What Is an International Standard Payload Rack, and Why Is It Important?

The International Space Station (ISS) is, without question, the most complex and expensive laboratory ever built. From 250 miles above the planet, it allows an international cohort of astronauts to conduct research that is impossible to perform on Earth. This research spans biology, physics, astronomy, and human health. But how does this science actually get done? When a pharmaceutical company wants to grow a protein crystal or a physicist wants to study a “cold” flame, they can’t just send a laptop and a beaker into orbit. The science itself needs a home, a support structure that provides it with power, cooling, data, and a physical place to exist.

Standardization

That home is the International Standard Payload Rack, or ISPR.

To a casual observer, an ISPR looks like a large, beige, or metallic locker, roughly the size of a telephone booth or a large vending machine. The interior of the station’s laboratory modules, like the U.S. Destiny laboratory or the European Columbus module , are lined with them. They are the standardized “bookshelves” of the orbital outpost. While they may appear as simple storage, these racks are the core of the station’s scientific capability. They are a triumph of logistics and engineering, a physical interface that allows dozens of experiments from dozens of nations to “plug in” and work. Without this standardization, the ISS would be a chaotic collection of incompatible, custom-built hardware. The ISPR is what makes the “International” part of the space station’s research truly possible.

The Problem ISPRs Solve

To appreciate the ISPR, one must look at the history of space-based research. In the early days of space stations, such as Skylab in the 1970s, and even on the Space Shuttle’s Spacelab missions, experiments were often one-off creations. Each scientific instrument was custom-designed and custom-built, with its own unique power connectors, data processors, and cooling systems. This approach was incredibly expensive and time-consuming. A university might spend years developing an experiment, only to find that integrating it with the spacecraft’s systems was a complex, bespoke challenge.

When the International Space Station was conceived in the 1980s and 1990s, engineers from multiple space agencies – NASA , ESA (European Space Agency), JAXA (Japan Aerospace Exploration Agency), and the Canadian Space Agency (CSA) – recognized this problem. The new station was going to be a partnership, with laboratory modules built by different nations. If ESA wanted to run an experiment in a NASA module, or vice versa, how could they ensure it would work?

The solution was a common standard, analogous to a USB port or a standard electrical outlet. The ISPR is that standard. The agreement stated that the modules themselves would provide a set of standardized connections for power, data, cooling, and ventilation, all terminating at specific points on the module walls. The ISPRs would be built to “plug into” these wall connections. In turn, the ISPRs would offer their own standardized connections inside the rack for the experiments themselves.

This “two-level” standardization completely changed the paradigm. A scientist no longer needs to worry about how to tap into the station’s main water-cooling loop. They just need to design their experiment to fit into a standard ISPR “drawer” and connect to the rack’s pre-built cooling port. The rack handles the rest. This system allows experiments to be developed faster, tested on the ground in identical mock-up racks, and swapped out on orbit with relative ease.

Anatomy of a Standard Rack

An ISPR is far more than an empty box. It’s a miniature, self-contained support system for science. While the exterior dimensions are standardized – roughly 79 inches tall, 41.5 inches wide, and 33 inches deep, with a mass of several hundred pounds on the ground – it’s the internal infrastructure that defines its purpose.

Physical Structure

The rack’s frame is typically built from advanced aluminum alloys, designed to be both lightweight for launch and incredibly strong. It must withstand the violent vibrations and G-forces of a rocket launch and then, once in orbit, serve as a rigid platform. On the ISS , racks are not just “placed” on the floor. The laboratory modules are essentially large cylinders, and the ISPRs line the “walls” (which are also the floor, ceiling, and sides in a weightless environment).

Each module, like Destiny or Columbus, is built with a grid system. The racks are guided into their designated slot and then locked in place using a “rack-and-pin” system. Astronauts use a special tool to drive large pins from the rack into attachment points on the module wall, securing it firmly. This ensures the rack can’t float away and, more importantly, that all the utility connections on the back of the rack are precisely aligned with the “feed-throughs” on the module wall.

The Life-Support Interface

The true genius of the ISPR is its “back panel.” When an astronaut installs a rack, they are also hooking up a complex set of “jumpers,” or flexible utility lines. These jumpers are the bridge between the station’s main systems and the rack itself.

