March 11, 2005

 

Broken Vehicle

 

We did a lot of vehicle tests in the past few weeks, but we ended up making a mess.

 

Since we were still having problems with the GPS cutting out due to vibration even after cushioning it as much as we could in the electronics box, we moved the GPS unit from the electronics box to the nosecone, and gave it triple isolation: lead acoustic insulation directly against the board, wrapped in foam, then attached to compact wire rope isolators.

 

We also added new circuits to monitor the regulated voltages coming from the DC/DC converters, because we had seen some suspicious behavior in one of the previous tests.

 

We tried a new warmup procedure to see if we could get the cloudy engine to run right – we left the nitrogen bleed open during pressurization to hopefully keep any drip leaks from igniting before we wanted them to. When we were ready to go, the engine come up to temperature (at the thermocouple, at least) fine, but it was still running very cloudy. We went ahead and hovered it, even though we could barely see the tip of the nose cone through the clouds, and the GPS did perform fine.

 

To try to fix the engine, we cut off the top to replace the cold catalyst. There was evidence that the spreading plate had been pushed up quite hard at some point, either during the rough running before we activated the bleed, or during one of the ignition pops during pressurization. The top of the catalyst was still pretty active, but the bottom was dead, and the lower inch of the two inch catalyst was shriveled quite a bit. When we replaced it, we also tried something new – we caulked around the edges with RTV. The cold pack shouldn’t get abover 250 F or so, so RTV should be fine, and preventing any leaks around the side should be an improvement. Because the engine was welded to the vehicle base framework, we weren’t able to braze the spreading plate back on under the hydraulic press, so we just put a heavy weight on it for the initial tacks. We also replaced the throttle valve with a brand new one to fix the leak.

 

We were quite disappointed to find that the engine still ran fully cloudy on the next test. I didn’t bother lifting it off for another hover, we just burned off the propellant load on the ground.

 

Process of elimination left the hot pack catalyst, so we plasma cut a hole in the side of the engine and shook all the catalyst rings out, then poured new ones in and closed the engine back up. Again, we weren’t able to put any pressure on the catalyst.

 

On the next test, the engine cleared up very rapidly, conclusively showing that the increased cloudiness of the engines was the wearing out of the hot pack catalyst.

 

When it throttled up for liftoff, the engine started chugging badly, which messes up the hovering auto-throttle, causing it to continue rising slowly instead of hovering or descending. I had to kill the engine when the nose started climbing past the top of the lift. When the vehicle fell down, we were extremely disappointed to see the recovery strap break, rather than stretching to absorb the fall, so the vehicle landed hard on its side. We had tied a knot in the strap to allow us to have a small loop above the GPS antenna, and the strap broke right at the knot. Lesson learned – never put a knot in a recovery strap.

 

The impact seems to have killed one of the gyro axis in the IMU, but the rest of the electronics is unharmed. One of the shocks was wrecked, and the frame is not salvageable, but most of the gear is fine. The tank took a hard enough hit that we will retire it to use as a water tank.

 

Several important lessons:

 

All engines need to be easily moved from the vehicle to the test stand. We have been building the big engines too integrally with the vehicle.

 

Any change to an engine needs to get a test stand run before it gets a vehicle run. Replacing the catalyst without pressure packing it gave us a rough engine, even with the nitrogen bleed on top.

 

We will never again use a coated catalyst in the hot section. They just don’t last. This engine had around eight drums of propellant run through it before it got unusable. While it would be possible to design an engine that was easy to remove and maintain, that just doesn’t fit with my goals for the program.

 

Both of our last two crashes were due to engine problems, rather than electronics problems. We need to get a truly reliable engine before we fly another vehicle.

 

We have one last direction to try for the mixed monoprop, using manganese compounds as the catalyst. Instead of using a thin coating of catalyst, the catalyst bed is made up of solid mineral crystals, so even if there is erosion, the surface remains catalytic.

 

If that doesn’t work, we are giving up on our 50% peroxide mixed monoprop and moving to LOX / methanol exclusively. If we have to do that, it is going to be a huge change, requiring a completely different vehicle design and control actuation system. We have laid out a lot of the changes we want to make for a next generation mixed-monoprop vehicle, but we probably won’t start building it until we have positive engine tests.

 

Our epoxy shelled LOX engine still leaks even after our patch attempts, so we are going to give up on that approach. We are looking into gun drilling the cooling channels, and various rapid prototyping fabrication technologies. So far, selective laser sintering is a bust for several reasons, and using stereolithography to make a casting pattern is also unsuitable for our sized cooling channels. We are looking into direct metal deposition next.