Other changes include modified propellant grain geometry in the forward segment of the reusable solid rocket motor, adding to those already made for the canceled Ares booster version. On the aft segment, the location of the attachment unit connecting the booster to the core has also been changed.
As with the upper stages, anticipated loads are impacting design decisions. “Analysis shows the tension loads on the separation bolt at the forward end of the booster are higher than expected and are higher than anything we saw on the space shuttle,” says Bevill. “So we will be making it as strong as we can.” The pyrotechnic charge and its housing groove have also been modified. The design of the SLS core stage meant the ring connecting the booster to the attachment had to be moved 240 in. farther aft, too. The redesigned attach ring has been demonstrated on a “pathfinder” aft segment.
Because the nozzles of the extended booster sit closer to those of the SLS main engines than they did to the space shuttle engines, the thermal curtain that protects the base of the nozzle may also have to cope with increased thermal and structural loads. Potential modifications are “in work” says Bevill. “We found that the outer layer of the curtain can be rapidly consumed, so we'll maybe use a reflective fabric on the outer surface to mitigate plume radiation, and we're working through those plans now.”
The close proximity of the nozzle also means there could have been a danger of debris from the seals covering the booster separation motors (BSM) impacting the core stage engines at separation. The designers fixed this by adopting the hinged, non-frangible seal design from the forward BSM. “We are also using a thermal barrier O-ring in the nozzle aft exit joint in place of an obsolete thermal barrier,” Bevill says.
Current work is focused on dealing with high ascent, liftoff and acoustic loads on the forward skirt area of the booster. “We do have a few loads-related challenges,” says Bevill. “There is a 35% increase in loads on the forward skirt and we could get buckling of the shell. We've looked at various solutions.” Skirt modification options will be tested at full scale to failure for additional model correlation. Acoustic load levels may also exceed the capability of the avionics boxes mounted near the forward skirt area.
“The expected load release is significantly higher than what has been seen on the side–particularly in areas at 90 degrees to the thrust post,” Bevill says. Proposed solutions include isolating or relocating avionics boxes.
Following three successful static-fire tests of the five-segment booster in 2009-11, the first full-scale qualification booster is being assembled in Utah for testing later this year. “We will be integrating avionics systems into the qualification motor,” Bevill notes. Flight control ground tests were run in March 2012 and in February 2013. The SLS boost element completed PDR in April and is “on target for CDR [critical design review] maturity,” says Bevill. System development testing is scheduled for September, marking the final hurdle before the CDR milestone.
Work to integrate the former space shuttle RS-25D main engines into the SLS is on track, says engines element chief engineer Katherine Van Hooser. NASA has an inventory of 16 flight engines and two development engines and, because they will be operating at a higher power than on the shuttle, “we are going through them, assessing life requirements and making sure we have enough life to meet the SLS manifest,” she says.
One of the main challenges in the meantime is long-term storage for the engines. “We've transferred them to [NASA] Stennis [Space Center] and some are in engine containers or bagged. All have been purged and are monitored,” says Van Hooser. Because of the proximity of the RS-25D nozzles to the SLS booster plume, the agency has also “done a bit of work analytically to make sure we can handle the hotter environments,” she adds.