The LM maneuvres pre docking

[ka9q]

I'm not sure that's really a monopropellant. 

Well...yes. I had the same thought. But it can be treated as if it were a monopropellant, passed over a catalyst bed to get it going and then fed into a nozzle much like straight hydrazine.

[Bob B.]

Bob, as a baseline how do propane (or LPG) and N₂O perform as bipropellants? I've always thought they would be a good choice for amateur high-power rocketry since both are readily available, liquids at room temperature under reasonable pressure that can be used instead of pumps, and essentially stable and nontoxic.

I've thought the same thing.  I believe I ran some numbers on this in the past but I don't remember what I got.  I'll check again and post the results (probably tomorrow).

[ka9q]

One practical complication with N₂O that some of the hybrid enthusiasts talk about is that its critical temperature is just above room temperature (like CO₂, though at somewhat lower pressure) so its pressure is a sharp, nonlinear function of temperature.

That pressure is about 50 bar, so the tank would be too heavy for a really large rocket. Propane is only about 8 bar. Still, N₂O does seem to be the oxidizer of choice for hybrid rockets, so it can't be too terribly difficult to work with.

[JayUtah]

It would be monopropellant only in the same sense that ATK's solid fuel is "mono" propellant.  Their stuff looks like a pretty homogeneous batch of goop, but it's just a composition of fuel and oxidizer that are chemically distinct until combustion appears.

Monopropellant is traditionally a single compound such as a cold-gas propellant, or something like hydrogen peroxide that undergoes a (possibly catalyzed) energetic decomposition.  Still 300 would be impressive if true.

[Glom]

Like the thrusters on Mercury? They were hydrogen peroxide monopropellent?

[Bob B.]

Like the thrusters on Mercury? They were hydrogen peroxide monopropellent?

Correct.  When hydrogen peroxide comes in contact with a catalyst it will decompose into hydrogen and steam and will release heat.  I believe the catalyst is usually silver.  It was used widely in early rocketry.  Unfortunately it doesn't have a real high specific impulse, only about 150 s maximum as I recall (depends on H₂O₂ concentration). 

Hydrazine provides much better performance as a monopropellant, about 230-240 s, but early tests with it weren't very successful.  After a short period it would eat up the catalyst, making restarts impossible.  This wasn't very good for a thruster that has to re-fire over and over.  Eventually they found a catalyst that could endure, alumina pellets impregnated with iridium.  Once that discovery was made, hydrazine pretty much became the standard.  Hydrazine will decompose into some combination of ammonia, nitrogen and hydrogen.  The amount of ammonia dissociation effects the temperature and performance of the engine.  By varying the design of the catalyst chamber it is possible to control the amount of ammonia dissociation to get the desired performance.

Another type of monopropellant is cold gas, usually nitrogen.  This is just pressurized gas released through a nozzle without any decomposition.  The performance is low but no heat is generated, which is sometimes more important than high thrust.  For example, a manned maneuvering unit.

I've heard of nitrous oxide as a monopropellant but I'm not familiar with any spacecraft that have actually used it.  It's much safer than hydrazine and provides better performance than hydrogen peroxide.  My own calculations suggest that the specific impulse of pure N₂O is about 170 s.  What ka9q has presented is a proposal to mix N₂O with a fuel that would be stored together.  It sounds like the arrangement would resemble a monopropellant system with a single storage tank and a catalyst to initiation the reaction, but it would technically be a bipropellant with separate oxidizer and fuel that would combust inside the engine.

[JayUtah]

Like the thrusters on Mercury? They were hydrogen peroxide monopropellent?

Yes.  Cold-gas propellant is basically WALL-E with the fire extinguisher.  Dirt-simple to make and fly, especially for small satellites.  And extremely reliable.  But you don't get much from your tank of prepressurized gas.

Most hydrogen peroxide motors are the catalyzed type.  As you've noticed from the bubbles, the extra oxygen pops off easily and releases energy as a result.  That's the reaction you want, but you want it to happen much, much faster and thereby increase the rate of energy release.  You use pure H₂O₂ (not the highly-diluted mixture you buy at the druggist/chemist), and you spray it across a plate or through a screen made of some transition metal (often platinum or silver) and the decomposition reaction rate ramps up to useful levels.  This is dirt-simple too, but it's that really nice potting-soil kind of dirt.  Mechanically speaking its just as reliable as cold gas.  But you typically want very, very pure catalysts.  And the platinum ones are costly.

The hypergols are problematic because the very properties that make them useful as propellants -- chiefly their reactivity -- are the ones that cause problems with handling or system engineering.  The Apollo ELS lost a parachute on Apollo 15 because the RCS hot-fire safing procedure burned through a shroud.  That in-flight safing procedure was made desirable by the difficulty in handling a fueled CM after splashdown.

Even so, you can never fully empty a propellant feed system simply by running it to fuel "depletion" because the limitations of tanking and plumbing mean some residue always remains.  And one of the post-landing CM safing crews suffered injury from just the very little bit of hypergols remaining in the CM RCS.  That stuff is just plain evil.  I think USAF lost a missile crew or two due to propellant leaks or tanking/detanking mishaps.

Finding propellant formulations that are energetic without being viciously toxic, and which let us have simple, reliable engines is why rocket science is still a going concern.

[Bob B.]

Cold-gas propellant is basically WALL-E with the fire extinguisher.

Or Sandra Bullock.

[JayUtah]

Don't get me started, Bob. 

[Luke Pemberton]

Correct.  When hydrogen peroxide comes in contact with a catalyst it will decompose into hydrogen and steam and will release heat.  I believe the catalyst is usually silver.

A little bit like this:

[JayUtah]

You use silver with moderate concentration H₂O₂ solutions, such as for naval torpedo turbine motors.  With high-concentration propellant, you need platinum because it has a higher melting point than silver and isn't contaminated by the stabilizers you have to put into high-concentration propellants.  Rocketry generally favors high concentrations due to mass budgets and the ability to delay tanking until just prior to launch.

[Glom]

At school we used potato or carrot.

[Luke Pemberton]

At school we used potato or carrot.

Saturated solution of potassium iodide is a great one, much cleaner than manganese (IV) oxide. Yeast also sets it going too.

[JayUtah]

At school we used potato or carrot.

Which can also be used as onboard batteries. 

[Luke Pemberton]

You use silver with moderate concentration H₂O₂ solutions, such as for naval torpedo turbine motors.

The Royal Navy used such torpedo turbines, but became concerned about the instability of H₂O₂ and potential oxygen build up. I believe they changed their motors. The Russians did not follow suit, and if I recall it is strongly believed that the likely cause of the Kursk disaster was a result of a fire fueled by oxygen from torpedo motors.