July 11, 2004 notes
We set up for another captive hover test of the big vehicle
on Tuesday. Everything proceeded well,
but as I started the flight control software up to warm the engine, the self-test
failed because one of the jet vane actuators was not responding. The automated self test on every startup now
is a Very Good Thing, because I very likely would have missed this condition,
as the vane position graphs arent on the first visible page of telemetry
results. On the driver board, two
actuator bit LEDs seemed to be stuck on We connected the ribbon cable to a brand new driver board, and
found that they were still on, so the fault was in the AccessIO A/D + DIO
board. We had moved to a Diamond A/D +
DIO board for the A/D, since we had fried four of the AccessIO boards (probably
from the motor inductive kicks, in hindsight), but the digital IO pinout was
completely different, so we had been continuing to use the AccessIO board to
run the motor drives, because the DIO part seemed to still be functioning fine,
even though the A/D was fried. Apparently
this wasnt a great idea. It is odd
that it seemed to give up sometime late in the vehicle pressurization, because
I had started it and let it pass self test several times during the loading
We decided to just quickly swap the board for another one
(also with blown A/D) that we had sitting on a shelf, so we could use the already
loaded propellant. The replacement
board seemed to operate fine, but as the engine throttled up to liftoff thrust,
it started chugging badly. We had
increased the tank pressure to 300 psi to give it a little more thrust margin,
which usually adds stability to an engine, but it may also have just been
settling some from the test runs. A
heavily chugging engine confuses the auto throttle software, because vehicle
acceleration bounces back and forth across the desired acceleration line no
matter what the vehicle is doing. This
usually leads to it not continuing to change the throttle, so the vehicle
wouldnt throttle up past the 60% mark.
This left it bobbing at the end of the tether after blowing out the metal
support stands (which worked perfectly after we added some T feet to keep them
from toppling over too easily).
While it was hanging there under thrust I shoved the
joystick hard over to see how the attitude control was working at the
almost-hovering point, and the vehicle rotated very rapidly. This change in attitude seemed to give the
control system just enough of a kick to let the auto throttle bring in a little
more throttle and let it lift off. It
straightened back up very rapidly, and started rising. The engine roughness also made it difficult
to bring it to a hover, and I thought it might slowly bump into the lift forks,
but it started descending a few inches shy.
What happened next was unexpected the tether looped itself underneath
an eye bolt above the hatch that we use for horizontal lifting of the
vehicle. We wound up with the vehicle
stuck hanging there, six feet above where it started. We lowered it back down with the lift and took stock of the
The bolted down road plate worked well as a blast deflector.
The automatic self test was a very good thing to implement.
We needed to remove all the protruding things that the
tether could hang up on.
The jet vanes provide lots of very rapid control
authority. The big vehicle seemed to
change attitude as fast as the little one.
We need to reduce the open area in the engine spreading
plate to bring the engine back to smooth operation.
We hung the vehicle from a scale for an up to date weight,
and in dry hover test form, the big vehicle weighed 1650 pounds. Add in the thin nose cone and the shock
absorbers, and it is a bit over 1700 pounds, plus we pump nearly 100 pounds of
nitrogen into it for pressurization (almost all of that can go away if we use
helium, but that would get expensive to throw away during testing). This is over our target weight. Each drum of peroxide plus methanol is about
The test stand 12 engine had made 3700 lbf at 378 psi tank
pressure, but the vehicle engine will be making less than that due to the
compressed hot pack (for better warmup), the reduced spreading plate area (for
better stability margin), and the jet vanes adding some drag.
Given that we missed both our target weight and target
engine thrust, it looks like it isnt going to be worth it to fly this vehicle
configuration under the burn time waiver, because it would just be creeping off
the pad with under a quarter G of acceleration. The new plan was to modify the engine, go ahead and do the
captive hover tests and the 15 second boosted hop locally as-is, then move
everything over to the 48 diameter tank, which saves enough weight to let it
do a decent long burn flight.
Conservative simulation shows it going to 10,000 and lading 120 seconds
after liftoff at the no-launch-license impulse limit, although it is still throwing
away most of the impulse as gravity loss and landing (it will just barely hit
terminal velocity in descent before powering up for landing). We need to build bigger engines.
I ordered new 12 spreading plates, but they werent going
to be delivered in time, so James took on the tedious task of manually closing
up a quarter of the spreading plate holes with braze.
Russ decided to weld in a couple more aluminum tubes to
stiffen up the landing support frame.
On Saturday, we got everything back together for another
test. The vehicle continued to pass the
self test all day, but after warming the engine up and starting the
rise-to-hover, we lost the computer.
Shit. We were worried that the
master cutoff hadnt worked, because the engine was still making quite a bit of
noise, but it turned out that the cutoff had worked fine, but it was
controlling the leaking ball valve, so there was a decent trickle of propellant
getting by the still-open throttle.
After the computer rebooted, I restarted the flight control software,
but the self test failed with all the jet vanes in off-scale positions. The throttle was still working fine, so the
engine finally quieted down when that was shut again.
We decided to just set it down on the road plate, vent the
tank pressure way down, and burn off the entire drum of propellant. In hindsight, this was the wrong thing to
do. The road plate held up fine, but
the exhaust backwash completely fried all of our vane actuators and caused a
hot crack in one of the aluminum landing frame struts.
When we laid the vehicle on its side, we found that all the
vanes were oriented almost perpendicular to the exhaust flow, and most of them
were somewhat bent. We may want to work
out some kind of physical stop for the vanes to keep them from ever being
pushed perpendicular, but it would have to cooperate with the internal limit
switches. The vanes are always fine
through tests when they are in the normal operating range, but if the computer
hangs and they get pushed perpendicular, they get bent. We are probably also going to try clipping
the top on the vane a bit to possibly make it self centering, but that will
increase the load on the actuators.
When we opened the cabin, there was smoke from the motor drive
board. We had smoked transistors before
when driving to a partially shorted motor, but much to Russs surprise, the diodes
were burned out this time. We think
that the jet force on the vane pushed it hard enough to generate sufficient
power to burn the diodes, and somehow upset the shared ground enough to reboot
We still have some snubbing components for the motors on
order, but we are going to completely separate the motor controllers from the
rest of the electronics anyway, both physically and electrically. We have had to share a common ground due to
the motor drive board also controlling the ignition, which has to ground into
the motor and the rest of the chassis, but I am going to add some separate
solid state relays to control the spark ignition from the main computer box so
we can truly isolate the motors.
We are going to rebuild most of the electronics and do just
about everything we can think of to improve reliability, because computer
failures have been the biggest problem on the big vehicle. I am going to replace the CPU board, because
it seemed like it was getting easier and easier to kill it when we were investigating
the inductive kick problem, and we may have permanently damaged it in a subtle
Since we have a lot of rebuilding to do, we are going to go
ahead and move to the 48 tank.
We also worked on the layout for our new trailer, which we
are going to have to outfit for the trip out to the southwest regional
spaceport for our waivered flights. We
should be able to carry enough nitrogen and propellants for several waivered
flights, but we also want it to be able to hold enough for a single space shot
flight. We had planned on using locally
supplied nitrogen and lift trucks, but after Neils scouting expedition, we
decided it was going to be worth the effort to be self contained. We should be able to just pull up, unload
and erect the vehicle with a truck carried A-frame, load it up and fly it.
We also got in the big batch of mini-nozzles for our
experimental multi-nozzle engine:
The support plates should be here soon.
A clarification some people took a comment I made about not
bothering to order something if Burt had a perfect flight the wrong way. We are carrying on with our work no matter
what happens with the X-Prize, we will just have a slightly different emphasis.