June 4 and 8, 2002 Meeting Notes
Neil Milburn (Tuesday)
Joseph LaGrave (Saturday)
Since we were packing 2 engines for the lander anyway, we tried
an experimental low-pressure-drop catalyst pack. The idea was to skip the stainless screens interspersed between
the silver screens, and instead use an anti-channel ring between every pair of
silver screens to give it an almost solid outer wall for structural strength,
and an air-gap between the screens to expose more surface area, and allow gas
to move around liquid. It still had a
spreading plate and 10 stainless screens at the top to spread the flow out, otherwise
a high pressure peroxide jet would just cut right through the weak silver
screens. We were only able to fit 22
doubled up 36 mesh screens (44 total) into the catalyst pack because of the
depth consumed by the rings, compared to 64 total silver screens in our normal
We had the quite-restrictive 0.070 jet in, which held the
flow down to a fairly low level, but it was still an abject failure, barely
foaming the peroxide before it got out the nozzle. We have seen behavior like that before, where some packs just
will not cleanly catalyze, no matter how small you jet the flow down, and then,
when you pass some magic threshold, they can take all the flow you can push
Our current formula is serving us well, but we will probably
try a few other things over time to improve the large pressure drop.
We repaired the bent leg from Saturdays test flights, and
finished packing and installing the 2 diameter engines on the seated
lander. With no jet (just the
restriction of a 4 AN fitting) at 600 psi, these engines make 100 pounds of
thrust, but we had an 0.070 jet and 500 psi pressure, which only gave about 32
pounds of thrust. This would still be
significantly more than the < 20 pounds thrust that the 1 motors would make
with 0.060 jets at that pressure.
We also carefully leveled the main engine bell with the
vehicle hoisted off the ground, so there wasnt any chance that a bent frame or
bent landing gear would give us an unintentional thrust vector. We also leveled the computer box, so the
initial attitude determination from the gravity vector would be aligned with
the leveled engine.
All the work paid off, as we made a perfectly well behaved
five gallon test flight. The attitude
engines had sufficient thrust to bounce it back and forth as needed, but not so
much that it was having huge oscillations.
The laser altimeter data was also all good, now that I am discarding the
occasional absurd value that gets sent out.
We could have flown it a couple more times like that, and
had a triumphant day.
We decided to go ahead and give the laser altimeter based
auto-throttle control a try. Instead of
using the joystick throttle, this is controlled by using the hat switch on the
joystick to just signal up 1 m/s, down 1 m/s, and hover. An initial tap in the up directions should
have brought the target altitude to about 2 off the ground, which would cause
the main engine to throttle up until it reached a small acceleration, then
throttle down as it approached the target altitude, then modulate the throttle
to hover there.
We set it all up, I tap the up button, the engine throttles
up, and KEEPS throttling up, with the lander leaping off the ground at a quite
rapid, and accelerating, clip. I
canceled the auto-throttle by bringing up the manual throttle within a half
second of it leaving the ground, and it started coming back down just before it
pulled the thethers taut. Our tether
system is much improved from the old one that broke, with solid welded attach
points on the frame, overhead lifting-rated shackles, and shock cord wound
through the chain links, so we are confident that it would have grabbed it
fine, but it probably would have given it a serious attitude adjustment. As it is, it came down with attitude control
and a little bit of main engine throttle, so it landed completely straight
again, bending all four legs and banging all the engine nozzles on the ground.
Here was my post-mortem the next day:
At first, it looked like we had a time lag on the altimeter
telemetry, but after looking at it more closely, we found that the altimeter
and accelerometers were all working correctly. The problem was that the
ball valve was opening slower than the computer thought it was, so when the
acceleration was such that it should begin to throttle down, it started moving
the target throttle point down from the current target of 75%, even though the
actual ball valve was only at 50%. This caused the throttle to continue
opening for another 200 ms until the target and actual crossed at around 60%.
I should have been checking for the target throttle position
getting farther and farther away from the actual valve position, which could
have bounded the time that it could overshoot. The only reason that it
maintained a target position at all was to allow the auto-throttle to move the
throttle at a slower rate than full-open / full-close. However,
simulation showed that the smoothest control was gained with it running the
valve as fast as possible, so there really isn't any reason to keep that target
throttle position at all, it should just be commanding the valve to open more /
close more based on the acceleration versus target acceleration. I will
change the code for this, after which I think that it will actually work.
The thing that bugs me is that I DID consider the issue of
the main valve lagging the target, and I had tested it on the simulator and
found it to not do anything too unusual, it just took a little longer to settle
down. Last night, I made the modifications to the simulator to let it
simulate the accelerometer sensor as well as the altimeter, and tried
again. It was just like my previous test, not too exciting.
However, after I changed the simulator engine thrust to be able to give a 1G
positive acceleration, and I had the simulator model the ball valve opening
curve a bit (very rapid changes around 40% throttle), I was able to get it to
behave just like the actual flight: it leapt three meters into the air, then
oscillated all the way back down to the ground.
There are two other mysteries with the results:
The telemetry log stopped just as I pulled it out of
auto-throttle mode. I may have clicked the pause button on the joystick
when I was canceling the auto-throttle, but there is no way of telling.
The computer lived through the whole thing, but we lost some useful data.
