Correction : According to the SES-8 launch broadcast, the F9 was not returned to the hanger for cleaning, but rather addressed on the pad due to ease of access to the engines.
Eight days after its initial launch attempt for the SES-8 satellite, and five days after a second effort on Thanksgiving day, Space Exploration Technologies is prepping its Falcon 9 V1.1 for an 86 minute launch window opening at 5:41 pm ET this evening.
Following the most recent attempt on Thursday, which was cut short at the moment of ignition when computers registered a slower than expected ramp-up in thrust, SpaceX drained the booster of propellants, lowered it, and wheeled it back into its horizontal integration building at SLC-40.
Subsequent analysis indicated the slow start up resulted when oxygen in the ground supply degraded the potency of the TEA-TAB ignition fluid which is fed to each of the 9 first stage engines. After inspecting each of the engines and cleaning the gas generators, engineers replaced the number 9, center mounted engine’s gas generator, the miniature rocket engine within an engine which drives the Merlin 1D’s turbopump feeding liquid oxygen and RP-1 to the main combustion chamber. According to a twitter update from Elon Musk, the generator swap out was taken as a “precautionary measure.”
Having tested the engines with ground pneumatics and found them to be “cycling cleanly,” SpaceX, which had been considering an attempt on Monday, added an extra day to the schedule to re-check everything in advance of the launch, says it has has cleared all known anomalies and is ready to try again. Weather is not likely to be a factor, as current forecasts estimate only a 10% chance of a range violation.
If there is a problem with today’s launch attempt, SpaceX has reserved tomorrow as a backup date.