The first full-scale booster qualification ground test was successfully completed in March 2015 and demonstrated acceptable performance of the booster design at 90 degrees Fahrenheit – the highest end of the booster’s accepted propellant temperature range. Testing at the thermal extremes experienced by the booster on the launch pad is important to understand the effect of temperature on how the propellant burns.
For Tuesday test, the booster's propellant was cooled to 41 degrees, a process that took more than a month using 25-degree air pumped into a roll-off hangar from multiple refrigeration units. The goal was to collect performance data at the lower end of the allowable temperature range.
These boosters will provide about 75 percent of the thrust needed to lift the rocket and its payload into low-Earth orbit. As implied by their name, the boosters burn a solid composite made largely of aluminum. During Tuesday's test, a single booster produced about 3.2 million pounds of thrust. By way of comparison, a Falcon 9 rocket with its nine engines produces about 1.7 million pounds of thrust.
Tuesday's qualification test was part of NASA's plan to continue developing the SLS rocket, consisting of two boosters and a core stage, in time for its initial test flight in late 2018. During a teleconference with reporters after the test, NASA's chief official for human spaceflight, Bill Gerstenmaier, said the flight would likely come in October or November of 2018. This uncrewed flight, Exploration Mission-1, will send the Orion spacecraft out beyond the Moon and back. "We’re in pretty good shape for EM-1 and we’re moving at the right pace," Gerstenmaier said.
With the initial version of the SLS rocket, NASA intends to have the capability to lift 70 metric tons to low-Earth orbit. An upgraded version of the rocket with a more powerful upper stage, scheduled to fly sometime in the early 2020s, will have the capability to lift 105 tons. That number compares favorably to the Falcon Heavy rocket being developed by SpaceX, with a capacity of about 54 tons to orbit.
