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Hydrogen vs. Helium

Posted by: John C. - Tue Feb 23, 2010 8:06 pm
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Hydrogen vs. Helium 
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Post Hydrogen vs. Helium   Posted on: Tue Feb 23, 2010 8:06 pm
Has anyone considered lobbying the government to get a variance to use hydrogen in the Station and Orbital Ascender instead of helium? The ban on hydrogen in man-carrying aerostats is due to the Hindenburg fire, which is now considered to have been due to the cloth skin catching fire rather than the lifting gas, and at 140,000 feet and up it's not like it's going to burn anyway. And hydrogen is a LOT less expensive than helium, as well as providing significantly more lift.


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Post Re: Hydrogen vs. Helium   Posted on: Thu Feb 25, 2010 7:40 pm
John C.,

John Powell discussed use of hydrogren at length in Floating to Space. JP Aerospace can probably get an airworthiness certificate for an experimental airship regardless of the lifting gas. However, you are right about the FAA not granting type certificates for production airships using hydrogen lift gas. An experimental aircraft certificate isn't allowed for the kind of passenger service discussed in JP's book, so it stands to reason that JP Aerospace will need to address the issue eventually if they opt for hydrogen.

Hydrogen will burn in air with oxygen partial pressure less than 1 mbar. That means it's still a combustion risk at 120000 feet. The FAA might see reason about that last 20000 feet, or they might not. There's no way to know without asking.

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Post Re: Hydrogen vs. Helium   Posted on: Thu Feb 25, 2010 9:36 pm
John C. wrote:
Has anyone considered lobbying the government to get a variance to use hydrogen in the Station and Orbital Ascender instead of helium? The ban on hydrogen in man-carrying aerostats is due to the Hindenburg fire, which is now considered to have been due to the cloth skin catching fire rather than the lifting gas, and at 140,000 feet and up it's not like it's going to burn anyway. And hydrogen is a LOT less expensive than helium, as well as providing significantly more lift.



You only get about 8% lift advantage with hydrogen over helium and there are a lot of arguments about the cause of the Hindenburg accident but i do think its time to reevaluate Hydrogen for airship type vehicles especially considering that when a modern plane falls burning out of the skies in most causes you get 99%+ fatalities whereas in all the cases of airship accidents that i know of the majority of the people they carried ran walked or at least limped away from the accedents.

I think that with modern materials it should be possible to build hydrogen airships that are safer than the planes we have now after all if the thermite theory of the Hindenburg is to be believed almost the entire vehicle was made out of what it would only take a few hundred grams of to take out a modern plane and in the case of the Hindenburg i think two thirds survived.

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Post Re: Hydrogen vs. Helium   Posted on: Thu Feb 25, 2010 11:12 pm
SANEAlex wrote:
You only get about 8% lift advantage with hydrogen over helium


Wikipedia lists the density of hydrogen as roughly half the density of helium. 0.08988 g/L vs 0.1786 g/L at 0 °C, 101.325 kPa
Could you explain why it only produces 8% more lift?

thanks,

johno


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Post Re: Hydrogen vs. Helium   Posted on: Thu Feb 25, 2010 11:46 pm
johno wrote:
SANEAlex wrote:
You only get about 8% lift advantage with hydrogen over helium


Wikipedia lists the density of hydrogen as roughly half the density of helium. 0.08988 g/L vs 0.1786 g/L at 0 °C, 101.325 kPa
Could you explain why it only produces 8% more lift?

thanks,

johno


The lift is got from the displacement of the air which is mostly Nitrogen. So i will do the calcs using Nitrogen so i can use moles . one of the standard rules is 1 mole of one gas at the same temperature and pressure will have the same volume as 1 mole of another gas. So a volume of N2 atomic weight 28 will be displaced by H2 Atomic weight 2 or He Atomic weight 4 so you either get 28-2=26 or 28-4=24 so the majority of the lift is the displacement of the N2 and the 2 difference between H and He only gives about 8%

I can do that math but the gravitational drag one is still beyond me ;-)

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Post Re: Hydrogen vs. Helium   Posted on: Sat Feb 27, 2010 11:28 pm
From what I gather, it won't matter whether the US government allow it or not - it's only going to be above US land for a fraction of the time. If it circles the North Pole, it needs overfly permits from Russia, Canada, Denmark, and America; if above the South Pole, potentially no-one.


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Post Re: Hydrogen vs. Helium   Posted on: Tue Mar 02, 2010 3:04 am
It does matter what the US government will allow - just as it will matter what the Canadian, Russian, and Danish governments will allow. The Ascenders must come down to Earth sometime, and the initial architecture must be registered somewhere during the years of development. The regulatory agencies of these countries tend to cooperate, so, "we'll just fly around it" is not a universal solution to regulatory problems.

Red tape can reach all the way to the edge of space.

