An Atlas 5 rocket slated to lift off in October will mark the first flight of a secondary payload ring the U.S. Air Force developed to carry up to six small satellites on any Evolved Expendable Launch Vehicle (EELV) that has room to spare.

The most high-profile payload on the October launch is Orbital Express, a two-spacecraft satellite-refueling experiment sponsored by the U.S. Defense Advanced Research Projects Agency. But in many ways, the five small, experimental satellites sharing the ride on the so-called EELV Secondary Payload Adapter (ESPA) ring is actually the primary payload.

While the rocket itself and lineup of satellites going along for the ride have changed in recent years, the one constant feature of the mission the Air Force is calling the Space Test Program 1 is the inclusion of the ESPA ring.

Up until 2003, the mission was assigned to Boeing’s Delta 4 rocket with the spot atop the ESPA ring reserved for a joint weather satellite known variously as Earth Observer-3 and GIFTS-IOMI, which is short for Geosynchronous Imaging Fourier Transform Spectrometer-Indian Ocean Meteorological and Oceanographic Imager. That satellite lost its spot amid budget uncertainties that eventually led to the project’s cancellation. The Space Test Program 1 launch, meanwhile, was one of several missions the Air Force stripped from Boeing for contracting improprieties and gave to Lockheed Martin.

The five payloads slated to fly on the ESPA ring in October are:

FalconSat-3: A 50-kilogram Air Force Academy satellite carrying out three Defense Department experiments.

MidStar1: A U.S. Naval Academy-built satellite carrying out two Defense Department experiments.

Naval Postgraduate School Satellite (NPSAT-1): an 80-kilogram satellite flying two military experiments.

Cibola Flight Experiment Satellite (CFESat): A Los Alamos National Laboratory-built satellite flying a Defense Department experiment.

Space Test Program Satellite-1 (STPSat-1):A 170-kilogram satellite built by Ashburn, Va.-based Aero Astro that hosts several military experiments and will deploy two 1-kilogram picosatellites at the end of its mission.

U.S. Air Force Lt. Col. Daniel Griffith, director of the Defense Department’s Space Test Program, said the long-awaited first flight of the ESPA ring is not only a positive development for small satellite builders hard up for affordable rides, but also is attracting interest from senior Air Force officials interested in making full use of upcoming EELV launches with excess capacity.

“There is a lot of interest in the ESPA ring across the community, and I will tell you the interest is growing,” Griffith said in a recent interview.

Griffith, whose job is to find rides for military space experiments, said the Air Force is taking a hard look at putting the ESPA ring on as many Altas 5 and Delta 4 launches as possible.

Griffith said the several Defense Meteorological Satellite Program spacecraft (DMSP) the Air Force plans to launch by the end of the decade are good candidates for including an ESPA ring.

“When you look at a DMSP satellite, when they fly on an EELV launch, they don’t come anywhere close to taking up the full capacity of the launch vehicle,” Griffith said. “Wouldn’t it be great to put an ESPA ring under the DMSP and take advantage of what the taxpayers have already paid for that launch vehicle?”

The idea also is finding support in industry. George Sowers, director of business development and the advanced space transportation program at Lockheed Martin Space Systems, said the company is pushing the Defense Department to include ESPA rings on missions with excess performance capability as a matter of policy.

In addition to the current generation of military weather satellites yet to launch, the Air Force plans to launch six to eight Global Positioning System satellites by around the end of the decade. Those missions, according to Sowers, are good candidates for taking an ESPA ring along.

Sowers agreed with Griffith that ESPA rides are a good fit for the smaller budgets that tend to go along with small satellites. He said ESPA slots could be made available to the research community for $1 million to $2 million each.

While Griffith, Sowers and others push the Defense Department to embrace the ESPA ring and include it on more missions, the next most likely military EELV launch to include the adapter is Space Test Program 2, which is tentatively slated for 2010.

Griffith said there are multiple candidates for the Space Test Program’s next dedicated EELV launch. The program is leaning toward flying an Air Force Research Laboratory project called the Deployable Structures Experiment, which aims to demonstrate the ESPA ring itself can be flown as a satellite with proper outfitting.

NASA’s Exploration Systems Mission Directorate also is looking at including an ESPA ring when it launches the Lunar Reconnaissance Orbiter in October 2008. The U.S. space agency expects to pick up to one piggyback mission in April from a list of four finalists. While NASA could have room to spare on the ESPA ring depending on which mission is selected, NASA exploration officials say they can only afford one of the $50 million to $80 million secondary payloads.

The ESPA ring was developed by CSA Engineering Inc., Mountain View, Calif., under a Small Innovative Business Research grant from the Air Force Research Laboratory.

The Air Force spent about $3.5 million on the ESPA development between late 1999 and December 2002, according to Peter Wegner, the responsive space technical area lead at the Air Force Research Laboratory’s Space Vehicles Directorate, Kirtland Air Force Base, N.M.

Joe Maly, the ESPA program manager at CSA Engineering, said the ring itself costs about $130,000 to manufacture, but flight qualification and requirements verification reports, quality assurance and other engineering oversight and paperwork requirements likely bring the estimated price tag for delivery of an ESPA to $250,000.