The U.S. Air Force has chosen four aerospace companies out of a field of more than 10 hopefuls to refine studies that could lead to the development of a partially reusable rocket, according to industry officials.

The Air Force still is negotiating the final terms of those agreements, and has yet to announce the winners, but one firm — Northrop Grumman Corp. of Los Angeles — issued an April 4 news release to announce that it was selected for a 14-month contract worth $1.5 million.

The other companies that were selected in late March to continue studies on the Affordable Responsive Spacelift effort are Lockheed Martin Corp., Orbital Sciences Corp. , and Andrews Space, according to industry officials.

The system, which is also referred to as the Hybrid Launch Vehicle (HLV), is envisioned as being capable of conducting missions that would take a total of about 48 hours or less between call up and completion, according to Dennis Poulos, HLV program manager for Northrop Grumman.

The Pentagon first mentioned the HLV concept to reporters when it unveiled its 2007 budget request in February.

The HLV is supposed to feature a reusable first stage that relies on rocket boosters to take off. After firing an expendable second stage into orbit, the first stage uses jet engines to return to the launch range, Poulos said during an April 5 interview here at the Space Foundation’s 22nd National Space Symposium.

The expendable second stage could come from one of the small rockets developed under the Pentagon’s Falcon Small Launch Vehicle effort, Poulos said. The Falcon rockets are envisioned as carrying payloads weighing up to 1,000 kilograms, but the HLV first stage could help boost that to as much as 6,800 kilograms when the two are combined, he said.

While a site has not yet been chosen, the Air Force would likely launch the HLV from Vandenberg Air Force Base, Calif., or Cape Canaveral, Fla., he said.

The 14-month study contracts could be followed by six-month contracts that would be worth about another $1.5 million, Poulos said. Those contracts would be intended to help contractors refine their own designs, as well as address issues associated with launch and return that could be used by whoever ultimately builds the HLV, he said.

The Air Force will likely issue a request for proposals for a follow-on design contract in August that will last 12 months, Poulos said. That contract will likely feature an open competition with two potential awards, with a single contractor chosen after that work is complete, he said.

A small-scale demonstration flight could take place around 2010, with a full-scale vehicle ready around 2018 or 2020, Poulos said.

Al Simmons, business development leader for the HLV effort at Lockheed Martin Space Systems in Denver, said while the company’s work in developing its concept for the Falcon Small Launch Vehicle program did not receive continued funding from the Pentagon last year to pursue a flight test, it could apply directly to the expendable second stage for the HLV.

Lockheed Martin’s work for NASA on concepts including liquid fly-back boosters for the space shuttle and the X-33 reusable launcher could apply as well, Simmons said during an interview at the National Space Symposium here.

Some of the most important work from the X-33 program that could apply to the HLV work includes technology and prototype hardware for thermal protection systems and vehicle health-management systems, Simmons said.

Officials in industry, Congress and the Pentagon expressed cautious optimism that the HLV effort will ultimately payoff in an operational vehicle — or even the flight demonstration in 2010. While the HLV could represent a useful and economical option for launching payloads up to 6,800 kilograms, budget constraints due to current operations in Iraq, the aftermath of Hurricane Katrina and existing space programs still in development could threaten the program on Capitol Hill or even within the Pentagon’s own budget offices, the officials said.