One year on – Review notes superb performance of STS-135’s SRBs

by Chris Gebhardt

One year to the day since Atlantis launched on her final mission, the exceptional performance by the Solid Rocket Boosters (SRBs) – which aided STS-135’s ride uphill during first stage flight – was noted in the seemingly independent flight performance review of STS-135 conducted by contractor agency United Space Alliance (USA), not NASA.

The stunning lack of an In-Flight Anomaly Review for STS-135:

For over 30 years, NASA’s Space Shuttle Program relied on the all-powerful and iconic white Solid Rocket Boosters to help propel every single Shuttle mission toward Low Earth Orbit.

For the entire life of Space Shuttle Program, one thing was certain: there would be an In-Flight Anomaly (IFA) review for all Space Shuttle missions to review all vehicle performance indicators and ensure maximum safety for the future.

And that constant held from STS-1/Columbia (April 1981) to STS-134/Endeavour (June 2011). Notably, though, STS-135/Atlantis is missing from the complete list of IFA reviews.

Some people within the program claimed it was related to NASA leaderships’ desire to kill the Shuttle Program as quickly as possible, following the highly successful and triumphant mission of OV-104 Atlantis on STS-135 (8 July – 21 July 2011).

What’s more, the non-performance an IFA review following STS-135 directly contradicted numerous pre-flight reports and statements by Shuttle Program managers, that specifically noted that all post-flight reporting and IFA reviews would follow normal procedures for the last flight of the program.

Essentially, the express desire by Shuttle Program management and NASA itself going into STS-135 was to treat it as if it were just another flight of the Program – as if there would be another flight, which in some sense is true given the large quantity of Shuttle hardware that will still be flown on the Space Launch System (SLS) rocket.

But with the desire to terminate the program as quickly as possible came the 31 August 2011 “official” end to the Shuttle Program – and with it any chance of performing an IFA… something quite baffling when it is considered that numerous Shuttle hardware elements will be used for the Space Launch System rocket (e.g. the SRBs, Space Shuttle Main Engines, External Tank design, Main Propulsion System, etc…).

The directive not to perform an IFA was clearly issued following the successful return of Atlantis from STS-135 as indicated by the Mission Evaluation Room (MER) compilation of an actual IFA list (available for download on L2 – LINK) on 1 August 2011 for review at a later date.

Thus, the appearance of a detailed flight performance and IFA report for any element of STS-135/Atlantis is refreshing – though surprising. 

An inspiringly safe final SRB flight for the Shuttle Program:

Following the launch of STS-135/Atlantis on 8 July 2011, the launch vehicle’s twin SRBs were towed back to the Cape Canaveral for disassembly and post-flight inspection.

This process led to the creation of a detailed flight performance report of Shuttle Atlantis’s Solid Rocket Boosters on 18 August 2011 – not by NASA or Shuttle Program departments but by the contractor agency United Space Alliance (USA).

As noted by the USA Volume II – Solid Rocket Booster presentation for the Space Shuttle STS-135 Flight Evaluation Report (L2 LINK), “All Solid Rocket Booster subsystems performed nominally during launch countdown, flight, and recovery.”

With liftoff of the final Shuttle flight registered at 1129.03 EDT, the launch occurred on the first attempt with “No SRB LCC (Launch Commit Criteria) violations during the time period of SRB power up through launch. No exceedances of the Countdown Experience Base occurred this flight, and all SRB subsystems performed properly during prelaunch testing and launch countdown.”

But perhaps even more impressive than the clean pre-launch performance of the SRBs is the fact that “There were no SRB related In-Flight Anomalies (IFA) on this flight.”

SRB separation loads/events from the launch pad at liftoff:

Designed to bear the entire weight of the fully-fueled Shuttle launch vehicle on the Mobile Launch Platform (MLP), and thus representing the only place to securely bolt the SSV (Space Shuttle Vehicle) to the MLP, each SRB for the Space Shuttle was bolted to the MLP by four hold-down bolts with frangible nuts.

These frangible nuts had to separate prior to liftoff to allow for a clean launch of the SSV from the MLP. Thus, the nuts were designed to separate into two halves via a NASA Standard Detonator – with the fire command being sent to each frangible nut at approximately T-0.3 seconds.

