Feb. 23, 2016
Early afternoon on February 20, 2016, BURPG successfully completed two ignition tests of the Lotus engine. The data obtained was highly sought after, showing engine performance and operating conditions. This is critical for further development of the engine for flight readiness, performance, and reliability.
The first test of the day was a final cold flow on the fuel system to verify previous changes had the desired effect. After a quick reset, the first 2 second hot fire was attempted, which resulted in a hard start and an abort 1 second into the burn. After inspection and some valve timing changes, a second 2 second hot fire was attempted. Ignition was much softer, though the support systems again threw an abort 1 second in. These aborts triggered as expected given the engine operating conditions, which was a good verification of the ground support system capabilities.
Fuel Cold Flow
The fuel cold flow had the objective of checking expected pressure drops across the system as well as getting valve timing for the ignition sequence.
Pressure drops were within expected ranges, and the time between the valve opening and fuel entering the chamber was calculated. This data is used for the valve timings for the hot fire.
Shortly after noon, preparations were complete for the first hot fire. Fill procedures were swift and smooth from the control station.
With ignition, there was a loud bang indicative of a hard start. An automatic oxidizer to fuel ratio abort also triggered upon being armed about 750 ms into the burn. Inspecting the engine after the test, it was clear that there was no damage. Pressure data from the ignition was used to alter the ignition sequence to introduce the propellants in the chamber at almost the same time in the next test with a slight oxidizer lead. Pressure data also verified that the ratio abort properly triggered due to the ratios being out of bounds.
The second ignition was much more gentle, with a much more desirable startup transient. Despite the better ignition, the ratio abort triggered again due to out of bounds data. This was not unexpected from the previous test’s data.
The following data comes from the second hot fire of the day.
The engine achieved 66% of the target thrust, which oscillated during the burn. The cause of the initial spike in thrust and the subsequent variance can be seen when looking at the chamber pressure. The actual oxidizer to fuel ratio the engine operated at is still in question due to these fluctuations disrupting the calculated mass flow data. To correct this, new flow meters will be added to our system in place of using tank mass to calculate mass flow. The flow meters will offer better data independent of vibrations and pressure oscillations from the engine.
The propellants entered at the correct time relative to each other, but ignition still happened about 100 milliseconds too soon. The igniter timing will be adjusted to correct this. The previously mentioned variation in thrust and chamber pressure begins after the first 100 or so milliseconds of operation. This is combustion instability, meaning waves of combustion are travelling through the chamber. This pressure variation is what causes the buzzing sound in the video as the engine fires.
The cause of the instability appears to be in the injector. A key parameter of injector design is “stiffness,” which is the ratio of the pressure drop across the injector and the chamber pressure. The stiffness on the fuel injectors was supposed to be 15%, but in this test ended up being more along the lines of 5% due to initial errors in fuel density calculations.
A new injector is already being made to increase stiffness, which should also help prevent instability as well.
Finally, shutdown of the engine was successful, though slow. This leads to the fiery shutdown seen in the video. This behavior was expected due to the fuel left in the regenerative cooling channels.
Above is data on the regenerative cooling system. The engine was far from steady-state at only 1 second into the burn, but it’s very gratifying to see decent temperature data! The upcoming longer burns will provide better data on regen performance and steady state engine operation.
Specific impulse is a measure of the efficiency of an engine, measured in seconds. The number provides the ratio between the thrust obtained and the weight of fuel per second needed to maintain that thrust. The engine did not reach the calculated 235 seconds. This is likely due to the aforementioned combustion instability as well as a skewed oxidizer to fuel ratio from the outgoing injector design. The specific impulse is expected to increase significantly with the corrected injector, which will allow the engine to operate much closer to the desired ratio.
To summarize, these tests provided very valuable data on the engine’s performance and behavior and answered some outstanding questions on fluids systems performance that could only be answered by igniting the engine. From the data, we were able to identify areas to improve on in the next few weeks. These largely focus on making a new injector to correct the oxidizer to fuel ratio and increase stiffness, which will in turn lead to more stable combustion, higher thrust, and much better specific impulse. Flow meters will also be added to the ground support system to provide better mass flow data, leading to more exact impulse and ratio calculations.
Stay tuned as we improve Lotus for the next round of testing!