There are lots of questions on here which ask "Why aren't rockets launched from railguns/mountains/balloons/etc." and lots of nice answers mainly dealing with the practical difficulties of doing so, which I fully appreciate.
I'd like to set that aside though, and just ask what benefits there would be for orbital launch if you could (for the sake of argument cost-free) start at various combinations of altitude up to 30km or so, and velocity up to 1 km/s or so (relative to the ground and in the direction of your choice)? Among the things that could be included in an answer if anyone can quantify them:
- Reduced Delta-V from altitude/gravity drag/iniitial velocity
- Lower initial thrust requirements (because you can choose to loose a little altitude if you like
- Possibility of lighter (or no) payload fairing
- Lower acoustical issues
- Possibility of using engines with something closer to vacuum nozzles
- Less complexity with engine ignition (no need for hold-down clamps etc.)