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RealKerbal3x

Would a vacuum airship be useful on Mars?

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Since in Mars' thin atmosphere hydrogen or helium just aren't gonna cut it, do you reckon airships using a vacuum for lift would be any good for Mars exploration? Considering that with electric propellers and solar power an airship would have practically infinite range, travelling at a much higher speed than any groundbound vehicle could, I think it would be excellent. I'd like to hear your opinions though :)

Note: I certainly didn't come up with this idea, I just read it somewhere. There's a NASA article on this actually, have a look.

Edited by RealKerbal3x

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Hydrogen doesn't differ a lot from the vacuum, but can oppose the external pressure.

The envelope mass decides, and it's probably much lower for hydrogen, than for vacuum inside.

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A vacuum dirigible as described in the accompanying article could well be useful, but my problem is, how would you get it there? It would surely have to be built in situ on the Martian surface and that would seem to entail technology beyond anything foreseeable in the near future. 

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Actually, inflatable RVs are very much a near future technology. This doesn't really differ. In fact, on Mars the added bonus is that evacuating the envelope of air becomes a non-issue. Just make sure it's sealed tight before entry, and design it so it remains sealed. Alternatively, put it in a conventional lander in a stowed configuration (again, sealed in space, after launch), and use an externally connected electric motor to "inflate" the envelope. It'd have to be heavy, but it can say on the ground. Mars has such a pathetic excuse for an atmosphere that this might actually be feasible.

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2 hours ago, Dragon01 said:

Actually, inflatable RVs are very much a near future technology. This doesn't really differ. In fact, on Mars the added bonus is that evacuating the envelope of air becomes a non-issue. Just make sure it's sealed tight before entry, and design it so it remains sealed. Alternatively, put it in a conventional lander in a stowed configuration (again, sealed in space, after launch), and use an externally connected electric motor to "inflate" the envelope. It'd have to be heavy, but it can say on the ground. Mars has such a pathetic excuse for an atmosphere that this might actually be feasible.

If said envelope was shaped to be lift body then even if you couldn’t get a lift state it would still make flying easier.

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Airships are not flying submarines. They are just balloon-shaped envelopes hardly keeping in air themselves. Even in the dense air of the Earth.

It looks attractive to use an inflatable ship instead of a metallic one.
Before they calculate. (Btw, I can't see that in the pdf)

So, while there "are" cargo airships, inflatable orbital modules, inflatable heatshields, inflatable Venusian skycities, inflatable Martian probes,
what we still have actually inflatable is several 1-t-capable guard airships, air baloon festivals, and two Venusian meteorological probes (Vega-1&2) with a sounding rocket payload.

Edited by kerbiloid

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A vacuum airship *might* be possible on Mars. The atmosphere there has a higher density to pressure ratio than here. This is important because density is what gives buoyancy but pressure is what crushes. This is because of the higher average molecular weight of the atmosphere there - CO2 is denser than N2. Also the gravity that needs to be overcome is less.

All other things being equal simply being in a thinner atmosphere doesn't work. As pressure reduces so does buoyancy.

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3 hours ago, kerbiloid said:

Airships are not flying submarines. They are just balloon-shaped envelopes hardly keeping in air themselves. Even in the dense air of the Earth.

This does describe *some* airships. Other airships, however, have a semi-rigid or potentially even fully rigid structure.

I agree that the real question would be whether a hydrogen-pressurized airship would be better than a fully rigid vacuum airship.

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12 minutes ago, mikegarrison said:

Other airships, however, have a semi-rigid or potentially even fully rigid structure.

Yes, others are balloons made of foil instead of rags, and reinforced with bars.
Still they mostly cause a false impression of something massive, solid, and tough, rather than what they really are.

Also, the megalophobia rules, when such enormous monster is flying above. This increases the false impression.
While actually this is a medium-sized airplane in sense of mass.

Airships are like this

Spoiler

frilled-lizard-638971_x700.jpg?v=1576590

 

Edited by kerbiloid

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Let's estimate.

***

Martian atmosphere at surface:
https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html

Surface pressure:  6.36 mb at mean radius (variable from 4.0 to 8.7 mb depending on season)  
                   [6.9 mb to 9 mb (Viking 1 Lander site)]
Surface density: ~0.020 kg/m3
Average temperature:  ~210 K (-63 C)
Diurnal temperature range: 184 K to 242 K (-89 to -31 C) (Viking 1 Lander site)

I.e. pressure ~600 Pa, density ~0.02 kg/m3, temperature 210 K.

***

Materials
https://en.wikipedia.org/wiki/Specific_strength

Carbon fiber (Toray)
Tensile strength = 4 300 MPa
Density = 1750 kg/m3.

Carbon nanotube (taking estimated average from the table and the note)
Tensile strength ~= 100 000 MPa (63..300)
Density ~= 1000 kg/m3. (0.037..0.55..1.34)

***

EnvelopeMass = MaterialDensity * Thickness * SurfaceArea = MaterialDensity * Thickness * pi * BalloonDiameter^2;

EnvelopeVolume = pi * BalloonDiameter^3 / 6;

EnvelopeAverageDensity = EnvelopeMass / EnvelopeVolume = MaterialDensity * Thickness * pi * BalloonDiameter^2 / (pi * BalloonDiameter^3 / 6)
= 6 * MaterialDensity * Thickness / BalloonDiameter;

EnvelopeAverageDensity = 6 * MaterialDensity * Thickness / BalloonDiameter;

MinimalThickness for a sphere ~= BalloonDiameter * Pressure / (4 * TensileStrength)

***

So, the average density of a balooon of any construction should be < 0.02 kg/m3 to float at least at the very surface.

