THE AIRFRAME THAT proved the concept for a viable racing rocket, no one will be surprised to learn, is a design by X-Prize winner and aeronautical magician Burt Rutan. XCOR took Rutan’s famous canard kitplane, the Long-EZ, and replaced the pusher propeller with two 400-pound-thrust rocket engines. The EZ-Rocket, as it was called, flew in 2002 at the Experimental Aircraft Association’s Oshkosh, Wisconsin airshow and again in October 2005, at the X-Prize Cup, an annual demo and trade show in Las Cruces, New Mexico, held to encourage private space ventures. The prototype racer is based on the Velocity SE FG kitplane, manufactured in Sebastian, Florida, a four-seat, composite derivation of the Long-EZ, which offers a cabin large enough to house the 39-inch-diameter liquid-oxygen tank in the aft section.
Before XCOR began modifying the Velocity, Searfoss put 25 hours of flight tests on the airplane—mostly sawtooth climb-and-descent patterns. He had written a computer program to analyze the rocket engine thrust and aircraft drag. From the data he collected, he could project what would happen when the piston engine is replaced with a rocket.
“You are going to have amazing climb rates and angles,” says Searfoss.
The calculations are not straightforward, but the league’s director of technology, Michael D’Angelo, says the 1,500 pounds of thrust from XCOR’s XR-4K14 rocket engine is—at around 220 mph—roughly equivalent to 1,000 horsepower. This is five times the power the SE FG airframe was designed to handle.
XCOR spokesman Doug Graham explains that the company is strengthening the airframe, which has a 200-knot indicated-airspeed limit, to handle the stresses of racing and also to handle the greater weight of the aircraft with a rocket engine—3,000 pounds instead of the 2,400-pound prop-driven Velocity. Because the rocket could easily push the aircraft past its rated speed, XCOR has also incorporated a governor that stops the engine from firing if a certain speed is exceeded.
Flying the EZ-Rocket gave Searfoss a good idea of what to expect in the operational vehicles. “The first thing you notice is how much smoother it is,” he says. “You turn that engine on and it’s just a great feeling to have that kick in the pants and not feel that shaking and vibration. It’s a whole heck of a lot easier than flying a Cub. You take the recip engine and the big swinging prop out of the equation and all those gyroscopic effects are gone.”
The rockets are simple: off or on. Power off, the Thunderhawk is a docile glider that offers a lot of leeway to a pilot who masters speed control. Power on, it’s a different animal.
At full fuel weight for takeoff the racer has a 0.6 thrust-to-weight ratio, which is comparable to that of an F/A-18 going to full power without afterburner. The feel of an afterburner comes later in the race, when the fuel load is lighter and the thrust-to-weight rises. The rate at which things will happen during takeoff is also comparable to what pilots face in an F/A-18.
XCOR DIRECTOR OF BUSINESS development Rich Pournelle is too young to have seen the Apollo missions on TV, but he is part of a movement—New Space—to replace big, government-funded space programs like Apollo with nimble, energetic space businesses that respond to a market. New Space entrepreneurs look with a mixture of disappointment and disgust at the broken promise of the complex, expensive space shuttle to provide routine access to space. Their goal is to make space travel airline-like, and XCOR’s Pournelle goes them one better: The ultimate goal of XCOR engine technology, he says, “is to power the Southwest Airlines of space.” Like Richard Branson’s Virgin Galactic, which has licensed Burt Rutan’s SpaceShipOne technology to produce rocketships for carrying tourists into space, XCOR is developing a reusable rocket vehicle, Xerus, that will travel to an altitude of 60 miles and back. The Rocket Racing League contract, says Pournelle, is “a perfect fit in our critical path.”
To solve the problems the Rocket Racing League requirements present, XCOR has turned the process of designing rocket engines on its head. Traditionally, rocket engine designers aimed for maximum performance whatever the cost. XCOR designs for safety and economy: The engines must be reliable and easy to maintain. In a race, crews must be able to load them with fuel quickly. And, with 200 seconds’ worth of fuel in a 14-minute race, the engines must be capable of reliable restarts—lots of them. “The requirements of racing can drive the technology in a sort of way that will have spinoffs for the suborbitals,” says Searfoss. Flying tourists on airline-like schedules will also require safe, reliable restarts, ease of maintenance and production, and fast fueling.
In the X-Racer engine, ignition begins with a a tiny spark plug, designed to ignite the fuel-air charge in model airplane engines. XCOR engineers repurposed the hobby shop item to ignite an equally tiny burner inside the XR-4K14. Once sensors confirm that this igniter rocket, essentially a blowtorch, is up and running, the engine control computer opens a sequence of valves to force a mist of liquid oxygen and kerosene into the combustion chamber through a proprietary pattern of spray nozzles. When the mist hits the igniter blowtorch, it ignites immediately and reliably, going from 0 to 1,500 pounds of thrust in less than a half a second. Because the blowtorch ensures that the LOX/kerosene mist cannot accumulate without ignition, the XR-4K14 cannot suffer what’s known as a hard start, an explosion in the combustion chamber instead of a controlled ignition. “Our engines don’t come apart without a wrench,” says Rich Pournelle. Even so, the engine installation includes a blast shield to keep shrapnel inside the cowling, just in case.