[X] My Assistant

[helvick]

Seems like Robert McNaught captured something else interesting in the Southern Skies last night.

[dvandorn]

Rocket stages in a vacuum are usually vented through the engine bell(s). That way, you don't need to design any separate venting system; all you have to do is activate the proper pumps (the ones that feed the fuels to the combustion chamber during flight) and your fuel or oxidizer goes flowing out of the bell. (You could actually vent hydrogen and oxygen at the same time, though I don't think they do that all that often... hypergolics, of course, you vent separately.)

Yes, these vents can be somewhat propulsive, but not nearly as propulsive as when the engine is actually operating (i.e., when the fuel and oxidizer are undergoing combustion). Apollo S-IVBs were sent on their post-TLI trajectories (often designed to crash into the Moon) using, in part, the propulsive effect of their fuel and oxidizer venting.

-the other Doug