Sandia National Laboratories' Mike Beabout and Patrick Barnes, left to right top, and Mark Stroman and Jamison Lee, left to right bottom, prepare a nitrogen-powered gas gun for the labs' Alternative Pyroshock Test by installing a resonant cone to a resonant beam. Sandia successfully demonstrated a more environmentally-friendly way to ensure that avionics can withstand the shock from stage separation during flight. Source: Sandia National Laboratories/Randy MontoyaSandia National Laboratories' Mike Beabout and Patrick Barnes, left to right top, and Mark Stroman and Jamison Lee, left to right bottom, prepare a nitrogen-powered gas gun for the labs' Alternative Pyroshock Test by installing a resonant cone to a resonant beam. Sandia successfully demonstrated a more environmentally-friendly way to ensure that avionics can withstand the shock from stage separation during flight. Source: Sandia National Laboratories/Randy Montoya

Sandia National Laboratories has successfully demonstrated a new and more environmentally-friendly method to test a rocket part to make sure its avionics can withstand the shock from stage separation during flight.

The new method is called the Alternative Pyroshock Test. This test uses a nitrogen-powered gas gun to shoot a 100-pound steel projectile into a steel resonant beam. This transfers energy through as resonant cone attached to the part that is being tested. The energy transfer that results mimics the conditions of stage separation in space. The first test using this method was completed in the spring.

Until now, pyro-shock tests had used explosives encased in lead to provide the impacts to parts needed for these experiments.

The lead and explosives were environmental hazards, and clean up was costly and time-consuming. The Sandia Labs team set out to find a better approach.

"We recognized early in the program that we need to seek out alternative test methods in order to reduce our hazardous work exposure, minimize environmental waste and develop a controlled and repeatable test capability," Pilcher said. "Investigating a large-scale nonexplosive gas gun test became a reality when we partnered with Sandia's large-scale mechanical test facilities. The combined team worked hard to get to this test."

Sandia mechanical engineer Bo Song turned to a 1-inch-diameter Hopkinson bar to research if there were alternative means of testing. The Hopkinson bar was first suggested in 1914 by Bertram Hopkinson as a way to measure pressure produced by explosives. It was modified further in 1949 for dynamic stress-strain measurements of materials.

Song and his team conducted small-scale testing in Sandia’s Experimental Impact Mechanics Laboratory. This testing uses a metal rod about 20 times smaller than the rods used in the full-scale test. The team discovered that the Hopkinson bar technology could provide frequency levels and mechanical energy that is needed in the large-scale test to recreate the flight conditions.

The team conducted more than 50 tests. They studied what types of projectiles to use, how fast the gas gun needed to shoot, how to design a Hopkinson bar-type apparatus called a resonant bar at a larger scale, how to design a steel resonant cone to transfer the energy to the object being tested and how to manipulate the pulse of energy using small copper “coins” called programmers or pulse shapers. The copper “coins” were placed on the surface of the resonant bar.

"The most difficult part was designing the programmers, or pulse shapers because we had to select the right material, geometry and dimensions," Song said. "We got a lot of experience through this kind of testing for the future large-scale testing. The same concept can be used for a variety of defense and space applications. This provides a new path for pyro-shock testing, but very clean and more controllable and will save a lot of costs."

The next phase of the Alternative Pyroshock Test is to apply the Hopkinson bar technology to a pneumatically driven gas gun.

For the test, the gas gun was not required to reach the maximum capacities. The 60-foot-long gas gun used compressed nitrogen gas to shoot metal projectiles into a resonant beam coupled with a resonant cone to expand the final diameter to interface with the rocket part. This is essentially a hybrid version of a large-scale Hopkinson bar.

"What's novel is the application of the Hopkinson bar," said mechanical engineer Patrick Barnes. "Typically the bar and test objects are really small, but in our case, we used a 1,500-pound, 8-foot-long, 8-inch diameter bar."

The resonant bar and the resonant cone needed to vibrate at certain frequencies to apply the right amount of energy to the text object.

Barnes’ team used an empty mock test object that was outfitted with accelerometers to measure the impact. Barnes changed the geometry and composition of the programmers to simulate the test conditions that were required for the program.

Now that Sandia has put in analysis and testing, future tests should require less development and also cost less.