May 30 and June 1, 2002 Meeting Notes
Phil Eaton (Tuesday)
Joseph LaGrave (Saturday)
An open question we have had for a long time was what the
appropriate ratio of catalyst pack area to throat area is. Our rule of thumb for the screen engines has
been to use a diameter ratio of 3:1 for the catalyst pack to throat. We ran back to back tests this week on a 1
diameter cat pack with two different nozzle diameters: a 0.33 and a 0.45.
Our previous attempts at measuring chamber pressure had not
worked out very well, because our pressure transducers started reading bad when
they got too hot. This time, we
connected the sensor to the engine with a piece of bent tubing, and poured some
brake fluid into transducer to buffer it.
We used our biprop fuel injection ring for the engine port, and we just
left the very small stainless jet in place, which also served as a low-pass
filter on the pressure trace. The
injector ring has gotten a bit warped after all the hot fires, so there was
probably some leakage, but the data all looked sane.
With the 0.33 nozzle and a 0.060 jet, we only saw 225 psi
chamber pressure from 600 psi feed pressure.
The run was slightly rough, because the 0.060 jet is a little marginal
for pressure drop.
With the 0.45 nozzle and a 0.060 jet, chamber pressure
dropped to 110 psi, giving slightly less thrust than the 0.33 nozzle, but
With the 0.45 nozzle and a 0.080 jet, the run was full
rough, indicating not enough pressure drop across the jet.
We put a pressure transducer off of a T between the jet
holder and the engine to see how much pressure was lost by the plumbing,
solenoid, and jet. With the 0.080 jet,
there was surprisingly little, showing 530 psi down from 600 psi tank pressure. A 70 psi drop is clearly not enough for
smooth operation. With the 0.060 jet,
we saw 460 psi, which seems to be sufficient for smooth operation.
So, we are dropping from 460 psi to 225 psi across the
catalyst pack, which is a very significant pressure drop, and the 3:1 diameter
ratio of catalyst pack to throat is probably as far as we can go with our
current screen packs.
Running at higher feed pressures has shown to give somewhat
better than linear increases in thrust with the screen packs, probably because
it pushes liquid farther down the pack, so the gas flow has less screens to
Our 4:1 nozzle expansions are all quite a bit overexpanded
for the current pressure ratios, so we could improve Isp somewhat by cutting
them down. We could alternately improve
chamber pressure by making smaller nozzles for our given pack diameters.
Our current packs have a total of around 70 silver screens
and 60 stainless screens. The 1 motors
did not work well with less screens, but the larger motors seem to be doing
well with 60 + 50 screens and a lower packing pressure, so they are likely
somewhat more efficient. We know for
certain that an all silver screen pack doesnt have the structural strength to
work, but packs that use silver plated screens exclusively can get by with lower
screen counts at the expense of shorter lifetime. We prefer the long life and repeatability of the pure silver
alternated with stainless.
We will eventually try some experiments with lower pressure
drop packs, but it may not really be an issue for us. Right now, our tanks are good for a whole lot more pressure, so
we can just run more tank pressure if we want higher chamber pressure. Our solenoids are good to about 1200 psi,
but we would have to change out our current 1000 psi burst disc if we go too
much higher. With biprop and hybrid
motors, you get the chamber pressure back up because most of the reaction takes
place after the pack.
Big Lander Testing
Several improvements have been made to the flight computer /
remote piloting software system, but the main loop is now taking three to four
milliseconds on the puny 100 mhz computer in the electronics box, which is
uncomfortably close to some of the sensor periods. I have a new 400 mhz panel-PC which we are going to use in future
vehicles, but we will probably be sticking with the current system for the
We tested with the laser altimeter today, but I still need
to get it to toss out some extraneous data samples before it can be used for
the auto-hover logic. We are using a
LaserAtlanta Advantage range finder, but it really isnt ideal for our
purposes. It has no convenient way to
mount (we wound up just strapping it to a tank), and while it can be controlled
over RS232, it cant be remotely powered up, and it has a power saving shutdown
This was our first time lifting the lander in months, with
the primary difference being that we now have stainless body silver screen
engines all the way around, and replacing the bent main engine. The engines are working great, with
completely clear catalyzation, and no sign at all of deterioriation. Russ commented that you couldnt even smell
peroxide in the exhaust. The fairly
rapid pack deterioration was the primary thing holding us back in the large
vehicle testing, and it looks like we have that well in hand now.
However, the little 1 engines had insufficient control
authority at 500 psi tank pressure.
They vehicle kept tipping shortly after liftoff, even with one engine
full on. When I went back and carefully
looked at the telemetry, I found that our main engine is not pushing along the
CG. This was shown by an axis rate not
being able to make any correction headway until the main engine was throttled
down, at which point it pulled around to where it should be. We hang balanced the entire lander, and the
new central engine is perfectly square, but the mounting brackets must not be
level. What we need to do is suspend
the vehicle, and shim the main engine until a level placed across the exit cone
is even in all orientations. We should
also level the computer box so the IMUs initial angle initialization based on
the gravity vector is true.
We bent one of the legs coming down on our second load of
We had a few options to increase the control authority:
Move the engines farther outboard, to get a better lever
arm. Putting them on the ends of the
legs would increase the lever arm by 50% or so. This would require new hoses, and would place the engines in
danger on a bad landing.
Run higher tank pressure.
We could go from 500psi to 800psi.
This would make the main engine throttle range lower, and makes any
catastrophic failures scarier, but would give at least 50% more control
authority, and a higher Isp on the attitude engines.
Move to the 2 engines, which could give us over 3x the
authority with the same plumbing.
We went ahead and started packing a full set of 2 engines,
although now that I have determined the thrust misalignment, I would be somewhat
more tempted to stick with the smaller engines. Too much control authority increases the amplitude of the
hovering oscillations, and the larger engines have somewhat more latency to
thrust, which decreases the period of oscillations. We will probably have some fairly large oscillations when flying
with the big engines, so we may wind up running them with jets not much larger
than used on the 1 engines.
We need to get some more stainless screens and spiral rings
to finish the last 2 engine, but if Bob can fix up the leg soon, we might by
flying again on Tuesday.