The propellants used in amateur rocket engines require a separate source for ignition. Because the engines are small, the use of an engine-mounted spark plug is not generally feasible. Even if it were, the ignition of incoming propellants in the combustion chamber by a small spark plug is dangerous and unreliable.
Propellant timing is extremely important in a bi-propellant liquid rocket engine.
The amateur engine using gaseous oxygen is not nearly as sensitive to hard starts as if the oxidizer were a liquid. A small amount of cotton is wrapped around, or attached to, thc wires very near the spark gap but not obstructing it. This ignition assembly is pushed through the nozzle into the combustion chamber of the rocket engine so that the spark gap is in the lower end of the combustion chamber but not blocking the nozzle throat. The wires outside the engine are bent or taped to hold the ignition assembly in position during the ignition phase. Figure 13 details this hot-source igniter.