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Capabilities and Accommodations on the ISS EXPRESS Pallet

Gene Cook, NASA, Johnson Space Center, International Space Station Payloads Office, Houston, Texas

ABSTRACT

The International Space Station (ISS) has several external attach locations to permit space research and observations outside the pressurized volume of the Space Station. To facilitate the installation of small individual payloads, NASA is developing a system that will provide the necessary data routing, power distribution, and structural interfaces, for the accommodation of these small external payloads. The system will be called the Express Pallet and will support 6 small adapter size payloads that can be individually removed and replaced on orbit. The purpose of this paper is to provide an overview of the system as proposed, detail the concept of operations of the system, and summarize the capabilities of the system.

OVERVIEW

The ISS will be equipped with several external attach sites to permit the science community to conduct

external (unpressurized) experiments and observations. There are small attach sites on the Columbus module, larger sites will be located on the Japanese Experiment Module Exposed Facility (JEM-EF), and large attach sites are located on the ISS truss.

There are 6 large ISS truss attach sites that can be used for storage of logistics carriers and large attached Payloads (Figure 1). The 2 sites located on P3 are scheduled for logistics use. Logistics usage includes the stowage of ISS Orbital Replacement Unit (ORU) spares and for the temporary stowage of unpressurized payloads.

The four attach sites located on S3 (starboard) are scheduled for attached payload use. Two are zenith facing and 2 are nadir facing. The ISS plans to use 3 of the S3 sites for Express Pallet Operations and the fourth attach site for other large truss class payloads such as the Alpha Magnetic Spectrometer (AMS).

Figure 1. ISS Truss Attach Sites

S3 Payload Sites P3 Logistics Sites

PAS No. 2PAS No. 1

PAS No. 3PAS No. 4

UCCAS No. 1

UCCAS No. 2MCAS

Mobile Base System (MBS)

S3 Payload Sites P3 Logistics Sites

PAS No. 2PAS No. 1

PAS No. 3PAS No. 4

UCCAS No. 1

UCCAS No. 2MCAS

Mobile Base System (MBS)

42nd AIAA Aerospace Sciences Meeting and Exhibit5 - 8 January 2004, Reno, Nevada

AIAA 2004-439

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

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ISS TRUSS ATTACH SYSTEM

Each of the six truss attach sites is equipped with an attach mechanism that is called the Payload Attach System/Unpressurized Cargo Carrier Attach System (PAS/UCCAS). Half of the PAS/UCCAS is attached to the truss (Figure 2) and has 3 guide vanes that interface with matching trunnions on the passive half that is attached to the payload or logistics carrier.

The mechanism used on the S3 payload sites is called the PAS and the mechanism used on the P3 logistics sites is called the UCCAS. The only hardware difference is in the design of the truss latching mechanism.

When the Payload/Carrier and the passive half of the attach mechanism are lowered onto the truss (Figure 3), the payload is latched to the truss by a device that is called the Capture Latch Assembly (CLA). The CLA closes on a bar on the passive half that is attached to the payload to lock the payload to the truss. The UCASS has redundant drive mechanisms on the latching mechanism. Redundancy for the CLA is provided on the S3 PAS mechanisms by a manual drive mechanism that can be actuated by an EVA crewman. Power and data services for both mechanisms are identical. Since this paper addresses the Express Pallet, which will be deployed to the S3 sites, the truss attach mechanism will be referred to as the PAS for the remainder of the text of this paper.

Figure 2. Payload Attach System (Active half)

After the Payload is attached to the truss attach site, the UMA active half is driven to mate with the passive half on the payload.

After the Payload is attached to the truss attach site, the UMA active half is driven to mate with the passive half on the payload.

Passive Half Assembly

Active Half Assembly

UMA Passive Half Not Provided

After the Payload is attached to the truss attach site, the UMA active half is driven to mate with the passive half on the payload.

After the Payload is attached to the truss attach site, the UMA active half is driven to mate with the passive half on the payload.

After the Payload is attached to the truss attach site, the UMA active half is driven to mate with the passive half on the payload.

After the Payload is attached to the truss attach site, the UMA active half is driven to mate with the passive half on the payload.

Passive Half Assembly

Active Half Assembly

UMA Passive Half Not Provided

Passive Half Assembly

Active Half Assembly

UMA Passive Half Not Provided

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There is one other truss class attach site on the ISS which is called the Mobile Base System Common Attach System (MCAS) that is located on the Mobile Base System that is used to shuttle large payloads from one end of the space station to the other. There are no plans to use this site for long term storage logistics storage or payload operations.

