Chilled “Fizzy” Propellants for Vapor Pressurized Rockets

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* 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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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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20 Responses to Chilled “Fizzy” Propellants for Vapor Pressurized Rockets

  1. I was wondering what “vapor fed” was all about.

    Thanks, Jon.

    Hope Gary Hudson provides some feedback.

  2. Gary C Hudson says:

    I think there is merit to blends of propellants to tailor vapor pressure, and as Jon says we did a fair amount of analysis on the fuel (vs. the oxidizer) side of the equation. I don’t think there is that much advantage to warmer propellants for air-launching; boil-off was never a limiting ore rally complicating factor in our CONOPS. I think blended propellants would be of more interest for long term in space storage, myself.

  3. Gary C Hudson says:

    Sorry, bad typing.

    “limiting ore rally complicating” was supposed to be “limiting or really complicating”

  4. Gary,
    I was thinking more for external-carry air launching. I’ve always been more a fan of that method than internal carry. Once you’re external, I would imagine the boiloff would be more of an issue, since the rocket is out in the fast flowing air.


  5. Gary C Hudson says:

    So was I. Internal is really good only for small launchers. Once you head to larger sizes, normal insulation works well for external carry of multi-hour duration.

  6. Gary,
    Interesting, I had always heard otherwise (but have never personally done anything air-launch, so it’s all second hand). You still need to have some extra propellant though, correct? Because you still get some boiloff, right? Or was it because you were doing VaPak and actually wanted the pressure to build a bit?


  7. Tim says:

    Is the fourth interesting property supposed to say ~-60C instead of ~60C?

  8. Tim,
    Good Catch.


  9. Gary C Hudson says:

    By saying “internal is only good for small launchers'” I was referring to the fact that you can only carry a small vehicle via internal carriage. The C-17 as an example maxs out at about 90K lbs with our Gravity Air Launch patent approach. It is obviously much better for insulation as far as wind is concerned, but since you want to keep the bay warm for people, external carry is actually better as it is around minus 50 F out there. External carry also allows one to build much larger vehicles with better surface to volume ratios.

  10. Eric Collins says:

    Just a couple of quick questions.

    How well has the chemical and thermodynamic properties of Nytrox been characterized?

    If you were to capture the ullage gases for use in a gas-gas RCS, how would the plumbing compare to that of the helium systems (and conventional RCS) that you are now able to do away with?

    Does the fact that you can now use composite tanks give you any other significant benefits, like say the ability to tailor the geometry of the tanks to your airframe (i.e. to take advantage of the black aluminum)?

  11. Eric,
    Taking you questions in order:
    1-Not sure. I imagine SPG has been doing a fair amount of experimentation, but I don’t know how thorough they’ve been so far.

    2-I would think it would be pretty simple. You’d have some similar parts, but there would be many you could leave out. My guess is that you’d only really need a prevalve, and a relief valve per branch, and then the on-engine solenoids. Compare that with accumulators, regulators, gas lines, valves, filters, pressurant fill plumbing, etc…

    3-Yes, composites give you a lot more freedom in shaping your vehicle. Typically, for shapes that can be fillament wound, there is little to no penalty for taking the shape and aspect ratio you want. Admittedly, for the use I was thinking of, mostly plain-jane cylinders with elliptical heads would be fine for the most part. But if you needed to do say a conical tank, composites would make it easier to do so.


  12. Tim says:

    Given that the gas phase of contains a greater amount of oxygen than the liquid phase (and I assume something similar would happen with a propane/methane mix), won’t the oxygen be used up before the nitrous? Would this mean you would lose vapour pressure as the tanks empty? If the pressurant fuel/oxidiser does vary and isn’t completely used up by the time you want to use the RCS, I would imagine the variable (and proably uncertain) propellant mixture would create some tricky control issues.

  13. Tim,
    As with all Vapak systems, as the propellant is forced out of the tank, a small bit of it vaporizes, keeping the vapor pressure nearly constant. In this case, most of the vapor boiling off will be oxygen. So yes, towards the end of the run, the liquid flowing out of the tank will have less and less oxygen, but the vapor left in the tank will have more and more oxygen (which keeps the pressure up).

    I’m not sure, but I think that the vapor composition actually ends up being relatively constant. But I could be wrong. It definitely will be a mix of vapors. You’d need to run a subscale test a few times to verify how consistent it would be, but my understanding is, it should be consistent.


