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

Author: Richard Nakka


In the duration since the previous test firing that occurred at the end of July, much work had been done to resolve the propellant disbonding issue that plagued earlier grain casting attempts. A satisfactory solution was eventually developed. This involved curing of the propellant under clamping pressure, in conjunction with a casting tube manufactured from high-temperature, absorbent gasket material. For this fourth firing, it was only necessary to cast six grain segments for reloading of the 1st phase motor casing. . The 2nd phase had not been fired in the previous test, due to an inadvertent failure to arm the ignition system. The grain segments for this test were made to a slightly larger diameter than previous, to achieve a close fit within the casing. This was done as it was decided not to line the 1st phase casing with thermal insulation, as had been done for the preceding test, due to the difficulty that was encountered in removing the degraded insulation material after firing the motor. The insulation had been epoxy-bonded to the casing. Removal required a great deal of peeling and scraping after soaking in solvent for several days. Other tasks that needed to be done in preparation for the fourth firing, besides propellant casting, were casting of a replacement pyrogen grain, and casting of the RNX delay plug inside the mid-bulkhead

The date for this fourth firing had been originally projected to occur in mid-September, but eventually slipped a week. By then, preparations had come together nicely and by the 22nd, everything was set for the test firing to take place the following day. Weather had been a factor during the last test, when rain made for complications and delays during setup of the test equipment. Although the forecast had suggested rain might once again show its unwelcome face, the forecasted probability was low enough that postponement of the test wasn't justified.

As such, early Saturday morning Tarun Tuli, Daniel Faber and I headed to the test site, a two hour drive to the Columbiad Commercial Test Range. This time the drive there was mercifully uneventful. We arrived just after noon hour. After resting for a short while, we proceeded to begin setting up for the test firing. Richard Graf, of Columbiad Launch Services, gave us a welcome hand in setting up the test stand, instrumentation shelter and with mounting the rocket motor in the stand. Preparations went very smoothly, without any noteworthy glitches. The only snag was a malfunctioning FRS radio, one of three brought along for communication. The rain held off, in fact, the sun broke through the clouds as we neared completion of the preparations.

After setting up the equipment, the rocket motor was assembled, the tandem load cells mounted in the stand and confirmed to be functioning, the two pressure transducers were mounted on the motor, and the eight thermocouples attached at their assigned locations to the motor casing and nozzle. Following this, it was time to fish out the final checklist that had been prepared in order to help ensure all final preparations would go smoothly and without oversight. This involved doing a confirmation that both ignition systems were fully functional, that data acquisition was activated, as well as confirmation that the video recording cameras were adjusted and turned on. I did a last minute walk around of the motor to get visual confirmation that nothing obvious had been overlooked or was askew. Following this, the others left the vicinity and took up positions at the firing safety area, while I stayed behind to do the final connections of the igniters to the ignition boxes.

Following the checklist procedure, electrical continuity to both igniters was confirmed, and then both ARM switches were thrown from the SAFE position to the ARM position. I alerted the others over the FRS radio that both systems were being armed, and subsequently hurried to the safety area to join the others. I had taken on the duty of announcing the countdown and, using a stopwatch, tracking the time following burnout of the first phase. The second phase was to be fired 18 seconds later. Tarun was once again assigned the task of pressing the firing button for both phases. After confirming that all participants were ready, and that Tarun had turned the key to arm the firing button, I announced the countdown.

Five - 4 - 3 - 2 - 1 - fire! Immediately the motor roared to life, firing with an exceptionally loud shriek and sending a large white smoke plume well beyond the treetops, high into the cool autumn air. The thrusting of the motor continued forcefully for about three seconds, then tailed off as the propellant was consumed. Black smoke could then be seen issuing from the nozzle from the burning delay plug and hot residue inside the motor. I pressed the "start" button on my stopwatch as soon as burnout occurred. When the stopwatch read "13" seconds of elapsed time, I began the second countdown: 5 - 4 -3 - 2 - 1 - fire! At this cue, Tarun pressed the second ignition button. After a brief moment, the motor once again awakened, roaring to life in much the same manner as it had moments earlier. The intensity of the sound was much the same, as was the burning duration, but in my mind, the three seconds seemed eternally longer, knowing that these were the critical seconds that had doomed the first two BEM firings. As the thrust tailed off and came to an end, accompanied by a weird, brief whistling sound, relief and awe silenced us all for but a moment. Then the cheers and hand-clapping of victory replaced the sudden silence, followed by hand-shakes all around.

The data acquisition systems for the thermocouples, load cells and pressure transducers once again worked well. 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 overpressurized to some extent, during the first phase firing, reaching a peak pressure of approximately 1700 psi (12 MPa). Initial thinking is that erosive burning once again factored into the performance of this somewhat temperamental propellant. Thermocouple data looked reasonable for the first phase burn, with the nozzle throat reaching a peak temperature of 450oC. (850oF.). However, the temperature data exhibited aberrant behaviour during the rapid temperature rise of the second firing. Both of these issues will have to be scrutinized carefully in the days and weeks to come.

A post-firing teardown of the motor showed it to be in excellent condition. Debris from the second phase grain inhibitor was found nested in the mid-bulkhead throat. This was likely the cause of the unusual whistling sound that was heard. Examination of the video showed a plume of black smoke that erupted briefly during the second burn, which was undoubtedly a sizeable fragment of ejected inhibitor material.

It was highly rewarding to see it all come together and culminate as well as it did. A tremendous amount had been learned over the course of the three prior BEM firings, as well as some valuable lessons in planning and preparedness. This allowed for many improvements to be incorporated. Undoubtedly, this will help us improve the odds of success for upcoming phases of the Sugar Shot to Space project. As it stands, this successful demonstration of the “two-phase”, re-startable rocket motor concept is a milestone in our quest for reaching space.

Photos

Preparations

  1. Casting setup with provision for clamping the propellant during cure
  2. 6 newly cast propellant segments for 1st phase motor
  3. A pair of freshly cast pyrogen grains

    Setting up for the test
  4. Unpacking our gear at the test range
  5. Tarun checking out the laptops used for data acquisition
  6. Daniel and Tarun adjusting the motor mounts
  7. Mounting the thermocouples is a three man job
  8. Daniel in the thick of things
  9. Motor just moments before firing

    Fire!
  10. Motor under full 1st phase thrust
  11. After 1st phase burnout, black smoke issues from nozzle
  12. Motor under full 2nd phase thrust
  13. Motor smoulders after burnout
  14. Close up of motor smouldering after burnout
  15. Nozzle close up
  16. Thumbs up!

    Post-firing teardown
  17. Large fragment of propellant inhibitor nested in mid-bulkhead throat
  18. Extruded o-ring residue at nozzle joint
  19. Heat affected paint tells the story
  20. Motor components were in pristine condition after cleaning

Video

Panorama view of motor firing (hi-res, 4.8 meg, wmv format)

Panorama view of motor firing (medium-res, 1.9 meg, wmv format)

Close-up view of motor firing (hi-res, 6.1 meg, wmv format)

Close-up view of motor firing (medium-res, 2.4 meg, wmv format)

Close-up view of motor firing (lo-res, 840k, wmv format)

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