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Jun 26, 2001 Meeting Notes

Jun 26, 2001 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Neil Milburn


New Supplies:


100 lb load cell

10,000 lb load cell

another 1000 psi heavy duty pressure transducer

More –6 hose and hose ends

-10 aluminum hose ends

More Teflon O-rings for compressed gas bottles

Silicone O-ring cord stock creation kit

Stainless machine screw / nut / washer sets




I thought I had found a good supplier of 70% semiconductor grade (unstabilized) peroxide with the Kanto Corporation, but it turned out they only had 35% in that grade. I am still interested in seeing what we can do with cheap 70%, possibly with hybrids or biprops.


We also looked at a video of Juan Lozano’s (http://www.tecaeromex.com/ ) peroxide distillation setup last week. I still like just getting our peroxide out of a drum, even if it is fairly expensive, but a team on a lower budget should probably consider it.


I am buying a fairly sizable lot of peroxide from X-L so we won’t risk running dry for a while. Flying the manned vehicle is going to go through four or five gallons per flight, which will chew through our supplies quickly.


We discussed possibly migrating away from the Speed Flex Teflon hose and AN fittings to one of the hose styles that McMaster carries, so we could reduce the number of suppliers I have to hit, and so we could just get permanently assembled hose lengths instead of building them ourselves and having half of them leak when bent.


I have requested rough quotes for some large items (4’ spherical tanks, 4’ by 8’ cabin, 4’ by 20’ tubing) from three filament winding companies:






After the first manned vehicle is worked out, we are probably going to do a narrow, high performance demonstrator, then build something pretty sizable. I am thinking that we would build the first cut with E-glass, work everything out, then build version 2.0 out of carbon fiber.



Broken Load Cell Analysis


I don't think we banged the load cell last week, I think we actually overpowered it with the equivalent of a monoprop "hard start".


The first blast was LOUD, louder than when we were firing the big motor at 90 pounds of thrust, so I would not be surprised if it made well over 100 pounds of thrust for a very brief time, which would be enough to wreck the load cell.


Under the right conditions at startup it is possible to generate much higher chamber pressures than during steady state operation, because without any chamber pressure, several times as much peroxide will flow as during steady state operation.


Normally this just results in the initial cloudy burst that comes from the engines for a quarter second or so, but if there is sufficient catalyst to operate on the much larger mass of peroxide, it may all go up as if it were in a much larger engine.


This is potentially dangerous when the motors get closer to the optimized weights, although it is somewhat balanced by the fact that the motor casing will still be at its room temperature strength instead of operating temperature strength.


I believe three factors contributed:


The uncompressed catalyst allowed a lot more to flow deeper into the pack than with our older designs.  We had the compressed foam spreader at the top, so we were still getting good distribution, rather than just tunneling straight through.  If the pack was compressed, it would have provided frictional back pressure to prevent too much peroxide from flowing in even without chamber pressure.


We had over twice as much catalyst as we needed, which provided a large volume to hold the peroxide as it flowed in, and it could work on decomposing a lot more at the same time.


We had just done a water run, so the pack was wet and cold, giving the peroxide time to flow all the way through the pack before reaction started.


With the low back pressure catalyst packs, 450 psi tank pressure, and the -6 plumbing, we could easily have pushed well over 20 ml of peroxide into the engine during the warm up pulse.


As we moved to larger plumbing, I had increased the warm up pulse to 200 msec from 100 msec, because 100 msec times were sometimes barely long enough to get anything from the tank to the engine.  This was dependent on the exact arrangement of the feed hose, and could vary.


My recommendations for future tests:


We should raise the test stand tank so the line to the engine can be strictly "downhill", avoiding most of the trapped air issue, allowing the warm up pulses to be shorter.  We may need to extend the angle iron we have the tank secured to.  At the same time, we may want to add a hook for the fill cart line on the test stand so we don't have to worry about it laying down and letting some of the tank drain back into the fill line.


After we have done water runs, I should manually make very short (25 msec) pulses until the plumbing is burped and the pack has gotten some amount of heating before starting the automated test runs.


This probably won't be an issue at all with the motorized ball valves, because they take long enough to open that they can't flood a cold engine.  However, the piloted solenoid valve may be particularly bad with this effect, because it can't make a short pulse, and it still opens completely full very rapidly.



Perforated Injector Plate Motor Tests





We tested a new engine configuration today, with very good results.


The motor has a ½” throat and a 2” exit cone, and a 3” chamber diameter. This is the same throat dimensions as our last 50 pound thrust motor, but the chamber is a bit wider so we don’t need to worry about the pack retainer hole area being near the area of the throat. It is also shortened to only the length needed to hold the catalyst pack, and the nozzle is integral with the chamber.


When we used the highly compressed catalyst packs, we could get by without any injector spreading plate at all, because the pack was dense enough that it didn’t really allow tunneling.


When we used uncompressed catalyst packs by themselves, we got very wet runs.


A couple weeks ago, we had very good results by placing four tightly compressed unplated foam discs on top of 15 uncompressed catalyst discs. We achieved a much higher flow and catalysis rate that with the other packs, but the thrust curve dropped off sharper than it should have, we believe due to the pack compressing itself as the peroxide pushed against the compressed discs at the top.


We concluded that with a loose pack, it is necessary to have a good spreading injector ahead of the pack, but it should be supported independent of the catalyst.


