August 7, 2001 Meeting Notes
Electronics Box Electrical Noise
Russ took a look at the electronics box with an oscilloscope
today, and it wasnt pretty. There were
50mV signals at 100 Mhz all over the box, which nicely explains the seven bits
of noise I see on the A/D channels.
We were planning on trying out some single pole resistor /
capacitor filters on the A/D breakout board, but it doesnt look like that
would help much, given that there is 18 of ribbon cable between the breakout
board and the A/D board.
If we move away from the PCMCIA 802.11b card to a separate
base station that communicates with the pc104 stack by Ethernet, we can enclose
the entire pc104 stack in a shielded box, with just a few slots for ribbon
cables to exit. There is still noise in
the A/D board, but that should prevent a lot of the noise that gets picked up
by all the other wires in the box.
The longer-term solution is to move the A/D out of the pc104
stack completely, and send the results over either USB or Ethernet. Im not sure of the perfect product for this
yet. I need proper documentation to
access it from Linux. It needs to be
very low latency, not just high bandwidth, which may not be the case for
communication over USB or Ethernet. I
want at least 32 channels, with the ability to individually pair them for
We tried several things to smooth out the roughness in the
70 pound thrust attitude engines, with mixed results.
All tests at 500 psi initial tank pressure, with one liter
of 80/20 peroxide, except test 3, which had only 500 ml of peroxide.
Test 1: reduced the catalyst pack thickness from 21
uncompressed discs to 13 uncompressed discs, adding a spacer underneath the
retaining plate to keep the pack at the top.
This was smoother than the runs with the full catalyst pack,
and produced a small amount more thrust, but it got a little rougher towards
Test 2: We ran the exact same parameters again, to see if
the roughness that showed up at the end was permanent.
It ran at least as good as the first time, so the reduced
pack height seems to be a solid improvement.
Test 3: We have never determined what the lower limit for
cat pack size is, so we cut the pack down to 7 uncompressed discs for a test
run. Because we thought it likely that
this would be a wet run, we only used 500 ml.
It still made a decent amount of thrust, but it was clearly
failing to catalyze everything, giving us the first cloudy plume we have seen
in a long, long time.
Test 4: We have always noticed a preferential wearing of the
cat packs in the center, even with our micro-etched spreading pates. We put a strip of metal over the center of
the spreading plate, so no peroxide would directly flow out of the fitting and
into the middle of the pack. We used 13
It was fairly smooth, but we were somewhat surprised to see
a bit of cloudy exhaust. We are led to
believe that pushing the peroxide away from the center may cause some
channeling. It made only slightly less
thrust, so it was probably only channeling a couple percent, but it was
Test 5: We took the
injector blocking strip off and ran it again, which should be identical
conditions to runs 1 and 2.
It was still showing some cloudiness in the exhaust, and
thrust was down a bit. Sigh.
Test 6: We
compressed 13 discs to the height of 7.
It had a slightly different pattern of noise, a slight drop
in peak thrust, and it also showed an effect we have seen before: compressed
packs have less thrust variance as tank pressure drops. We arent sure what to make of this. I favor compressing the packs somewhat,
because they will compress themselves during operation, and doing it ahead of
time should cut down on run-in variation.
Test 7: We ran one of the other engines from Saturday with
the 21 disc packs for direct comparison.
It was definitely rougher across the board, so we are going
to go to 13 disc packs for all the engines.
We are unsure of the lifetime of these cat packs. The engines in the lander are still working
well after over two minutes of firing times on the various hops. The second batch of foam we had plated used
slightly different parameters, which we think might be allowing the silver to
be worn off faster.
On the last run, we also ran into the valve problem we had
seen a few weeks ago the NOS Pro-Race solenoid stuck on. It does have a warning about not sealing
bubble-tight below 450 psi, but sticking open (until you tap it with a wrench)
is damn poor behavior. If it had just happened
with one valve, we were willing to let it go as a bad valve, but now that we
have seen it on two different valves, we arent going to use these for the
manned vehicle. We may still hop the
frame unmanned with them (as far as we know, it only shows up as a failure to
close after venting the tank down to low pressure after all the firing is done),
but I am now actively looking for more valves again.
Our required specs are:
303/316 stainless steel body
Viton or Teflon seals
12V, direct acting solenoid drawing less than 15 amps
Able to open and close within 10 msec, and capable of
surviving continuous pulsing
As much flow as we can get
We can move down one step on the NOS solenoids to a model
that flows a little less, but hasnt given us problems, but this is probably a
good time to shop for some higher end valves anyway.