How Do You Test Hypersonic Systems?
One Key Test is Time
By Albert McKeon
The engineers and technicians of Northrop Grumman's Aerothermal Research and Testing facility recognize the irony of their jobs.
They work slowly, following exacting methodologies and taking painstaking care to properly test hypersonic and supersonic systems and products in environments that mimic high altitudes and pressures. (Supersonic speed is a rate of travel that's faster than the speed of sound, starting at Mach 1. Hypersonic speed is travel at five times or greater than the speed of sound, starting at Mach 5.) If their extremely measured approach doesn't plan for and predict success in early testing stages, then the technology ultimately won't work at real-life high Mach speeds.
Assessing the performance and durability of engines, missiles, vehicles and other items withstanding stresses of hypersonic speeds indeed takes time. With cutting-edge technology at their disposal — including a new test bed that will simulate conditions of up to Mach 8 for as long as two minutes, a ground test facility that doesn’t exist anywhere else— the Aerothermal Research and Testing employees have seen that precision makes perfection.
“We're dealing with different customers and different materials,” said Dean Feola, manager of the technician group at the facility. “These aren't simple, cookie-cutter things.”
Replicating Extreme Conditions at High Mach-Speed
The Aerothermal Research and Testing facility is one of the few programs that tests the capabilities of electronics and equipment at conditions simulating speeds from Mach 0 to Mach 8.
With significant money and time invested in the early development stages, many companies and government agencies focus attention on testing here on earth, wanting to keep costs and complications of testing in the upper reaches of the Earth’s atmosphere to a minimum. So they turn to Northrop Grumman to recreate extreme air pressure and temperatures in the controlled environment of one of five test cells. The process includes testing while connected to a 60,000-cubic-foot vacuum sphere and ejector vacuum system.
Data-Driven Decisions in Hypersonics
Tests can run from 20 seconds to 20 minutes, but organizations seek more than reproducing atmospheric conditions; they also crave data. “They want to learn something about their vehicle, fuel system or material,” said Dan Cresci, chief engineer of the facility's Test Services group and a Northrop Grumman employee of 34 years. “We collect more data today than 30 years ago. You're now able to make better conclusions. The instruments are not only more plentiful but more accurate.”
The time the Northrop Grumman team spends preparing for a test might number in the weeks – all in the name of achieving just 20 seconds of testing. Cresci, for instance, will first learn what, and how, a client wants to test at high Mach speeds, and will suggest modifications if any specifications are outside achievability. Initial discussions are just the start of several methodical processes.
“Very rarely are any test articles plug and play,” Cresci said. “Sometimes you'll have a dozen instruments that have to connect; other times you have hundreds….You have to make each of those connections exactly right.”
Clean Air Mach 8 Tests Running Longer Than Anywhere Else
Rarely does the methodical nature of the job feel monotonous, Feola and Cresci said. Even though the high expectations of testing protocols never change, each client has different goals and that makes the job exciting. “We're learning something new every day. Part of the fun is teaching the new team members. They're young and eager,” Feola said.
The new Hypersonic Aero Thermal & Propulsion Clean Air Testbed (HAPCAT) will add to the excitement of work.
Testing facilities simulate high Mach speeds in different ways, Cresci said. Testing tunnels that rely on high-powered fans can reach only Mach 1, he said, whereas other tunnels that replicate Mach 5 to Mach 8 speeds use combustion, or arc plasma, to recreate the air of Earth's atmosphere, these methods inevitably introduce a touch of CO2 or NOx, which also introduces subtle uncertainties in the engine testing results.
This shortcoming is overcome in HAPCAT by heating ceramic bricks, that in turn generate high-temperature clean air, Cresci said. But making that kind of “heat exchanger” for long stretches of time hasn't been accomplished anywhere — until now. When it goes online in late 2020, HAPCAT will create unvitiated, or pure, “clean” air from Mach 5 to Mach 8 conditions for up to two minutes; an exact environment for hypersonic scramjet engine performance testing.
“To do that for several minutes is leading-edge technology,” he said. “It will be very unique in the world. We’re very excited.”