Altitude is the height of the vehicle above the ground. When a spacecraft reaches low earth orbit, it is generally at an altitude of one hundred and forty kilometers ( one hundred and forty thousand meters ). 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. Altitude is used to calculate air density and orbital velocity.

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

orbital = ground orbital * sqrt( radius / ( radius + altitude ) )
 

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

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