August 18, 2001 Meeting Notes
In attendance:
John Carmack
Phil Eaton
Russ Blink
I have added dust caps to all the AMP CPC connectors on the
electronics box. They aren’t
waterproof, but it is still an improvement over having open pin holes to the
inside of the box. We did have once
case a while ago where hosing down the engines got a few drops of water inside
the box.
I have tested a 0-15 psi absolute pressure sensor for
barometric altitude determination. This
will be integrated into the electronics box to supplement GPS before we do a
high altitude launch. Notably, this
sensor is 1/10th the price of most other Omega pressure transducers,
because it is used in automotive manifold applications.
We now have a delivery account set up with a local nitrogen
distributor, because we were up to refilling our tanks every week. We reworked the fill cart some more so that
it can handle the largest high pressure bottles. One bottle is still only good for a bit more than two fillings of
the big lander tank to 600 psi. Once we
get the main lifting engine in, we will probably reduce the pressure a fair
amount, and loading more than a gallon at a time will also reduce nitrogen use,
so we will probably get three flight tests per nitrogen bottle change.
I wanted to cut off a 2’ section of the sonotube so we could
lay it flat for evaluating some things.
I started with a cutoff grinder, but that made a smoky mess of the
cardboard tube, and didn’t cut very well.
We got a little jig saw from home depot, and that went through it pretty
quickly.
Phil was working most of the day on drilling and tapping all
the flange holes for the 700 pound thrust motor. We didn’t quite have enough time to fire it, but we should on
Tuesday. We will run it initially with
restricted plumbing, so it is only expected to make 150 pounds of force. When Russ finishes the 1/2" tank
manifold, we will water test it flat out with the big valves to see if we
rupture the spreading plate, then we will take it out to our test range for a
flat out run.
Big Lander Hops
Analysis of the last test hops showed that the control law
gain that was appropriate for the small lander was too high for the big
one. While the total thrust to weight
is the same, the polar moment is relatively smaller on the big one, and the
lever arms are longer. The high gain
was making it far overshoot the desired rate when making corrections, which
caused it to far overshoot again on the way back. It flew, but it was very lurchy.
The first hop today was with the gain dropped from 0.025 to
0.01, with four liters of peroxide at 500 psi.
It couldn’t get stably into the air with the gain set this
low and the offset CG, and it tipped over again on landing.
We decided to add the outrigger legs to the vehicle to avoid
tipping it again. That added 40 more
pounds to the vehicle, bringing it up to 175 pounds dry. We didn’t add the cross braces, which would
have tacked another 40 pounds on it. It
looks like the extension legs are strong enough for the attitude engines, but a
very hard landing might bend something without the braces.
I left the gain at 0.01 for the next hop, because the
engines had moved much farther out, increasing their torque. We filled the tank to 600 psi to compensate for
the additional mass. It got into the
air, but it started rotating almost immediately. It turned out that we had canted the engines the wrong way. Anything that can be backwards, will be. We drew arrows on the legs to hopefully
prevent this from happening again.
There was still a fair amount of overshoot at this gain
level with this engine configuration, but it was less than at 0.025 with the
short legs.
For the third hop, we corrected the roll angles and I
dropped the gain slightly to 0.008.
The engines started clouding up again on this run. They still made effectively their full
thrust, because I was able to lift it off the ground, but I set it back down
quickly because my view was getting covered up. This gain level was probably a bit too low, because the lander
had a very lazy correction rate. I
think I will leave it at 0.01 for our next set of tests.
http://media.armadilloaerospace.com/misc/BigWideFrame.mpg
We still aren’t sure why these engine cat packs aren’t
lasting very long. The cat packs in the
small lander have run dozens of times without problems, but the big ones seem
to be wearing out after four or five runs.
The foam was plated slightly differently on our current batch, which is
the leading guess. The foam in the old
packs was plated so thick that the foam would crack when bent, while it is more
flexible on the current foam. The other
difference is that the old packs were highly compressed, nearly two to
one. It would be nice to get rid of the
questions regarding plating. We still
have pure silver screens we could use, and I have made another inquiry to a
different company about pure silver porous materials.
We need to increase our minimum propellant load to at least
six liters for each hop to get a reasonably long hover time. Once we get a lifting engine in the center
so we can tolerate more pressure drop, we should be able to load up to 16
liters or so. With a pilot, that will
still only be about 15 seconds of flight.
We are probably going to get a longer tank to extend that.
I am working on a completely different flight control
algorithm that treats the solenoids as binary devices, instead of using PWM to
simulate a proportional valve. This won’t
really have a gain parameter, and should be adaptable to a wider range of
vehicles, but it will rely on very low latency in the sensing and
actuation. We will probably test this
out first on the little lander to save peroxide during the tests.
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