This timing allowed the nuts to separate and fall into their designated receptacles where they were “caught” and prevented from recirculating in the aerodynamic environment around the vehicle induced at SRB ignition – thus preventing the frangible nuts from contacting the launching SSV stack and causing potential damage.

To verify the safety and continued functionality of this system, several post-flight and flight ascent data was collected on each Shuttle mission to verify a clean separation of the SRBs from the MLP and the eight hold-down bolts.

This verification process was performed for STS-135. As noted by the SRB flight performance report, “Post-flight inspection showed that all eight frangible nuts performed nominally, separating into two major halves. All four frangible nut major webs, and both minor webs, were identified in the debris from all eight blast containers.”

Furthermore, there were not stud hang-ups during liftoff, and all hold-down stud loads for STS-135 were well within family for the Space Shuttle Program – indicating a clean separation of the SRBs from the MLP at liftoff.

The maximum hold-down stud load for STS-135 was recorded on Post #2 with a value of 771.5 kips; likewise, the minimum stud load was recorded on Post- #5 just prior to SSME (Space Shuttle Main Engine) start with a value of 609.6 kips.

Both of these values were well within the maximum flight experience base of 841 kips (registered during the December 1988 launch of Atlantis on STS-27) and a historical maximum of 849 kips (registered during the STS-26/Discovery Return to Flight mission’s Flight Readiness Firing of the SSMEs).

Overall SRB ascent performance:

All ascent data points for both SRBs were well within family and consistent with previous nominal flights of the SRBs.

Both RSRM (Reusable Solid Rocket Motor) Ignition PICs (Pyro Initiator Controllers – the hardware that actually ignites the SRB propellant at T-0) met all performance requirements and functioned nominally.

All SRB rate Gyro performances were nominal, and correlated rate data from the Left and Right SRBs and the Orbiter’s rate gyro subsystems were within Shuttle specifications during powered ascent.

Moreover, “All reviewed measurements from the Operational Instrumentation performed properly throughout their respective mission phases for the launch,” notes the SRB performance report.

However, in terms of overall performance, of particular note were the Significant Event Times for STS-135’s SRBs.

While several of the manually initiated commands in the pre-launch timeframe deviated from their predicted time (an expected occurrence), of particular note were four flight event times that deviated ever so slightly from the pre-flight predictions.

The first was the actual time of ignition of the twin SRBs.

Pre-flight expectations showed a 0.006 second delay between the ignition command and the actual ignition of the twin SRBs.

Post-flight analysis of video from the launch pad revealed an actual 0.008 second delay between the ignition command (clocked from the SRB ignite command at T-0) and the actual, simultaneous ignition of the SRBs – a statistically insignificant 0.002 second difference.

Additionally, there was a 0.16 second difference in the time it took both the Left and Right SRBs to drop to a PC less than 50 indication – much better than the pre-flight prediction of a 0.3 second difference.

Likewise, there was a statistically irrelevant 0.2 second different in the safing of the Range Safety System (destruction system) between the two SRBs.

SRB nozzle null commands for SRB separation were nearly identical to each other and to pre-flight predictions; however, physical separation of the SRBs from the External Tank differed from the identical pre-flight prediction of MET (Mission Elapsed Time) 122.97 seconds. 

Physical separation of the Left SRB was registered at MET 123.08 seconds, with the Right SRB’s physical separation recorded at 123.12 seconds.

SRB separation performance:

Nevertheless, all SRB separation commands and performances were classed as nominal during STS-135.

Release of all structural attachments (8 total; 4 on each SRB – 3 on the aft attach bolts to the ET and one on the forward attach bolt to the ET) was completed within 30 milliseconds of the issuance of the separation command from orbiter Atlantis’s General Purpose Computers (GPCs).

Booster Separation Motor (BSM) – thrusters that push the SRBs away and clear from the ET – firing was nominal, with all 16 BSMs firing to completion.

SRB post-separation flight profile:

Both SRBs were tracked by ground-based radar assets from their separation from the ET to their loss of signal (LOS) due to the curvature of the Earth.

