Jan 23, 2001 meeting notes
Jan 23 meeting notes:
Location: Long Range Systems
Next meeting on Tuesday at LRS for a VTVL system checkout with water. If the
planets align correctly, we may be able to do a tethered power up.
We have a tentative attitude sensing solution. Gyration (www.gyration.com)
will sell their dual axis rate gyro and support chipsets in small quantities.
In lots of less than 10, the gyro is $150 and the support chip is $18. I
got four of each in today, and we had a look over the specs and application
notes. A GyroMouse only retails for $99, so there is a bit of gouging
going on, but buying the real parts does give you access to tech support.
The support ASIC turns out to not be doing much of anything special, just 12 bit
A/D. I thought it might be doing some application specific compensation or
correction, but it doesnt seem to be.
I have been testing with a butchered GyroMouse as a two axis rate sensor, and we
will probably do initial manual liftoff with that logging data. It has an
unwanted radio link, only updates 30 samples a second, and needs one more axis,
so we intend to build a custom board for later use. We will probably
incorporate three DC accelerometers as well, giving us a full 6DOF rate sensor.
An interesting side note: during my testing with the GyroMouse connected
as a serial device, the default Windows 16550 configuration caused noticeable
problems. Serial mice send three (or four, in some cases) byte packets,
but the default FIFO limits are set so that you only get an interrupt when 12 -
14 characters have been received, or when four character clocks pass without new
data. At 1200 baud, this causes very long delays and getting updates in
lumps of three or four at a time, giving very jerky graphs. Either
disabling the FIFO, or setting it to interrupt on the very first character fixes
it. I still need to grunge around in Linux to find how to set this there.
Also interesting, on Win98, the updates come very precisely at 33 or 34 msec
apart when I have the scheduling priorities cranked all the way up. On
Win2K, they com at 30 or 40 msec intervals, implying a 10 msec scheduling
granularity. I havent tested on Linux yet.
The orientation of the sensors is nicer than most of the dual axis
accelerometers one of the axis comes out of the board, so we can get the
third axis by just rotating the second gyro chip instead of having to install it
sideways on a riser.
It handles 150 degrees a second rate, which is way more than our craft should be
doing, and the drift in place doesnt seem bad at all on the Gyro Mouse.
The biggest limitation is that they are only good up to 2 to 3 Gs of
acceleration. That is completely fine for our first generation VTVL, but
not as much as we would want for a high performance rocket.
We will see how bad the drift characteristics are in operation, and see how we
can use other sensors, like GPS and magnetometers, to keep it calibrated on
longer flights. For remotely piloted flights, it isnt going to make a
bit of difference, because drift will just move the joystick centering position,
which is easy for an operator to compensate for.
I have decided that my initial purchase of an EBX/PC104 form factor computer for
our flight system was a mistake. I liked getting everything I needed for
development on a single board, but I am now pretty sure I would prefer a couple
pure PC104 boards in a stack instead. I have a separate document on flight
computers that I am putting together as I learn things.
I brought a new NOS pressure guage for the VTVL manifold, so we can leave the
other one on the test stand. It is 1/8 NPT, but the gauge didnt quite
clear the installed hose ends. I stuck some adapters together to space it
out, but in hindsight, I bet we could remove the hose below it, and install it
with the proper clocking (gauge facing up) so that the hose can be put back on
without clearance problems.
The fill connector still requires an obnoxious chain of adapters. If we
could tap the manifold for 4, we could screw it directly in with an O-ring.
I brought an NOS nitrous filter that we were going to use as a nitrogen filter,
but when we looked at it, it didnt seem all that useful the screen
inside wasnt as fine as we would like. I still want to get a good
filter, because Bill Colburn has reported that there have been incidents of
corrosion inside nitrogen tanks getting blown into a peroxide tank.
I brought a couple NOS blow off valves, but we are going to need to get some
adapters to fit them to the manifold. Russ the best solution would
probably be to get larger taps and just cut the manifold to take it directly.
You might want to check it out and see if it is feasible. The blow off
valves are rated at an incredible 3000 psi for our tanks. I would like to
find some that are more like 600 psi, because the hoses we are using are only
rated to 1500psi.
The lander3d.exe program can now deal with the PWM actuation and GyroMouse
sensing for the initial VTVL liftoff, and has a bunch of customizations for our
first test flight. It is now configured so that no engine will drive
unless the joystick trigger is held down, so just releasing the joystick should
shut everything down instantly. I made a 100 parallel extension cable
for the tethered tests, and it checked out fine.
We cut some light chain for tethers for the first test. We are only going
to give it about a foot of freedom until we have some sense of the vehicle
dynamics. Once I get some attitude and engine logs, I will try and make
the simulator model the actual behavior better, then we can think about giving
it some more leash.
