Propellant inlet conditions also differ because of the taller vehicle and resulting higher liquid oxygen (LOx) inlet pressure. “We will modify the engine to adapt to the higher pressure and modify the start sequence,” says Van Hooser. The higher pressure surges through the engine as it starts and, as a result, “we have to teach the controller as it works its way through those pressure surges,” she notes. “So we will have to modify the start sequence and that's touchy.” The team “melted about 13 turbines at the beginning of the program before we got it right,” adds Van Hooser.
She says the LOx is also “going to be colder, so there is a potential for damage from temperature spikes.” This affects the start sequence as well, because the colder flow will be denser and therefore have a higher mass, which in turn will affect the mixture ratio in the turbines. A solution has been devised that includes adding heat locally and reducing pre-start bleed flows. The revised start sequence will be modeled, evaluated on the test stand and verified with ground tests.
Tests of the upper-stage Aerojet-Rocketdyne J-2X at Stennis in June included an evaluation with the engine gimbaled up to 7%. “We're getting these test stands ready to go for RS-25, which will support the vehicle PDR in June 2014,” says Van Hooser, who notes that the J-2X program had completed 50 tests on two engines and two powerpacks as of late June.
Reporting on progress with the core stage, Boeing SLS chief engineer Michael Wood says the vehicle is on-cost, five months ahead of schedule and “well on the way to CDR in mid-2014,” with drawing release underway. The large core stage will measure 200 ft. in length overall, and comprises a forward skirt, LOx tank, intertank, liquid hydrogen tank and an engine section enclosing the SLS's four main RS-25Ds. The company expects to complete CDR on all major components by year-end and has finished acceptance testing on all major tooling.