Why hasn't anybody used superheated water as rocket fuel

[Rune]

Actually, Isp with water is whatever you can get it to be. For a given engine, yeah, water will have worse Isp than lighter propellants like hydrogen, for example, but note that "for a given engine". If you are using water as propellant, one has to assume that the energy is provided in some other way, since water is very non-reactive. An NTR, as K^2 mentions, would work fine, getting around 500s if memory serves, at the temperatures a solid-core Nerva can get before melting. But. You could go a bit fancier, using other method (some other kind of nuclear reactor, most likely, not limited by stuff like its innards melting or vaporizing), to really heat that water, turn it into a plasma, and handle that with some short of magnetic field nozzle. Then Isp is only limited by your powerplant efficiency, and your cooling capabilties. Mind you, nobody has really figured out a better way to get heat into a propellant than NTRs, so...

In any case, yeah, water can be a propellant. You could encase nukes in ice and actually use that in an Orion-style drive for stupendous Isp, even. The bad news is that you still need your fancy drive, and said drive would still get more Isp (but less thrust!) out of using pure hydrogen.

 

Rune. Resistojets, for example, get NTR-like Isps with awful thermal efficiency.

[K^2]

It should be noted that I_SP of an LH₂/LOX rocket is already very close to the 500s limit on the NTR. Both operate very close to thermodynamic limits of conventional materials. So applications for a water NTR are going to be somewhat limited.

[Temstar]

39 minutes ago, Rune said:

But. You could go a bit fancier, using other method

Like say, mix the water with 90% enriched weapons grade uranium-235 salt, then you'll really get some Isp!

Zubrin's funny drive aside, I think in the original Project Orion there were ideas to use water ice harvested from Enceladus as propellant for coming back to Earth. As in, you pack all that ice around each bomb so that when they go off, the water plasma will then hit your pusher plate. It won't be as good as tungsten that will be used for the bombs leaving Earth but using ISRU means you only need to bring the bombs for the trip home and not the propellant for the bombs.

Edited February 2, 2016 by Temstar

[K^2]

40 minutes ago, Temstar said:

Like say, mix the water with 90% enriched weapons grade uranium-235 salt, then you'll really get some Isp!

Forget the pressure chamber and the nozzle. How would you store that fuel in the first place?

There's also significant loss of I_SP due to the weight of all that uranium, meaning temperatures and pressures are going to have to be absolutely insane to get an actual I_SP leverage over, say, H₂ NTR.

[Temstar]

16 minutes ago, K^2 said:

Forget the pressure chamber and the nozzle. How would you store that fuel in the first place?

In thin tubes surrounded by neutron absorbing material so the uranium don't start the party until it's time.

 

16 minutes ago, K^2 said:

There's also significant loss of I_SP due to the weight of all that uranium, meaning temperatures and pressures are going to have to be absolutely insane to get an actual I_SP leverage over, say, H₂ NTR.

It's 2% solution of uranium salt, so 98% of the propellent you shoot out the back is water. Being basically a continuous nuclear explosion the Isp of a NSWR will be something like 479,103s for the 90% enriched version. Zubrin says the engine can survive this as peak neutron flux will be outside of the nozzle and he says the engine will have a ring of water jets (just water, not nuke juice) firing water next to the wall of the engine to create a film to project them.

But then again it's Robert Zubrin, a person known to be too optimistic with his designs.

Edited February 3, 2016 by Temstar

[K^2]

Huh. Not bad. Storing fuel in long tubes would be pretty inefficient in terms of fuel/tank weight, but at well over 400ks impulse (I'll check this number when I have a moment), it'd be shame to complain even if that ratio gets as silly as 1:1.

Yeah, it's definitely a bit on the far side, but I would invest some research into it when we get to the point where we could make practical use of something like that.

[magnemoe]

4 hours ago, K^2 said:

It should be noted that I_SP of an LH₂/LOX rocket is already very close to the 500s limit on the NTR. Both operate very close to thermodynamic limits of conventional materials. So applications for a water NTR are going to be somewhat limited.

True, however you don't have to splitt the water into hydrogen and oxygen. Benefit of water is that its very easy to store. It also common making isru easy. 
On the other hand if you don't have an nerva but an reactor or solar panel splitting water and then burn it works too.  

 

[wumpus]

6 hours ago, K^2 said:

That's stupid. Converting energy directly to heat is way, way more efficient than electrolyzing water and burning the mix to generate the heat. If you are trying to use water as propellant, an NTR is going to be several times more efficient than what you are proposing.

Rockets aren't magic. They don't get I_SP from some mystical reaction between fuel and oxidizer. Fuel and oxidizer burn, generating heat. Nozzle allows heated exhaust gas to expand and converts internal heat energy of the gas into thrust. If you can generate the same heat without bothering with a chemical reaction, you've just saved yourself some trouble.