This LOS due to the curvature of the Earth nominally occurs at approximately T+340 seconds. But during STS-135, LOS for the Left SRB occurred at T+328 seconds, 12 seconds earlier than normal. Likewise, LOS with the Right SRB occurred at T+308 seconds, a full 32 seconds earlier than normal.

Post-flight review revealed this was due to a problem with the radar tracking site and not the SRBs’ post-separation flight profile.

Additionally, a temporary LOS was registered with the Left SRB at T+202 seconds. The signal was re-acquired at T+239 seconds. This LOS was traced to the same radar tracking site issue that resulted in the early, permanent LOS.

Radar tracking data of SRBs indicated a post-separation apogee altitude (highest altitude achieved) of 223,100 feet for the Left SRB at T+192 seconds and an apogee altitude of 223,200 feet for the Right SRB at T+192 seconds.

This deviated from the pre-flight prediction and Program nominal apogee of 226,836 feet by more than 3,636 feet.

Likewise, peak dynamic pressure on the Left SRB also deviated slightly from historical norms with a registered peak dynamic pressure of 1,500 pounds per square feet (psf) at T+309 seconds v. the historical norm of 1,456 psf at T+313.9 seconds.

Peak dynamic pressure for the Right SRB was not recorded due to the radar track site issue that led to an early LOS.

Nonetheless, both the peak dynamic pressure deviation and shallow apogee of the SRBs were “within the experience base of ascent trajectory planning and post-separation radar tracking.”

Drogue parachute deployments and associated loads were nominal, as was main parachute deployment, inflation, and associated loads.

Horizontal ribbon damage on five of the six main parachutes was reported during post-flight reviews; however, it was not noted during recovery operations if the damage was already present on the parachutes – indicated in-flight damage – or if the damage was caused during recovery operations.

Both SRB extension nozzle jettisons after main chute deploy and before water impact were nominal.

SRB splashdown:

With three fully inflated main parachutes each, both SRBs achieved nominal velocity for splashdown into the Atlantic Ocean.

Deriving splashdown times from “acceleration data recorded by the on-board data acquisition systems,” the Left SRB impacted the water at MET 398.98 seconds. The Right SRB followed with splashdown at MET 403.70 seconds.

Both SRBs hit the water at a relative speed of 76 ft/sec with a horizontal wind velocity measured at 30 ft/sec and seas at 4-6 ft at both splashdown locations.

Information from the Data Acquisition System (DAS) – a system installed in the parachute camera canister in the forward skirt of each SRB to record vehicle acceleration loads from just after liftoff to splashdown in the +/-125 G category – indicated a nominal +11.7 g water impact of both SRBs.

According to the SRB performance report, “This is within the range of normal SRB rigid body axial accelerations experienced for three fully open main parachutes.”

Likewise, post-splashdown cavity collapse occurred 1.17 seconds after splashdown in the Left SRB and 1.10 seconds after splashdown in the Right SRB.

Both cavity collapse times corresponded to in-family parameters from previous flight experience.

Interestingly, accelerometers in both SRBs did not record a “hard SRB splashdown” event that is nominally recorded approximately 3 seconds after water impact.

Thrust Vector Control system performance during flight:

For STS-135, all in-flight performance readings for the critical Thrust Vector Control (TVC) system indicated the proper positioning of all TVC actuators as commanded. 

As noted by the presentation, “During prelaunch and ascent, all actuators positioned as commanded. Actuator rates and duty cycles also correlated with the issued commands.”

All TVC actuator duty cycles correlated well the 134 flight experience base for the Right and Left SRBs, and all rock/tilt numbers fell well within the flight experience envelope.

Nevertheless, there were six minor exceedances in the second-to-second experience base… four for the Left SRB and two for the Right SRB.

The first second-to-second exceedance for the Left SRB occurred at T+12.5 seconds. “The LH Position TVC Tilt Actuator Position B58H1151C exceeded the second-to-second High Experience Base. The experience base high value was 1.89 inches, and the actuator position was 2.71 inches,” notes the USA SRB flight performance report.

This 2.71 inch actuator position was still well within the overall Left SRB maximum flight experience envelope during the roll maneuver.

