When NASA’s Space Launch System (SLS) enters service in 2017, it will replace the historic Saturn V as the largest, most powerful space launch vehicle ever flown. To lift its initial 70-metric ton (77 ton) payload into orbit, the SLS will rely on additional boost from a pair of the largest solid rocket motors ever to be built for flight. The first of these boosters is being assembled for NASA in Brigham City, Utah by ATK Space Systems using new manufacturing methods intended to make these behemoths safer and less expensive.
When the SLS enters service in 2017, it will have ten percent more thrust than the Saturn V rocket at liftoff and is intended to launch the Orion manned spacecraft and for lifting large payloads for deep space missions such as a possible space outpost orbiting beyond the Moon. In addition to the four RS-25 former space shuttle main engines that will power the first stage, the SLS will also use two advanced solid rocket boosters.
These boosters are derived from the Space Shuttle boosters, though they are larger and of an improved design. Where the Shuttle boosters were made in four segments, the SLS boosters are made in five. These segments contain the fuel, which is composed of ammonium perchlorate, powdered aluminum, iron oxide, a polymer (such as Polybutadiene acrylonitrile (PBAN) or hydroxyl-terminated polybutadiene (HTPB)) and an epoxy curing agent. Like the Shuttle’s, the SLS boosters are reusable, though they are much more powerful. Where a shuttle booster put out 2.8 million pounds of thrust (12,000 kN), and SLS booster reaches 3.6 million pounds (16,000 kN).
The engine currently being assembled by ATK is called Qualification Motor-1. It’s not intended for flight, but rather as a test rocket to evaluate design and construction, with the first test firing scheduled for the spring of 2013. In manufacturing the booster, ATK cut costs by 46 percent by streamlining the assembly process and replacing x-ray inspections with an ultrasonic examination of the booster's nozzle. This allowed the tests to take place on the factory floor rather than moving the nozzle to a special location.
The basic idea was to provide a more hands-on approach toward assembly and inspection to keep an eye out for any flaws and to move the booster as little as possible during assembly. In one case, part of the assembly process that required 47 moves now only required seven. This greatly reduces the chances that the booster might be damaged during assembly.
This effort to avoid damaging the booster is more than just a cost-cutting exercise. It’s also a way of improving the safety of the booster. The solid rocket motor puts out extreme heat and pressure that must be kept contained and controlled. It was the failure of a simple rubber o-ring that turned one of the Challenger’s boosters from a rocket into a blowtorch that destroyed the spacecraft. In order to keep the SLS from going from the biggest rocket to a very big bomb requires a lot of care and a lot of attention to detail.