News Archive

X-Prize Cup, LOX Engine Work

Cotronics flexible epoxy

April 18, 2005 notes

X-Prize Cup

We have committed to doing demo flights at the X-Prize cup event in October of this year. While they would have been fine with us just dusting off our 2’ diameter mixed-monoprop vehicle (the “flying crayon”), we are going to use this as a firm deadline for having the new LOX vehicle in robust flying shape. We will be aiming to do several back to back 15 second boosted hops to demonstrate operational efficiency. Flying on a biprop, this should be quite exciting.

It looks like we are going to use our last 15 degree crush cone as the aeroshell for the little vehicle, which will make it look like a little SSTO model. We would be a lot more aerodynamic packing it into a 2’ diameter cylinder, but I want to give it a wider base for better landing stability.

LOX Engine Work

We have been doing lots of work in many different areas.

We finished a next-generation lox preburner and engine mount that is much more compact and easy to inspect. This burner can attach either directly to the test stand, or to our gimbal mount. It also uses a larger methanol injector, http://www.bete.com/products/pages/cw.htm CW25, because we had to throttle very low to get autoignition temperatures with the smaller nozzle. This nozzle is actually a bit too large, so we had to add a restrictor jet in the plumbing to keep the gox temperatures reasonable even at full throttle.

Our measured Isp was quite low with the previous gox injector plate. At only around 300 pounds of thrust, the flow was a long way from sonic, and we weren’t getting high quality combustion. I made a new plate that has smaller holes that come down closer to the horizontal fuel injection points, and also include a splash plate backup, so any fuel that isn’t atomized by the gox should splash back into the stream. This seems to work well, giving us a measured 188 Isp. We don’t expect to do too much better than this until we make a longer chamber, because an L* of only 34 is almost certainly on the short side, even for gas / liquid injection.

Having the preburner, injector, and chamber all interchangeable at the flange joint is really convenient, especially since we are still having trouble with the chambers.

We still don’t have a satisfactory solution for our test stand blast deflector. A half-inch thick stainless steel plate was fine for 10 seconds or so, but on a 30 second test run we cut completely through it (and almost cut off the test stand uprights). We tried making an aluminum deflector that was cooled by a water hose, but that burned through rather quickly. We tried “extra high temperature fire bricks”, McMaster 9355K1, but they were mostly burned through at the end of a 30 second run. Flying molten steel and firebrick also have a tendency to start grass fires nearby. We are going to try the “5000 degree cement” next, but we might have to go to a water cooled deflector supplied with high pressure water.

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The flight computer now runs the test stand, controlling the burner solenoid, spark coil, purge valve, lox valve, and methanol valve. At the moment we are still logging pressures, thrust, and flow rate on the separate data acquisition system, but we intend to move all that to the flight computer (we won’t fly with the load cell and flow meters). At that point, the test stand will be completely wireless, which will be convenient. One thing we found while investigating low throttle operation was that there was an over 10% difference in the initial cracking points on our fuel and oxidizer valves.

We have our gimbal system almost done. We are using Electrak 12v linear actuators, but we had to swap out the motors, because the factory motors had very conservative thermal cutouts that shut down after only a few seconds of the back-and-forth hunting that the gimbal is required to do.

Our biggest issue is that we continue to burn through our regeneratively cooled chambers.

We made two chambers with normal epoxy for the saddle fill, but we did get the Cotronics flexible epoxy in, which we used for the next chamber. Neat stuff. All of the epoxy filled chambers had some apparently low flowing cooling channels when we blew compressed air through them. We originally attributed this to possibly not getting all the wax out, but expected that when it got hot it would self clean. After cutting some of the engines apart, we found that epoxy had been able to get into some of the channels that apparently weren’t completely filled with wax. We tried using adhesive Teflon tape to cover the channels before epoxy filling, but it wasn’t sticking well enough to give us much confidence. A pretty interesting attempt was made at using heat shrink tubing to cover everything, which looked good, but we could only find a polyolefin based tube in the dimensions we needed, and that probably isn’t high enough temperature. A Teflon tube would probably work well, but I can’t find anything at that diameter with a large enough shrink ratio.

We finally decided to just make a metal outer jacket for the engine, but it had to be done by hand (many more hours of work), and it wasn’t a completely perfect fit. We eventually burned through this engine as well, but this is probably the right direction to follow.

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We know our coolant velocity is too low, only about 13 ft/sec (not even counting the slop in the outer jacket). We were originally working on engines 2x – 3x this size with the same cooling passages, which would have been more reasonable. I’m going to go to cut the passage width in half for the next chamber. My mill’s rotary chuck has a large amount of runout, so I am hesitant to make the channels much shallower, which would amplify the variability.

Based on comments in Clark’s book “Ignition!”, we tried mixing silicone oil into the methanol to form internal chamber coatings, but it came out of solution very rapidly. We have some ethyl silicate on order, which has been reported to work with methanol.

I am getting some larger, accurately CNC machined C145 copper chambers and jackets quoted, which will probably be the real solution, but we are going to try one more aluminum chamber before Space Access.

Lots of unsorted photos:

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