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Skip reentry


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Atmospheric entry trajectory illustrating the basic phases of flight in
a skip reentry.

*Skip reentry* is a reentry technique involving one or more successive
"skips" off the atmosphere to achieve greater entry range or to slow the
spacecraft before final entry, which helps to dissipate the huge amount of
heat that is usually generated on faster descents. The range modulation
made possible by skip entry allows a spacecraft to reach a wider landing
area, or to reach a designated landing point from a wider range of
possible entry times, which is especially important in abort situations.
Like aerocapture , skip reentry requires precise guidance. An overly
shallow entry angle will result in the spacecraft retaining too much of
its velocity, possibly escaping into space permanently if this is more
than escape velocity . An overly steep entry, on the other hand, results
in more intense heating and stress that could exceed the design limits of
the spacecraft, potentially destroying it.

The basic concept is to 'clip' the atmosphere at such an angle that the
craft is 'pushed' back out into space, conceptually similar to a pebble
skipping across the surface of a lake. Each time, the craft's velocity is
reduced so that it can eventually drop into the atmosphere at a low
suborbital velocity.

In theory, any craft could perform skip re-entry as it does not require
much lift, but in practise it requires precise guidance. Although the
space shuttle is capable of skip reentry, NASA has carried it out only in
computer simulations (Scott Horowitz, NASA interview, Jan. 25, 2007). It
is unclear how thermal shielding would fare under the rapid heating,
cooling and reheating. In theory STS-107 might have survived if a skip
trajectory had been attempted - giving more time for heat dissipation -
but this cannot be proven.

Skip entry was first imagined in the 1930s, when a suborbital skipping
trajectory was planned for the German Silbervogel bomber, which never
flew. The technique was used by the Zond series of circumlunar spacecraft,
which planned for one skip before landing. Zond 6 and 7 made successful
skip entries, although Zond 6 was largely destroyed on impact for
unrelated reasons. The Apollo Command Module , when returning from the
moon, was capable of a one-skip entry. The Orion spacecraft crew module
will be capable of skip entry, to allow targeting the landing site from a
greater variety of abort trajectories.

Guidance of a skip trajectory can be tricky due to trajectory sensitivity.
The Apollo Skip Guidance ^[1] was engineered, but never utilized in a
manned mission. More recent work relies on advances in computing
technology to compute a trajectory onboard the vehicle. ^[2] ^[3] ^[4]



[edit ] Skip Reentry in Fiction

Robert A. Heinlein 's 1940s and 1950s fiction made frequent use of skip
reentry for spacecraft, before ablative heat shields became the preferred
real world method.


[edit ]
References

1. *^ * Apollo Skip Guidance

2. *^ * Skip Guidance Example

3. *^ * Skip Guidance Example

4. *^ * Skip Guidance Example
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