Starting velocity is the velocity at 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 velocity helps because less velocity has to be made up by the spacecraft on its way to orbital velocity. This is especially important in spacecraft with a limited performance which can barely reach orbit, in these cases the extra starting velocity provided by an aircraft can be the difference between reaching orbit with a small payload and not reaching orbit at all. Starting velocity is used to calculate the initial x and y velocities.

x velocity = cos( angle ) * starting velocity
y velocity = sin( angle ) * starting velocity
 

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

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