Water Plasma as Propellant: Feasible?

[Sepulchrave]

If you brought water to a plasma state - say 12,000K-15,000K - and it dissociated into a bunch of ions, how would it perform as a thrust agent?

Would there be an economic case for using it above something like xenon, given the ubiquity of water?

I'd like ships to use water for thrust fuel - assuming that the ship's main reactor can somehow transfer this much heat to it.

 

[aramis]

The only real advantages of water over xenon in such applications is that water doubles as a source of fuel for the fusion plant, and oxygen for the LS system, and its ubiquity.

 

[Spinward Scout]

Here's the current state-of-the-art for Real World thrusters. Maybe that could give you a few ideas.

http://www.newscientist.com/article/dn17476-ion-engine-could-one-day-power-39day-trips-to-mars.html

 

[Sepulchrave]

The only real advantages of water over xenon in such applications is that water doubles as a source of fuel for the fusion plant, and oxygen for the LS system, and its ubiquity.

I'm assuming that the power plant is a closed system D-D or D-T fusion reactor at the moment, and that PP fuel isn't a consideration - maybe top it off once a year.

I'm considering a kind of 'universal' engine capable of working in turbojet, ramjet, scramjet and rocket mode.

It would work inatmo like those crazy nuclear-powered jets proposed back in the 50s, with heat transference through liquid metal loops. No propellant would be required. Exatmo, the ducts would be sealed off and the ship would switch to rocket mode - this is where the water would come in.

It strikes me that, economically, the sheer plentifulness of water would make this viable BUT I have no idea how water behaves as a plasma: it would dissociate into OH-, H20+, O-, free electrons and protons. Would this cocktail perform ok?

Being 14 times as dense as L-Hyd, water wouldn't suffer from annoying relativistic or superluminal exhaust velocities (like HEPlaR) in order to get a decent Isp. For a variety of reasons, I want the Isp of rockets to be in the 500,000s - 1,000,000s range.

Does this sound even remotely plausible?

 

[mike wightman]

Wouldn't water become O2- ions, H+ ions and free electrons in its plasma state?

 

[aramis]

Water in the plasma would dissipate into O and H ions; plasmas generally are atomic cloud, not molecular.

 

[DFW]

It would work inatmo like those crazy nuclear-powered jets proposed back in the 50s, with heat transference through liquid metal loops. No propellant would be required.

So, part time air breather, part time outer space?

 

[Zutroi]

It would work inatmo like those crazy nuclear-powered jets proposed back in the 50s, with heat transference through liquid metal loops. No propellant would be required. Exatmo, the ducts would be sealed off and the ship would switch to rocket mode - this is where the water would come in.

Why not heat the air by blasting a small quantity of the plasma into it? You lose a bit of your 'bunkerage' in the process, but you eliminate the need for the seperate heat-transfer loops...

 

[Sepulchrave]

Why not heat the air by blasting a small quantity of the plasma into it? You lose a bit of your 'bunkerage' in the process, but you eliminate the need for the seperate heat-transfer loops...

Good question. At ram/scram speeds and altitudes, this would probably be fine: fuel consumption overall wouldn't suffer, as the ship would presumably be climbing toward orbit anyway.

At lower altitudes, it would have to remain a regular airbreathing engine with a rotary fan. Ships chugging out free radicals at these altitudes would probably be frowned upon.