Why does the heatshield have to be on the outside?

Question

To what extent has internal insulation been tested to deal with the heat of re-entry?

In the case of Starship, is the existing steel strong enough (to be non-ablative!), or would a different type of material be necessary? The melting point of most steel alloys seem to be right at the limit... can it be raised with paints and resins containing things like aerogels or glass? Can steel with higher temperature tolerances be made? - what are the limits here?)

Why not just fill the shell with aerogel? Has this been tried? To what extent is the weight (or cost) of such concepts, prohibitive?

Are there practical limitations besides weight, that might limit the scale, such as heat dissipation, or thermal expansion? What techniques have been tried to counter them?

Or why not have a fuel tank hug the perimeter... or expand it into a more elaborate refrigerator... It seems pointless to land with fuel, unless you want to take off again and either not land again, or just refuel in space... how much fuel would be necessary by surface area? Would current tank materials be able to accommodate such a design, and has it been tried?

Or what about some sort of perspiration of ablative material?

Answer 1

To answer the title:

Why does the heatshield have to be on the outside?

Because most materials commonly used in spacecraft construction melt at the temperatures encountered during reentry. And most of the materials that can withstand that heat are not suitable for structural use: the heat shield tiles on the Space Shuttle are very fragile, for instance. So you can't build the structure from heat shield materials. Which means you have to build a structure from materials with lower melting points, and put an outer cladding on top of those that has a higher melting point.

Answer 2

To answer some other aspects:

In the case of Starship, is the existing steel strong enough (to be non-ablative!), or would a different type of material be necessary? The melting point of most steel alloys seem to be right at the limit...

In general, metals do not have good mechanical or corrosion properties when at the edge of the melting point, and the transition between cold and red-hot and back again will also often be "fun" -- after all, this is how metals are hardened, stress relieved, and annealed. According to the MakeItFrom materials database, various 300-series stainless steels have temperature limits due to corrosion or mechanical loss of strength between 400 deg C and 1000 deg C. Reentry temperatures seem to often be over 2000 deg C.

To be clear, there are plausible spacecraft designs where the structural outer hull is simply made of materials able to withstand the temperature of reentry, and the inside is insulated with appropriate insulation materials. These tend to be excessively expensive and hard-to-machine alloys involving nickel, molybdenum, tungsten, cobalt, etc. My impression is that the cancelled X-20 Dyna-Soar spaceplane was to use this strategy, with lots of exotic (for the 1960s) metals.

Can it be raised with paints and resins containing things like aerogels or glass? Can steel with higher temperature tolerances be made? - what are the limits here?)

You are describing an ablative heat shield. (many of these involve fiberglass and phenolic resin). Spray-on ablators for re-usable spacecraft have been studied.

Coatings able to reusably withstand high reentry temperatures will have to be ceramic, metallic, or metalloceramic. This sounds like the so-called "thermal barrier layers" used in jet engines.

Why not just fill the shell with aerogel? Has this been tried? To what extent is the weight (or cost) of such concepts, prohibitive?

Aerogel is a very good insulator, but despite it being a ceramic-type material I do not see it usually proposed for extremely high temperature applications. It is extremely fragile as a bulk material and often therefore is used as powder or chips integrated into an insulation blanket. In any case, this would be a "hot structure" type of application as described above. Also, hot structure designs seem to have issues with weight (possibly because the high temperature alloys have poor strength-to-weight ratios and it is hard to build strong, light structures that can also survive being cooked).

Are there practical limitations besides weight, that might limit the scale, such as heat dissipation, or thermal expansion? What techniques have been tried to counter them?

Thermal expansion can make structure-building, especially using multiple materials, a lot harder. The Dyna-soar, for example, was to have its structural elements connected by hinges rather than normal rigid connections.

The heating profile over time also matters. Ballistic reentry tends to lead to very high heat for a short time, lifting reentry (like the Space Shuttle) leads to less heat flux but over a much longer time. "Heat sink" and heavy ablator heat shields seem to work well with the former option.

Answer 3

"perspiration of ablative material

This appears in science-fiction and allows for the possibility of easy maintenance and multi-use, but these benefits are only realizable if some very big hurdles are overcome.

1. A liquid material is required that has a phase change temperature at least as high as existing solid materials. It is very difficult to think of one. Just starting as a liquid instead of a solid is a big handicap here because, all things being equal, it takes less energy to vapourize a liquid than to sublimate a solid.

   A perspirating abalative layer would not be chosen over a single-use solid heatshield unless it had a weight advantage. But with current material science, the aforementioned issue requires more abalative mass. Even if a viable liquid material existed that required less mass, it would need to be light enough to also offset the additional mass of the pumps and ductwork. If the weight is only parity, then not much changes because it is effectively still single-use and instead of always wearing your solid heat shield, you just keep your liquid heatshield in tanks until you want to put it on.

2. This liquid material also needs the capability to be applied reliably in-situ. This greatly raises the bar for the potential maintainability advantage. To put things into perspective, existing solid heatshields are installed and inspected on the ground. You don't get to inspect your perspirating heatshield before using it so it would need to be that reliable.

This is rather easily determined by knowing how much energy needs to be dissipated, and at what temperatures then just looking at the thermal characteristics of available materials to determine the material mass required. No need to even consider the extra weight of pumps and ductwork yet. Thus far, my understanding is that it would never get past the paper napkin stage.

Answer 4

In early 2019 after SpaceX changed the Starship design from carbon fiber to stainless steel, Elon Musk said they were considering using transpiration cooling for Starship, essentially sweating liquid methane through pores in the hull. In an interview with Popular Mechanics in January 2019 Musk said,

On the windward side, what I want to do is have the first-ever regenerative heat shield. A double-walled stainless shell—like a stainless-steel sandwich, essentially, with two layers. You just need, essentially, two layers that are joined with stringers. You flow either fuel or water in between the sandwich layer, and then you have micro-perforations on the outside—very tiny perforations—and you essentially bleed water, or you could bleed fuel, through the micro-perforations on the outside. You wouldn’t see them unless you got up close. But you use transpiration cooling to cool the windward side of the rocket.

In a January 2019 tweet Elon mentioned the advantage of using methane instead of water for this purpose:

When going to ~1750 Kelvin, specific heat is more important than latent heat of vaporization, which is why cryogenic fuel is a slightly better choice than water

Eventually of course SpaceX decided to instead use hexagonal ceramic tiles attached to the hull as a heat shield. However shortly after making this switch Musk indicated in a March 2019 tweet that transpiration cooling was not completely ruled out for some parts of the ship:

Transpiration cooling will be added wherever we see erosion of the shield. Starship needs to be ready to fly again immediately after landing. Zero refurbishment.

As recently as a tweet in October 2020 Musk seemed to indicate that the possibility still exists:

It might be used in some areas. ITAR laws prevent us from being too specific about solutions.

Although as far as I know nothing has been said about it since 2020, Starship has not yet attempted reentry from space and so it is possible that SpaceX has not ruled out at least some amount of transpiration cooling as they wait to see the results of how well the tiles work. And as Musk indicated in his March 2019 tweet, just one successful reentry might not 100% settle the question until they are able to see the longer term effect on the tiles of multiple reentries.

Note that I am not making any claims about the practicality of this approach, I am only pointing out that until we hear something different from SpaceX there is a possibility that transpiration cooling at least at some level is still possible for Starship.