| Re-entry
of Space Vehicle |
Definition
The successful exploration
of space requires a system that will reliably transport payload such as personnel
and instrumental etc. into space and return them back to earth without subjecting
them an uncomfortable or hazardous environment. In other words, the spacecraft
and its payloads have to be recovered safely into the earth. We have seen the
re-entry capsules and winged space vehicles approach the earth followed by safe
landing. However, this could be accomplished only after considerable research
in high speed aerodynamics and after many parametric studies to select the optimum
design concept.
Re-entry systems were among the first technologies developed
in 1960s for military photo-reconnaissance, life science and manned space flights.
By 1970s, it led to the development of new refurbish able space shuttles. Today
space technology has developed to space planes which intend to go and come back
regularly from earth to space stations. USA's HERMS and Japan's HOPE is designed
to land at conventional airports. Few significant advances in current proposed
re-entry capsules are ballistic designs to reduce development and refurbishable
cost, to simplify operations. For entering
into atmospheric and non-atmospheric planet the problem involves is reducing the
spacecraft's speed . For an atmospheric planet the problem involves essentially
deceleration, aerodynamic heating, control of time & location of landing.
For non-atmospheric planets, the problem involves only deceleration and control
of time & location of landing. The vehicle
selected to accomplish a re-entry mission incorporates a thick wing , subsonic
( Mach < 1 ) airfoil modified to meet hypersonic (Mach>> 1 ) thermodynamic
requirements. The flight mechanics of this vehicle are unique in that rolling
manoeuvres are employed during descent such that dynamic loading and aerodynamic
heating are held to a minimum. Therefore re-entry
technology requires studies in the following areas:
1. Deceleration
2. Aerodynamic
heating & air loads
3. Vehicle stability
4. Thermal Protection Systems
(TPS)
5. Guidance and Landing.
Re-entry mission profile, constraints
And vehicle requirements The safe recovery
of the spacecraft and its payloads is made possible by the re-entry mission. According
to the different constraints the mission profile can be divided into three distinct
flight segments:- 1. Deorbit and Descent to
sensible atmosphere at an altitude of nearly 120kms.
2. Re-entry and hypersonic
glide fight.
3. Transition flight phase, final approach and landing.
The unguided first flight segment (Keplarian trajectory) initiated by a rocket
deboost maneuver at a specific orbital point determines the flight condition at
re-entry. The second flight segment covers the atmospheric glide at an altitude
of 120 km to 30 km during which the re-entry vehicle's high initial kinetic energy
is dissipated by atmospheric breaking. The third flight segment does the final
approach and landing. The various forces acting
on the re-entry vehicle are:-
1. Gravitational force acting towards the centre
of the planet.
2. Gas dynamic force opposite to the direction of motion of
the vehicle.
3. Centrifugal and gas dynamic lift force acting normal to the
direction of
4. motion of the vehicle.
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