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Armadillo Aerospace News: Engine alternatives, Preburner work

Published by Sigurd De Keyser on Mon Sep 13, 2004 3:11 pm
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chabot imageMatt made a video of last week’s GOX-GH2 rocket engine test:


This is a great example of the abrupt transition from subsonic to supersonic (choked) flow.

Engine Alternatives

We tried another test with the 7” engine, giving it a very short 1.5” section between the packs, and the same 1/8” perf plate flameholder that the smooth running big engine used. It still ran rough. We welded a new nozzle on the old 12” motor and ran it at rather low pressure on the test stand, and it did still run smoothly, so we haven’t imagined our previous successes. There are only two remaining different in that engine versus our new ones: It has a single 2” thick 900 cpsi catalyst on top, while the new ones have either a single 1” thick, or two 1” thick 900 cpsi noloiths. Instead of using the square-grid water jet cut support plates, it used heavy perf plates backed up by a cross of square bars in the hot pack, and a milled pie-section support plate under the cold pack.

We have been getting so frustrated with our rough running mixed-monoprop engines that we have decided to do some experimenting with other alternatives.

We would like to do some 70% peroxide / kerosene biprop tests, but we are having difficulty getting a few hundred gallons of unstabilized 70% for basic tests. We can definitely get a tank-car load, but I don’t want to commit to that without actually having some successful engine firings.

I used ISP to run the numbers for 70% peroxide / kerosene at 150 psi chamber pressure and sea level operation:

O:F mass Isp density Chamber K Exit K
10 : 0.5 156 1.24 1569 1008
10 : 0.6 165 1.23 1732 1133
10 : 0.7 172 1.23 1885 1253
10 : 0.8 179 1.22 2026 1368
10 : 0.9 185 1.21 2152 1479
10 : 1.0 189 1.21 2220 1531
10 : 1.1 187 1.20 2172 1474
10 : 1.2 186 1.20 2109 1420
10 : 1.3 184 1.19 2046 1369
10 : 1.4 183 1.19 1984 1319

10 : 1 by mass is nice and easy to remember…

For comparison, our current 50% peroxide / methanol mix is:
10 : 1.6 (lean) 146 1.11 1304
10 : 1.3 154 1.10 1434

At best, we might see a 25% performance improvement, but we aren’t likely to be able to maintain an exact mixture ratio, especially while throttling, so the total will be less. The additional tankage and system complexity would further erode the benefit, but it would still be a somewhat higher performance setup. The performance wouldn’t justify the change, but more repeatable engines would.

An exit temperature of about 1200 C is at the upper end for making jet vanes out of superalloys, but we could always go to refractories.

The other combination we are considering is LOX / methanol.

For 150 psi chamber to sea level operation:

O:F mass Isp density Chamber K Exit K
10 : 5 208 1.0 3042 2486
10 : 5.5 211 0.99 3070 2530
10 : 6 214 0.98 3087 2558
10 : 6.5 217 0.97 3096 2570
10 : 7 219 0.97 3096 2566
10 : 7.5 220 0.96 3086 2540
10 : 8 221 0.96 3067 2480
10 : 8.5 221 0.95 3037 2385
10 : 9 220 0.95 2993 2269
10 : 9.5 219 0.94 2937 2151

Going to higher alcohols, like isopropanol, would slightly improve Isp and bulk density and shift the O:F ratio higher, but cooling would get somewhat harder. Going to a kerosene would improve performance somewhat more, but you need a special grade to keep it from gunking up your cooling channels, and cooling gets still more challenging.

With LOX / methanol you wind up with two roughly equal sized tanks, which is more of a packaging problem than the big tank / tiny tank layout of 70% peroxide / kerosene (a cluster of four skinny tanks might be the most convenient layout). The exit temperature is so high that jet vanes would have to be made out of a coated refractory metal if we want them to be reusable (graphite would ablate away during our long, continuous burns). We couldn’t use our polyethylene lined fiberglass tanks for LOX. Alcohol isn’t as good of a coolant as peroxide, there is less of it, and the combustion temperatures are much higher, so you really have to push it through the cooling jacket rapidly to pull enough heat out, resulting in the need for a significantly higher fuel tank pressure, which probably doesn’t let us use our fiberglass tanks for the fuel, either.

