In 1968, as Apollo neared its culmination - the first manned landing on the moon - various organizations within NASA and their contractors sought to chart the U.S. civilian space program's post-Apollo future. Three underlying cost-cutting approaches guided much of their work.

The first was re-application of hardware developed for Apollo. This was the approach proposed for the Apollo Applications Program (AAP), which had been endorsed by President Lyndon B. Johnson in 1964-1965. AAP was rapidly shrinking in 1968, following a half-billion dollar cut in its budget in August 1967.

The second approach to NASA's post-Apollo future was reusability. AAP largely rejected this option, though it did explore reuse of the Apollo Command Module (CM) capsules that would be used to transport AAP crews from Earth's surface to space and back again. Reusability became the basis for the Space Shuttle Program, Apollo's eventual successor. This approach has, however, not been commonly used in space programs.

The third approach was commonality; that is, the development of a suite of common-design spacecraft modules that could be combined in different ways to achieve diverse post-Apollo space goals. The approach reached its grandest expression in the Integrated Program Plan (IPP) proposed in 1969-1970.

The IPP was not the first time that space planners had invoked the principle of commonality, nor would it be the last. In late 1967-early 1968, for example, Charles Davis and Joseph Tschirgi, with Bellcomm, NASA's Washington, DC-based Apollo planning contractor, proposed a "Common Space Fleet" based on modified and upgraded Saturn V rockets and new-design hardware evolved from Apollo spacecraft systems. The Common Space Fleet would, they wrote, enable all the piloted missions that NASA was likely to be called upon to perform until about 1990.

Most Common Space Fleet hardware would reach Earth orbit on two variants of the Apollo Saturn V rocket, Davis and Tschirgi wrote. The first, the "product improved Saturn V," would be essentially identical to the Apollo Saturn V. It would comprise an S-IC first stage, an S-II second stage, and an S-IVB third stage, all with only modest uprating. The second Saturn variant, the INT-20, would employ an S-IC as its first stage and an S-IVB as its second.

The INT-20 would be more powerful than the two-stage Saturn IB rocket used in Apollo Earth-orbital tests but less powerful that the Saturn V. It would, like the Saturn IB, be used mainly to launch crews into Earth orbit. Though Davis and Tschirgi did not attempt to justify their decision to replace the Saturn IB, their rationale was almost certainly mainly economic. The Saturn IB would require manufacture of a fourth type of Saturn rocket stage, its S-IB first stage, which would play no other role in the Common Space Fleet, as well as continued operation of the Complex 34/Complex 37 Saturn IB launch pads. The INT-20, on the other hand, would comprise two Saturn V rocket stages and could launch from the same Complex 39 launch pads as the Saturn V.

An illustration in their paper also showed a Saturn V variant comprising an S-IC and an S-II; this Saturn variant, known as INT-21, resembled the Saturn V used to launch the Skylab Orbital Workshop, the last vestige of AAP, in May 1973 (image at top of post). The Bellcomm engineers envisioned using it to launch a Common Space Fleet "space base" into Earth orbit.

Cutaway drawing of Propulsion Module-I. Image: Bellcomm/NASA

The Common Space Fleet would include two chemical propulsion modules (nuclear propulsion, Davis and Tschirgi noted, was unlikely to be needed before the 1990s). The first and largest, Propulsion Module-I (PM-I), would burn liquid oxygen/liquid hydrogen propellants and have a gross mass of 140,000 pounds. It would include one main rocket motor, eight spherical liquid oxygen tanks, one liquid hydrogen tank, four liquid oxygen/liquid hydrogen-fueled docking propulsion modules with four small motors each, and a ring-shaped Instrument Unit containing avionics. Equipped with lunar landing gear, it would be capable of placing more than 21.5 tons of cargo on the moon.

PM-I would in addition be used to perform major maneuvers during manned planetary missions. Major maneuvers would include Earth departure, capture into planetary orbit, and planetary orbit departure for return to Earth. Davis and Tschirgi proposed also that PM-I be used to launch robotic planetary probes from Earth orbit and for "general maneuvering propulsion in cislunar space for rescue missions," though they did not describe these uses in any detail.