I am going to add logged information about what terminates graphing, but I also
want to start having a completely separate computer that does nothing but
capture every data packet that comes over the wireless net, allowing us to go
back and extract whatever we need irrespective of what the remote pilot system
chose to save.
We saw over 9 m/s^2 of acceleration on the vehicle, and it
wasn't clear that the telemetry we got was even showing peak
acceleration. It moved FAST. We don't understand exactly why it was
capable of moving quite that fast. The last time we weighed it, the
vehicle was 350 pounds dry, and we added 30 pounds of ballast and at least 40
pounds of peroxide, so it would need at lest 760 pounds of thrust. From
our last bench test of the big motor, at slightly under 500 psi tank pressure,
it should make 590 pounds of thrust. The attitude engines made 32 pounds
of thrust at that pressure, and two of them are on at all time, giving a maximum
thrust of 654 pounds.
The four attitude engines running half the time may give
more than 2x32 pounds of thrust because of initial inrush, which I seem to
recall was the case when we tested 50% duty cycles a long time ago, so that may
give another 30 pounds or so.
Still, it seems like we have some set of sensors
miscalibrated, either the load cell we used for the big engine, or one of the
scales we were weighing the lander with. The big engine's center of
thrust on the test stand may also have introduced a pivoting force that caused
it to read lower than actual at the load cell. I am pretty sure the
accelerometer is accurate, because I checked the 1G gravity vector both ways on
When we rebuild this vehicle, we are going to experiment
with using wire rope isolators from http://www.enidine.com
as landing gear. We are going to move
the mount points inboard so they are directly under the triangulated load
points. Extended legs in single shear worked
well for the 40 pound lander, but they just dont cut it on a 400 pound
lander. We are also going to make
engine mounts that keep everything high enough that the frame will bottom
before an engine nozzle does. On
Saturday, we pulled all the engines off, swapped out the dinged nozzles, and
replaced the broken fittings, so the propulsion system is ready to go again.
One thing we did notice was that the accelerometer data was
very smooth, cleanly ramping up to 9 m/s^2 now that I am smoothing of 8 samples
(40ms). This is going to be a whole lot
better for the auto-throttle than the noisy double-derivative (quadratic
regression, actually) of the laser altimeter data that I was doing. The flight computer software is updated to
just drive the valve as fast as possible, so we are ready to give it another
test in a new vehicle.
We spend most of Saturday working on the tube vehicle.
We pulled one of the tanks off the seated lander frame, and
finally got around to measuring it properly.
It weighs 62 pounds, and holds 115 pounds of water, which is 13.8
gallons. I didnt measure the length,
but it is 14 in diameter. That is a
3000 psi rated tank. For comparison,
the 45 gallon 150 psi rated fiberglass tank that I am trying to buy only weighs
46 pounds, and is 22 in diameter. 668
psi*gal/lb for the carbon tank is obviously way better than 146 psi*gal/lb for
the fiberglass tnak, but I havent found any stock carbon tanks built for <=
1000 psi usage.
We got centering rings made to hold the tank inside the
tube. We still need to epoxy coat them,
and probably add a liner of gasket material to snug the fit down.
We got all of our airfoil fins / legs cut and mounted to the
bottom bulkhead. We still need to pin
them so they cant twist, add metal plates for load spreading, and mount the
rubber bump stops on the tailing edges.
We got the main engine mounted on the top of the bottom
bulkhead, so the nozzle doesnt protrude as far. We have a Kevlar-phenolic ring under the flange, and alumina-silica
insulation clamped around the catalyst chamber, but we still plan on doing a
full-vehicle static test to make sure nothing catches fire from the heat inside
We made holes and a mounting strap for the laser
altimeter. We still need to put a piece
of lexan underneath the laser altimeter lenses so they dont get peroxide on
We plumbed up our full upper manifold with pressure gauge,
pressure transducer, temperature RTD, manual purge valve, quick connect, and
quick connect bleed valve. We still
need to do a little work on the lower manifold to plumb to the attitude engines
on the bottom side of the lower bulkhead.
We vented another servo ball valve, because the one on the
lander was killed in the crash. We made
a long enough cable to reach from the valve at the bottom to the electronics
box on the top bulkhead.
We discussed how we want to set up the parachute system and
We have a number of tests to do before flying the vehicle:
Full power captive test to smoke check the vehicle. We will probably flip the tube upside down
so the main engine points straight up, and run the propellant from the
trailer. We will run a full five
gallons through it like that, and see how hot all the different parts get.
Parachute shock cord drop tests. Joseph can suspend the vehicle off the ground from the shock
harness with his tractor, and we can let it come up hard on the cord from
progressively greater drops to simulate parachute opening shocks. We dont want this big thing pulling apart
several thousand feet up.
Landing gear drop tests.
If we get wire rope isolators on the bottom bulkhead, it should land
nicely from hover tests, but if we only have the rubber bumpers on there, we
wont expect too much from it. It will
be interesting to get accelerometer logs from these tests.
Swinging attitude control.
We should give it a good push while it is suspended, then engage the
attitude engines to see how much it will be able to stabilize itself while
coming down under parachute. We hope to
be able to throttle up the main engine to an on-gear soft landing after coming
down most of the way on the parachute.