On February 24, 2018 MIT Rocket Team fired our first ‘P’ motor. As a class 3 motor, this represents our first step beyond the world of hobby rocketry and into the stratospheric heights of actual rocket science. This test produced 65,077 Ns at a peak pressure of 1440 psi and 22,970 N of thrust. The test was full duration, full impulse, but not entirely according to plan.
At T+2.9s we ejected thermal liner material from the rocket. This material came from the thermal liner and casting tube of the aft-most grain of the rocket. As the material was being ejected the motor registered a sharp pressure spike. This pressure spike resulted in the axial cracking of our nozzle, and a leak path that increased the effective area of the throat. For the remaining 5s of the burn we still produced over 1000 lbs of thrust, but at a reduced pressure of 200 psi. This was a minor efficiency loss, but the motor still provided almost 100% of designed impulse, leading us to conclude that no propellant was ejected with the liner.
(First liner material ejection)
At T+6.1 s additional liner material was ejected as the second aft most grain burned out.
As the design pressure and thrust of this motor were greatly exceeded, we think it likely that aluminum combustion (triggered by the increased residency time in a longer motor) lead to the unplanned increase in burn rate, the increase in combustion pressure, and then triggered the cascade of failures.
Even though this burn did not go as planned, the failure modes were all safe to humans and (most) hardware. Good safety practices are critical if you want to get second chances at building rockets. As the adage goes: “A good rule for rocket experimenters to follow is this: always assume that it will explode.” – Astronautics Magazine (October 1937)