Landing

The landing process onto
planetary bodies with sufficient atmosphere has three phases – the Entry,
Descent and Landing, always involving a Viking-inherited method consisting of a
series of aerobreaking mechanisms utilising the atmospheric drag by employing
parachutes and blunt aerobreaking heat shields. The entry and descent phases
reduces almost 97% and 2.5% of initial energy.  

The landing process onto
planetary bodies with very low atmospheric density (almost vacuum) is almost
completely done by retrothrusters.

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Various EDL alternatives include:

Heat shields – major role in
entry. They provide a large surface area at the nose end to provide a very
efficient air-breaking. Made up of layers of heat-withstanding materials (upto
2000  ?C, they keep the rover inside at room
temperatures. Requires a good atmospheric density.

Hypersonic Inflatable Aerosynamic
Deccelerator  – entry and descent. Being
inflatable, can provide much larger surface area than heat shields. Functions
like a heat shield. Most applicable for larger payloads like human missions.

Suersonic parachutes – Made of
strong, yet lightweight materials like Nylon and Polyester and straped to the
payload by Kevlar. Requires a good atmospheric density to function.

Ballutes – descent. Torus shaped
air balloons. Funtions similar to a parachute.

Air bags – Landing. A bunch of
airbags surround the payload, which when inflated and dropped from heights as
much as four storeys, bounce and roll on the surface to a stop. Orientation of
rover is unpredictable on ground. Less safer for the payloads.

Sky crane – Landing. Activated
near the surface, when the velocities have reached almost 100-200 m/s. They
utilise a small system of retrorockets to gently place the payload onto the
surface. Used in EDL of Curiosity rover.

Vertical landing – landing.
Involves retrothrusters which slow down the payload to almost zero velocities
and gently places it onto the surface. Mention SpaceX

Retrothrusters – entry, descent
and landing.

Ganymede has a low value of g =
1.4 m/s^2 and the atmosphere  is tenuous.
This necessiates the use of retrothrusters even in supersonic velocities.  The final phase of landing will be enabled by
a skycrane which will gently place the lander onto the precise location.

Why skycrane instead of vertical
landing?  

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