Power: The ISS provides several different voltages of Direct Current (DC) power. An ISPR can tap into the station’s 120 VDC and 28 VDC power grids. The rack contains its own internal power distribution and conversion systems, taking the “raw” station power and converting it into the specific voltages required by the different experiments housed inside. This means an experiment designer only needs to build their payload to accept the rack’s standard power output, not the station’s.

Cooling: Any complex experiment, especially furnaces, freezers, or high-performance computers, generates a lot of heat. In a vacuum, heat doesn’t dissipate easily. The ISS has a sophisticated liquid cooling system. The ISPRs are a key part of this. Most science racks have an air-cooling system, using internal fans to circulate the cabin air through the experiments and a heat exchanger.

For more demanding payloads, the rack plugs directly into the station’s water-cooling loop. This “Moderate Temperature Loop” (MTL) circulates water through pipes in the module walls. The ISPR taps into this, and the experiments within the rack can then dump their waste heat into this water, which is eventually radiated away from the station’s external trusses.

Data: An experiment is useless if its data can’t be retrieved. Every ISPR plugs into the station’s data network. This is typically a 100 Mbps Ethernet connection that links the rack to the Payload Multiplexer/Demultiplexer (MDM). This is the station’s “router,” which manages all scientific data. This connection allows experiments to be controlled in real-time by scientists on the ground at a Payload Operations Integration Center (POIC) , a concept known as “telescience.” It also beams the resulting terabytes of data back to Earth via the station’s Tracking and Data Relay Satellite System (TDRSS) connection.

Vents and Vacuum: Some experiments, like those involving combustion or material processing, produce waste gases that can’t be released into the crew’s living space. Other experiments need to be exposed to the hard vacuum of space. ISPRs can be connected to the station’s Waste Gas System, which safely vents these gases overboard. They can also connect to the Vacuum Exhaust System (VES), which is essentially a pipe to the outside, allowing scientists to create a vacuum environment without a mechanical pump.

The EXPRESS Rack: The Workhorse of the ISS

While “ISPR” is the general term for the standard, the most common type of ISPR on the U.S. Orbital Segment (USOS) is the EXPRESS Rack. The name is an acronym for “EXpedite the PRocessing of Experiments to Space Station.” It perfectly describes its purpose.

The EXPRESS Rack is not an experiment itself. It’s a hosting facility. It’s a general-purpose ISPR designed to hold multiple, smaller experiments simultaneously. The front of an EXPRESS Rack is divided into mounting bays that can hold a combination of payloads. These typically include eight Mid-Deck Lockers (MDLs) and two larger “drawers.”

The Mid-Deck Locker is another key standard, a carry-over from the Space Shuttle program. It’s a standardized box, roughly the size of a microwave oven, that can hold a self-contained experiment. An EXPRESS Rack acts as a “landlord” for these lockers, providing each one with its own power, data, and cooling connection.

This system is incredibly flexible. A university can develop a small experiment that fits inside a single locker. Once it’s ready, it’s flown to the station, and an astronaut simply slides the locker into an empty slot on an EXPRESS Rack and connects a few cables. The experiment can then be operated from the ground. Next to it, in the same rack, could be a locker from a different university, a different country, or a private company, all sharing the rack’s resources.

Each EXPRESS Rack has its own “brain,” the Rack Interface Controller (RIC) , which manages the distribution of resources, monitors the health and status of the experiments, and communicates with the station’s main computer.

The International Family of Racks

The ISPR standard forms the basis for research on the U.S. , European, and Japanese segments of the station. While the EXPRESS Rack is a general-purpose host, many ISPRs are single, dedicated research facilities.

European Space Agency (ESA) Racks

The ESA Columbus module is filled with ISPR-compliant racks.

