A truly reusable, quick-response launch system has been an elusive dream of the U.S. Air Force since the dawn of the space age, but now the service is taking the first steps toward a real capability as plans for a reusable booster system (RBS) come together.

Dubbed Pathfinder, the demonstrator is a subscale vehicle aimed at proving the concept of a vertical-launch-and-horizontal-landing first stage. Researchers hope to show that by returning to an aircraft-like landing and being fully reusable, the system could cost up to 50% less to launch than the Air Force's current Evolved Expendable Launch Vehicle (EELV) family.

As all missions take place from coastal locations, the vehicle can only be recovered if it returns to land after launching the second-stage payload at a point somewhere above Mach 5.5 and 150,000 ft. altitude. For such a concept to be feasible, it must therefore be capable of executing a demanding about-turn or “rocket-back” maneuver, which has never been demonstrated before, and returning from downrange using its own rockets.

The Pathfinder vehicle—which is being developed under the Air Force Research Laboratory's (AFRL) RBS Flight and Ground Experiments (RBS-FGE) program—is designed to prove the concept. Pathfinder is expected to lead to a larger-scale demonstrator and, ultimately, a full-scale reusable successor to the current EELV family, which is being retired in 2030.

“The big driver is the rocket-back mission,” says Bruce Thieman, leader of AFRL's Affordable and Responsive Space Access Technology Transition effort. “Every launch you do has to go out over the water, so everybody has to be able to get back to land. So we have the problem of what to do at the staging point around Mach 5.5. How do we flip it back while still firing the engine and how to control the propellant inside the vehicle? So Pathfinder is going to help us understand the aero-thermal issues and guidance-and-control problems of doing that flip,” he says.

The project emerges from the Air Force's Strategic Master Plan for a range of satellite and counter-space launch capabilities. These include responsive spacelift; the ability to reposition, recover and service on-orbit assets; transport of objects from one terrestrial location to another using space; and the regeneration of satellite constellations. The RBS builds on the groundwork of the AFRL-led Future Responsive Access to Space Technologies (FAST) program, which focused on a series of ground experiments of airframe, structural health-monitoring and ground developments. FAST is now coming to an end, and the emphasis is shifting to the flight demonstrator.

AFRL expects to select a winning concept in the September-November period from the three designs submitted by Andrews Space, Boeing and Lockheed Martin. The first phase of the flight experiment in 2014 will be a vertical takeoff and a horizontal landing and probably going downrange at a little more than Mach 1 for an operational concept demonstration, says Thieman. “The next phase is to go downrange at higher speeds and go to higher altitudes. Flight-envelope expansion will take place in 2015.”

Testing will likely be conducted at either Edwards AFB, Calif., or White Sands Missile Range, N.M. “Each team will tell us what they plan to do, and it is part of their proposal,” he says. All are aimed at concepts similar to the 15-ft.-long reference Pathfinder revealed by AFRL in 2010, and are expected to test different profiles and gather data to map out a matrix for future flights.

More details of the three concepts are also emerging. The Andrews system resembles a simple, stub-winged version of a SpaceX-like Falcon booster, while the Boeing concept features a cranked delta wing with tip-mounted rudders. The Lockheed Martin concept is also delta-winged but is differentiated by the addition of canards. For propulsion, Lockheed Martin is developing a 20,000-lb.-thrust liquid oxygen/kerosene engine, while both Boeing and Andrews are believed to be studying the use of growth versions of XCOR engines.

Lockheed Martin is just beginning engine runs using a reactivated test site at its Space Systems Waterton facility near Denver. The rocket is already proving “very stable” during initial tests, says John Karas, vice president for human spaceflight and general manager. Although only running for 1-sec. bursts up until now, the plan is to extend run time until “we do full-duration firings by the end of the year.” The requirement for Pathfinder is an 80-sec. burn, he adds.

Lockheed's plan appears to be ahead of the schedule set by AFRL, which calls for engines to be up and running by February 2013. “We have to have an engine firing before we continue to go anywhere,” says Thieman. Individual proposals will also detail how they plan to boost and perform the rocket-back maneuver. “We've done it subsonically with the [McDonnell Douglas] DC-X at altitudes around 10,000 ft.—but that didn't involve the aerodynamic problems you are dealing with at these higher speeds and altitudes.”

Evaluation will also include “looking at how small the vehicle can be and still make it really traceable.” Teams must provide detailed analysis showing that their designs would represent a vehicle that could be sized up to the staging point of between Mach 5 and 6, and 150,000-200,000 ft. “So they're going to have to figure out a Pathfinder that's scalable,” says Thieman. The design must also be capable of cruising back at Mach 2-2.5 and be gliding back at sufficient speed and altitude when the fuel is exhausted.

The next stage beyond Pathfinder will be determined by the outcome of several studies now underway. The National Research Council is conducting an independent review that will be published in August. A commercial options study is also underway involving industry.

“The original plan was a large demonstrator in the $70-80 million range. The Rocketback Demonstrator [RBD] was going to do a Pathfinder function, and a decision was made to break it into two. So the decision is, should it be F-16-sized or bigger—around the size of the [F-15] Strike Eagle?” asks Thieman.

Payload or separation demonstrations will be performed by the larger variant and not by the Pathfinder. “The question is, can you save money by doing this? But there's no real hard data—this will be jointly done by AFRL and Space and Missile Systems Center [SMC] because the large demonstrator impacts what the prototype will look like. SMC has money in both, regardless of whether it's a Pathfinder followed by a small demonstrator or one that goes directly from a Pathfinder to a full-up vehicle,” says Thieman.

A large-liquid-engine group comprising the Air Force Space Command, propulsion researchers from AFRL and SMC's launch systems program office is also meeting to discuss engine options. The group will consider whether to focus on developing oxygen-rich, staged combustion engines like the Air Force's long-running Hydrocarbon Boost program, use an existing design or evolve a newer engine such as Pratt & Whitney Rocketdyne's J-2X. Whatever path is chosen would result in building a full-up “brassboard” engine capable of roughly 250,000 lb. thrust by 2020. The technology readiness level would be in the TRL 5 to 6 range at this stage, requiring several years of additional development to raise it to production-ready hardware readiness.