January 2024

Test Stand Activation, Fuel System Testing

Test Stand

Over the course of this month I’ve been getting the test stand ready for real tests. I finished writing a 1.1 version of the test stand control software that supports data logging, manual valve and motor control, and sequences with aborts and automated valve and motor control. Channel naming and sensor scaling are controlled by an excel file that is imported to the code and can be easily changed making the sensor/actuator configuration flexible. Sequences with aborts are also generated by an excel sheet and imported. The code also runs a GUI that plots from the sensors in real time and has buttons and entry-fields for the valves, motor, and other commands.

I mostly finished the fuel feed system, installing the run tank, pump, and various valves and tubes. I partially built up a pneumatics system so piloted valves can be actuated. Soon I want to hotfire the igniter so I installed it on the test stand and connected it to the fuel system.

Fuel is stored in a half-gallon unpressurized tank. At the nominal flow rates it can support a hotfire of about 7 seconds at maximum. I installed a clear tube connecting to the top and bottom ports of the tank so I can easily see the liquid level. The tank feeds into the inlet of the fuel gear pump which is driven by a brushless motor. The outlet of the gear pump feeds directly into a poppet valve that I designed which acts as the main fuel isolation valve. An orificed tube also recirculates some flow from the outlet to the inlet side of the pump so that when deadheaded, the discharge pressure does not spike to insane levels. This slightly reduces the volumetric efficiency of the pump but is much simpler and cheaper than a back pressure regulator. The igniter taps off fuel from the pump discharge. The igniter fuel valve is a small 500psi skinner/parker direct-acting solenoid I found for about $18 at local industrial surplus. Downstream of the igniter valve is an inline filter and 0.010” diameter orifice. With previous igniter tests I found that the orifice was extremely sensitive to blockage hence the filter. For the time being, the outlet of the main fuel valve dumps overboard into a bucket, but once the TCA is ready it will feed into the fuel injector manifold

Main Fuel Valve (MFV) (RV-2)

The MFV I designed basically consists of a round bar with an orifice in the middle. An air cylinder actuator can plug and unplug the orifice with a nylon acorn nut threaded onto the end of the piston. Flow from the fuel pump enters the bottom of the valve and travels vertically. I drilled and tapped ports upstream of the seat for the igniter, recirc orifice, highpoint bleed, and RPT-3. Downstream of the seat there are 2 ports, one for the fuel injector purge line and one to actually go to the fuel injector manifold. Pressure from the fuel pump acts in favor of opening the valve but the pressure-area of the actuator is about 25% greater than force from the deadhead pump pressure plus I added a small compression spring to help bias the valve closed. Additionally, when the purge is running, even more force will keep the poppet pressed into the seat.

Tests

I started running some fuel pump tests starting on 1/25/24 to roughly simulate engine startup and observe the pressure behavior under different conditions since I’ve never run the pump deadheaded before. I ran the pump at lower duty cycles when deadheaded to make sure the torque and current didn’t go crazy. The lower resulting shaft speed ended up creating pressure pulsations I had not seen before. I initially presumed this was from the gear pump ripple flow however the frequency matches the shaft speed of the pump (once per rotation), not the number of gear teeth times shaft speed (number of discrete volumes per second). Perhaps this is due to asymmetrical resistance or friction in the stroke of the motor/pump. It will be interesting to see if this causes the igniter to chug and if it will be an acceptable quirk long-term. At higher speeds these pulsations disappear or at least the amplitude is negligible. During the first test, RV-3 leaked through and I later figured out I had installed it backwards and it performed normally in the later tests. MFV behaved normally. After MFV was commanded open in sequence, duty cycle was ramped up much higher and the pump reached hotfire speeds and flow rates but at a lower pressure due to the real injector flow area being simulated instead of using an orifice sized to simulate hotfire fluid power like in September. In tests conducted on 1/27/24 I installed a long 1/4” tube on the RPT-1 standoff as a dead-end high point with the hope that it would trap air and help dampen the pressure pulsations but it did not help. One chronic problem has been communication speed and reliability to and from the motor. Sending and receiving data from the motor takes in the range of 100ms which is not inherently terrible but this means it cannot be done in series with the normal loop cycle of the software. In python I had to implement some multithreading to allow the motor control to run at 10hz in parallel/simultaneously to all other features which are at 50hz. For some reason communications tends to drop out after running the software for 20-30s and sometimes there are serial timeouts and delays. This is something I will be working on improving before conducting igniter tests. One inconsistency between the test on 1/25 and the tests on 1/27 was the magnitude of discharge pressure after MFV opening. When the pump ramps to a duty cycle of 0.75, on 1/25 the discharge pressure rose to about 130-150psi while on 1/27 the pressures were lower. I think it could be due to the runline downstream of MFV not being primed or it could be something like the gear teeth slipping. I still need to investigate more deeply