Overlooked airship gasses

[farmerben]

Everybody knows hydrogen is explosive, helium is expensive and rare, hot air is ponderous.  But there are other lifting gasses.  Steam was used exactly once in 1792 to launch a few animals over Paris, but never again.  Methane to the best of my knowledge has never been used as a lifting gas for humans or animals.  

It's been a while since I studied this so double check it.

Methane and air cannot ignite if the methane is below 5% or the oxygen is below 12%.  Hydrogen is similar, and the mix of CH4 and H2 is similar.  Somewhat richer combinations could be safe in some circumstances.  

https://en.wikipedia.org/wiki/Flammability_diagram#/media/File:Flammability_diagram_methane.svg

Steam is essentially an inert gas toward methane and hydrogen, below about 3 atm pressure and 1000 C and certain catylists.  The ignition point for methane and air under ideal ratios is about 540 degrees C.  The temperature to maintain steam falls with altitude  (from 100 toward about 70 degrees C).  Therefore it is possible to maintain methane above boiling temperature without any risk of accidental ignition.  

Now a fuel cell can process hydrocarbons on one side and release hydrogen on one side of a membrane.  On the other side of a membrane oxygen from the air is converted to steam, and much heat is released.  

 

A balloon inside a balloon is called a balloonet.  Now suppose you had a ballonet that was about 40% methane and hydrogen mixed with 60% steam as an inert gas filler.  Surround that with a larger balloon of nearly 100% steam.  Surround that with an open balloon of air only the appendix of the balloon is part of the fuel cell so the air in the appendix will be close to 10% oxygen, 10% steam.  I would predict it would be virtually impossible to explode the gasses.

Methane and Steam despite being 6 times denser than helium, have half the bouyancy, because what counts is the mass of the air displaced.  

 

Assuming it is possible to keep the steam up.  The limiting factor would be slowly converting all the methane to hydrogen and the rate at which hydrogen escapes.

Edited April 2, 2019 by farmerben

[Shpaget]

7 hours ago, farmerben said:

Methane and Steam despite being 6 times denser than helium, have half the bouyancy, because what counts is the mass of the air displaced.  

Unfortunately, the math says it's not going to work.

This guy explains why even helium is a poor lifting gas.

 

 

 

[Scotius]

Steam? Seriously - it's considered a "lifting gas"? But it's just hot air, with added hassle of having to vaporise a lot of water. Guess someone looked at the cloud and had a lightbulb moment

[kerbiloid]

But the steam molar mass is 18, while air is 29.

As pressure ~ density * temperature / molar_mass, so density ~ pressure * molar_mass / temperature, and at the same pressure and temperature the steam density is 1.6 times lower.

[Shpaget]

10 minutes ago, Scotius said:

But it's just hot air, with added hassle of having to vaporise a lot of water.

Uh, no. Steam is not hot air. It is just water, and the point of it being use as a lifting gas is exactly the opposite of "having to vaporize a lot of water". You actually need very little of water to fill up a big balloon. There is no air (nitrogen and oxygen gas) in steam.

10 minutes ago, Scotius said:

Guess someone looked at the cloud and had a lightbulb moment

Clouds are also not steam. They are liquid water droplets in suspended air.

[farmerben]

What the video showed was that the old airships of the WWI era had a huge dry mass.  Something like 600 g of bulk for every 1m^3 of lifting gas.  Actually the cow intestine that held the pure hydrogen was much much lighter than that.  Those intestine balloonets were contained inside essentially a (hot?) air balloon made of canvas with metallic paint over an aluminum frame.

It is better to compare it to modern helium blimps.  However helium blimps may use thicker materials than what is needed for methane and steam.  Helium is never wasted, so these blimps need thick walls, and they compress their own gas supply instead of venting it.  Steam may be vented.  

I don't have permeability data handy.  Isn't it the case that hydrogen and helium leak through almost everything, quantum tunneling is significant. While for steam and methane the leak rate is hundreds or thousands of times less.   

[farmerben]

Suppose you want 150,000 kg of total lift.  A spherical balloon of pure steam 50m in radius has a little more than that.    Call it 8000 m^2 of surface area.  

Try 6 mil polyethylene.  The balloon weighs about 1,600 kg.  We are up to about 1% of the old zeppelin dry mass.  

 

Now consider that helium retails for about $1.50/ft3 while methane retails for about $0.015/ft3.  Presumably steam can be even cheaper than methane.  

