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# Building an LTA structure with controllable lift

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Hey folks. I am wondering if I can borrow some of that wonderful brain stuff that is in abundance here :phones:

I am going to be building a Lighter Then Air structure for a project I am working on. I need to be able to control the lift generated by the gas, without dumping the gas.

The concept I have come up with is something along the lines of the following.

The idea is that the inner bladder can contain a cheap lift gas like Hydrogen. the while the outer envelope is filled with a cheap inert gas like Nitrogen (is there a more appropriate gas I could use here?).

In full lift, the outer envelope is completely unpressurised, allowing the inner bladder to expand to it's full size.

To decrease lift, increase pressure of the inert gas to collapse inner bladder.

No loss of lift gas.

Outer envelope cheaper to manufacture as it does not have to contain small molecule gases. (Laminated rip stop nylon should suffice?)

Inner bladder is Aluminized Polyester ("Mylar" rebranded name). It is cheap, and commonly available.

Inner bladder needs minimal structural strength as it is not pressurized. Bladder is larger then the envelope so even if outside pressure drops, the ripstop provides the mechanical strength, not the Mylar.

Modular. Add as many as you require.

The inert gas in the envelope should help offset concerns about the flammability of the hydrogen.

Lastly, I was hoping that an envelope containing the inert gas under pressure would help to contain or reduce losses of the lift gas. Modern containment systems for Hydrogen generally have a 5% loss per year, and I wanted to reduce that to 1% and the most.

Thoughts?

Would a system like this work?

How much pressure would be needed to cut the lift in half?

Is there a better system out there?

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Hey folks. I am wondering if I can borrow some of that wonderful brain stuff that is in abundance here :phones:

I am going to be building a Lighter Then Air structure for a project I am working on. I need to be able to control the lift generated by the gas, without dumping the gas.

The concept I have come up with is something along the lines of the following.

The idea is that the inner bladder can contain a cheap lift gas like Hydrogen. the while the outer envelope is filled with a cheap inert gas like Nitrogen (is there a more appropriate gas I could use here?).

In full lift, the outer envelope is completely unpressurised, allowing the inner bladder to expand to it's full size.

To decrease lift, increase pressure of the inert gas to collapse inner bladder.

No loss of lift gas.

Outer envelope cheaper to manufacture as it does not have to contain small molecule gases. (Laminated rip stop nylon should suffice?)

Inner bladder is Aluminized Polyester ("Mylar" rebranded name). It is cheap, and commonly available.

Inner bladder needs minimal structural strength as it is not pressurized. Bladder is larger then the envelope so even if outside pressure drops, the ripstop provides the mechanical strength, not the Mylar.

Modular. Add as many as you require.

The inert gas in the envelope should help offset concerns about the flammability of the hydrogen.

Lastly, I was hoping that an envelope containing the inert gas under pressure would help to contain or reduce losses of the lift gas. Modern containment systems for Hydrogen generally have a 5% loss per year, and I wanted to reduce that to 1% and the most.

Thoughts?

Would a system like this work?

How much pressure would be needed to cut the lift in half?

Is there a better system out there?

What you are proposing is a lot like the way submarines control their buoyancy. It should work but remember hydrogen leaks out of containment a lot faster when it's contained in a gas bag than it dose under ideal containment. If you could get around that problem you could have a much improved lighter than air craft.

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I was hoping the moderately pressurized outer envelope would reduce lift gas losses as it would be at an equal pressure to the inner bladder.

This assumes of course that the rate the gas escapes the bladder is determined by the difference between internal and external pressures. As soon as you add the inert gas to the external envelope then the 2 vessels should be equal, instead of the bladder being at higher pressure.

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Sorry about the giant picture folks.

Anything I can do about it now?

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Sorry about the giant picture folks.

Anything I can do about it now?

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Ahh, Thanks

Unfortunately I can not correct Moontanman's :phones:

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Hey folks. I am wondering if I can borrow some of that wonderful brain stuff that is in abundance here :)

I am going to be building a Lighter Then Air structure for a project I am working on. I need to be able to control the lift generated by the gas, without dumping the gas.

