Three ignitions for J-2

[Schnobs]

Opening that can of worms again. Under the current system, three ignitions is less wrong than two.

See http://ksp.schnobs.de/stuff/notes.txt for the time being. I'm aware that there's supposed to be a proper solution someday, these notes were compiled for the issue/discussion about the would-be proper solution, but that fails to happen and I'm growing impatient.

[A1Ch1]

Thanks for putting together those notes :)

One thing I noticed in the S-IVB section: the cold helium spheres didn't contain liquid helium, they contained gaseous He at the temperature of LH2 (PV=nRT - assuming the pressure and volume the spheres could contain were constant, reducing T increases n by the same factor. But cold He isn't good for pressurizing tanks; assuming V and n are constant, reducing T means reducing P so the cold He has to be reheated somehow). To elaborate a little on S-IVB pressurization:

Pre-pressurization = ground pressurization of tank for first burn
Pressurization = maintenance of tank pressure during burns
Re-pressurization = pressurizing tanks prior to restart

All flights had both cold He bottles and ambient temp He bottles. Initially (prior to AS-503), ambient temp He bottles were used for re-pressurization of the H2 and O2 tanks. Gas from the cold He bottles was heated by the J-2 during burns and used solely for pressurization of the O2 tank (the H2 tank was pressurized with gaseous H2, also heated by the J-2). After the introduction of the H2/O2 burner, cold He was also used for re-pressurization and the ambient He was relegated to backup. Thus all flights after the introduction of the H2/O2 burner carried enough He for three ignitions (one from pre-pressurization, one from cold He, one from ambient He as backup) as demonstrated by Apollo 9.