Armstrong drive? Possible propulsion

[Lorenzo A.]

Hey Everyone, newbie here. 

I'm aiming way over my head here, but was wondering about something has been bugging me all day. 

At 70000 feet ( 21km) passed the Armstrong Line fluids start to boil, due to the atmospheric pressure, producing steam and therefore energy. Now, its like -56C° at that altitude and possibly colder in the Mesosphere. Does Liquid Hydrogen create enough energy once it has reached the Boiling point of -252C°?

Even though liquid hydrogen has low energy density, and has high specific energy. 

Could a plane with a fully pressurised cabin, something larger than a U2 jet, be powered by a  Liquid Hydrogen steam powered  engine/jet propulsion? 

Without any combustion to boil the Liquid? 

[swansont]

Boiling does not produce energy. Liquids will boil at low pressure if they possess enough energy to change phase at the temperature they have. e.g. at 0.5 atmospheres, water boils at ~80ºC. At 0.25 atm, it's about 60 ºC. Keep getting lower and it will boil at room temperature.

What is a hydrogen-powered steam engine? Are you boiling hydrogen, or making steam?

 

[exchemist]

1 hour ago, Lorenzo A. said:

Hey Everyone, newbie here. 

I'm aiming way over my head here, but was wondering about something has been bugging me all day. 

At 70000 feet ( 21km) passed the Armstrong Line fluids start to boil, due to the atmospheric pressure, producing steam and therefore energy. Now, its like -56C° at that altitude and possibly colder in the Mesosphere. Does Liquid Hydrogen create enough energy once it has reached the Boiling point of -252C°?

Even though liquid hydrogen has low energy density, and has high specific energy. 

Could a plane with a fully pressurised cabin, something larger than a U2 jet, be powered by a  Liquid Hydrogen steam powered  engine/jet propulsion? 

Without any combustion to boil the Liquid? 

I had to look up the Armstrong Limit, so I've learnt something as a result of your post. So thanks for that. 🙂

But this limit is, specifically, the pressure at which water, not any other liquid, boils at 37C, which is the temperature of the human body. The relevance of the Armstrong Limit is solely to do with the survival of human beings at high altitude without a pressure suit. It does not tell you anything about other fluids, or about boiling at other temperatures. 

Also, you get no energy from something boiling. You have to put energy in, in order to make it boil, because the Latent Heat of Vaporisation has to be supplied to enable the change of state to occur. What will happen when a liquid subjected to low pressure boils is that the liquid will become colder than the surroundings, as it extracts the necessary Latent Heat input from its environment. So I'm afraid this idea won't work. The First Law of Thermodynamics tells you "there is no free lunch". 

 

[J.C.MacSwell]

If you had liquid hydrogen in outer space, you could configure it's evaporation to produce thrust, if that is what you are asking. It would not be a very efficient use of the "fuel" but with no oxygen available it might be the best "McGiver" available.

It's not free lunch, the entropy of your system increases.

 

You still have the latent heat of fusion to work with, as a source of energy. Just make sure to save some of the hydrogen snowballs produced...throwing them might give you more thrust.