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Author: Richard Nakka
July 29th, 2006 had been
firmed up as the date for the test firing some weeks ago.
This was to accommodate a TV film crew that had requested
to be present to film the event for a future documentary
on rocketry. Work on rebuilding the motor, constructing
a test stand, as well as design and fabrication of the
measuring instrumentation had gone really well. The only
real glitch was related to preparation of the propellant
grains. It was noticed after the propellant had cooled,
that it had slightly shrunk and consequently disbonded
along one side of the casting tube. As such, the casting
tubes were abandoned in favour of a post-cast inhibitor
consisting of multi-ply cotton fabric impregnated with
polyester resin, which was applied to the bare propellant
grains. The problem of partial disbonding was found to
have re-occured. It seemed that the cotton fabric swelled
from immersion in the liquid resin and "grown"
slightly, causing pockets of disbond (as determined by
the coin "tap" test). Only half the grains had
been affected. Repairs were made by filling the disbond
area with cyanoacrylate or by injection of liquid epoxy.
This appeared to eliminate 90-95% of the disbond. These
grains were loaded into the more robust 1st phase motor,
in case some overpressurization resulted. By the evening
of July 28th, all preparations for the test were completed
as planned.
Although the summer, to date,
had been a hot & dry one, the forecast called for
a mix of sun and rain for the 29th, and continuing hot
weather. Feeling confident the rain would not be an issue,
early Saturday morning Tarun Tuli and I headed to the
test site, a two hour drive to the Columbiad Commercial
Test Range. Misfortune struck (literally) not long after
our departure. While stopped on the expressway as traffic
had come to a halt, we were rear-ended, with the resulting
impact pushing my car into the vehicle ahead of us, which
was towing a boat. The propeller made its indelible and
rather ugly marks in my car's hood and grille. The exchange
of license and insurance information between the three
parties involved set us back a half-hour, and had us wondering
if this was an omen of things to come.
Just as we came within 5 kilometres
of the range, it started to rain. Light at first, the
rain's pace quickened as we came to a halt at the test
location. The film crew was already there and we apologized
for the unavoidable delay. Luckily, the rain soon stopped
and after being "wired for sound" we were able
to begin assembly of the test stand and instrumentation
shelter. With three of us working (Richard Graf of Columbiad
Launch Services kindly assisted us), things proceeded
smoothly despite having to do (or say) certain things
twice. The rain then started to fall again. However, we
took advantage of this wet spell to assemble and load
the rocket motor in the dry and comfort of the range trailer.
The igniter/pyrogens were also installed at this point.
This process took well over an hour, at least partly due
to accommodating the wants of the film crew.
The rain continued ceaselessly
for at least another hour making further progress impossible.
The instrumentation could not be set up until the rainfall
ended and until we could be confident that further rain
would hold off. Good fortune then came our way as the
rain stopped and the sky soon partly cleared. Knowing
that the weather could change at any time, we worked in
a timely and efficient manner to set up the instruments,
mount the motor, install the pressure transducers and
loads cells, mount the thermcouples, and lay out the ignition
systems (two, one for each motor phase). Everything proceeded
smoothly without significant problems. Nearly as we wound
up the preparation, I discovered that my digital camcorder
was malfunctioning. It appeared the aperture was stuck
in the closed position, a problem I suspected was caused
by the high humidity. After several minutes of troubleshooting,
aided by consultation with the professional cameraman,
I realized that a solution was not at hand and I abandoned
the camcorder. We instead mounted Tarun's analog camcorder
on the tripod. I also had my digital still camera which
I then decided to use to take a videoclip of the firing,
as backup footage.
At last it came time to commence
data acquisition and to prepare to connect the igniters
to the ignition box. I took on the task of attaching the
igniters and arming the two ignition systems. Since the
cameraman wanted footage of this, I "faked"
the process a couple of time while the camera rolled.
Everyone except me then headed to the viewing site (where
the remote firing box was placed) 500 feet distant. I
then repeated the ignition setup procedures for real,
then departed to join the others.
Countdown was delayed for several
minutes while the film crew tried to decide how to best
cover the key moments that were to follow. Eventually,
we got things sorted out and Tarun (who was assigned the
task of pressing the firing button for both phases) announced
the countdown to ignition. After a miscue, the count was
repeated: 5 - 4 - 3 - 2 - 1 - fire. Shortly after, a "pop"
sound was heard as the burst diaphragm ruptured and nearly
immediately the motor roared to life, firing with a loud
shriek and sending a large white smoke plume high into
the air. The powerful thrusting continued for about three
seconds, then rapidly tailed off as the propellant was
consumed. Black smoke could be seem issuing from the nozzle
from the burning delay plug (and possibly also the inhibitor
material). I pressed the "start" button on my
stopwatch as soon as burnout occurred. The planned delay
period between burnout of the first phase and ignition
of the second phase was 17 seconds. As such, when the
stopwatch read "12" seconds of elapsed time,
I announced the second countdown: 5 - 4 -3 - 2 - 1 - ignition.
Tarun then pressed the ignition button. Nothing happened.
We frantically tried the button again and switched the
safety key off/on several times, but to no avail. I then
performed a "bypass" of the switches by shunting
the ignition leads with the brass key, but even that left
the motor lifeless.
After several minutes had elapsed
I cautiously approached the test stand and as soon as
I was close enough to see the ignition box, it became
immediately and painfully clear what the problem had been.
I had failed to throw the "safe/arm" switch
of the second ignition box to the "arm" position.
The idea of then arming the system and proceeding to fire
the second phase was briefly considered. This was decided
against, as it would not prove to be of enough value to
fire the second phase after the motor had several minutes
to cool down. Instead it was felt that most benefit would
be gained by aborting the test at this point and later
reloading the first phase for a repeat test in the near
future.
I was disappointed in myself
for allowing such a thing to happen. I'd done this procedure
literally a hundred times before and had not previously
forgotten to arm the system. I did realize that the distraction
of the film crew played a role, and as such, learned a
lesson about something that we'll likely have to cope
with for future stages of the SugarShot project, when
film crews will almost certainly be once again present.
Despite the incompleteness of
the test, there were some positive outcomes. The data
acquisition systems for the thermocouples, load cells
and pressure transducer worked flawlessly. Good data was
collected which will be analyzed in detail in the days
to come. A brief look at the data showed that the motor
did overpressurize to some extent, reaching a peak pressure
of approximately 1300 psi (9 MPa). As such, some breaching
of the inhibitor clearly did occur, as had been considered
a possibility. However, also as expected, the extent of
such was not serious. Maximum temperature on the casing
reached just over 100 Celsius. A post-firing teardown
of the motor showed it to be in excellent condition. The
delay plug was fully consumed and the mid-bulkhead diaphragm
that isolated the two casing sections was found to be
intact and effectively isolated the second phase from
delay plug combustion heat.
Overall, I found the whole experience
to be exhilarating and rewarding, and having the film
crew there added a whole new element of excitement and
challenge. It was great to see it all come together so
well and nearly work out as planned. Lessons were learned
and needed changes (mostly small ones) will be made to
ensure the odds of the next firing being fully successful.
Photos and video: http://sugarshot.org/gallery/showcase_motor_bem3.html
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