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Good warmup test

March 28, 2004 notes

March 28, 2004 notes


Good Warmup


We tried building another 7” engine with only 600 grams of ring catalyst in the hot pack, since the 700 gram engine from last week seemed to still be running at full temperature after the screen burned away and let the rings jump around.  The engine came up to temperature fine, but on throttle up it lost a lot of temperature and never gained it back.  We decided to stick with 700 grams of rings in the hot packs.


We built up a complete ship set of engines like this, and tested them individually.  They all make a bit over 800 lbf at 250 psi feed pressure, generally losing about 18 psi through the hot pack and 36 psi through the plumbing / spreading plate / cold pack.  The temperatures still aren’t quite as high as the engine that burned the retaining screen (the stainless burning may well have added the extra heat), but they are plenty good enough.  Two of the engines are running a bit rough, probably because the rings weren’t able to be packed as solidly in the recycled engine chambers with stubs of old plates still on the sides.  The next time we build fresh engines, we are probably going to try vibration settling, then weld the retaining plate on top under a fair amount of hydraulic pressure.




I did a significant amount of rewiring on the vehicle to set it up for spark ignition instead of glow plugs.  The MSD Small Engine Controllers only draw about 1.25 amps each, so there isn’t any need for the dedicated battery and relays we used on the glow plugs.  The vehicle is also all wired up for a thermocouple for each engine now, but the amplifiers haven’t arrived yet.  To avoid having to make new electrical connectors for each engine, I started sharing power and ground for the three sensors at each engine.  It turned out that it probably would have been easier to just make new connectors, but there was a benefit I hadn’t considered: driving the valve potentiometers with 12V (regulated) instead of 5V more than halved the sensor noise.  We are completely maxed out on the current A/D board now:  Each engine has a pot feedback, a pressure transducer, and a thermocouple.  We also have a pot feedback on the master cutoff valve, tank pressure, battery voltage, and recovery tank pressure for a drogue cannon if we decide to add one.


We spent quite some time chasing two sensor problems:  The tank pressure signal had been coming and going for a while, which was finally tracked to a problem we have seen before – someone had twisted the AMP CPC off by the strain relief instead of the locking ring, which twists the wires internally.  When I pulled out the contacts, the signal wire had a section visibly abraded through.  Cutting it off, stripping it back, and replacing the connector fixed it.  The other problem took longer to track down, but Russ finally figured out that we needed a capacitor across the driver board power terminals to avoid some noise issues.


On James’s suggestion, we bought a bunch of steel to make some serious work tables.  The cheap banquet tables we have all over the shop have been severely protesting their use as welding and engine assembly tables.  James welded up two tables while we were chasing electrical issues.





We finally got everything out for the warmup test in the parking lot, and loaded 20 gallons of propellant.  Since we don’t have the vehicle thermocouple amplifiers yet, we used one of the test stand sensors to display the temperature of one of the engines.  All the engines lit right up and ran clear, but I had selected too high of a throttle setting for the warmup, and we didn’t reach the desired temperature for full flow.  After the run, we found some heat damage under the vehicle: the pressure gauge face had bubbled up, and the master cutoff valve indicator and wiring harness was cooked.  We added some insulation around the cutoff valve, but we are probably going to look into getting a stem extension for it so we can get the actuator out from the middle of the base.  We will locate a new tank pressure gauge remotely.








We added another thermocouple so I could watch two engines temperatures, and loaded up another 20 gallons.  The engines again all lit right up, and at a lower throttle setting one of the two temperatures I was watching rapidly climbed to proper startup temperature (800C), but the other one was having trouble getting over 600C.  I throttled up anyway, and all four engines came evenly to pressure, bucking the vehicle up a bit even at the 125 psi we had the tank at.  Engine zero was still running rough, but as far as we can tell, the vehicle should be flyable right now.


We aren’t going to try and hover it until we get the thermocouple amplifiers for the vehicle, because that data is too critical for us to guess at during warmup.  We also want to replace some of the heat damaged components and insulate things better, but a hover test next weekend is a possibility, weather permitting.


Our drum pressurization propellant loading has been working well, but when the drums bulge up we often wonder how much safety margin we actually have, so we filled a drum up with water and started pressurizing it (on the other side of a wall) to see what would happen.  It was very anti-climactic: At 40 psi, the drum deformed enough that the bung seal no longer worked, and pressure leaked out.  When it dropped down to 35 psi, it sealed itself back up.  We had only been pressurizing the drums to 7 psi, so we don’t have anything to worry about.


We have made a lot of interesting melted parts over the last several months while developing the mixed-monoprop engines.  Get your very own Armadillo Droppings:





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