Space Launch System: Core Stage Thrust Vector Control Systems Engineering Challenges in Reusing Heritage Hardware

The Space Launch System (SLS) Core Stage (CS) Thrust Vector Control (TVC)
system is comprised of 8 mechanical feedback Shuttle heritage Type III TVC
actuators and four RS-25 engines, each attached to a Shuttle heritage gimbal
block/bearing. Two actuators are used to move each engine in two planes perpendicular
to one another (i.e., pitch and yaw). The TVC system design leverages
hardware from the Space Shuttle program as well as new hardware designed
specifically for the Core Stage.
The Space Shuttle heritage hardware directly reused on SLS includes the Orbiter
TVC hydraulic servo-actuators (with two slight design modifications), the
Orbiter hydraulic circulation pumps, the Orbiter gimbal block/bearing, and the
Solid Rocket Booster hydraulic pumps. The Core Auxiliary Power Unit (CAPU)
is derived from the Orbiter Auxiliary Power Unit (APU). The Orbiter and Solid
Rocket Booster APU turbines are powered by hot gas produced by catalyzed hydrazine
decomposition. On the SLS Core Stage, the CAPU turbine is spun using
cold gas tapped-off from the RS-25 to CS liquid hydrogen autogenous pressurization
line.
While direct reuse or slight modification of existing hardware may seem to be
a triple-win for a program in cost, schedule, and technical risk mitigation, those
benefits can only be realized when its degree of application in a new system is carefully
and thoughtfully managed. The heritage hardware reuse should be prescribed
within the heritage design capability and reuse environments must lie within the
envelope of heritage qualification testing. Despite the significant test and flight experience
of the Shuttle heritage hardware components, successful integration with
the newly designed CS TVC components and incorporation into the stage design
proved to be a challenge which required re-qualification of the heritage hardware
as well as thorough integrated testing to support flight certification.
Examples of the challenges that were overcome include: re-qualifying heritage
hardware to survive new shock and vibration environments, certifying performance
of extensively modified heritage hardware, regenerating design insight
due to lack of available heritage vendor data, showing compliance to modern structural
design standards, translation of heritage requirements for analog avionics to
modern digital avionics, and interfacing heritage mechanical hardware with newly
designed avionics. This paper is the second installment in a seven-paper series
surveying the design, engineering, test validation, and flight performance of the
Core Stage Thrust Vector Control system. This paper will discuss several engineering
challenges encountered during the development process for SLS CS TVC
and how they were successfully overcome to reach flight readiness.