Inflated gossamer parachute
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
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
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
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
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
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
The NASA experiments and inflatable... heat shield!: www.nasa.gov/topics/aeronautics/features/irve.html
July 2 2009