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1/4 Scale Ballistic Evaluation Motor (BEM)
Third Test Firing)

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 Tulli 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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