There’s a piece in the New York Times this morning about a German project to send a balloon-borne rover to Mars that’s got a little something for everyone. It’s a Mars mission, which never gets old, but it’s also a privately financed project, and thus a nice demonstration of the power of private enterprise, for those who favor private space exploration. It’s not quite clear to me that what they’re proposing will work (one or two of the elements sound a little goofy), but they’re certainly fired up about it.
And really, why shouldn’t they be? Balloons on Mars are just cool.
1) “…a vehicle inspired more by dirigibles like the Hindenburg…” Riiight.
2) Earth air has average MW = 29/760 torr. One liter masses 1.295 g/STP. Helium has MW = 4.003. One liter masses 0.1787 g. A helium balloon has 1.116 gram/liter of lift (less balloon mass).
Mars air is 95% CO2 5% nitrogen at 7 torr maximum. Average MW = 43.21. One liter masses 0.01777 g. Helium at 7 torr masses 0.001646 g/l. A Mars helium balloon has 0.01612 g/liter of lift.
Temperature corrections from STP won’t save you.
3) A 170 lb craft with a 33-foot diameter balloon is claimed. 170 lbs = 77,273 g needing 4.8 million liters of helium for neutral boyancy at 7 torr. 33-foot diameter sphere is 502.9 cm radius or 533,000 liters at 7 torr. Ain’t even close. Inflating the balloon in vacuo from a 24 liter 3000 psi tank is cute (tribocharging!). Helium’s Joule-Thompson inversion temp is ~50 kelvin. Turbine expansion for cooling a half-millon liters during fill?
http://en.wikipedia.org/wiki/Joule-Thompson_effect
http://www.aero.org/publications/donabedian/donabedian-1.html
Tell Uncle Al why a Zylon paraglider (double Kevlar’s strength; decomposes at 650 C) weighing as much as the balloon plus included gas is not a better drag solution – especially form factor.
http://www.aramid.com/aramid_industrial_fibers_zylon.asp
Ah… but what spreads the paraglider prior to re-entry? A little inflation. For the comparatively small inflation volume needed a liquefiable gas like ammonia is good enough – easy to store and dispense.
http://history.nasa.gov/SP-4002/images/fig32.jpg
1960s Gemini paraglider
http://www.coatingapplications.co.uk/paragliders.htm
http://members.core.com/~flyboy/para.jpg
http://uplink.space.com/attachments/307940-paragliders.jpg
statistical spread
Bottom line: Who gives a rat’s ass about desiccated irradiated Mars?