  14. Randy Campbell says:

    Jon, and all ;

    SPG caught my interest a bit ago as the are the company that is commercializing the paraffin hybrid booster technology. I suspect this is going to fold into that concept. I’d originally heard of the concept they are working with from Dr. Bruc Dunn’s site:

    Under “Low Cost Rockets” it has the information on his initial work on the concept:

    While he was working on the assumption of upper-stage engines, for the longest time (since I couldn’t find any actual information on the process :o) I was under that assumption that VPAK WAS this concept! Now that VPAK has been explained to me in simple terms, (by the Jon… just an FYI but some of us with ADOS or ADD can’t STAND twitter so I don’t suspect I’ll visit much, though I recall my wife said the same thing about Facebook and she’s got a page there now so….) I see the differences and this concept looks very interesting. Both for hybrid oxidizers and full-up liquid propellants. Thanks for point it out.
    (Oh and as to Eric question on the vapor pressure the Dunn page details how that works very well :o)


  15. Dan Moser says:

    Nytrox seems like an innovative VaPak choice. In the 60s Aerojet reportedly had some success with N2H4/NH3 fuel and N2O4/NO oxidizer blends, some seriously nasty stuff.. a classic trade off between higher performance cryogens and moderate performance mild cryogens. CH4/H2 “Mythrogen” fuel blends take that to an extreme. I used C3H8/C2H6 mixtures with H2O2 on the Comp-L system tests years ago. The blended fuel feed system worked fine, but we ended up going to all ethane anyway. You might want to investigate propylene or Dimethyl ether fuels.

  16. Bruno Berger says:

    We did that once a while back. The idea wasn’t to get a high density propellant, the idea was to get a subcooled oxidizer which isn’t at it’s saturated point and therefore doesn’t form bubbles at the slightest pressure drop (preventing “water hammering” effects). This was at ambient temperature (this was about 10 °C at that time) and pressurized it with nitrogen and later with oxygen to about 60 bar. As a nice side effect (We didn’t knew that then) SPG showed with their great research work, that diluting the vapor phase with something else like helium, nitrogen or oxygen makes it more or less immune to vapor phase detonation.

    I think cooling it down to -80 °C makes it very unhandy. Imagine you have your rocket or VTOL vehicle sitting on the pad and the sun shines on it. With the cheap LOX you just vent it and so it keeps its temperature. You can’t do that with Nytrox because you will loose the O2 fist (without much cooling effect because the N2O is still not at its vapor pressure). Cooling down to -80 °C isn’t something you can do with a “fridge-like” aperture (maybe I am wrong, what is the lowest temp you can reach with a compressor fridge and a suitable coolant?).
    But I see a nice application for planetary missions (GLXP for example) where you probably can sustain the temperature without much effort, combined with blended fuel like propane/methane or ethane/methane.
    Just a hint about o-rings: Viton (and other brands) is chemical compatible, but not physically… it swells when exposed to N2O for a longer period. Our sealant supplier recommended silicone o-rings to us. The chemical compatibility isn’t perfect though.


  17. anon says:

    “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.”

    Do you have test data on this, or was this a chemical sim?

  18. Jonathan Goff Jonathan Goff says:

    Yeah, I wouldn’t take it all the way down to -80C. -60C at the most, and -40C would be better. Fluorosilicone gives you most of the benefits of silicone while being oxidizer compatible…

    Anyhow, it’d be interesting to try the stuff.


  19. Dan Moser says:

    Bruno hit on a key issue: pressure drops in plumbing cause 2-phase flow which can cause serious flow rate fluctuations. Truax told me once that having one propellant in a near-saturated state should be OK, with proper precautions taken, but that having both propellants saturated causes flow stability problems. He didn’t elaborate or provide back up data, so take it for what it’s worth. My current system concepts, AVOx and ECPS, both attempt to use vapor pressurization with heat exchange between fluids so as to feed them through the plumbing & injector at sub-saturated conditions.

  20. Jonathan Goff Jonathan Goff says:

    Yeah, having never tried a system like that for real, I don’t know what all the complexities end up being. That said, protoflight had both propellants at saturation I think, and they had several runs whose pressure traces were flat enough to eat off of. Not sure what they did to deal with those potential instability issues. I do know they used a pintle-injector, but that wouldn’t solve feed-system instabilities…


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