NASASpaceFlight.com Forum
General Discussion => Q&A Section => Topic started by: cube on 05/24/2022 08:08 pm
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Hello,
I was wondering why do falcon 9 rockets have their rp1 tanks under the lox tanks? I believe that the lox is denser than the rp1, it would be more stable to have the heavier tank (lox) at the bottom than at the top, right?
Thank you !
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Heavier on the top is more stable than heavy on the bottom
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But then why on the starship and the superheavy the lox tank is at the bottom unlike the falcon 9 ?
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But then why on the starship and the superheavy the lox tank is at the bottom unlike the falcon 9 ?
RP-1 vs Methane tank size
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Starship sidewalls carry much less load with LOX on the bottom. Stability isn't really a factor.
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If you have RP-1 at the top you have to worry about it freezing in the down comer surrounded by much colder LOX. Liquid methane might have the same problem, but it is significantly colder than liquid RP-1 so it should be easier to manage.
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This may explain why the second and third stages of the saturn v have their lox tank below their liquid hydrogen tank despite the fact that the lox tank is heavier than the hydrogen tank.
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LH2 forward and LO2 aft also has much lower loads and structural mass than the reverse arrangement. Nested or common bulkheads save structural mass, eliminating the Intertank.
F=ma
Edited - oops! lol
This may explain why the second and third stages of the saturn v have their lox tank below their liquid hydrogen tank despite the fact that the lox tank is heavier than the hydrogen tank.
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LOX forward and LO2 aft
Liquid oxygen is not a good mono-propellant (you have a typo).
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Wouldn't it be rather the LOX below the LH2 which would require a lower structural mass?
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Note that the shuttle external tank had LOX on top, but the downcomer was outside the LH tank.
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The Saturn V's first stage was lox above RP-1, which has the benefit of better aerodynamic stability and the drawback of higher structural mass.
The two upper stages placed LH2 above lox, which hinders aerodynamic stability and reduces structural mass.
Though, even with fins, the Saturn V was aerodynamically unstable, it was desirable to limit the degree of instability to give the crew a bit more time to escape in the event of a major failure. Furthermore, mass efficiency is not so critical on a first stage, since it is not accelerated to high speed. An extra kilo on the first stage might reduce the payload to LEO by only a hundred grams (the first stage, unlike the upper stages, does not even bother with common bulkheads). Lox above RP-1 makes sense for the first stage.
Structural mass is much more of a drag for the upper stages, since they are accelerated to higher speeds. Obviously there is a kilo-for-kilo trade-off between final stage mass and payload mass. Aerodynamic stability is also irrelevant, since the upper stages operate only in a vacuum (shifting an upper stage's center of mass forward would slightly shift the entire vehicle's center of mass during first-stage operation, thereby improving aerodynamic stability, but since the first stage is so much more massive, the benefit is slight). LH2 over lox makes sense for stages 2 and 3.
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In the Saturn V, all tanks are pushed from behind. In the Shuttle, everything in the external tank below the thrust beam is pulled from the top. Structural mass is minimized with lox above the beam and LH2 below it.