Are there any viable hot structure orbital spacecraft designs capable of re-entry?

Question

The current design of all orbital spacecraft capable of re-entry is cold-structure, using ablative materials, insulating tiles other approaches to keep the structure of the spacecraft relatively low during re-entry. The X-15 and Spaceship One are hot-structure spacecraft, where the materials absorb the heat of friction. The X-15 used special alloys to withstand heating while Spaceship One is composite, but distributes heat by continually exposing different parts of the structure. Neither the X-15 or Spaceship one got to orbital speeds, however. A hot-structure design would seem to have significant advantages over thermal protection systems in re-usability and maintenance, however the heat generated from re-entry is far more than suborbital flight, even at high mach speeds.

1. Are there true advantages in a hot-structure design in cost or reuse of the spacecraft, or would the costs of the materials and manufacturing outweigh the benefits?
2. Are there any viable hot-structure spacecraft designs that could survive re-entry using current materials?

Answer 1

The early ICBM Reentry Vehicles, which while not orbital at least approach orbital velocity, used a heat sink design, where the heat was stored in to the system to keep it from being used. The problem with these is that if you exceed the load slightly, they will not work, and it adds considerable mass

Of some interest is this DoD paper that discusses reentry techniques for hypersonic and orbital planes.

The bottom line is, there are 3 techniques that one can use to mitigate the heat of reentry:

1. Ablative Shielding- The most commonly used technique.
2. Absorb the heat- Might add considerable mass to the spacecraft, works well for low speeds. Also might cause stress, as metals could get quite hot.
3. Spread out the reentry heat more gradually.

If there was going to be such a system, it would most likely be a plane design, as that could glide more and spread out it's heat.

Also, I would challenge your assumption that it would be easier to maintain. There is a high amount of heat that happens in reentry, and that heat has to go somewhere. It could warm metals up considerably, which can cause a number of issues, which might affect the lifetime of not just an external component, but the internal frames, which are likely to be more difficult to replace.

In the one orbital system well designed, the Dyna-soar, it was expected that the structure could withstand about 4 worst-case reentries. I'm not sure what the distribution of worst-case reentries is likely to be, but this is still below the SpaceX estimation of 8-10 reentries for it's heat shields.

Of some note is the Space Shuttle. It is a plane, and thus can maneuver, but at the same time it has a shielding that puts it somewhere between a "hot" and "cold" reentry system. The tiles can be reused many times, but need to be examined after every flight, and some of them needed to be replaced. So it achieved partial reusability, and were easily the most reused vehicles to orbit in space as a result, although much work was required to refit each shuttle between launches.

The bottom line, if you can slow reentry, and absorb a certain amount of heat into the structure, you can reduce the maintenance cost. You might be better off still using at least some ablative shielding to further improve the lifetime rather then absorbing all of the heat.