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Inflated gossamer parachute







All landings on Mars have used a parachute but only to brake from supersonic speed down to still high speeds like 60 m/s. Rockets have always been used to end the braking. The last bit of speed is ultimately harvested by landing legs with shock absorbers or by airbags. The parachutes were quite big but the problem is that the Martian atmosphere has about one hundredth the density of the Earth atmosphere. Hence to generate the same parachute drag, the speed must be ten times higher (the drag is proportional to the speed squared; ten times more speed means one hundred times more drag).

Can the speed be brought down to say 6 m/s by a parachute? Sure! Use a parachute with 100 times more surface. A standard parachute is made out of tissue with a thickness of about 100 microns. Plastic sheets with a thickness of 1 micron can perfectly keep a probe descending gently. Some reinforcement by a criss-cross of threads is necessary... maybe the parachute will be two times heavier... no problem. Such a very wide and vaporous parachute is called a gossamer parachute.

No problem? Big problems... First of all, a gossamer parachute has no chance to survive the opening at very high speed. It will be torn apart in a flash, like an exploding soap bell. This is the easiest to solve. Intermediate drogue parachutes can be used. Maybe the most pleasing would be a parachute that opens by stages, for example a parachute made of concentric circles of fabric. The innermost discus is made of robust tissue while the outermost is made of 1 micron plastic. The circles are released in sequence. The size of the deployed parachute grows while the speed decreases. At the end most of the deployed parachute is gossamer while the inner circle hangs lower because it cannot lift even its own weight. Many variants are possible... but this is not the main problem.

The main problem is that 1 micron plastic sticks to itself. The deployment at high speed is also needed to blow the parachute open. There is no chance that the gossamer parachute opens wide under a wind of a few tens of m/s in the Martian atmosphere. Lots of kids have that problem when playing with toy plastic parachutes. It simply won't open...

Using an appropriate surface deposit on the plastic sheet can help to decrease the stickiness but the only reliable solution is to force the parachute to inflate. My favorite idea would be inflatable ribs in the parachute. The parachute would first be stretched between the drogue parachute and the probe. Then a few kilograms of liquid CO2 or butane would be injected in the radial ribs to inflate them and force the sideways opening of the parachute. Or a turbine can inject the Martian atmosphere in the ribs...

Many other approaches are possible, like charging the metalized plastic electrostatically (possibly using two parachute, one for each polarity), use telescopic rods (moving parts... no way), deploy the gossamer parachute first as a vertical surface then pull or release it to horizontal (high risk of wrapping up)...

One idea I like would be to let the gossamer parachute have a hot air balloon shape and be inflated by the air stream at the bottom opening. The balloon shape is aerodynamic and it inflates steadily, limiting the forces acting on the plastic sheet. Once the balloon is inflated, the bottom is released and the balloon is now hold by its outer rim. The lower hemisphere of the balloon rises and becomes concave, yielding a standard parachute shape with a maximum drag. (A variant would be to close the bottom opening and have lines pull the balloon to a concave shape to use the pressure to inflate an even bigger parachute.)

A problem is the sideways movement of the whole, due to the Martian wind or to an asymmetry in the deployment of the parachute. Probes surrounded by airbags don't fear this too much but probes intended to land on legs do.

Next problem would be to avoid the parachute covering the probe. If the rope between the two is long enough, the probability of this happening would be low. Yet a more active approach would be to keep the gossamer parachute somehow inflated and cut its lines so none of them forms a loop. The wind will blow the huge jellyfish away. The parachute can also be pulled to a little volume by lines, to become a meter-sized object with a low probability to cover the probe.

Maybe the best solution for a legged probe would be to use a propeller and a rudder (or two propellers). And possibly a winch to decrease or increase the length of the rope towards the gossamer parachute (or a long elastic rope and a horizontal propeller). Below a long rope, the propelled probe can hover around, compensate for the horizontal speed, select an adequate landing site and make the descent speed really gentle. Then possibly, once the probe landed, the propeller(s) and the (possible) winch can stay attached to the parachute rope and wind it back up to control the parachute in all sorts of ways. The propeller can also be used from the start on, to inflate the ribs of the gossamer parachute...


The NASA experiments and inflatable... heat shield!: www.nasa.gov/topics/aeronautics/features/irve.html



Eric Brasseur  -  July 2 2009
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