2002-10-29

The test stand is coming along nicely. I finally settled on a propellant tank design (I think this is version 4). I just used an E-size medical aluminum oxygen cylinder, similar to the scuba tank I was playing around with earlier. I made some brass fittings for the top and put a 1/4 NPT fitting in the bottom of the tank. There are aluminum manifolds a few inches above the tanks where the vent, pressure transducer, and helium supply all connect. You can also see the associated poly tubing for the air cylinders used to control the ball valves.

I had to get some flow control mufflers for the air cylinders because the ball valves were opening and closing too fast. The flow control mufflers are little valves that go on the air solenoid block and have a setscrew to vary the orifice size on the exhaust port of the cylinder. It's pretty cool - you can still get the full force of the cylinder but you get a high degree of control over the speed.

I've checked out the data acquisition system with the pressure transducers and things appear to be working properly. I can also operate the air solenoids under software control. The products from National Instruments are perfect for this kind of stuff! I'll be working on some preliminary LabVIEW control software during the next couple of weeks. I've been getting increasingly nervous being around the tanks when they're pressurized so now that I have computer control over the test setup, I'll be using remote video viewing when possible. Hopefully in a couple of months I'll be able to do some liquid nitrogen/water tests to get an idea how cryogenic liquids will work with this setup.

2002-09-22

I've settled on an electrical connector type for use in the ground test. The CPC series by AMP are very inexpensive ($7 for a pair of plug and panel-mount pieces with strain relief) compared to MS series connectors ($20-40 per connector). Fry's Electronics stocks one type (Series 1, 16-17 type) along with the associated pins.

Started on the other two ball valve actuators. Now I just need to order two more air cylinders and I'll have all the hardware I need for remote control of the test.

Started a list of all the things I need to get/build/do before the ground test. Some of these things I can borrow, others I have to buy or build.

  • Propellant tanks (build)
  • Burst disks (buy), holder (build)
  • Load cell (build or buy)
  • Helium supply check valves (buy)
  • Various electrical connectors (solenoids, load cell, pressure transducers) (buy)
  • Ignitor (build)
  • Pressure tap for combustion chamber
  • Air compressor (borrow)
  • Signal conditioning for transducers (borrow)
  • Failsafe relay and control box, watchdog timer (build)
  • Data acquisition and control software (can you say LabVIEW?)
  • Video cameras (borrow or buy)
  • Video overlay for test data (thrust, pressures, etc.)
  • Double-check stress calculations for fasteners holding injector onto the combustion chamber
  • Test with liquid nitrogen/water - make sure ball valves, burst disks still work and don't freeze up
  • Check the fine print on life insurance policy to make sure I'm covered :)
  • Lots more that I haven't thought of yet

2002-09-15

Wow, has a whole year gone by? I've actually been somewhat busy working on components for the test setup. I borrowed a digital camera and took some digital photos of the various parts I've made so far - see sidebar for the links.

Scored some good deals off EBay: cryogenic ball valves (for the LOX), high pressure stainless steel ball valves (fuel), 3000 psi solenoid valve (helium supply). Got a bank of 7 double-acting air solenoids and air cylinders from Burden's Surplus Center for remote control of the ball valves.

Tried to make my own propellant tank but didn't have too much luck the first time. The guy who supposedly was an "expert" at aluminum welding did a terrible job. It leaked from four places during my hydrostatic proof tests. I took it back, he worked on it some more but it still leaked. See Propellant Tank Version 1.

Version 2 of the Propellant Tank is a weld-less design. It's a 4 inch OD aluminum pipe with a 0.25 inch wall. The end caps are made from 4 inch aluminum bar stock and use an o-ring to seal. The radial holes are for 0.25 inch steel pins to hold the ends on. I hydrostatically tested this design up to 450 psi with no leaks. But, since the aluminum pipe isn't perfectly round (up to 0.020 inches runout), I'm worried about the o-ring in the LOX tank sealing when filled due to shrinkage of the o-ring. I may go with a hybrid design that has a flat plate welded to the bottom and uses my design for the top so I can easily get it on and off. But first, I need to find a better welding shop!

Version 3 (sort of) the Propellant Tank uses a converted aluminum scuba tank with a special fitting I made. It worked really well and I used it for several cold water tests. The spray pattern looked really cool (still waiting for the pictures to get developed!).

Found a good source for rupture disks (Oseco). For $100, I can get 5 rupture disks made to my exact specifications. The disks will be the last line of defense on the propellant tanks in case something goes wrong and I can't open the vent valves remotely. The idea is to have disks rupture at a pressure high enough above the working pressure so they don't fail prematurely but low enough below the maximum pressure of the tank so the tank itself doesn't burst.

Performed some cold water tests of the injector a few months ago to check the performance. The orifice discharge coefficients looked pretty good with my crude setup (water bucket, garden hose, and cheap pressure gauge). I hope to get a more stable test setup going soon here and then I can do some serious tests on the injector performance.