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January 8 and January 12, 2002 meeting notes

January 8 and January 12, 2001 meeting notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Neil Milburn

Joseph LaGrave


Silver screen engine testing


We test fired the little 0.65” by 2.55” cat pack engine again this week, with two changes: added a top injector plate with two 1/16” holes in it to provide a pressure drop, and alternated stainless steel screens with the pure silver screens.


Cutting the stainless steel screens (20 mesh) was much harder than expected. We first tried cutting them on the press with the little turned down socket that we cut the silver screens with, but it just dug into the socket edge when pressed. We cut a few by chucking the now-notched socket on the drill press, but it tended to grab and tear. I tried cutting discs with various tools held in the boring head on my little mill, but it always grabbed before it cut.


What finally worked was making a punch and die set, where you slide the screen in a slit across a thick walled cylinder bored to the correct ID, place a tightly fitting punch into the cylinder on top of the screen, then whack it with a hammer. I made the cylinder out of stainless steel, and I intended to make the punch out of stainless, but I mis-measured and botched the part. I made a quick one out of brass, which worked well for a while, but slowly got chewed up as it cut screens. Eventually it was wedging the screens down without cutting them cleanly off, making it difficult to remove the punch. The next time I do this, I will take the time to make the stainless punch.


We packed the engine with 50 pure silver screens alternated with 50 stainless screens, which required mild compression force. The stainless screens are drastically stronger than the pure silver screens, so we expect them to make up the bulk of the strength of the pack, with the silver screens just fitting between them.


Our first test was at 400 psi, and while it still took a very long time to warm up with manual pulses, once it was active and I let it run, it catalyzed cleanly and ran extremely evenly. It made 13 pounds of thrust, which was about what we expected given the deep, restrictive cat pack.


In hindsight, I wish we had done a high pressure (800 psi) run with the engine at that point, but we chose to do a long duration test firing instead. The last long duration test run we made was with one of the 75 pound thrust attitude engines with a foam pack, and it started clouding up after 45 seconds of firing.


We ran this engine for 85 seconds at 400 psi, and the power and catalyst stayed good the entire time. There was a hint of uncatalyzed peroxide, with an occasional cloud a couple feet from the nozzle, but there was no roughness to the run. It dropped about a pound and a half of thrust 20 seconds into the run, which turned out to be when the copper gasket blew out.


After it cooled down, we examined the motor and found out that we have finally reached the limits of alloy 360 brass for the engines. Two of the retaining threads had pulled themselves almost free from the engine, unwinding out of the bar like a helicoil when we took it apart. The 1” square bar stock had also bulged three hundredths of an inch on the hot end.


Our current vehicle motors have the closure at the cool end, so they aren’t likely to have this exact problem, but they are also thinner wall, and we run them at higher pressure, so it is something to keep in mind. Closure-at-the-bottom has the benefit that if you have a leak, you just leak steam and oxygen instead of raw peroxide, and it is easier to swap nozzles, but it does make the retention and sealing a bit more challenging..

That evening, I tried to turn down the square bar down to a cylinder, leaving just the bottom part to act as a flange that we could use a nut behind instead of internal threads, but there wasn’t enough wall thickness left if I cut it deep enough to hold a socket head or nut instead of just the thread.


The next screen test engine will have a 1” diameter by 2” long cat pack for a 0.25” nozzle throat, which should be a lot less restrictive. It will use bolts and nuts across a flange instead of the internal bolt threads. It looks like there is no getting around using stainless for production rotor tip engines, because they will see extremely high pressures (4000+ psi) and have run times of potentially a couple minutes.



New lander frame buildup


The legs aren’t done yet, but we got the main frame back to start assembling things. The base tubes are thicker wall, and there is a bit more triangulation in it, so it is stronger (and heavier) than before. There are extremely strong attachment points for tethers and a top parachute.


We cut a special bracket to make the top seat attach point stronger.


The bottom distribution manifold has been reconfigured so we could remove a half inch cross fitting, which saves about three inches of plumbing height under the seat, making it impossible to hit the seat bottom even if the engine is pushed level with the frame bottom.


We built a new set of plumbing mounted to the back of the vehicle that gets rid of the hanging valves off the tops of the tanks, and allowed us to relocate the pressure transducer (and pressure gauge) to the gas side instead of the liquid side, which should give us much more level readings during operation.




Russ finished the new power supply board.


The flight computer is all configured and tested with the Esteem 1000 mW wireless units.


Phil did some maintenance on the vacuum pump, and replaced a leaking ball valve on the fill cart.


Joseph sunk really good tether attach points into the concrete. We had been removing the eye bolts after each test session, but that was weakening the threads in the ground, so they are now permanently epoxied in. The only way a tether is coming up is if it takes a good-sized chunk of concrete with it.


Russ made brass anti-channel rings for the attitude engines.


We should fly again next week.




The rotary test stand has the tachometer configured for it.


The TZM bar stock arrived. A $1100 hunk of metal. We are discussing exactly how we want to configure the hybrid test motor.





I went to the Oklahoma Space Industry Development Authority again, pushing for near term waivers from the FAA. I will be briefing the Oklahoma FSDO about our development program next month. We still haven’t gotten anywhere on our local waiver.




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