Starting altitude is the altitude from which the spacecraft is launched. A large balloon is defined as a large tapered black balloon at least three hundred meters high with air vents at the nose to reduce drag as it ascends and the air within expands. One of the most practical turbojet / rocket planes built is the NF-104 Starfighter Space Pilot Trainer. It has a slim airframe and a hydrogen peroxide / kerosene rocket engine. It was capable of taking off from conventional runways under its own power and reach altitudes of over thirty six thousand meters. Unfortunately the Starfighter had stability problems, a wind tunnel tested design which was stable at speeds up to at least seven hundred and eighty meters per second is the Lippisch Interceptor. In my opinion, a design with the lippisch frame and the NF-104 trainer turbojet / rocket combination would be an inexpensive, safe hypersonic aircraft.

A high starting altitude is desirable because the air pressure and density decrease as the altitude increases. A lower air density means the aerodynamic drag will be lower and a lower air pressure means the exhaust velocity will be higher. A high starting altitude especially helps small spacecraft which have a relatively large aerodynamic drag to mass ratio. Air density at altitude can be calculated from the sea level density given the altitude and column height.

air density = sea level density * exp( - altitude / column height )
 

This is used in astropolis, atmospheric spacecraft, multi stage spacecraft, and spacecraft cost.
 

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