The point is that you use solar arrays to crack the water (presumably when you are sciencing down on the surface of a planet), and then when you return there is plenty of hydrogen and oxygen.  You either ditch the oxygen (because it will corrode everything it touches) or toss it first into your NERVA*.  Once the oxygen is gone, you start shoving the hydrogen into your NERVA.  Since you aren't trying to move any of the Oxygen, your ISP will be around 800 (instead of less than 400 with the oxygen).

The point of cracking is that you just want to carry the hydrogen (or possibly just use it as an "upper stage"), the oxygen is never going to be as efficient in a NERVA as hydrogen.  It isn't about energy efficiency (who cares how many watts your solar panels waste), it is about mass efficiency and rocket efficiency.

* expect some pretty bad ISP (actually, I'd guess between LH/LOX and RP1/LOX, but maybe a little lower.  Nowhere near LH/nukes).  But presumably something is better than nothing.

[Temstar]

LOX is still potentially useful for NTR if your NTR engine is a LANTR engine. A LANTR NTR is a regular solid core NTR with an amusing "afterburner" stage where LOX is optionally injected into the hydrogen steam after its shot out of the reactor. The LOX then reacts with the super hot hydrogen to further increase the exhaust temperature slightly which goes some way to offset the Isp disadvantage from high molecular mass exhaust.

Basically on "LH2 only" mode the LANTR behaves as a normal solid core NTR with the ususal 800-1000s Isp. When it runs on "afterburning" mode the Isp drops to something like 500s but the thrust increases a lot because of much high mass flow, with the final result that you get more delta-V per unit of water split by your ISRU kit running on afterburner since you are no longer throwing the LOX away. Plus high thrust makes the engine better at taking advantage of Oberth effect.

But if the amount of water harvested is not an issue than throwing the LOX away and keeping only the LH2 gives much more delta-V for the same vehicle wet mass. Having LANTR gives you flexibility to fine tune LOX/LH2 ratio to optimize for a particular mission.

[kerbiloid]

An outdated but still amazing Vertical Propulsion Emporium mod is exactly about superheated water (usually they call it a "steam") -powered KSP vehicles. So, somebody has.

Also, nuclear engines can use heated water as a propellant.

[fredinno]

10 hours ago, Temstar said:

Like say, mix the water with 90% enriched weapons grade uranium-235 salt, then you'll really get some Isp!

Zubrin's funny drive aside, I think in the original Project Orion there were ideas to use water ice harvested from Enceladus as propellant for coming back to Earth. As in, you pack all that ice around each bomb so that when they go off, the water plasma will then hit your pusher plate. It won't be as good as tungsten that will be used for the bombs leaving Earth but using ISRU means you only need to bring the bombs for the trip home and not the propellant for the bombs.

Why would you need tugsten or water for bombs?

4 hours ago, wumpus said:

The point is that you use solar arrays to crack the water (presumably when you are sciencing down on the surface of a planet), and then when you return there is plenty of hydrogen and oxygen.  You either ditch the oxygen (because it will corrode everything it touches) or toss it first into your NERVA*.  Once the oxygen is gone, you start shoving the hydrogen into your NERVA.  Since you aren't trying to move any of the Oxygen, your ISP will be around 800 (instead of less than 400 with the oxygen).

The point of cracking is that you just want to carry the hydrogen (or possibly just use it as an "upper stage"), the oxygen is never going to be as efficient in a NERVA as hydrogen.  It isn't about energy efficiency (who cares how many watts your solar panels waste), it is about mass efficiency and rocket efficiency.

* expect some pretty bad ISP (actually, I'd guess between LH/LOX and RP1/LOX, but maybe a little lower.  Nowhere near LH/nukes).  But presumably something is better than nothing.

Where would you carry the Lox? You probably would only have large LH2 tanks.

Also, it's probably easier just to waste the oxygen, as most solar system objects have lots of water, and you get more delta-v overall that way.

[shynung]

29 minutes ago, fredinno said:

Why would you need tugsten or water for bombs?

Propellant. A detonating nuclear bomb emits its energy mostly in the X-ray spectrum, which is useless for thrust.

Pictured above is a typical bomb/pulse unit design for a nuclear pulse drive, AKA Orion. At detonation, the nuclear device releases most of its energy in the X-ray spectrum. The radiation case, opaque to X-ray, channel the emissions to one direction. The channel filler absorbs this mad flurry of X-rays, and transforms it into a mad flurry of heat. This intense heat turns the slab of propellant, which can be made out of any solid material, instantly into ionized plasma, while simultaneously accelerating it to 150,000 meters per second. This jet of plasma smacks right into the ship's pusher plate, transferring its momentum to the ship.

