Launch Environment Water Flow Simulations Using Smoothed Particle Hydrodynamics

John F. Kennedy Space Center, Florida

One of the crucial ground structures employed at the launch pad during the Space Shuttle program is the rainbird nozzle system, whose primary objective is to suppress acoustic energy generated by the launch vehicle during pad abort and nominal operations. It is important that the rainbird water flow does not impinge on the rocket nozzles and other sensitive ground support elements. For the new Space Launch System (SLS) vehicle, the operation is similar, regardless of the new mobile launcher and new engine configurations. The goal of the rainbird nozzle system remains sound suppression (SS), and the rocket engines still cannot get wet. However, the rearrangement of the rainbird water system for the SLS mobile launcher locates the rainbirds closer to the first-stage rocket engines, which are positioned above the exhaust hole. The close proximity of the rainbird nozzle system could potentially cause vehicle wetting during liftoff.

Numerical simulation of water flow was conducted using Smoothed Particle Hydrodynamics (SPH) in an attempt to determine if rainbird water would have an impact during abort and launch operations. SPH is a meshless computational method used for simulating fluid flows. The meshless, nodal collocation, spatial discretization, kernel approximation method eliminates the pre-processing step, and grid quality is no longer an issue. Like other particle methods, SPH was known for being computationally expensive in problems using fine description. However, with recent advances in hardware acceleration and parallel computing such as Graphical Processing Units (GPUs), SPH technology has become more useful and versatile. Compute Unified Device Architecture (CUDA) is a parallel programming method and software for parallel computing with some extensions to C/C++ language. The parallel power computing of GPUs can also be applied for SPH methods where the same loops for each particle along the simulation can be parallelized.

The object-oriented programming paradigm makes the code easy to understand, maintain, and modify. Sophisticated error control is available. Furthermore, better approaches are implemented; for example, particles are reordered to give faster access to memory, symmetry is considered in the force computation to reduce the number of particle interactions, and the best approach to create the neighbor list is implemented. The CUDA language manages the parallel execution of threads on the GPUs.

Several test cases have been performed. Water can be rejected through the nozzle by plunger, of which the motion is described by acceleration, velocity, or position. Simulation results of water flow through an actual rainbird nozzle matched predictions obtained from other methods such as 1D projectile and Volume of Fluid (VOF) analyses. During a nominal launch, water spray gradually develops over the exhaust hole, and the launch vehicle already lifts off when the water flow is fully developed and completely covers the hole. There might be some water splashing off the deck and sprinkling on the rockets; however, the impact is minimal and acceptable.

During a pad abort event, it is important that the Hydrogen Burn-Off Ignitor (HBOI) is kept dry. The simulations showed that the HBOI will get doused in the baseline design unless there is some blockage to fill the gap between the launch deck and the blast shield in the vicinity of the HBOI. A wedge underneath the blast shield deck is designed to prevent water from deluging the HBOI.

This work was done by Bruce Vu, Jared Berg, and Michael Harris of Kennedy Space Center. KSC-13900