I'm not smart enough to be an engineer, but I don't understand the relentless pursuit of this design. Same as the Osprey, just looking at it, if it is in the climb or descent phase, and loses an engine either due to malfunction or ground fire, it flips over and kills everybody. Stupid. Someone on FR has posted another design that looks like a helicopter with a pusher-prop. Seems more sensible to me. At least with a helicopter, if it gets hit in the engine, it can auto-gyro downward, and give the troops a chance to survive impact. This thing, no way. It loses an engine, and it flips over, killing everybody. What is up with that? This 4-engine design, maybe it can compensate for loss of 1 engine. But it seems that would still be very difficult.
Very simple.
1) It's the only forward-based troop transport in development, so it has no competition, and
2) It's components are slated to be produced in 350 different congressional districts, so Congress won't let it be killed.