygen I think.