Computational Simulations of the Space Launch System Scale Model Acoustic Test

Overview

NASA is currently designing a new heavy-lift rocket, the Space Launch System (SLS), which will be used for deep space exploration. Because the SLS uses a new configuration of solid rocket boosters (SRBs) and liquid engines, testing and analysis are needed to ensure that the acoustic pressure waves generated during liftoff will not compromise the vehicle. The Scale Model Acoustic Test (SMAT) is a 5% scale model of the SLS that is being used to help predict the acoustic launch environment and assess the effectiveness of the launch platform’s water sound suppression system for the new configuration. Computational simulations of the SMAT are being employed to better understand and interpret the test data and to gain additional insight into the predicted flow physics of the SLS launch environment.

Project Details

Computational fluid dynamics (CFD) simulations are being performed in conjunction with the SMAT tests. These simulations enable analysis of the full, three-dimensional fluid dynamics around the scale model, whereas the test data is only recorded at distinct locations. Parametric CFD studies were also performed to distinguish different sources of overpressure waves and to investigate the effects of SRB ignition timing variations, which can occur due to uncertainty in the delay between the ignite signal and actual motor ignition. An offset in the ignition timing between the two boosters alters the acoustic environment and the resulting ignition overpressure (IOP) waves, and this effect is more amplified at the 5% test scale than it would be at full-scale. In addition, the SMAT data will also be used to assess the accuracy of the CFD results and validate the simulation methods for future analysis of the full-scale SLS launch environment.

Results and Impact

Two parametric CFD studies were performed for the SMAT configuration. The first study focused on understanding the effects of the SRB ignition timing uncertainty during testing. Simulations were performed for five different ignition timing offsets from 0.0 milliseconds (ms) to 16.5 ms, yielding a suite of flow field data throughout the model. This data was then analyzed to determine the booster timing effects without requiring costly additional testing. The computational results identified that the peak-to-peak magnitude of the overpressure environment was reduced as the timing offset increased up to the 5.75 ms offset, after which point additional offset had minimal effect.

The second CFD study used a segregated computational domain, which divided the domain above the mobile launch platform without significantly altering the flow physics, to isolate the IOP waves from the duct overpressure (DOP) waves. These results were compared and contrasted with a CFD simulation that used the standard domain to determine the locations and magnitudes of the IOP and DOP waves on the vehicle. The comparison provided insights that enabled better interpretation and classification of the SMAT test data.

Ultimately, these simulation results are providing key launch environment data needed to help design the SLS and its water sound suppression system, and assure safe liftoffs for the next-generation heavy-lift rocket.

Why HPC Matters

Accurate analysis of the complex SMAT geometry and intense rocket plume flows required high-resolution computational models with approximately 250 million grid cells. Each of the SMAT CFD simulations was performed using 2,000 parallel computing processors on NASA’s Pleiades supercomputer. These resources enabled the large simulations to be turned around within just weeks, which allowed adequate time to assess the results and provide feedback to the SMAT engineers during testing.

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