The smaller drum-shaped Propulsion Module-II (PM-II) would have a gross mass of 25,000 pounds with its tanks full of methane and exotic high-energy fluorine-oxygen propellants. The 12.5-foot-diameter, 12.2-foot-long module would have even more uses in Common Space Fleet missions than would PM-I. Envisioned as an advanced rocket stage, PM-II would include "high-performance" thermal insulation to prevent its propellants from boiling and escaping, removable attitude control system rocket motor clusters, and three rocket motors, each with a retractable engine bell ("skirt") measuring three feet in diameter.

The Common Space Fleet would include two kinds of crew modules. The largest would be the Common Mission Module (CMM). This would comprise a pair of drum-shaped single-deck modules stacked one atop the other, with a 21.7-foot-diameter cylindrical shroud encasing and linking them together. One of the single-deck modules, the living quarters module, would serve as "home," Davis and Tschirgi wrote, while the other, the command and operations module, would serve as "office."

A tunnel with hatches at either end would link the decks. Each would include beneath its floor a "crawlspace" containing life support and other systems. Either deck could support the crew in the event that the other became uninhabitable. A pair of lozenge-shaped Brayton-Isotope nuclear power systems would protrude from the CMM's side; if one failed, the other would be sufficient to power the mission.

Cross-section of two-deck Common Mission Module. Image: Bellcomm/NASA Cross-section of two-deck Common Mission Module. Image: Bellcomm/NASA

The two-deck CMM could support four men for two years with no resupply, Davis and Tschirgi estimated. For shorter lunar missions and missions with smaller crews, a single-deck CMM could be used. A second living quarters module could be stacked onto a standard two-deck CMM (thus creating a three-deck CMM) if an eight-man mission were contemplated.

Davis and Tschirgi's second crew module was the conical, four-man Earth Depart and Entry Module (EDEM) outwardly resembling the Apollo CM. It would be capable of remaining dormant for up to two years while docked with an interplanetary spacecraft or space station or parked on the moon, and of operating for two weeks as an independent spacecraft. This would give astronauts ample time to rendezvous with and return from an Earth-orbiting space station, to reach and return from the moon, or to separate from an interplanetary spacecraft and reenter Earth's atmosphere at a speed of up to 55,000 feet per second (15,000 feet per second faster than the Apollo CM) at the end of an interplanetary voyage.

All astronauts would lift off from Earth inside an EDEM; in the event of a launch vehicle failure, an abort system would boost EDEM and astronauts to safety. When a lunar or interplanetary mission departed from Earth orbit, the astronauts would ride within an EDEM attached to a PM-II. If a malfunction during Earth-orbit departure forced the astronauts to abandon the lunar or interplanetary spacecraft, then they would detach from it in the EDEM/PM-II combination and fire the PM-II's engines to decelerate and return to Earth.

Davis and Tschirgi described launch vehicle, PM, and crew module combinations for four Common Space Fleet missions. The simplest was the aforementioned INT-21-launched space base. On the launch pad, the Common Space Fleet modules would be arranged as follows (top to bottom): an EDEM, a PM-II, a two-deck CMM, and a drum-shaped experiments compartment (the last being not a standard Common Space Fleet module). Upon arrival in Earth orbit, the Saturn INT-21 S-II stage would separate, then the EDEM with its attached PM-II would detach from the CMM, turn, and dock nose-first with the CMM. The four-man crew would then enter the CMM to live and work on board the space base.

Although they provided no details concerning long-term space base operations, Davis and Tschirgi implied that space bases would support consecutive crews and be resupplied. Crews would reach space bases on Saturn INT-20 launch vehicles in EDEMs attached to PM-II propulsion modules. Unmanned INT-20-launched EDEM/PM-II combinations would deliver supplies and equipment, as might unspecified spacecraft launched on Titan-III rockets. Bell and Tschirgi offered no details concerning the Titan-III cargo launch option.