...

Thoughts?

Would a system like this work?

How much pressure would be needed to cut the lift in half?

Is there a better system out there?

Is this staying connected to the ground? I recommend Helium over Hydrogen because of the fire risk. :ebomb:

I have often wondered why airships dump lifting gas rather than repressurizing it into tanks with a compressor? :shrug: If you plan to have this connected to the ground, then I see no major problems with this scheme. If you want to go up, fill the balloon from the tank; to go down, suck the gas out of the balloon and push it back into the tank with a compressor. :phones: :shrug:

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Ahh, Thanks

Unfortunately I can not correct Moontanman's :shrug:

If you fix your, I'll fix his. :phones:

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Turtle, this is staying connected to the ground.

Helium is very expensive, and will be getting more so being a rather finite resource.

Hydrogen is cheap, and can even be produced on site.

The greatest danger comes from mishandling the stuff.

From further research it appears nothing can stop the damned stuff from escaping.. not even glass.

Still, does the concept of compressing the gas to control lift seem sound?

Is there a better way?

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Interesting concept!

I would advise against hydrogen, because containment is indeed an issue. If you could succesfully create such an LTA, and you use helium instead, it will be more expensive - but only initially. You won't be pumping the gas out and have to refill it continuously, after all. Having to keep on stocking with Hydrogen to replace the lost gas will, in the end, be more expensive. Not to mention more dangerous, what with the fire hazard et al.

Alternatively, you can just coil a thin, strong cable around the whole thing and winch the cable on a strong step-down gearbox. The cable coils will then decrease the volume of the bladder, python-fashion. If you free the winch, the expanding gas bladder will then pull the cable back. This will probably be easier and cheaper than messing around with piezoelectric metal foils.

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Would a system like this work?

Is there a better system out there?

I’m pretty sure it would, and that there are.

Other than the gasses used and the orientation of the envelopes, it’s pretty similar to the design used in blimps for the half century or so (eg, see the diagrams in the wikipedia article “Skyship 600”)

Taking a lesson from blimp design, I’d guess that the design could be improved by having only one or two smaller ballonets, rather than the entire outer envelope a single large one. The simplest design I can think of would be something like this:

(Red = hydrogen, blue = nitrogen)

With a heavier, inflexible, less gas permeable outer envelope, and a thinner, elastic inner one into which a pure, non-reactive gas like nitrogen is pumped or released, it should be possible to control the buoyancy of the whole rig with fairly low-power pumps.

Pure hydrogen is a great lifting gas - with a 1 atm density of 0.090 g/L, it’s $\frac{1.2 - 0.090}{1.2 - 0.18} =$ about 9% more buoyant than helium. The problem with hydrogen, however, is that reacting as readily as it does with other common atmospheric gases, it’s rarely pure. “Insulating” it from the atmosphere by filling its ballonet, and possibly an outer layer, with nitrogen rather than air, might be a very good idea.

AFAIK, since lighter-than-air aircraft were pushed to the margins of aviation, interest and research in lifting gases other than helium has been very low, despite the potential cost saving. Hydrogen is potentially very inexpensive – if loss rates can be kept low, a combination of a water collector, electrolysis machine, and solar or other “free” power source could make it effectively free. If the LTA machine is some sort of power generator, such a built-in hydrogen replacement scheme seems very feasible.

How much pressure would be needed to cut the lift in half?
Lift equals the difference in density of the lifting gas vs. air times volume. The density of a gas is, ideally, and pretty close to actually, proportional to pressure and temperature, so to cut lift in half, you need to double the pressure. Since most large lifting envelopes are only slightly (less than 10%) above 1 atm, to cut the lift of one in half would require pressurizing it to about 2 atm.

Practically speaking, the main need to vary the density of the gas in a lifting envelope is to allow it to fly at a wider range of altitudes. Because the density of air varies strongly with altitude, LTA machines that don’t have envelopes that allow them to expand a lot tend to have low maximum altitudes – 1000 to 2500 m above sea level. Hence, during the golden age of airships, you didn’t see them operating over very mountainous terrain.

