One of the topics I like to blog about is promising new space technologies, especially those that relate to reusable launch vehicles and space transportation infrastructure. Clark Lindsey recently linked to the website for the Space Propulsion Group, where a friend and former intern (Jonny Dyer) now works. They mostly do hybrid propulsion systems, but while I’m not personally more interested in all-liquid systems, they did have an interesting oxidizer concept that I figured merited a blog post.
Nytrox for Vapor Pressurization Systems
This concept seems aimed at overcoming one of the key tradeoffs of VaPak systems–you typically can choose either high propellant density or high vapor pressure, Nytrox gives you both. The concept is pretty simple (here’s a link to the patent, which includes a lot of useful charts). Nitrous at room temperature is a high vapor pressure, low density liquid. But if you chill it down into the -40 to -80C range, it’s vapor pressure drops to almost nothing, and it’s density increases till it’s higher than LOX. If you then dissolve in a bunch of oxygen gas, you can get the vapor pressure back up to where you want it with very little drop in density. You can easily have vapor pressures in the 400-1000psi range while still getting near LOX densities, at non-cryo temperatures.
Other interesting properties of this mix:
- Isp and optimal mixture ratios tend to be between that of nitrous and that of LOX
- The vapor phase tends to be very oxygen rich (greater than 50%) even if the liquid only has 5-10% oxygen by mass
- The fact that oxygen comes out preferentially in the vapor acts as a stabilizer against vapor phase detonation events. Unlike nitrous, oxygen won’t exothermically decompose if heated, so it acts like a heat sponge. Theoretically you need 10,000x as much energy to initiate a detonation in a oxygen/nitrous vapor compared to pure nitrous.
- If you keep the mixture above ~-60C, it’s warm enough that you can use standard o-rings, valves, regulators, as well as bog-standard composite tanks.
- If a Nytrox mix warms, oxygen will boil out (raising the pressure), but if you cool it, it will dissolve back in.
The main drawback is that at moderate pressures and temperatures (ie -40 to -60C and pressures under 800ish psi), you’ve only got a tiny amount of oxygen dissolved in the nitrous (about 5-10% by mass), so your performance as a biprop is fairly “meh” when compared to using pure oxygen as the oxidizer. Nitrous is also a lot more expensive than LOX, and any time you have to make your own propellant, even when it’s just chilling something down and pumping in some gas, it’s going to cost a lot more than something you can just call up praxair and order.
One Fizzy Propellant Deserves Another
As I was thinking about Nytrox, I realized that you could do the same thing on the fuel side (in fact the idea of fizzy propellants was discussed years ago on the sci.space.* newsgroups). It turns out that propane also loses almost all of its vapor pressure right in the same temperature range (about -40C), and you could dissolve methane into it as the vapor pressurant. Density won’t be as good as sub-cooled propane, but now you have a second non-cryogenic vapor pressurized, moderate density propellant. Note, this also gets around the key drawback of propane for VaPak systems that AirLaunch/Protoflight found–with pure propane, in order to get the vapor pressure to a good point, you actually end up heating the fluid. It’s much easier to let the fluid heat itself up, and use a vent to keep the pressure from building up too far. In fact, Gary indicated that they had actually looked at just that option, but hadn’t had the chance to try it.
Combining the two, the overall performance and bulk density is actually almost identical to the equal-volumes LOX/IPA bipropellant combination I’ve worked with at Masten. The Isp is two or three seconds lower (my combustion analysis software only allows me to input three fluids, so I don’t know for sure how much benefit having the methane in it gives you), but the density is a little bit higher, so the density Isp ends up being a tiny bit better. But you get the following benefits:
- Composite tanks for both propellants. We can currently get custom carbon fiber composite tanks cheaper than we can get aluminum tanks made, and they tend to weigh about 20% as much, even without going to advanced fibers.
- Get rid of all helium on the vehicle.
- You might be able to get rid of all inert gasses on the vehicle, if you can use the ullage vapors for anything pneumatic on-board. Or at worst you just need a small pony bottle.
- The ullage regions are mostly oxygen and methane respectively, which should make a gas-gas RCS engine a lot easier (since you know you’re getting a flammable vapor instead of a liquid).
- No need for cryo valves or cryo seals, which opens you up to a lot more possibilities on the valves side (low-cost butterfly valves and poppet valves for instance).
- Boiloff is a much smaller issue.
Fizzy Propellants for Air Launched Vehicles
All told though, I’m not sure if the benefits outweigh the costs for a typical ground-launched system. Where they really shine though is for air-launched systems. For air launch, one of the key headaches when using a cryogenic propellant is keeping it from all boiling off as you fly off to your launch point. But with a propellant that is only moderately chilled, it becomes a lot easier to just subcool it a little bit, and let it warm up as you fly. Actually, depending on what temperature you’re using, the outside air at 30-50,000ft is actually cold enough that the liquid might be able to fly indefinitely at that altitude without boiloff issues. Also, since both propellants are warm enough, it might be possible to do in-air refueling, if that’s what floats your boat.
Anyhow, that’s the concept. For the Nytrox stuff, SPG has a patent application in, so you’d need to license propellant production from them if the patent is awarded. But for the fuel side, the idea has been out in the public domain for years, so there’s nothing preventing someone from dabbling in that now. Heck, I might even try some.
For more information, I’d check out the SPG site linked to above.
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