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Armadillo Aerospace News: Engine Tests

Published by Sigurd De Keyser on Mon Aug 30, 2004 8:36 am
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chabot imageMatt was out of town all week, getting trained on and picking up his new Cirrus SR20, so no photos this time.

Engine Tests

We are still having chugging problems with the 7” engine at full throttle. We have tried a great many things that made no difference at all (still good data points), and a couple things that made a difference, but still didn’t fix it. We aren’t going to finish the new 12” engine until we definitively fix this on the 7” engine. The worry is that the catalyst monoliths we have on hand are not exactly the same as the ones that were in the known good engine configurations. If we have to order more, that would be a five week lead. We do have another 12” monolith exactly like the one in the engine that worked well, but again, I really want to get the 7” engine working properly first.

All the configurations we have tried:

Two 1” thick 900 cpsi monoliths with 1676 x 0.022” spreading plate
Two 1” thick 900 cpsi monoliths with 940 x 0.022” spreading plate
One 1” thick 900 cpsi monoliths with 940 x 0.022” spreading plate
One 1” thick 900 cpsi monoliths with 584 x 0.022” spreading plate

50% open area perf plate under the cold support plate and spark plug.
25% open area plate under the cold support plate.
Choke plate with only 8 x 0.5” holes, underneath spark plug.

Hot pack with single layer of 1000 grams, compressed to 4000 psi gauge.
Hot pack with two layers of 500 grams each, compressed to 3300 psi gauge
Hot pack with three layers of 250 grams each, uncompressed

Optional 10 extra 20 mesh screens below monolith.
Domed or flat top above spreading plate.
More or less volume above spreading plate.
With or without support pegs between the spreading plate and top.

0.25” Cavitating venturi
0.3” Cavitating venturi
1” valve
½” valve

I gathered all of our notes and data into one place and reviewed them again tonight.

The initial runs with two monoliths were stable at much higher thrust levels that all the configurations with only a single monolith.

Something I hadn’t noticed before was that the double monolith runs were also over 100C hotter than the later runs. That might have something to do with the fact that it ran at a higher pressure, but it does look like I had runs that were at stable temperatures both ways.

The lighter, segmented, and uncompressed hot pack did have about 10% less pressure drop than the single layer compressed pack, and it was able to run slightly higher thrust without instability.

These support our theory that the pressure drop needs to be higher through the cold pack than the hot pack, but we are having difficulty getting more drop in the cold pack. Adding ten screens underneath the monolith didn’t give any measurable increase in the pressure drop, rather to our surprise. Neither did adding a 25% open area perf plate. With a good amount of the flow from the cold pack still being liquid, the velocities must be quite low. The choke plate did increase the pressure drop a lot, but it didn’t move the stability point, probably because it was below the spark plug, and the pressure drop needs to be before any combustion area.

The temperature result does imply that the double thick monolith is still catalyzing more peroxide than the single monolith, which should make the pressure drop a lot greater than simply doubling the drop of a single monolith.

Our next test will be rebuilding the top with a double monolith and adding even more screens below it, which should push the top pressure drop higher than the hot pack. We might even try three monoliths, with the thinking that we might as well put active elements in instead of inert screens, but we sometimes have difficulty getting the flameholder started on engines with very strong cold packs, because the methanol starts catalytically breaking down before it gets a chance to burn.

In other work, Russ and Tommy have been working on custom board layouts for our new electronics systems. The new boards will be much smaller than our old electronics systems, and won’t have any of the on-board cabling we used to have. There will be short ribbon cables coming down from the PC104 boards, but that will be about it.

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