J-2 Rocket Engine Augmented Spark Igniter
The J-2 rocket engine augmented spark igniter (ASI) is mounted at the center of the J-2's injector face and consists of a small combustion chamber with two spark plugs. It provides the flame to ignite the propellants in the main thrust chamber. When the engine is started, the spark exciters energize the spark plugs as oxidizer and fuel flow to the spark igniter. As the oxidizer and fuel enter the combustion chamber of the ASI, they mix and are ignited.
The ASI operates continuously during entire engine firing, is uncooled, and is capable of multiple reignitions under all environmental conditions.
The augmented spark igniter was at the center of what was probably the most spectacular malfunction of the entire Saturn program: On SA-502 (which launched the Apollo 6 mission), approximately 3 minutes and 45 seconds after launch, temperatures in the aft section of the S-II (second) stage began to change. At 5 minutes and 18 seconds, the thrust of the number 2 engine decreased suddenly. Premature cutoff occurred at 6 minutes and 52 seconds.
A second engine shutdown one second later. The vehicle remained stable and the three remaining engines burned to near fuel depletion to compensate for the two engines out. Complete second stage shutdown occurred 58 seconds later than originally planned.
Third stage first first-burn performance was normal. After two revolutions in the parking orbit, third stage restart was attempted but not achieved.
The anomalies involving both the second and third stage engines were identified as failure in the fuel line feeding the J-2 engines' augmented spark igniter. Ground testing demonstrated that fluid flow-induced vibration in the flexible bellows section of the line was the cause of the failure. This vibration phenomenon had been masked in previous ground testing because of the damping effects of liquid air condensed on the exterior of the bellows. Condensation of liquid air on the bellows surface occurred in the ambient conditions of the ground tests but not in the vacuum environment.
The igniter lines were redesigned to eliminate the bellows section and a qualification test program verified the correction. The corrected J-2 engine was later flown successfully in the second stage of the Saturn IB launch vehicle on the Apollo 7 mission and on the Saturn V upper stages during the Apollo 8 mission.
The following diagram shows the details of the redesigned ASI fuel line installed on the J-2, as well as the original ASI fuel line:
And both propellant lines, as they would be installed on the J-2:
The premature cutoff of the second engine of the Apollo 6 vehicle's second stage was traced to erroneous wiring of the engine prevalve to the cutoff circuit of the engine that failed first. Procedures were established to prevent recurrence of this kind of error.
Note: This is a preliminary version of this page, because some other page made reference to the ASI lines. Some day, I'll add additional text and diagrams. The bulk of the text on this page was taken, most of it verbatim, from the Saturn V Press Kit and NASA's Manned Space Flight Program.