The Marshall Space Flight Center (MSFC) is constructing a scaled version of the Space Launch System (SLS), ahead of test firing it later this year. Known as the Scale Model Acoustic Test (SMAT), the mini-version of the SLS will have functioning rockets mimicking both the core engines and boosters.
Continuing the heritage of testing future launch vehicles at the scale model level, NASA engineers have test fired scaled versions of rockets to gain data on the acoustic environments endured during ignition and launch.
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.
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.
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.
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Notably, the FRR presentations noted they lacked key historical data, given the 6.4 percent model tested during the 1970s only fired motors that mimicked the Solid Rocket Boosters and not the SSMEs, 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.
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 the SLS now providing the role of NASA’s flagship launch vehicle, the Heavy Lift Launch Vehicle (HLV) will enjoy its turn on the test stand for the acoustic environmental tests – known as Scale Model Acoustic Test (SMAT).
Work begin in 2012 at Marshall’s test stand 116, with the construction of a working water-based sound suppression system.
“This water system will be used during the planned hot fire testing series that is planned for SMAT, which utilizes small-scale solid rocket Boosters and Lox-Hydrogen thrusters,” noted L2’s rolling SLS updates.
“Based on discussions with NASA/KSC Ground Systems Development and Operations (GSDO) engineers, MSFC is satisfied that this properly represents the water flow rates and coverage of the full-scale system and will meet the test needs for SMAT.”
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As with the tests on the previous vehicles, the data will provide a good baseline ahead of the actual SLS firing into life later this decade.
Notably, the SMAT will involve the most technically advanced sub-scale rocket used on such a test.
“Ignition overpressure (IOP) is a significant transient low-frequency pressure event caused by the rapid pressure rise rate of the solid rocket motor,” opened an extensive presentation on the SMAT (L2). “Lift-off acoustics (LOA) noise is caused by the supersonic steady jet flow interaction with surrounding atmosphere and launch complex, persisting for 0-20 seconds as the vehicle lifts off.
“Scale Model Acoustic Test (SMAT) objectives: Verify predicted LOA environments, obtain data to update the lift-off acoustic environments. Verify predicted IOP environments, obtain data for use in IOP analytical models for updated environments, and improve IOP analytical models.
“Verify SLS deflector design. Characterize Ground Acoustic (GA) environments, provide data to support GA environment predictions. Obtain Spatial Correlation (SC) data for use in vibro-acoustic models. Obtain data for Computation Fluid Dynamics (CFD) validation, and evaluate water sound suppression systems, determine water suppression attenuation.”
As such, two Rocket-Assisted Take Off (RATO) motors will simulate SLS boosters, with the test requirement calling for the motors ignite simultaneously, as the SRBs would during launch.
Testing has already begun on the small thrusters that will provide the role of the four RS-25 liquid main engines on the core.
A single thruster – 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 last year at Marshall’s Test Stand 115.
Fabrication then began for a “fourthruster cluster” set, mirroring the four RS-25s that will power all versions of SLS’ core stage.
“Hot-fire testing was initiated for the thrusters that will simulate the Core Stage Engines for the Scale Model Acoustic Test (SMAT). All four thrusters have been tested together for the first time in a single cluster in the same configuration that will be used for the Core Stage of the SMAT model,” added SLS’ rolling update section (L2).
“Testing is being conducted at Test stand 115 in the Marshall Space Flight Center (MSFC) East Test area. The first start ignition test was conducted on March 7, 2013. Two low thrust main stage tests were conducted on March 8, 2013. All test hardware is in excellent condition so far and (will continue testing during the Spring).”
When the actual SMAT model is completed and integrated on the test pad, a number of tests can be expected, not least because the maximum lift-off acoustic environment during an SLS launch will not be endured in the vehicle starting position, but at some elevation above the Mobile Launcher.
As such, tests will probably attempt to simulate a lift-off, without the SMAT model actually launching.
For the Ares I Scale Modelling Acoustic Tests, the vehicle model was set at a number of 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.
Also, as expected, the test vehicle will be heavily instrumented, with five primary instrumentation suites resulting in over 325 sensors on the SMAT rocket.
It will be outfitted with B&K 4944-B microphones, pressure transducers on the tower/mobile launcher. It will include far field measurement devices, accelerometers, thermocouples and strain gauges on vehicle, thermocouples, flow meters and chamber pressure instrumentation.
The first test fire is expected to take place either in the summer of fall of this year.
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