The 6 truss attach sites and the MCAS have power and data availability that can be remotely connected as a part of the procedure to attach the payload to the ISS truss. After the payload is attached to the truss site, the Umbilical Mating Adapter (UMA) is driven into position to provide power and data services to the

Figure 3. Passive PAS being lowered onto the Active PAS∗∗∗∗

payload. Each truss site UMA provides 120 VDC power, 1553 C&DH interfaces, and a High Rate Data Link (HRDL) optical fiber pair.

THE EXPRESS PALLET SYSTEM

The ExPS is composed of three main functional elements: Express Pallet Adapters (ExPAs), the Express Pallet (ExP), and ExPCA.

EXPRESS Pallet Adapters

One of the most important features of the Express Pallet is the feature that allows individual payloads to be changed out on orbit. This feature is made possible by the use of a standard plate assembly that is called an Express Pallet Adapter or ExPA (Figure 4). Express Pallet Adapters are provided to the payload developer as Government Furnished Equipment and the payload developer integrates his payload onto this standard plate.

The ExPA provides standard structural, mechanical, electrical and communications interfaces for payloads. The ExPA provides the structural and mechanical interface with the ExP with a capability for robotics or EVA manipulation, connection, and attachment of the Adapter-to-pallet related interfaces.

Figure 4. EXPRESS Pallet Adapter

The different components of the ExPA are shown in Figure 5. The ExPA top surface serves as the payload interface and has a flat aluminum surface with a grid of threaded inserts on 70 millimeter spacing. The plate is identical to the Columbus External Payload Adapter (CEPA) plate that was developed for use on the Columbus Exposed Facility. The threaded inserts provide the structural interface between the payload hardware and the adapter. The plate surface is anodized aluminum, which is non-conductive, and therefore bonding points are provided to assure that no electrical potential develops between the adapter and the payload.

Figure 5. ExPA Components

The ExPA has data and power connections available for the payload via connector panels located on the top of the adapter. One connector panel is used for power connections and the other connector panel is used for data connections. The Payload data and power are routed from these connectors via a cable assembly that terminates at what are called blind mate connectors located on the bottom of the assembly (Figure 6).

Passive half of the Payload PAS (Lockheed Martin AMS Design)

Active half of the PAS

Capture Latch

Passive half of the Payload PAS (Lockheed Martin AMS Design)

Active half of the PAS

Capture Latch

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Figure 6. Blind Mate Connectors

When the payload and adapter system is placed on the Express Pallet, a drive mechanism is turned by the 7/16 ” drive mechanism on the SPDM or is turned by an EVA crew man. The drive mechanism moves components on the bottom of the assembly engaging the lock mechanisms and engaging the blind mate connectors to transmit power and data.

Express Pallet System

The ExP provides a structural platform for the integration of ExPA mounted payloads. It will provide for a structural and mechanical interface with the ISS Truss Site (S3 or P3), with a capability for EVR or EVA manipulation, connection, and attachment of structural, electrical, and data interfaces. One of the concepts for the Express Pallet is shown in Figure 7.

Figure 7. EXPRESS Pallet (Brazil Concept)†

The structure consists of a deck that is populated with the interface hardware that allows the ExPAs to be attached, supporting structure that is attached to the deck and to the PAS system. Grapple fixtures are provided to maneuver the system out of the Shuttle and onto the truss. The Pallet is also equipped with Orbiter sill and keel trunnions to manage the loads during the Shuttle phase of the mission.

The EXPRESS Pallet Payload Controller Assembly

The ExPCA provides data/command routing, status monitoring, and power control, distribution, and conversion to the 6 ExPAs. The ExPCA is mounted on a FRAM mechanism so that it will be EVR and EVA compatible for manipulation, connection and replacement. Data and power services provided to the individual adapters are summarized in the following section.

Express Pallet Services

The Express Pallet system provides both Nadir and Zenith viewing. The Zenith sites provide full Zenith viewing capability and the Nadir viewing Express Pallet sites provides extensive earth viewing coverage. The 52 degree inclination of theISS carries the Express Pallet over 75% of the earth’s surface (Figure 8).

Figure 8. ISS Coverage

Payload volume – Each Payload (less adapter) shall fit within a 46 (l) x 34 (w) x 49 (h) inches (1.1 x 0.86 x 1.2 m) volume. This volume has been determined to be within the ISS robotic handling capability. This payload volume is shown in Figure 9.

51.6° Inclination

Equator

KSCDFRC

-Lat.

-Lat.

Earth Surface Coverage

75%

Blind Mate ConnectorsBlind Mate Connectors

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Figure 9. EXPRESS Pallet Adapter Payload Allowable Volume

Payload weight - up to 500 pounds (227 kg). This weight does not include the weight of the ExPA which is approximately 240 pounds.

