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Bent jet vanes, Liquid catalyst tests

May 22, 2004 notes

May 22, 2004 notes


Bent jet vanes


Some shots I didn’t have time to put up last week:









We pulled the engine back off the vehicle to rectify our error of leaving the paper towel plug in the valve top last week.  Unfortunately, only half of it was left above the valves, and there were little pieces visible in the spreading plate holes.  We decided to go ahead and cut the top plate off so we could clean everything out well, which turned out to be absolutely mandatory, because there were shreds of towel in every single spreading plate hole.




After cleaning everything out, we welded the engine back together and reassembled the engine on the vehicle.  We added an extra layer of Teflon sealant around the double O-ring seal between the engine valves and the tank manway.  This was the thing we were most worried about sealing, because they are really tiny 1/16” thick O-rings, but it turned out to work perfectly.  We tightened up the fittings on the drain valve and pressure gauge, so the plumbing is now leak free except for a tiny drip on the loading connector.  Unfortunately, there is a slight leak past both the master cutoff valve and the throttle valve, so propellant trickles into the engine when the tank is pressurized.  This will eventually auto-ignite and start heating the engine, which is good from a warmup standpoint, but we think this is also how we cook the cold packs, by having such a low flow rate that actual burning can take place above the flameholder.  We usually get these leaks after we drill vent holes in the ball valves, either due to a metal chip or a burr scuffing the seats.  I have rebuild kits on the way, but it will be a hassle to dismount everything.


All of the wiring was completed on the vehicle base, and everything looked like it was ready to go, but the A/D board went out on us again.  We thought we had a pretty good theory this time: it was working earlier in the day when I set up the vane calibration, but between then and when we found the board to be bad again, we had TIG welded a bracket onto the landing legs.  The high frequency starting current on the welder may have found its way into the wiring harness somehow.  We got yet another A/D board for the next day of tests.


The 1’ foam cubes had worked so perfectly for supporting tethered takeoffs of the little vehicle that we had 2’ foam cubes made for the big vehicle to replace our various attempts at fall-away supports.




We added a vacuum gauge to the new adjustable high flow venturi pump, which turned out to be a very good idea, because the interaction of the inlet pressure and the pump adjustment covered a very wide range of efficiency, and when we were randomly adjusting it we were probably wasting half the gas.  I had started to use our normal gear for the vacuum valve and gauge adapters, but I realized that I could save a couple hundred dollars by using CPVC fittings and valves instead of very expensive 316 SS 1” hardware.  The combination of the stronger vacuum pump and the hotter weather causes the fiberglass reinforced PVC hose to get squashed pretty flat, but it starts pulling propellant in much faster than the smaller two stage pump.


The leak past the valves was preheating the engine fairly well, and when I brought the throttle up it was hot and clear almost immediately.  The throttle up burned quite a bit away from the inside edges of the foam cubes, depositing lots of sticky melted plastic goo on everything.  We need to put some insulation or coating on the inside edge to prevent this next time.


Unfortunately, we lost the computer at throttle up.  The master cutoff worked perfectly, shutting off the propellant flow and avoiding what would have been a rather exciting swing of the vehicle pulling on the tether.  I was able to telnet back in to the computer a minute later, which showed that it had rebooted itself, but the Crossbow wasn’t responding.  We saw this exact behavior on a previous test of the big vehicle a while ago, and a real power cycle did bring the crossbow back to life.  When this happened before, I wasn’t 100% sure if the computer had rebooted, or if I had just lost the telnet connection, but this time I checked the system date, which I have to manually reset after booting if I want to do any code building, and it had indeed been reset to 1970, positively indicating a reboot.


A momentary voltage drop is probably the cause.  The battery voltage drops from 12.5V to 11.7V when all the motors (vanes and throttle) are being driven, but the DC/DC converters for the computer and other gear should work down to 9V source.  There isn’t any indication in the telemetry that it is dropping below about 11.5V, but that is only a 20hz sampling, and it seems reasonable that the jet vane motors will draw more current when they are working against the exhaust stream, which is our current theory.


We set the vehicle back up on the foam blocks, but because the Crossbow was still down, we had to take it back down and get the ladder out so we could power cycle everything.  All this time, the leak past the valves was cooking propellant off in the engine, and we were getting quite concerned that this trickle flow was going to burn up the cold catalyst pack.


