What Are the Key Challenges in Developing a Nitrogen-Powered Plane Prototype?

[Cheesus128]

Hey,
I saw a documentary about the first Nitrogen powered plane.
It was being developed in Germany and is more a prototype than anything else.
They said one of the problems is that the tank is insanly expensive for liquid nitrogen but "Cheap" for nitrogen gas.
Well I don´t understand why it is so expensive and why they need to insulate the tanks. I mean considering you kept the liquid nitrogen under preasure it could not turn into gas until it is being burnt.
Am I right?

 

[mgb_phys]

If you don't insulate the tank you end up with a plane covered in several tons of ice.

ps - Are you sure you mean nitrogen and not hydrogen?

 

[Cheesus128]

Wait so the Air around the tank would freeze too?

 

[mgb_phys]

Nitrogen boils (at standard pressure) at 77K, Hydrogen at 20K
At normal pressure neither of them will form a solid, you can make solid N2 and H2 under vacuum - you use this for cooling camera below 77K for example, but it's an engineering pain.

 

[Algr]

If it is nitrogen, the air around the tank wouldn't freeze, but it would collect lots of water vapor.

A nitrogen plane would use compressed air motors to turn propellors, wouldn't it? Hydrogen could be burned in jets.

 

[cjameshuff]

Nitrogen boils (at standard pressure) at 77K, Hydrogen at 20K
At normal pressure neither of them will form a solid, you can make solid N2 and H2 under vacuum - you use this for cooling camera below 77K for example, but it's an engineering pain.

Nitrogen freezes at 63 K at standard pressure, hydrogen at 14 K. You might have been thinking of helium, which only freezes at higher pressures (assuming you misspoke by saying vacuum, which makes very little sense in this context).

A nitrogen powered plane seem very unlikely. The energy density of compressed nitrogen is pretty pathetic even compared to alternatives like batteries, which struggle to be useful for transport here on the ground. Liquid nitrogen might be a bit better, but will require heat exchangers to boil the nitrogen, which will be heavy and ice-prone...it'll also be relatively energy-intensive to produce. Such a vehicle might fly, but it won't be anything that'll actually see wide use.

Also, liquid nitrogen tanks are cheap, a styrofoam cup can be used for working with nitrogen, but a high-pressure gaseous nitrogen tank would be both expensive and heavy. This isn't so for hydrogen, it's deeply cryogenic enough that you need special tanks with sufficient insulation, made of materials that can handle those temperatures, etc. Aside from some material compatibility concerns (avoiding hydrogen embrittlement), a high pressure hydrogen tank is just like one for nitrogen, oxygen, or air. Plus, hydrogen has rather higher energy density than liquid or compressed nitrogen. I also suspect this was a misunderstanding of a hydrogen powered aircraft project.

 

[Phrak]

If it is nitrogen, the air around the tank wouldn't freeze, but it would collect lots of water vapor.

A nitrogen plane would use compressed air motors to turn propellors, wouldn't it? Hydrogen could be burned in jets.

Interesting factoid: A nitrogen line at the boiling point of nitrogen (77K) can liquefy the oxygen out of the air (BP 90K). If this occurs upon reducible substance, it can represent a significant fire hazard.

 

[Algr]

Oxygen has a higher boiling point then nitrogen? Why does everyone use nitrogen for cooling things then? Is liquid oxygen really corrosive or something?

 

[Phrak]

Oxygen has a higher boiling point then nitrogen? Why does everyone use nitrogen for cooling things then? Is liquid oxygen really corrosive or something?

This is wandering off topic but basically nitrogen is relatively safe. It's already in air (78 percent or so), so obviously nontoxic to breath. Oxygen in concentration would cause a lot of fires set off by a small energy spark. Oxygen is very reactive. Nitrogen isn't. Remember the historic http://en.wikipedia.org/wiki/Apollo_1" ? However, nitrogen can displace the oxygen in the air, and cause blackout and affixiation without warning, so regular use should require sensors.

