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Why not GH2/LOx ? Curious French question

Posted by: RolandP - Mon Oct 03, 2011 9:17 pm
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Why not GH2/LOx ? Curious French question 
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Post Why not GH2/LOx ? Curious French question   Posted on: Mon Oct 03, 2011 9:17 pm
I've been following the evolution of JPAerospace these last years,
and like everybody, I'm impatient about the development of the giant ATO,
the Ascender to Orbit.
In the book FLOATING TO SPACE, the author writes that the most probable motor (or thruster) will be a Symphony Engine, a kind of small chemical
engine which would use electric energy (via MagnetoHydroDynamics ?) to accelerate the hot gas produced, so that the ISP of the engine would be artificially augmented.
Pure electric thrusters are also discussed, kind of like the VASIMR engine, I suppose, since the 'traditional' ion engines have a very feeble thrust.
Wouldn't be better to use Gaseous Hydrogen (GH2) which would be absorbed by a compressor from the envelope of the airship to the engine, where it would be mixed with Liquid Oxygen (LOx) ?
The LOx would cool the engine while furnishing the oxygen needed for the chemical reaction which gives water (H2O).
Or course, since the Hydrogen in the envelope would be at a very light pressure (10g/m3, perhaps even 1g/m3), it would take hours to use all this fuel, but since space's vacuum is a good thermic isolator (as specified in the book), we wouldn't need to expel the LOx very quickly before it would vanish in space by heat (it could be protected from sun's heat by the huge body of the airship).
This system would certainly bring a much more powerful thrust than the pure electric engines, perhaps enven the Symphony Engine.
How do you find the idea ?

Roland, France (on the French Riviera)

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Post Re: Why not GH2/LOx ? Curious French question   Posted on: Thu Oct 06, 2011 8:29 pm
Well, if it's going to accelerate over the course of hours or days, then you need the buoyancy of the envelope, otherwise you'll end up holding the thing up using rocket fuel, and that gets inefficient really quickly. So, you need the hydrogen in the envelope! Taking it out will make the airship sink. Of course, you start at a lower altitude, where either the envelope will be smaller (but an expanding envelope would be tricky, or maybe not?) or the pressure will be higher, so you could compress de GH2 as you rise instead of letting it bleed, and then use it in an engine.

I think the main reason for JP to not do that is that the efficiency of a pure rocket engine is much lower than that of an electrical one, or a rocket engine with an electric afterburner. I have to say though that I still don't see how this thing is going to work at all, as far as I can tell the drag will keep it from getting up to orbital speed. Maybe I should buy the book :-).

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Post Re: Why not GH2/LOx ? Curious French question   Posted on: Thu Oct 06, 2011 9:08 pm
Hello again, Lourens,

I see that you've not only answered to my topic at the N-Prize Forum,
but that you've come to this one at JPAerospace's Forum !

Your own reflections are interesting, so it's a great pleasure for me to exchange with you here !

Concerning JPAerospace's ATO project, it's indeed a Hydrogen inflated airship, and a lighter than air, not a beast like the one I've been proposing at the N-Prize Forum : They work on small rockets, at that Forum, often using the hybrid rocket engine technology, less dangerous than the explosive Hydrogen, so I thought the path I proposed there would be more seductive for passionate amateurs with little money.

My proposition uses what's in John Powells's book : They will have many small inflated bags in the airship's envelope. These bags will contain Hydrogen gas, while the volume surrounding them in the envelope will be, at the beginning of the trip, filled with air, a very tenuous air, 1/100th or even 1/1000th sea level's pressure, avoiding the risk of an explosion.

The same system will be used in the Dark Sky Station, for maintaining its altitude and attitude.

These bags will be, in their lower part, rolled or unrolled from a roller, this roller turning in one direction of rotation of another because of a small electric motor controlled by a computerized system.

Being rolled or unrolled, these Hydrogen gasbags will be compressed or, at the contrary, liberated. When they will be liberated from the roll, the bags will inflate and push the air around them. The air will be pushed outside of the envelope throught vents.

When the bags will be compressed, they will put less pressure on the surrounding air, and so more air will enter the envelope through the same vents.

The bags will be connected between them, letting the Hydrogen gas go from one to another, making it possibled to move the center of gravity of the airship in many axis, and even to make it turn on itself ! (slowly, but JPAerospace estimates their airship will be a slow reacting airshi).

These bags put in pressure by the rolls will be able to inject Hydrogen gas at higher pressure for the engines, even if I thing a compressor will be necessary to put hight pressure input of the gas in these ones, while a traditional motorpump will inject the liquid oxygen.

Yes, the airship will have a tendency to begin to go down while the bags will be compressed and the air will enter, but the aerodynamic lift due to the speed of the airship when it accelerates (remember that at its beginning, the aerostatic force will already be equilibratind the weight of the airship) will lift it.

Of course, when the airship will attain Hypersonic speeds, the aerodynamic lift, now predominant over Archimede's force, at these hight altitudes where these speeds will be possible, the lift on drag ratio of the airship will have diminished.

But since the airship will be loosing a lot of mass during the acceleration (it could ultimately finish at 14%, even 10% of its initial mass) because of the relatively low efficiency of the Hydrogen/Oxygen mix compared to electric thrusters, this loss of weight will compensate the loss of aerodynamic lift.

And when approaching orbital speed, more and more weight will be lost too.

And finally, orbital speed could be attained - with a lower final mass than for electric thrusters, OK, and less payload - but it would be mush faster to attain this speed !

What do you think about my concept ? Is there a fact I've forgotten ?
Your thought interest me.

Concering what you said, you're right, Lourens, you should read the book, it gives a lot of revelations about their goals and ways to attain them.
I'm sure you'll love to discover it, like me !

It's always a pleasure to exchange with you, and reading you again will be pleasurable.

I'm getting tired.

Bonne nuit (Good Night)


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