Suppose there is a comet of radius 500m made of mostly ice, with the rest being rock, dust, and other frozen gases. The comet is being hollowed out in order to use the water as reaction mass (giving it a few miligees of acceleration). How could the comet be strengthened such that it does not collapse onto itself/from the forces of acceleration? This should preferably be done with materials on the comet itself.
- Thaw and re-freeze in more uniform, solid building blocks. Water ice on the outside to increase albedo, gases and gravel on the inside.
- Bag it in a big plastic sheet. Carbon dust and soot plus hydrogen from water are the raw material for polyethylenes (or polystyrenes).
- If you can manufacture them, ropes of carbon nanotubes both as nets around the surface and as anchors inside.
Ice at the temperatures at the edge of the solar system is hard enough although a bit brittle. If you blow the gases generated in the centre through a long pipe to reduce the contact with the outer surface the external shell should remain strong enough. The pipe would be also useful to bypass the gravity of the comet itself and prevent the ejecta from falling back. If the wall remain thick enough it would act as a thermal insulator keeping the outer layer cold and strong. Furthermore the small pressure would let the water sublimate immediately keeping no liquid water with latent heat around.
The only problem is that you don't have just ice, but a mix of ice and rocks. You would have to find a way to blow away also some rocks.