NASA Johnson's Plan to PEP Up Shuttle/Spacelab (1981)

Early artist concept of a Space Shuttle Orbiter with a "Sortie Lab" at the front of its Payload Bay. The Sortie Lab pressurized module is shown as a cutaway illustration. At this point in its history, the Sortie Lab was expected to be manufactured by a U.S. aerospace contractor. The Sortie Lab depicted is dedicated at least partly to astronomy, as evidenced by the large telescope attached to the aft end of its pressurized module. Image credit: NASA.
On 29 November 1972, NASA Administrator James Fletcher abolished the Space Station Task Force formed in early 1969 by his predecessor, Thomas Paine, and formed the Sortie Lab Task Force. The "Sortie Lab," a concept that emerged during Phase B Space Station planning in 1970, was envisioned as a pressurized laboratory module which would be carried in the Shuttle Orbiter's Payload Bay.

Fletcher's move acknowledged that the Space Shuttle, conceived originally as a vehicle for transporting crews and cargoes between Earth and an Earth-orbiting Space Station at low cost, would need to become a Space Station — or, at least, an interim space laboratory that could demonstrate that a Space Station would be a desirable new NASA goal after the Space Shuttle became operational.

Strapped for funds and encouraged by President Richard Nixon to use spaceflight as a vehicle for international cooperation, NASA asked the European Space Research Organization (ESRO), a predecessor of the European Space Agency (ESA), to provide the Sortie Lab in exchange for European astronaut flights on board the Shuttle. In August 1973, ESRO and European aerospace contractors agreed to build the Sortie Lab, which became known as Spacelab.

Cutaway illustration of a drum-shaped, ESA-built Spacelab module (center) with a pair of U-shaped Spacelab pallets (left). A bent tunnel with an airlock on top for spacewalks (note space-suited astronaut atop pallet at left) links Spacelab with the Shuttle Orbiter Mid-Deck, the main living space for the crew. Above that is the Flight Deck, the Orbiter cockpit. Image credit: NASA.
Spacelab would provide scientists with ample pressurized volume in which to conduct research, but it would rely on limited resources — for example, electricity — provided by the Shuttle Orbiter. Orbiter electricity came from a trio of liquid oxygen/liquid hydrogen fuel cells that in early 1981 were expected to generate 21 kilowatts continuously for just seven days. Of this, 14 kilowatts were required for Orbiter systems. The Orbiter could thus supply only seven kilowatts to Spacelab. Of those seven kilowatts, between two and five kilowatts would be needed for basic Spacelab systems, leaving a paltry two to five kilowatts for Spacelab experiments.

In 1978, NASA Johnson Space Center (JSC) in Houston, Texas, launched the Orbital Service Module Systems Analysis Study, which looked into ways that the Space Shuttle Orbiter could be augmented to enable it to better support Spacelab research. An early product of the study was the Power Extension Package (PEP) concept.

Stowed PEP components in the Space Shuttle Orbiter Payload Bay, between the front of a Spacelab module (right) and the rear of the Orbiter crew cabin. Image credit: NASA.
The PEP deployed in orbit. PEP displays and controls were meant to be located on the Shuttle Orbiter Flight Deck. Image credit: NASA.
The PEP concept was linked with NASA's extensive efforts in cooperation with the U.S. Department of Energy to justify the construction of enormous Earth-orbiting Solar Power Satellites (SPSs). It was portrayed as an experience-building experimental test-bed for SPS technology in the Von Karman Lecture JSC director Christopher Kraft presented to the 15th meeting of the American Institute of Aeronautics and Astronautics in July 1979. The PEP may also have been conceived as a rival for NASA Marshall Space Flight Center's Power Module (see "More Information" below).

The PEP Project Office (PEPPO) at JSC pitched the PEP in a brief report published one month before the first Space Shuttle flight (STS-1, 12-14 April 1981). The PEPPO envisioned the PEP as a "kit" that could be installed easily in the Shuttle Orbiter Payload Bay over the tunnel that would link the Orbiter Mid-Deck with the Spacelab pressurized module.

One hour after launch from Earth, an astronaut on the Orbiter Flight Deck would use the Canada-built Remote Manipulator System (RMS) robot arm to grapple the PEP's Array Deployment Assembly (ADA) and extend it out over the Orbiter's side. The ADA would then unroll a pair of lightweight solar array wings that together would measure more than 100 feet wide. PEP deployment would require about 30 minutes.

The PEP arrays would track the Sun automatically no matter how the Orbiter became oriented, so almost no astronaut intervention would be needed after they were deployed. The RMS and arrays would be sufficiently sturdy to remain deployed during Orbiter attitude-control maneuvers, but the crew would need to stow them before Orbital Maneuvering System burns lest the acceleration cause damage.

The twin arrays would generate a total of 26 kilowatts of electricity. A cable built into the RMS would carry the electricity from the ADA to the PEP's Power Regulation and Control Assembly (PRCA) in the Payload Bay. The PRCA would then distribute it to the Orbiter's electrical system.

The three Orbiter fuel cells would "idle" while the PEP arrays were in sunlight. Each would generate one kilowatt of electricity, bringing the total available on board to 29 kilowatts. Fifteen kilowatts would be available for Spacelab, of which between 10 and 13 kilowatts could be devoted to experiments.

Keeping the Spacelab electricity supply constant throughout each 90-minute orbit of the Earth would require that Orbiter fuel cell output ramp up rapidly from three to 29 kilowatts as the PEP arrays passed into darkness over Earth's night side. To achieve this output, each fuel cell would need to exceed its normal maximum by nearly three kilowatts. The fuel cells would then return to their idle state as the PEP arrays passed again into sunlight. Although it would almost certainly place unusual demands on the Orbiter fuel cells, the PEPPO judged this approach to be "feasible."

