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In 1997 the Mars Pathfinder probe hit the ground of Mars protected by airbags. Those airbags covered the whole surface of the outer spheric protective shell of the probe. They were inflated just a moment before impact using gas generators. Thanks to them the impact deceleration was kept well below the maximum. After a few bounces and rolling, the probe stand still. The airbags were deflated and redrawn by inner kevlar wires. The protective shell of the probe was opened a way that forces the probe to stand upside (yet it was already upside). Web pages: 1 2 3 4 5 6 7 8 9 10. A pdf document: 1.

Maybe an enhancement to the airbags would be to allow them to release gas during the first impact. This can be done either two ways:

• Suddenly release the gas when the probe reaches 0 m/s velocity towards the ground. Thus when the pressure reaches its maximum. This can be done by an explosive wire on the bottom airbags external side.

• Let the gas stream out of the airbags throughout the impact. The pressure would be held below a given maximum. This can be obtained by calibrated holes in the airbags or valves that open at low pressure.

That way the probe will not bounce back after the first impact. It will stand still or almost.

This would have some advantages:

• The probe will endure only one impact. And the deceleration forces will only occur towards the bottom of the probe. It will not have to withstand deceleration forces towards random directions.

• The airbags themselves just have to withstand one impact. They can be less rugged against stone puncture and abrasion. Holes punctured by stones would be of no importance (so long the airbags material resists further tearing apart of the holes). Thus the airbags can be more lightweight. Reciprocally they can be bigger and provide a softer landing. Maybe in some cases they can become big enough to avoid the need for a huge parachute and final rocket break system.

• The probe will land upside and stay that way. So there is no need for airbags on the upper side of the probe shell. Either they can be removed or very light airbag-shaped balloons can be used for aerodynamic reasons. A ballute can be used around the sides of the probe. A second advantage is the system that makes the probe stand up can be removed or simplified. Deflating the remaining gas in the airbags in a right sequence should be enough to guarantee the probe stands upside.

• Making just one hit the probe shell will less likely hit dangerous big stone pikes and will less likely end its bouncing trajectory halted by a big stone or wall that could hamper the probe mission. Nevertheless this does not guarantee the probe will hit a clear zone. Maybe an optical system should be used during the descend to check the landing site and use some light sideways rocketry to displace the shell even just a few meters aside.

• If bigger airbags are used this means the system to redraw them has to be bigger too. Or that other technological choices have to be made to free the probe from its airbags. One way would be to keep the side airbags inflated, release them and let them roll away blown by the wind. Maybe carrying some equipment. (Such device may be best for little probes. The little probe shell should be surrounded by say four very big lightweight airbags. This allows for a high impact velocity. Should some of the airbags be punctured this will be of no importance. The first hit would not be braked completely, the airbag pressure control would be designed so the probe bounces away a few tens of meters further. Yet the airbags would be released just when the shell starts that bounce trajectory. That way there would be no need for a complex airbag redraw system. The probe shell just uses a remain of its kinetic energy to get free from the airbags. There is a little risk, if one airbag is punctured and will not be blown quickly away by the wind, that it will be blown towards the probe and cover it one day. Minute radio emitters should be latched to the airbags to check for the airbags departure before opening the probe systems. Or the probe should be mobile and go away. Probably best is to use no airbag at all below the shell. Only airbags on the side yet very big ones. Maybe one on the upside for aerodynamics and gas pressure repartition. The shell would be ducked between the side airbags. When the whole hits the ground the shell will hang between the airbags yet it will not touch the ground. When the gas pressure amongst the airbags comes to its maximum and the shell stands still, the airbags are released by explosive bolts. The shell then just falls to the ground from a meter hight. But the airbags, due do their pressure, will bounce away at high speed. One sole enormous airbag can be used, placed above the shell and kept in shape by ropes in order to be bend around the shell. Only the bottom parts of the airbags have to be protected against rocks. A little part of the shell can stay latched to the airbag. It will be accelerated towards a high speed upwards. Once it reaches its maximum velocity an explosive wire can open the whole bottom of the airbag, freeing the gas at once. So the airbag will become just a long and small shape with a solid head that will make it jump far away. (Maybe the gas pressure inside the airbags can be used to make them blow further away like kid balloons. But best is they keep their shape so the wind can easily drive them away.))

• If the airbags are removed from the upper side of the probe it becomes aerodynamically more complex and needs close tests and computations. Maybe a parachute or ballute can be sufficient to keep it upside. Furthermore if the parachute needs to be latched to the probe till impact it can hinder the probe by falling upon it. Maybe a parachute redraw system should be used just at impact to roll the parachute back up. (A variant of this system can help brake the impact : the rope between the parachute and the probe shell should be made very long. A few tens of a seconds before impact an electric motor can roll up the cable very quickly. This will increase the parachute force on the probe shell and slow down the probe before impact. Then the system can go on after impact to roll up the parachute completely.)

To avoid the shell to start rotating sideways on impact, the pressure in the bottom airbags should be held equal amongst them. So big gas ducts should exist between the airbags. Or the bottom should be made of one huge airbag kept in shape by internal ropes and partitions. A more high-tech way would be to keep the airbags separate but to explode them open at slightly different moments. The airbag with the highest pressure should be opened first, in order for the other airbags to make the shell rotate back in place.

The shape of a folden probe or a big payload often can be made relatively flat. In such case, one single big airbag below the shell, held to a flat shape by tendons or open partitions, can be used. With gas outlets on the sides of the airbag and an explosive wire all around for the final halt. The bigger the load, the bigger the airbag can be and the softer the landing will be. With even no need to redraw the airbag afterwards.

A simplification would be to build the bottom airbag so it breaks open by itself. The part of the airbag facing the ground would be rugged against stones yet its sides would be build to explode by themselves like a kid balloon once an impact pressure close to the maximum is reached. Such passive system does not allow for a high precision. Using an explosive wire lets the probe electronics decide actively and precisely when to break the bottom airbags open. Nevertheless this could be usable for little simplified shells. Most of the kinetic energy would be evacuated using such a passive self-exploding airbag and the shell would only make a few little bounces, needing few protection.

The role of the airbag gas exploding sideways during the final airbag blast is to carry away the kinetic energy of the falling shell. When the airbag touches the ground its gas is compressed by the probe motion. During the compression process the kinetic energy of the shell downs to zero while the shell is halted. Meanwhile the pressure rises inside the airbag and so its internal energy. The final blast open of the airbag sends the compressed and hot airbag gas away in the surroundings before it could transfer its energy back to the probe and would make it jump upwards.