Atmosphere braking
Under atmospheric braking ( engl. Aerobraking or atmospheric braking ) refers to an orbital maneuvers the space with which the velocity of a spacecraft purposefully reduced by repeated dipping into the upper atmosphere of a planet and thereby gradually the trajectory is approximated to the planet. An atmosphere braking is technically less demanding than an atmosphere capture (Aerocapture), since the spacecraft is slowed down less and not continuously and thus heats up less.
process
The spacecraft to be braked is initially in an elliptical orbit. The periapsis (smallest distance) of the orbit lies in an area of the high atmosphere of the celestial body. Then the height of the apoapsis (furthest distance) is reduced with each round trip . This reduction is due to the flow resistance of the atmosphere. If the spacecraft is not supposed to land, when the desired apoapsis is reached, the thrusters are ignited in order to lift the orbit of the periapsis out of the atmosphere. To improve the braking effect, the solar panels of a planetary probe can be used in a similar way to airbrake flaps or blades, with which the resistance in the upper layers of the atmosphere is increased or the heat input is distributed over a larger area. The duration of the atmospheric braking maneuver can be several months.
In contrast to the atmosphere capture ( aerocapture ), the atmosphere braking involves the injection from a hyperbolic orbit into a (highly elliptical) orbit by means of the engines. When catching the atmosphere, braking takes place at an escape speed with a single flight through the atmosphere, so that a heat shield is necessary due to the high braking power .
control
Requires an atmosphere braking
- a celestial body with a notable atmosphere (in the solar system these are: Jupiter , Saturn , Uranus , Neptune , Venus , Earth , Titan and Mars ) and
- a spacecraft that has already braked below the escape speed (more precisely: after the first braking maneuver, the escape speed must have fallen below).
If, in addition to the use of engines for pivoting into an atmospheric braking path, additional braking by engines is required, it is most effective if this is done shortly before, during or shortly after the first atmospheric braking maneuver, because
- by effects of the Oberth Effect it is most efficient to use fuel at high speeds and
- the escape speed must be undercut after the first braking maneuver.
Further characteristics of atmospheric braking:
- The braking takes place primarily in the high atmosphere of the celestial body at pressures of 0.02 Pa up to a maximum of 40 Pa. What is underneath (dense atmosphere / gas planet, oceans or rocks) is irrelevant except for investigating the causes of failed atmospheric braking.
- In the case of unmanned spacecraft, braking is usually carried out in many cycles (with the Mars Reconnaissance Orbiter : 426 cycles), the braking effect and temperature increase are moderate, and the braking maneuvers can take several months.
On aerodynamic braking maneuvers in the EDL phase (Entry, Descent and Landing):
- In the case of manned spacecraft / return capsules, usually only one braking cycle is carried out in the earth's atmosphere, followed by a landing. A comprehensive heat shield is required for this.
- It is critical to maintain a precise flight path in order to reach a prepared landing area
- The greatest load and braking effect occurred when the Galileo daughter probe entered the Jupiter atmosphere with 228 g and up to 15500 K. The probe was decelerated from 47 km / s to subsonic speed within 2 minutes.
- It can also be used to dispose of disused spacecraft by incineration.
Since atmospheric braking is self-reinforcing and the density in the high atmosphere also fluctuates considerably due to solar activity, atmospheric braking is usually initiated conservatively and permanently readjusted through further small course or position corrections using rocket engines. Successful atmospheric braking ends with a landing or is canceled by acceleration at the far point of the flight path. The latter raises the near point so that the orbit no longer leads through the atmosphere.
Areas of application
Since the late 1990s, atmospheric braking has been increasingly used to correct the trajectory of interplanetary space probes .
Aerobraking is used to swing into a planet-near, less elliptical orbit around a planet in a two-stage process. For this purpose, when approaching the planet for the first time, the speed is reduced by rocket engines to just below the escape speed, and then further reduced by means of aerobraking.
The method was first tested in 1993 by the Magellan Venus probe and first used in 1997 by the Mars Global Surveyor mission. Since then, this method has been used on all US probes that have been orbited into Mars.
The Starship and Super Heavy rocket is supposed to use aerobraking to slow down the return from Mars flights in the earth's atmosphere.
See also
Individual evidence
- ^ Donald Rapp: Human Missions to Mars: Enabling Technologies for Exploring the Red Planet . Springer, 2015, p. 246 ( limited preview in Google Book search).
- ↑ Munk, MM: AEROCAPTURE DEMONSTRATION AND MARS MISSION APPLICATIONS. (PDF; 282 kB) NASA Langley Research Center, accessed on January 25, 2014 (English).
- ↑ Mars Reconnaissance Orbiter at an altitude of about 330 km
- ↑ At the Mars Climate Orbiter , the limit of destruction was 85 km and an estimated 40 Pa air pressure.
- ↑ The air pressure at normal altitude on Mars is about 600 Pa, estimates from the gas equation result in a halving of the air pressure about every 22 km.
- ↑ The main braking maneuver of the space shuttle took place between 55 and 70 km altitude at 4 to 40 Pa air pressure. The highest temperature of the heat protection shield is reached at an altitude of 70 km, at an altitude of 55 km 75% and at 25 km 99% of the kinetic energy is dissipated. The Columbia broke apart at 63 km.