NASA to test world’s largest crew-rated solid rocket booster
On June 28, 2016, Orbital ATK will once again test the new five-segment solid rocket booster (SRB), dubbed Qualification Motor-2 (QM-2). A pair of these boosters will be used to help launch NASA’s Space Launch System (SLS) on the agency’s “Journey to Mars”, beginning with the rocket’s first, uncrewed launch, EM-1, currently slated for the latter half of 2018.
Although similar in appearance to the Space Shuttle’s SRBs, the motors for the SLS have an additional segment—for a total of five. This extra section allows for more propellant and thus increases the rated thrust from 2.8 million pound-force (12,000 kilonewtons) on the Shuttle-era booster to 3.6 million pound-force (16,000 kilonewtons) on the SLS-designated booster.
Other design changes from the Shuttle’s SRBs are the elimination of an asbestos-based casing insulating liner, upgraded avionics that are 1,900 pounds (860 kilograms) lighter, and the fact that it will not be recovered after splashdown.
Using data acquired from last year’s successful QM-1 test, in addition to information validated through multiple simulated launches, the booster’s upgraded flight control software will take command of SRB’s functions, including ignition and nozzle actuation.
Booster engineers at NASA’s Marshall Space Flight Center will receive data from QM-2 shortly after the conclusion of the test, which will be incorporated into further flight simulations.
Unlike QM-1 where the booster was warmed to 90 degrees Fahrenheit (32.2 degrees Celsius), the QM-2 will be a “cold soak” firing. As part of the qualification regime, NASA requires the booster be tested at the extreme limits of its launch criteria. Last year’s successful QM-1 test saw the booster tested at its upper operational temperature limit. QM-2, though, will see the booster tested at the lower operational temperature limit of 40 degrees Fahrenheit (4.4 degrees Celsius).
Temperature can have an impact on the characteristics of the propellant, and it’s important to understand how it may alter the performance of the booster. Due to the mass of the SRB’s casing and propellant, it takes more than a month for the internal temperature to reach the desired reading.
The solid fuel in the new booster is produced at the same Orbital ATK plant which handled boosters during the Shuttle program. The components are mixed in large bowls, greatly resembling an over-sized kitchen mixer. Unlike home cooking, however, the ingredients blended to produce the mixture is considerably more explosive than what would be found in a household kitchen.
The Ammonium Perchlorate Composite Propellant (APCP) is produced in small batches in hardened bunkers, with the equipment being operated remotely. These safety precautions are not unfounded. In June 1985, at the then Morton Thiokol-owned facility, one of the propellant mixing buildings was destroyed after the fuel mixture was detonated by a lightning strike, causing millions of dollars in damage.
Unlike liquid-fueled engines, which have their fuel pumped to them when needed, a solid rocket booster needs to have the fuel cast into a casing. This casting occurs in a large, isolated building, with deep pits designed to hold the huge motor segments while the putty-like fuel is poured into casings. Since SRBs cannot be actively throttled, varying levels of thrust can be passively achieved by casting the fuel in a particular shape.
As the fuel burns from the center of the casting outward, the fuel’s surface area will change over the duration of the burn with the resulting change in surface area directly impacting the amount of thrust being generated. This precise casting is achieved by placing a mold into the segment casing while the fuel is being poured, which creates the desired shape. The three middle sections use the same mold, while the forward and aft segments require molds specifically designed for them.
The road to QM-2 was not always smooth. The boosters flown during the Shuttle program used an insulating liner which utilized an asbestos-based component. Special dispensation had been given to continue using the banned insulation so long as the Shuttle program was active, but something else had to be used going forward.
Early insulating products exhibited voids in the propellant caused by a reaction of the solid fuel and the insulating material, which could have potentially caused significant damage to the booster. Engineers eventually developed a liner for the booster which performed as desired, while also reducing weight. Analysis of the casing after QM-1 validated the effectiveness of the new insulation.
Engineers at Orbital ATK’s Promontory, Utah, location have performed a multitude of tests on the booster in addition to the static firings. The booster attachment points have been stressed to—and beyond—nominal flight loads, ensuring the vehicle can withstand the greater stresses predicted from the upgraded boosters. The gimbal joints in the nozzles, which are larger than what was on the Shuttle’s SRBs, have also undergone their own battery of tests.
The solid rocket booster components being used for the QM-2 test has a long history of service, with all five segments having been used on prior motor tests and Shuttle flights. Three of the cylinder segments have flown on six previous Shuttle missions while the aft dome flew on STS-134, the penultimate Shuttle mission. In fact, the only new major component is one of the stiffeners used on the aft segment.
Although the SRBs for the Space Shuttle were recovered after splashdown, the ones used on SLS will not share the same fate. No longer being outfitted with recovery hardware, such as parachutes, the new boosters will not be recovered after use.
Analysis of the five-segment solid rocket booster used in the 2009 Ares I-X test revealed significant casing damage, suggesting recovery would not be practical. That, coupled with the lower flight cadence of SLS and the cost of keeping a standing recovery crew and assets, it was decided to allow the SRBs to splashdown hard and sink to the ocean floor, eventually becoming an artificial habitat for sea creatures.
Tuesday’s static firing will be a full-duration test with a burn time of slightly greater than two minutes. Barring any significant anomalies, the next firing of a five-segment solid rocket booster will occur in 2018 when a pair are ignited to loft the EM-1 mission into space on the first flight of the SLS.
Curt Godwin has been a fan of space exploration for as long as he can remember, keeping his eyes to the skies from an early age. Initially majoring in Nuclear Engineering, Curt later decided that computers would be a more interesting - and safer - career field. He's worked in education technology for more than 20 years, and has been published in industry and peer journals, and is a respected authority on wireless network engineering. Throughout this period of his life, he maintained his love for all things space and has written about his experiences at a variety of NASA events, both on his personal blog and as a freelance media representative.