New Mars Forums

kbd512,

I see, I think I get the intentions of your design now, that's intruiging!

In which case, bearing in mind tahanson43206's request for number crunching, I have the following thoughts to contribute:

In the northern side of the dichotomy, say Acidalia Planitia, you might have an altitude at the surface of 6,000 m below "sea level" (at which the pressure is 610 Pa). With a scale height of 11.1 km the pressure near the surface roving across Acidalia Planitia is thus something like 610*e^(6/11.1) ~ 1 KPa. During the day (when you need it up most to catch sunlight) this corresponds to a density of some 20 grams per m^3. A hundred metre wide sphere thus holds ~(100^3)/2 ~ 500,000 m^3, with a mass of some 10 tonnes atmosphere - if you had H2 instead it's about 22 times lighter at the same temp and pressure so even ramping up the pressure to keep it rigid H2 lifting gas still should mass less than a tonne. The skin has to cover 30,000 m^2 so if you want to lift any solar cells etc. it can't mass more than about 6 tonnes, giving 200 grams per metre. That's about enough to buy you 100 microns PET coated with 20 microns Aluminium film to hold in the H2. With an internal pressure of 1,500 Pa the internal stress trying to push the sphere apart is the cross sectional area exposed by cutting it in half times the pressure: some 7,500 m^2 * 1,500 N/m^2 ~ 11,000,000 N. This must be held by the skin exposed across the circumference of the cut, which is ~300 m. At 100 um thick (0.1 mm) the load-bearing PET surface across the circumference is ~0.3 m^2, meaning it’s loaded at some 33 MPa. That’s within PET’s 50-ish MPa tensile strength but I wouldn’t push it when winds and local stresses from cables etc are included so a little Keflar or carbon fibre (or basalt fibre, much cheaper and almost as good) are probably needed in practice but that’s well within what’s possible. A quick perusal of Wikipedia ( https://en.wikipedia.org/wiki/Power-to- … tovoltaics ) gives ~70 W/kg for solar cells in Earth orbit (where it’s much brighter than Mars) so even at 40 W/kg that 2 tonnes or so left over (with 1 tonne’s weight on Mars lifting force for margin of error) hefts a good 80 KW of photovoltaics.

So: a 100 metre wide balloon with 6 tonnes reinforced aluminised plastic skin, around 0.7 tonnes H2 and a capacity for lifting an additional 3 tonnes on top of this and still floating, of which it’s practical to get something like 100 KW photovoltaics up. What about wind?
I can’t find good data on the wind speed at moderate altitude on Mars but on Earth the no-slip condition makes it much slower on the surface than even a few tens of metres up (hence chimneys). Taking it as 10 m/s provincially, since this has been reported sometimes at the surface and you could always take it down if you had serious winds developing that day:

Reynolds # ~ (0.02*100*10)/(10*10^-6) ~ 2,000,000 -> the flow is pretty turbulent!

Turbulent flow around a sphere gives a drag coefficient of around 0.15, so the drag force for 10 m/s wind is something like:

F ~ (0.15*0.02*7,500*10^2)/2 ~ 1,000 N

This is around the magnitude of the net upward force I’d let the balloon have in order to keep the tow cable tight and it very definitely floating rather than falling so no problems - provided it’s reliable at this altitude you can even afford to carry more stuff with it by using it as a kite. The problem is if you’re driving against the wind.

At your top design speed of 40 km/h (about 11 m/s) and assuming relatively gentle 10 m/s wind again the analysis gives something closer to 4,000 N which is getting pretty dangerous (that’s around the weight of a tonnes on Mars). Since the force goes as the square of relative wind speed this can get progressively worse if wind speed is above 10 m/s in practice - at 20 m/s + 10 m/s driving speed it’s closing in on 10,000 N. Besides the obvious danger of high stresses on the tow cable and rover etc, aeroelastic flutter might put undue stresses on the balloon also.

Provided it’s relatively peaceful at our altitudes it’s surely possible to just bring it down for a while if the wind picks up too much. A winch firmly attached to the rover can reel the balloon down to accomplish this but decompression is much more difficult, requiring pumps and a thicker storage balloon etc. (turning H2 into a liquid is a very difficult proposition for a machine that must fit casually on the back of your rover!). I guess with accurate weather reporting giving you a few hours warning you could tie it down though, hammering in tent pegs and hooking on trailing ropes etc.

How much do you/others here know about the wind speeds at altitudes a few tens to hundreds of metres up from the surface on Mars? I can’t find much information, assumed it just isn’t possible to get with modern equipment presently on the planet. Under the right conditions a kite/dirigible hybrid could serve at least the observation post or communications purposes of the previous (I wouldn’t put delicate equipment like thin film solar cells on a kite of course) for much lower mass, cost and complexity.

Ah, sorry, I get it now! Having a blimp carry your solar cells at high altitudes (these could also hang down below the blimp if needed) could also give you longer hours in sunlight per day (since you're not so quickly blocked by the horizon). It might even mean getting above dust storms, which otherwise terrify me: barring proper nuclear reactors (which I’m told we might not be allowed to use!) are we supposed to go three weeks without electricity or heat?

Yes indeed, I’m going off the data in “The Case for Mars” by Dr Zubrin where it just quotes the performance for H2/CO2 but I can’t see any other exothermic reaction route that’s better than the following:

CO2 + 4H2 -> CH4 + 2H2O

The general idea being that you only have to bring H2 with you which is very light so that even though the energy released per mass of reactants isn’t that great the energy released per unit mass of H2 is pretty competitive. Storage of LH2 is also much easier since anything that boils off need not be vented during flight - it goes straight into the engine.