Quote:
Originally Posted by Extravoice
Perhaps Mugaliens' point was that the ullage motors are a one-shot deal (because they are solid). If the liquid fuel hasn't settled by the time the ullage motor burns out, you don't get another chance to restart the liquid engine.
|
Yes, I think I may have misread what he was saying.
Quote:
Originally Posted by Extravoice
Of course, you could always include spare ullage motors, or use a restartable liquid fueled ullage motor that relies on a pressure bladder. (From Larry's link, it appears that the Apollo third stage did that, using hydrazine as the fuel.)
|
The Saturn third stage used both solid motors and liquid engines for ullage. I believe the solid motors were used prior to the first engine burn only and in combination with the APS (Auxiliary Propulsion System) engines. The APS provided ullage not only at the start of each main engine burn, but also at the end of the first burn to keep the propellant from separating too much in the tanks. The APS also provided three-axis control for the third stage.
The Saturn first and second stages used solid ullage motors only.