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Turtle, this is staying connected to the ground.

Then forget varying the gas in the bag. Put in as much gas as you need to lift your load and then simply winch it up and down.

Helium is very expensive, and will be getting more so being a rather finite resource.

Hydrogen is cheap, and can even be produced on site.

The greatest danger comes from mishandling the stuff.

Ohhh the humanity! :)

From further research it appears nothing can stop the damned stuff from escaping.. not even glass.
It's corrosive too.

Still, does the concept of compressing the gas to control lift seem sound?

Is there a better way?

Yes, but only if the machine is free-flying. Keep in mind also that the outer envelope must withstand the pressure you propose in a multiple envelope system and adding that strength adds weight. For a free-flying craft, I'm sticking with my idea of having 1 envelope and pressurizing/depressurizing it with a compressor/tank setup.

As I say, if your machine is moored then just winch it up & down and forget messing with the gas pressure.

By-the-by, it is against FAA regulations to fly a tethered balloon above 500 feet without getting official clearance. Here's some info on that : >> FAA regs :)

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Boerseun

With Hydrogen AND Helium there is a loss of approx 5-10% per year on a perfectly sealed envelope.

This structure will need to remain airborne for 5-20 years if possible. Every time it needs to be brought down will be costly in lost revenue.

The constriction idea is interesting. I will ponder on that.

CraigD:

Informative as ever, Thank you.

If I am not mistaken, current airship design uses power to ascend and descend, and they rely on the ballonet’s only to equalize pressure and some measure of pitch control. Once they achieve neutral buoyancy then they tend to stay that way.

The reason for the orientation of the ballonets is to allow material on the interior to be optimized to reduce gas loss, and the outside envelope can be optimized to deal with the pressure without having to be concerned about the gas loss (Nitrogen molecules are huge I guess).

Apparently (I forget where I read this now) the lift gas of choice will migrate through both envelopes. It will not even build up in the nitrogen envelope much beyond the percentage of what is in the air outside unless there is a leak in the inner bladder.

If the LTA machine is some sort of power generator, such a built-in hydrogen replacement scheme seems very feasible

Nail… head. Once the system is running the hydrogen is almost free.

Lift equals the difference in density of the lifting gas vs. air times volume. The density of a gas is, ideally, and pretty close to actually, proportional to pressure and temperature, so to cut lift in half, you need to double the pressure. Since most large lifting envelopes are only slightly (less than 10%) above 1 atm, to cut the lift of one in half would require pressurizing it to about 2 atm.

Thank you. It seems obvious in retrospect, but then again what doesn’t :hyper:

Turtle:

For various reasons I can not go into, this system is not able to be winched :doh:

Apparently Canada just copies the FAA regulations for the most part. I have looked into it and the relevant things are that it not be in controlled airspace or direct flight path, the system have an emergency deflation system, and have lights every 50’ (I thought that was funny seeing feet measurement in a Canadian document) that blink at least 1 time per second.

I will be dealing with between 1,400 and 5,200 cubic meters of lift gas, so replacement of lost gas can equal a significant cost if I use Helium.

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Turtle:

For various reasons I can not go into, this system is not able to be winched :(

Apparently Canada just copies the FAA regulations for the most part. I have looked into it and the relevant things are that it not be in controlled airspace or direct flight path, the system have an emergency deflation system, and have lights every 50’ (I thought that was funny seeing feet measurement in a Canadian document) that blink at least 1 time per second.

I will be dealing with between 1,400 and 5,200 cubic meters of lift gas, so replacement of lost gas can equal a significant cost if I use Helium.

Mmmmm...Not able to winch? Can't tell why? That seems rather unscientific. :doh:

How much will 1,400 to 5,200 cubic meters of Hydrogen cost initially? Where will you get it? What are the regulations for handling/having that volume of flammable gas? Is it legal to put that much H in a balloon even? Or were you planning on not telling the authorities? What happens when lightning strikes your balloon? What material is your tether?

