Stretched between growing operational space demands and shrinking budgets, the U.S. Air Force is funding the first major research phases of a reusable booster system (RBS) intended to replace its costly expendable launchers.

Although the value of the initial contracts to Andrews Space, Boeing and Lockheed Martin is only $2 million each, the agreements are potentially worth up to $250 million over the next five years. More importantly, the awards come on the eve of unprecedented budget cutbacks and appear to underline the importance the Air Force attaches to a concept that promises to slash launch costs by more than 50% compared to the conventional Evolved Expendable Launch Vehicle (EELV).

The RBS consists of a vertically launched reusable first stage and expendable upper stages. After deploying the upper stack containing the payload, the rocket-powered, winged first stage will return to make an autonomous, aircraft-like horizontal landing near the launch site. Although this and similar jet-powered concepts have been proposed over the years, the Air Force Research Laboratory's RBS Flight and Ground Experiments (RBS-FGE) program is the first to support a funded demonstrator, as well as the first to form part of a sanctioned Air Force Space Command spacelift plan.

However, the RBS faces major technology hurdles on the path to planned deployment beyond 2025, when it could begin to replace the current Atlas V and Delta IV vehicles. At the top of the list of challenges is a preferred “rocket-back” maneuver, which was selected by the AFRL over first-stage designs that glide back to land or return using high-speed turbine engines. The focus for the RBS-FGE design is a liquid oxygen/kerosene rocket-powered vehicle that will be able to achieve staging at a higher Mach number than the other options.

Following release of the second stage, which could also evolve to a fully reusable vehicle, the first stage will turn around 180 deg. so that its rocket engine is firing in the direction of the velocity vector. This rocket-back maneuver, involving extremely high angles of attack and sideslip, will be tested by a sub-scale RBS Pathfinder vehicle built by one of the three initial RBS-FGE contractors.

The AFRL will select the winning Pathfinder design in the second half of 2012, with flight tests expected to begin in 2015 and run into 2016. Individual contractor designs remain under wraps, but all are similar to the 15-ft.-long reference target Pathfinder that AFRL revealed in 2010, and all are expected to be ground- or air-launched on up to three flights to test different rocket-back maneuvers.

A follow-on reusable booster demonstrator (RBD) is expected to follow the Pathfinder into the air within five or six years. The RBD, likely to be allocated as an X-plane demonstrator, will be highly representative of the operational unmanned, reusable booster. Overall configuration is expected to be similar to that outlined by the Air Force in 2010, which indicated an overall length of 60 ft., span of 34 ft., 9-ft.-dia. body and gross liftoff weight around 230,000 lb. Early concept details also showed the demonstrator could be powered by a liquid-oxygen/kerosene RD-180 EELV engine. Another AFRL demonstration program, called Hydrocarbon Boost, is developing a large liquid-oxygen/kerosene rocket engine for the full-size booster.

“The RBD will be a good X-vehicle that could demonstrate all the parts of the puzzle, including the ability to fly and operate the RBS flight profile as well as ground operations,” says Slater Voorhees, Lockheed Martin RBS program manager. “Some of that will be demonstrated by the Pathfinder, but the RBD will be much closer to the operational vehicle.” Lockheed Martin's design will incorporate “a good mix” of design know-how from the company's fixed-wing and space vehicles ranging from the SR-71 to the X-33. Voorhees says the design will combine expertise in “materials, systems and solutions—not only for flying maneuvers, but also for affordability, operability and responsiveness.”

Initial steps under the RBS-FGE program will include flight and ground experiments as well as demonstrations to address “aeromechanics, configuration, and flight performance; structures and materials; flight controls and health management; flight systems and propulsion; and ground systems and operations,” the Air Force says. The technology road map is expected to support the eventual development of two versions of the RBS: a single, reusable first stage and expendable cryogenic upper stage for medium-lift missions; and two reusable boosters, cryogenic core stage and upper stage for heavy-lift and growth missions.

The latest contracts follow parallel research studies conducted into the RBS concept by Andrews, Boeing and Lockheed Martin for the Air Force Space and Missile Systems Center (SMC), from November 2010 to August 2011. These aimed to achieve a better understanding of the RBS trade space and, in particular, the flight dynamics, flight control and technological impacts associated with several unproven RBS flight maneuvers. The studies focused not only on the rocket-back and return-to-launch sites, but also the booster-phase abort and upper-stage separation.

Andrews Space, based in Tukwila, Wash., has been involved in several reusable booster studies including, most recently, the SMC RBS risk-reduction study earlier this year. It is a long-time proponent of advanced space systems, having worked on an air collection and enrichment system for a two-stage-to-orbit reusable launch system as well as ballute-based reentry systems and advanced materials. The company is also involved in the development of a variety of technologies ranging from unmanned ground vehicles to recoverable space cargo logistics modules and nanospacecraft.

Boeing, which has worked closely on several RBS concepts with Maryland-based research and development company Astrox, is expected to leverage its experience gained with the X-37 Orbital Test Vehicle.

Lockheed Martin's RBS team is led by the company's Space Systems unit based in Denver, Colo., and includes the Skunk Works operations in Palmdale, Calif., and Fort Worth. Also part of the Lockheed team are: Science and Technology Applications of Moorpark, Calif.; UP Aerospace of Highlands Ranch, Colo.; and JFA Avionics Systems of Newbury Park, Calif. For the RBS Pathfinder, Lockheed says it has entered into an agreement with the New Mexico Spaceport Authority to conduct flight-test operations from Spaceport America, in Las Cruces.