On May 2nd, 2015, BURPG attempted the first hot fire and full systems test of the Mk. V hybrid rocket engine, the most powerful amateur rocket engine ever built. The hot fire was intended to test the throttling profile to be used during flight, as well as characterize the TVC response. It would also verify the durability of the combustion chamber and nozzle. Following the test, the results would be used to optimize throttling for maximum altitude, and adjust gains in the TVC control algorithms to obtain stable flight.
The hot fire test did not go as planned. A software crash, and ensuing abort, led to valves not operating as intended, causing the combustion chamber to rupture. A full breakdown of the events during the test follows.
During fueling, the telemetry and control stream between the control station and the ground support electronics was lost. This triggered a system-safe abort, which functioned as intended. All fueling valves closed, and vent valves opened to start releasing pressure, in the event that contact could not be reestablished. Connection was reestablished and lost propellant replaced. At this point, a TVC sweep test was done, to verify valve control before the hot fire began. During this, one of the TVC servos stopped working, leaving the TVC valve open. This was seen as a non-issue, since its effects would not interfere with the rest of the TVC sweep test during combustion, and the nitrous oxide ice forming on the nozzle would be burned off during the ignition sequence by the long preheater burn.
The autosequence was initiated, and the preheater burned as intended during the ignition process. The pyrotechnically actuated valve then opened, allowing the oxidizer to enter the combustion chamber. However, the engine stayed at idle. This was due to the throttle valve not opening for the ignition process, but rather remaining closed. It is not designed to seal fully, so oxidizer did leak into the combustion chamber. The cause of the throttle valve not responding still remains to be determined, though it was not something that came up in normal testing. Most likely explanations stem from some effect of the software crash.
After about two minutes of the engine idling, a burn through was noticed in the post chamber, directly above the nozzle. A combustion abort was called, which would shut the throttle valve. The throttle valve instead opened, flooding the combustion chamber with oxidizer. The flame near the base of the combustion chamber ignited the engine from the bottom up, over-pressurizing the chamber and causing it to rupture.
The safety shield on the trailer kept all propellant bottles safe. The fire department was on-call, so the burning combustion chamber was extinguished within minutes. All that was lost in the test that was not disposable were the TVC valves, servos and bottom combustion chamber bulkhead. The trailer was undamaged, except for scorching on the safety shield. Due to the way BURPG operates the rocket fueling and tests, no one was in danger at any point. All operations are handled from a safe zone more than 700 feet away from the rocket from the moment the propellant tanks are opened.
Anyone familiar with rocket engine development is aware that engine failure is always an option. The only way a test can be a failure, however, is if nothing is gained from it. While the first hot fire of the Mk. V may not have been successful in terms of testing the engine performance, it is important to still reflect on what was gained. Our members now have invaluable experience in designing, assembling, and testing of highly complex rocket engines, on par with what is seen in professional aerospace. We also identified problems and their solutions. New networkable digital servos, with feedback, are now going to be used on the rocket. With the addition of feedback, servo position can always be verified, allowing better diagnosis of the system and closing the loop on the controls. A weak point in the software is also being massaged out to provide better reliability.
Moving forward, the Mk. V will be rebuilt and retested. A second combustion chamber is nearly complete. TVC valves, servo brackets, and combustion chamber bulk plate are all being redesign and remachined over the summer. Upon returning to school, fuel grains need to be cast and a new nozzle made. We would then make a second attempt at a hot fire, and hopefully be ready for a flight following that.
All of us here at BURPG are incredibly excited to continue the Mk. V campaign, and all look forward to the day this thing flies. We thank the Boston University College of Engineering for their continued support and encouragement, as well as our many sponsors and backers. It has been a lot of work to get to this point, but incredibly rewarding for all involved. We are excited for what the future holds for this project.
Rockets are high risk, high reward. We all understand that; it’s also what makes all of this so much fun.