The ESPA ring was developed and flight qualified in the early 2000’s as the Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter, to utilize excess launch capacity by mounting additional payloads, up to 400 lb each, below a primary spacecraft up to 17,000 lb.   This reduces launch costs for the primary mission and enables rideshare missions with minimal impact to the original mission.  The original mount interface is a Ø15-inch circular “port” with 24 fasteners that has become a standard interface for small satellites.  The concept was embraced by the emerging small-sat community, and in 2004, a more capable version of the ring, the ESPA Grande, was designed with Ø24-inch ports for spacecraft up to 700 lb each.  Qualification test programs in 2016-2018 have documented substantially increased capabilities for both the ESPA and ESPA Grande ports, described in detail in Moog’s ESPA User’s Guide.  Moog has developed a modular product line around ESPA, and, as the launch vehicle industry transitions to reusable stages, the ESPA acronym has been re-purposed as the Evolved Secondary Payload Adapter.

ESPA was designed, built, and tested by CSA Engineering (now Moog Space and Defense), under a 1999 SBIR contract to the Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS) for the DoD Space Test Program (STP, of the Space and Missile Systems Center, SMC).  STP developed technology requirements and operational inputs; AFRL/VS led the research and development effort and integrated the technology development team.  In 2001, CSA designed and built a static test facility at AFRL for ESPA qualification that is still in use today; the original ESPA test occurred in April 2002.  Following the flight program for the first ESPA launch, the Ø15” port was re-designed to facilitate satellite integration; this design change also strengthened and stiffened the ESPA structure (thus prompting the Delta Qualification test that occurred in 2016).  The ESPA Grande Ø24” port was qualified by analysis under Moog internal funding and a 2008 SBIR contract to NASA Ames Research Center (and eventually qualified by test in 2018 at AFRL under Air Force funding).  In 2012, Moog received a Phase 3 SBIR contract from AFRL to modify ESPA for the EAGLE Program (ESPA Augmented Geostationary Laboratory Experiment), and a 4-point-mount satellite interface was developed for this Propulsive ESPA Flight Program.  EAGLE was launched in 2018 and has become the prototype for annual Air Force missions built around the ESPA ring.  And Moog is developing a family of Orbital Maneuvering Vehicles (OMVs) for Government and commercial customers.

• STP-1, the maiden voyage of ESPA, launched on Atlas V in March 2007 from Cape Canaveral Air Force Station.  STP-1 was also the first Atlas V EELV Mission for the Air Force and the first Atlas V with three Centaur burns.
• LCROSS (Lunar CRater Observation and Sensing Satellite) launched on Atlas V in June 2009, as a secondary payload on the Lunar Reconnaissance Orbiter (LRO) mission, and it impacted the moon in October of that year.  LCROSS was the first free-flying propulsive ESPA.
• OG2 (ORBCOMM Generation 2) Mission 1 in July 2014 was the first time a stack of ESPA rings was used to launch part of a constellation.  It also marked the first launch of ESPA Grande, and the first ESPA on Falcon 9.
• AFSPC-4 (Air Force Space Command 4) in July 2014 was the first ESPA launched on the Delta IV.
• EAGLE (ESPA Augmented Geostationary Laboratory Experiment) is an AFRL propulsive ESPA mission currently on orbit.  The satellite was designed using the Moog ESPA by Northrop Grumman for AFRL.  EAGLE was the first launch of the ESPA 4-point-mount interface.
• DSX (Demonstration and Science Experiment) was launched on a Falcon Heavy in June 2019 as part of the STP-2 Mission.  The DSX ESPA is the hub of an AFRL free-flyer spacecraft in Medium Earth Orbit.