Gravity drag
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In astrodynamics, gravity drag (or gravity losses) is inefficiency encountered by a spacecraft thrusting while moving against a gravitational field. It describes an inefficient use of thrust, not some magical effect on the efficiency of the operation of the engines.
In very simple terms, the purpose of a rocket engine is to change the velocity of the rocket. It is logically better to burn expensive fuel at locations in the orbit where the resulting change in velocity will be largest. If choosing the location is not an option, then choosing the rate at which fuel is burned may lead to a change in efficiency.
As an extreme example, consider a launch from Earth. One could plan a rocket that climbs to 1000 feet above the ground and then hovers there for an hour before proceeding onward and eventually into orbit. Certainly, this rocket will burn more fuel than one which proceeds directly to orbit without a hover. The reason for this inefficiency is that the thrust is simply supporting the weight of the rocket (during the hover) and not increasing the rocket's speed. This illustrates the rule of thumb that a time-consuming launch is inefficient.
If the gravitational acceleration vector is g and the thrust vector 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 of the vehicle used 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.
When applying delta-v against gravity to increase specific orbital energy, it is advantageous to spend delta-v at the highest speed 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 instantaneously.
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 Low Earth orbit (LEO) or from LEO to an escape orbit.
[edit] Vector considerations
It is important to note that acceleration is a vector quantity, and the direction of the acceleration has a large impact on the overall efficiency. For instance, gravity drag would reduce a 2.6 g thrust directed upward to an acceleration of 1.6 G, for an efficiency of less than 62%. However, the same 2.6 G thrust could be directed at such an angle that it had a 1 G upward component, completely cancelled by gravity drag, and a horizontal component of 2.4 G, unaffected by gravity drag. Achieving 2.4 G acceleration with 2.6 G thrust gives an efficiency of over 92%.
Note, however, that the objective of the craft is not only to maximize acceleration, or else it would direct its 2.6 G thrust downward, achieving 138% efficiency but never reaching orbit. Rather, the objective is to achieve the necessary specific orbital energy to sustain the desired orbit. On a planet with an atmosphere, the objective is further complicated by the need and to achieve the necessary altitude to escape the atmosphere, and to minimize the losses due to atmospheric drag during the launch itself. These facts sometimes inspire ideas to launch orbital rockets from high flying airplanes, to minimize atmospheric drag, and in a nearly vertical direction, to minimize gravitational drag like in the above calculations. For example, an AN-225 plane could launch a 250 ton rocket like that.
