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In astrodynamics, gravity drag is inefficiency encountered by a spacecraft thrusting while moving against a gravitational field.
If the magnitude of the field is g and the thrust per unit mass (acceleration produced by the engine) is a, then the actual acceleration of the craft is a - g, while using delta-v at a time-rate of a; that is, the delta-v is a / (a - g) times the actual increase in speed. In the case of a very large thrust during a very short time, a desired speed increase can be reached with little gravity drag, while for a only slightly more than g, the gravity drag is very large.
For example, during launch from Earth, a rocket does not have a horizontal speed yet (which provides a centrifugal force), so just staying aloft costs a delta-v of 9.8 m/s for every second. Therefore a slow launch would be inefficient.
When applying delta-v against gravity to increase specific orbital energy, it is advantageous to spend the delta-v as early as possible, rather than spending some, being decelerated by gravity, then spending some more, or spending it at less than full capacity. Gravity drag can be described as the extra delta-v needed because of not being able to spend all the needed delta-v at once, because of the finite power of the rocket engine.
This effect can be explained in two equivalent ways:
- The specific energy gained per unit delta-v is equal to the speed, so spend the delta-v when the rocket is going fast; in the case of being decelerated by gravity this means as soon as possible.
- It is wasteful to lift fuel unnecessarily: use it right away, and then the rocket does not have to lift it.
These effects apply whenever climbing to an orbit with higher specific orbital energy, such as during launch to LEO or from LEO to an escape orbit.
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