The mini-me version of the Space Launch System (SLS) is now being put through its paces as part of the Scale Model Acoustic Test (SMAT) program. The heavily instrumented five percent scale model of the SLS is being used to provide data on the acoustical energy that could be expected during launch, in turn feeding into the design of Pad 39B’s Sound Suppression System that will be tasked with mitigating the impact on the vehicle.
A better definition of the lift-off acoustic environment can be determined from hot fire testing of dynamically scaled models of the launch vehicle and stand.
That heritage – which is continuing with SMAT – ranges back as far as the early days of the Space Shuttle Program (SSP).
During the Space Shuttle development program, a 6.4 percent scale model of the launch vehicle, propulsion system, launch stand, and exhaust duct system with water suppression was used to refine the analytical/scaling estimates of the lift-off acoustic environment.
The resulting data provides a very useful template for the full scale rocket, although final verification of the environment is only fully provided by full static firings or launches of the actual vehicle.
Notably, the debut launch of the Space Shuttle Program (SSP) – with Columbia on STS-1 – showed the importance of understanding the acoustic environments, as the orbiter’s heat shield was damaged when an overpressure wave from the SRBs caused a forward RCS oxidizer strut to fail. Her body flap was also pushed five degrees out of position.
The subject was also raised during STS-129’s Flight Readiness Review (FRR), as a potential issue with a very small area of the orbiter – known as a stinger attach point between the RCS and OMS Pod – raised concerns that recent acoustic environment analysis of the Space Shuttle Main Engines (SSMEs) during ignition could cause stressing that potentially leads to cracks in the attach pins/stinger.
Although those concerns were based on old and overly conservative data, managers showed their usual due diligence in gathering an array of updated information, via new computational models, borescope inspections on the fleet, and the installation of sensors in the area in question – all of which would be used to completely allay the potential fear of life fatigue on the stinger.
Notably, the FRR presentations noted they lacked key historical data, while Main Engine Ignition (MEI) Acoustic & SSME Ignition Overpressure (IOP) Environment data was classed as “continually evolving” during the 30 years of the program – leading to the concern ahead of STS-129.
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The vehicle that was set to replace the Space Shuttle, Ares I, also underwent IOP testing – with the Ares I Scale Model Acoustics Test (ASMAT) tested during 2010.
Numerous tests, each using a different pad configuration – such as with and without water bags within the launch mount – were conducted at MSFC.
Quick look test results indicated that the overall noise levels measured on the vehicle were within predicted ranges and the data compared favorably between the firings. However, Ares I was cancelled shortly after the ASMAT firings.
With SLS now under the acoustical microscope, the SMAT program is using updated approaches for harvesting data from the test firings.
“This test is unique because it’s like going through the steps of a true launch, only on a much smaller scale,” said Jeremy Kenny, acoustics engineer at MSFC.
“The noise the engines and boosters generate is so great that it can impact the rocket, and the crew, during liftoff. We have to ensure we have the proper suppression system to basically turn that noise down to a safe level.”
The primary source of the acoustic field is the fluctuating turbulence in the mixing region of the rocket exhaust flow – known as Engine Generated Acoustics.
Engine generated noise is a function of the exhaust flow parameters, launch stand configuration, and to a lesser extent atmospheric conditions.
Preliminary estimates of the engine generated acoustics at a specified location on the vehicle can be determined by scaling measured acoustic data from previous launch vehicle programs, taking into account the above mentioned flow, configuration, and atmospheric parameters.
“When you’re building the largest rocket in the world, you have to take everything into consideration,” said SLS Chief Engineer Garry Lyles.
“Acoustic testing is a very critical part of that. We’re using testing techniques that were highly successful during the space shuttle era, and tailoring them to SLS design specifications. It’s getting us where we need to be for the rocket’s first flight.”
The test stand at MSFC allows for the mini-SLS to be placed at fixed elevations for individual test firings, these being 0, 2.5, 5.0, 7.5, and 10.0 feet. Based on the scale of the ASMAT, these distances corresponded to full-scale elevations of 0, 50, 100, 150, and 200 feet.
The test vehicle is heavily instrumented, with five primary instrumentation suites resulting in over 325 sensors on the SMAT rocket.
The instrumentation includes B&K 4944-B microphones and pressure transducers on the tower/mobile launcher. It also includes far field measurement devices, accelerometers, thermocouples and strain gauges on vehicle, thermocouples, flow meters and chamber pressure instrumentation.
The tests will ramp up over time, with the opening SMAT firing at the start of this year – which lasted five seconds – involving an article that was without its twin imitation boosters.
Instead, SMAT began testing with small thrusters of its aft, mimicking the RS-25D (Space Shuttle Main Engines) that will be placed on the core of the real life SLS.
These thrusters – similar to vintage hardware originally designed in the 1960’s and tested during the Space Shuttle program – successfully met all test objectives during Phase I scale model acoustic testing at Marshall’s Test Stand 115.
Testing has now moved on to using the two “booster” rockets on each side of test article, which utilizes two Rocket-Assisted Take Off (RATO) motors, simulating SLS boosters, with the test requirement calling for the motors ignite simultaneously, as the SRBs would during launch.
Once the test program is complete, the data will feed into the final design of the Sound Suppression System for SLS at Pad 39B.
The initial model of the system was one of the first items to be built at MSFC for the SMAT tests.
Work begin in 2012 at Marshall’s test stand 116, with the construction of a working water-based sound suppression system that is now being employed by the SMAT testings.
The build was finalized after discussions with NASA/KSC Ground Systems Development and Operations (GSDO) engineers, who are responsible for the full scale system that will be used during SLS’ debut launch at the end of 2017
(Images via NASA and L2)
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