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Redesigned Field Joints - Shuttle Solid Rocket Motor

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A failed O-ring in the Right Solid Rocket Motor caused Challenger to explode and Mission STS 51-L to fail on the 28th of January, 1986. 

Unfortunately, numerous individuals knew that the O-rings had a propensity to fail when weather on the day of a shuttle launch was too cold. On the day of Challenger's launch, the weather in Florida was unseasonably cold. It was so cold that there were icicles at various places on the launch pad.

When members of the Rogers Commission investigated the reason for Challenger's explosion, one member in particular - Dr. Richard Feynman - conducted a skillful examination of the witnesses. Using a simple experiment - something any high-school student would understand - he revealed why the O-ring failed on launch day.

Because he believed the Commission's report did not state, strongly enough, how NASA and its suppliers should communicate together - to avoid disasters like the Challenger explosion - Dr. Feynman wrote his own report. He concluded his technical analysis with simple words that anyone can understand:

For a successful technology, reality must take precedence over public relations, for nature cannot be fooled.

A new design had to be created, and implemented, before America could send another group of astronauts into space aboard one of the remaining Orbiters.

Allan McDonald - a Morton Thiokol engineer who had earlier recommended that NASA should not launch the shuttle if temperatures where lower than 53 degrees Fahrenheit, was placed in charge of the redesign. A book which he wrote twenty years later - entitled Truth, Lies and O-rings: Inside the Space Shuttle Challenger Disaster - tells us more about the redesign (and why it was needed).

This NASA image depicts the original design, for the impacted field joint, compared with the redesign. 

We learn the technical details, about the changes, from NASA’s 1988 NSTS Shuttle Reference Manual:

The SRM field-joint metal parts, internal case insulation and seals were redesigned and a weather protection system was added.

In the STS 51-L design, the application of actuating pressure to the upstream face of the O-ring was essential for proper joint sealing performance because large sealing gaps were created by pressure-induced deflections, compounded by significantly reduced O-ring sealing performance at low temperature.

The major change in the motor case is the new tang capture feature to provide a positive metal-to-metal interference fit around the circumference of the tang and clevis ends of the mating segments. The interference fit limits the deflection between the tang and clevis O-ring sealing surfaces caused by motor pressure and structural loads. The joints are designed so that the seals will not leak under twice the expected structural deflection and rate.

The new design, with the tang capture feature, the interference fit and the use of custom shims between the outer surface of the tang and inner surface of the outer clevis leg, controls the O-ring sealing gap dimension. The sealing gap and the O-ring seals are designed so that a positive compression (squeeze) is always on the O-rings. The minimum and maximum squeeze requirements include the effects of temperature, O-ring resiliency and compression set, and pressure. The clevis O-ring groove dimension has been increased so that the O-ring never fills more than 90 percent of the O-ring groove and pressure actuation is enhanced.

The new field joint design also includes a new O-ring in the capture feature and an additional leak check port to ensure that the primary O-ring is positioned in the proper sealing direction at ignition. This new or third O-ring also serves as a thermal barrier in case the sealed insulation is breached.

NASA and its contractors also decided to eliminate the use of putty in the sealing process:

The field joint internal case insulation was modified to be sealed with a pressure-actuated flap called a J-seal, rather than with putty as in the STS 51-L configuration.

NASA and its contractors also decided to improve the field-joint mating pins, as another layer of fail-safe protection:

Longer field-joint-case mating pins, with a reconfigured retainer band, were added to improve the shear strength of the pins and increase the metal parts' joint margin of safety.

The joint safety margins, both thermal and structural, are being demonstrated over the full ranges of ambient temperature, storage compression, grease effect, assembly stresses and other environments. External heaters with integral weather seals were incorporated to maintain the joint and O-ring temperature at a minimum of 75 F. The weather seal also prevents water intrusion into the joint.

Click on the image for a better view of the diagram.


Media Credits

Image and quoted passages from NASA’s 1988 NSTS Shuttle Reference Manual.  Online, courtesy NASA.

PD

 

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