Headlines > News > Flawless big hover, Propellant disposal, 7" engine tests

Flawless big hover, Propellant disposal, 7" engine tests

Published by Sigurd De Keyser on Tue Aug 3, 2004 1:31 am
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chabot imageFlawless Big Hover

We finished the assembly of the 48″ diameter vehicle on Tuesday, and it did a perfect 16 second hover with no hint of any problems. This is the very first 100% successful test of a big vehicle. Hopefully all the new isolation and shielding has banished the electronics gremlins that have plagued us. We drained out the residual propellant instead of burning it off, so when we checked the temperature of the various components, even the outsides of the insulating boxes were barely warm.

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The on-vehicle camera view is pretty boring pointing at the ground, but it is interesting to watch the water on the ground instantly vanish as the vehicle slides over it. It would be great for clearing snowy driveways. :-)

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Our vehicle loading equipment has been rearranged so that it just connects directly onto a nitrogen six-pack, instead of being a part of the test stand loading equipment on the trailer. We moved back to using the small dual-stage venturi vacuum pump, because the smaller tank won’t take as long, and it uses a lot less nitrogen. It still takes four nitrogen bottles to get it loaded, pressurized, and water washed, but that is a lot better than the nine it took on the 63” tank.

We found that it is actually easier to unscrew the landing shock absorbers (with a power drill) for tipping the vehicle up and down than to compress them down and lock them in place with the little caps we made. We are concerned about damaging the threaded bodies if we handle them a lot, so we made a custom box to hold them in when they aren’t on the vehicle.

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We wanted to do a boosted hop on Saturday, but it rained all day Thursday, so the remote site wasn’t in good enough shape. We were going to do some more hover tests at the shop to try out some code modifications for the upcoming waivered (long duration) flight, but we had a surprising incident: we were just starting to erect the vehicle up by the nose when the entire epoxy bond holding the cone adapter on popped lose.

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We were glad that it happened while the vehicle was still in the cradle instead of hanging off the ground, but it was still a shock, given how large of a bond area there was. We had heard a pop during pressurization on Tuesday, which was probably most of the bond letting go. The primary difference on this cone was that we had only prepped the aluminum with a sanding disc instead of a grinding disk, so the epoxy / flox evidently didn’t get enough bite. We pulled the cone back off, re-sanded the tank epoxy, ground deep grooves in the aluminum cone, drilled holes in it for good measure, and bonded it back on.

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We had planned on doing hover testing on Sunday after the epoxy cured, but I managed to get the flight control code pretty broken while I was working on it, and I didn’t get it fixed in time. A number of issues had been cropping up with it – I added graphing for the integrated velocity position that is used for the position hold, and found that it was reversed from the GPS position value, so I flipped that and negated the gains, but that had me wary enough that I decided to thoroughly go over it and work out all the known issues. My integrated 6DOF vehicle simulator hadn’t been working correctly since we switched to jet vanes and ECEF GPS initialization, and when I started fixing that up, I wound up making several changes in the flight control code for improved clarity. This turned into an entire day of working out all the axis transformations and integrations again. It was obviously functioning just fine before, but there were some important things I wanted to simulate before our longer boosted hops, so the rework was justified. Some subtle errors in the initialization and coast-with-GPS-failure modes were uncovered.

Everything should be ready to go now, so we expect to do one more hover test, then a boosted hop next week. After that, we are going to do an endurance hover in preparation for our waivered test flights. If the engine, vanes, or actuators are going to give us any problems after a 60+ second burn, we would rather have it happen under the lift instead of a mile up in the sky at the southwest regional spaceport.

Propellant Disposal

We drained the propellant out of the vehicle after landing to avoid cooking things in an extended ground burnoff, but there is still the question of what to do with it. We don’t like to leave the pre-mixed propellant around, and we don’t want to carry enough water to remote sites to dilute it (10:1). We added some water to it to bring the concentration down a bit, then tossed some small chunks of old catalyst into the drum with it. The result was surprisingly unexciting, and did not seem to be getting rid of the peroxide at a very rapid rate. We eventually dumped in a little bit of potassium permanganate, which did finally get it decomposing at a pretty good rate, but at the high rate of decomposition it was also kicking out droplets of liquids, which was making a mess. The permanganate also gets consumed, reacting a certain mass of peroxide, then turning into an inert brown gunk. We left the solid catalyst chunks in the drum, but it was a self-limiting reaction, because as the peroxide was decomposed the water fraction increased, which slowed the peroxide decomposition rate. It was still bubbling away a few days later, down to a concentration of a couple percent peroxide.

We decided to take one of our old engines and make a propellant burner that we would just run a hose to the vehicle vent line for. We’ll just set this a ways away from the vehicle and crack the vent valve until it starts cooking off. Without a spark igniter in it, it won’t run at high enough temperatures to completely cook everything off, but the foamy mess will be under 10% peroxide, so it should be fine to just let it run off. If we aren’t happy with the residue, we might put together a full fledged engine with an igniter and hot pack, but no nozzle.

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7” engine tests

After changing the big vehicle loading equipment, we were able to move the test stand loading equipment off of the trailer and onto a dedicated sheld:

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We replaced the spreading plate in the 7” engine with a more restricted one (948 x 0.022 holes), but it still ran extremely rough over 550 lbf or so, and was difficult to get started. We believe that the 2 x 1” thick 900 cpsi catalysts in the cold pack is too much. The current 12” engine is running very well with a single 1” 900 cpsi catalyst, and the previous engines used 2 x 600 cpsi catalysts. Since it ran ok at low throttle, we took this opportunity to try out an experiment I had been meaning to do for a while: replace the methanol mixture ( 7.5 : 1 by mass ) with an isopropal mixture ( 12 : 1 by mass ), under the theory that the methanol breaking down in the cold pack before the flameholder is the cause of the difficulty in getting the initial warmup going on some engines. We were never able to get ethanol or isopropal to run steadily before, but now that we had spark igniters it was worth another try. Didn’t work. It was just as difficult to get the warmup to catch as with methanol, but it did burn ok once it was going.

We cut the engine apart again, and we found an interesting piece of evidence: the top of the second catalyst monolith (between the two monoliths) was severely beaten up. It wasn’t melted, just crushed in places like it was hit with a small hammer. This was almost certainly the location of our combustion instability. We are going to put it back together with only a single 1” 900 cpsi monolith and test again next week.

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