Could the Space Shuttle have Replaced the ISS?

Jun 26, 2020 · 9 min read
International Space Station from STS-132 (Image courtesy: NASA/Crew STS-132)

The International Space Station (ISS) is by far the largest and most expensive man-made structure ever launched and constructed in space. It weighs nearly 400 tons and covers an area as big as a football field. A collaborative effort between 15 countries and five space agencies (NASA — United States, Roscosmos — Russia, JAXA — Japan, CSA — Canada), the ISS has evolved from a government research lab designed to study the effects of prolonged zero gravity on the human body and the effects of radiation and the vacuum of space on materials; it is now open to both commercial and educational research extending the boundaries of material manufacturing and the production of biochemicals, pharmaceuticals, and even beer. This highly versatile and extensible system comes with an astounding price tag: estimated costs of nearly 100 billion dollars for the development of the modules and assembly of the station on-orbit. Additionally, the International Space Station requires 3–4 billion dollars a year to maintain its operational costs; this is usually about half of the entire human space flight budget each year. These operational costs include the launches to keep the station stocked with supplies for the various astronauts and expeditions, the delivery of additional science modules and experiments, and routine maintenance and personnel costs of the vast network of organizations and individuals that help keep the ISS and its crew running.

These high development, assembly, and operational costs were well known before the first ISS module was completed and launched in 1998 (Zarya) when the space station was still undergoing design changes and final assessment in the early to mid-1990s as a part of the Space Station Freedom project. Before the International Space Station was born out of the Space Station Freedom architecture in 1993, NASA entertained proposals for a less expensive interim space station that could be finished quickly and at a fraction of the cost until funding allowed the full space station to be realized. This led to a case study known as the STS-Lab.

STS-Lab: A Shuttle Derived Space Station

Since the creation of the Saturn V launch vehicle, NASA has always favored the idea of lowering costs through the reuse or repurposing of existing flight-ready hardware. This ideology is apparent in the development of Skylab (1973) which reused Saturn V assets, and the Constellation (2005–2010) and Artemis (2019 — Present) Programs which have explored reusing Space Shuttle components to create Shuttle-Derived Heavy Launch Vehicles. A design team led by Randolph Ware, the President of the External Tanks Corporation (ETCO) and Aerospace Engineering Professor at the University of Colorado, and Phillip Culbertson, Senior Vice President of the External Tanks Corporation and former NASA Space Station Program Director and General Manager, proposed an innovative solution in 1991 that promised low-cost and rapid construction of a space station platform. The ETCO was a company focused on transforming Space Shuttle External Tanks into independent, free-flying labs using them as orbital research stations instead of discarding them after every Space Shuttle flight. This design team used these principles of Shuttle-derived technology and External Tank reuse proposing the STS-Lab as a low-cost “interim space station concept” to bridge the gap in funding and time until Space Station Freedom.

“Just as Skylab was derived from parts of the Saturn rocket, a capable cost-effective space station can be constructed from the Space Transportation System (STS).”

STS-Lab consisted of two primary modules: a modified Space Shuttle Orbiter, known as Orbiter-II, and an External Tank. The case study selected the Columbia Space Shuttle for STS-Lab’s Orbiter-II, the oldest operational Shuttle in NASA’s fleet. It would be heavily modified to include a permanent laboratory and power module inside of the cargo bay, and the External Tank would be transformed into a “wet workshop” outfitted with docking ports and internal pressurized tunnels providing a large working area for astronauts and a connection hub for visiting vehicles. The Orbiter would remain permanently attached to its External Tank, and the entire system would launch without crew as a single unit to a 400 km circular orbit. Although this seems far-fetched, NASA had done studies on the Space Shuttle’s capabilities to bring an empty External Tank into orbit and determined it was, in fact, possible. Additionally, the modifications to the Orbiter for STS-Lab would help achieve this capability, but more on that in a moment.

STS-Lab Conept with Visiting Orbiter (Ware and Culbertson, 1992)

Orbiter-II System and Weight Loss

The Space Shuttle Orbiter was a perfect starting point for the space station as it already came with many necessary components and systems “out of the box.” The Orbiter already contained existing life support systems, avionics, EVA air locks, thermal control systems, communication systems, attitude control hardware, and Remote Manipulator System (RMS) with many other existing components and capabilities that could be retained and fully utilized for the STS-Lab platform. For repairs and component placement, the Shuttle airlock provided a well-controlled method for astronauts to perform EVA missions while the RMS could handle berthing operations for supply vehicles just like the Canadarm does on the ISS today. Additionally, the Space Shuttle’s Cargo Bay provides an 18.3 m long and 4.6 m wide (60 ft by 15 ft) volume to mount additional assets and modules. Although a small living space by itself, the Orbiter contained much of the needed hardware to support a space station in a flight ready package.

To transform the Space Shuttle Orbiter into a larger, sustainable orbital research platform, modifications were needed to certain systems. For instance, Orbiter-II’s carbon dioxide removal system and environmental control systems would need to be reconfigured to improve operational efficiency and to accommodate the larger pressurized volume of the External Tank workshop. Additionally, Orbiter-II’s cargo bay would have been put to good use as it would be modified to carry the “Laboratory Module” and the Solar Powered Extended Duration Orbiter (SPEDO) package… NASA does love their acronyms.

