2015-01-04

I ran a round of cold tests with LN2 this week on the new main, vent, and fill valves. It wasn't perfect but after some adjustments, I think they will work good enough for a flight vehicle. For all the tests the sequence was as follows:

  1. Close the main valve, then open the fill and vent valves
  2. Fill the test stand tank through the fill valve until liquid squirts out the vent valve
  3. Close the fill valve (manually operated) and open the valve to drain the fill hose
  4. Close the vent (with an AX-12A servo), checking for stickiness
  5. Pressurize the system using N2 in steps at 50, 100, 200, and 300 psi to check for leaks
  6. Open the main valve for a couple of seconds to check for leaks on video and verify smooth operation of the servo/gear setup
  7. Check for leaks in between cycles of the main valve
  8. After the tank was empty, open the vent valve

On the first day of testing, I was able to perform one fill and three main valve actuations before the tank burst disc popped. I didn't have a gauge hooked up after the pressurization check valve but it shouldn't have been much over 300 psi. It had been sitting for about 10 minutes closed up so you would expect the pressure to increase somewhat. However, that particular disc had been on the test stand for several years so maybe it failed due to fatigue (it was rated for 580 psi). A few days later when I continued testing, the fill valve which had worked OK the first day started was leaking liquid like crazy out the stem during the low pressure fill. I took the valve apart and discovered some damage to the spring loaded stem seal which was likely the cause of the leak. I can't explain why the same valve worked fine a few days earlier without being disassembled in between. I also noticed the four body bolts on all the valves were not as tight as they were originally so I re-torqued them. After that they stayed tight for the rest of the testing.

Inserting the stem seal into the valve bore is very tricky because there is a small step (0.005 for the LOX valve) that the bearing sits on. So the seal has to be pushed past that step without damaging it. I had been inserting the seal at an angle to get past the step (which puts it into an oval shape) then straightening it out. I discovered this process tends to put a small kink in one of the edges which creates a leak path later. I also noticed some excessive extrusion in the ID caused by just inserting the stem into it. I experimented with using aluminum foil as a sleeve to push the seal in past the step but then it was hard to get the foil out without tearing it and leaving some in the valve. I finally made made a little plunger out of some PTFE bar stock to just push the seal in past the step (after polishing up the step edge a bit). In general, this design doesn't lend itself well to inserting the seal without damaging it.

I was using vented balls in the valves to allow any trapped cryo fluid to expand either upstream or downstream. Normally the vent is pointed upstream when the valve is closed to keep the ball pushed against the downstream seat. However, this exposes the ball cavity and stem seal to cold liquid the entire time. If the upstream seat spring is stiff enough, the ball vent can be pointed downstream but if not, there is the chance that liquid will leak past the upstream seat, through the side of the ball, then out the vent downstream. I found in my testing with the main valve vent pointed downstream that there weren't any leaks until after the first open/close cycle of the valve. And even then the leak was extremely small - with 300 psi upstream of the valve, you could just barely feel the gas coming out of the exit. Since the main valve will only be operated once in the flight vehicle, this may be acceptable. But then I got to thinking about whether the ball really needs to be vented. The only reason for the vent is to release the trapped liquid but what if there was another way to release the internal pressure? Ben Brockert mentioned a while back that these little spring loaded seals can act as check valves so I thought if the seal was installed backwards, then any trapped liquid could burp itself around the seal as it expands to a gas. Another seal in an opposing configuration would keep anything from getting out the bearing end. I had designed in #10-32 ports in the stem bore of all the cryo valves so any leaks can be sent overboard in a controlled manner through a tube. I replaced the balls in the vent and main valves with a solid ball and the idea seemed to work. This eliminated all the liquid leaks out of the stem at the expense of an extra seal in each valve. The main valve already had two opposed seals so the cavity could be pressurized as it is the only valve that will be actuated under pressure. This revised arrangement (along with the new valve bore dimensions to compensate for shrinkage) seemed to dramatically reduce the stem leakage from the fill and vent valves. Only a very small gas leak is audible from the stem instead of the liquid leaks there were last year.

I also noticed a very small gas leak from the body seals of the main valve after it had been opened and closed once under pressure. The original Swagelok design uses a regular o-ring as the body seal so it's not surprising it leaks a bit when cold. At one point, I used an air gun to blow off the frost and the leak stopped so it must have pushed some ice into the gap between the end piece and body which sealed it back up. Without a drastic redesign, a second o-ring or maybe just a tighter o-ring groove may get rid of the leak completely.

I didn't have time to fabricate new male -6 AN fittings for all of the valves so both sides of the vent and the main valve exit were using an older female -6 o-ring boss fitting I had on hand. I had thought the o-ring fittings were working fine at cryo temps but on every run there was a liquid leak around the fitting at the main valve exit. I went back and looked more closely at the 2nd camera view from the 2013-09-15 hot-fire test and saw a similar but much larger LOX leak at the injector fitting. About halfway through that video you can see LOX dripping down onto the load cell bracket. Not good but at least I caught it before having LOX spray all over the inside of the rocket. The fix will be to either weld an aluminum AN fitting to the LOX injector cap or switch back to using NPT threaded fittings. I had sworn off NPT fittings after ruining an injector on the 100 lbf engine by stripping out the threads but maybe if I'm careful, they will work. The LOX cap is easier to make than the injector so I could always make a spare or two. The problem with NPT fittings though is if you accidentally back it off at all while tightening the tube nut, then you have to pull out the whole fitting to re-tape it. And then you run the risk of FOD from the PTFE tape.

The AX-12A servos seemed to have no problem opening the main valve (3:1 gear ratio and linked to the fuel valve) and vent valve (direct connection). On the very last run, the vent valve was a bit sticky and didn't close initially. Some alternating open/close cycles eventually freed it up but I suspect condensed moisture left over from previous runs got into the bearing and froze it up. I also found a "feature" with the AX-12A where it reports that it isn't moving anymore when it gets stuck prior to reaching its destination. The control code was using that flag to decide when it was done moving and disable the servo. I changed the code to also check the position error when deciding if the move is complete or not. There were two other changes that eliminated the intermittent comm problems I saw during previous tests. A scope on the half-duplex TTL line showed that the default 1 Mbps baud rate didn't leave enough time with long cable lengths for the logic low signal to be recognized. Turning the rate down to 500 kbps seemed to be enough. There was also a logic level problem that I needed to address properly. The AX-12A is a 5V TTL part but the 74HC126 that is performing the TX/RX switching is connected to the MOD54415 runs at 3.3V. The 74HC levels at 3.3V meet the TTL spec but the reply back from the servo exceeds the 3.3 Vcc of the buffer. There are dedicated chips for this but most require +5 operation. It turns out the 74HC parts have ESD protection diodes on the inputs so with an appropriate series resistor to limit the current, the extra voltage can be safely shunted through the diode to Vcc. Certain 74HC vendors had the resistor built in but you can't count on that so I added a 470 ohm resistor on the data line in between the servo and buffer. Checks with a scope afterwards show all logic levels were met and I haven't seen any comm errors since then.

Next up will be some tests of the Aqua regulators and shop work on the flight propellant tanks.


For archived articles, check out the News section.