January 31, 2005 notes
The time on Sundays that I used to spend writing updates and
such has been taken over by family duties, so updates are probably going to
remain sporadic for the foreseeable future, except for when we get something
really exciting done.
We had to build three separate motor drive boards to finally
get something that works reliably. The
first two used integrated motor drive chips that had supposedly helpful
features like slow ramp up and overcurrent protection. We got the board up and running pretty
quickly, but we had intermittent faults when using it to actually drive the jet
vanes. If the overcurrent protection
just limited the current draw to a maximum, that would have been fine, but both
chips would trigger a fault condition and require a reset signal whenever
We tried adding inductors in the lines to limit the current
spikes, but it didnt help.
When we tried a different motor drive part for the second
board, we also added LEDs on every conceivable point on the board, which was
We tried adding software minimum turn-around delays for
changing motor directions, which helped, but we could still occasionally get a
chip to fault after extended running even with 50 millisecond delays, which is
a lot more delay than I wanted to add. With
a small delay, none of the motors would fault if they were all driven together,
but with random directions on each of four motors it would still get a fault
fairly quickly. There is evidently
enough noise that the smart chips arent doing so well. As a confirmation about our earlier
electronics problems, we did have a computer reset while testing the computer board
and motor drive board from a single battery.
We havent had any problems whatsoever when they are running from
isolated batteries in their own isolated and shielded cases.
For the third board, Russ went back to a discrete transistor
design like our original motor drives, and this works fine. Thankfully, the company that we are working
with for the boards now has been turning them around for us in less than a
We did a practice load and warm-up test of the full vehicle
on Saturday, which went fine. The one
thing we had an issue with is freezing the loading check valve during nitrogen
pressurization. We plan on trying to
hover the vehicle on Tuesday if the weather cooperates. There are still two things we want fixed
before we do boosted hops: The
differential pressure transducer we are using as our fuel gauge failed again,
and we are waiting for it to be repaired.
Since taking off without enough propellant was the cause of our last
crash, we arent going to fly without it.
The other thing we havent finished is the isolated voltage signals for
the motor drive and master cutoff batteries, which I really want to get into
(The spindly thing hanging off the right is the detachable
vacuum pump for propellant loading)
The other thing we have done on the vehicle is set up a
mounting point for up to 100 pounds of lead ballast on the nose of the
vehicle. We are interested in doing
disruption pulse recovery testing while hovering both with and without the
ballast, which will move the CG considerably.
We will probably make the first boosted hop with the ballast weight,
which will give the jet vanes a lot more authority relative to aerodynamic
(The GPS antenna mounts on the top plate)
We are attempting to fix the last LOX engine by covering the
cracked epoxy shell with a thick layer of epoxy reinforced with milled fiberglass
and covered with some fiberglass cloth.
Very short chopped fiberglass would have been better, but we didnt have
any on hand. It might have been
sufficient to just use reinforced epoxy with the fiberglass cord. The hoop-wound cord doesnt have any
strength in the axial direction, but it is a convenient way to get a uniform
thickness layer over the troublesome nozzle section.