roll joint

External Stowage Platform

The External Stowage Platforms (ESP) are three components of the International Space Station (ISS). Each ESP is an external pallet that can hold spare parts, also known as Orbital Replacement Units (ORU), 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. The ORUs are attached to the ESP with Flight Releasable Attachment Mechanisms (FRAM). The structure of the platforms is based on the Integrated Cargo Carrier (ICC) originally designed for the Space Shuttle and produced by Spacehab, Inc.


The first of the External Stowage Platforms, called ESP-1, was installed on the 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 FRAMs to store ORUs.

ESP-1 holds the following ORUs:

  1. Direct Current Switching Unit (DCSU)
  2. Pump Flow Control System (PFCS)


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 FRAMs creating room for up to eight spare parts (ORUs). Like ESP-1, it is powered by the Unity Module. However unlike ESP-1, the ESP-2 is attached to the Quest Joint Airlock using a specialized ESP Attachment Device (ESPAD). As of March 2008, the ORUs on ESP-2 are:

  1. Flex Hose Rotary Coupler
  2. Main Bus Switching Unit
  3. Utility Transfer Assembly
  4. Battery Charge/Discharge Unit (BCDU)
  5. Pump Module Assembly (PMA)
  6. SPACEHAB/Oceaneering Space Systems - Extended Deployable
  7. Main Bus Switching Unit (added on October 23 2007 during STS-120)
  8. Direct Current Switching Unit (DCSU) (added on 17 March 2008 during STS-123)
  9. Direct Current Switching Unit (DCSU) (added on 17 March 2008 during STS-123)
  10. Yaw Joint ORU for Canadarm2 (added on 17 March 2008 during STS-123)

The four Video Stanchion Assemblies initially stored on ESP-2 were installed on the P1 truss segment on 7 November 2005, on the S1 truss segment on 3 August 2006, on the S3 truss segment during EVA 4 of STS-117 on 17 June 2007, and on the P1 truss segment during an EVA on July 23, 2007. The empty FSE/FRAM (Flight Support Equipment/Flight Releasable Attach Mechanism) was then removed from ESP-2 and jettisoned overboard on July 23, 2007. The empty VSSA FSE/FRAM was 96 kg in mass and 1.4 x 1.2 x 0.9m in size.

A failed Control Moment Gyroscope was stored on ESP-2 from August 13 2007 (when it was removed from service during an EVA) until February 13, 2008 (when it was returned to Earth on STS-122).


ESP-3 was installed on August 14 2007 during the Space Shuttle STS-118 mission. It has eight attachment sites capable of holding up to seven ISS spare parts and assemblies. 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). Astronauts robotically installed the platform onto the station's P3 truss segment during the STS-118 mission’s seventh day.

As of June 2008, the following ORUs are installed on ESP-3:

  1. Battery Charge/Discharge Unit (BCDU) - A BCDU charges batteries and provides conditioned battery power to power buses during eclipse periods.
  2. Pitch Roll Joint (P/R‐J) – The P/R-J is an ORU provided by the Canadian Space Agency for the Canadarm2, the Space Station Remote Manipulator System (SSRMS).
  3. Nitrogen Tank Assembly (NTA) – depleted. Added on 8 June, 2008 during STS-124 afer removal from the S1 truss.

The Control Moment Gyroscope (CMG) and its associated support frame that were on ESP-3 at launch were removed on 13 August 2007. The support frame was placed on ESP-2 and the CMG was installed in the Z1 truss.

The Nitrogen Tank Assembly (NTA) that was on ESP-3 at launch was moved to the S1 truss on 8 June, 2008 to replace a depleted tank.


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