ESA ExoMars Vented Airbag Control Logic
Ciambottini, Daniela¹; Giovangrossi, Giacomo¹; Battocchio, Luciano¹; Tenore, Amedeo Giancarlo²; Laine, Benoit³; Bolz, Joerg⁴; Strauch, Hans⁴; Bown, Nick^5
1Aero Sekur S.p.A; ²Thales Alenia Space; ³ESA/ESTEC; ⁴EADS Astrium; ⁵Vorticity Ltd

ExoMars is the first ESA mission to land on Mars. Its aim is to further characterise the biological environment in preparation for robotic missions and then human exploration. Data from the mission will also provide invaluable input for broader studies of exobiology - the search for life on other planets. The Lander, containing the rover, will be released on Mars surface using an EDLS composed of heat shields, parachutes, breaking thrusters and a vented airbag, which will cushion payload impact during final landing phase.

The airbag system scope is the absorption of the residual kinetic energy of the Lander by means of compression of the gas it contains. To avoid the elastic transfer of the energy back to the Lander and to prevent potential rebound, the kinetic energy must be dissipated releasing the gas within the airbag by the means of dedicated vent valves. This new technology enables the landing of the payload in a predefined attitude, since the airbag is foreseen to be placed only on the bottom side of the Lander. In order to control the rotational dynamic of the Lander, the airbag is composed by several sectors that can be vented at different times.

The vented airbag requires a control logic that conveniently activates the valves to bring the payload to rest at first impact. Two vent control logics have been investigated and tested, based on two different kinds of sensors. Since the vent must ideally occur when the kinetic energy is at minimum level the trade-off has been conducted on sensors that can indirectly measure the velocity: laser sensor and accelerometers. Based on the trade-off, the accelerometer turned out to be the optimum sensor to control the vent.

The trade-off has been carried out considering two parameters, regarding the delay of the electronic actuating the valve and the valve opening, and the effective area of the valve allowing gas venting. The assumptions have been validated by means of a test campaign carried out in vacuum chamber that showed a good fitting between analytical and experimental data.

Once the design has been improved, based on the first tests results, the accelerometers based control logic has been implemented on a breadboard system. A set of drop tests simulating different Mars impact conditions has been carried out to validate the new control logic.

The two different control logics, based on accelerometer and laser, will be presented together with the simulations carried out during the trade-off. Moreover the results of the vacuum test campaign, validating the trade-off results, and the results of the drop test campaign, will be presented.