Things learnt about the real space program from KSP

[Space Peanut]

Was watching a documentary last night and looked like on Apollo 11 they didn't empty the second stage (looked like it was still firing) when they started the third.

Amused myself greatly to catch myself critiquing them for inefficiently wasting fuel!!!

Anybody else noticed that they have some surprising new insight into the space program after playing KSP for a couple of months?

[theattackcorgi]

Yes, I do notice that a lot. When I see stuff on the news about rockets and planes, I actually know what their talking about. And i usally start thinking stuff like " you should have turned on Sas".

[goldenpeach]

They didn't empty the second stage for redundancy: in case of a problem (like an engine shutting down(like on Apollo 13)) they could still finish the mission.

We don't do that in KSP because we don,t have those problem(and anyway, who care about the safety of the kerbals )

[anraj761]

I was very pleased to find this site. This is an intelligent and well written article, you must have put a fair amount of research into writing this. Thank you

[Nibb31]

They didn't empty the second stage for redundancy: in case of a problem (like an engine shutting down(like on Apollo 13)) they could still finish the mission.

No. The F-1 and J-2 engines were not restartable. Restartable rocket engines are quite more complex, so that level of complexity is only implemented when necessary, for the S-IVB or the CSM for example. The Shuttles OMS engines were restartable, but not the SSMEs.

The reason there was propellant left in the stages is because it's preferable to shut down the engine with a bit of extra propellant at the end of the burn than to run out of propellant prematurely and starve the engines. Turbopumps don't like starvation and can fail catastrophically.

The S-II kept on burning at low throttle in order to maintain positive acceleration for a clean separation event. You don't want things accelerating and decelerating all over the place. Also, rocket engines take several seconds to cut their thrust. It must have been around 2% throttle at separation time.

The actual staging sequence was something like this:

- S-IC burn for a pretermined time.

- Cut off S-IC engines.

- Fire explosive charges for S-IC separation.

- Fire ullage motors on the interstage.

- S-II engine ignition

- Fire explosive charges to jettison the interstage.

The interstage was jettisoned separately because they were afraid about the clearance with the J-2 engines. If it had stayed attached to the spent first stage, they were afraid that it might hit the engine nozzles. The interstage itself contained the ullage motors and was a heavy piece of kit that needed to be jettisoned. Failure to drop it would have prevented the rest of the rocket from reaching orbit.

The ullage motors were small solid rockets that gave a jolt to the S-II (and S-IVB) before main engine ignition. This was designed to settle the propellant at the bottom of the tanks to prevent cavitation in the turbopumps, which could be catastrophically destructive. As you can see, the whole sequence is a tad more complex than "press space"

Edited November 4, 2013 by Nibb31

[Mr Shifty]

They didn't empty the second stage for redundancy: in case of a problem (like an engine shutting down(like on Apollo 13)) they could still finish the mission.

The engine didn't shut down on Apollo 13. They were cruising to the moon when the accident happened, under no thrust. The accident blew up one of the oxygen tanks in the service module. The SPS actually was probably still usable (though Mission Control wanted to reserve it for last ditch only due to ultimately valid concerns about structural damage to the SM, and ended up never firing it.)

[Nibb31]

The engine didn't shut down on Apollo 13. They were cruising to the moon when the accident happened, under no thrust.

Goldenpeach was referring to an incident during the launch that was unrelated to the oxygen tank explosion. One of the second stage engines suffered a shutdown. The other engines and the S-IVB burned slightly longer to compensate and they got into orbit fine.

[Mr Shifty]

Goldenpeach was referring to an incident during the launch that was unrelated to the oxygen tank explosion. One of the second stage engines suffered a shutdown. The other engines and the S-IVB burned slightly longer to compensate and they got into orbit fine.

Ah. I hadn't known that. So, apparently this is what I learned about the real space program today!