Jan 14, 2022

Start Your Engines

Get a Sneak Peak Into XB-1 Engine Runs

Plane spotters in the Denver area started noticing something different on the taxiway at Centennial Airport last year, and it wasn’t typical airport activity. It was Boom’s ground crew conducting engine runs on its supersonic demonstrator, XB-1.

Engine runs are a mission-critical phase of all ground testing programs, which ensure new aircraft are ready for flight testing. XB-1’s program includes about 10 engine runs worth of data collection.

Among other things, XB-1’s engine runs are establishing the installed performance of each of its three General Electric J85 engines. These are small single-shaft turbojet engines previously installed in Canadian F-5 aircraft (the Canadair licensed-built version of the American Northrop F-5 Freedom Fighter).

A look inside one of XB-1’s J85 engines. The panels were removed during initial engine runs to make observation easier.

While XB-1’s J85 engines are a tried and true design, they have never been installed and operated in this particular experimental aircraft with this specific inlet. Hence, the need for extensive testing.

The team is using engine runs to determine that everything functions as designed, from the stall susceptibility (due to the unique installed configuration), to the performance of the engine’s accessory gearbox that powers hydraulic pumps, generators and more. This, in turn, is enabling the checkout of all the other systems on the aircraft, such as the flight controls and avionics.

Here’s a behind-the-scenes look at XB-1 engine runs.

On an early morning, Crew Chief Tom Grindle tows the aircraft on the taxiway at Centennial Airport (IATA: APA; ICAO: KAPA) near Denver, Colorado. It’s next door to the Boom hangar. Airfield Operations escorts XB-1 towards its destination.

In order to access the taxiway directly from the Boom hangar, the airport granted Boom permission to build a gate between the two. In a matter of minutes, XB-1 is on the taxiway.

The ground crew tows XB-1 to a large apron (ramp) that has reinforced anchors in the ground for tethering the aircraft. The tethers prevent XB-1 from moving during all of the events, which will eventually include all three afterburners (which is the highest thrust the aircraft will generate). Other aircraft can also use the apron, so the airport puts out a Notice to Airmen (NOTAM) that the team is performing testing.

Grindle checks to ensure the pilot is correctly strapped in before XB-1’s first engine run. During testing, he’s usually on the ground observing the operation and speaking with the pilot via a headset.

Engine runs are conducted in a build up fashion: first one engine at idle; then all three at idle; then power settings all the way up to full afterburner; then various rates of throttle movement. The process eventually builds up to a rapid throttle movement from idle to afterburner that mimics what a pilot might do in a late go-around maneuver.

Boom CEO Blake Scholl watches the second engine run. The ground crew typically includes 15 to 20 members on the taxiway.

Back at Boom’s hangar, the flight test team monitors the tests from the control room. They watch video captured by a camera on XB-1’s telemetry antenna while monitoring the weather and the test pilot’s multifunction display. The team also has displays showing different parameters of the aircraft that are also transmitted via the telemetry antenna.

XB-1 has a beautiful view of Denver’s Front Range as it’s towed to the testing apron.

The vehicle following XB-1 carries yellow hoses, which the team uses to run air into the engines to start them. They also use similar hoses to run cooling air into the aircraft bays to ensure electronics maintain the correct temperature.

The ground crew runs air into the electrical system and conducts additional checks. XB-1 engine runs begin with the aircraft anchored to the ground, and are followed by a number of taxi tests that examine aircraft performance and handling qualities.

Taxi tests at Centennial Airport will go from barely moving to 60 knots, which is about 70 mph (112 km/h).

To the right, a power generator provides 28V of electric power. The team uses it when the aircraft’s generator or batteries aren’t providing power.

Not all ground tests happen on the airfield. The first time the team “swings” the landing gear, they will conduct the test on a special stand in the hangar. The stand allows the aircraft to be held off the ground while the landing gear is moving.

Boom Chief Test Pilot Bill “Doc” Shoemaker concludes XB-1’s an engine run. At Boom, test pilots serve a number of different functions. They’re involved with the design of the aircraft, and along with performing the actions required to operate the aircraft, they observe real-time behavior to assess if it’s meeting the designed intent. Test pilots also evaluate the aircraft in other ways, ranging from conducting human factors in the cockpit, to the actual handling qualities of the aircraft, to the performance of all the systems they’re able to observe.

For Doc, the road to flying XB-1 has encompassed military service, a Stanford doctorate in aeronautics and astronautics, and being the test pilot for several prototype aircraft. A former U.S. naval aviator, he’s a graduate of the United States Naval Test Pilot School and has flown more than 5,000 flight hours in 50 aircraft types and has 900 carrier arrested landings (landing on an aircraft carrier deck).

Flight Testing Continues in Colorado and California

Ultimately, XB-1 ground testing will demonstrate the functionality of all aircraft systems, and determine that each system meets or exceeds specifications. The next step is flight testing in California’s Mojave Desert.

Once the aircraft arrives in Mojave (via flatbed), the team will work their way up to higher taxi speeds on the long, wide runway of the Mojave Air & Space Port. During these tests, XB-1 will likely reach 135 knots, which is around 155 mph (249 km/h). XB-1 is expected to leave the ground on the first flight at around 185 knots, around 212 mph (341km/h).

Plane spotters: Let us know when you spot XB-1 on the runway.

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