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Armadillo Aerospace News: Graphite nozzles, Vehicle work

Published by Sigurd De Keyser on Wed Jan 4, 2006 7:31 pm
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Armadillo AerospaceGraphite Nozzles

Since we have determined that the side-injecting throatless engines do not scale up well, we wanted to build some test engines with converging nozzles (2:1 area contraction ratios for now). I started working on a new design for a cooled aluminum nozzle, but we decided that the easiest thing to do for testing would be to try some graphite nozzles, because we could swap different ones out on the same cooled engine tube.

I ordered “superfine isomolded” graphite rods from http://www.graphitestore.com/, and I also purchased some of the zirconium oxide and silicon carbide based coatings to try out on the graphite. I was pretty surprised to find that even high end graphite is cheaper than the phenolic rods we had been using for the ablative engines. Machining the graphite wasn’t as big of a mess as I had feared. Russ and I just made sure someone else held the shop vac nozzle as near the cutting point as possible.

Our first engine design used two radial seal o-rings on the outside of the graphite nozzle, submerged in the cooled aluminum engine tube and retained by an internal snap ring. This wasn’t a good design for several reasons: It required more machining work on the graphite (which are expected to be disposable at some level) for a precise OD and o-ring grooves, you can’t pull the nozzle out past the snap ring groove without tearing the o-rings (and we had problems tearing the o-rings putting it in until we completely smoothed out that side of the snap ring groove), the snap ring ears stick in a fair amount and limit the expansion ratio you can put on the nozzle exit cone, and, most importantly, the o-rings just won’t last very long against the hot graphite.

We applied the zirconium oxide coating to the machined graphite nozzles, but we didn’t have a way to accurately give it the two hour / 200 degree cure, which may have effected how well it stuck. We did a couple test firings, and preliminary results were encouraging. Our Isp was improved a decent amount with the contraction ratio / larger L*, but the o-rings cooked off and allowed some leakage. After a total of about 40 seconds of firing over a 3:1 throttle range, we pulled it apart to examine it. The coating was completely gone everywhere. The throat had only eroded by about 0.050” on each side, but there was some significant erosion along the sides, probably due to the burning of the o-rings and resulting leakage.

We decided to modify the engine to a different configuration, essentially hanging the graphite nozzle below the engine just like we held the ablative chambers, and using a ceramic face seal at the top instead of o-rings. I was looking for a fine felt mat style ceramic gasket material, so I was a bit disappointed on ordering McMaster part number 1687T21 and finding it to be a fairly coarse cloth weave, but we cut rings out of it to insulate and seal the engine flange and to insulate the retaining flange. I want to try machining a retaining ring directly from ceramic in the future, but the insulated aluminum flange seems to work for our current testing.

I finally bought a lab oven for the shop. We have wanted one for various things for a long time, but properly curing coatings was finally a good enough reason for me to buy one. We made a new graphite nozzle and did the proper cure cycle on the coating.

This design seemed to work much better, giving us higher Isp and thrust, with no visible leakage, but we only got to fire it for about four seconds before we had a little mishap with our test stand. We are building a new test stand with larger tanks to better deal with these higher thrust motors, and will continue testing of this engine soon. After the short burn, the coating still looked good except in one place on the top of the converging section where it had flaked off. The gasket seemed to have sealed perfectly, with no signs of leakage.

This engine is making around 1000 lbf, which would be good for a boosted hop with the X-Prize Cup vehicle, or hover tests with the big vehicle, but we already have all the pieces on hand to put together a 2000+ lbf engine, which is what we intend to do most of the big vehicle work with

When we are happy with our thrust and Isp, I will go back to trying to make a completely regeneratively cooled motor that can run indefinitely.


Vehicle Work

The big vehicle has had all the plumbing completed, and has been fully leak tested. The only thing left to do before flight testing is a bit more wiring, getting higher power DC motors installed in the gimbal linear actuators, and putting a well-tested engine underneath it.

This vehicle has used hard line tubing for everything except the hoses going to the gimbaled engine, which is a first for us. Previously we have used flexible hoses for convenience, but the hard lines are lighter, cheaper, don’t require an anti-chafe sleeve, are probably more robust, and we can make high quality tubes of any length and bend in the shop. Very early on we did our own braided hose assembly, but it always involved a lot of holes poked in fingers and our leak rate was far from perfect, so I moved to ordering factory assembled hoses from McMaster. There service is good, so I could always have hoses in hand by the next shop day, but we have been planning to move to hard line for a while now. Phil is Chief Tubing Bender.

We already have a couple things noted to do differently on the next vehicle –

For some reason, I had always thought quick disconnect hose couplings had fairly low pressure ratings, but it turns out that there are plenty of them available with 3000+ psi ratings. Our current vehicles have had an AN fitting backed up by a valve for filling the high pressure tanks, but I definitely want to replace that with straight through quick connect couplings next time, to avoid the need for a wrench and the chance of messing up a threaded connector. I still want to use a straight-through connector backed up by a valve instead of one with internal shutoff valves, because it allows us to vent the high pressure tanks from the filling port if necessary. If we really wanted to save the weight of the valves, we could use screw-on shutoff couplings that can be connected and disconnected under pressure, but that would be less convenient to use.

We had discussed capping and pressurizing the structural cross members in the vehicle that the high pressure manifolds and regulators are mounted to, and actually using the cross member as a low-pressure manifold, but we decided not to. We should have. The side pipes taking ullage down the sides of the vehicle for vent valves, roll control, burst disk, engine purge, and pressure gauges would have mounted much cleaner if they could have just punched a hole in the side of the cross members and been welded in place.

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