Apollo 13’s Barbecue Mode

[BDL]

During Apollo 13, it is said that the spacecrafts had become very cold inside (because they had to turn off most of the electronics to save power, and could therefore not afford to have the AC on).
The reason it grew cold was because they had the spacecrafts in Barbecue Mode, where they were rolling and rotating.

I’m not entirely sure I understand how rolling would help keep the heat down. I thought that rolling would still result in the place heating up regardless but I’m probably incorrect. Can someone explain to me? Also, did the Apollo 13 astronauts keep on their EVA suits to help keep warm? It seems like that would be logical. Anyways, thanks again!

[Peter B]

G'day BDL

I think it's a bit more complicated than that.

1. The A13 spacecraft got cold because a lot of electrical systems had been shut down to save electricity. The spacecraft's equilibrium temperature was low because the outside was highly reflective and therefore didn't absorb much solar radiation. In order to maintain the spacecraft at a habitable temperature it relied on the heat generated by the normal operation of its electrical systems. So when these systems were shut down the spacecraft lost the heating they provided, and the temperature drifted down towards its (low) equilibrium temperature.

2. The barbecue roll was to ensure even heating of the spacecraft, in particular to ensure no part of the heat shield dropped to too low a temperature. The danger here was that testing had shown the heat shield material didn't perform as intended if its temperature got too low. This, therefore, was the reason the damaged SM wasn't ditched straight away - to avoid directly exposing the heat shield to space which would cause it to chill.

3. IIRC the astronauts tried wearing their suits but found they started to sweat in them. This then gave them chills when they became exposed to the cold air of the spacecraft. They therefore decided it was better to be dry if cold than to be wet and cold.

I'll now step back and let someone with actual expertise to correct me where necessary.

[raven]

I heard also that sweating in the spacesuits would have meant their water situation that much worse, as the potable supply in the LM was very limited and was shared with the supply for cooling the electronics. <oreover, it was meant for two guys and the LM for two days, with much water for the astronauts coming as 'waste' from the SM fuel cells generating electricity from hydrogen and oxygen.

[PDI-11]

Regarding whether to wear their spacesuits or not, here is some information from Andrew Chaikin's A Man on the Moon (pg 324):

The men had donned a second set of cotton long johns, and they discussed the idea of putting on their space suits to stay warm, but had decided against it. They wouldn't be able to turn on the suit fans, because they ate up too much power. Without any ventilation they'd overheat and perspire, then run the risk of getting seriously chilled if they had to take them off.

[Peter B]

Ah, thank you for clarifying that, PDI-11.

And welcome to the board!

[Count Zero]

Good first post, PDI-11.  Welcome!

[Allan F]

Another heat-related issue - the LM was designed for the surface of the moon, where IR radiation from the surface was an important component of the heat budget.

[BDL]

Thank you for explaining! I understand how the Barbecue Mode works now.

When the Lunar Module was on the moon, did it also have shielding from the radiation? I’m not sure but I think the shiny foil stuff on the lower part of the Lunar Module was probably used to deflect radiation reflected from the moon itself and directly from the sun (correct me if I’m wrong). Did the top portion of the Lunar Module (ascent stage) also have a protection against heat and radiation? Some of the missions had spent a couple days on the moon, so it makes sense that they would have some sufficient shielding to keep the Lunar Module at a fair temperature.

[Allan F]

The kapton foil was for temperature control - it reflected IR radiation away as needed. Enough to keep the ascent module from absorbing too much energy to be safe, but also to alow it to absorb enough energy to keep the ascent propulsion fuel/oxidizer within its liquid temperature band.

If you're referring to IONIZING radiation - no, it was not designed to do that. Since it had almost zero mass, it would be very bad in that aspect.

[Echnaton]

Did the top portion of the Lunar Module (ascent stage) also have a protection against heat and radiation?

The upper stage had insulation too, but you don't see it because of the micrometeoroid shielding panels on the outside. Getting rid of heat from the electronics was really more of a problem for the upper stage though. The panels also covered some equipment that was mounted outside the airtight body.  The micrometeoroid panels are attached with stand offs from the underlying spacecraft to minimise heat transfer, so they also as part of the thermal protection insulation.  Some people wrongly assume those panels were the wall of the airtight part of the spacecraft and see them as flimsy. But the panel movement seen on lunar takeoff is inconsequential.

[ka9q]

The Kapton on the descent stage was in the form of multilayered blankets. The visible outer layer was a "second surface mirror", i.e., the thin aluminum layer was on the inside, which is why they appeared gold instead of silver. Kapton has an emissivity of essentially unity, so it appears black in the far infrared. So while the aluminum reflected the sunlight that passed through the Kapton, the Kapton itself absorbed significant thermal radiation from the lunar surface and became warm.

This is why there were multiple layers. Each was crumpled to minimize heat transfer by conduction and the aluminum coatings blocked transfer by radiation (they're in vacuum so there's no convection - think of a Thermos bottle). The overall effect is to thermally decouple the structure inside from the environment, so any heat-generating equipment (e.g., the batteries in the descent stage) had to be actively cooled with a pumped heat transfer loop. That heat was dumped in the form of steam from the top rear of the ascent stage.