Apollo Re-entry and Splashdown

[kryton]

I was watching some splashdown footage and I had a question about the heat of the heatshield. Was it 5000 F during re-entry? In the footage I watched, the capsule did not cause any steam after splashdown. During re-entry did the heat shield cool down enough not to cause any steam? I assume the shield must have been able to cool down very quickly for the divers to attach the floatation ring around the capsule. Correct?

[captain swoop]

one word 'ablation'

[JayUtah]

It all matters what you mean by it. The actual heat transfer in aerodynamic re-entry TPS systems is quite complex and involves many different temperatures in many different substances (e.g., the shield itself, the different layers of air in front of it).

The Apollo TPS was ablative, meaning it was made of a consumable substance that burned and melted slowly. Those processes consumed a great deal of heat, but also created gas and carbon layers chemically that prevented heat from transfering from the superheated shock wave in front of the spacecraft.

After the ablative heat shield had slowed the craft substantially, further aerodynamic braking used drogue parachutes. During this phase of re-entry the spacecraft was subjected to far less aerodynamic heating, and in fact the heat transfer reversed. Any residual heat in the heat shield is carried away by convection into the atmosphere. No part of the spacecraft is dangerously hot at splashdown, except perhaps for the CM RCS jets that have just finished burning off the remainder of the RCS fuel as a pre-landing safing procedure.

[captain swoop]

wow! as usual an answer that defies expectations!

[JayUtah]

You got it right with one word. That's pretty impressive too.

[Donnie B.]

Hmmm, I wasn't aware of the RCS burnoff. How did they work that, burning two (or more) opposed engines so there was no net effect on the attitude?

It must have taken a few minutes to do that, since surely there was a considerable margin of safety in those consumables.

And while I'm asking dumb questions, were there pyrotechnics to sever the parachute shrouds after splashdown? You wouldn't want them dragging the capsule under, but then you also wouldn't want those pyros to go off too early.

[JayUtah]

How did they work that, burning two (or more) opposed engines so there was no net effect on the attitude?

Doesn't matter when you're dangling under the main parachutes. But yes, the procedure was essentially to burn every RCS jet at once until the fuel was exhausted.

And while I'm asking dumb questions, were there pyrotechnics to sever the parachute shrouds after splashdown?

Yes, although they actually disconnected the shroud tie-offs from the spacecraft structure on a 14-second delay after splashdown. The general procedure for parachute-braked landing is to jettison the parachute as soon as it is no longer needed. It would indeed be critical not to prematurely jettison the parachute, but that mode of criticality was well within human-rated engineering knowledge at the time and we know how to design for it. In general they are designed fail-safe, so that if anything goes wrong the parachute does not jettison -- e.g., the pyro bus would be de-energized.

One general Apollo engineering rule was that manual switches would back up any crew-safety circuit, so the pilots could elect to control parachute disconnection manually.

Apollo 15 is infamous for landing on only two main parachutes, the shrouds for the third having been burned through inadvertently during the RCS burn-off.

[hplasm]

[b]
Apollo 15 is infamous for landing on only two main parachutes, the shrouds for the third having been burned through inadvertently during the RCS burn-off.

Aha! I remember that, but never questioned/forgot the reason why.

Thanks!

[Joe Durnavich]

Doesn't matter when you're dangling under the main parachutes. But yes, the procedure was essentially to burn every RCS jet at once until the fuel was exhausted.

On the subject of RCS firings, if I may, when the LM fired an RCS jet to cause a roll, say, how did they exactly null that roll out? Did the opposite RCS jet have to be built to a tolerance that guaranteed the exact amount of counterthrust? Or, was there some way to "fine tune" the rates?

[Nicolas]

PUUUFF PUUUFF puff puff puff puff

My ("educated") guess is that the jets were made as identical as possible (and thrust can be remarkably constant for engines), the jet burn time could be regulated, and in order to bleed off rest roll rates when they would occur, it would require some very short puffs.

But given the thrust precision I've seen quoted for engines, I think that if they give 2 thrusters the same burn time, there wouldn't be much significant rotation left at all.

[Graham2001]

Just as an aside, is there a map that shows the splashdown points for all the Apollo missions, including the Skylab flights and the ASTP?