There is a cone shaped 'plug' at the front of each engine
In other words, a large articulated structure that must be insulated but still function perfectly over an extremely large range of temperatures, pressures, and airspeeds (including some combinations of which we've never made such a thing work - see the recent news about the HTV-2 failure board report). It must operate reliably within strict tolerances both before and after being subjected to the dynamic stresses of launch and the thermal stresses of re-entry, and it must be economically re-usable, re-workable, cleanable and inspectable. I can see the "oh, shits" already.
Those are some serious engineering challenges. I'm not saying they're insurmountable, but they're certainly not to be treated casually either.
There's an engineering definition of the word "risk," which includes factors like probability of failure and effects of failure. Right now, both of those factors are large. Clever and diligent engineering can reduce the probability of failure, but there's not much one can do to mitigate the effects of failure of the main engine on a manned spacecraft, especially when it's required for both liftoff and landing. The best you can do is add redundancy and backup systems... ... and there goes your improvement in payload fraction that gives you competitive advantage, right out the window. At some point it may still be technically feasible, but the economic reason for doing it becomes moot. You will not find any customers because a Soyuz or Proton is cheaper and more reliable, and their insurance companies will insist on the reliable part, that's for sure.
It sucks that it's that way. I am another "space nutter" who wants to see the human race develop better options than being confined to one gravity well.* But the technical and economic realities are harsh, and produce many victims. Any entrepreneur who is not aware of these realities is making a grave error.
(And the pessimists and nihilists who believe that it's a wasted effort are welcome to stay here at home, but we won't let you hold us back. We'll send home goodies, we promise...)