Edited February 3, 2016 by shynung

[RizzoTheRat]

On ‎22‎/‎01‎/‎2016 at 7:04 AM, SomeGuy123 said:

On ‎22‎/‎01‎/‎2016 at 2:56 AM, pincushionman said:

Water injection has been used in the past to increase jet engine thrust. A good example is getting early BUFFs off the ground. But there are serious drawbacks in weight and efficiency for doing it.

I thought this was actually to cool the incoming air, the water itself is besides the point.  I don't know.

In the case of turbocharged piston engines, you inject water mist into the cylinders and adjust the turbocharger "boost" pressure.  Essentially, the air from the turbocharger is at higher pressure and so each cylinder-full has more actual air.  This lets you inject more fuel.  The water mist flashes to steam as the cylinder of fuel-air compresses, cooling it.

 The purpose of this is to let you have more fuel and air and not get predetonation from the "overfilled" cylinder heating up the air inside too fast during the compression stroke.

So you get more power: weight, but the energy lost from the water injection (and the weight of the water tank) reduces efficiency.

Dunno about the B52 but in the Harrier's Pegasus engine it's injected in to the combustion chamber to lower the turbine entry temperature, meaning they inject more fuel and get more power out without cooking the high pressure turbine, they carry about 1/4 of tonne (a significant amount when MTOW is about 14 tonnes and max VTO weight is about 8 tonnes) and it only lasts 90 seconds, which apparently means it can get a bit scary if you cock up a carrier landing as once you hit the button you've got 90 seconds to put it on the deck.

Edited February 3, 2016 by RizzoTheRat

[sevenperforce]

It's not stupid to suggest cracking the water into hydrogen and oxygen; it's just a different approach for different missions. Your propellant choice will depend on what kind of craft you have, what goal you're trying to achieve, what energy source you're using, and the availability of fuel.

Liquid water is fairly easy to store as long as you have at least some mild source of waste heat to keep it from freezing. It's also quite dense. It offers pretty good thrust if you have a powerful heat source like a nuclear reactor. Its thrust is a little less impressive if you're using something like a solar reflector (e.g. solar moth) but still respectable. Isp sucks, though, so you'll need a lot of it...but depending on your mission and the availability of water, that might not be a problem.

Cracking water into hydrogen and oxygen can allow you to skip carrying an energy source, if you want. Might be useful if you have a deep-space mission far from the Sun but don't want to use a nuclear reactor (for whatever reason). Unfortunately, neither hydrogen nor oxygen are particularly dense. If you are going to carry your own energy generator and merely use the hydrogen as propellant, you need it to be fairly dense when it's injected into the reaction chamber, which either means compressing it mechanically (requiring energy for the compressor, a heat sink for the waste heat from the mechanical compressor, and resulting in preheated hydrogen which decreases the effectiveness of the engine) or keeping it liquid in the first place. But liquid hydrogen has very low density and is difficult to handle or store for long periods of time. So you may be better off just using water unless you need a high Isp.

The autonomous space tugs of the future may well be giant water balloons with nuclear-thermal reactors on one end...

[wumpus]

6 hours ago, fredinno said:

Why would you need tugsten or water for bombs?

Where would you carry the Lox? You probably would only have large LH2 tanks.

Also, it's probably easier just to waste the oxygen, as most solar system objects have lots of water, and you get more delta-v overall that way.

Why not?  It has 8 times the mass and takes a much smaller tank.  The ISP might be in the ~300 range, but with 8 times the fuel it wouldn't be a trivial amount of delta-v.  And then you have all your hydrogen to 'burn' after that.  This mostly depends on how much water you mined and how much power you are getting from your solar panels/TNGs.  If you can easily get enough hydrogen, there is no need to worry about the oxygen (and all the corrosion it causes).  What the cracking buys you is vastly better staging: the high mass/low ISP fuel goes first, followed by the low mass/high ISP fuel.

Don't forget, that hydrogen is going to leak (probably not that much, but it will add up).  You can't simply assume that it will completely build up as the solar cells slowly crack the water.  The oxygen will stay put.

[fredinno]

2 hours ago, wumpus said:

Why not?  It has 8 times the mass and takes a much smaller tank.  The ISP might be in the ~300 range, but with 8 times the fuel it wouldn't be a trivial amount of delta-v.  And then you have all your hydrogen to 'burn' after that.  This mostly depends on how much water you mined and how much power you are getting from your solar panels/TNGs.  If you can easily get enough hydrogen, there is no need to worry about the oxygen (and all the corrosion it causes).  What the cracking buys you is vastly better staging: the high mass/low ISP fuel goes first, followed by the low mass/high ISP fuel.

Don't forget, that hydrogen is going to leak (probably not that much, but it will add up).  You can't simply assume that it will completely build up as the solar cells slowly crack the water.  The oxygen will stay put.

