Liftoff acceleration is defined as the acceleration of the spacecraft at liftoff. In these scripts, the thrust is assumed to be constant, therefore the acceleration is at a minimum at liftoff and at a maximum when the fuel is exhausted. In a real spaceship, the engines are throttled back towards the end of the flight to keep the acceleration below around seven gravities, so that neither the passengers nor the rocket gets crushed.

A higher liftoff acceleration means relatively less acceleration will be spent countering gravity; but, the engines will be heavier and there will be higher aerodynamic forces, both simple air resistance and destructive dynamic forces. One and a half gravities at liftoff is a typical compromise for spacecraft lifting off through an atmosphere.

In atmospheric spacecraft, multi stage spacecraft and spacecraft cost liftoff acceleration is used to calculate duration and final acceleration.

duration = exhaust velocity / liftoff acceleration * ( 1.0 - ( 1.0 + payload ratio ) / empty mass ratio )

final acceleration = liftoff acceleration * empty mass ratio / ( 1.0 + payload ratio )
 

In pumped rocket and rocket cost liftoff acceleration is used to calculate fuel flow.

fuel flow = mass * liftoff acceleration / exhaust velocity
 
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