Gravity loss

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As gravitational loss in space physics of the part of the physical will work refers to a rocket against the gravitational field has to raise one central body. Other losses such as air friction , which also plays a major role in a rocket launch from the earth's surface, are not taken into account . The decisive effect is that a rocket needs a certain time to reach the orbit speed. During this time, the missile continues to fall back onto the central body along a ballistic path. For this reason, the energy required to reach the orbit speed is greater than the actual potential difference that results from the height difference.

As a rule, in space travel, instead of the energy, the part of the delta v that a rocket has to generate in order to reach the necessary circular orbit speed is specified for the gravitational loss. The orbit speed of an earth orbit is about 7.8 km / s, but to reach the orbit a rocket must have energy for a significantly higher final speed. However, the gravitational loss is less than the energy loss due to air friction. Therefore, rockets start vertically from the earth to quickly traverse the atmosphere and then slowly pivot into the horizontal trajectory.

Basic explanation

In the following, all other influences, such as B. the air resistance, neglected and only the influences of gravity taken into account.

A body that starts perpendicularly from the surface of a celestial body at a sufficiently high speed will reach a certain height. If the body could immediately pick up the necessary speed for an orbit in a horizontal direction at this highest point, it would swivel into a stable orbit and the gravitational losses would correspond exactly to the energy required to reach this height.

But if this body builds up the necessary speed for an orbit in the horizontal direction with a rocket engine at its summit, then this takes a certain time and the body falls during this time. In order to compensate for this, the body not only has to accelerate in a horizontal direction, but also has to be lifted again and again at the same time. The weaker the thrust of the drive, i.e. the longer it takes to reach orbital velocity, the greater the effort to maintain the altitude.

A missile that z. B. starts from the surface of the earth, must of course at least cope with the potential difference to reach the desired height. Reaching the orbital speed makes up the greater part of the energy, but until this speed is reached, gravity keeps falling back. This gravitational loss is therefore dependent on the time in which a rocket can reach the target.

Practical examples

  • The Ariane 5 ECA has a relatively weak upper stage to be able to reach the desired orbit. However, the stage also has a comparatively long burning time. Here a high orbit is controlled at the beginning in order to then use the falling as kinetic energy for a GTO transfer.
  • The Dawn asteroid probe used its weak ion thruster for a very long time , but since it actually only had to slow down to reach its goals in higher orbits around the sun, it only worked against the sun's gravity.

literature

  • Martin JL Turner: Rocket and Spacecraft Propulsion: Principles, Practice and New Developments . Springer, 3rd edition 2009, ISBN 978-3-540-69202-7 ( online ).

Web links

  • Gravitation losses calculation examples on Bernd Leitenberger's blog (2009)