A look at FT-13's heat shield by NASA Expert

The week is off to a slow start with most talk still about Starship's FT-13 last Friday, with one or two smaller, less-interesting (at least to me) stories. The big story about FT-13 is about the heat shield efforts the company has been going through, and a conversation with a former NASA engineer who studied heat shield materials for nearly four decades at NASA Ames Research Center. This expert warns that the current Starship heat shield tech is a “dead end” for rapid reuse. Even video channel Alpha Tech did a discussion that touched on the subject. 

From the quote about, "nearly four decades at NASA", I have to assume that was starting before the shuttles flew for the first time, so (in rough numbers) 1975 until 2015. The design work was done (or declared that way) by the late '70s, and my impression is they may have done some monitoring of the shuttle heat shields but this wasn't like "continuous process improvement" as much as it was "keeping an eye on things." 

So what's up? Why? A point both Alpha Tech's video and Ars Technica's article make is the Shuttles had an aluminum chassis that the heat shield was mounted on. SpaceX uses stainless steel, but not a conventional austenitic alloy like 304. That means that they developed their own alloy with improved properties (including cost!) over the common, COTS (Commercial Off The Shelf) alloys. This source says SpaceX patented it as 30x. (Do I need to mention that Musk seems to like the letter X in product and business names?) 

The powerful advantage to using stainless steel instead of something like the aluminum alloys the Space Shuttles used is a much higher tolerance for heat. On top of that, it's better at the cryogenic temperatures of the fuels and oxidizers the ship uses. 

“Starship’s current thermal protection system is a dead-end for all missions that require full and rapid reusability,” Dan Rasky, a former NASA engineer who studied heat shield materials for nearly four decades at NASA Ames Research Center, told Ars. Rasky is credible: He co-invented PICA, the material used for Crew Dragon’s heat shield.

NASA confronted this same issue half a century ago during development of the Space Shuttle, which used thousands of lightweight ceramic tiles to insulate the vehicle’s aluminum structure. Although the tiles worked fairly well, the US space agency was never able to crack the code on “rapid” reuse. Each tile required post-flight inspections, so reuse of Space Shuttle orbiters remained expensive and time-consuming.
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However, according to a new analysis by Rasky and two other space industry officials, former NASA astronaut Charles Camarda and the head of the NASA transition team for the Trump-Vance White House, Charles Miller, significant challenges remain for rapidly reusing Starship.

They give us a handy photo to concentrate on. Notice that whitish streaks seem to diverge from edges and corners (points) on the hexagonal tiles. Seams or boundaries between the tiles. That implies that it's the super hot gas from the upper atmosphere, melting and pushing out material from under the tiles. SpaceX has shown in their own videos that they put something like a white plastic foam under the tiles. 

A close-up of Starship's heat shield after it landed in the Indian Ocean last week. Credit: SpaceX

The problem is that, with the signs of damage, such a heat shield would appear to require a fair amount of inspection and refurbishment before another launch. In other words, SpaceX has a ways to go to reach “full and rapid” reuse of Starship. “Full” means reusing both the booster and the Starship upper stage, and “rapid” reuse means turning each Starship around in days or weeks, instead of months, at considerable expense.

Here, the person that's saying this is completely missing the story. “Full rapid reuse” doesn't mean landing and flying again “in days or weeks” to Elon and crew. It means taking off again in a few hours. More like a commercial jet than a spaceship. 

On various podcasts and X social media postings, SpaceX founder Elon Musk has repeatedly stated that solving the reusable heat shield problem is one of the toughest problems the company faces. Back in February on a podcast with Dwarkesh Patel, Musk expounded on this, saying a reusable heat shield is “the biggest problem that remains” for Starship. “If you want to be able to land it, refill propellant, and fly again, you can’t do this laborious inspection of 40,000 tiles type of thing,” he said.

The Ars article reduces this down to the reasonable conclusion that Starship's heat shield will likely be good enough to deploy the third generation Starlink constellation like the ones FT-13 carried to orbit. But the experts are concerned that it will be unable to hit the high flight rate needed, with several hundred to thousands of low-cost launches annually, for large settlements on the Moon and Mars, orbital data centers, or large-scale space-based solar power. 

This sounds like the kind of "advancing the art and science of spaceflight" that we think NASA could help with, but the problem with that is that NASA dropped this kind of research decades ago. 

The experts noted that NASA has not made substantial investments in thermal protection research for vehicles entering from low-Earth orbit (speeds of Mach 25) for three decades. The underlying technologies on Starship, they note, are all based on work done for the space shuttle, National Aerospace Plane, or X-33 programs. No one today, they said, is seriously investing in new, advanced thermal materials for orbital reentry. 

I think that SpaceX has shown that they might be the best place to go for this sort of development. I've seen video clips where Elon discussed how they've altered the recipe for the heat shield tiles and gotten better performance. Perhaps they just haven't gone far enough down that adventure. Or perhaps it's something else entirely. Maybe instead of the white plastic foam sheet they have the tiles mounted on, perhaps they need a layer of something else there. Another tile? In other words, a sandwich of tile, foam, tile, and foam?