e>HALO Report -- Rocket Motor Test Day #6
Held on Saturday, July 8, 1995 at the HALO Rocket Motor Test Facility
The following text was taken, with permission, from an article by Ronnie Lajoie, which was published in the July-August 1995 issue of the Southeastern Space Supporter, newsletter of HAL5.
It was Saturday, July 8, and we were back --- yet again --- for that full day of rocket testing we had missed now twice previously. This time, we had plenty of N2O, two full supply bottles, which were now covered with a plastic tarp last used as a rain cover. Tim had made one last change to the oxidizer system, based on a concept developed by Herman Pickens. The useless gaseous N2O cools as it is vented. Herman determined that we could take advantage of this cooling to keep the oxidizer tank cold. Tim wrapped 10 feet of the vent copper tubing around the green oxidizer tank, then encased it with insulation surrounded by duct tape. Now each time Steve vents the oxidizer tank, the tank gets colder, creating a pressure differential with the supply bottle; thus allowing more N2O to enter the tank.
We also had plenty of hybrid motors. Gene had volunteered to pour and mix hybrid motors for Project HALO, as he was still interested in retesting his little hybrid rocket motor. During the break, Gene produced two all-asphalt motors, and two others containing 15% aluminum powder (by weight). Being a conductive metal, the aluminum would increase the heat transfer rate throughout the asphalt, thereby increasing the propellant burn rate, and hopefully increasing the chamber temperature and associated rocket thrust and density-impulse. We also had two more McDonnell Douglas motors left untested.
The only major change for the tests would be in the motor ignition system. Greg Warren of Advanced Concepts, supplier of squibs and Thermolite for HALOs igniters, joined HAL5 and volunteered to serve as the pyrotechnic safety officer for Project HALO. He persuaded Tim to abandon the wire-cylinder igniter insert in favor of a thick wafer of solid propellant sliced from a K-motor. The wafer would be placed on top of the motor like a rubber washer. A squib would be used to set off the wafer. The heat from the burning solid propellant would rush down the core as the oxidizer started to flow, causing ignition. Ron Creel did his usual excellent job of preparing the squibs and bonding them to the wafers.
Finally, all the hard work of the entire HALO Propulsion Team paid off. Unlike Test Day #4, when we batted 0 for 6, this time we batted 9 for 10! Three HALO motors, two McDonnell Douglas motors, Genes hybrid motor, and three solid rocket motors (brought by James Mitchell and Rick Kauffmann of Tennessee) all were fired successfully between noon and sunset. It was a very good day for testing motors.
I arrived just prior to lunch. The only item I missed was the one failed attempt to fire Gene Hornbuckles little hybrid motor, now placed in its own miniature wooden test stand. After lunch, we began testing in earnest --- and did not stop until the cows came home (i.e., sunset).
First up was a HALO asphalt motor with a 1.25-inch core diameter. The remote oxidizer tank loading and weighing system worked like a charm, although the occasional POP-HISS of the release valve (to vent unwanted gaseous N2O) made more than just a few of us jump. After the Safety Officer verified that the range was clear, the countdown commenced: 5-4-3-2-1- Ignition! The familiar snap of the squib was immediately followed by the new poof sound of the solid motor igniter, which released a small cloud of smoke at the base of the motor. As in the old TV show I Dream of Jeannie, the poof was soon followed by the release of the genie --- but ours was a roar of flame instead of a beautiful woman (too bad) --- or even Robin Williams! The test was stopped at exactly 3.0 seconds; the motor yielded an average thrust of 162 pounds and an Isp of 167 seconds.
Next up was one of the HALO asphalt-and-aluminum motors with a 1-inch core diameter. Preparations began well, the stage was set, the director said Action! (or was it Ignition!?) --- but nothing happened. Closer inspection revealed that the squib failed to fire. We confirmed among ourselves that we would be using two squibs on the flight motor. We would not want a $1000 flight ruined by a $1 part!
With a new squib, ignition went off perfectly, and the motor fired for 3.0 seconds before Steves sequencer automatically cut-off the oxidizer flow, ending the test. The motor yielded an average thrust of 163 pounds and an Isp of 168 seconds.
This was not much performance improvement compared to the previous test. Steve believes the less-than-desired results were because the oxidizer-to-fuel (O/F) ratio was not optimized for the asphalt-and-aluminum propellant. He recommended that we not fire the second aluminum motor until after he had time to study the data.
