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Engine development, Methane work

Move completed

June 11, 2008 notes:


We are working hard on the Rocket Racer program and making good progress, but the Rocket Racing League has requested that we allow them to manage the public dissemination of information about the project, so I won’t be discussing it here. Since that has been a large part of our work lately, this update is a bit light.


We are finally completely moved in at our new location. Getting our three-phase power hooked up took a lot longer than expected, but we are finally fully operational – parts have been milled, turned, and welded. It will be another month or two until everything finds its proper place here, but things should start moving along at an accelerated pace now.


Engine Development


We have run over ten thousand pounds of propellant through the test sled this month, and I think everything is at essentially final form.


I had known for a while that the alignment of the angle table we use for drilling the injector elements wasn’t quite right – we had noticed that the elements weren’t perfectly impinging, but things still seemed to be working ok. When I finally got around to dialing it in very precisely and made another injector with it, we got a pleasant surprise – performance jumped significantly, and interestingly, the engine actually ran significantly cooler! Previous engines must have had stray oxygen burning with the film cooling fuel at the wall, while the new engine gets all the oxygen properly burned with fuel from the elements before it has a chance to get to the film cooling.


We accidentally did an interesting test with the new engine. We were trying out different regulators for the startup / shutdown purge gas, but we wound up removing the regulator on the fuel side without changing the code to only pulse the purge solenoid, resulting in a continuous, large amount of helium being dumped into the fuel manifold while the engine was running. Surprisingly, it still ran perfectly smoothly, but thrust was down a bit, and the exhaust bell started glowing red hot after ten seconds of firing. We ran it a few times, and eventually eroded a little bit of metal out of the exhaust bell when we were seeing if it was a steady-state red hot or not (not quite). We were very puzzled why the engine had just changed performance and heating dramatically until we realized the issue with the purge solenoid. Upon analysis, we found that the helium had displaced over a third of the fuel, resulting in the engine running right around stoichemetric mixture ratio with reduced film cooling. The fact that the engine could almost run steady state under those conditions was extremely encouraging. We could certainly lean the properly fueled engine out a fair amount with no consequences at all.


We had been chasing some igniter problems for a while now, where the igniter would light properly, but abruptly flame out after a few hundred milliseconds when operated at feed pressures below 200 psi. We had never run into that before, because our vehicles never had to do restarts after the initial pressurization. Because we eventually intend to shutdown the engines and relight for landing after high altitude VTVL flights, this was obviously something that had to be solved eventually. Our ignition interlocks handled the flameouts properly (important point: don’t assume that an igniter will stay lit, a single point check for pressure is not sufficient!) every time, but it was frustrating. It looked like it was related to the flow inrush when the main propellant valves started opening, so we spend several tests moving around the igniter propellant pickup points and adding accumulators, but we eventually fixed it by changing the igniter injection geometry. We now light reliably down to 100 psi (and probably lower), but our spark plug is taking a lot more heat with the new geometry, so the last thing we are probably going to change is to move that a little farther away.


At this point, I can say that the decision to go to the film cooled engines was absolutely the right thing to do. We are improved on every metric from the cooled graphite engines of last year.


One other interesting point that all the restarts in our testing lately showed up: normally, we don’t really have to worry about our lox heating up too much in the test stand tanks unless we take twenty or thirty minutes to fix something, but the situation changes a lot if you pause when a spherical tank is nearly empty. The high surface area to volume ratio of a shallow puddle at the bottom of a sphere sucks heat up surprisingly quickly. We had a case where we were paused before the last ten second run on a tank, and the lox managed to heat up enough to self-pressurize beyond the 125 psi that the tanks were sitting at from the end of the blowdown run. This generally isn’t a real problem, because the mixture ratio just gets richer as the lox gets “fluffy”, but we try to avoid it.


Methane Work


We tried to do another long, high thrust methane run this month, but it didn’t work out well. There were several loud bangs as it started up, then the engine chamber peeled itself off the injector. Upon inspection, it turned out that the weld penetration wasn’t very good, and it was a purely pressure related failure, rather than a burn through, but the engine clearly wasn’t running right. The methane engine that fired ok two months ago had just a single ring of unlike doublets, but the last two ones we have had problems with have been injectors with two rings of split-triplet elements. Our current belief is that the separate ring manifolds are causing problems during startup as the methane chills first the center, then the middle, then finally the outer ring. The lox travels around in one manifold on the top, feeding all the elements at about the same time. This means that the inner ring starts at about the right O/F ratio, but the outer ring is extremely lean, and the film cooling holes just get gas for some time. The outer ring may well have been too lean to ignite cleanly and promptly, resulting in the bangs.


It is so nice to be testing at our shop again – we were able to clean everything up from the somewhat destructive failed methane engine test and perform two more (successful…) alcohol test firings later that same day.


We currently dump methane from the plumbing in front of the main fuel valve to make sure we have liquid up to that point, but we are going to try dumping some methane through the engine next time to pre-chill the manifolds before starting. We may yet go back to a single ring of elements for methane, but we had experienced injector face heating problems with that, and also generally lower performance. It is possible that with a 310 alloy face, possibly with a coating, and the proper alignment on the holes, it may not be a problem now.



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