With the overwhelming majority of heat shields being ablative and expendable in nature, it is fascinating to read about the Shuttle's TPS, its design and some of the problems that it faced. I believe that at some point in the (likely distant) future, developing reusable heat shielding systems will be of interest. Assuming for a moment that my speculation is accurate (please comment if you think that it is not), what would be the primary design challenges facing such a development?
Making it cost effective
One of the large expenses in the shuttle program was to inspect and overhaul the heat shield after each flight. A major condition for re-use to be feasible is that the equipment is expensive, and still useful after flight. Even if they are specialized and single-purpose, thermal tiles are not exactly expensive to manufacture, and may benefit more from a higher production rate than re-use. (A small market for used shuttle tiles seems to exist, but it is not that important for the overall economy.)
Another issue is reliability, you have to be tediously careful to find that one invisible crack or weakening that may end in a catastrophe. A heat shield defect is not something to be careless about, and to guaranty quality, an enormous amount of manual labour is required.
We remember Columbia
1$\begingroup$ Am I correct in my impression that shuttle tiles are somewhat similar to the fibrous refractory block material that's in basically every industrial furnace ever? $\endgroup$ Feb 19, 2020 at 5:29
1$\begingroup$ Industrial furnaces don't have severe weight constraints. $\endgroup$ Feb 19, 2020 at 13:58
$\begingroup$ @OrganicMarble Certainly, but the fibrous refractory insulation I'm describing is a very lightweight material, even if not to the degree of Space Shuttle tiles (since being lightweight makes it a better insulator). $\endgroup$ Feb 20, 2020 at 7:53
Hohmannfan detailed the basic problem behind developing a fully reusable heat shield.
I believe a more viable alternative developing an inexpensive, simple to apply ablator. Even some kind that an astronaut could spray on, while on a spacewalk or after landing on a planet.
Ablator as such is really not a very complex idea chemically, and leaves a lot of room for improvement in other aspects than 'ablative'. Applying it to shuttle tanks was done by a spray-on method. So - make the firm, hard non-ablative part of the heat shield reusable, but reapply the ablator whenever needed - even in field conditions. Just treat it as consumable like fuel or food, not as a part, like an engine - the heat shield "consumes" ablator, the way engine consumes fuel.
1$\begingroup$ USAF tried reusable ablative coating on the X-15 but it was a nightmare "...it was impossible to restore it for use during another flight. A completely new coating would be required for any new mission." ref: Aerofax Datagraph 2 North American X-15 by Guenther / Miller/ Panopalis. Also the ET coating was not ablative except in very limited locations nasa.gov/sites/default/files/114022main_TPS_FS.pdf $\endgroup$ Feb 14, 2016 at 22:31
1$\begingroup$ I would really question how a spray could work in a vacuum. Remember, most materials sublime in a vacuum, so either you spray a powder (which structurally would not survive hypersonic shockwaves) or you spray a gas (which structurally would not survive... time/Brownian motion). $\endgroup$ Feb 15, 2016 at 18:09
$\begingroup$ Seems to me like you could use a low-vapor-pressure solvent such as fomblin, that would only gradually evaporate, or perhaps a solvent-free epoxy type material that would stay goopy or liquid until crosslinked by chemical reactions or sunlight. $\endgroup$ Feb 19, 2020 at 5:28
The OP is missing the point of what an ablative heat shield is actually doing.
The heat shield's primary function isn't to protect the capsule. An aerodynamic shell would fare far better than a heat shield.
The main function of a heat shield is to slow a capsule down from orbital velocity.
It does this by converting kinetic energy into a huge amount of heat.
In fact, much of the heat generated never touches the heat shield.
To protect the capsule by pure insulation is rather difficult. As a huge amount of energy needs to be dissipated, or the heat shield would have to insulate the huge temperate differences for the lengthy reentry sequence (over 10 minutes long).
Additionally the skin temperature is at about 1600C, which is above melting point of many materials. Most materials would structurally fail at that temperature.
My point is of course is that you are in effect trying to build a reusable meteorite catcher (in terms of energy).
$\begingroup$ Yes, spacecraft rely on friction during reentry to slow down and land. However, the heat shield's primary purpose is to dissipate the heat load incurred during this process. The blunt shape of the heat shield is twofold. One, the heat load is inversely proportional to the shield's drag coefficient. Two, the blunt shape traps the shock wave forward of the vehicle, prohibiting the hot gases from coming into direct contact with the vehicle. If a blunt shape were not more effective at dissipating the heat and protecting the vehicle, it would not be the design shape. $\endgroup$– yeemonicFeb 15, 2016 at 18:59
$\begingroup$ @Beanre I avoided the word friction on purpose because it's misleading. The mechanism is actually aerodynamic in nature. My point is that the thermal load is something akin to a meteor strike in energy range. $\endgroup$ Feb 15, 2016 at 19:04
1$\begingroup$ I'm not sure I am following. What is misleading about using the word friction in the context of aerodynamics? My point is that the heat shield's primary purpose is not to slow down a capsule from orbital velocity - it is called a heat shield for a reason. The main reasons that an aerodynamic shell is not used is because it would fail to handle the heat load and protect the vehicle. $\endgroup$– yeemonicFeb 15, 2016 at 19:15
$\begingroup$ @Beanre an aerodynamic shell would fail when the aerobraking transitions to lithobraking. It's not friction when most of the energy disapated does not occur on the skin on the capsule. $\endgroup$ Feb 15, 2016 at 19:19