Fwd: Shuttle SSMEs: Looking Back at the Flight Readiness Firings

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From: "Gary Johnson" <gjohnson144@comcast.net>
Date: September 20, 2015 at 9:20:37 PM CDT
To: "Gary Johnson" <gjohnson144@comcast.net>
Subject: FW: Shuttle SSMEs: Looking Back at the Flight Readiness Firings

 

 

AmericaSpace

For a nation that explores
September 19th, 2015

Firing Up the Shuttle: Looking Back at the Flight Readiness Firings (Part 1)

By Ben Evans

 

Each shuttle was powered by a trio of RS-25 Space Shuttle Main Engines (SSMEs). These were test-fired before each orbiter's maiden voyage, during the Flight Readiness Firing (FRF). Image Credit: Aerojet Rocketdyne

"And we have a Go for Autosequence Start. Atlantis' on-board computers now controlling the final sequence."

Thirty years ago, last week, NASA entered the final period before the maiden launch of its fourth space shuttle, named in honor of the world's oldest serving oceanographic research vessel, the two-masted RV Atlantis. In time, Space Shuttle Atlantis would dwarf her oceangoing namesake's 1.3 million miles (2.1 million km) traveled, flying no fewer than 33 missions into low-Earth orbit, voyaging 126 million miles (203 million km), and circling the Home Planet no less than 4,848 times. However, on the morning of 12 September 1985, as the spacegoing Atlantis was readied for her first flight on Pad 39A at the Kennedy Space Center (KSC) in Florida, one particularly critical milestone lay ahead. It was a milestone which would be performed seven times over the span of the shuttle era and it sought to impose launch conditions on the Space Shuttle Main Engines (SSMEs), as well as testing the orbiters' suite of Auxiliary Power Units (APUs) in high-speed mode. Executed prior to the maiden voyage of each of the five orbiters—and on two occasions by Challenger and Discovery—the Flight Readiness Firing (FRF) was perhaps the most visible example of the behemoth straining against its shackles and yearning to fly.

"T-15 seconds … 12 … Go for Main Engine Start … "

In a manner which had become instantly familiar over the preceding 4.5 years, ever since the first flight of the shuttle era, STS-1 in April 1981, a flurry of hydrogen burn igniters produced a shower of sparks, after which the three dark SSME bells at the extremity of Atlantis' aft fuselage suddenly sprouted a sheet of translucent orange flame. This was quickly replaced by the striking trio of dancing Mach diamonds, as the shuttle's on-board General Purpose Computers (GPCs) commanded the engines to full power.

" … minus 5, 4, 3, 2, 1 … we have three engines up. Three engines up. All going well … "

And so it was. However, as Atlantis flexed against the struts which held her, steadfast, against her rust-colored External Tank (ET) and twin Solid Rocket Boosters (SRBs), the key difference was that on 12 September 1985 she was not yet headed for space. Instead, as part of the FRF protocol, virtually all of her systems would undergo the most rigorous testing, under the closest possible conditions to actual flight, with her SSMEs fired for 20 seconds, all control elements of the Main Propulsion System (MPS) required to hold pressure in the engines and ET, the flight control instrumentation providing proper throttle and gimbal functions and validating the integrated performance of the shuttle "stack" and the compatibility of the on-board GPCs with ground-based computers.

" … Cutoff command should be issued at this time … "

It had been a remarkable morning; a morning whose procedures and processes had followed many of the routines which would occur on an actual launch. At T-31 seconds, the Ground Launch Sequencer (GLS) had handed over primary control of the countdown to Atlantis' GPCs and the SRB Hydraulic Power Units (HPUs) had been activated shortly thereafter. At T-18 seconds, the SRB nozzle positions were verified and the quartet of giant "rainbirds" around the base of the launch pad were activated at T-11 seconds, ushering a flood of water to reduce the reflected energy, ahead of Main Engine Start.

Atlantis' three Space Shuttle Main Engines (SSMEs) roar to life on 12 September 1985, clearing a critical milestone before her maiden voyage. Photo Credit: NASA

As vast clouds from the engines obscured Pad 39A and drifted upward into the clear blue Florida sky, the shutdown commands were issued. The SSMEs fell silent after a perfect test. "Our Flight Readiness Firing is completed," it was announced. "All three engines came up. We ran for the full planned duration of this test. All preliminary indications would be we had a completely successful test this morning." Not for another three weeks, until 3 October 1985, would Atlantis' SSMEs again roar to life … and on that occasion, they would do so with five humans aboard and with the intention of spearing into space.

