Loads Challenges Remain For SLS Design

By Guy Norris
Source: Aviation Week & Space Technology
July 29, 2013
Credit: NASA

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.”


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