The Space Shuttle Program – in its near 29-years of service to the U.S. and world communities – has seen numerous successes: the Hubble Space Telescope repair missions, joint operations with the MIR space station, invaluable research in the scientific and medical fields, and the construction of the International Space Station.
At the same time, the Shuttle Program has taught NASA and the space community the limits and specifications of the space vehicle’s systems. While some of these lessons have been learned through non-lethal failures/anomalies of Shuttle systems (such as the Flow Control Valve failure and resolution last year), two of the biggest lessons from the Space Shuttle Program were learned through the loss of orbiters Challenger and Columbia and their respective crews.
Challenger: O-rings, Field Joints, and Heaters:
The storied nature of the events leading up to the disintegration of the orbiter Challenger and the STS-51L stack on January 28, 1986 is one that will forever remain in the forethought of those in the Space Shuttle Program.
Nonetheless, while the reasons behind the disaster are remember to this day, it is important to remember the changes to the Solid Rocket Booster O-rings and field joints that came about as a result of the Rogers Commission investigation.
During the Rogers Commission hearings in 1986, Richard Feynman famously placed a small O-ring into a glass of ice water. When he removed the O-ring from the glass, he shocked many people by breaking the O-ring with little effort.
While this demonstration served as a visual representation of the brittle nature of the O-rings when exposed to cold temperatures, the display also served the engineering and physics communities in their understanding of the chemical composition of the O-rings.
By breaking the O-ring with ease, Feynman showed the inability of the FKM (Fluoroelastomer) component of the O-rings to maintain pliability in cold temperatures. This failure showed that when the O-rings were cooled below their Tg (glass transition temperature), they lost their elasticity – thereby eliminating their ability to flex and seal the joint casings of the SRBs during ignition and flight.
As the Rogers Commission determined, once the SRB O-rings are cooled near but not beyond their Tg limit, they became compressed and require a “longer than normal amount of time to return to their original shape” (in the case of Challenger, a circular shape with enough pliability to seal any gaps created from the flexing of the SRB cases during ignition and flight).
Since the temperatures at Challenger’s launch pad dipped well into the 20s (degrees F) the night before liftoff, the O-rings lost their pliability. As the temperatures on January 28 rose above freezing levels, a majority of the O-rings regained enough pliability to sufficiently seal the joint casing at SRB ignition.
Furthermore, while the ambient temperature at the time of Challenger’s launch was within Launch Commit Criteria guidelines, the temperature of the O-rings remained well below the ambient air temperature.
As a result, two of the O-rings on the Right Hand SRB did not regain enough pliability and failed across 70 degrees of arc at the moment of SRB ignition at the aft field joint at a circumferential point near the aft ET/SRB attach strut.
A complete blow-through of the SRB propellant flame was prevented in the early stages of Challenger’s flight due to aluminum oxides from burned solid propellant that became lodged within the secondary O-ring, thereby creating a temporary seal of the field joint.
Abnormally strong wind sheer encountered between T+37 seconds and T+64 seconds dislodged the temporary O-ring-like seal and allowed a hot gas penetration through the SRB field joint. The slipstream around the shuttle stack diverted the escaping SRB flame plume onto the External Tank and SRB/ET attach strut.
At T+64.66 seconds, the plume impingement on the ET weakened the structural integrity of the LH2 (Liquid Hydrogen) portion of the tank to a point permitting an LH2 leak. Within two seconds, pressure in the LH2 tank began dropping.
A T+72.284 seconds, the plume burned through the Right Hand SRB/ET attach bolt, severing the strut and allowing the SRB to rotate around its forward attach strut and causing a sharp, lateral acceleration to the right.
The aft dome of the External Tank’s LH2 tank failed at T+73.124 seconds, resulting in a massive propulsive force that pushed the LH2 tank into the LO2 (Liquid Oxygen) tank. At roughly the same time, the Right Hand SRB completed its rotation forward and struck the Inter-tank region of the ET.
Structural failure and breakup of the Challenger/51-L stack began almost immediately. Contrary to popular opinion and initial reports, the Challenger stack did not explode. A majority of the External Tank disintegrated from the loss of the structural integrity and the burning of LH2 and LO2 while the Challenger orbiter was forced from its ‘correct’ attitude in relation to the local slipstream and torn apart by divergent aerodynamic forces in excess of 20g — four times the maximum structural limit of the orbiter.
Following the loss of Challenger, NASA spent 2½ years upgrading the shuttle’s safety systems, including a redesign of the SRB field joint to include the addition of a third O-ring, the inclusion of a new, internal metal latch, and a redesign of insulation. These enhancements were designed to fortify the ability of the O-rings and fields joints to prevent hot gas penetration through the SRB joints.