Likewise, the all the other instances of SRB TVC actuator duty cycles spiking above the second-to-second maximum flight experience base were still contained within the overall maximum flight experience envelope.

SRB Thermal Performance:

Thermal performance reviews for the two SRBs for STS-135 further indicated a very clean and safe flight, with the actual flight thermal environment being “less severe than the current Performance-Enhanced Space Shuttle ascent and 95 percentile descent design environments.”

Click here for recent SRB/RSRM Articles:

No breach to the structural temperature limit exceedances were observed on the recovered SRBs, and all post-flight TPS (Thermal Protection System) conditions were well within family and consistent with previous nominal flights.

SRB in-flight debris environment:

A debris assessment review from ground cameras and on-board flight/engineering cameras from the SRBs and External Tank revealed a nominal performance and no major areas for concern.

In fact, only one squawk, 135-001, was written for “suspect ascent impacts in the RH ETA (External Tank Attach) Ring froth-pak foam,” notes the SRB performance report.

Post-flight analysis of the impact area revealed no foreign debris, and the impact areas themselves were consistent with previously-observed ice impacts to ETA ring froth-pak foam.

Moreover, all observed post-flight material loss was within flight experience and easily attributed to post-separation, splashdown, and recovery activities.

SRB onboard camera performance:

All four SRB engineer cameras and four Solid State Video Recorders (two per SRB) functioned nominally during first stage flight.

Likewise, ET-facing engineering cameras on the SRBs to monitor engineering targets of interest during SRB separation from the ET functioned as expected and collected good engineering images of the ET intertank panel during separation.

The camera feeds from the ET-facing cameras automatically switched to the parachute cameras 5mins 50seconds after liftoff. Nominal observation of the parachute deployment sequence was observed on both SRBs by the parachute engineering cameras (one per SRB).

An excellent final performance of exceptional hardware/propulsion element:

Overall, STS-135 appears to be the cleanest and safest flight of the ATK-produced Solid Rocket Boosters in the history of the Shuttle Program with ZERO In-Flight Anomalies indicated during preliminary post-flight inspections and only ONE squawk for damage that most likely was caused from ice from the External Tank, not the SRBs.

However, it is impossible to verify if this was, as suspected and indicated, the cleanest and most-successful flight of the Solid Rocket Boosters as no formal IFA review was held.

Nonetheless, the SRBs ended their service to the Shuttle Program as tried, true, and safe propulsive elements – thanks in great part to NASA’s and ATK’s intense desire to safely use the SRBs.

Obviously, this level of safe use changed significantly in 1986, finally morphing into what it always should have been.  But throughout the life of the Shuttle Program, continuous efforts were taken – at a different level of enthusiasm in the early days of the Program – to learn about and improve upon the SRB design.

This strategy and desire was greatly aided by the fact that 133 of the 135 SRB flight sets were recovered after launch, disassembled and painstakingly scrutinized for any defects or off-nominal indications.

Only two flight sets (4 SRBs total) throughout the life of the Program were not recovered: those on STS-51L/Challenger and those on STS-4/Columbia. The SRBs on STS-4/Columbia suffered parachute deployment failures and impacted the Atlantic Ocean at terminal velocity, shattering into multiple pieces and sinking to the bottom of the Atlantic.

Through this recovery and post-flight inspection process, safety levels continuous improved – even reaching the point of suspending Shuttle flight operations in the mid-1990s to address a partial burn-through of an inner O-ring on just one SRB.

And NASA’s use of SRBs is not over.  SRBs will continue to serve manned and unmanned launch endeavors for NASA as the Shuttle’s successor vehicle, the SLS rocket, makes use of twin 5-segment SRBs, verse the Shuttle’s 4-segment Solids, for its debut series of flights currently anticipated for the latter years of this decade.

To read about the orbiters –  from birth, processing, every single mission, through to retirement, click here for the links:

(Images: L2’s STS-135, SRB, RSRM and IFA sections – all highly expansive collections of presentations, photos, video and data. Additional via NASA and Brian Papke – MaxQ Entertainment/

(L2 and NSF are continuing to follow the orbiters through to their final resting places. To join L2, click here:

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