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Armadillo Aerospace News: Engine Work, Vehicle Work, Supporting Armadillo

Published by Sigurd De Keyser on Thu Dec 1, 2005 6:15 pm
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Engine Work

The 2.75” ID throatless engine with twice-around spiral cooling channels worked fine without melting, but Isp dropped from the 2” ID engines even though the cooled section of the chamber had been lengthened from 9” to 12”. The fact that it didn’t melt even without film cooling was a good sign that the spiral cooling passages were a good improvement, but the drop in Isp here, especially after seeing the huge drop in Isp on the 6” ID engine, shows that this injector design just isn’t scalable with increases in diameter.

We are pretty sure that what happens is that all the lox vaporizes to gas in the one inch long section of tube between the lox injector ring and the fuel injector ring, making a uniform fast moving gas stream for the fuel to inject into. The fuel stream can only penetrate a short distance, probably under an inch, before it is strongly swept downstream. In a 2” diameter motor this gives decent mixing, but as we get larger and larger, it leaves a huge hole in the center that is basically just pure oxygen with no fuel mixed with it, and an excessively rich mixture near the walls. In the video last month looking down the throat of the 6” ID engine you can actually see a ring of combustion fire around the outside edge and a dark center.

To avoid this problem, we need to get the propellants both injected very close to each other in axial distance. We will probably be going to top mounted injectors for the 6” and larger engines, but at the smaller sizes our igniter, pressure transducers, and engine mount assembly takes up too much room on top. I only just recently realized that I can crank the entire spindle on my mill off at an angle, which allows me to automate drilling of unlike impinging injectors around the tube. To get them to impinge very close to the wall, this does require a single welded plate between the lox and fuel injectors, which I worry a bit about. Russ makes a good, deep weld from both sides, then I mill the welds down flat and drill the injector holes. I will feel better when we go to bigger engines and an injector fabrication strategy that allows an open air gap between propellant welds or seals.

Going to an unlike impinging injector did not have the results I expected.

I was concerned that it might run rougher, since the conventional wisdom is that combustion stability is worse on unlike impinging versus like impinging, but the run was positively eerie smooth — only a single pixel jitter on the thrust curve, under 1% total roughness. However, the Isp was worse than the unlike impinging injector.

The chamber did burn through after about 15 seconds, when the fuel started boiling in the cooling channels. Watching the fuel flow meters tells you what happened in a burn through — if the fuel flow notches up when the first signs of trouble show up in the plume, then the wall burned through because enough heat wasn’t being removed from it and liquid sprayed into the chamber, so you need to increase the coolant velocity. If the fuel flow starts going down somewhat smoothly before things go to hell, the fuel is starting to boil in the cooling channels and you need to reduce the coolant velocity. We probably lost the intrinsically fuel rich layer near the wall with the better mixed propellant.

Interestingly, in the two seconds that the fuel was starting to boil before the chamber melted through, Isp climbed steadily. I have an instantaneous Isp graph, which is often interesting to look at in cases like this. Even though the mixture ratio was getting extremely lean of peak, the fact that the fuel was coming in as a gas was letting a lot more of it combust in the short chamber. At this very lean ratio, Isp came fairly close to the theoretical value.

We have reached the conclusion that an L* of 12 just doesn’t give the propellant enough time to burn and give a decent Isp, and making throatless engines with much longer tubes becomes a worse boring job and more difficult to cool. The next engine we are testing has a 2:1 area contraction rato, and will have an L* of about 20. I am also going with slightly deeper cooling channels and only a single wrap spiral to reduce the heat transfer to the propellant.

This was kind of interesting: Matt took two photos of our injector water flow test on the new engine. The one without a flash shows how it looks to the naked eye, with smooth fans, but with a flash, it shows a much more turbulent instantaneous view.


Matt has been busy with work at his day job lately, no we don’t have any nice engine test videos…

Vehicle Work

We went through several designs of ground contact sensors before settling on a spring contact threaded over our shock absorbers, but we had a little surprise when we welded studs onto the springs for electrical contact – every one of the springs broke at the weld point when we just pressed it together with our bare hands. I would have thought welding would have just annealed the spring at that point, but it made it extremely brittle instead. We rebuilt them with soldered on brass studs instead of welding, and that seems to have worked fine.


(old version, new version has the plate on the end of the shock, which comes down and touches the spring, instead of the plat on the spring which comes down to touch the shock tip)

In retrospect, we should have put a fountain tube on the bottom vehicle tank as well, so we could pull ullage gas out on the bottom without running another tube down the side of the vehicle.

While we will continue to have our independent watchdog computer that can override the main computer’s fuel valve commands to shut things down if the main computer dies, we are adding another completely independent valve that can be triggered to close by a radio signal independent of our normal telemetry stream. This will be the “big red button” that a range safety officer can control without having to go through the remote pilot. This makes us triply redundant for flight termination, and is key to our plan for hopefully being able to demonstrate some high flying and powerful rockets in front of spectators next year.

As seen in this picture, we are using a lot more sanitary clamps for our larger plumbing sizes, which have been quite nice to work with. The only downside is that we had to make aluminum versions ourselves for welding onto our tanks and pipes, because they only come in stainless. This vehicle is plumbed to flow for a > 5000 lbf engine, but we expect to be doing most of our flying next year on a smaller 2000 lbf engine.


Eight helium bottles are mounted directly to the intertank wall with custom milled brackets:


The computer plate (with wire rope vibration isolators) bolts to mounts welded directly to the lox tank: (having extra hemispheres around for fit up is a big help!)


We still have to bend all the hard line for the high pressure bottles, but everything is pretty much in place:


The vehicle has a six foot base, and the CG is about seven or eight feet off the ground:


We are still waiting on a couple parts, but it should be functional relatively soon.

Supporting Armadillo

I get asked fairly often what people can do to support Armadillo. Unfortunately, there isn’t a really great answer. The best way to support a company is to buy their products, but we don’t have any consumer products at the moment. We shut down the web store a little while ago because it wasn’t worth the hassle for a hundred dollars a month or so. It will probably come back when we do some more publicity oriented things next year, but for now there is only an indirect way.

Roughly 1% of the retail price of Id Software products provides the funding for Armadillo Aerospace. Retail stores, publishers, partners, and family are first in line, Armadillo gets what is left of my share. Buy Doom 3 or Quake 4, or Doom RPG on your cell phone, and a bolt goes on a vehicle.

The X-Prize Cup was our first serious paying customer, and we are working together to do much more impressive things next year, so I would also encourage people to attend the X-Prize Cup events in the future. They should get pretty exciting fairly soon.

We are in this for the long haul. Armadillo is operating at a sustainable level for me, and the team members (all still unpaid at the moment) are just as driven as they ever were. We continue to make steady progress, learning lessons and expanding our skill sets with each new vehicle generation. We feel that something fairly close to the current designs will soon give us capabilities that are marketable.

I’m not really interested in attracting investors, but if someone wanted to put several million dollars behind us, we could certainly talk about stepping up the pace of things.

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