I think after the landing the reentry is probably the most problematic. If you can prevent them from burning up and you can land them (and one is implied by the other) you are on a good road.
I think the Space Shuttle engines would have been capable of more direct relights. They test fired them quite a bit and the were reusable. NASA was just taking a now risk approach and did not really push the technology forward.
Boeing has just received a contract for a first stage with wings that should fly 10 times in 10 days and it essentially uses a SSME. So they seem to believe that they don't need to do that much to make it work.
I think the Space Shuttle engines would have been capable of more direct relights. They test fired them quite a bit and the were reusable. NASA was just taking a now risk approach and did not really push the technology forward.
Boeing has just received a contract for a first stage with wings that should fly 10 times in 10 days and it essentially uses a SSME. So they seem to believe that they don't need to do that much to make it work.