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A Raptor engine has about 1/3 to 1/4 of the thrust of an F-1 engine. However, the Raptor is far more efficient and much more impressive technically. They are re

A Raptor engine has about 1/3 to 1/4 of the thrust of an F-1 engine. However, the Raptor is far more efficient and much more impressive technically. They are really a marvel of modern engineering and science.

JumpCrisscross 2y ago
> Raptor engine has about 1/3 to 1/4 of the thrust of an F-1 engine Correct for Raptor 2. Raptor 3 is closer to half [1][2]. [1] https://en.m.wikipedia.org/wiki/SpaceX_Raptor https://en.m.wikipedia.org/wiki/SpaceX_Raptor [2] https://everydayastronaut.com/raptor-engine/ https://everydayastronaut.com/raptor-engine/
RoddaWallPro 2y ago
I have what is probably a dumb question: How can a Raptor turbopump need almost double the HP of a F1 while putting out 1/3 or 1/4th the thrust? (Assuming Elon's 100k HP number was correct, and/or hasn't changed). That just doesn't settle out for me. If it's got double the power, it should be moving double the fuel, so double the power, no?
- TaylorAlexander 2y ago
  Maybe something to do with pressure. Maybe it is higher chamber pressure and maybe higher pressure even with lower flow rates could require more power.
  - deleted 2y ago
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- tliltocatl 2y ago
  Raptor has much higher chamber pressure (35 MPa vs 7 MPa of F1) and hence higher Isp (380 vs 304 in vacuum).
  - jcims 2y ago
    35MPa is ~5000psi for anyone wondering lol.
    - tliltocatl 2y ago
      And I took the effort to convert it to metric… Seriously, NASA uses the metric system and so should you.
- evo 2y ago
  Perhaps it has to do with the relative densities of RP-1 versus methane?
- dotnet00 2y ago
  Raptor is much smaller, F-1 has a diameter of ~3.7m, Raptor has a diameter of ~1.3m, so it's pumping out 1/4th-1/3rd the thrust in 1/8th the area.
- _0ffh 2y ago
  Maybe fuel might play a role. The Raptor burns methane, the F-1 refined petroleum. Another possible reason is that the designs may make different efficiency vs power trade-offs.
- avmich 2y ago
  The formula for Isp - the important measure of efficiency of the rocket engine - says that the speed with which the engine throws away hot gases grows with the difference of pressures - before nozzle and at the exit of the nozzle. The whole idea, by the way, of the full-flow combustion is to extract some more energy from the fuel - before sending that fuel to the chamber, and at the temperature which the turbine of the turbopump can tolerate - so that energy could be used for pumps and more pressure could be created in the chamber. More energy than "more conservative" closed-cycle engines. The pump power is equal to the volume flow (how many cubic meters, or, say, liters of fuel the pump transfers per second) multiplied by the pressure (which pressure is at the exit of the pump). So, it's not the flow - it's the pressure where Raptor has a big advantage over F-1, and that pressure allows to have a better Isp. And of course the better Isp allows to reach bigger characteristic velocity (or just a velocity in a free space, where gravity or atmosphere don't get in the way) using the same amount of fuel. The logic goes roughly like that. Every rocket engine designer wants to reach bigger Isp. For that, using a particular fuel, one need to reach bigger pressure in the chamber, and we move from pressure-fed engines (like the first French orbital rocket, Diamant, which had pressure-fed first stage) to pumps, because high-pressure tanks are too heavy. Pumps initially are open cycle, or gas generator cycle, but we throw away enough gases after the pumps' turbine, so next improvement is we get a closed cycle. With closed cycle we can choose to use all fuel or all oxidizer to move the turbine, but as soon as some component is used fully, we can't get more energy for pumps. Eventually we go to the more complex full-flow cycle, which uses both components and reaches the highest pressure in the chamber. The next step would probably be a detonation engine :) which uses somewhat more efficient process to convert chemical energy into speed, but it's not yet developed enough. We can also talk about more heat-resistant turbines which would allow to extract more energy from the fuel and to increase pressure some more... but there we also have a lot of R&D ahead of us.
dotnet00 2y ago
Raptor is also a lot smaller. F-1s were intentionally designed as large and high thrust because it'd have been simpler to build, test and control a handful of F-1s at the time, compared to say, 30 smaller but more efficient engines. The useful metric on this front would be thrust density, where Raptor 1 is a bit under twice that of F-1A, Raptor 2 is a little over twice of F-1A, and Raptor 3 should be ~2.5x of F-1A.
- capitainenemo 2y ago
  But with more engines they can tolerate a failure or two (or even more) per launch. If a larger engine fails and you only have 3 of them, you're going to have a bad day. And, they do seem to test the heck out of their engines, even with 30 of them on a ship.
  - sbradford26 2y ago
    Yes those benefits can now be realized now with modern controls. Back when the Saturn V was designed the control systems necessary to manage 30 engines didn't really exist. Digital control was in it infancy and was only really implemented with a backup on the whole Apollo stack. Trying to manage that many engines while technically possible with controls of the era (check out the N1) means your control system would be introducing reliability issues instead of adding fault tolerance through redundancy.
    - capitainenemo 2y ago
      Didn't the soviets give it a try? I'd swear they had a large number of engine design way back when also for fault tolerance. Ok, they didn't get it working but I'm pretty sure it wasn't due to lack of digital control... Surely they wouldn'tve even attempted it if it was impossible :) [edit] ah. That was the N1 you referred to. Ok. So you're saying it was possible, but it introduced more failure points.. So is that why it failed...
      - dotnet00 2y ago

