From Monolithic to Modular: NASA Establishes a Baseline Configuration for a Shuttle-Launched Space Station (1970)

Modular Shuttle-launched Station in the 1980s. Image credit: NASA.
On 22 July 1969, two days after Apollo 11's triumphant landing on the Moon's Sea of Tranquillity, NASA issued a pair of Phase B Space Station study contracts. One, under the direction of NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, went to McDonnell Douglas Corporation (MDC), while the other, under the direction of the Manned Spacecraft Center (MSC) in Houston, Texas, went to North American Rockwell (NAR).

Both companies looked at 33-foot-diameter, barrel-shaped "monolithic" stations. These were designed to be launched in one piece into low-Earth orbit atop a two-stage Saturn V rocket. Both companies assumed that a logistics vehicle — commonly called a Space Shuttle — would resupply the Station, rotate its six-to-12-man crews, deliver experiment equipment and small experiment modules, and return experiment results and experiment modules to Earth.

Plan drawing of NAR's Phase B "monolithic" Space Station design. Image credit: NAR/NASA/DSFPortree.
Elsewhere in this blog (see "More Information" at the bottom of this post) I have described the monolithic Space Stations and efforts in the early 1970s to preserve a Space Station Program in the face of rapidly shrinking NASA budgets and rapidly changing national priorities. In this post, I will describe a little-known study performed in-house by NASA personnel at MSC for the NASA Headquarters Space Station Task Force. The study helped to pave the way for a sea-change in Station planning in late July 1970.

In January 1970, as negotiations toward the Fiscal Year (FY) 1971 NASA budget got under way between NASA, President Richard Nixon's White House, and the Congress, NASA Administrator Thomas Paine announced that, to accommodate proposed funding cuts, NASA's Saturn V rocket test and assembly facilities would be mothballed. He was not specific about when this would happen, stating only that it would occur after the last Saturn V ordered for the Apollo Moon program — the fifteenth — was completed and tested. That was expected to occur before the end of 1971.

The Mississippi Test Facility at Bay St. Louis, home of test stands for Saturn V engines and rocket stages, would be hardest hit; from about 2000 its staff would shrink to from 150 to 200 "caretaker" personnel. The industry publication Aviation Week & Space Technology explained in its 9 February 1970 issue that, if NASA proceeded with these Saturn V plans and then received funding for new Saturn Vs in its FY 1972 budget, it would need four years to restore its assembly and test capabilities. The first Saturn V after the last Apollo Saturn V — the sixteenth — would not launch before July 1975.

On 4 May 1970, the Space Station Task Force asked MSFC and MSC to direct MDC and NAR to devote some attention during their Phase B studies — which were set to conclude in two months — to assessing a new method of launching the Space Station: specifically, by boosting it into Earth orbit in pieces in the payload bays of Space Shuttle Orbiters. At about the same time, MSC began to organize its in-house Shuttle-launched modular Station study, which commenced officially on 1 June 1970.

One ground rule of the MSC study was that the modular Station should be able to accomplish the same research objectives as its monolithic counterpart. Another was that MSC should seek to "exploit the unique capabilities of multiple Shuttle launches."

By June 1970, NASA had, in exchange for U.S. Air Force political support, largely settled on a 15-foot-by-60-foot payload bay for its winged Shuttle Orbiter design. Engineers at its Houston center had, however, not fully reconciled themselves to these payload bay dimensions. Some sought a shorter — and sometimes wider — payload bay.

The habitat modules they considered for their Space Station during June 1970 reflected this. They looked at five modules; then, in a second round of analysis, they emphasized four. The initial five measured 12 feet in diameter by 39.5 feet long; 12 feet in diameter by 29 feet long; 14 feet in diameter by 29 feet long; 16 feet in diameter by 22.2 feet long; and 18 feet in diameter by 17.4 feet long. The four "second-pass" modules measured 12.5 feet in diameter by 30 or 40.5 feet long; 14.5 feet in diameter by 30 feet long; 16.5 feet in diameter by 23.2 feet long; and 18.5 feet in diameter by 18.4 feet long.

MSC's four "second-pass" circular floor plan Shuttle-launched Space Station Modules. Image credit: NASA with dancing stick figures by DSFPortree.
MSC's four "second-pass" horizontal floor plan Shuttle-launched Space Station Modules. In this image and the image above, the stick figures indicate the positions of the floors in the modules, not necessarily the presence of artificial gravity. Image credit: NASA/DSFPortree.
MSC looked at both "horizontal" and "circular" floor plans for the four second-pass habitat modules. The former yielded a rectangular floor and ceiling aligned with the long axis of the module. Space above the ceiling and below the floor could hold supplies, spare parts, and equipment. The latter, a stack of floors, each as wide as the module's maximum internal diameter, tended to have more floors and less equipment space.