The only problem is that H2 and O2 tanks are different, so you would have to throw the O2 away anyways.

[K^2]

9 hours ago, shynung said:

Propellant. A detonating nuclear bomb emits its energy mostly in the X-ray spectrum, which is useless for thrust.

That's bad to throw as a blanket statement. For a moment, picture a perfect world with perfect energy to thrust conversion. Suppose that you have a drive with energy source separate from propellant. You will consume quantity of fuel dM to produce quantity of energy dE, at some constant dE/dM = kc², which is used to accelerate propellant of mass dm. Since we expect really high energy, we need to consider relativistic limit. As such, dp = sqrt(dE²/c²+2 dm dE) = sqrt(k²c² dM² + 2 k c² dm dM)

For convenience, lets also define fraction x = dm/dM, so that dp = c dM sqrt(k² + 2kx). Now, we wish to maximize impulse gain, dp, while minimizing fuel+propellant we've expelled. Which is equivalent to maximizing sqrt(k² + 2kx)/(1 + x).

This function has very interesting behavior. For "low" values of k, such as we see with nuclear reactions (k ~ 0.01), this function has a maximum at just bellow x = 1. (Wolfram Alpha Plot) Which means that the amount of propellant you should bring should be equal to the amount of fuel on board. (This assumes you'll be ditching spent fuel as you go along, by the way, so that the ratio stays constant.) So an Orion drive should have a damper roughly equal to the mass of the bomb.

This behavior starts to change as fraction of energy gets closer to 1. At k = 0.5, you should already bring significantly less propellant (Wolfram Alpha Plot), but things only really get interesting when you get to k = 1, which is the case for matter-antimatter drive or a black hole drive. Here, with total conversion of fuel to energy, you get optimum at x = 0. In other words, you shouldn't bring any propellant at all and use radiation to propel yourself. (Wolfram Alpha Plot)

So it's not that radiation is useless for thrust. It can be absolutely the perfect propellant. But its efficiency depends on your conversion ratio, and photon drives only make sense when conversion ratio is damn near 1.

[mikegarrison]

On 1/22/2016 at 7:33 AM, pincushionman said:

From what I understand, the temperature reduction (throughout the engine, not just the incoming air) allows increased RPM and works for both piston and jets. But on turbojets and low-bypass-ratio turbofans, you also increase the mass flow rate, which is a key variable in the thrust equation.

In turbojet engine water injection back in the day, the water was an attempt to increase thrust by increasing mass flow. Turbojets have lousy static thrust.

There is a concept being talked about today for water injection of jet engines again, but now it's not for thrust -- instead, the idea is to use the water to cool down the air in the combustion chamber and make less NOx.

Edited February 3, 2016 by mikegarrison

[shynung]

9 hours ago, K^2 said:

That's bad to throw as a blanket statement. For a moment, picture a perfect world with perfect energy to thrust conversion. Suppose that you have a drive with energy source separate from propellant. You will consume quantity of fuel dM to produce quantity of energy dE, at some constant dE/dM = kc², which is used to accelerate propellant of mass dm. Since we expect really high energy, we need to consider relativistic limit. As such, dp = sqrt(dE²/c²+2 dm dE) = sqrt(k²c² dM² + 2 k c² dm dM)

For convenience, lets also define fraction x = dm/dM, so that dp = c dM sqrt(k² + 2kx). Now, we wish to maximize impulse gain, dp, while minimizing fuel+propellant we've expelled. Which is equivalent to maximizing sqrt(k² + 2kx)/(1 + x).

This function has very interesting behavior. For "low" values of k, such as we see with nuclear reactions (k ~ 0.01), this function has a maximum at just bellow x = 1. (Wolfram Alpha Plot) Which means that the amount of propellant you should bring should be equal to the amount of fuel on board. (This assumes you'll be ditching spent fuel as you go along, by the way, so that the ratio stays constant.) So an Orion drive should have a damper roughly equal to the mass of the bomb.

This behavior starts to change as fraction of energy gets closer to 1. At k = 0.5, you should already bring significantly less propellant (Wolfram Alpha Plot), but things only really get interesting when you get to k = 1, which is the case for matter-antimatter drive or a black hole drive. Here, with total conversion of fuel to energy, you get optimum at x = 0. In other words, you shouldn't bring any propellant at all and use radiation to propel yourself. (Wolfram Alpha Plot)

So it's not that radiation is useless for thrust. It can be absolutely the perfect propellant. But its efficiency depends on your conversion ratio, and photon drives only make sense when conversion ratio is damn near 1.

Interesting. So, you're saying an Orion pulse drive with matter-antimatter bombs are better off without a propellant slab than with?

If so, how would that affect the drive design?