We did a lot of engine tests today, with somewhat mixed results. We got
good power and perfect catalyzation out of every run, but many were rough.
What we do seem to have found is that compressing the catalyst packs more always
increases the smoothness of the run, but drops the power output a bit.
A large pressure drop would not be acceptable for a high performance vehicle,
but since we are a long way from caring about optimization, we have two workable
paths: either just run much higher tank pressure, or just open op the
throat more and live with a low chamber pressure. Our tanks are good to
3000psi, so we could certainly go that route, but I like staying around 400 psi
for safety, and I dont mind the Isp drop.
From our previous tests with highly compressed packs, it showed that we dont
need to fill the engine with them, but if we cut the size down, I do want to add
spacers so we keep the pack all the way to the inlet fitting.
A useful result we got was that the front injector plate can be omitted entirely
if you have a dense enough cat pack. Having catalyst right up to the inlet
fitting prevents a puddle of peroxide from forming in the engine at the end of a
horizontal test run, which I believe was the cause of the thrust bumps at the
end of our previous runs.
The new modifications to the PWM static test program (which we use even when
doing 100% duty cycle runs) worked great. It now clips off some of the
initial cruft, allows comments in the scripts, and can continue to vent the tank
after it has stopped logging.
Results are at: media.armadilloaerospace.com/misc/jan23.xls
Run 1: Uncompressed pack of cut
disks, new check valve in place of pipe union, no spreading plate. This
run was extremely ragged.
Run 2: Go back to the old check valve. Still extremely ragged.
Run 3: Add spreading plate, slightly compressing the pack. Smoother, but
still not as good as we have seen.
Run 4: Compress pack a quarter inch, add more disks, keep the spreading
plate. Nice and smooth.
PWM run: After the initial warm up pulse, this run two seconds of 10 ms on
/ 15 ms off, then two seconds of 15 ms on / 10 ms off, then two seconds of 20 ms
on / 5 ms off.
The small duty cycle seemed to ramp into the medium duty cycle. I need to
extend the test program to log the duty cycle by the thrust curve to see better
what is happening/. The medium duty cycle was decent. The high duty
cycle intermittently failed to operate the solenoid inside the 5 msec, making it
My current theory on the PWM is that there may only be a narrow range of
off times that do not either have solenoid opening problems, or allow the
chamber pressure to blow down to the point that the nozzle flow separates.
We may not be able to deeply throttle smoothly, but we should be able to get
high percentages by stretching the total cycle time instead of shrinking the off
time more. More experimentation is needed.
Run 5: No spread plate. This started out the smoothest run, but got ragged
at the end.
Run 6: 450 psi instead of 400 psi. This stayed ragged the entire time.
Something may have happened to the pack at the end of run 5. We dont
Run 7: Test one of the new engines with a new pack compressed a half inch and no
spreading plate. This has exactly the same throat area and expansion ratio
as the old engine, but has a smoother radius in the nozzle, and a 15 degree
expansion cone instead of the 30 degree that was on the original (because Russ
didnt have a small enough tool at the time). The nozzle does seem to
make a bit of a difference.
This run also seems to give some evidence that my load cell meter is point
sampling at 12hz instead of doing any averaging, because the warmup pulse is
completely missing. This is likely contributing to making the PWM runs
look worse on the graph than they really are. I need to get a high
frequency load cell meter.
Stuff to do:
Order more peroxide from X-L. We have enough to do a bunch more engine
tests, but the VTVL will chew through our supply rapidly.
Adapter for blow off valves, or drill out the manifold.
Possibly tap one of the manifold ports to 4 for the quick connect.
Cinder blocks for the tether chains.
Finish the rear plates for the new engines.
Cut catalyst for all the new engines. I think we should go with highly
compressed (at least the half inch we ended up with), but we could probably use
half the total depth or less if we can get a good spacer and want a little more
Bring in the pressure transducer and specs so I can see about getting the
Check my glassware box to see if we have something suitable to replace the
broken beaker, or buy a new one.
Is it possible to make a chamber pressure tap in the current test engine?
Go through Linux From Scratch on the embedded system.
Allen screws and nuts so we can get the swivel 90 fittings on the engine mounts.
Various female pipe and AN fittings, which we always seem to need to double up
other fittings to simulate.
Find a big viton O-ring for the bottle necks.
Protective plate on the bottom of the VTVL manifold to protect the hose ends.
Mount the multiple solid state relay board on VTVL.
Build a couple sets of batteries and work out mounting on the VTVL.
Mount the butchered GyroMouse on the VTVL.
Put everything in a Tupperware bin in case we want to hose down the VTVL?
Fix up the original single solid state relay for the test stand so we dont
need to pull the multi board from the VTVL.
Build the 6DOF rate sensor board.