Marshall Space Flight Center

Consistent with its mission to develop spacecraft technologies and with its heritage as the site where Wernher von Braun and his team had

worked since 1950, Marshall Space Flight Center has always had a strong technical/analytical organization, engaged in science and engineer­ing research as well as advanced design studies. Research areas have included basic finite element methods, shells, fluid-structure systems, and nonlinear structures, as well as quick-turnaround non-FEM meth­ods for early design and feasibility studies.[930]

Applications have usually involved the structural and structural – dynamic problems of launch vehicles. As an example, computational techniques were used to help resolve "pogo” oscillations in both the first and second stages of the Saturn V launch vehicle. As the name implies, the pogo mode is a longitudinal tensile/compressive oscillation. Flight data from the unpiloted flight of the second Saturn V in 1968 showed severe vibrations from 125 to 135 seconds into the first-stage burn. The pogo mode is not always harmful, but in this case, there were concerns that it could upset the guidance system or damage the payload. The structural frequency was dependent on fuel load, and at a certain point in the flight, it would coincide with a natural frequency of the engine/ fuel/oxygen system, causing resonance. Using the models to evaluate the effects of various design changes, the working group assigned to the task determined that accumulators in the liquid oxygen (LOX) lines would alter the engine frequency sufficiently to resolve the issue. Subsequently, engineers examining flight data from the Apollo 8, 9, and 13 missions noticed a similar occurrence in the second stage. This was studied and resolved using similar techniques.[931]

The first-stage pogo issue occurred at a point in the Apollo program when time was of the essence in identifying, analyzing, and resolving the problem. The computer models were most likely no more complex than they had to be to solve the problem at hand. Marshall Space Flight Center has continued to develop and use fairly simple codes for early con­ceptual studies. Simple, quick-turnaround tools developed at Marshall include Cylindrical Optimization of Rings, Skin and Stringers (CORSS, 1994) and the VLOADS launch loads and dynamics program (1997). VLOADS was developed as a Visual BASIC macro in Microsoft Excel. When released in COSMIC in 1997, it was also available in PC format.

It was distributed on a single 3.5-inch diskette.[932] This was a remark­able development from the days when the problem of launch vehicle dynamics occupied a sizable fraction of this Nation’s computing power!

Like researchers at Langley, Marshall’s personnel moved swiftly from single or limited application tools to finding ways to integrate them with other tools and processes and thereby achieve enhanced or previ­ously unattainable capabilities. The Coupled Eulerian Lagrangian Finite Element (CELFE) code, developed collaboratively with NASA Lewis Research Center in 1978, included specialized nonlinear methods to cal­culate local effects of an impact. It was coupled to NASTRAN for calcu­lation of the far-field response of the structure. Applications included space debris, micrometeor, and foreign object impact studies for air­craft engines.[933] Marshall developed an interface between the PATRAN finite element preprocessor (normally used with NASTRAN) and the NASA Langley STAGS shell analysis code in 1990.[934] Marshall sponsored Southwest Research Institute to develop an interface between Lewis – developed NESSUS probabilistic analysis and NASTRAN in 1996.[935] Both STAGS and NESSUS have been widely used outside NASA. This review of NASA Centers and their work on computational structural analysis has offered only a glimpse of the variety of structural problems that exist and the corresponding variety of methods developed and used at the various NASA Centers and then shared with industry.