(Courtesy of Star Rocketry)
How the Basic Fill / Fire Launch Control (BF/FLC) System Works
The basic fill/fire launch control system performs three functions. It fills the oxidizer tank remotely, provides for dumping the flight oxidizer tank in the event of an aborted mission (remembered you forgot to turn on your electronic recovery system!), and ignites the hybrid rocket non-pyrotechnically.
The flight oxidizer tank is filled through the center coaxial tube in the launch stem, this tube mates and seals with the o-ring in the injector bell forming the Kline Valve. After the rocket is placed on the fill stem assembly, it is attached to the launcher with a tie strap placed through the slots in the bottom of the motor grain and through the rings in the launcher. This strap prevents the fill tube from being forced out of the Kline Valve when the tank is pressurized with N2O.
The fill solenoid is activated by setting the mode selector rotary switch on the launch control box to "Fill" and pressing the activate switch. This opens the N2O solenoid, allowing the liquid N2O to flow through the hose, up through the fill tube on the launch stem assembly, through the Kline Valve, and then into the oxidizer tank. The oxidizer tank has a vent tube which runs from the top of the tank to a port on the side of the injector bell. When the liquid N2O reaches the top of the vent tube, it vents from the port and out a vent hole in the side of the rocket. This venting N2O is visible from the control area and indicates that the rocket is ready to launch. This will take about 35 seconds for a 440cc motor system. When the tank is full you are now ready for the launch sequence.
If the launch must be aborted for any reason, a filled flight oxidizer tank may have to be dumped. This is accomplished by moving the rotary switch on the launch control box to the "Dump" position and pushing the activation toggle switch. This activates the dump solenoid, located on the N2O solenoid assembly, which allows the N2O to escape through the Kline Valve, back through the hose, and out the dump solenoid.
When the oxidizer tank is filled and the venting N2O is visible, the rocket is ready to fire. Have the count down commence and when the count down gets to "2" release the toggle switch so that N2O is no longer filling the oxidizer tank. Move the rotary switch on the launch control box to "Fire" position and at "0" the activation toggle is pushed. This initiates the fire sequence causing the ignition module and the oxygen solenoid to activate simultaneously. The high voltage from the ignition module transformer causes the wire in the combustion chamber to arc instantly, a few milliseconds later the oxygen reaches the combustion chamber, and in the presence of oxygen, the insulation on the wire burns rapidly, igniting the fuel grain.
Keep the activation toggle pushed until the rocket launches from the pad. A flame will be coming out through the motor nozzle from the fuel grain and oxygen burning inside the combustion chamber. This will eventually burn the tie strap holding the fill stem in the Kline Valve. When the tie strap is burned through the pressure from the N2O in the oxidizer tank forces the fill tube out of the Kline Valve and the N2O mixes into the preheated combustion chamber causing the rocket to launch.
On heavier rockets it is wise to have the rocket setting on a holding device (example, a pair of vice-grips clamped to the launch rod with the bottom of the rocket resting on the handles). Have the launch stem assembly rigged to drop about ONE INCH down after tie strap burn through so that the fill stem comes out of the Kline Valve to allow the N2O into the combustion chamber. [Having the launch stem assembly drop any further may cause damage due to the greater velocity reached traveling a longer distance!] The reason for this is that the N2O does not have enough pressure to push the fill stem assembly away from the Kline Valve. I have seen fill stem assemblies actually melt away before the rocket finally launches due to the weight of the rocket not allowing the fill stem assembly to come away from the Kline Valve.
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