So I think we realistically approach 1% of the total cost of previous airships. 

 

 

Edited April 3, 2019 by farmerben

[Ultimate Steve]

Wait, never mind, I think I get it now, you need less thrust to rotate the further from the COM you are, so if you had one translation thruster close to the COM firing at 100% power, one further away would only need to fire at 70% power to counteract the rotation torque, leaving a 30% translation thrust.

EDIT: This is the wrong thread, ignore.

Edited April 4, 2019 by Ultimate Steve

[mikegarrison]

Steam is never just steam. It is always in equilibrium with water. Very hot steam is more "dry" than cool steam, but it always has some liquid water percentage. You have a giant bag, which has tremendous surface area to radiate away heat. The higher you go, the colder it gets outside. How do you keep your steam dry near the bag? You would need a very high bulk temperature and a *lot* of convection to make sure that the steam at the edges doesn't get too "wet".

[LordFerret]

10 hours ago, Shpaget said:

This guy explains why even helium is a poor lifting gas.

It might be a poor lifting gas, but there was a time when it was used extensively.  Living near the Lakehurst airbase, where the Hindenburg crashed, there's a lot of history passed around about dirigibles.  One such interesting story is about the USS Akron, a helium airship.

[Bill Phil]

12 minutes ago, LordFerret said:

It might be a poor lifting gas, but there was a time when it was used extensively.  Living near the Lakehurst airbase, where the Hindenburg crashed, there's a lot of history passed around about dirigibles.  One such interesting story is about the USS Akron, a helium airship.

Both the Macon and the Akron crashed...

So did Shenandoah. Luckily Los Angeles didn’t...

If I recall the blimps used to monitor the coast used helium.

[mikegarrison]

4 hours ago, Bill Phil said:

Both the Macon and the Akron crashed...

So did Shenandoah. Luckily Los Angeles didn’t...

If I recall the blimps used to monitor the coast used helium.

US airships used helium because the US was essentially the only producer of helium. It was considered a strategic material.

[farmerben]

6 hours ago, mikegarrison said:

Steam is never just steam. It is always in equilibrium with water. Very hot steam is more "dry" than cool steam, but it always has some liquid water percentage. You have a giant bag, which has tremendous surface area to radiate away heat. The higher you go, the colder it gets outside. How do you keep your steam dry near the bag? You would need a very high bulk temperature and a *lot* of convection to make sure that the steam at the edges doesn't get too "wet".

Great questions!  This is the very heart of the problem.  We need to know the rate at which heat radiates away.  I do not know it.  

As you go higher the condensation temperature drops and solar radiation increases.  So the colder atmosphere is not a big problem.  If the balloons utilize the reflective properties of solar hot water heaters, they will get better with altitude.  My experience with 6 mil plastic greenhouses says they can maintain a 15-20 F temperature difference overall  While the vaporization temperature is going to drop almost 50 F going up.  Convection will fall going up. Solar radiation increases.  Ascending requires lowering pressure, thus venting steam or water while going up.  

The outer edge of the steam bag would benefit enormously from insulation.  My idea is to surround the steam with hot air.  The exhaust from a fuel cell is a steamy oxygen depleted air with no heavy CO2, so it would be perfect.  Condensation of the outermost balloon only would drip back down to the fuel cell, and if it does not revaporize, then it will fall out.  The inner balloons containing steam would be kept super hot and dry if possible.

Using methane to insulate steam would work better much better from the heat perspective, the reverse of an ignition mitigation strategy.  

I'm estimating that fuel cells are about 10 kW per ton, plus about 2 kW of "waste heat".  

So a 100m diameter sphere of steam would get over 1000 kW of solar power (potentially twice as much with reflectors) , and say 100 kW of total fuel cell heat (that is a very modest gross weight fraction).  The surface of the steam bag is 32,000 m2.  So at equilibrium the steam bag will dissipate 35 W/m2.  From this we can calculate the insulation value for the outer balloon we need.  The outer balloon will have a temperature gradient less than 100 degrees, so it's insulation value could need R as high as 3.  I'm not sure exactly how much air that is, but the air gaps in 3.5 inch studs count as R=1.  So I think that a 3 layer onion of air 1 foot thick has enough insulation such that the inner balloonet would have almost zero condensation.  

 

Edited April 3, 2019 by farmerben