Module design Concept Selection took place on 1 July. MSC chose a horizontal habitat module 14 feet in diameter by 29 feet long, which could launch in a 15-foot-diameter Shuttle Orbiter payload bay as short as 30 feet long. MSC assumed that the module — which it called a Basic Structural Element (BSE) — would weigh about 8000 pounds empty and about 20,000 pounds fully outfitted.

MSC then included the selected habitat module concept in six modular Space Station configurations (shown below). Five of the six would provide their crews with a weightless living and working environment. All six would feature one Solar Power Boom with a pair of two-part solar arrays, one or two Central Assembly Element (CAE) core modules with 10 docking ports each, eight BSE modules, and two Expendables Storage Element (ESE) logistics and crew carriers. MSC calculated that all six modular configurations would provide roughly the same workspace as the NAR monolithic Station design.

Illustrations of four configurations MSC considered and then put aside are labeled 1 through 4 below. X, Y, and Z axes and Station ground tracks are indicated. The designs are of two classes: the Configuration 1 and 2 BSE modules form arms and the Configuration 3 and 4 BSE modules form bundles. In Configurations 3 and 4, a single nadir-facing plus-Z BSE module is provided for Earth-observation experiments.

On 15 July 1970, MSC engineers briefed the Space Station Task Group on its progress at NASA Headquarters. They included in their presentation — which, being an interim product, contains its share of internal inconsistencies — the four designs they had put aside plus a preliminary revolving artificial-gravity baseline design with a specialized telescoping CAE (fifth image above). Most of their presentation was, however, devoted to a preliminary assembly sequence for their baseline Shuttle-launched Station configuration (bottom image above — click to enlarge).

The baseline configuration illustration includes no ESE, though the modular Station would always operate with at least one — and often two — attached to CAE Y-axis ports. Though its length was not given, the ESE was described as shorter than the CAE, BSE, and Solar Power Boom modules.

The ESE was intended as a temporary Station module — it would ride into space inside a Shuttle Orbiter payload bay, then would transfer to a Station Y-axis port under its own propulsion bearing supplies, equipment, spare parts, and astronauts. It would remain docked with the Station after the Orbiter departed. While attached to the Station, it would serve as a "pantry" or "warehouse." It would later move under its own propulsion to another Shuttle payload bay bearing experiment results and astronauts for return to Earth. The ESE was the only module designed to dock with the Station under its own power.

Shuttle Orbiters would dock the Solar Power Boom, BSE, and CAE modules with the Station. All three module types would include a docking port at either end. After reaching orbit, the Orbiter crew would pivot the module out of the payload bay and, using one of its end ports, attach it to a docking port atop the Orbiter cabin. They would then rendezvous with the Station and dock with it using the port at the module's other end. When time came to return to Earth, the Orbiter would undock from the module, leaving it attached to the Station.

MSC estimated that 14 Shuttle flights would be needed to launch and assemble its baseline modular Station. The first flight would place into Earth orbit 20,412-pound CAE 1 (labeled 1 on the drawing). CAE 1 would have nothing yet with which to dock, so it would be released directly from the Orbiter payload bay without first linking to the docking port atop the Orbiter crew cabin. It would include electricity and propulsion systems that would keep it operational until Shuttle flight 2.

The second Shuttle flight would see a 19,351-pound ESE dock with one of the two plus-Y CAE 1 ports, giving the growing Station an "L" configuration. It would provide electricity and propulsion for the CAE 1-ESE combination, and would carry food sufficient for 12 men for 90 days. It would, however, carry no astronauts.

Shuttle flight 3 would see the 19,154-pound Solar Power Boom (3) join to the CAE 1 plus-X end port (the end port nearest the docked ESE). Its solar arrays (2) would unfurl after the Shuttle Orbiter that delivered it moved away, This would block the Boom's plus-X docking port (4) and double the Station's length.

Shuttle flight 4 would place into space the first BSE, a 17,209-pound module containing the Station's main control & science data processing facilities. It would be attached to the CAE 1 minus-Y port nearest the Solar Power Boom; that is, to the Y-axis port on the opposite side of CAE 1 from the ESE. It is not shown in the baseline configuration illustration above; an arrow, however, marks the port to which the first BSE would be docked. Except for the Solar Power Boom and the one or two CAEs, the Shuttle flight 4 BSE would be the only permanent module not docked to a CAE Z-axis port.

