NASA fueled space shuttle Atlantis Wednesday (June 15), conducting a pre-flight "tanking test" to verify the integrity of the shuttle's external fuel tank.
The launch team at the Kennedy Space Center in Florida began the test at 11:15 a.m. CDT, filling Atlantis' external tank, ET-138, with about 535,000 gallons of liquid hydrogen and liquid oxygen, just as they will when Atlantis launches the STS-135 mission, now targeted for July 8.
For today's test, instead of lifting off, the tank will be drained overnight after being allowed to sit filled with the cryogenic propellants for two and a half hours.
The tanking test is to confirm if the rib-like stringers in Atlantis' tank are able to endure the super-cold temperatures it will be exposed to before and during launch without developing cracks like those that delayed Discovery's STS-133 launch last year.
Last night, teams changed the start of today's test from 6 to 10 a.m. after severe storms delayed their preparations. Overnight, a power substation that supports the launch pad went offline at about 2 a.m., which caused a number of systems to lose electrical redundancy.
Engineers have been working to fix the problem and in the meantime have developed a method to get the redundant power feed restored to critical systems. That extra work this morning caused the further delay in starting the tanking test.
During the fueling process, the main fuel valve (MFV) on Atlantis' main engine three (SSME No. 3) saw temperatures below normal levels, possibly indicating a leak. Teams isolated the engine valve and continued to fuel Atlantis with no issues. Temperatures returned to normal readings.
Engineers will evaluate any necessary work on the engine fuel valve following the tanking test.
The last time that a main engine fuel valve needed to be replaced, STS-73 in October 1995, the work delayed the launch about a week.
Should an inspection team find no other issues with Atlantis or its external tank, NASA plans to rollout STS-135's payload to the launch pad on Thursday evening.
The cargo, the multi-purpose logistic module Rafaello packed with supplies and equipment for the International Space Station, will be loaded into the pad's payload changeout room on Friday morning, in preparation for it being loaded into Atlantis' bay.
Questions about the tanking test? See: STS-135: Viewing, questions, and comments