# Active liquid cooling for heat shield

I have not found much about active liquid cooling for heat shields using cryogenic fuel. It has been mentioned but not researched in depth. It may never have been done. Now that the Starship seems to be going that way, I'm curious.

Has anyone studied active liquid cooling of heat shields?

• I realize there was one question about this subject, but it was a while ago and not exactly on target with mine. It was not answered to much satisfaction, so I think this is still relevant. – Johnny Robinson Jan 18 at 16:18
• As far as I know cryogenic fuel was never used within reentry capsules, therefore no cryogenic active liquid cooling for their heat shields. Producing extra heat by releasing hydrogen into the hot compressed air seems to be not a good idea. – Uwe Jan 18 at 16:26
• Space -X is planning on cooling their stainless steel hull with cryogenic methane. The big question is where does it go after heating up? Does it bleed out through tiny holes in the hull? For that they could use nitrogen. Would it just heat sink into the whole mass of fuel in the tanks? In the BFR days Musk said the tank and hull were going to be the same thing. Maybe they could spray it on the inside of the tank wall from the inside (as a partial solution). Heated fuel could go to the engines for retro propulsion. But during reentry, the ship orientation is wrong for that. Hard to picture. – Johnny Robinson Jan 18 at 17:04
• Johnny, you've got enough posts to know this, but -- while Organic Marble's answer is good, it might be better to wait a day or so before accepting it to see if you get more answers. People are much less likely to add answers to a question with an accepted answer. – Bear Jan 18 at 18:13
• @JohnnyRobinson I think a new and interesting question would be "How did SpaceX plan to use cryogenic methane on Starship for cooling during atmospheric entry?" or something similar. – uhoh Oct 22 at 6:42

There have been many such. Here's one from way back in 1957 for the Brass Bell vehicle.

The primary or load-bearing structure was of aluminum and relied on cooling in a closed-loop arrangement that used water-glycol as the coolant. Wing leading edges had their own closed-loop cooling system that relied on a mix of sodium and potassium metals. Liquid hydrogen, pumped initially to 1,000 pounds per square inch, flowed first through a heat exchanger and cooled the heated water-glycol, then proceeded to a second heat exchanger to cool the hot sodium-potassium. In an alternate design concept, this gas cooled the wing leading edges directly, with no intermediate liquid-metal coolant loop. The warmed hydrogen ran a turbine within an onboard auxiliary power unit and then was exhausted overboard. The leading edges reached a maximum temperature of 1,400ºF, for which Inconel X was a suitable material.

Source: Facing the Heat Barrier: A History of Hypersonics, T. A. Heppenheimer, NASA SP-2007-4232, Part 2 (p. 139) Also viewable here. Part 1 is available here.

• Closed loop works, steady state, while you have an atmosphere to dump heat into. But not so well when you have just a few minutes of re-entry. – Camille Goudeseune Oct 22 at 5:35
• Pardon the edit. Your answers are really valuable and the linked documents are as well. I've just added some identifying information so that if the link breaks they can still be recovered, per space.meta.stackexchange.com/q/1356/12102 – uhoh Oct 22 at 6:34

In the most recent (as of Oct. 2019) SpaceX Starship media event, (SpaceX 29-sep-2019 YouTube video Starship Update) Elon Musk stated after about 21:05 that they were ditching the "transpirational cooling method" which was supposed to pump cryogenic fuel out of "pores" to form a cushion, often described as similar to the " "Leidenfrost effect" . Instead, they will be using hexagonal heatshield tiles constructed out of PICA-X, a similar material to what SpaceX uses for the Dragon 1 and Crew Dragon capsules.

I'm pretty sure that since they are no longer planning on using the "pores" system, they won't be circulating cryogenic liquids through the heatshield.

• I hope that's what you meant, feel free to roll back. – uhoh Oct 22 at 13:19
• – uhoh Oct 22 at 13:20