Furthermore, electric heaters were installed into each joint. These heaters maintain O-ring temperatures during periods of cold ambient temperature. In this manner, the O-rings remain pliable and retain their ability to form a complete seal of the SRB joints.
These heaters remain a vital part of the SRB system to this day and are activated whenever the ambient temperature drops below 50-degrees F.
Furthermore, today’s Weather Launch Commit Criteria state: “Prior to external tank propellant loading, tanking will not begin if: a. The 24-hour average temperature has been below 41 degrees or b. The temperature has fallen below 33 degrees at anytime during the previous 24 hours.”
Moreover, “After tanking begins, the countdown shall not be continued nor the shuttle launched if: a.) The temperature exceeds 99 degrees for more than 30 consecutive minutes or b.) The temperature is lower than the prescribed minimum value for longer than 30 minutes unless sun angle, wind, temperature and relative humidity conditions permit recovery.”
This prescribed minimum value is determined by a 5-minute average of temperature, wind, and humidity levels. The prescribed minimum value temperature consideration becomes valid when the ambient temperature reaches 48-degrees F.
As a result, the lowest acceptable ambient temperature – with wind speed and ambient humidity factored in – is anywhere between 48-degrees and 36-degrees F.
This table can also be used to “determine when conditions are again acceptable for launch if parameters have been out of limits for 30 minutes or less. If longer than 30 minutes, a mathematical recovery formula of the environmental conditions is used to determine if a return to acceptable parameters has been achieved. Launch conditions have been reached if the formula reaches a positive value.”
Since the redesign of the SRB joints, only one serious O-ring anomaly has occurred. On STS-71 (June 1995), “evidence that hot exhaust gases had strayed dangerously within the booster nozzle” was discovered during post-flight inspections.
After discovery of this issue, Endeavour’s STS-69 mission was delayed indefinitely to allow NASA engineers as much time as necessary to investigate the failure and correct the problem.
Repairs to the O-ring nozzle seals on the STS-69 stack took place at the launch pad and involved “vacuum back-fill operations around the nozzle seals” and the installation of nozzle plugs.
STS-69 launched successfully on September 7, 1995. Post-flight SRB inspections revealed nominal performance of the O-rings around the nozzle seals.
To this day, NASA continues to closely monitor all O-ring and SRB field joint conditions pre- and post-launch. The addition of new thermal cord barriers to the SRB field joints along with the ongoing replacement of O-ring materials to increase the resiliency and design of the O-rings serves as the best demonstration of NASA’s commitment to SRB design and safety following the lessons learned for from the 51-L accident.
Columbia – Like losing a family member:
Lessons learned from the loss of Columbia in 2003 have also played a major role in improving the shuttle’s safety record.
The vehicle and her seven member crew were lost after a liberated piece of External Tank’s insulating foam impacted and breached a RCC (Reinforced Carbon Carbon) panel on Columbia’s left wing during the January 16th launch, Columbia stood no chance of surviving the unforgiving environment of re-entry on February 1.
While vast improvements have been made to the mitigation of foam releases from the ET during ascent are well documented, additional safety protocols were included in the post-Return To Flight “NASA Culture”, such as “Time Outs” – where any engineer can stop work on shuttle hardware if he/she seems anything they aren’t happy with – have aided the safety of the remaining fleet.
“We have a very strong timeout policy on the floor,” noted KSC Launch Director Mike Leinbach. “If one of the processing guys seems something wrong, they’ll call a time out, we’ll stand down and talk about it – and that’s heralded throughout the program. It’s an amazing program and it works well.
“The thing that really makes this all come together are the people that process these vehicles. I’m extremely proud of each and every one of them, and happy to represent them.
Many of these engineers are faced with the final year of working in the space program, as the shuttle comes to a close. However, their dedication to the vehicles is obvious, with the orbiters treated as beloved vehicles to such an extent Mr Leinbach noted it felt like losing a member of the family when Columbia was destroyed during re-entry.
“We couldn’t do all this work if we didn’t enjoy it and it’s is the love of the people doing the work. It’s an amazing amount of work, some of it is very repetitive – but you’d be amazed how often a repetitive job becomes a job of love by the people themselves.
“‘Yeah, I’ve torqued this bolt down on this ship 100 times, but by God I’m going to do bolt 101 the same way.’,” added Mr Leinbach as an example. “You have to see and feel it to understand, I wish I could explain it – just an amazing team to see at work.
“It comes from within and one of the times that I like to reflect on – even though its sad – is the Columbia accident. We lost seven astronauts and that was awful, just devastating, but we also lost an orbiter.
“It’s hard to explain to people but when we lost Columbia it was like losing a family member. It’s almost that deep when you work on these machines day in and day out.”