EELV Secondary Payload Adapter
The EELV Secondary Payload Adapter (ESPA) is an interstage adapter ring for launching secondary payloads on US DoD space missions that use the Evolved Expendable Launch Vehicles (EELV) Atlas V and Delta IV. This reduces launch costs for the primary mission and enables secondary and even tertiary missions with minimal impact to the original mission. The adapter design has become a de facto standard and is now planned for use on some private spacecraft missions as well.
Development was funded by the Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS) for the United States Department of Defense (DoD) Space Test Program (STP) under a Small Business Innovative Research (SBIR) grant in the late 1990s. Moog CSA Engineering teamed with AFRL to design, build and qualify the ring in the early 2000s. Additional studies have been done on ESPA applications for lunar and science missions under an SBIR from NASA Ames Research Center  As of 2010[update], the ring is produced by Moog CSA Engineering.
ESPA was designed to support a 15,000 pounds (6,800 kg) primary payload and up to six 400 pounds (180 kg) secondary payloads. Each secondary spacecraft is mounted radially on a 15 inches (380 mm) diameter port and is allocated 24 inches (610 mm) x 28 inches (710 mm) x 38 inches (970 mm) volume. This has led to the colloquial designation of "ESPA-class" payloads. The design includes a standard electrical interface for the attached payloads; however mission-specific requirements may preclude each secondary payload from receiving more than a single, non-redundant payload separation signal.
A number of missions have used the ESPA ring. The ESPA ring’s maiden mission was on STP-1 in 2007. When NASA upgraded its Lunar Reconnaissance Orbiter (LRO) mission’s launch vehicle to an Atlas V, it freed around 2,200 lbs. of additional mass for what would later become LCROSS. NASA held a competition to see how best to use the space and a number of proposals came from the Ames Research Center. The winning proposal included Moog CSA Engineering’s ESPA ring launching the LCROSS as a secondary payload under the LRO. LCROSS ultimately impacted the lunar surface and confirmed the presence of water ice.
The LCROSS Lunar-impact water detection mission in 2009 took advantage of the structural capabilities of ESPA ring to attach all six of its science experiments, command and control systems, communications equipment, batteries, solar panels, and even a small monopropellant propulsion system to implement pre-impact payload separation and control.
Commercial derivatives of the ESPA Grande ring are being developed. Named the Spaceflight Secondary Payload System (SSPS), the system is being developed and manufactured by Andrews Space under contract to Spaceflight Services. It includes five 24 inches (61 cm)-diameter ports, each capable of carrying payloads weighing up 300 kilograms (660 lb). "The SSPS operates very similar to a standalone spacecraft with a flight computer, electrical power system, orbit determination capability, and payload power switching."
- Perry, Bill. "ESPA: An Inexpensive Ride to Space for Secondary Payloads". July 2012 Edition of MilsatMagazine. MilsatMagazine. Retrieved July 2012.
- Evolved expendable launch vehicle secondary payload adapter (PDF). AIAA. Retrieved 2011-08-10.
- "ESPA: The EELV Secondary Payload Adapter Heavy Lift. Excess Capacity. Small Satellites.". MOOG. 2010. Retrieved 2011-12-25.
- Foust, Jeff (2011-08-22). "New opportunities for smallsat launches". The Space Review. Retrieved 2011-09-27. "SpaceX ... developed prices for flying those secondary payloads ... A P-POD would cost between $200,000 and $325,000 for missions to LEO, or $350,000 to $575,000 for missions to geosynchronous transfer orbit (GTO). An ESPA-class satellite weighing up to 180 kilograms would cost $4–5 million for LEO missions and $7–9 million for GTO missions, he said."
- Perry, Bill. "ESPA: An Inexpensive Ride to Space for Secondary Payloads". July 2012 Edition of MilsatMagazine. Retrieved 26 September 2012.
- Lo, Amy (2009). "Secondary Payloads Using the LCROSS Architecture". AIAA. Retrieved 2011-09-27.
- Spaceflight Secondary Payload System, retrieved 2012-05-10.