• Fluid Science Laboratory (FSL): This is a full-rack facility dedicated to studying the strange behavior of liquids in microgravity. On Earth, gravity causes denser fluids to sink and lighter fluids to rise. In space, this doesn’t happen, allowing scientists to study phenomena like foam stability, emulsions, and fluid mixtures that are “hidden” by gravity on Earth.
• European Drawer Rack (EDR): This is ESA’s version of the EXPRESS Rack , designed to hold standardized “drawers” and lockers. It provides the same flexible, multi-user hosting capabilities.
• Materials Science Laboratory (MSL): This rack contains a high-temperature furnace. Scientists use it to create new metal alloys and semiconductor materials. In microgravity , molten metals don’t separate by density, allowing for the creation of perfectly uniform, “ideal” alloys that are impossible to make on the ground.

Japanese (JAXA) Racks

The JAXA Kibo module also uses the ISPR standard. It houses several dedicated facilities.

• Ryutai (Fluid) Rack: JAXA’s advanced fluid physics facility, studying complex fluid dynamics.
• Kobairo (Gradient Heating) Rack: A high-precision furnace used for growing large, flawless semiconductor and protein crystals.
• Saibo (Cell Biology) Rack: A facility containing microscopes and incubators for performing advanced biological research on living cells in orbit.

Specialized U.S. Racks

Not all NASA ISPRs are EXPRESS Racks. Many are highly specialized facilities built into the standard ISPR frame.

• Human Research Facility (HRF) Racks: The ISS is a laboratory not just for science, but for studying the astronauts themselves. The HRF racks contain medical-grade equipment like an ultrasound scanner, blood sample collection hardware, and cognitive test batteries to understand how the human body adapts to long-duration spaceflight.
• Combustion Integrated Rack (CIR): This rack is a sealed chamber for safely studying fire in space. Flames behave very differently without gravity; they are rounder, slower, and can burn at lower temperatures. This research is vital for improving fire safety on future spacecraft and for understanding the fundamentals of combustion.
• Minus Eighty-Degree Laboratory Freezer for ISS (MELFI): This is one of the most in-demand racks on the station. MELFI is a set of four large, independent freezers that can hold biological samples (like blood, urine, and cell cultures) at temperatures as low as -80°C. This allows samples from astronauts or experiments to be preserved for months before being returned to Earth for analysis.
• Materials Science Research Rack (MSRR): NASA’s primary materials science furnace, complementing ESA’s MSL.

This table highlights some of the key scientific and life-support racks built to the ISPR standard that are (or have been) operational on the International Space Station.

Infrastructure in a Rack’s Clothing

One of the most important design decisions made for the ISS was to use the ISPR standard not just for science, but for the station’s own core infrastructure. The station’s key life-support systems are built into ISPR frames.

• Oxygen Generation System (OGS): This rack pulls water from the station’s water supply and uses electrolysis to split it into hydrogen and breathable oxygen for the crew.
• Water Recovery System (WRS): This system, housed in two ISPRs, is the station’s advanced recycling plant. It takes wastewater, crew perspiration, and even urine, and purifies it through a series of filters and a distillation process back into clean, potable water.
• Carbon Dioxide Removal Assembly (CDRA): This rack contains the “lungs” of the station, a system that scrubs the carbon dioxide exhaled by the crew from the air.

Using the ISPR standard for these vital systems was a brilliant move. It means that a broken life-support component can be removed and (in theory) replaced with a spare rack, just like a scientific payload. It streamlines logistics, training, and maintenance across the entire station.

The Logistics Lifecycle of a Payload

The ISPR standard governs the entire life of an experiment, from an idea on Earth to a result from orbit.

Phase 1: Ground Development

A scientist at a university or company designs an experiment to fit the known ISPR standard. They don’t have to invent a power supply or data logger; they just need to build their experiment to plug into the EXPRESS Rack’s standard “drawer” interface. They can test their payload on the ground using a high-fidelity EXPRESS Rack mockup at a NASA facility, ensuring that it will work correctly long before it ever gets to space.

Phase 2: Launch and Transport

Getting a refrigerator-sized ISPR to orbit is a major challenge. During the Space Shuttle era, racks were pre-installed in Multi-Purpose Logistics Modules (MPLMs) , which were large pressurized “moving vans” that flew in the shuttle’s cargo bay.

In the post-Shuttle era, this job falls to Commercial Resupply Services (CRS) vehicles. The Northrop Grumman Innovation Systems Cygnus spacecraft and JAXA’s HTV were specifically designed with a wide enough hatch to accommodate a full-sized ISPR. SpaceX’s Dragon 2 cargo vehicle can also transport racks. The racks are carefully packed inside these vehicles, launched, and then docked at the ISS.