The PEPPO estimated that a PEP could extend Shuttle/Spacelab endurance in Earth orbit by four days (that is, to a total of 11 days). If other Orbiter resources (for example, life support consumables) could be augmented, then mission duration might be stretched to 45 days.

The PEPPO explained that it jointly managed PEP solar cell development with NASA's Lewis Research Center. Industry involvement in the PEP project was, it added, already "extensive," with several companies working on small NASA contracts or funding PEP-related work themselves. It estimated that the PEP could power a Spacelab module in Earth orbit as early as 1985 for a total development cost of only $150 million.

Spacelab 1 in Columbia's Payload Bay during STS-9 as viewed from the Flight Deck windows. Cables linking the Orbiter to Spacelab 1 are visible at lower right. Image credit: NASA.
The first Spacelab, appropriately designated Spacelab 1, reached orbit in the Payload Bay of the Orbiter Columbia on 28 November 1983, as part of the ninth Shuttle mission. Columbia's crew for mission STS-9 included ESA's Ulf Merbold, the first non-U.S. astronaut to reach space on board a U.S. spacecraft. Merbold was part of a six-man crew that also included Gemini, Apollo, and Shuttle veteran John Young, Skylab 3 veteran Owen Garriott, and spaceflight rookies Brewster Shaw, Robert Parker, and Byron Lichtenberg. Columbia landed at Edwards Air Force Base, California, on 8 December, ending a busy 10-day mission.

Columbia's fuel cells powered Spacelab 1, and all of the 27 Spacelab missions that followed relied on Orbiter fuel cells for their electricity. PEP work had ended in late 1981 as NASA Headquarters took charge of and terminated Shuttle augmentation and Space Station development efforts across the agency. It did this in part to clear the decks as it began formally to seek approval for a Space Station, which it billed as the "next logical step" after the Space Shuttle. President Ronald Reagan called on Congress to approve new-start funding for a Space Station during his annual State of the Union address in January 1984, less than two months after STS-9. 


Power Extension Package (PEP) Concept Summary, JSC-AT4-81-081, NASA Johnson Space Center, PEP Project Office, March 1981.

The Solar Power Satellite Concept, NASA JSC 14898, Christopher C. Kraft; Von Karman Lecture, 15th Annual Meeting of the American Institute of Astronautics and Aeronautics, July 1979.

"Spacelab joined diverse scientists and disciplines on 28 Shuttle missions," Science@NASA, 15 March 1999 ( - accessed 25 March 2017).

More Information

Electricity from Space: The 1970s DOE/NASA Solar Power Satellite Studies

Evolution vs. Revolution: The 1970s Battle for NASA's Future


  1. I always thought the shuttles should have been solar powered. That could have extended the shuttles on orbit time, empty fuel cells were the main hindrance for the short shuttle flight if memory serves me well.

    1. I'm not sure why NASA never relied on solar power during a Shuttle flight. I suppose a large solar array might have been considered fragile enough to be damaged during maneuvers, or perhaps Orbiter attitude constraints were judged to be too limiting. There's also the possibility that a solar-powered Orbiter might have been seen as too self-sufficient, perhaps undermining the argument in favor of a Space Station (that's wholly speculation on my part; please don't quote me).


  2. While I defer to David on the actual history, I can imagine a number of possible reasons (remembering that the design was finalized in the early 1970s):

    Fuel cells were simple, proven and sufficient for what the shuttle was expected to do. Orbiters weren't intended for long-duration flights, and if it were later decided that this capability was desired, the necessary equipment could be carried as payload as needed. It didn't need to be designed into the orbiter as such.

    On the other hand: Deploying and stowing large solar arrays in flight had never been demonstrated. It introduced potentially mission-killing failure modes (as seen with Skylab's OWS panels). Orbiters would never leave LEO, so solar panels would be dead weight half the time, and the orbiter would need an alternative power source or battery system, which adds mass.

    Basically, solar panels are a lousy way to power a large spacecraft UNLESS it's going to be up there long enough to make fuel/battery mass an issue. People much smarter that me presumably did the math and figured out that the orbiter would rarely (if ever) be up there long enough to justify making solar power a design feature.

  3. CPK:

    I always like it when people who are smarter than me step in and answer questions - thanks!

    This brings to mind a couple of things. First, why would the JSC folks propose and pursue the PEP, given the constraints you mention? I expect it had to do with Chris Kraft's obsession with Solar Power Satellites. He wanted to make some progress in that direction, but was limited in what he could do. The PEP study, performed in-house, was one thing he could do which, if it could gain support, could provide an incremental step toward SPS.

    Also, as I mention in the post, the long-standing rivalry between MSFC and JSC might have played a role. The "Evolution vs. Revolution" post linked at the end of this post gets into that in more detail.

    JSC "alliance" with LeRC - also mentioned in the post - is not surprising because LeRC had become for all practical purposes NASA's lead center for solar power. The PEP arrays would have been just about right for powering an electric-propulsion Comet Halley rendezvous mission, which was a high priority for LeRC (in alliance with JPL) at this time. I haven't seen PEP tied to Comet Halley before, but I suspect that there's an indirect link.

    Finally, the USAF space plane currently orbiting the Earth has a solar array. Though it is in some ways Shuttle-like, it proves your point. It is intended for long-duration orbital flight, and there's no way it could carry enough reactants. So a solar array is a requirement. (Also it's small, so its array can be small, and small arrays tend to be less problematic than large ones.)

    Thanks again for your comment.



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