Shuttle flights 5 through 8 would also deliver BSE modules. Module placement would alternate between minus-Z and plus-Z CAE 1 ports. A pair of robot arms on CAE 1 would aid Shuttle astronauts in safely docking the closely spaced BSEs.

Shuttle flight 5 would place in orbit a 20,605-pound BSE containing mainly life support and personal hygiene equipment (5). This would bring total Station mass to 96,731 pounds.

Shuttle flight 6 would deliver a 20,302-pound BSE outfitted with crew staterooms and communications equipment (6). Shuttle flight 7, midway through the assembly sequence, would attach to the Station a lightweight (13,367-pound) BSE containing crew recreation and dining facilities and a galley (7).

The Shuttle flight 8 module, a BSE dedicated to crew health and biomedical studies (8), would also be a lightweight (13,324 pounds). Its arrival at the Station would signal completion of one of the modular Station's two redundant, independently pressurized volumes. MSC's modular Station would at that point be equivalent to two decks, an equipment bay, and the Solar Power Boom of the NAR monolithic Station. It would weigh 143,724 pounds.

Redundant, independent volumes reflected the Station's crew safety philosophy. If one volume became uninhabitable, the entire crew could retreat to the second volume to await an Orbiter that would provide repair assistance or rescue. The modular Station would not be permanently staffed until both volumes were completed.

Shuttle launches 9 through 14 would boost into space the Station's second redundant, independent volume. It would be equivalent to a monolithic Station equipment bay and two more decks.

Shuttle flight 9 would place into space the 18,645-pound second CAE (9), the plus-X end port of which would be attached to the CAE 1 minus-X port. This would enable attachment of four more Z-axis BSEs. The MSC team did not specify whether CAE 2 would include its own pair of robot arms or if it would use the pair launched on CAE 1. Shuttle flight 10's 16,395-pound BSE would include a maintenance shop and laboratory space (10), while Shuttle flight 11's 19,024-pound BSE would contain a general-purpose lab (11).

The Shuttle flight 12 BSE would provide backup Station control & data processing (12). Like its twin delivered during Shuttle flight 4, it would weigh 17,209 pounds. The payload for Shuttle flight 13 would be a 15,756-pound BSE containing crew quarters (13).

Shuttle flight 14 would complete MSC's baseline modular Station. An Orbiter would release from its payload bay a 20,551-pound ESE containing the Station's first six long-term resident astronauts and food for 12 men for 90 days. Like the first ESE, the second ESE is not shown in the drawing above; it would, however, be attached to the CAE 2 Y-axis port marked on the drawing by a star. With the addition of the 14th Shuttle payload, Station mass would total 251,304 pounds.

MSC Baseline Shuttle-Launched Station Assembly Sequence

  1. Shuttle flight 1: CAE 1
  2. Shuttle flight 2: ESE 1 (to CAE 1 +Y port)
  3. Shuttle flight 3: Solar Power Boom (to CAE +X port)
  4. Shuttle flight 4: BSE 1 (to CAE 1 -Y port)
  5. Shuttle flight 5: BSE 2 (to CAE 1 -Z port)
  6. Shuttle flight 6: BSE 3 (to CAE 1 +Z port)
  7. Shuttle flight 7: BSE 4 (to CAE 1 -Z port)
  8. Shuttle flight 8: BSE 5 (to CAE 1 +Z port)
  9. Shuttle flight 9: CAE 2 (to CAE 1 -X port)
  10. Shuttle flight 10: BSE 6 (to CAE 2 -Z port)
  11. Shuttle flight 11: BSE 7 (to CAE 2 +Z port)
  12. Shuttle flight 12: BSE 8 (to CAE 2 -Z port)
  13. Shuttle flight 13: BSE 9 (to CAE 2 +Z port)
  14. Shuttle flight 14: ESE 2 (to CAE 2 -Y port)
The image at the top of this post (click to enlarge) shows MSC's modular Station as it would appear by the mid-1980s. It would include at least five more BSEs than the baseline configuration. Four would link to the Z-axis ports of a third CAE attached to the CAE 2 minus-X port.

In the painting, an ESE makes an appearance: it includes a pair of robot arms. One of the four BSE modules attached to CAE 3 is a dedicated Earth-observation module (an open round end-hatch and extended instruments are visible below the ESE arms).

