As work progresses around the country toward the debut of NASA’s SLS rocket in 2018, Orbital ATK is preparing for the final qualification test of the five segment Solid Rocket Booster motor that will power the SLS through first stage flight. That second qualification test, known as QM-2, is targeted for late spring 2016 at the company’s proving ground in Utah.

QM-1: Fixing the propellant void issue

As part of a phased development plan for the massive five-segment Solid Rocket Boosters (SRBs) that will be used in combination with four liquid fueled main engines to power the SLS rocket through first-stage flight, Orbital ATK has progressed through a series of ground tests of the rocket motor.

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The most recent test was theQualification Motor -1 (QM-1) test on 10 March 2015.

That QM-1 test was delayed indefinitely when Orbital ATK discovered small voids between the propellant and the motor casing of the aft segment during standard x-ray inspections to evaluate an insulation lining material change in the SRB motor segments.

The bond between the propellant and insulation, which provides thermal protection for the SRB case, is a critical design feature that undergoes rigorous x-ray inspection to ensure its acceptability for flight.

Given the rigorous test objectives of QM-1 at the upper operating temperature range of SRB certification, Orbital ATK – with NASA’s consent – rejected the original aft segment for QM-1 and proceeded to cast another aft segment.

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Curiously, that recast aft segment also contained voids between the propellant and the motor casing.

With two aft segments now displaying the same issue, ATK expanded their investigation to determine what was causing the propellant to de-bond from the casing wall and form voids.

Evidence quickly began to mount that the issue was related to the use of a new material in the insulation lining between the propellant and the motor casing wall.

Originally, Shuttle SRB motor segment insulation lining was made of chrysotile-filled rubber: chrysotile being the most-common mineral form of asbestos.

While asbestos minerals contain useful sound absorption, strength, and resistance to fire, heat, electrical, and chemical damage properties, the silicate mineral holds severe health risks when inhaled over a prolonged period of time.

Non-asbestos (chrysotile-free) rubber insulation was therefore baselined for use on the SLS SRBs.

As development of the SLS 5-segment SRBs continued between 2011 and 2013, then-ATK conducted three DM (Demonstration Motor) tests, all of which occurred with the new insulation and were free of out-of-family propellant void issues.

As the SLS booster design matured from the DM stage to the QM stage, standard development processing and design changes were undertaken as the SRB design moved toward flight maturity.

Read the full article on NASASpaceflight.com.