“Three engines up. All going well.”
Thirty years ago, last week, the three Space Shuttle Main Engines (SSMEs) of the orbiter Atlantis roared to life and burned fiercely for 20 seconds, thereby clearing a significant milestone, prior to her maiden launch into orbit on Mission 51J. All told, Flight Readiness Firings (FRFs) were performed on seven occasions during the shuttle program’s 30-year history—once apiece by Columbia, Atlantis, and Endeavour and twice each by Challenger and Discovery—and sought to validate all of the vehicle’s systems under the closest possible conditions to actual flight, without actually leaving Earth. As described in yesterday’s AmericaSpace history article, by the time of Atlantis’ FRF on 12 September 1985, the process ran with exceptional smoothness, but the earlier test firings of Columbia and Challenger had led to several changes, including the timing of hydrogen burn igniters ahead of Main Engine Start, and a second FRF prior to STS-6 had been ordered, due to excessively high levels of gaseous hydrogen in the shuttle’s aft compartment.
By the spring of 1984, two more orbiters, Discovery and Atlantis, had concluded their final production and were being readied for their maiden voyages. As outlined in another AmericaSpace article, Discovery’s first flight would come in August 1984, on the heels of the shuttle program’s first Redundant Set Launch Sequencer (RSLS) abort of the SSMEs on the pad. Discovery’s engines were to be test-fired for around 20 seconds at Pad 39A on 2 June, prior to her scheduled launch on Mission 41D, later that same month. Under the conditions of the FRF, all control elements of the shuttle’s Main Propulsion System (MPS) were required to hold pressure, the flight control instrumentation was expected to provide proper throttle and gimbal functionality, and the integrated performance of Discovery, her ET and twin Solid Rocket Boosters (SRBs), and associated computers were to be validated.
The morning of the FRF proceeded in a fashion which the casual observer might be forgiven for thinking was the countdown to a real launch, with the notable exception that no astronauts boarded the shuttle for the test. The minutes preceding the test were hampered by only the most minor of issues, notably extended discussion on the flight control loop over Auxiliary Power Unit (APU) exhaust duct temperatures, which proved not to be a violating factor in the successful conduct of the FRF. At T-31 seconds, the Ground Launch Sequencer (GLS) handed over primary control of the countdown to Discovery’s on-board suite of General Purpose Computers (GPCs), and the SRB Hydraulic Power Units (HPUs) came online a few seconds later. At T-18 seconds, SRB nozzle positions were verified and the quartet of giant “rainbirds” around the base of Pad 39A were activated, ushering a torrent of water to reduce the reflected energy, ahead of Main Engine Start.
“Preparing to enter the final sequence,” came the call from the Public Affairs Officer (PAO). “T-15 … T-10, 9, 8 … firing … ”
All at once, a flurry of hydrogen burn igniters produced a vast shower of sparks beneath the three dark SSME bells at the extremity of the shuttle’s aft fuselage as the engines unleashed a sheet of translucent orange flame. This was quickly replaced by the striking trio of dancing Mach diamonds, as Discovery’s GPCs commanded the engines to full power.
“ … 7 … Main Engine Start sequence … we have Main Engine Start … we have three engines up and running … T-0.”
It seemed a peculiar sight, as the countdown clock touched zero and the shuttle stack failed to rise from Earth, but the SSMEs continued pounding the launch complex with 1.2 million pounds (540,000 kg) of propulsive yield. Eleven seconds later, the No. 1 engine at the “top” of the SSME pyramid was shut down, as planned, followed by its siblings—the No. 2 lower-left engine and the No. 3 lower-right engine—about 4 seconds thereafter. As vast clouds from the engines obscured Pad 39A and drifted upward into the clear blue Florida sky, the SSMEs fell silent after a picture-perfect test.
“ … All three engines have shut down,” came the welcome confirmation. “Shutdown sequence appears to have been nominal.” This was followed by a successful high-speed test of the APUs, which concluded at T+2 minutes. One significant outcome of Discovery’s FRF was that the so-called “twang” effect—the forward motion of the orbiter, as it flexed against its External Tank (ET) struts during the Main Engine Start sequence—was “quite pronounced,” although well within specific limits.