Yet the roar of Atlantis' engines, that September morning, 30 years ago, was not the first occasion on which a shuttle had shaken the marshy Florida landscape, without actually leaving the ground. The first FRF took place on 20 February 1981, in the run-up to STS-1, the first orbital voyage of the shuttle program, as Columbia stood on Pad 39A, and was labeled as the "Wet Countdown Demonstration Test (WCDT)/Flight Reading Firing (FRF)." Key aims were to test the complete shuttle system—Columbia, her ET, and SRBs—"in a real-time launch countdown which will culminate in the firing of the orbiter's three main engines and a simulated launch to insure their proper integration prior to the STS-1 flight." Fundamentally, the FRF sought to "verify the capability of the launch facility to provide propellants to the shuttle under launch conditions," with the ET and SRB stack "exposed to the same thermal environment they will experience during STS-1 launch preparations." All MPS control elements were "required to maintain pressure in the External Tank and in the main engines during the test firing as they would during an actual launch," whilst the ability of the APUs, hydraulic systems, and flight controls were verified as being able to throttle the three SSMEs at between 94 and 100 percent, as well as gimbaling them to effect steering.

When 20 February 1981 came around, the six-day WCDT was nearing its conclusion and would culminate in the FRF. In readiness for the test firing, launch controllers started the countdown clock at T-53 hours, when they powered up the SRBs, the Ground Support Equipment (GSE), and the shuttle's on-board systems. Four seconds prior to the simulated liftoff, Columbia's SSMEs roared to life at 120-millisecond intervals, reaching 90 percent of rated performance within three seconds and hitting 100 percent precisely at T-0. Three seconds later, engineers simulated the retraction of the ET umbilical and the SRBs' hold-down posts; and after 15 seconds of stable thrust from the SSMEs, shutdown commands were issued to the engines. The test was a great success and cleared another critical milestone, ahead of the STS-1 launch.

One significant aspect of the FRF was to evaluate and measure the effect of the "twang"—the amount of movement in the orbiter, against the rest of the stack, during the SSME ignition sequence—and in the case of Columbia this induced a forward shift by about 25.5 inches (64.7 cm), which was somewhat greater than the 19 inches (48.2 cm) predicted. However, the twang remained within anticipated structural limits. Also, the spark-like hydrogen burn igniters were started at the same time as SSME ignition, but post-FRF analysis led to a decision to initiate this sequence at T-4.4 seconds ahead of Main Engine Start on STS-1 and all subsequent missions.

Challenger's three main engines ignite on 18 December 1982 for the first Flight Readiness Firing (FRF) of STS-6. Photo Credit: NASA

By the time the next FRF occurred at 11 a.m. EST on 18 December 1982, Columbia had flown five missions and her sister orbiter, Challenger, was approaching her maiden voyage. However, as circumstances transpired, Challenger became the only vehicle to undergo two FRFs in the same mission processing flow. Drawing close parallels with STS-1, Challenger's systems were powered up and at the 60-second mark the Public Affairs Officer (PAO) picked up the final coverage: "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 main engines, 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 … 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, 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."

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 and the identification of the leak source this ultimately proved unnecessary.

A year later, in the spring of 1984, two more orbiters—Discovery and Atlantis—neared completion and headed into their own processing flows for their maiden voyages, capped by their own FRFs. As will be discussed in tomorrow's AmericaSpace history article, these firings added valuable new data and experience to engineers' understanding of SSME functionality. Moreover, beyond the opening flights of Discovery and Atlantis, an additional two FRFs would also be performed before the end of the shuttle era: one in the weeks prior to STS-26, the first post-Challenger mission, and another to herald the career of the newest orbiter, Endeavour.

 

Copyright © 2015 AmericaSpace - All Rights Reserved

 

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AmericaSpace

For a nation that explores
September 20th, 2015

Firing Up the Shuttle: Looking Back at the Flight Readiness Firings (Part 2)

By Ben Evans

 

For the final time in the 30-year shuttle program, Endeavour's engines blaze for their Flight Readiness Firing (FRF) on 6 April 1992. Photo Credit: NASA

"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 … "

Discovery completed two Flight Readiness Firings (FRFs) during the course of her career. Had Challenger not been lost, she would have also performed an FRF ahead of her maiden voyage out of Vandenberg Air Force Base, Calif. Photo Credit: NASA

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.

All five orbiters performed FRFs in the weeks preceding their respective maiden voyages, with Challenger and Discovery performing two apiece during their careers. Photo Credit: NASA

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. 

 

Copyright © 2015 AmericaSpace - All Rights Reserved

 

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