After the reboot, I throttled up the engine and found that there was a stream of liquid coming out the center.  We thought that we had probably burned out the engine, but I was eventually able to get it running clean.  When I throttled up, it didn’t crash, but it showed up another problem:  the lift truck was taking most of the vehicle weight on the tether, so as soon as the throttle came up enough to lighten it just a little bit, it blew the foam blocks away, but allowed the vehicle to bounce on the tether.  This up and down acceleration confused the auto-hover software, causing it only increase the throttle during parts of the negative G sections of the bouncing, resulting in it only creeping the throttle up, and not getting high enough to actually hover before it went into auto-land mode.  I did move the vanes around while it was sort of bouncing there, and the directional control was basically working, but it didn’t make it up in the air.  Next time, we are going to let it rest most of its weight on the foam blocks.




After pausing briefly to reset, I tried again, and it rebooted the computer again.  The master cutoff again did its job properly.  This time, when it came back, the A/D board was hosed again.  That was it for our testing.  We didn’t weld anything on the vehicle, and I didn’t even use the spark ignition system, so I am going to move to a different brand of A/D board and see if we have better luck there.


When we laid the vehicle back down, we found that all the jet vanes had been bent to various degrees, even though the engine had only seen about 2000 lbf of thrust.  The long heat soak under the trickle burning engine before the high thrust run certainly weakened them.  We were more-or-less expecting this, and we have ¾” shafts and 3/16” thick vanes ready to weld together.  ¾” bearings happen to fit in exactly the same mounts as the ½” bearings, so we don’t need to change any of that.






The actuator that was downwind of the engine was badly burned.  The protective cover box has a slot that the shaft slides down, so hot exhaust and (during warmup) propellant can still get through.  We actually saw a puddle of burning goo under the actuator at one point, and I wasn’t sure if it was melted foam pad that started burning up there, or melted actuator casing.  The entire mounting plate was quite hot, but conduction through the shaft and mounting brackets didn’t seem to be hurting anything.  As usual, convective heating is the killer.  We are probably going to add some insulation boards around the shaft to block off more of the flow.  The burned motor may have had something to do with some of our problems.




All the melted polyethylene foam came off the vehicle pretty easily.





We were also prepared to finally do boosted hops with the small vehicle, but we can’t fly that vehicle either until we fix the A/D board.


We should have things back in shape to try again on Tuesday.


We will have the stouter vanes completed and installed.


We will get a different brand of A/D board and rewire the breakout board for it.


We are going to go back to a separate battery for the actuators, which should prevent anything from the actuators hurting the computer power.  There isn’t room on the electronics board for another batter, so it is going to have to be externally mounted somewhere in the cabin.


We are near the upper end of the current supply on our 5V and 12V DC/DC converters, so we are going to build a new power supply board with converters that are twice as large to give us plenty of margin.



Liquid catalyst testing


We have done experiments with liquid catalyst injection in the past, and generally found it inferior to catalyst packs for our purposes, but now that we only have a single engine on the vehicle and we are looking at making 20,000 lbf class engines, we thought it might be worth a little more experimentation.  The upside would be that there wouldn’t be five week waits for custom catalyst monoliths, and we would probably pick up a little chamber pressure and some engine thrust to weight.  The downside is that you need to manage twice as many propellants, you get grit clogging up your plumbing, and you wind up staining everything you work with.


Because we need deep throttling capability, a classing liquid-liquid atomization injector scheme won’t really work for us.  We are experimenting with using static pipe mixers to perform the mixing, which might be an interesting option with 50% peroxide mixed-monoprop.  The catalyst and peroxide are flowed through the static mixer together, which then exhausts into the main chamber with flameholders and a spark plug ignition.


We hacked together a test rig for this, and worked our way up from just the static mixer, to open flameholder tests, to a welded on engine nozzle.




Results were rather mixed.  We got a nice burn with the open flameholder, but when we added an extension chamber below it, it chuffed very badly, and with the nozzle on, we clearly weren’t getting full combustion.  We are going to add pressure and temperature instrumentation and try adding additional layers of flameholders in the chamber.


One interesting point:  we are using potassium permanganate as the catalyst, and we were surprised to find that while we could only dissolve 0.5% by weight in tap water before condensate started to form, we could dissolve over 5% by weight in de-ionized water.  The tiny little impurities in tap water make a huge difference.


We also have some sodium borohydride coming for various tests.



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