 

[mgb_phys]

Sorry I slightly mixed up thermodynamics and engineering there.
You can cool N2 and H2 to a solid, but you would need extra cooling in place, you can easily solidify both by just pulling a vacuum but if you just need to store them it's not worth the extra effort over a liquid.

As Phrak points out -the reasons for not using liquid O2 are pretty obvious!
Even hydrogen has problems, it makes a lot of steels brittle.

 

[rcgldr]

It's not that new of an idea. Some old free flight aircraft models used motors powered by small pressurized carbon dioxide cannisters.

 

[mgb_phys]

It's not that new of an idea. Some old free flight aircraft models used motors powered by small pressurized carbon dioxide cannisters.

This idea is to burn the hydrogen in either a fuel cell (to make electricity) or directly in a turbine not just use it as a water rocket.
Liquid hydrogen is currently the only thing that gets near to liquid dead dinosaur as a fuel in terms on energy density.

 

[Phrak]

It's not that new of an idea. Some old free flight aircraft models used motors powered by small pressurized carbon dioxide cannisters.

This idea is to burn the hydrogen in either a fuel cell (to make electricity) or directly in a turbine not just use it as a water rocket.

I had seen one of these model airplanes once. Rather than using pressurized CO2 as a jet, it drove a small piston to rotate a propeller.

 

[mgb_phys]

Rather than using pressurized CO2 as a jet, it drove a small piston to rotate a propeller.

Doesn't make a lot of difference, the total energy in a gas pressurized to 200bar in a 11L scuba tank is only about 1MJ - about the same as a tablespoon of jet fuel.

 

[Phrak]

Doesn't make a lot of difference, the total energy in a gas pressurized to 200bar in a 11L scuba tank is only about 1MJ - about the same as a tablespoon of jet fuel.

That settles it. On my next dive, I'll use jet fuel. But seriously, it is a great deal more efficient, using the energy available, to push a lot of air a little faster with a propeller, than throwing it out in a narrow stream such as a jet. Of course, the compressed CO2 powered planes were surely a novely that didn't have a large market compared to burning nitro methane.

 

[mgb_phys]

Some dim memory of a thermodynamics lecture is that you have a maximum efficency of 66% for air expanding into a piston (can't remember if this is adiabatic) better than ICE but still not going to lift a 747.

There is a proposed compressed air driven car (french company le air-car?) which uses basically scuba tanks and a piston engine.

 

[rcgldr]

It's not that new of an idea. Some old free flight aircraft models used motors powered by small pressurized carbon dioxide cannisters.

I had seen one of these model airplanes once. Rather than using pressurized CO2 as a jet, it drove a small piston to rotate a propeller.

Which is what I meant by motors powered by the small CO₂ cannisters.

Of course, the compressed CO2 powered planes were surely a novely that didn't have a large market compared to burning nitro methane.

Free flight models only use a short burst of power, perhaps 15 to 30 seconds of power, then they glide in big circles. They needed a big field. There was no need for extended motor run times, so the CO₂ motors were good enough.

 

[alxm]

Mgb_phys: Yup, maximal efficiency of a Carnot cycle is somewhere around there.

It seems every so often, someone gets the (very unoriginal) idea of using compressed gas as energy-storage. It's an idea so old, it was old in the 19th century.
Yet somehow people seem to think they've found some way around the very obvious inefficiencies that stopped it in the past.

Now, find me some tetrahedral N₄ (aka T_dN₄) and then we're talking some serious nitrogen fuel - Rocket fuel, even! An energy density of about 54 MJ/kg for its decomposition into two N₂ molecules. No other components needed, and it's environmentally friendly too!
Just one slight problem.. We don't know how to make it. So far, it only exists in theory, although some http://dx.doi.org/10.1016/S0009-2614(01)00866-1" have been floated..