JumpCrisscross 2y ago

> Raptor engine has about 1/3 to 1/4 of the thrust of an F-1 engine Correct for Raptor 2. Raptor 3 is closer to half [1][2]. [1] https://en.m.wikipedia.org/wiki/SpaceX_Raptor https://en.m.wikipedia.org/wiki/SpaceX_Raptor [2] https://everydayastronaut.com/raptor-engine/ https://everydayastronaut.com/raptor-engine/

RoddaWallPro 2y ago

I have what is probably a dumb question: How can a Raptor turbopump need almost double the HP of a F1 while putting out 1/3 or 1/4th the thrust? (Assuming Elon's 100k HP number was correct, and/or hasn't changed). That just doesn't settle out for me. If it's got double the power, it should be moving double the fuel, so double the power, no?

TaylorAlexander 2y ago
Maybe something to do with pressure. Maybe it is higher chamber pressure and maybe higher pressure even with lower flow rates could require more power.
- deleted 2y ago
  [deleted]
deleted 2y ago
[deleted]
tliltocatl 2y ago
Raptor has much higher chamber pressure (35 MPa vs 7 MPa of F1) and hence higher Isp (380 vs 304 in vacuum).
- jcims 2y ago
  35MPa is ~5000psi for anyone wondering lol.
  - tliltocatl 2y ago
    And I took the effort to convert it to metric… Seriously, NASA uses the metric system and so should you.
evo 2y ago
Perhaps it has to do with the relative densities of RP-1 versus methane?
dotnet00 2y ago
Raptor is much smaller, F-1 has a diameter of ~3.7m, Raptor has a diameter of ~1.3m, so it's pumping out 1/4th-1/3rd the thrust in 1/8th the area.
_0ffh 2y ago
Maybe fuel might play a role. The Raptor burns methane, the F-1 refined petroleum. Another possible reason is that the designs may make different efficiency vs power trade-offs.
avmich 2y ago
The formula for Isp - the important measure of efficiency of the rocket engine - says that the speed with which the engine throws away hot gases grows with the difference of pressures - before nozzle and at the exit of the nozzle. The whole idea, by the way, of the full-flow combustion is to extract some more energy from the fuel - before sending that fuel to the chamber, and at the temperature which the turbine of the turbopump can tolerate - so that energy could be used for pumps and more pressure could be created in the chamber. More energy than "more conservative" closed-cycle engines. The pump power is equal to the volume flow (how many cubic meters, or, say, liters of fuel the pump transfers per second) multiplied by the pressure (which pressure is at the exit of the pump). So, it's not the flow - it's the pressure where Raptor has a big advantage over F-1, and that pressure allows to have a better Isp. And of course the better Isp allows to reach bigger characteristic velocity (or just a velocity in a free space, where gravity or atmosphere don't get in the way) using the same amount of fuel. The logic goes roughly like that. Every rocket engine designer wants to reach bigger Isp. For that, using a particular fuel, one need to reach bigger pressure in the chamber, and we move from pressure-fed engines (like the first French orbital rocket, Diamant, which had pressure-fed first stage) to pumps, because high-pressure tanks are too heavy. Pumps initially are open cycle, or gas generator cycle, but we throw away enough gases after the pumps' turbine, so next improvement is we get a closed cycle. With closed cycle we can choose to use all fuel or all oxidizer to move the turbine, but as soon as some component is used fully, we can't get more energy for pumps. Eventually we go to the more complex full-flow cycle, which uses both components and reaches the highest pressure in the chamber. The next step would probably be a detonation engine :) which uses somewhat more efficient process to convert chemical energy into speed, but it's not yet developed enough. We can also talk about more heat-resistant turbines which would allow to extract more energy from the fuel and to increase pressure some more... but there we also have a lot of R&D ahead of us.

dotnet00 2y ago

Raptor is also a lot smaller. F-1s were intentionally designed as large and high thrust because it'd have been simpler to build, test and control a handful of F-1s at the time, compared to say, 30 smaller but more efficient engines. The useful metric on this front would be thrust density, where Raptor 1 is a bit under twice that of F-1A, Raptor 2 is a little over twice of F-1A, and Raptor 3 should be ~2.5x of F-1A.

capitainenemo 2y ago
But with more engines they can tolerate a failure or two (or even more) per launch. If a larger engine fails and you only have 3 of them, you're going to have a bad day. And, they do seem to test the heck out of their engines, even with 30 of them on a ship.
- sbradford26 2y ago
  Yes those benefits can now be realized now with modern controls. Back when the Saturn V was designed the control systems necessary to manage 30 engines didn't really exist. Digital control was in it infancy and was only really implemented with a backup on the whole Apollo stack. Trying to manage that many engines while technically possible with controls of the era (check out the N1) means your control system would be introducing reliability issues instead of adding fault tolerance through redundancy.
  - capitainenemo 2y ago
    Didn't the soviets give it a try? I'd swear they had a large number of engine design way back when also for fault tolerance. Ok, they didn't get it working but I'm pretty sure it wasn't due to lack of digital control... Surely they wouldn'tve even attempted it if it was impossible :) [edit] ah. That was the N1 you referred to. Ok. So you're saying it was possible, but it introduced more failure points.. So is that why it failed...
    - dotnet00 2y ago