Jul 24, 2001 Meeting Notes
Twist grip Magura throttle for manned vehicle
Dremel cutting discs
More hazmat suits
Degreed angle bubble level
New pressure gauge for the demonstrator
Battery for master cutoff computer
Battery for test stand box
Bolt down wire ties
Ratchet terminal crimper
Russ standing by our two vehicles:
The electronics box is almost totally complete for all the
capabilities it needs for the next few vehicles.
The external connectors to the electronics box are now:
Four engine solenoids
Main engine ball valve / position potentiometer
Tank temperature RTD
Tank pressure transducer
Twist grip throttle
All of the connectors are AMP circular plastic connectors
(CPC), which are a pleasure to work with if you have the right crimper for the
I still need to add a connector for the master cutoff
watchdog signal, a connector for the manned vehicle pilot joystick, and a
barometric pressure sensor.
I was going to add a magnetometer for initial earth relative
orientation (it goes nuts when the engine solenoids are operating), but I dont
have three free A/D channels any more.
I have a different magnetometer that talks over a serial port, but I dont
have a free serial port, either.
The current box doesnt have room for another board in the
PC104 stack, so I am probably going to shop around for some more integrated
boards that will let me get four serial ports some way. Worst case, I can add USB serial ports if I
need to, but I dont trust that as much for real time things as a normal serial
port, and Im not sure how much Linux grief I will have to go through to use
them. One thing that we are going to
have to do eventually is move away from using the PCMCIA 802.11b wireless
adapter to something higher power, which will probably talk to the CPU over Ethernet
(that will also let us shield the entire PC104 stack, which should help some
noise issues). There are PC104 CPU
boards that have Ethernet and VGA (for pilot instrument panel displays) on a
single board, which would allow me to ditch the PCMCIA board and the messy
supporting linux infrastructure for it.
We added mounting brackets to the box today, so it is now
rigidly screwed to the platform, and we dont need to mess with the restraining
hose clamps we used to use.
The current box weighs 20 pounds fully loaded. All the little things add up
I have extended our remote pilot laptop application to
visually display all the data the computer collects, as well as a 3D graphical
representation of the vehicle attitude.
It doesnt update during actual flight, because the screen redraw can
cause a noticeable increase in control latency and variability.
Master Cutoff Computer
I have all the parts now for the master cutoff watchdog
computer. It consists of a motor drive
board, a basic stamp board, its own private battery, and some connectors. The master cutoff valve goes before the
attitude engine manifold, which is before the lifting engine throttle (on the
manned vehicle). The idea is that if
anything catastrophic goes wrong, like the main computer crashing or losing battery
power, the cutoff computer will notice the loss of a watchdog signal and shut
off the peroxide to prevent any additional thrust. This also addresses any stuck on valve conditions.
Im not sure yet if I want to spend a serial port to
communicate with it with feedback, or if I just want to have a digital IO line
The big frame is at Long Range for fitting now. We measured lines for some plumbing and
electronics tonight, and we will be doing the necessary cable building on
The large foam blocks for the landing gear will be here by
Russ has all four attitude engine bodies finished.
Phil has the drawings for the main lifting engine done, and
the metal stock should be here soon.
Bob is going to take the frame back on Saturday for any
final modifications, then we are going to get it all powder coated so it doesnt
rust as easily as the small vehicle.
I have the twist throttle and main engine ball valve hooked
up to the computer, but I have some more work to allow the software to run with
either the four engine demonstrator or the five engine manned vehicle (both remotely
piloted and physically piloted).
The vehicle is turning out heavier than we expected (that
seems to be chronic with rockets). With
all the engines, foam, and electronics mounted, it is going to be right around
200 pounds dry. We may remove some of
the bracing tubes, because the main cross members are a lot stronger than they
need to be. Once we have some
experience with how the attitude engines deal with the offset CG, and we arent
too afraid of tipping it over, we might be able to move the engines inboard
quite a bit and get rid of most of the external structure, which would save
nearly 70 pounds.
Starting the pipeline for the next two vehicles after this,
I found a supplier with a lot of online data about their filament wound pipe:
These are considerably heavier than we would want for an
airframe (even a pressurized one), but they can probably make lighter runs.
We are looking at a 2 diameter demonstrator that will
basically be the propulsion system from the manned vehicle in an aeroshell with
side firing attitude engines. This
should go supersonic and let us learn about a lot of new issues.
Looking even farther down the road, we are getting some 48 Sonotube
to allow us to start seeing how we can seat people inside larger frames. A real vehicle would be made out of
composites, but for something to crawl around in and drill holes in, Sonotube
should work just fine.
Pressure Falloff Log
We finally got around to logging pressure falloff during a
tank blow down.
It was interesting to notice that paying close attention to
the pressure showed that the pressure was dropping visibly within a few seconds
of ceasing pressurization. The tank
gets rather hot when pressurized, and it cools off rapidly enough that 20 or 30
psi is lost within 30 seconds. We
should make another log of just letting the tank temperature equalize to see
how much it changes over several minutes.
During blow down operation, the tank gets cold enough that
we have formed ice in the past.
The tank is 8.5 liters, we loaded 2 liters of water, and
pressurized to 600 psi, then let the engines pulse at a rate that was roughly
equal to the flow during VTVL flight (about 10 seconds to expend 2 liters).
The pressure graph is a bit rough while there is liquid
being expelled, then it smoothes out when it starts to vent just gas. The
transducer is in the distribution manifold, so the valve pulses are probably
showing up as lots of little water hammer reverbrations.
It drops from 600 psi to 400 psi when the water is vented,
which is over 50 psi of additional drop due to temperature changes.
We will probably repeat this test with 4 liters and (if we
can draw a good enough vacuum) 6 liters of water, so we can gauge the limits of
pure blow down tank loading for a pressure range.
Fiber Optic Gyro Bias Issue
We caught an interesting effect tonight. I had the remote pilot dashboard running
next to the lander inside the garage with no liquid in it, and we were testing
the various sensors and solenoid actuations.
The drift on the gyros by themselves is about a degree a
minute right now (better A/D will improve this) when it is just sitting there,
but when I held down the button to fly it, the drift increased very
significantly, to nearly a degree a second.
Looking at the data logs, we could see that powering all
four solenoids drops the main battery voltage from 12 to 10 volts. When the voltage dropped, the gyro bias
points changed. This was somewhat
unexpected, because it explicitly takes an unregulated supply voltage.
Ideally, the FOGs should be read with a differential A/D
converter, but the A/D board I am using doesnt let me select that on a
per-channel basis, and I dont have enough channels to put the entire board
into differential mode, so I was reading the signal single ended. The docs mention a slight loss of precision
when this is done, but they didnt elaborate.
I still had the reference voltages wired up to the A/D
board, so I was able to look at them in the data logs. The reference voltages arent supposed to
move due to any physical action, but they do move when the supply voltage
It looks like I can do my own differential A/D by just
taking single ended measurements of both the signal and reference voltages and
subtracting them myself. That is going
to take my last three A/D channels, but it should make this problem go away.
I never noticed this before, because my bench testing never
had 35 amps of solenoid current draw during operation.
It is a little surprising that this didnt cause more
noticeable problems during our flight tests.
Having all this data logged is a Good Thing. A minor interesting observations was that
the 5v regulated level actually moved slightly in the opposite direction from
the battery voltage when the battery was at 12v, the 5v supply was at 4.91v,
but when the battery dropped to 10v, the 5v supply was at 4.94v.