Sandia National Laboratories in Albuquerque, New Mexico has been up to big things lately. Just a few days ago, the facility reported replicating the energy found on the edges of black holes, upending a theory about how we measure the big gravity sinks and possibly leading to a lot of recalculations by astrophysicists. Now, it's just delivered proof of concept for a method of testing rocket parts using a 60-foot-long (18 m) compressed nitrogen gun that makes the process greener and safer.
Traditionally, to recreate the incredible stress rocket parts are put under during launch and various stages of flight and stage separations, rocket makers have relied on something called a pyroshock test in which explosives encased in lead are detonated. However, the costs and clean-up time after such tests are costly, as researchers have to deal with the hazardous byproducts of their work.
"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," said mechanical engineer Mark Pilcher. "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."
The new method just carried out at Sandia – called alternative pyroshock testing – makes use of the giant nitrogen gun and a setup known as a Hopkinson bar.
First put forward by British electrical engineer Bertram Hopkinson in 1914, the setup actually consists of two bars with a testable material placed between them. A shock is sent through the first bar by some kind of striker (a projectile), which creates a wave that travels through the material and into the second bar. Measurements are taken from both the incident bar and the transmission bar to derive information about the material's performance. Hopkinson bars are often fairly small, so when then Sandia researchers decided to give it a try in rocket part testing, they needed to scale things up quite a bit after experiments with a bar measuring one inch in diameter.
"What's novel is the application of the Hopkinson bar," said Sandia 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." (That's 680 kg, 2.4 m, 20 cm.)
Sandia mechanical engineer Bo Song amended the standard Hopkinson bar into a metal tube known as a resonant bar. He then engineered the system to transfer the shockwave created in the first part of the bar by a 100-lb (45-kg) steel projectile to a steel resonant cone that would then pass the force onto the object being tested. He also used items called pulse shapers, which are small coin-sized objects made from copper, on the outside of the bar to sculpt the wave produced into just the right form. Song and his team conducted over 50 tests before getting things right.
"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 pyroshock testing, but very clean and more controllable and will save a lot of costs."
The team is hoping to create a facility where the alternative pyroshock testing apparatus can be installed permanently to allow easy, efficient and cost-effective rocket part testing in the future.
You can see the test below, which replicated the force applied on a rocket part from a stage-separation pyroshock event.
Source: Sandia National Laboratories