main tank insulation solution
Before the first shuttle went up, they carried it on the back of a Boeing 747 and a bunch of tiles blew off at a mere 300 mph. They said, "We'll have to get a different glue for the tiles". I thought, "Yeah, right ... you can go to Ace Hardware and pick up some glue that will hold them on at 18,000 mph".
The underside of the shuttle is in pieces rather than one continuous shield because of the temperature difference between the top and bottom during re-entry. The tiles have a "coefficient of expansion" just like everything else in existence. Thus, as they heat up they expand ... and ... you have to leave gaps for them to expand into ... else the bottom would just buckle or pop rivets at the connection with the rest of the ship's fuselage. They leave some spacer material that can be squeezed when the tiles expand. That's the stuff that's sticking out. They are presently afraid that it might direct the re-entry plasma into the empty space and send a jet of hot gas into the meltable stuff under the tiles. They just don't know ... so ... they're taking the safest route they can conceive of.
The bigger problem is that insulation that keeps coming off the main tank at around 2000 mph. Here, you have a tank of cold hydrogen that needs to be kept cold so that it will stay liquid at low pressure. They could run with room temperature liquid hydrogen ... but ... that would require a very strong container (like a scuba tank). But that would be soooooo heavy that you'd have no appreciable payload capacity for the mission.
So the 24 dollar question is, "How do you make rock hard insulation that won't come unglued?"
Of course the answer is simple ... tile the large tank (at least the front half of it) with the shuttle tile material. It NEVER comes off ... unless it's knocked off. This would be expensive ... but ... it would most definitely work. You'd have to recover the main tank after each mission ... and ... would the tiles come off on splash-down? They stay on in 18,000 mph air, so maybe they can take water at 300 mph?. Do the tiles float so you can pick 'em up and re-use them? ;o)
I don't know.