That doesn't mean that a national ban on hydrogen lift gas is insurmountable. It just means that it must be addressed at some point.

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Post Re: Hydrogen vs. Helium   Posted on: Wed Mar 03, 2010 5:31 pm
Actually, national airspace ends at 60,000 feet - under 20km. According to the FAI (I think)...

The Ascenders can use Helium if required, although there's always the possibility of placing the port in the middle of the Arctic, in international water, or in the Pacific or Atlantic. Maybe Russia might be fine with using Hydrogen, as well.


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Post Re: Hydrogen vs. Helium   Posted on: Sun Mar 21, 2010 1:45 am
Terraformer,

After a little more reading, I've found that you're correct on both points.

International recognition of national airspace cuts off at 12 nautical miles altitude, by the same legal reasoning that national waters are only recognized 12 miles from the coast. Above that, it's effectively international airspace. As long as a vehicle can stay above that altitude permanently, no country has an internationally recognized right to regulate its lifting gas.

JP's book Floating to Space also covers helium & hydrogen use on page 88 and others. The first stage Ascender airship will use helium, and the components of the other two vehicles will be lifted using helium gas. Then, once they are well above national airspace, the helium will be gradually replaced with hydrogen during the course of construction & maintenance. This meets the legal requirement of the FAA's hydrogen lift gas ban, but allows JP Aerospace to use any lifting gas they want once the components are in position.

The type certificate is a design registration, and lifting gas will probably need to be specified. However, if the design lifting gas is "helium and/or hydrogen" that arguably meets the legal guideline, particularly if the normal operating conditions of the assembled vehicles could be certified to never intrude into either national airspace or any region of the atmosphere for which there is an explosive limit for hydrogen gas at ambient pressure.

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Post Re: Hydrogen vs. Helium   Posted on: Sun Mar 21, 2010 8:31 pm
A quick note about the buoyancy aspects discussed above. The buoyant force is equal to the weight of the displaced gas (or water, Archimedes' law). It doesn't matter whether you fill your airship with hydrogen, helium, or lead, the buoyant force equals the volume of the airship times the density of the medium it flies in.

The trick to building an airship is to build a craft that occupies a lot of space (i.e. has a big volume) without having a lot of weight. It will only get off the ground if its weight is less than the buoyant force. Or, equivalently, if its density (weight / volume) is less than that of the surrounding medium. Hence airships being lighter-than-air (for an equal volume) craft.

So, how big a difference would a change in lifting gas make? That depends on the composition of your airship. I have no idea what is realistic, but two random examples to explain the idea: if your airship consists of (by weight) 80% lifting gas, 10% structure and 10% payload, then halving the weight of the lifting gas will quintuple your payload (from 10% to 10 + (80 - 40) = 50%). If it's 20% lifting gas, 30% structure and 50% payload, then it will only give you an increase of 20%.

I'll leave looking up the specs of an actual airship and doing the calculation as an exercise for the reader :-).

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Post Re: Hydrogen vs. Helium   Posted on: Thu Apr 22, 2010 1:39 pm
Quote:
International recognition of national airspace cuts off at 12 nautical miles altitude, by the same legal reasoning that national waters are only recognized 12 miles from the coast. Above that, it's effectively international airspace. As long as a vehicle can stay above that altitude permanently, no country has an internationally recognized right to regulate its lifting gas.

I thought that was the case, but this paper disagrees. Hmmm. I do recall reading that national airspace ends at 60,000 feet...


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Post Re: Hydrogen vs. Helium   Posted on: Sun May 02, 2010 5:43 pm
US regulatory agencies do not recognize the 60,000 foot line in any way for US citizens. There is no getting around it. They are of the legal view that their jurisdiction is driven by citizenship not any geographic location. You can spend your resources fighting that fight or spend your resources building spaceships. You can't do both. You just need to build that into your plan and work with them.


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Post Re: Hydrogen vs. Helium   Posted on: Thu Jul 22, 2010 9:40 pm
They may not recognise it for US citizens... but all that means is that you have to register your airship and station with another nation. They'd be breaking international law if the then decided to throw their weight about and interfere with such craft. Same as if they tried interfering with a ship in international waters, or an orbiting satellite which crossed over the US.


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Post Re: Hydrogen vs. Helium   Posted on: Fri Jul 23, 2010 1:11 pm
Terraformer wrote:
They may not recognise it for US citizens... but all that means is that you have to register your airship and station with another nation.


And that still might not be enough if they determine that this falls under ITAR (International Traffic in Arms Regulations). It doesn't matter if the company is registered in another country, if there are US citizens working for or even associated with it, they can get snagged by the same red tape as if it were based in the US.


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Post Re: Hydrogen vs. Helium   Posted on: Fri Jul 23, 2010 10:41 pm
I don't think Airships fall under ITAR. Besides, it's pretty stupid of the US gov. to try and get people for using off the shelf technology.

Not that that would stop them trying, though.


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