Next most complex was the Common Space Fleet lunar base mission, which would require a pair of uprated three-stage Saturn V launch vehicles. The payload for Launch Vehicle (LV)-1, which would lift off without a crew, would comprise a single-deck CMM atop a PM-I with lunar landing gear. A streamlined nosecone would cover the CMM's top, which would carry lunar surface exploration equipment and a crane for lowering it to the lunar surface. LV-2's payload would comprise an EDEM bearing the four-man crew, a PM-II, and a PM-I with lunar landing gear. The LV-1 lander would descend to an automated landing, then the crew would arrive in the LV-2 lander. When the time came for the astronauts to return home, they would ignite the LV-2 lander's PM-II to launch their EDEM directly back to Earth.

Third was the multi-planetary flyby mission, which would need three uprated Saturn V rockets. LV-1 would launch a nosecone and a PM-I. LV-2 would launch the crew in an EDEM, a PM-II, and a two-deck CMM. LV-3 would launch a nosecone, a compartment outwardly similar to the space base experiment compartment holding automated planetary probes for release at the target planets, and a second PM-I. The separately launched Common Space Fleet modules would dock in Earth orbit to form a single manned planetary flyby spacecraft. This would comprise (fore to aft) the EDEM, the PM-II, the CMM, the planetary probe compartment, and the second and first PM-Is.

Earth-orbit departure would expend the two PM-Is in turn, then the EDEM/PM-II combination would detach, turn, and dock with the CMM/planetary probe compartment. The PM-II would be used to perform any necessary course corrections. Near the end of the mission, as the flyby spacecraft neared Earth, the EDEM/PM-II would undock from the CMM. The PM-II would ignite its engines to slow the EDEM to a safe Earth-atmosphere reentry speed (if necessary), then would be discarded. The EDEM would reenter Earth's atmosphere and land.

The most complex Common Space Fleet mission Davis and Tschirgi proposed was the Mars or Venus landing or orbital mission. It would need four or five uprated Saturn V launchers depending on the launch opportunity used. LV-1 and LV-2 would both launch a nosecone and a PM-I. LV-3 would launch an EDEM, a PM-II, and a two-deck CMM. LV-4 would launch a nosecone, a compartment holding automated planetary probes or a manned Mars lander, and a PM-I. If four PM-Is were needed, then a fifth Saturn V would launch a payload identical to that of LV-1/LV-2.

The modules would dock in Earth orbit to form a spacecraft comprising (from fore to aft) the EDEM with attached PM-II, the two-deck CMM, the planetary probes/manned Mars lander compartment, and the three or four PM-Is. One or two PM-Is would be expended to push the spacecraft out of Earth orbit. The EDEM/PM-II would then separate, turn, and dock with the CMM. Upon arrival at its destination planet, one PM-I would be expended to place the remaining PM-I, the probes/manned lander compartment, and the EDEM/PM-II into orbit. The last PM-I would place the spacecraft on course for Earth. Near Earth, the EDEM/PM-II would undock from the CMM. The PM-II would slow the EDEM for reentry into Earth's atmosphere, then would be discarded. The EDEM would then reenter Earth's atmosphere and land.

Davis and Tschirgi wrote that the Common Space Fleet would at a minimum be capable of manned missions to any destination between half of Earth's distance from the Sun (that is, between the orbits of Venus and Mercury) and two times Earth's distance from the Sun (beyond the orbit of Mars). This would take in Venus, Mars, and the moon. Ideally, however, Common Space Fleet capabilities would be extended to permit missions to destinations from between one-quarter of the Earth-Sun distance to beyond Jupiter. The expanded range would, they wrote, make available before 1990 the option of U.S. manned flyby missions to Mercury, asteroids in the Main Belt between Mars and Jupiter, and Jupiter and its family of moons.

Reference:

The Common Space Fleet - a Brief Description, Case 730, C. Davis and J. Tschirgi, Bellcomm, March 1968.