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.