A quarter-century ago, on 6 April 1992, the roar of three Space Shuttle Main Engines (SSMEs) echoed across the marshy landscape of the Kennedy Space Center (KSC) in Florida, as Endeavour showcased her muscle, ahead of her maiden voyage into orbit. Built as a replacement for her lost sister, Challenger, the new vehicle would go on to fly 25 missions—supporting the first (and only) three-person spacewalk, servicing the Hubble Space Telescope (HST) and building the International Space Station (ISS)—and cement her credentials as the fourth-most-flown member of the shuttle fleet. Within 22 seconds of Main Engine Start, her engines fell silent, as planned, to close out the final Flight Readiness Firing (FRF).
As outlined in yesterday’s AmericaSpace history article, FRFs were executed before each orbiter’s maiden voyage, and were performed twice by shuttles Challenger and Discovery. The intention was to impose approximate launch-like conditions on the SSMEs, as well as testing the Auxiliary Power Units (APUs) in high-speed mode. Visually and acoustically, it was perhaps the closest example—other than a launch itself—of the orbiters straining against their shackles, yearning to fly. All control elements of the Main Propulsion System (MPS) were required to hold pressure in the engines and the External Tank (ET) and the flight control instrumentation was expected to provide proper throttling and gimbaling functions. This would serve to validate the integrated performance of the Space Shuttle “stack” and the compatibility of the on-board General Purpose Computers (GPCs) with ground-based computer systems.
Endeavour’s FRF on the morning of 6 April 1992 proceeded relatively smoothly, with the notable exception of high vibration levels in one of the SSMEs’ high-pressure liquid oxygen turbopumps. Taken as an indicator of hydrogen ingestion into the fuel injector, it was decided to replace the engines with another set.
It was the final FRF of the shuttle era, coming 11 years after the engines of Endeavour’s sister ship, Columbia, roared to life on 20 February 1981. Interestingly, the second orbiter to undertake an FRF actually did so twice. At 11 a.m. EST on 18 December 1982, shuttle Challenger was almost ready for her maiden voyage, with launch targeted to occur in the second half of January 1983. With a minute to go before Main Engine Start, the Public Affairs Office (PAO) announcer reeled off the timeline, as Challenger came alive and her vehicle systems were brought online.
“T-1 minute and counting…the firing system that releases the sound suppression water onto the pad has been armed…T-50 seconds and counting…T-45 seconds and counting…T-40 seconds and counting; SRB development flight recorders are being turned on…T-37…gaseous oxygen vent arm will not be retracted on this particular test…T-31 seconds, we have a Go from LPS [Launch Processing System] for auto-sequence start…[Challenger’s] four primary flight computers taking over control of the terminal count…final LPS command for engine start will occur at approximately 10 seconds…T-15 seconds and counting…”
At this stage, the relative silence on the pad changed markedly. Firstly, the sound suppression system gushed water across the launch pad. “T-10…Go for Main Engine Start…we have Main Engine Start…” as the now-familiar sheet of orange flame gave way to a trio of shock diamonds from the three SSMEs, combined with a thunderous roar and vast cloud of smoke. The engines ignited in a ripple-like sequence, starting up at 120-millisecond intervals, reaching 90 percent of rated performance within three seconds and hitting 100-percent at zero.
“T-0, engines throttled at 100 percent, all engines up and burning,” came the second-by-second updates from PAO. “T+5 seconds, engines continuing to burn…T+10 seconds…twelve…first [engine] cutoff at T+15 seconds…[Number One] engine cutoff…and engines Two and Three also cutoff at 16.8 seconds…T+25 seconds; GLS safing now in progress…” However, the FRF was not yet over, for the APUs were run up to T+2 minutes in high-speed mode, after which GLS safing of the vehicle was completed.
According to NASA Launch & Landing Operations Director Al O’Hara, it was anticipated that about 48 hours after the test, by 20 December 1982, the initial data was expected to be in place, after which the actual physical inspection of the SSMEs could commence. In the immediate aftermath of the FRF, O’Hara described it as “a resounding success”, but it later became clear that everything did not run according to plan. “Let me caution you that this is based upon the real-time information from the firing room and from the support rooms that we got on-net about 30 minutes after T-0,” he told journalists later that morning. “So as the day progresses and more information get available, that may change.”
And indeed it did.