The power services provided to the ExPAs are summarized as follows:

• The ExPS shall distribute up to 2.5 kW of combined nominal 120 volts direct current (Vdc) and nominal 28 Vdc operational electrical power to the ExPA complement.

• The ExPS shall distribute up to 2.5 kW of nominal 120 Vdc operational electrical power to the ExPA complement.

• The ExPS shall distribute up to 750 W of nominal 120 Vdc operational electrical power to any ExPA location.

• The ExPS shall distribute up to 1.0 kW of nominal 28 Vdc operational electrical power to the ExPA complement.

• The ExPS shall distribute up to 500 W of nominal 28 Vdc operational electrical power to any ExPA location.

• The ExPS shall provide a single (1) 120 Vdc feed and single (1) 28 Vdc feed for a total of two (2) operational electrical power feeds to each ExPA location. Additional stay alive and contingency electrical power feeds are also required at each ExPA location.

• The ExPS shall provide independent on/off control of each of the two (2)

operational electrical power feeds to each ExPA location.

• The ExPS, while in the Orbiter cargo bay, shall distribute Orbiter-provided electrical power as ExPS stay alive electrical power to each ExPA site and the ExPCA adapter site for stay alive heaters only. Up to 280 Watts of Orbiter-provided 28 Vdc electrical power to each stay alive heater.

• When only reduced electrical power is available from the ISS, the ExPS, while attached to the Truss PAS or MCAS, will distribute ISS-provided 120 Vdc electrical power to the ExPA and ExPCA locations.

Data Services• The ExPS shall provide for two (2) MIL-

STD-1553B bus interfaces at each ExPA location for command and control.

• The ExPS shall provide six (6) analog input channels at each ExPA location.These channels shall be compatible with 1000 Ohm Resistive Temperature Devices (RTDs), and +/- 5 Vdc differential voltage interfaces.

• The ExPS shall provide six (6) discrete channels at each ExPA location. Each channel shall be capable of operating as an input or output, programmable on-orbit. Each channel shall be capable of discrete logic voltage levels of 0V (zero Volts), 5V, and 28V, programmable on-orbit. These channels shall be capable of providing pulsed outputs.

• The ExPS shall provide for two (2) IEEE 802.3 Ethernet 10-Base-T interfaces in accordance with ISO 8802-3 at each ExPA location for medium to high rate payload science data, accommodating an effective rate of six (6) Mbps.

OPERATIONS CONCEPT

The three Express Pallets will be integrated with 6 adapter payloads prior to initial launch and deployed to the S3 attach sites. The pallets will remain at these locations for the life of the program and the individual adapter payloads will be removed robotically and replaced with payloads delivered by Unpressurized Logistics Carriers (ULC). The payloads that are removed from the pallet will be returned via ULCs.

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The ExPAs are supplied to the Payload Developer integrated into an Express Pallet shipping container that is sized to provide adequate shipping volume for the Adapter as well as the payload. Test ports are provided that are connected to the Blindmate connectors on the adapter to provide a path for ground testing of the payload during payload buildup and prior to launch. The payload assembly can also be connected for testing at KSC via these ports.

The Payload developer is also provided with a test system known as the Suitcase Test Environment for the Express Pallet (STEP-EP). This system emulates the Express Pallet/ISS data stream for use by the payload developer during payload and adapter integration.

After the payload and adapter integration is completed, the payload developer ships the integrated adapter and payload to KSC where it is integrated onto the Express Pallet on the first launch of the Express Pallets or ULCs for subsequent adapter payload transport to the ISS.

The first Express Pallet payloads will be integrated onto the pallet itself and thus will have an opportunity for ground testing that will not be available for subsequent Express Pallet payloads. After the initial launch of the Express Pallets, all Express Pallet adapter payloads will be changed out on orbit and therefore the first time that the replacement adapter payloads will be interfaced with the pallet will be when they arrive on orbit. To provide an equivalent level of testing for replacement payloads, a provision has been made for an Express Pallet Payload simulator that will be situated at KSC.

The Express Pallet simulator will have a flight equivalent ExPCA and will be interfaced to the KSC Payload Test and Checkout System. This facility will provide an opportunity to test each adapter payload with the Express Pallet flight software load.

During the Shuttle transportation phase, the Express Pallet does not provide data connectivity to the Shuttle and heater power is supplied to each adapter payload to assure that payloads stay within limits. The Shuttle only provides 28 volt power for Express Pallet use so payload developers will have to size heaters used in the Shuttle to interface with a 28VDC power source.

