I'm coding up the ground abort logic in the flight computer for my
liquid biprop rocket and I'm trying to determine the best abort
sequence. The main fuel and oxidizer valves are on a single servo
actuator, each tank has a separate servo controlled vent valve, and the
helium valve is servo controlled. There is no way to vent the helium
tank except through the regulators, into the tank manifolds, and out the
tank vents. The augmented spark igniter has separate solenoid valves
for each propellant and separate control for the CDI used by the spark
plug. There are a variety of different conditions that can generate an
abort (valve failure to open/close, low chamber pressure, no CDI sense,
etc.) but I was thinking of the following general sequence:
1. Close the main propellant valves.
2. Wait the expected time for the main valves to close, about 1 second.
3. Close the igniter solenoid valves and turn the CDI off. The idea
behind waiting a second before closing the valves is to encourage any
propellants currently being injected a chance to burn off in a
controlled manner. If the main valves got stuck and only partially
opened, the main chamber pressure might not reach the expected value so
that would trigger an abort. There is a single pressure transducer on
the igniter that I'm using to verify igniter operation as well as main
chamber operation. The expected igniter pressure is 130 psia and the
main chamber pressure is 250 psia so I can tell when the main chamber
starts up.
4. Close the helium valve and open the vent valves. Leaving the helium
valve closed doesn't leave the vehicle as safe as dumping it through the
tank vents but if there is a failure of the main valve to close, then it
seems that shutting off the high pressure supply would help keep the
fire from being fed by propellants under pressure. They still would all
leak out, just more slowly. But if there is a fire that eventually
destroys the ability to control the helium valve, then the tank will
remain pressurized so it seems it would be better to go ahead and
depressurize the helium tank. All tanks have burst discs on them
including the helium tank.
Suggestions? I could also tailor the sequence based on the pre-abort
conditions. For example, if I know the main valves successfully closed
and the vents opened, leave the helium valve open. On my test stand,
the abort would simultaneously close the main valves, close the igniter
solenoids, open the vent valves, and shut off the helium valve (it was
supplied from a cylinder through a solenoid valve).
-Bob