Don't forget shutdown transients - local "pops" as the shutdown mix
varies in time and space - can also to some degree send stuff back into
the propellant feed system.
Maybe not send stuff as far as the tanks, but still potentially
interesting degrees of inappropriate backflow...
Henry V
On 2/8/2018 8:56 AM, Doug Jones (Redacted sender randome for DMARC) wrote:
No, reverse flow is extremely unlikely in any event other than a hard start- which is over and done before the control system (even an automatic one) can react. Combustion happens and pressure builds in the chamber because the feed system delivers propellants to the chamber, stop the feed, stop the flame. The panic button can all at once close what's open, open what's closed* and the chamber will flame out far faster than the tanks can blow down.
For ground test, it would be really nice to have purges stubbed out for hardware protection, then remove the QDs for flight. Cascaded purges cover a multitude of sins.
* Igniter valves excepted, they just get turned off. For any spark torch igniter, turn it off as soon as your main valves are open, there's no need to roast it and jam your avionics with the EMI. Running a single gaseous igniter fluid (GOX, methane, whatever) during the main burn is nice but not essential, the igniter is a stagnant cul-de-sac with no flow from the chamber unless you have blown seals. For a simple one-and-done vertical flight, you'll inspect everything before trying to run the engine again anyway, so no need to keep the igniter dry.
On 2018-02-08 6:13 AM, Robert Watzlavick wrote:
I hadn't thought about reverse flow during the shutdown but that's a very good point. I guess it depends on the amount of ullage in the tanks. The main propellant lines and the LOX vent are 0.5 inch while the fuel vent is 0.25 inch. I don't have a purge on the vehicle.
-Bob
On 02/07/2018 11:21 PM, Bruno Berger SPL wrote:
That was my first thought too. But then I realized that it might be
possible that the propellant lines and maybe the tanks can be
contaminated from pushed back combustion products. It depends how fast
the vent valves are able to depressurize the tanks. We also close the
propellant valves and open the vent valves at the same time, but the
tanks are big enough, so they are still a bit pressurized when the
propellant valves are closed.
Bruno
Am 08.02.2018 um 04:23 schrieb Robert Watzlavick:
I already found a bug. There's no reason the vents shouldn't be
immediately opened:
1. Close the main propellant valves, open the vent valves, and close
the helium valve (they may have to be staggered for current limit
reasons though).
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.
4. If the main valve closed and the vent valves opened, re-open the
helium valve to vent it.
-Bob
On 02/07/2018 09:10 PM, Robert Watzlavick wrote:
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