Earlier engines had a coarsely drilled metal plate at the top, but that had little effect.


This current engine is designed around using a sheet of micro etch perforated stainless steel (McMaster 92315T101) clamped between the top closure and the catalyst pack. The perforations are only 0.006”, but they are so close together that the steel is semi-transparent.


It can be cut with scissors, so we were concerned that it might just rupture on the initial valve opening. We did a water test without any catalyst in the engine, and the metal sheet held up fine.


We loaded up 200 ml of peroxide and fired the engine. And broke another 100 lb load cell. Sigh. This time we banged the cell on the initial pulse.


We switched to the 500 lb load cell, and wrapped some heavy rubber bands around the rear of the slide so it could never pull away from the load cell. This gives us a four to five pound bias in the graph numbers, but it will prevent us from ever banging a load cell again. Interestingly, if you look closely at the data logs, you can see the recoil after thrust terminates as it stretches the rubber bands back by several pounds.




Run 1: 200 ml at 450 psi gave 58 pounds of thrust and ran smoothly

Run 2: 500 ml at 450 psi started out the same and dropped slowly with blow down, but was a little more ragged

Run 3: 500 ml at 600 psi gave 75 pounds of thrust, a somewhat surprising linear scale up with pressure

Run 4: 500 ml at 600 psi with pro race (8.8 amp) solenoid was only slightly lower than the big shot (30 amp) solenoid


The pro race solenoid has a poor characteristic in that it leaks a bit when below 400 psi (their docs mention this) due to a hard Teflon seal.


The engine starts catalyzing FAST. Most of the thrust is there within 50 msec of the valve opening. We can probably do away with warm up pulses completely, except after we have run water through the engine.


When we took it apart, the catalyst pack was completely even under the injector plate. It looks like we have quite a bit more catalyst than we need in this motor.


There was a slight dishing of the perforated steel plate when we opened the motor, but not much.


The bottom pack retainer plate holes were too large, because they allowed some of the pack foam to be pushed down enough to rupture. This may have been why the later runs were a bit rougher than the first one. We are going to look at using perforated steel for the retaining plate as well, either on top of a normal plate with larger holes, or clamped as with the top closure.


Russ and Phil are going to work with this design a bit more, then make four identical motors to this spec for the manned vehicle attitude control engines. The next engine will be the 1.5” throat engine, which should make around 600 pounds of thrust. We won’t be able to fire that until we get the new test stand set up on my new land.





Pwm2.exe, the test stand software, now has switchable calibrations for all of our load cells – 100 lb, 500 lb, 3000 lb, and 10000 lb.


I wrote some code to directly compare the FOGs and our old Gyration gyros. A graph of the simultaneous rate output when I am spinning them together is at:




It doesn’t look like a lot of difference between the $100 and $1500 gyros (my wife’s reaction: “Honey, you got ripped off!”), but the slight lag is important for stabilizing the flight control, and the little bumps that are smoothed out do make a difference when accumulated. The shock and G sensitivity are also critical issues.


I have a test program that properly handles the 3D rotations of the axis instead of just integrating the rates. I need to get the flight computer moved over to this before we start doing any significant translational maneuvers. I draw a 3D view of the axis live as you rotate the sensor board, and display both the integrated and derived values. Our old sensor board does noticeably drift as you move it around. I can’t test the three FOG axis directly on my laptop, because I only have two A/D channels, so I am going to have to set up a sensor forwarding program on the flight computer to compare.



New Electronics Box






The new electronics box built around the KVH fiber optic gyros is basically finished.


I am not going to do this again without getting a decent drill press. I put the last two together with a hand drill, but I have had enough of not-quite-straight and not-quite parallel holes…


It’s fairly heavy at 14 pounds. The gyros are bulky, necessitating a larger box, we have a few more boards inside, and I added a larger lead-acid 12v battery because the 40 amps we could draw when all solenoids were open was quite a bit more than the lithium batteries we were using are supposed to put out.


I’m using AMP Circular Plastic Connectors for everything now. You need a special crimper for the contacts, but they work very well. On the electronics box right now I just have the four connectors for the engine solenoids, but we will eventually be adding several more for the motorized ball valves, tank sensors, and remote GPS (because the box is too electrically noise to have it mounted inside). I also have all our load cells wired up with CPC for the test stand.


In the process of building the box out of Home Depot machine screws, I ran across two completely defective screws, and two other screws broke while I was tightening them. I ordered a large set of stainless steel machine screws from McMaster that will hopefully be of better quality.


When I was finally all set to start logging the data from the flight computer, I was appalled to see up to 8 bits of noise in the data from a 16 bit A/D.


I was scared that the gyros ($1500 each axis!) were messed up, but even wiring a battery directly to the A/D still had all the noise.


I moved the wires and cables around, but it didn't make much difference.


When I powered the PC104 stack directly with a 5v bench power supply instead of through the 12v DC/DC converter, the noise went away (well, down to two bits, which is fine).


I had been concerned about DC/DC converter noise as it would apply to being used as a reference signal to some of our sensors, but I had been assuming that the A/D board would condition its own power signal well enough. Bad assumption.


Russ made a few quick modifications to the DC/DC converter board today that I am going to check out, but we are probably going to have to add our own power supply to the SSR driver board.


We probably won’t have things together enough to fly this weekend, but it should be next week for sure…






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