On the upside, LOX is dirt cheap and readily available, and the combination does give performance more than high enough to compensate for the increased tank mass.

Preburner Work

Based on our need to do fairly deep throttling, we have started to do some tests on a LOX preburner, which we would use to supply hot oxygen gas to a primary cooled combustion chamber. We took delivery of two big dewars, one of liquid oxygen and one of liquid nitrogen for testing. Both support up to 500 psi pressure.

We built a new concentric gox/gh2 torch, but with an insulator around the core and various connections so we could avoid a spark plug altogether, making the spark jump between the two feed tubes. We had a couple unexpected current paths, and we had to wind up putting a plastic fitting between one of the solenoids and the torch to keep the spark current from traveling back through all the plumbing. When we got it all fixed up, it worked great. Instant push-button flamethrower.

We had been using hydrogen gas running quite rich, with equal sized orifices and pressure to both the oxygen and hydrogen. When we were doing the gox/gh2 cooled rocket test we noticed that the regulator needle buzzed around a lot while the hydrogen was flowing. I thought it might have been chamber pressure related, but we were also seeing the same thing on open air burner tests. The regulator wasn’t specifically for hydrogen, so we assumed that the low molecular weight was causing some issues with it. It didn’t seem to be causing a problem, but we had just received a cylinder of ethane to switch over to. The liquefied ethane has at least an order of magnitude more mass than an equal sized hydrogen cylinder, so we can get a lot more tests out of it.

The ethane ran smoothly through the same regulator, but we had another little surprise from it. After making a run and closing the solenoid, we saw the line pressure rise up from the 50 psi we had it set at, sometimes going up to 100 psi or more. It took us a little while to figure it out, but apparently the ethane bottle was full enough that our pulling the gas out from the top fairly rapidly was entraining some liquid ethane droplets, which would then vaporize in the closed line. It seemed to decrease in intensity as the tests went on, and I bet that once the bottle gets a quarter or more used up, it won’t happen at all any more.

The original idea was to flow the LOX (liquid nitrogen for testing) out an annular ring around the torch flame, with the intent of letting the high velocity torch entrain and vaporize the LOX.

It turns out this doesn’t work. Any reasonable flow of liquid nitrogen would promptly extinguish the torch. In retrospect, this makes perfect sense. It takes several inches of space for the torch to actually burn the gox and ethane, and if too much non-participating gas is added, combustion can’t continue.

We added a choke plate so the torch had it’s own combustion chamber, then welded another section of tube on the end to be the vaporizer for the injected cryogen.

Matt caught a nice picture of the first firing, which ejected all of our machining chips at high temperature:

I burned a thermocouple off trying to get temperature readings before the LiN flow was started, so we began letting an initial low flow of LiN go in before lighting the torch off. With a 140 psi oxygen flow through a 0.1” jet and a 50 psi ethane flow through a 0.1” jet, the full flow from the LiN dewar at 350 psi was raised to 200 C on exit. As the LiN flow was increased, the torch flame out the end would shrink down and down, until nothing but warm gas was coming out the end.

The torch chamber was getting very hot in the last inch or two before the choke plate, and it looked like we were going to burn through. We cut out the choke plate, but we then found out that an initial flow of nitrogen would prevent the torch from lighting without that amount of backflow prevention.

We realized later that the right way to do this is to have the vaporizer tube concentric around the burner chamber to keep it cool, then let the burner just exhaust into the rest of the vaporizer chamber after it has had enough private combustion space to completely burn. We will get this going on Tuesday, then attach the preburner to a flange and actually measure some thrust from the gas flow. After that, we will be ready to bolt the cooled chamber onto it and try injecting some fuel.

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