To oppose the external pressure, the balloon should be either filled with hydrogen at 600 Pa,
or be a (let's presume for an estimation) "inverted sphere" of vacuum with a carbon envelope of 600 Pa strength.

***

A hydrogen balloon.

Pressure ~= 8.31441 * Density * Temperature / MolarMass

Density ~= MolarMass * Pressure / (8.31441 * Temperature) ~= 0.002 * 600 / (8.31441 * 210) ~= 0.0007 kg/m3

Say, the balloon is 100 m in diameter.

EnvelopeVolume = pi * BalloonDiameter^3 / 6 ~= pi * 100^3 / 6 ~= 524 000 m3;

Max total mass = 524 000 * 0.02 ~= 10 500 kg

HydrogenMass = HydrogenDensity * EnvelopeVolume ~= 0.0007 * 524 000 ~= 367 kg;

So, total mass of the envelope and the payload is ~10 t.

The envelope doesn't withstand any pressure, it just keeps the hydrogen inside and the attached rope with cargo.

So, the envelope thickness is limited by the hydrogen leakage, while the rope diameter is limited by the cargo weight.


***

A vacuum balloon.

MinimalThickness ~= BalloonDiameter * Pressure / (4 * TensileStrength)

EnvelopeAverageDensity = 6 * MaterialDensity * Thickness / BalloonDiameter
= 6 * MaterialDensity * BalloonDiameter * Pressure / (4 * TensileStrength * BalloonDiameter)
= 3/2 * MaterialDensity * Pressure / TensileStrength = 900 * MaterialDensity / TensileStrength;

So, EnvelopeAverageDensity = 900 * MaterialDensity / TensileStrength;

Carbon fiber (Toray)
EnvelopeAverageDensity = 900 * 1750 / 4.3*10^9 ~= 0.0004 kg/m3;

Carbon nanotube (taking estimated average from the table and the note)
EnvelopeAverageDensity = 900 * 1000 / 10^11 ~= 10^-5 kg/m3;

In any case the envelope mass is negligible.

But the vacuuum envelope needs a rigid structure to save it from deflation.

The structure mass is defined by the payload weight.

So, it's similar to the rope in hydrogen version.

The envelope still has to prevent the gas from passing through, just from the outside.
So, it still has to be enough thick.

***

So, as we can see, both balloons have nearly same payload capacity,
but in case of hydrogen you have to carry a hydrogen tank to keep the balloon filled with hydrogen while it's venting out;
while in case of vacuum you have to carry a pump to pump out the carbon dioxide which has passed into the vacuum balloon.

So, no variant is more capable, but the hydrogen one is obvioulsy more simple and familiar.

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It's more familiar, but hardly simpler. Not only does it have a hard limit on its endurance (hydrogen will run out, unless you have an ISRU), you need to carry hydrogen, and it will boil off and escape over you journey to Mars, meaning you might need to carry a lot of it, or use a ZBO tank. Any quantity of hydrogen is troublesome on a Mars probe. If you can make a light enough pump, then the vacuum one is actually the simpler choice, not to mention lighter at launch.

Also, it might be easier to get people onto a vacuum Zeppelin than a hydrogen one (nevermind that Martian atmosphere won't actually support combustion...). :) 

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If carry vacuum, it will escape as well, just in opposite direction.

And a gas balloon doesn't need a support structure to keep its shape.

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8 hours ago, kerbiloid said:

If carry vacuum, it will escape as well, just in opposite direction.

And a gas balloon doesn't need a support structure to keep its shape.

This, now how do you unpack an vacuum airship? you can easy inflate an blimp in the air 
As I see it vacuum airships takes the problems with rigid airships up to 11, the main airship killer is turbulence or shear winds, and you can not make an very flexible vacuum airship. 

 

 

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1 hour ago, magnemoe said:

This, now how do you unpack an vacuum airship? you can easy inflate an blimp in the air 
As I see it vacuum airships takes the problems with rigid airships up to 11, the main airship killer is turbulence or shear winds, and you can not make an very flexible vacuum airship. 

With mechanical actuators. You might need a powerful motor, but it only has to work once, after that the structure can be locked into place mechanically.

Neither turbulence nor shear winds are a problem on Mars. Air there isn't thick enough to present a problem to a structure capable of holding vacuum.

11 hours ago, kerbiloid said:

If carry vacuum, it will escape as well, just in opposite direction.

Yes, but you can restore it without carrying replacements. Vacuum can't "run out" as long as the pump is working. Hydrogen, on the other hand, is guaranteed to, unless you have ISRU.

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4 hours ago, Dragon01 said:

With mechanical actuators.

Mass. Complexity.

4 hours ago, Dragon01 said:

Yes, but you can restore it without carrying replacements.

A pump. Mass. Complexity.

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Yes, because a hydrogen tank weighs nothing and the system to gassify and get into to a balloon is simple. Here's a hint: they're not. Carrying any quantity of hydrogen, in any form, to Mars is a non-trivial problem. You simply move mass and complexity elsewhere in the design.

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The surface area of a sphere .

S = pi * D2.

D = 100 m
S ~= 31400 m2.
P = 600 Pa.
Total compression force = 31400 * 600 = 18.8 MN ~= 2000 tf
Payload ~10 t.

D = 10 m
S ~= 314 m2.
P = 600 Pa.
Total compression force = 314 * 600 = 188 kN ~= 20 tf
Payload ~0.01 t.

Good luck with the support structure and the vacuum pump!

In case of hydrogen the hydrogen itself is what withstands the external pressure.

Edited by kerbiloid

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