The original pallet specification required that the pallet have a dropped deck to accommodate tall payloads. This requirement was subsequently eliminated since it would only benefit payloads on the initial outfitting since the pallets would be positioned permanently on the Space Station. The only impact to Express Pallet operations is that the initial launch of adapter payloads will have a slight volume constraint on the side adapter payloads due to interference with the Orbiter Payload Bay Envelope. This interference is shown in Figure 10. The constraint is no longer applicable after the ExPRESS Pallet is deployed to the truss attach site.

Figure 10. Payload Bay Interference on the Initial Launch of the ExPRESS Pallet

A typical robotic operation for the installation of an Express Pallet is shown in the attached drawings (Figure 11): 1). The Express Pallet is grappled but the Shuttle Remote Manipulator Arm (SRMS); 2). The Express Pallet is removed from the Cargo Bay by the Shuttle RMS; 3). The SRMS presents the Express Pallet to the Space Station Remote Manipulator Arm (SSRMS); 4). The SSRMS grapples the Express Pallet; 5). The Express Pallet is released by the SRMS; 6&7). The Express Pallet is maneuvered to the S3 location; 8). The Express Pallet is installed on the S3 truss.

The initial time estimates for the completion of this activity indicated that the Express Pallet would be without power for 4.5 hours however this requirement is enveloped by the EVA requirement for external payloads to be able to be without power for 8 hours.

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

Shuttle Cargo Bay envelope

Full Adapter Volume is not available on the initial launch of the Express Pallet due to interference with the Shuttle Cargo Bay Envelope

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Figure 11. Typical EXPRESS Pallet Installation

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Subsequent change out of Express Pallet payloads is performed using the Special Purpose Dexterous Manipulator (SPDM) shown on the end of the SSRMS in Figure 12. The SPDM is a robotic tool that is placed on the end of the SSRMS that permits more intricate robotic operations to be performed than could be performed by the SSRMS. The SPDM has two arms that are used to remove small items and a series of cameras to position the arms. The SPDM is controlled from the robotics workstation in the Space Station. Each arm of the SPDM has a device on the end called the ORU and tool change out mechanism (OTCM). The OTCM has a clamping device called gripper jaws that grasps SPDM compatible fixtures on the payload or ORU. The OTCM is also equipped with a motor driven 7/16” hex drive that is used to remove FRAM attached devices. Power and data services are also available through the OTCM if the necessary connections are provided on the payload.

Figure 12. SPDM Shown Attached to the End of the SSRMS

To replace a payload on the Express Pallet, the replacement payload is first removed from the Carrier after the carrier is removed from the Shuttle Cargo bay and placed on a stowage location on the Space Station. As of the writing of this paper, this step is necessary since the SPDM is not deemed to have sufficient fault tolerance to remove a payload directly from the Orbiter Cargo bay. After the carrier has been placed at the desired location, the SPDM is maneuvered to the worksite by the SSRMS. The SPDM will then attach one OTCM to a fixture that is called a stabilization fixture located on the carrier. The purpose of the stabilization fixture is to reduce the amount of movement transmitted to the SPDM through the SSRMS since it is cantilevered on the end of the SSRMS.

After the SPDM is stabilized, the SPDM maneuvers the other OTCM to attach to the fixture on the ExPA. The

7/16” hex drive in the SPDM is engaged to rotate the ball screw mechanism that is part of the active FRAM mechanism on the ExPA. This action releases the FRAM from the Carrier.

After the payload has been released from the Carrier, the SPDM maneuvers the payload and places it on a square grid fixture on the base of the SPDM. This location is provided since the OTCM arms do not have sufficient rigidity to control the payload while the SPDM is being translated by the SSRMS.

When the payload has been temporarily stowed on the Square Grid Location on the SPDM, the SPDM is maneuvered to the Express Pallet. The SPDM then stabilizes on a stabilization fixture that is on the Express Pallet. The payload that is to be replaced is removed in the same fashion as the new payload was removed from the carrier.

At this point the SPDM releases the stabilization fixture and that OTCM now removes the replacement payload from the SPDM square grid location and the payload that is to be returned is placed on the SPDM square grid by the other OTCM. This OTCM is not free to attach to a stabilization fixture on the Express Pallet. After the SPDM is stabilized, the payload is installed on the Express Pallet by the other OTCM.

At this point, the SPDM returns to the ULC and installs the payload to be returned in the same fashion as the Express Pallet installation was performed.

The Express Pallet will have the crew aids necessary to perform these operations via EVA. The 7/16” hex can be actuated by a crew EVA tool and the FRAM is provided with soft dock magnets to hold the adapter and payload assembly in place while the action is completed.

∗ Lockheed Martin Company† AEB