Although the external differences were hardly noticeable, the internal differences were anything but! The aluminum did indeed increase the heat transfer, for the propellant had clearly regressed much faster than we had previously seen. The propellant burned axisymmetrically, but ripples were noticed all the way down the length of the unburned core. Steve found a layer of aluminum-oxide coating the nozzle, but fortunately found no damage once it was removed. The group agreed that more tests would have to be performed before a final determination could be made on the benefits of adding aluminum to asphalt.
Next up was a McDonnell Douglas motor with a 1-inch core diameter. After a successful ignition, the motor burned for 3.0 seconds before shutdown. Herman Pickens remarked that he could clearly see the Mach cones in the very clean exhaust plume. Dr. Dean smiled and replied that he designed it that way. The motor yielded an average thrust of 157 pounds and an Isp of 167 seconds.
Next up was a HALO all-asphalt motor with a 1-inch core diameter. Another successful 3.0 second test yielding an average thrust of 154 pounds and an Isp of 162 seconds.
Next up was a McDonnell Douglas motor with a 0.75-inch core diameter. This test would be an intentional repeat of the first M-D motor back in May, when gaseous N2O was used due to a low supply bottle. Again, another successful 3.0 second test; this one yielding a low average thrust of 97 pounds (as expected) but a high Isp of 204 seconds (as hoped).
Next up was Genes little hybrid motor, back for a second attempt. Gene placed his miniature test stand against the concrete firewall and I laid bricks in the lower portion to keep it from moving. While the rest of the group struck the test stand, Tim and Steve borrowed the 50-pound load cell (used for weighing the tank) to use as a thrust-monitoring device for the test. After setup, most of us moved our cameras up closer for this little rocket motor firing.
Upon ignition, we realized that maybe moving closer was not necessarily a smart idea. The little motor roared to life, producing a very clean (and very LOUD) thrust, which the load cell claimed was close to 125 pounds! The motor continued to burn for about 6 seconds, until its little oxidizer tank was depleted. It was definitely one of the highlights of the day. The sound was so loud that a neighbor from miles away called to check on us. Chris Pickens assured her that it was just those rocket guys. The 50-pound load cell was damaged by the high thrust. Paul Paelian of Toroid agreed to see if he could repair it for us.
The last tests of the day were three solid-propellant motors custom-made by James Mitchell of Tennessee. The three sizes of motors were said to be equivalent to J, K, and L type motors. Due to the sizes of the motors, we decided do the tests with the motors horizontal rather than the usual vertical. The heavy steel rocket motor holder was placed horizontally across two bars of the test stand. For each test, a solid motor would be bolted to the holder aimed such that, if it broke loose, it would strike the firewall first.
As James and Rick Kauffmann prepared the J-motor, those of us with cameras returned to our original more distant locations. I saw David Dean setting up his tripod out in the open and reminded him that we were about to test solid motors. He said OH! and (knowing more about the dangers of solid rockets than me) proceeded to retreat farther back.
After Steve shouted Ignition! there was a multi-second delay before the J-motor actually lit. When it did, though, it was powerful and short-lived, about 2.4 seconds. The thrust averaged 95 pounds, but peaked close to 140 pounds. Isp was 194 seconds, the second highest of the day.
James and Rick removed the J-motor and bolted into place the larger K-motor. Chris Pickens road up to the front of her property to monitor the noise level from a distance. Upon Ignition! the only delay was while the K-motor revved up, a sound like someone taking a deep breath. This was immediately followed by a loud roar as the motor set out a large plume of white-hot flame. The motor burned for 2.3 seconds, yielding an average thrust of 151 pounds, and a peak close to 230 pounds. Isp was 181 seconds.
James and Rick removed the K-motor and bolted into place the even larger L-motor. Chris was still monitoring the noise level from a distance. Upon Ignition! the L-motor revved up with a sound like a fire-breathing dragon taking a deep breath! This was immediately followed by a very loud roar as the motor set out an even larger plume of white-hot flame. The motor burned for 2.6 seconds, yielding an average thrust of 268 pounds, and a peak close to 380 pounds, the largest recorded thus far at the HALO test facility. Isp was 183 seconds.
The motor holder showed no sign of wear, giving us confidence that it would hold even larger motors. Chris returned and reported that the noise sounded like distant thunder from her remote vantage point.
The final test equipment was removed and the group retired to the home of Herman & Chris Pickens, where we were treated to dinner and videos of our rocket motor tests. It was the end to a very good day at the HALO Rocket Motor Test Facility.
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This file was last modified on Saturday, 15-Apr-2017 13:19:40 EDT