Torch Igniters

The RL-10 engine has been fired with a spark plug. The spark plug is energized for 4 
seconds (from t = 0 to t = 4 sec). Replacing the spark plug with a torch igniter was one of 
the proposed Air Force reliability improvements to the ignition system of the RL-10. With 
fast data acquisition, it was discovered that a potential problem with the second start of 
the engine following a long coast time could occur. The reason for the problem was that the 
first firing would produce water which could freeze. Coupled with a transient backflow 
problem, the igniter oxidizer supply valve would freeze. The engine still got enough O2 to 
start from elsewhere.

Benefits of the torch ingniter:
1. The torch igniter eliminated the need for the valve. 
2. The torch igniter extends the mixture ratio over which the engine can operate.