Sunday’s Falcon 9 launch of a Dragon capsule to the space station went off, but not without a hitch. During the rocket’s ascent, one of its nine Merlin engines shut down prematurely one minute and 19 seconds after launch.
In the ensuing press release, SpaceX pointed out that engine malfunctions are neither abnormal nor crippling, and pointed out that the Saturn V experienced two engine malfunctions. It did, and neither were fatal to the mission.
More than one Saturn V had problems during launch. The Saturn V that took Apollo 12 to the moon, for example, was struck by lightning twice in the first minute of its flight. And two rockets did experience engine failures, Apollo 6 and Apollo 13. In both cases it was second stage engines that failed, and in the case of Apollo 13, it didn’t affect the spacecraft’s orbital insertion at all.
Apollo 6 launched on April 4, 1968. It was an unmanned mission designed to demonstrate compatibility between the Saturn V rocket and the Apollo Command and Service Module (CSM). The mission would also give ground crews a chance to practice tracking and controlling a spacecraft in orbit.
The Saturn’s third stage, the S-IVB, was to be reignited in orbit simulating a translunar injection burn (the burn that would send the spacecraft to the moon) before freeing the CSM. The spacecraft would stay in orbit for six hours, cold soaking the heat shield to give engineers data on reentry heating after returning from the moon.
But problems with the Saturn’s second stage, S-II, took a major toll on the mission. Three hundred and 19 seconds after launch, a fuel line leak in the S-II’s engine 2 caused a loss of thrust. After 94 seconds, the thrust had dropped off further; both engines 2 and 3 lost 40 percent of their power, triggering a shutdown of both engines. The dual shutdown was accompanied by a power surge, which caused a loss in pressure in engines 4 and 5. Only engine 1 made it through launch unscathed.
The result was a significantly altered trajectory for the spacecraft. It was much lower than expected. In the end, 6 of the mission’s 16 primary goals were only partially accomplished and one — re-ignition of the S-IVB stage for the simulated translunar burn — wasn’t done at all. But overall the mission was successful enough that NASA felt confident to put a crew on the next Saturn V — Apollo 8 took three men to the moon in December 1968.
Apollo 13 launched on April 11, 1970 with a crew bound for the moon’s Fra Mauro highlands. It was to be the third lunar landing but an explosion of an oxygen tank 55 hours and 54 minutes into the mission crippled the spacecraft and put the crew in mortal peril. But long before the oxygen tank exploded, a launch malfunction briefly threatened the mission’s lunar goal.
During its ascent, the Saturn’s S-II stage’s central engine, engine 5, shut down 2 minutes and 12 seconds ahead of schedule. The remaining four engines were unaffected, and burned for an extra four minutes to compensate for the lost engine and the spacecraft achieved the desired orbit. Once the S-II’s four outboard engines were determined to be stable, there was no question of aborting the mission. Only later did the severity of the engine malfunction come to light.
No one was entirely sure why the S-II’s center engine shut down, but many suspected it was an automatic response to pogo oscillations; test data and past experience supported this theory. As propellant flows through the rocket stage’s plumbing on its way from the fuel tanks to the engines, low-frequency disturbances can form. This is similar to the groaning noises residential plumbing makes, only on a rocket they’re less strong. But these disturbances can cause variations in propellant flow rate, which in turn affects the thrust of the engine fed by that propellant. Thrust in that engine can fluctuate several times a second. The resulting oscillation can make a rocket “bounce” like a pogo stick.
Telemetry from Apollo 13’s S-II stage suggested engine 5 was experiencing a significant pogo — up to 68g vibrations. A pogo this strong could cause the engine frame to flex by a few inches, enough to threaten to tear the second stage apart. Engine shutdown was likely triggered by sensed thrust chamber pressure fluctuations.
Similar, though smaller, pogo oscillations had been seen in previous rockets, including Apollo 6’s Saturn V. But on Apollo 13, the oscillations were amplified by interactions with other hardware. Subsequent Saturn V’s were modified to dampen the pogo effect.
Pogo oscillations are one kind of issue that can threaten a mission and cause an engine’s early shutdown, but there are more. As for the culprit in the Falcon 9 launch mishap, SpaceX hasn’t yet determined the cause of its premature engine shutdown.
Image: The five F-1 engines of the huge Apollo/Saturn V space vehicle’s first (S-IC) stage leave a gigantic trail of flame in the sky above the Kennedy Space Center seconds after liftoff. The launch of the Apollo 6 (Spacecraft 020/Saturn 502) unmanned space mission occurred at 07:00:01.5 (EST), April 4, 1968. This view of the Apollo 6 launch was taken from a chase plane. Credit: NASA