The LM maneuvres pre docking

[Bob B.]

Why not run at the higher pressure? N₂O has a vapor pressure of 51.5 bar @ 20C, so you might as well use it. Propane is only about 8 bar, but you could conceivably use N₂O's much higher vapor pressure to push it out. As long as the propellants leave their tanks as liquids, the propane tank would absorb no heat and relatively little heat would be absorbed by the N₂O tank to boil enough liquid to make the gas to fill the ullage space.

Solid or hybrid propellants just sound like a much simpler solution for about the same specific impulse.

However, if we're going to use N₂O to pressurize the fuel, then fuel wouldn't necessarily have to be propane.  Perhaps something else would produce a higher specific impulse that is still relatively safe to handle.
   

[ka9q]

You have a very good point there. What's the I_sp of N₂O oxidizing acrylic or PVC, two of the more popular hybrid fuels? (I think I'd prefer acrylic as it wouldn't generate HCl or possibly chlorinated hydrocarbons).

One of the main problems with these hybrids is igniting them. N₂O is so stable that it can be difficult. The units I saw in the desert maybe 10 years ago were lit with steel wool and oxygen gas. That seemed a little crude, but it worked. Sort of.

I like Surrey's heated catalyst, but an arcjet seems like it should be even better. They make a big deal about being able to turn off the power after ignition occurs, and I can see how that's important for a small satellite, but if the power can be found I'd think keeping an arc on during the burn would increase I_sp substantially. It would be a hybrid (so to speak) of chemical and electric propulsion.

[Peter B]

Well, thanks everyone.

As a result of this thread I've now learned about both Bayesian Search Theory and SCAPE suits.

[cjameshuff]

Why not run at the higher pressure? N₂O has a vapor pressure of 51.5 bar @ 20C, so you might as well use it. Propane is only about 8 bar, but you could conceivably use N₂O's much higher vapor pressure to push it out.

Well, higher pressure tanks are heavier. I don't know what tank pressures are typical for pressure-fed systems, it presumably depends on size.

Here's a useful reference: http://encyclopedia.airliquide.com/Encyclopedia.asp?LanguageID=11&CountryID=19&Formula=&GasID=55

The vapor pressure varies widely with temperature, and there's a critical point at 36.37 °C, 72.45 bar. It might be a bit...optimistic to keep it as a liquid in a 51.5 bar tank.

[ka9q]

Yes, the vapor pressure of nitrous oxide varies sharply with temperature because it's close to its critical point. The same is true for carbon dioxide.

I wonder how large hybrid rockets like those on Space Ship One got around the oxidizer tank weight problem.

[cjameshuff]

Yes, the vapor pressure of nitrous oxide varies sharply with temperature because it's close to its critical point. The same is true for carbon dioxide.

I wonder how large hybrid rockets like those on Space Ship One got around the oxidizer tank weight problem.

They didn't. They use gaseous nitrous oxide, and don't get much performance from it. SS1 only had to hop up above 100 km altitude, and it appears SS2 can't even do that. There's rumors that they're looking at all-liquid systems now.

[Bob B.]

I don't know what tank pressures are typical for pressure-fed systems, it presumably depends on size.

Here are a couple examples.

LM DPS
Helium regulator inlet pressure:  320 to 1750 psia
Helium regulator outlet pressure:  245 +/- 3 psia
Nominal propellant tank ullage pressure:  235 psia (full throttle)
Propellant tank proof pressure:  360 psia
Nominal combustion chamber pressure: 103.4 psia

LM APS
Helium regulator inlet pressure:  400 to 3500 psia
Helium regulator outlet pressure:  176-190 +/- 4 psia (depending on which regulator)
Nominal propellant tank ullage pressure:  184 psia
Propellant tank proof pressure:  333 psia
Injector inlet pressure:  145 psia (steady-state operation)
Nominal combustion chamber pressure: 120 psia

[ka9q]

Here are a couple examples.