Two BSEs are shown attached to Y-axis CAE ports; one is the BSE delivered during Shuttle flight 4 (it is displayed here attached to the CAE 1 plus-Y port nearest the Solar Power Boom rather than the minus-Y port), while the other, attached to a CAE 2 minus-Y port, is probably a temporarily docked free-flyer with an independent propulsion system. This would detach from the Station periodically to provide a stable platform for materials science and astronomy experiments; such experiments could be adversely affected by vibration caused by crew movement within the Station.

The approaching Shuttle Orbiter is an MSC design with straight wings a little more than 90 feet across, internal liquid oxygen and liquid hydrogen tanks, twin main engines, and a payload bay shorter than 60 feet. It bears atop its crew compartment, attached to its docking port, a BSE module - probably another freeflyer, or perhaps a temporary attached lab - bound for a CAE Y-axis port.

After its 15 July presentation at NASA Headquarters, the MSC team apparently halted its activities. The artificial-gravity baseline design, for example, seems not to have been developed further. I have found no evidence that briefings scheduled for 1 August and 7 September at MSC and 15 September at NASA Headquarters actually took place.

NASA extended the NAR and MDC Space Station Phase B contracts by six months on 30 June 1970. On 29 July 1970, Charles Mathews, chair of the Space Station Task Force, requested that MSC and MSFC direct their respective Phase B contractors to expend more effort to study Shuttle-launched modular designs. This direction became formal on 1 February 1971, when NAR and MDC began Phase B Extension studies almost entirely focused on modular designs. When unveiled in late 1971, the NAR modular design resembled the baseline design from MSC's May-July 1970 in-house study.


BSE = Basic Structural Element
CAE = Central Assembly Element
ESE = Expendables Storage Element
MDC = McDonnell Douglas Corporation
MSC = Manned Spacecraft Center
MSFC = Marshall Space Flight Center
NAR = North American Rockwell


Shuttle-Launched Space Station Study Interim Review, NASA Manned Spacecraft Center presentation to NASA Headquarters, 15 July 1970.

"Curtailing Field Centers Limits Saturn 5 Options," Aviation Week & Space Technology, 9 February 1970, pp. 26-27.

"Space Station and Space Platform Concepts: A Historical Overview," J. Logsdon and G. Butler, History of Space Stations and Space Platforms - Concepts, Designs, Infrastructure, and Uses, I. Bekey and D. Herman, editors, Volume 99, Progress in Astronautics and Aeronautics, American Institute of Aeronautics and Astronautics, 1985, pp. 226-233.

Space Shuttle: The History of the National Space Transportation System - The First 100 Flights, Third Edition, D. Jenkins, Specialty Press, 2008, pp. 101-108, 137.

More Information

A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)

An Alternate Station/Shuttle Evolution: The Spirit of '76 (1970)

Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1970)

McDonnell Douglas Phase B Space Station (1970)

Think Big: A 1970 Flight Schedule for NASA's 1969 Integrated Program Plan

"A True Gateway": Robert Gilruth's June 1968 Space Station Presentation


  1. I want both. Huge HLLV launched hubs with modular ring segments

  2. You'll get no argument from me! :-)


  3. The ESE reminds me of the Leonardo module used with the International Space Station. The ESE has capabilities Leonardo lacks, but the concepts seem similar.

  4. Indeed they do seem similar. The ESE is a smaller version of a functionally similar module proposed during the 1969-1970 Phase B Station studies. MDAC called their version of that the "Crew/Cargo Module." I write about it here - - which is the URL for the fourth "For More Information" item above.

    The MDAC module was a relative of 1962 Gemini and the Gemini modular stations - Their One-Man Station structure could be outfitted to carry cargo and remain attached as a "pantry/warehouse" module.

    In the early 1960s, there was another approach - NASA expected to have Apollo CSMs available for its station program, so developed a six-man CSM with cargo capability. I wrote about that here - At that point, cargo was going to be moved in small containers from external compartments in the SM.

    For Mir and ISS, we've seen version of both approaches - that is, both item transfer and "pantry/warehouse" modules. The Destiny node doubled as a cargo carrier, so I guess the rule is, get supplies in any place you can!


  5. You are always very giving with information. Thank you for that.

  6. Phillip:

    You're welcome. I love to talk about spaceflight - especially with knowledgeable and enthusiastic people such as yourself. I could do that all day. One reason my current archives job (which I'll have been at for 10 years in October) is growing a bit stale is that I spend too much time *not* talking with people. It's great to dig through old records, discovering fascinating things, don't get me wrong. I'd like to present that info to people, however, and so far my efforts in that direction haven't been too satisfying. Oh, well. We shall see what happens.



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