May 6, 2006 notes
continued to have problems with the current engine melting lots of wires on the
vertical test stand, so we finally broke down and made a horizontal test
stand. I have resisted this for a long
time for two reasons: When possible, you
should test the same way you fly, and we fly with the rockets pointing
down. More importantly, it is easier to
have catastrophic hard starts in a horizontal configuration, because a
significant amount of propellant can pool inside before running out the nozzle. Several years ago when we were testing 90%
peroxide / kerosene biprops, we had a hard start due
to a leaking solenoid letting kerosene seep into the catalyst pack. After collecting all the pieces from that, I
swore off horizontal test stands. At
this point, our torch ignition system and pressure transducer interlocks seem
to be reliable, so we are guardedly going to do some development work
we fired the engine horizontally, it was clear why we were having heat
problems: the plume was 35 feet long!
The film cooling on the current injector (20% of fuel) was, if anything,
far more effective than necessary, since the graphite throat really didn't get
very hot after a couple twelve second burns.
The film cooling shows distinctly in the exhaust plume as the cloudy
layer around the mach diamonds.
are going to angle the engine down into the water more in the future, and add
some water spray directly into the nozzle to reduce noise.
been closing the valves with a lox lead, so the fuel always shut off last. This was intended to prevent raw lox from
ever shooting at the hot graphite nozzle, but the effect was to always give us
a big puff of flame on shutoff, often leaving some burning fuel backwashed onto
the motor. I am now shutting them off simultaneously,
which avoids the flaming fuel at the end of a run.
carbon-fiber-with-special-binder reinforced graphite chamber / nozzle
combination from Cessaroni Aerospace is in, but we
havent fired it yet:
film cooling, I have little doubt that this will run for 360 seconds, but the
latest rules for the Lunar Lander Challenge will allow us to swap chambers
between the two 180 second hops if necessary.
Hold Down Test
set out to our 100 acre test site to do first a hold down test, then a hover
test under the lift. We had four
concrete blocks cast with a cubic yard of concrete each (4000 pounds) and a lot
of internal reinforcement to act as the hold down points.
had an initial problem with getting the engine started. It turned out that the last test we did with
pressure loading water into the fuel tank left a good bit of water in the fuel
line to the engine. We had blown the
entire helium load out through the engine, but the feed line is a corrugated 1
line, and the gas velocity just wasnt high enough to entrain all the
water. We had the vehicle up in the air,
but the igniter just wouldnt start. The
safety interlocks worked fine, so it never did anything other than make a tiny
puff of vapor come out the bottom of the engine, but even after a dozen
attempts, it wouldnt light. We let most
of the pressure out of the tanks and investigated, finding that the fuel coming
out of the solenoid barely smelled of alcohol, so we purged the lines until it
seemed that we had gotten all the water out.
re-pressurized and lifted the vehicle up again, and the engine lit right
off. I throttled up, and it immediately throttled
back down to idle. We had left a little
bit of slack in the chain to the blocks, with the intention that it would lift
up a few inches or so and come taut on the chains. Unfortunately as it pulled on the first
chain, it rotated the vehicle enough to trip a roll abort condition, which put
it into auto-land mode. This has been
happening more and more to us there are so many automatic no-go, abort, and
shutdown conditions that most of the time we try to test something, I wind up
having to disable something. This is
probably a good thing.
went back out and carefully lifted the vehicle up until the chains were taut,
and we started it again and ran it until lox depletion. The fact that it was lox depletion was a bit
of a surprise we expected it to run out of fuel first. When we analyzed the telemetry, it was clear
what happened. The roll thrusters were
going essentially 100% duty cycle, so the tank pressure dropped a fair amount,
but the lox had been sitting there long enough while we debugged the ignition
issue that it was self pressurizing to a fair degree. This told us a few interesting things: the burn was still stable with the fuel
pressure less than 10% over the chamber pressure, and the film cooling kept the
ablative chamber and graphite nozzle from eroding even though the mixture ratio
went from fairly rich, through stoichemetric, to very
lean at the end of the burn.
it was too late, and we didnt have the helium for pressurizing again, so we
werent able to put the vehicle on the stands for a hover test. All the telemetry looked perfect, so it very
likely would have flown fine. Im sure
well have to tweak the gains a bit, but this vehicle is probably capable of
doing the 90 second flight legs for the level one lunar lander
challenge right now.
second unfortunately was that we pissed off a neighbor with this test. Our site is 100 acres, but based on the
decibels at the point we were at, it was probably still 102 db at the property
line, which I agree is probably excessive for unsympathetic neighbors. Doing it at 8:45 in the evening probably didnt
help, either (we didnt expect to be that late
). Testing rockets is a dubious enough activity
that any determined person could probably get us shut down on some grounds, so
we arent going to push them on the issue.
We are looking at leasing some land down in McGregor, TX,
right next to the Space-X test site. If
their big engine tests dont bother folks, they wont even notice us. J
have 4000 pounds of anchor chain to use as the "tether" on our
upcoming flights without the lift. We
will let the vehicle fly with a reasonable length of 6000 pounds strength steel
cable connected between a leg and the anchor chain, so if the vehicle takes off
uncontrolled for any reason, it will just jerk up a few chain links, then
progressively lift a heavier and heavier length of chain until it rotates over
and comes crashing down. The vehicle
will be destroyed, but the damage will be strictly localized.
65" tanks are in. They are 65 ID,
3/8 original thickness 5086 aluminum, and weigh just under
400 pounds per sphere. The 36 ID
spheres we have been using are 81 pounds dry, 950 pounds full of water, and
burst at 760 psi.
By direct scaling, the 65 ID spheres should hold 5.86 x more volume,
and burst at 640 psi if the weights are distributed equally.
(note that if you cut the legs down a bit, you cut fit the
entire X-Prize Cup vehicle in the background inside the sphere...)
have made a big band clamp to sinch the hemispheres
together, with holes for tack welding every two feet or so. We were able to bang the 36 spheres pretty
close by hand, but the 65 spheres didnt have as close of tolerance.
realized something useful -- the old mill has a very stout rotary chuck on it
that we are going to be able to adapt to rotating tanks for welding. All we have to do is turn it sideways, crank
the table down, and rig up some sort of giant tailstock for it, and it should
be able to spin even the big tanks. We
bought a wire feed gun and a water cooled torch, and we have a new high end
welder coming in soon, so we expect to be able to weld complete tanks with one
continuous, high quality bead.
made a rendering of what the final 65" tank vehicle will look like:
differentially throttled 65 diameter vehicle is our current bet for commercial
service, but it looks like we are going to be detouring and building a
different vehicle first that is specifically targeted for the Lunar Lander
Challenge. We might actually have three
flying vehicles at the Cup this year, but that is a lot of work still ahead of
wiring note: I have started using the
shrink sleeve strain relief for CPC connectors instead of the screw-down clamps. I like them, because they provide two
significant advantages: Used in
conjunction with the CPC receptacles with an o-ring in the bottom, you get
water-tight wiring, and the strain relief doesnt unscrew. It has happened too many times to be
coincidence: every time someone new starts working with CPC connectors, they
will inevitably at some point unscrew the strain relief instead of the plug,
twisting all the wires badly.
couple other random pictures:
I am swamped the next month, and any update may be late.