A year later, in September 1985—as highlighted in yesterday’s article—Atlantis underwent what was expected to be her first, and last, FRF, prior to her maiden voyage. For her sister ship, Discovery, however, another FRF was always anticipated, for in the pre-Challenger era she was earmarked to begin a series of shuttle flights out of Space Launch Complex (SLC)-6 at Vandenberg Air Force Base, Calif. The first of those flights, designated “Mission 62A,” was provisionally targeted for July 1986 and would have featured an FRF in the weeks ahead of launch. (Interestingly, Discovery’s sister orbiter, Columbia, would have been flown to Vandenberg in the late spring of 1986 for pad “fit checks” at SLC-6 and would herself have supported a 20-second FRF.) However, in the aftermath of Challenger’s loss, all shuttle missions from the West Coast were canceled, but Discovery proved to be the first vehicle to return to operational flight status in September 1988 and wound up performing another FRF in the lead-up to her STS-26 mission. This tied her with Challenger for having completed as many as two FRFs during the course of her career.
The enforced down time after Challenger included substantial SSME enhancements and safety upgrades which necessitated an FRF ahead of the STS-26 return to flight. This was in marked contrast to STS-114—the second return to flight, in July 2005, executed in the wake of the Columbia tragedy—when the ET was the primary focus of modifications and repairs and an FRF was unnecessary. On 4 August 1988, Discovery aimed to perform the first FRF of the shuttle era on Pad 39B, but the attempt was postponed when a sluggish SSME valve aborted the test at T-6.6 seconds; this caused some consternation on the part of the PAO, for the hydrogen burn igniters had already kicked into action, but no Main Engine Start took place. After repairs, the FRF was successfully conducted on the 10th, with the shuttle’s engines running perfectly at full power for 20 seconds.
With Challenger gone, another orbiter was under construction and Endeavour was delivered to the Kennedy Space Center (KSC) in May 1991, for a projected maiden launch in the late spring of the following year. On 6 April 1992, three weeks after rolling out to the pad, Endeavour’s SSMEs were test-fired in what wound up to be the seventh and last FRF in the 30-year shuttle program. As her engines reached 100 percent of rated performance at T-0, vast clouds of steam billowed from the pad, as engineers simulated the retraction of the ET umbilical and the SRBs’ hold-down posts, before eventually transmitting shutdown commands after 15 seconds of stable, continuous thrust.
The final FRF of the shuttle era—which came ahead of the 47th flight of 135 total missions—had proved a great success, with the exception of a couple of technical issues. High vibration levels were detected in one of the engines’ high-pressure liquid oxygen turbopumps, whilst another exhibited a loud “popping” noise, shortly after it had been shut down, indicating possible hydrogen ingestion into its fuel injector. Prudently, on 8 April, NASA opted to replace all three of Endeavour’s engines with a set previously earmarked for a subsequent mission and a second FRF was not considered necessary.
“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.”
The SSMEs posed major headaches for NASA, both during their development—when agonizing failures and explosions were experienced in the late 1970s—to no fewer than five RSLS abort situations between June 1984 and August 1994, in which the engines were shut down on the launch pad, with a crew aboard, seconds before liftoff. Yet over the course of their career, they were extensively modified and proved themselves to be one of the safest and most reliable elements of the shuttle system. Their contribution to the progressive exploration of space will continue beyond the end of the shuttle era, however, for heavily upgraded and reconditioned SSMEs will someday form the Main Propulsion System (MPS) of the first stage of the Space Launch System (SLS) booster, tasked with delivering humans beyond low-Earth orbit for the first time in more than five decades.
This is part of a series of history articles, which will appear each weekend, barring any major news stories. Beginning next week, AmericaSpace will look back at the upcoming 30th anniversary of the maiden voyage of Space Shuttle Atlantis. This will begin with a look back at Mission 51J, a classified Department of Defense mission, followed by an in-depth reflection on Atlantis’ career during the anniversary of the flight itself, on 3-7 October.