Atmospheric entry

In the aerospace re-entry (English reentry ) denotes the critical phase of entry into the atmosphere. As soon much kinetic energy is converted into heat, objects are destroyed without a heat shield. The hot plasma also interrupts a radio link.

The term is used not only for manned space vehicles, but also for space probes warheads from ICBMs, capsules with sample material, as well as for objects that may burn or intended as burnt rocket stages or retired satellites. Often the object is previously in orbit and begins the descent to the brake ignition contrary to the direction of flight. Re-entry does not count the later stages of descent in which the thermal load is low. For the same reason the term is used for objects that have reached only a small fraction of the orbital velocity, not used.

Examples

In human spaceflight, there are recycle capsules (Apollo, Soyuz, Shenzhou ) and reusable space shuttles (eg Space Shuttle ), which must survive the re-entry each harmless, so as not to endanger astronauts.

Each start of a multistage rocket leaves burned upper stages that occur after task is fulfilled in the atmosphere and partially burn up. Likewise ( disused ) satellites are in a controlled crash completely or mostly destroyed to avoid further space debris. The entrance path is possible chosen so that large parts that could survive re-entry plunge into the sea. A spectacular example of such a process was the Russian Mir space station. Even the Hubble Space Telescope could be brought to a controlled crash after the end of its working life, since his recovery no longer shows up due to the crash of the space shuttle Columbia in the plans of NASA and it would be too costly by other means.

More atmosphere admissions are landings of planetary probes ( Cassini -Huygens, Mars Rover ) and the aerobraking or Aero capture so-called.

A re-entry also undergo re-entry (English: reentry vehicle) ICBMs and SLBMs of that move over large areas in space and then re-enter at high speed into the atmosphere.

Conditions for safe re-entry from space shuttles and return capsules

Entrance angle as the angle will be referred to in the space among the inlet, a spacecraft relative to the horizontal in the dense strata of the atmosphere of a celestial body. The height of this point is determined arbitrarily. NASA gives for example for entry into the Earth's atmosphere a height of 400,000 feet ( about 122 miles) to ( Entry Interface ).

On re-entering high demands on the materials used and the structure of the spacecraft cell are provided. The temperature at the heat shields reached when entering the Earth's atmosphere over a thousand degrees Celsius, as well as the airspeed is decreased rapidly, so that major delays occur.

If the missile the heat load through undamaged, so usually with reusable spaceships heat-resistant materials with a low thermal conductivity such as ceramics are used in heat protection tiles that provide sufficient insulation. In addition, the heat must be radiated back; for themselves as well as ceramic materials are metallic. If you want to use materials whose melting point is too low, there is the possibility of cooling by an ablative heat shield. This sublimate or pyrolyze the materials used in the heat shield. The resulting relatively cool boundary layer insulates the underlying layers and carries away a large part of the heat. An ablative heat shield is technically easier and less expensive than a reusable heat shield; with an appropriate design (still) higher entry speeds ( more kinetic energy that must be converted ) are possible. If an ablative heat shield on a reusable spacecraft are used, then a renewal is required after each flight.

Entrance angle and speed of the missile must be calculated exactly when a controlled, safe descent and landing in the intended landing area should be ensured. The entrance angle is usually between 6 ° and 7 °. If too shallow entry, the spacecraft leaves the atmosphere again ( after every other mission in orbit, it would indeed further decelerated, the target area is, however, so missed ), with a steep entrance to the thermal load is too large. When entering the Apollo spacecraft after the return from the moon, the entrance angle was ideally 6.5 °, with a tolerance of plus / minus 0.5 ° was.

Calculation of the trajectory

Since the early days of space travel, it was an important task, predict the reentry reliable and particular time and place of Verglühens and the landing site to be determined. Depending on how it comes to re-entry occurred or were different difficulties. The Apollo Lunar Lander had no fuel to decelerate before the recurrence of a low orbit that would have been then first measured precisely. Path corrections needed at a great distance, take place before the separation of the capsule, very accurate for the conditions at that time.

In a descent from a low orbit out of the deorbit burn must be able to be accurately dosed. For example, used the American Space Shuttle weak OMS engines to reduce the train speed by 1 % within three minutes. This delta v of only 90 m / s is sufficient in an elliptical orbit on the other side of the world - again rotated in the direction of flight - enter the atmosphere. Shape and angle of the space shuttle create lift, flattens the first steep descent before the occurrence of the largest load. The power distribution is time limited, which reduces the heat absorption.

Particular difficulties in the calculation of very flat sheets are / were:

• Insufficient knowledge of current air density along the track. This problem was completely solved in 1960 and has led to forecast errors of up to 2 days. The ionosphere also varies regionally with solar activity.
• Changing air resistance of the wobbling and rotating missile - has yet to be solved
• Modeling of disintegration of the missile (smaller parts are decelerated more )

In severe or regularly shaped bodies, the calculations are more reliable than for light satellites with a variety of arms. Some crashes have already been to a few minutes, and the track can be accurately predicted on some miles.

Spacecraft that will land a payload safely back, are therefore shaped accordingly. The return capsule thereby takes flight in an aerodynamically stable position, so that the missile with the heat shield forward dips into the atmosphere ( Soyuz spacecraft, Mercury spacecraft ).

Until the 1970s there was a own network of visual observers called Moon Watch, which was supervised by the U.S. Smithsonian Astrophysical Observatory (SAO ) and worldwide several hundred volunteer teams involved. The support of the satellite cameras (especially the Baker / Nunn stations) by relatively simple equipped amateur astronomers was necessary because the cameras align despite technical complexity in certain conditions little, in which visual observers can react much more flexible.

Such problem areas are, among others,

• Measurements at dusk ( missile only still in sunlight but long exposure times impossible)
• Very low-lying trajectories
• Inaccuracy of the forecasts just before the re-entry, which complicates the programming of the cameras.

Risks

Generally, the start and the landing of a ( rocket-powered ) spaceship the critical phases of flight, for which there is an increased risk of accidents.

In the case of the U.S. Space Shuttle is well known that the heat protection system used (mainly consisting of reinforced carbon - carbon panels and ceramic tiles), although very resistant to high temperatures, mechanical influences but is very sensitive. In February 2003, the Space Shuttle Columbia NASA during re-entry burned up at the end of mission STS -107, in part because at the start of the shuttle at least one of the most polluted parts of the thermal protection system on the left wing leading edge by a foam piece the size of a briefcase has been damaged. Since this damage was not discovered during the mission (some cautions of NASA employees were ignored by the flight line or trivialized ), could adversely affect its aluminum structure thus far in re-entering the penetrating into the support surface plasma that the left area, and then the entire shuttle were destroyed.

Landings on Mars are more difficult to perform due to the low density of the Martian atmosphere, so landing probes sometimes crash with too high speed on the surface and can be damaged. For the same reason there are limitations in the landing heights on the surface of Mars, as probes may currently be landed only at altitudes below 2 km, which some of the interesting regions of Mars can not be achieved. In contrast, the landings on Venus or Titan, due to the dense atmosphere are much easier to carry out, but the high pressure and the high temperature of Venus's atmosphere poses a further threat to the landers.