Lotus Engine Hot Fire 3

The weekend of April 2 saw the first test of Lotus with a corrected triplet injector. From the previous ignition tests 1 and 2, it was clear that a known design flaw had reduced the injector stiffness to the point where the feed system was able to couple to the engine, resulting in oscillations much lower than any acoustic chamber modes.

The new injector attempted fixed these issue with properly sized fuel ports. Unfortunately, the day’s events did not provide useful data on engine performance. However, the day was a good shakedown of a heavily upgraded fluids system and an exercise in rapid response to operational changes.

Upgraded pressurization system and oxidizer fill panel

Upgraded pressurization system and oxidizer fill panel

Testing was to start with a 2 second ignition test, followed by a 5 second burn to obtain regenerative cooling data. The first attempt resulted in a manual abort being called due to non-nominal tank pressurization. The pop valves had been moved closer to the bang-bang pressurization solenoids on the upgraded fluid system, meaning they saw higher pressure than before and were opening. The tanks run at 900 psi, but the pressurization line immediately after the solenoid valve was exceeding the valves’ rated 1000 psi. Due to the amount of GN2 vented through the pop valves, the GN2 battery pressure dropped from 2700 psi to 1500psi in under 5 seconds.

It was determined that the test could be run without the pressure-relief backup of the pop valves due to the automatic aborts programmed into the ground software, so the valves were removed. Due to the loss of GN2, a conditional abort was added to shut down the engine if GN2 pressure dropped too low during a run.

Draining the oxidizer tank during a system-safe abort

Draining the oxidizer tank during a system-safe abort

Everything was reset for a second attempt, which ended on an automatic system triggered abort. An existing conditional abort to shut down testing on oxidizer/fuel ratio was triggered early with the activation of the GN2 pressure abort flag. The oxidizer/fuel ratio abort then shut down the system, since the not-yet-ignited engine was not providing nominal data. The system went into system safe while the abort configuration file was quickly rewritten to remove the oxidizer/fuel ratio abort, being deemed unnecessary for a 2 second ignition test. With the corrected aborts in place, control of the system was regained and testing reset for a second time.

Combustion occurring outside the chamber

Combustion occurring outside the chamber

The third attempt of the day went smoothly operations-wise. However, due to a check of the main fuel valve that was run after the fuel tank had been filled, the fluid level in the fuel feed tube was different than normal. This advanced the fuel timing, which led to fuel and oxidizer entering the chamber at the same time. Proper ignition of the engine relies on the oxidizer having decomposed when exposed to the pre-burner ahead of fuel entering the chamber, which did not happen effectively in the presence of the fuel that was entering early. This caused the mixture to ignite outside the engine once mixed with atmospheric oxygen, creating a very fiery, low thrust burn. Video footage seems to suggest that the fuel/ox mixture was not igniting until after leaving the chamber.

Data Analysis and Results:

Due to the combustion occurring outside the chamber, the engine performance was obviously not valid. Due to not having choked flow, the thrust data was very low. The thrust level matches that expected by the pure momentum of propellant discharge.

All the operating pressures were non-nominal as well. The ideal are as follows: 480 psi oxidizer manifold, 660 psi fuel regen inlet, 460 psi fuel manifold, and 400 psi chamber. From further analysis of the fluid system, the line losses appear to be higher than initially expected. Taking the data seen here, the lines are being upgraded appropriately to decrease pressure drop to an acceptable level. A filter on the fuel system is also being removed, and replaced by a filter on the tank fill inlet.

Along with the upgraded fluids lines, there are operational changes planned. A fuel bleed process is being added, to make sure the fuel level in the feed lines is consistent from test to test. The software has been upgraded to make the abort system more predictable and flexible, as well as easier to modify.

The water suppression system is also being changed. Instead of a pneumatically pressurized deluge tank, a larger plastic tank and pump is going to be used. The water suppression also includes a new blast plate with water channels for plume suppression and to prevent severe heating of the plate.