Anyway, why not just lay it all out and we can have an all out evaluation. :hyper:

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Mmmmm...Not able to winch? Can't tell why? That seems rather unscientific. :doh:

Not unscientific, just necessarily cautious :(

The entire system is semi rigid and that is all I can say on that for now.

How much will 1,400 to 5,200 cubic meters of Hydrogen cost initially? Where will you get it? What are the regulations for handling/having that volume of flammable gas? Is it legal to put that much H in a balloon even? Or were you planning on not telling the authorities? What happens when lightning strikes your balloon? What material is your tether?

All very good questions.

• Total cost should be $120 to$900 US until I can generate my own.
• I will get it next door. The welding supply guys tells me he can order it in any quantity I need.
• Regulations are the same as for Propane/Methane/Gasoline.
• All laws in each country must be abided by as this will be a commercial venture.
• Lightning is ALWAYS an issue. The structure avoids use of conductive materials where possible, and the use of inert gas envelope and a moderately flame retardant exterior envelope. More study is required though. Does anyone have any insights in this area?
• I will probably use a Polybenzoxazole fiber (Zylon) or polyalirate fiber (Vectran), but this will be driven by design constraints and cost. Any other suggestions would be helpful :)

Anyway, why not just lay it all out and we can have an all out evaluation. :hyper:

Because several parts of this concept are patentable, and once I confirm that the entire system is viable I will be applying for one. Until then I will unfortunately have to remain rather cryptic:(

Any suggestions for dealing with freezing rain? That one is pretty much a killer as far as I can see, and is driving the need for a controlled lift system so that the structure can be "Laid down" if need be.

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Not unscientific, just necessarily cautious :)

The entire system is semi rigid and that is all I can say on that for now.

Roger; mums the word. :zip:

All very good questions.

• Total cost should be $120 to$900 US until I can generate my own.
• I will get it next door. The welding supply guys tells me he can order it in any quantity I need.
• Regulations are the same as for Propane/Methane/Gasoline.
• All laws in each country must be abided by as this will be a commercial venture.

• Lightning is ALWAYS an issue. The structure avoids use of conductive materials where possible, and the use of inert gas envelope and a moderately flame retardant exterior envelope. More study is required though. Does anyone have any insights in this area?
• I will probably use a Polybenzoxazole fiber (Zylon) or polyalirate fiber (Vectran), but this will be driven by design constraints and cost. Any other suggestions would be helpful :)

Flame retardancy might help keep a fire from starting; it is now thought the covering of Hindenberg is the material that caught fire first. However, once the Hydrogen is burning, there is no putting anything out.

Non conductance isn't necessarily a protection from lightning; anything that's wet will do nicely as a conductor thank you very much. :hyper: :eek: Moreover, in dry conditions a non-conductive envelope may build up a considerable static charge. :doh: :)

Because several parts of this concept are patentable, and once I confirm that the entire system is viable I will be applying for one. Until then I will unfortunately have to remain rather cryptic:(

Any suggestions for dealing with freezing rain? That one is pretty much a killer as far as I can see, and is driving the need for a controlled lift system so that the structure can be "Laid down" if need be.

Roger. Until then I must contiue to probe you. :Alien: :eek: :D

Freezing rain? How about a heater? :ebomb: :doh: What altitude do you see flying it at? Your bag may not be big enough to lift the mooring line. How much lift do you get from that bag in your drawing if filled with H?

That's all I got. Keep that imagination flowing!

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Flame retardants should help as this is a segmented design. Keeping one part from igniting another is the main concern and only has to last a few seconds to a minute before fuel from one segment is exhausted or dispersed.

That pesky lightning will be an issue, I know. Any ideas on mitigating this?

Static should not be to much of a problem so long as the hydrogen is contained in an inert gas envelope and no ground is available, but I had not considered the problem should I need to bring the structure to ground. Hmm

The core will be heated, but this will not be available to the exterior. I was hoping for a solution involving some magical water and ice shedding ultra light super strong material :hyper:

Total lift is approx 1 Kg per cubic meter of lift gas. I anticipate I will have 20% (minimum) excess lift on this structure.

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