Floaters – Onward to Mars etc

Floaters – Onward to Mars etc

In an earlier exchange I convinced myself that a dirigible on Mars was simply not practical. The issue is the low atmospheric density. It is much easier to design something to float in a dense medium. You and I can swim quite easily in water, but have difficulty flying, well, I do anyway.

The reason I am in favour of gas balloons is that once they are in flight they do not need energy to maintain them there. That makes them economical. This does not rule out hot-air (or other gas) balloons or gliders but they are issues for another day.

I don’t give up easily, and I thought I would investigate the possibility of using balloons on other bodies.

Bodies I investigated briefly and rejected:

Body:            Atmospheric Density:    Composition:

Mercury        “tenuous”                        -

Mars            0.02 kg/m3                        95% CO2

Ganymede    “micro pascals”                -

Europa           “tenuous”                        -

Jupiter            90% Hydrogen.             “Hydrogen and Helium in roughly solar proportions”

All the data is sourced from Wikipedia or other basic websites because I’m lazy.

Balloons are favoured by dense atmosphere composed of gases with a high molecular weight. Low gravity helps too.

Balloons are not practical on Mercury, Mars, Europa or Ganymede because of the low atmospheric density.

Jupiter is an interesting case. The atmosphere is so deep that there will be a wide range of density. The composition being mostly hydrogen means that something relying on pure “gas buoyancy” will not work (because you can’t get less dense that Hydrogen). On the other hand, the pressures are so high that the penalty for solids is less. Jupiter needs its own special, radical approach. Jupiter is not really rejected, but it needs a completely different approach.

Venus

On the other hand, gas balloons may have real potential on Venus!

Surface Temperature:    464 deg C

Pressure:                        92 Bar

Gravity:                          8.87 m/s2, 0.904 g

Atmospheric Density:    65 kg/m3

Composition:                  96.5%

If we assume ideal gas laws apply and the Venusian atmosphere is entirely CO2, then the density of a gas is simply going to in ratio to the molecular weight of the gas relative to CO2 (the other conditions, temperature and pressure remain constant).

This gives us the following:

Density of Atmosphere:                                65 kg/m3

Lift from 1m3 of vacuum:                            65 kg

Density of H2 at same conditions (2/44)      2.95 kg/m3

Lift from 1m3 of H2 at same conditions.      62 kg

Density of He at same conditions (4/44):      5.91 kg/m3

Lift from 1m3 of He at same conditions:       59.1 kg

There you are! If my sums are mostly right, that means that the high pressure and CO2 atmosphere means that on Venus 1 m3 of enclosed Hydrogen can lift 62 kg of load.

The Venusian atmosphere contains Sulphuric Acid. I would propose manufacturing hydrogen “in situ”. Possible routes are: bringing a reactive metal like lithium with you and reacting it with sulphuric acid, or breaking down the acid with energy. Suddenly making this practical becomes a materials-science problem.

Next Steps:

I prefer “back of a beer-mat” to “back of a fag packet” but that exercise convinces me that balloons are worth investigating for Venus. Next stage would be “back of an envelope”.

Do you know anything about aerodynamics and lift? (because I don’t). Are you aware of any equations I could use in a similar way to screen glider devices for different bodies?

I think gliders will be impractical on Mercury/Europa/Ganymede because of the low atmospheric density. I think they will probably either have to move far too fast or be vast. I’d like to do the sums for Mars (just to “show it won’t work”).

On the other hand, hot-“air” balloons and gliders (or “fish”) might be really interesting for Jupiter, but I’m still working out where to start. Any suggestions?

(Original 5th June 2021)