Think about a time you've been a passenger in a car and stuck your hand out the window. As your speed increases, so do the vibrations in your hand. Trying to keep those fingers steady as the wind whips around them at 75 mph gets pretty tricky, right?

You've just had a quick lesson in unsteady aerodynamics, something engineers are researching and testing on a much larger scale and with supersonic speeds using wind tunnel technology. The wind tunnel tests, recently conducted at NASA's Ames Research Center in Moffett Field, Calif., will be used to enhance the design and stability of the Space Launch System (SLS), NASA's new heavy-lift launch vehicle.

The SLS capability is essential to America's future in human spaceflight and scientific exploration of deep space. Only with a heavy-lift launch vehicle can humans explore our solar system, investigate asteroids and one day set foot on Mars.

"The aeroacoustic tests we just completed at Ames are all about unsteady aerodynamics," said John Blevins. Blevins is the lead engineer for aerodynamics and acoustics in the Spacecraft and Vehicle Systems Department at NASA's Marshall Space Flight Center in Huntsville, Ala., where the SLS Program is managed for the agency. "Local vibrations can have a major impact on the rocket and critical hardware."

"You don't fly hardware, especially with people on board, unless you can verify the environments they fly in," he added. "There are standard practices we've learned from past successful programs. Wind tunnel testing is a cost-effective way to set the requirements needed for all the rocket's components to sustain the flight."

Four models of three different crew and cargo variations of the SLS, including the 70-metric-ton (77 ton) configuration, were tested in a series of wind tunnels at Ames. The 70-metric-ton configuration will be used for the maiden flight of SLS.

Crews of engineers worked around the clock to accomplish the test objectives. "Since vibrations are very localized, they may affect how hardware on the rocket will work," said Andy Herron, lead data analyst for the aeroacoustics tests at the Marshall Center.

"Our job is to figure out what these vibrations are, so when another team is designing something -- for example, an avionics box -- we can determine if that hardware needs to be moved or isolated on the vehicle. Or, it may be that the design needs to be tweaked a bit - all with the goal that the parts will work the way they are intended."

For the tests, the models were affixed with pressure transducers, or sensors, that measure pressures on the model at specific locations. They were first put in the 11-by-11-foot transonic wind tunnel, with wind speeds ranging from Mach .7 to Mach 1.4. A Mach figure represents the ratio of the speed of an object to the speed of sound in the surrounding medium, like air.

Also included in this test series were critical buffet tests, which determine how air affects the vehicle at low frequencies.

The models were then put in unitary 9-by-7 wind tunnel, with winds ranging from Mach 1.55 to Mach 2.5. This test was high-supersonic flow and more focused on local vibrations. Shock waves attach throughout the vehicle at different protuberances, like the feed line or the boosters.

"This is the fastest acoustic test we've ever done, in terms of Mach speed," said Marshall's Darren Reed, lead engineer for the acoustics test. "We tested a wide range of configurations, Mach numbers, angles and more than 4,000 data conditions -- each one with hundreds of transducer measurements."

The next step will be to analyze the test data and share it across the SLS Program for use on the design and development of different components -- including the core stage -- on the actual vehicle. The core stage, towering more than 200 feet tall with a diameter of 27.6 feet, will store cryogenic liquid hydrogen and liquid oxygen that will feed the vehicle's RS-25 engines.