A method is examined for safely deploying and inflating helium balloons for missions at Mars. The key for making it possible to deploy balloons that are light enough to be buoyant in the thin, Martian atmosphere is to mitigate the transient forces on the balloon that might tear it.
A fully inflated Mars balloon has a diameter of 10 m, so it must be folded up for the trip to Mars, unfolded upon arrival, and then inflated with helium gas in the atmosphere. Safe entry into the Martian atmosphere requires the use of an aeroshell vehicle, which protects against severe heating and pressure loads associated with the hypersonic entry flight. Drag decelerates the aeroshell to supersonic speeds, then two parachutes deploy to slow the vehicle down to the needed safe speed of 25 to 35 m/s for balloon deployment. The parachute system descent dynamic pressure must be approximately 5 Pa or lower at an altitude of 4 km or more above the surface.
At this point, a pyrotechnic device will break the retaining mechanism and open the balloon container. The parachute force will pull the balloon upwards out of the container while simultaneously the payload module (containing the helium tanks and flow control system) freefalls and pulls the bottom of the balloon down. This causes the balloon to stretch out to its maximum length. Transient shock loads are generated in the balloon when its maximum length is reached. These shock loads are held to safe values by ripstitch elements in the flight train that break at a prescribed force and dissipate energy. After a short delay, a valve opens to start the helium flow into the bottom of the balloon through a flexible hose connector. Pyrotechnic cutting devices fire at the end of inflation to stop the helium flow and to separate the parachute and payload module from the balloon. By design, the balloon ends inflation below its nominal float altitude to avoid over-pressurization. It will then rise to its nominal float altitude, typically 2 km or more above the surface, before leveling off. This may require some venting of excess helium through a pressure relief valve.
At the time of this reporting, this technology is at the prototype testing stage. Further development is needed, particularly with end-to-end flight tests showing the balloon surviving deployment and floating afterward, as well as increasing the size of the balloon from its current 10-m diameter to an ultimate size of 20 m in order to support equatorial Mars missions.