3 ms·

Given how successful our Saturn package was, it seems curious to me that we wouldn’t just follow the ullage motor method.

panick21_ 3y ago
One needs to put success into perspective. Saturn V only launched a very small amount of times and had some near misses in that time. How successful it would have been if it had flown 100+ times is a question. Also, If you are not reusable you have much more margin to play with. SpaceX is optimizing this thing to an incredible amount. Liftoff thrust is 2x as much as Saturn V and they are aiming at 3x as much. Total payload to orbit is 2-3x larger while being reusable. SpaceX could have done what they did on Falcon 9 (200+ successful launches in a row) but Hot-Staging like the Soyuz is also successful. SpaceX optimizes for long term performance and operational simplicity. Ironically that leads to more Soviet way. The N1 would also have used hot staging.
avmich 3y ago
Ullage motors have to be solid - this way they don't need ullage themselves to fire - or to be fed from separate tanks - this is another reasonably complex subsystem which needs to be refueled somehow between flights. If ullage motors are solid, that needs to be re-loaded between flights. So, overall the ullage system is a certain complexity to design, build, refuel, a weight to carry in flight - clearly some drawbacks. Hot firing simplifies things in this regard.
dotnet00 3y ago
Ullage motors as they were on Saturn V (and in most other cases) are solid fueled. Making them incompatible with reuse. SpaceX emphasize reusable systems even when not recoverable because that allows for testing of the exact flight hardware (eg mechanical separation systems over explosive bolts). A reusable approach would involve some form of gas thruster, so might as well just try hot staging.

3 ms·

Given how successful our Saturn package was, it seems curious to me that we wouldn’t just follow the ullage motor method.

by dclowd9901 3y ago

panick21_ 3y ago
One needs to put success into perspective. Saturn V only launched a very small amount of times and had some near misses in that time. How successful it would have been if it had flown 100+ times is a question. Also, If you are not reusable you have much more margin to play with. SpaceX is optimizing this thing to an incredible amount. Liftoff thrust is 2x as much as Saturn V and they are aiming at 3x as much. Total payload to orbit is 2-3x larger while being reusable. SpaceX could have done what they did on Falcon 9 (200+ successful launches in a row) but Hot-Staging like the Soyuz is also successful. SpaceX optimizes for long term performance and operational simplicity. Ironically that leads to more Soviet way. The N1 would also have used hot staging.
avmich 3y ago
Ullage motors have to be solid - this way they don't need ullage themselves to fire - or to be fed from separate tanks - this is another reasonably complex subsystem which needs to be refueled somehow between flights. If ullage motors are solid, that needs to be re-loaded between flights. So, overall the ullage system is a certain complexity to design, build, refuel, a weight to carry in flight - clearly some drawbacks. Hot firing simplifies things in this regard.
dotnet00 3y ago
Ullage motors as they were on Saturn V (and in most other cases) are solid fueled. Making them incompatible with reuse. SpaceX emphasize reusable systems even when not recoverable because that allows for testing of the exact flight hardware (eg mechanical separation systems over explosive bolts). A reusable approach would involve some form of gas thruster, so might as well just try hot staging.