External stowage platforms (ESPs) are key components of the International Space Station (ISS). Each platform is an external pallet that can hold spare parts, also known as Orbital Replacement Units (ORUs), for the space station. As a platform it is not pressurized, but does require electricity to power the heaters of some of the stored equipment. ORUs are attached to the ESP via Flight Releasable Attachment Mechanisms (FRAMs), matching witness plates that mate the ORU to the platform. The structure of the ESP-2 and ESP-3 platform is based on the deployable version of the Integrated Cargo Carrier (ICC) which was designed to fly inside the Space Shuttle’s cargo bay and is owned and operated by Astrium North America, Inc.
The first of the external stowage platforms, called ESP-1, was installed on the port side trunnion pin on the outer hull of the Destiny Laboratory Module on March 13, 2001 during the second EVA of the STS-102 Space Shuttle mission. It is powered by the Unity Module and has two attach points to store ORUs. ESP-1 was carried into orbit on the underside of an Integrated Cargo Carrier. It is smaller than the other ESPs and ELCs, with dimensions approximately 0.46 m wide by 2.4 m long, and is differently shaped.
ESP-1 holds the following ORUs: FRAM-1 Pump flow control system (PFCS) added by the STS-102 crew.; FRAM-2 Direct-current switching unit (DCSU) added by STS-100 crew.
ESP-2 was installed with the assistance of Space Shuttle Discovery’s robotic arm and two spacewalkers during the STS-114 mission. It is much larger than ESP-1 with eight FRAM sites creating room for up to eight spare parts (ORUs). Like ESP-1, it is powered by the Unity Module. However, unlike ESP-1, ESP-2 is attached to the Quest Joint Airlock using a specialized ESP Attachment Device (ESPAD). ESP-2 and ESP-3 are deployable versions of the integrated cargo carrier and have the same dimensions, approximately 8.5 feet (2.6 m) long and 14 feet (4.3 m) wide.
The ORUs on ESP-2 are:
FRAM-1 (top side) Failed Pump Module PM-004 relocated here by the ISS-41 US EVA-27 crew in Oct. 2014; FRAM-2 (top side) Direct Current Switching Unit (DCSU) (added by STS-123 crew) FRAM formerly held the VSSA.; FRAM-3 (top side) CTC-3 container moved here via SPDM. DCSU added by STS-123 crew) had been relocated via SPDM Jan. 30, 2013 to ELC-2.; FRAM-4 (top side) Main Bus Switching Unit (MBSU) launched on ESP-2.; FRAM-5 (keel side) Pitch/Roll Joint (P/R-J) added by STS-123 crew. FRAM formerly held a CMG.; FRAM-6 (keel side) Main Bus Switching Unit (MBSU) added by STS-120 crew. This unit was swapped with a failed unit MBSU #1 from the SO truss, by the Exp 32 crew in late 2012.
FRAM-7 (keel side) Flex Hose Rotary Coupler (FHRC SN1003) launched on ESP-2.; FRAM-8 (keel side) Utility Transfer Assembly (UTA) launched on ESP-2.
ESP-3 was installed on the P3 Truss at UCCAS-1 on August 14, 2007 during the Space Shuttle STS-118 mission. It has seven attachment sites for ISS spare parts and assemblies, called Orbital Replacement Units (ORUs). The platform also has handrails and attachment points for tethers and foot restraints that astronauts can use while working with the ORUs on the ESP-3. ESP-3 was the first major station element to be installed completely by robotics, using only the shuttle and station’s robotic arms, an external berthing camera system (BCS) and a Photovoltaic Radiator Grapple Fixture (PVRGF). On January 12, 2010, the station’s robotic arm was used again to move ESP-3 from the P3 truss segment UCCAS-1 site. It was grappled by the arm and then transferred down the station’s backbone on the mobile transporter. ESP-3 was then attached to its new location on the lower part of S3 truss segment at the PAS-3 site. Moving the storage platform cleared the way for ExPRESS Logistics Carrier-3 to be installed during STS-134.
The ORUs installed on ESP-3 are:
FRAM-1 (top side) Pitch/roll joint (P/R‐J) launched on ESP-3; FRAM-2 (top side) Flex Hose Rotary Coupler (FHRC SN1004) added by STS-126 crew; FRAM-3 (top side) empty (future home of failed PM SN0004 currently stored on the MBS POA, by the Expedition 38 crew Dec. 2013.; FRAM-4 (top side) Linear Drive Unit (LDU) added by STS-127 crew; FRAM-5 (keel side) Space-to-Ground Antenna (SGANT) added by STS-127 crew; FRAM-6 (keel side) Battery Charge/Discharge Unit (BCDU) launched on ESP-3; FRAM-7 (keel side) ATA Flight Support Equipment (FSE) added by STS-118 crew.