Carrying this extra tonnage to orbit, the cargo bay modules and the entire External Tank, the Space Shuttle needed to lose some weight. In fact, the Space Shuttle needed to lose between 30,000 to 40,000 pounds! That is nearly a quarter of the Orbiter’s entire dry mass. To lose this weight, Orbiter-II, Space Shuttle Columbia, would be modified to “remove systems required only for manned ascent and for reentry and landing.” Since Orbiter-II would be launched unmanned and would remain on orbiter as a space station, much of the Space Shuttle’s structure could be removed. These unnecessary systems included the wings, tail, body flap, thermal protection tiles, landing gear, avionics equipment, crew flight controls, certain displays, and other flight hardware. In short, Space Shuttle Columbia would lose just about everything that made it a “Space Shuttle.”

Space Shuttle Atlantis During Landing (Image courtesy: NASA/Chuck Luzier)

Laboratory Module and SPEDO

The Laboratory Module would call for a structure similar to the Spacelab and Spacehab modules used in previous Shuttle missions. Additionally, the proposal mentioned the Laboratory Module could even be a Space Station Freedom Module. The lab module would be completely assembled with all necessary hardware and components on the ground before launch. Since Orbiter-II wouldn’t need to worry about being balanced for a landing, the lab module could be extended and much more massive. Completely outfitted for microgravity experiments, the Laboratory Module would have provided NASA with its much-desired research platform.

The International Space Station has eight solar arrays. Each solar array is 112 feet (34 meters) long by 39 feet (12 meters) wide and the four sets arrays can generate 84 to 120 kilowatts of electricity. Although smaller, Orbiter-II would need to generate much more power, and more sustainably, than its conventional fuel cells could manage. The SPEDO package would consist of deployable booms that would swing out once the cargo bay doors were opened on-orbit. These booms contained extendable solar arrays sized for 30 kW of power and large battery storage banks capable of supplying a minimum of 18 kW of power on average to the Orbiter across the entire orbit. Additionally, SPEDO carried its own momentum wheels to control the vehicle pitch to maintain constant sun pointing to maximize power production. Each of these systems would be fully integrated into the Space Shuttle’s existing radiator system keeping everything within normal operating temperatures while in the vacuum of space. Perhaps most impressive of all, SPEDO was designed for emergency separation meaning that the entire power system could be removed and replaced once in space.

Spacehab in Columbia Cargo Bay, STS-107 (Image courtesy:

The External Tank

The Space Shuttle External Tank is by far the largest component of the entire launch system consisting of about 5,000 cubic feet (141.5 cubic meters) of unused space. After reaching its operating orbit, the STS-Lab would vent the remaining oxygen and hydrogen to the vacuum of space. The empty tanks would then be modified on orbit and outfitted with pressurized tunnel systems to allow astronauts to pass through the structure. This system created a “wet workshop” structure allowing additional volume for astronauts to conduct operations.

The External Tank’s Intertank creates an excellent hub for connecting additional modules and visiting vehicles. The Intertank would be modified to accommodate three docking ports, each connected to a pressurized tunnel. Using a Space Shuttle External Tank as a primary structure for space station construction was a widely considered approach, but that is another article in its entirety. These docking ports would allow other Space Shuttles to rendezvous and dock with STS-Lab and provided a platform for payload modules, and even Space Station Freedom modules, to remain at the STS-Lab as a staging point for further construction and usage.

Space Shuttle External Tank (Image courtesy: NASA)

Could STS-Lab Have Replaced the ISS?

STS-Lab was a proposal, from the ground up, set on getting a low-cost, sustainable space station into Low Earth Orbit (LEO), quickly, reusing Space Shuttle technology. It aimed to reuse the older Space Shuttle Orbiter, Columbia, with Space Shuttle Main Engines (SSMEs) near their end of life. Existing equipment could be modified to augment the system like the existing Spacelab hardware and an External Tank. Most importantly, since it used flight proven Space Shuttle components, the expensive and time-consuming process of man-rating new space station modules could be largely avoided. Overall, the STS-Lab development costs were estimated to be around $3 billion USD following a fixed-price contract, and estimated to be fully operational within four years of the contract award This still would have been two years before the first ISS module was ever launched.

STS-Lab Cost Estimate ($ Billion)

It was technically feasible to develop, launch, and maintain a low-cost, Space Shuttle derived space station. Much of the needed hardware already existed and the modifications were straightforward on the flight proven Shuttle systems. Although the $3 billion estimate would most likely have been exceeded, this proposal was, and still largely is, attractive, promising an operational space station in just a few short years in comparison to the timeline of Space Station Freedom which was already seeing vast schedule overruns and budget issues.

Ultimately, STS-Lab was rejected in favor of continuing to build Space Station Freedom. By the time this proposal was presented, NASA and many organizations had already spent significant amounts of time and money to develop and certify new station modules. Additionally, the thought of losing another Space Shuttle Orbiter after Challenger (leaving the fleet to just 3 as Endeavor was still a year from flying) was unattractive. Of course, Space Station Freedom was also abandoned and transformed into the smaller and more manageable International Space Station design that used the station modules already under development. The STS-Lab was never meant to replace Space Station Freedom. It was merely meant to provide an interim LEO research platform and staging point for future Space Station Freedom construction. Space Station Freedom promised much more capability with additional future plans that required the larger design.

Although a Space Shuttle space station would have been amazing to see, perhaps it was best that STS-Lab did not get funded and built. If you’re a Shuttle fan like me, the thought of mutilating the Space Shuttle to this end, clipping its wings off and removing just about every component that made the Space Shuttle the Space Shuttle, is too horrific.


Randolph Ware, Phil Culbertson. “STS-Lab: A Low Cost Shuttle-Derived Space Station.” External Tank Corporation, 1991.

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Written by

Aerospace Engineer, CTO, and big space enthusiast. Check back here for stories about space, space exploration, and technology topics.

The Startup

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Aerospace Engineer, CTO, and big space enthusiast. Check back here for stories about space, space exploration, and technology topics.

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