During the test, engineers detected levels of gaseous hydrogen in the shuttle’s aft compartment which grossly exceeded allowable limits. When it proved impossible to pinpoint the cause or location of the leak, the Mission Management Team (MMT) elected to perform a second FRF. New instrumentation was installed inside and outside Challenger’s aft fuselage to better determine if the leakage was from an internal or external source, with suspicion initially focusing on the latter possibility, because vibration and current had found their way behind the SSMEs’ heat shields.
Extra sensors and a higher-than-ambient pressurization level were installed to prohibit penetration by “external” hydrogen sources and the second FRF, lasting 23 seconds, took place on 25 January 1983. It too revealed high concentrations of hydrogen gas, necessitating the replacement of one of the SSMEs and, subsequently, the replacement of the other two engines, due to detection of cracked welds, fractured fuel lines and generic “seepage” in an inconel-625 tube within the ignition system. Described by NASA Associate Administrator for Space Flight James Abrahamson as “a real detective job”, a third FRF was briefly considered, but by mid-February this proved unnecessary.
Over the course of the next three years, two more orbiters—Discovery and Atlantis—also embarked on their FRF rite-of-passage, ahead of their maiden voyages. Discovery, which wound up as NASA’s most-flown shuttle, saw her SSMEs burned to full power on 2 June 1984. Operationally, her FRF was not dissimilar to its predecessors, with the exception that the forward motion of the vehicle (nicknamed “the twang”) was more pronounced, although well within limits. And on the morning of 12 September 1985, Atlantis’ engines roared for her own FRF, ahead of her inaugural mission a month later.
It was expected that this would be the last, first-time FRF for a member of the shuttle fleet. However, with NASA and the U.S. Air Force planning to initiate Space Launch Complex (SLC)-6 at Vandenberg Air Force Base, Calif., for West Coast-based flights from the summer of 1986, it was intended that Discovery would perform another FRF prior to Mission 62A. In the aftermath of the Challenger accident in January 1986, Vandenberg missions were canceled, but Discovery was tasked with her second FRF in August 1988, prior to STS-26. This met with several delays, including a false start on 4 August, when a sluggish SSME valve led to an abort at T-6.6 seconds. Some consternation was apparent in the voice of the PAO, for the engines’ hydrogen burn igniters had already kicked into action, but no Main Engine Start occurred.
Six days later, on 10 August, there were no such problems and the shuttle’s engines fired smoothly for 20 seconds of continuous thrust. This allowed NASA to clear another milestone in getting the fleet back to post-Challenger operations in late 1988. The principal reason for an FRF before STS-26 was to evaluate more than 200 SSME and other vehicle modifications and, with Endeavour’s test in April 1992, no further plans were laid. Significantly, STS-114—which marked the post-Columbia return-to-flight mission in July 2005—did not require an FRF, because the External Tank (ET) had been the principal focus of modifications, rather than the SSMEs themselves.
“On every FRF that we conducted, we learned something new about the vehicle, which made our process and flight hardware better,” said deputy shuttle processing chief engineer Jorge Rivera. “It’s definitely a good practice in reducing the risk of the actual flight.” Critically, Rivera added that the FRF was in keeping with the “test as you fly, fly as you test” mentality. “Test is the best control or mitigation for hazardous conditions that could impact the mission,” he explained. “Subsystems that tested fine in isolation may interface with each other in a different way, which could create a bigger problem.”
Across their 30-year-plus career in the Space Shuttle Program, the SSMEs posed major headaches for NASA. Agonizing failures and explosions in the late 1970s, coupled with no fewer than five Redundant Set Launch Sequencer (RSLS) abort situations—premature engine shutdowns, with crews aboard, seconds from T-0—contributed to a pre-Challenger belief that they would be the lead contender in any launch failure. However, the SSMEs were extensively modified and proved themselves to be one of the safest and most reliable elements of the shuttle system. Their contribution continues, for heavily upgraded and reconditioned SSMEs will soon form the Main Propulsion System (MPS) of the Space Launch System (SLS), when it flies its inaugural voyage in the second half of the present decade.
This is part of a series of history articles, which will appear each weekend, barring any major news stories. Next week’s article will focus on the 15th anniversary of STS-110, a shuttle mission which saw Jerry Ross become the first human to launch into space, from Earth, a total of seven times.