LM DPS
Helium regulator inlet pressure:  320 to 1750 psia [22-120.7 bar]
Helium regulator outlet pressure:  245 +/- 3 psia [16.9 +/- 0.2 bar]
Nominal propellant tank ullage pressure:  235 psia (full throttle) [16.2 bar]
Propellant tank proof pressure:  360 psia [24.8 bar]
Nominal combustion chamber pressure: 103.4 psia  [7.13 bar]

LM APS
Helium regulator inlet pressure:  400 to 3500 psia [27.6 - 241 bar]
Helium regulator outlet pressure:  176-190 +/- 4 psia (depending on which regulator) [12.1 - 13.1 +/- 0.3 bar]
Nominal propellant tank ullage pressure:  184 psia [ 12.7 bar]
Propellant tank proof pressure:  333 psia [23 bar]
Injector inlet pressure:  145 psia (steady-state operation) [10 bar]
Nominal combustion chamber pressure: 120 psia [8.27 bar]

There. Now I can read it.

[ka9q]

They didn't. They use gaseous nitrous oxide, and don't get much performance from it. SS1 only had to hop up above 100 km altitude, and it appears SS2 can't even do that. There's rumors that they're looking at all-liquid systems now.

Really? How could you get anywhere near enough gaseous N₂O on board to burn the fuel? The fuel grain usually requires a liquid N₂O tank of roughly the same size to burn it.

Yes, the hype around SpaceShip One really bugged me, especially all the breastbeating about how great the private sector was. People simply don't understand that just getting to altitudes we call 'space' takes only about 4% of the energy required to stay there, i.e., achieve orbit. Nearly all of the energy in low earth orbit is kinetic, not potential.

[ka9q]

It might be a bit...optimistic to keep it as a liquid in a 51.5 bar tank.

Well, I suppose you could always cool it. Not the first time that's been done in rocketry.

The vapor pressure of N₂O is 16 bar at 242K. That's the ullage pressure Bob gives for the LM descent engine. 242K is only -31C, not that cold.

[Bob B.]

There. Now I can read it.

I just gave it to you as it appears in the NASA News Reference.

[Bob B.]

However, if we're going to use N₂O to pressurize the fuel, then fuel wouldn't necessarily have to be propane.  Perhaps something else would produce a higher specific impulse that is still relatively safe to handle.

You have a very good point there. What's the I_sp of N₂O oxidizing acrylic or PVC, two of the more popular hybrid fuels? (I think I'd prefer acrylic as it wouldn't generate HCl or possibly chlorinated hydrocarbons).

I've run a few sample computations and it looks like just about any hydrocarbon fuel is going to give a similar performance.  It looks like those with a slightly higher ratio of hydrogen to carbon are a little better (i.e. methane the best), but they all seem have specific impluses within a few seconds of each other.  Alcohols are a little worse, but still in the ballpark.  It looks like the only fuels that provide a significant increase in performance are the hydrazines, but that creates the safety issues that we were trying to avoid in the first place.

[VQ]

Silane, SiH₄ is commonly used in fabrication...

To bring the discussion full circle, Silane is hypergolic in air (pyrophoric) at STP (presumably even more readily hypergolic in pure O2), which incidentally makes pipe leaks easy to locate. Bob, what kind of performance could one get from SiH4 and the usual oxidizer suspects?

PS - Wikipedia tells me that with SiH4 as a fuel, CO2 is an oxidizer. The more you know...

[ka9q]

I tested liquid nitrous oxide (N₂O) with liquid propane (C₃H₈).  It seems to work moderately well

Hey, while you're at it, why not try cyanogen and hydrogen cyanide? Cyanogen's enthalpy of formation is +309.07 kJ/mol and HCN's is +109.9 kJ/mol. That's still less on a weight basis, but HCN boils at a convenient +26C while cyanogen boils at -21C so it would have to be kept under pressure to stay liquid at room temperature.

There is the minor problem of their affinities for cytochrome c oxidase, but hey, if you're willing to play with hydrazine and nitrogen tetroxide...

[ka9q]

To bring the discussion full circle, Silane is hypergolic in air (pyrophoric) at STP (presumably even more readily hypergolic in pure O2)...

Well, there's experimental evidence to show that it is not pyrophoric with N₂O...