The U.S. Defense Advanced Research Projects Agency (Darpa) plans to seek industry interest next month in an Experimental Spaceplane (XS-1) which will be capable of delivering a payload up to 5,000 lb. to space for less than $5 million per launch.

The XS-1 is targeted at flying at Mach 10 plus and generating a sortie rate of up to 10 times over 10 days. The program compliments the agency’s ongoing Airborne Launch Assist Space Access (Alasa) program, which is developing an air launch system for small satellites, and appears to be a partial revival of the Air Force Research Laboratory’s abandoned Reusable Booster System (RBS) Pathfinder.

Like RBS, the XS-1 would be based on a hypersonic first stage which would deliver payloads to low Earth orbit via one or more expendable stages. The first stage would return autonomously to the launch site for reuse. Darpa, which is expected to issue a Broad Agency Announcement for XS-1 shortly, says “modular components, durable thermal protection systems and automatic launch, flight, and recovery systems should significantly reduce logistical needs, enabling rapid turnaround between flights.”

The initiative was unveiled at the recent Space 2013 conference in San Diego by Darpa Tactical Technology Office Deputy Director Pam Melroy. Although the agency has released artist’s impressions of winged XS-1 concepts, Melroy emphasizes the goal is reusability and the method of achieving that is up to interested parties. Darpa also believes that some reusable technology features of the Alasa contenders could also feature in the larger capacity XS-1. The agency has awarded Alasa concept study contracts to Boeing, Lockheed Martin and Virgin Galactic, as well as separate technology development contracts to other contenders.

Ironically, it was at the same conference last year that news emerged of the termination of the Pathfinder project. By that time one of the three contestants, Lockheed Martin, had begun hot fire tests of a rocket engine designed to power its RBS demonstrator. The sub-scale Pathfinder was being developed under the AFRL’s RBS Flight and Ground Experiments (RBS-FGE) program. The Pathfinder was expected to lead to a larger-scale demonstrator and, ultimately, a full-scale reusable successor to the current Evolved Expendable Launch Vehicle (EELV) family beyond 2025.

Consisting of a vertically-launched reusable first stage and expendable upper stage, the RBS was designed to cut launch costs by more than 50% compared to conventional Delta IV and Atlas V rockets. Other contenders included concepts from Andrews Space and Boeing. All three were exploring rocket-powered, winged designs that were to demonstrate a vertical launch followed by an autonomous, aircraft-like horizontal landing near the launch site.

Flight tests of the Pathfinder were originally expected to begin in 2015 and run through into 2016. No projected time frame for the proposed XS-1 has been announced.