Last month, Elon Musk tweeted a design test of his regenerative heat shield that, he hopes, would spare Starship from its return to the Earth's atmosphere, thus removing the disposable ceramics used up to then by shuttles and capsules to absorb the intense heat. But what is a regenerative cooling system?
We have regenerative heat shields for a while now
– Elon Musk (@elonmusk) January 25, 2019
If you've ever run down the street, lying on a beach, or survived in a clogged apartment without air conditioning, you've used pretty much the same system that Elon Musk has been using for Starship. Essentially, Starship sweats to stay cool.
SEE ALSO: ELON MUSK COATED STARSHIP SHAT SHIELD WITH TWO FLAMETHROWER TESTS
In the Musk system, microscopic holes cover the windward side of the craft; the side that faces the atmosphere by coming back from the space. On this side of the boat is a double-walled "sandwich" that contains a liquid that fills the space, either water or liquid methane.
As the windward side of the unit heats up, the liquid is drawn through the microscopic pores, thereby cooling the surrounding metal. When it evaporates from the heat, it sucks more fluid behind it, which continues to keep the stainless steel side of the vessel cool.
What is important is that even if water or liquid methane can burn, the rest of the metal does not, which means that all that should be done in theory is to fill the reservoir of heat shield and that it would be good to leave, as opposed to replacing all the lost insulating material that traditional heat shields use.
Will it work?
As Business Insider points out, the proper functioning of such a system faces significant obstacles. Clogged pores on a thermal shield of perspiration could be catastrophic.
According to Walt Engelund, director of NASA Langley's Space Technology and Exploration Branch, experimental models with transpiratory cooling are particularly vulnerable to clogged pores.
"I have seen cases where you will get a clogged channel … and that will immediately cause burns," he said. "A model will disappear in a hypersonic wind tunnel, it vaporizes almost, there is so much energy and heat."
Yes. In addition, despite the high external heat, rapid water vaporization can counter-intuitively freeze and block the cooling channels.
– Elon Musk (@elonmusk) January 22, 2019
This is problematic for both proposed refrigerants up to here, water and liquid methane. Like Elon Musk recognized, water vapor may freeze quickly, which could clog the coolant channels.
With methane, as with other hydrocarbon compounds, high temperatures can cause carbon atoms to stick together in a process called "coking," making the whole thing solid and easily blocking enough channels to cause a catastrophic failure of the heat shield. . .
Engelund is not too dissuaded. He proposed a bold solution in the US for SpaceX to implement Starship's heat shield: add more holes than needed, just in case.