NASA is revealing its most detailed assessment yet of the design challenges that are being tackled as part of its plan to develop and test the heavy-lift Space Launch System (SLS) vehicle for human exploration from 2017 and beyond.

While the overall SLS effort remains on track, and even ahead of schedule in some cases, NASA says significant design issues have had to be overcome in some areas to cope with the unexpectedly high liftoff and ascent loads of the powerful vehicle. The key challenges have been encountered, and so far successfully addressed, in adapting the modified space shuttle five-segment solid booster to the SLS core stage, as well as in designing the interim cryogenic propulsion system (ICPS) that will power the Orion multipurpose crew vehicle out of Earth orbit.

The ICPS sits atop the main core stage and forms part of the integrated spacecraft and payload element (ISPE) of SLS-1000X, which is the designation for the initial variant. The overall stack comprises the crew vehicle, a stage adapter, separation system and launch vehicle stage adapter. The propulsion unit, sitting between the two adapters, will be based on a modified 2016 production version of the Delta IV cryogenic second stage with an Aerojet-Rocketdyne RL 10B-2 engine.

However, analysis indicates that on liftoff and during initial ascent, the “vehicle twangs and imparts lateral loads” says Rene Ortega, SLS spacecraft and payload integration office chief engineer. The lateral loads at liftoff are expected to be produced mostly by north-south winds over the launch pad, while ascent lateral loads will be generated by aerodynamic buffeting. To reduce the potential impact on the RL 10B-2 and the rest of the ICPS, NASA plans to incorporate a stabilizer liftoff restraint and release system at “T-zero” as well as additional system damping, as recommended by the ICPS developers Boeing and United Launch Alliance. Further liftoff wind limits may also be considered in later design iterations, while for tackling the ascent loads, NASA says an SLS “aero-buffet” team continues to study potential damping from electro-mechanical actuators and “other more complicated options.”

The design team says that although loads are decreasing and capability improving as the configuration matures, “the challenge with using heritage hardware is that the capability is mostly fixed.” Overall progress on the spacecraft and payload element continues on track, however, with a preliminary design review (PDR) for the combined unit completed in June.

The design has been slightly modified with a larger hydrogen tank for added stage performance, and manufacturing has begun of the adapter that will be used for the Delta IV-boosted exploration flight test of the Orion in late 2014. First flight of the SLS is targeted for 2017 with Exploration Mission-1 (EM-1), an unmanned Orion test flight beyond the Moon. The first crewed flight, EM-2, is set for 2021.

The challenges of adapting heritage design hardware to the new SLS have also led to modifications to the ATK-developed booster. Describing the changes at the American Institute of Aeronautics and Astronautics (AIAA) Joint Propulsion Conference in San Jose, Calif., NASA SLS booster element chief engineer Mat Bevill said: “everything associated with the recovery system [parachutes, sensors and so on] has been removed. The forward skirt is using heritage hardware from space shuttle but the avionics under it are all new.”

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.

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.