Phase 3: Installation On-Orbit

This is where the astronauts’ work begins. After a cargo vehicle arrives, the crew must “unpack” it. Moving a massive, 800-pound rack (on Earth) is impossible for a person. In microgravity , it’s not about weight, but inertia. The rack is still just as hard to start and stop moving.

Astronauts will carefully unbolt the rack from the cargo vehicle’s restraints and, with gentle pushes, “fly” it through the station’s modules. They must navigate tight corners, often rotating the rack to fit through the circular hatches of the station’s nodes, like the Harmony module. Once at its destination (e.g., the Destiny lab), they slide it into its designated slot and use a power tool to drive the pins that lock it to the station structure.

The final step is connecting the jumpers. The astronaut goes “behind” the rack (in a sense, through side access panels) to connect the large, flexible hoses for cooling water, the power cables, and the data lines. The rack is then powered on, and ground control takes over.

Phase 4: Operations and Return

Once the rack is “online,” scientists at the POIC at Marshall Space Flight Center can “talk” to it. They can activate experiments, download data, and troubleshoot problems. Astronauts may be required to interact with the rack, suchg as swapping out sample cartridges in the MSRR furnace or retrieving biological samples from the MELFI freezer.

When an experiment is complete, its samples or hardware are often the most valuable cargo. Sub-payloads (like lockers) are removed from the host rack, packed in special foam-lined bags, and loaded into a return vehicle like the SpaceX Dragon 2. This is the only current vehicle that can return a large amount of sensitive hardware intact to Earth via a gentle splashdown. The ISPR itself almost always stays in orbit, ready to receive its next “tenant.”

The “Non-Standard” Standard: The Russian Racks

It’s important to note that the “I” in ISPR primarily applies to the United States Orbital Segment (USOS) , which includes the American, European, and Japanese modules. The Russian Orbital Segment (ROS) , which includes the Zvezda and Nauka modules, is a separate engineering heritage.

The ROS uses its own system of payload racks. These racks are based on the standards developed for the Mir space station. They have different physical dimensions, different attachment mechanisms, and use the Russian-standard 28 VDC power. This isn’t a “better” or “worse” system, just a different one.

This difference highlights the power of the ISPR standard. It is very difficult to move a USOS experiment (like one designed for an EXPRESS Rack) and install it in the Zvezda module. It would require complex and costly power adapters, data converters, and custom-made hardware. The ISPR’s value is that it creates a common “language” that all partners on the USOS can speak, dramatically lowering the barrier to international scientific collaboration.

The ISPR Legacy

The International Space Station will not orbit forever. As it approaches its retirement, the engineering lessons it taught are already shaping what comes next. The ISPR standard is arguably one of the station’s most important and enduring legacies.

The Lunar Gateway , a smaller space station planned for orbit around the Moon as part of the Artemis program, is being built around this concept. The Habitation and Logistics Outpost (HALO) module, one of the Gateway’s first elements, will use international standard racks. This allows NASA and its partners to leverage the decades of experience gained on the ISS.

Perhaps more importantly, the ISPR standard has created a commercial market. Companies like Axiom Spaceare building commercial space stations. They are not reinventing the wheel; they are actively designing their stations to be compatible with ISPR payloads. A pharmaceutical company that has already spent money developing an experiment for an EXPRESS Rack locker on the ISS can now easily fly that same experiment on a future commercial station. This continuity is what will allow a true low-Earth orbit economy to flourish.

Summary

The International Standard Payload Rack is the circulatory and nervous system of the International Space Station’s scientific enterprise. It is a concept that turns a complex, multi-national laboratory into an efficient, “plug-and-play” research park. By providing a standardized interface for power, data, cooling, and physical space, the ISPR system allows scientists on Earth to focus on the science, not the engineering of getting to space. It allows astronauts to act as efficient lab technicians, swapping out experiments like memory cards in a computer. These humble, refrigerator-sized boxes are what transform the ISS from a simple outpost into the most productive scientific laboratory in human history.