Stable and Light?

guest blogger john hare

My reference on combustion stability goes into great detail on injectors controlling droplet sizes to get them to do what they supposed to.  I really couldn’t follow the reasoning much beyond that it is crucial to get the droplet sizes consistent and properly mixed. Gas-gas mixtures seem to have far less problem.

Why not have the fuel and oxidizer boil out together and mix as gasses? Inject as liquids in a container in such a way that they stay in the container as long as they are liquid, and when they boil, they come together for rapid combustion just above the liquid surface. The rapid combustion just above the liquid surface boils them faster so that the end result is very short residence time as liquid. It also eliminates the super precision injection process in favor of a very few large ports for each propellant.

 I have not seen anything exactly like this, though the vortex engines come fairly close. I’ve also read that the vortex engine had fairly low combustion efficiency with the LOX swirling in from the bottom to mix with fuel injected from the top. It seems to me that eliminating the injection manifold and most of the regenerative cooling passages makes a large L* less of a liability. Regenerative cooling would be required for the expansion nozzle and throat, after that the propellants inject at high velocity from the ends of the cooling passages. Liquid velocity will keep the liquid separate with centrifugal force from the gas until they surface impinge at the top of the chamber.

With the LOX and RP both injected at high velocity along the surface of the sphere, the LOX will tend to hug the outer wall with the RP as a thin layer on top. The LOX will cool the chamber wall and be boiled off by the warmer RP plus some radiant heat from the combustion zone. The RP will be cooled by the LOX even as it is boiled off by the chamber heat and disrupted by the GOX escaping the LOX. The gasses will mix just above the surface of the liquid and start combustion just above the mixing layer as enough heat becomes available.

The pure gas-gas injection should be considerably more stable than most classic injection systems with no droplet variation problems. With the much shorter cooling path and injector elimination, a high pressure drop is available relative to normal plumbing, which should reduce feed system instabilities. I’m told spherical chambers have less harmonic instability problems. It really seems that this should be a system that would have good cooling characteristics, and inherent stability.

Construction of the chamber should be relatively simple. A thin copper sphere to distribute heat with a composite wrap. One pass cooling from nozzle exit to just above the throat.

Combustion efficiency is a big question. Will this have decent efficiency? If not, will a large L* correct the problem? If it works at one size, does it scale? Does this layout attach to the vehicle well? It would be interesting to find out if all that precision injector work is really required. It would be nice to have all passages large enough that propellant filters are slightly less important. Of course I have a pump in mind for later.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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About johnhare

I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.
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8 Responses to Stable and Light?

  1. Paul Breed says:

    Two big problems…
    The fuel and Ox will not boil at the same rate, so it will go rich lean rich lean etc…..

    An Injecter provides negative feedback, IE if the chamber pressure increases the injectors flow less…..this has positive feedback more
    energy more boiling more energy etc…..
    Leading to an ungodly heavy chamber or Rapid unplanned dis assembly.

  2. Kerosene mixed with liquid oxygen is an explosive.

    Mixing is a serious problem in rocket engines. The basic problem is that large volumes of propellants must be mixed in a few milliseconds.

    Gas phase mixing is a problem, if at reasonable pressures, because the volume flow is large. Liquid phase mixing leads to a dense explosive. Gas/liquid mixing seems best. I think the shuttle has many annular injectors that inject liquid oxygen droplets at low velocity surrounded by a fast-moving stream of hot gaseous hydrogen. The oxygen evaporates off the surface of the droplet and mixes with the hydrogen in the turbulent wake of the droplet.

  3. john hare says:

    I hadn’t thought of the positive feedback problem, though the variable boil off seemed serious enough to think of increasing the L* to compensate. I wonder if the positive feedback could actually lead to a very short chamber. It just might require too many dollars of testing to make it worth investigating.

    I believe liquid kerosene and liquid oxygen together are well below ignition temperature and the different densities will tend to keep them separated for the few milliseconds in question. As the temperature rises, they boil off into the combustion zone before ignition.

  4. Paul Breed says:

    Mixing Liquid Lox and Liquid Keroscene in the presence of flame is “Bad” The amount of energy released and the physical destruction of hardware from a minor Lox/RP! hard start is spectacular….
    This was less than a liter of mixed propellants and it broke a bunch of grade 8 high strength bolts like they were baslawood.
    The bolts were not stripped, were fractured in the middle, this is in a pretty beefy piece of steel. See some really nice LR101 pictures by looking up ebay item 200276795091

  5. James says:

    I’ve been wondering why one of the NewSpace startups don’t try the following trick in a rocket engine:

    Seems like it should provide the same kind of improved combustion efficiency that it does in ICEs, and it would not require much more than wrapping a coil around the fuel and oxidixer input lines.


  6. Tim says:

    Rather than having the fuel and oxidiser in two layers, you might be able to have alternating channels containing fuel and oxidiser flows spiraling up the combustion chamber. That way the two liquids would be separated so they don’t form an explosive. I suspect you might get better combustion with alternating spiraling layers of fuel/oxidiser than with bubbles of GOX haphazzardly breaking through the kero. The channels would also add some structural rigidity to the chamber, but could cause a bit of a cooling challenge. Another way to picture this is as channel wall regenerative cooling with the wall facing the combustion chamber removed, and with both oxidiser and fuel flow instead of just one.

    Speculatively, I wonder if there is some benefit in injecting the fuel right at the throat, as is done on the vortex engines. This might allow you to direct some of the flow down the nozzle to both cool the nozzle and provide some thrust augmentation. This would of course create many many concerns that need to be addressed.

    I suspect that you can get around the positive feedback problem if you enrich the flow with the propellant that has the higher evaporative rate. If the chamber gets too hot, the mixture enriches and cools; if it get too cool, the mixture goes towards stochiometric and heats up. I suspect that increasing chamber pressure will reduce the rate of evaporation to provide a bit of negative feedback as well, but my thermodynamics is a bit hazy in this area.

    One final tangent, how insulative is RP-1? An old idea I had was to wrap the kero tank around the LOX tank. This gives you less surface area to insulate (if any), and a less sticklike LV. Problems are heat transfer from the LOX to the kero (you might get around this by prechilling the kero), and probably higher tank weight.

  7. john hare says:

    I’m trying to think my way around the problem you pointed out.

    You have given me a few ideas to check on.

    As far as I can tell, that concept is only useful on vehicles with really bad injector design. It isn’t really useful on newer vehicles with better designs. The flow rate on rockets is such that the device wouldn’t have time to do it’s trick.

    Several contracts activated a couple of weeks ago which put me to the six day week of pushing instead of the short weeks of the last several months. These idea posts will be infrequent as long as the push is on. Happy holidays all.

  8. Martin Chiaverini says:

    Your comment about low combustion efficiency is incorrect. I worked on this engine for many years, and it achieved high effiencies and very stable combustion.

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