U.S. Air Force Lays Out Air-Breathing-Engine-Powered Launcher Studies
As part of its pursuit of breakthrough propulsion systems for high-speed flight and potential access to space, the U.S. Air Force Research Laboratory (AFRL) has spent more than two years studying the novel air-breathing rocket engine system concept invented by UK-based Reaction Engines Ltd. AFRL’s studies of the Synergetic Air-Breathing Rocket Engine (SABRE) cycle, which uses atmospheric oxygen and liquid hydrogen from a standing start to above Mach 5 when it switches to onboard liquid oxygen, have shown the concept is thermodynamically feasible. The engine and innovative precooler at the heart of the cycle are ultimately aimed by Reaction at a single-stage-to-orbit (SSTO) launch vehicle dubbed Skylon.
However, while SSTO remains a long-range goal, Reaction has recrafted its ground demonstrator to reflect smaller-scale potential applications in the nearer term. AFRL, which is meanwhile working in parallel with Reaction under a cooperative research and development agreement, is for the first time outlining details of how SABRE might be used to support orbital launch missions when configured as part of a two-stage-to-orbit (TSTO) launch system. Working with Atlanta-based SpaceWorks Enterprises, AFRL has defined two initial next-generation launch system concepts: A partially reusable TSTO with a SABRE-powered booster and expendable rocket-powered second stage, and a second fully reusable option incorporating a SABRE-powered first stage and a rocket-powered upper stage. “We asked, ‘What’s a nearer-term approach to do access to space with this very interesting engine idea and precooling technology?’ and came up with this two-stage-to-orbit system,” says Barry Hellman of AFRL’s High-Speed Systems Division.
The first system is designed to place a 5,000-lb. payload into a 100-nm orbit with a 28.5-deg. inclination from Cape Canaveral AFS.. Two missions were also analyzed to determine performance to a sun-synchronous orbit at 378 nm flown from Vandenberg AFB, California, and a proposed UK spaceport site in Newquay, England.
The partially reusable concept is based on a twin-SABRE-powered winged booster. Measuring 150 ft. long, or about 4 ft. longer than a B-1B bomber, the vehicle would carry the rocket-powered second stage in a lower payload bay. “The notional concept of operations is like an aircraft with a horizontal takeoff and landing,” said Hellman at the American Institute of Aeronautics and Astronautics (AIAA) Space 2016 conference and exhibition in Long Beach, California, Sept. 13-16.
“The vehicle would accelerate to almost Mach 4.5 before transitioning to rocket mode, when it would pull up while still accelerating,” he said. “We carry the upper stage inside so it does not have to have a payload fairing or thermal protection system (TPS). That also means it has to stage at a very high altitude of over 260,000 ft. and a low dynamic pressure of about 0.8 psf. It then boosts away to orbit while the vehicle turns away and returns.” Staging would occur at about Mach 8 before the vehicle begins a gliding turn toward the launch site and briefly restarts the engines in air-breathing mode for additional boostback.
“We assumed we would have conformal hydrogen tanks and that would be another technology challenge that would have to be solved to make this design work,” Hellman said. By placing the payload underneath, no large cranes are needed to load the vehicle, and “when we get to staging, we use gravity and the payload just falls away,” he added. Hellman conceded, however, that payload bay door design will require special consideration for the TPS to handle reentry.
The vehicle’s propellant mass fraction (mass of propellant divided by the weight of the whole stage apart from the payload) “is about 0.43, and that is unheard of for access to space systems,” said Hellman. “That is what an air-breathing engine allows you to have. The bottom line is, the vehicle can make the mission with extremely low propellant-mass fractions.”
The second, fully reusable option consists of a 190-ft.-long scaled-up variant of the smaller booster and a reusable upper-stage booster. Designed to carry a 20,000-lb. payload to orbit, the 115-ft.-span booster and upper stage would collectively weigh about 1.3 million lb. at takeoff. The system would launch horizontally, and both stages would return for a horizontal landing. After deploying its payload, the upper stage would continue around the world and, with the cross-range capability of its 40-ft.-span, X-37B-like wing, recover to the launch site.
For the immediate future, the Air Force continues to focus on further evaluation of the precooler. “That is the biggest interest we have right now at AFRL,” Hellman said. “We have been looking at trying to get the funding to test the heat exchanger at higher temperatures, simulating conditions behind the inlet at Mach 3.5-5. It’s a very fascinating technology that has a lot of senior Defense Department and NASA officials excited. We are moving along slowly, though hopefully we will make more progress in the next few years.”