During its first dozen years, the U.S. piloted space program pursued an evolutionary course, with simple missions and spacecraft leading to more complex and capable ones. Single-man Mercury suborbital missions led to Mercury orbital missions of increasing duration, then in 1965-1966 two-man Gemini missions progressively added maneuverability, an ability to rendezvous and dock, spacewalk capability, and flight durations of up to 14 days.

Next came Apollo, which saw four piloted non-landing preparatory missions in 1968-1969 ahead of the first lunar landing attempt. Apollo 7 (September 1968) tested the Command and Service Module (CSM) in Earth orbit. As in biological evolution, contingency played a role; Apollo 8, intended originally as a high-Earth-orbital test of the CSM and the Lunar Module (LM) moon lander, became a CSM-only lunar-orbital mission after the LM was delayed and the Soviet Union appeared close to launching a cosmonaut around the moon. The Apollo 8 CSM orbited the moon 10 times on 24 December 1968. Apollo 9 saw the first Earth-orbital test of the LM and CSM. Apollo 10 (May 1969) was a dress-rehearsal in low-lunar orbit for Apollo 11 (July 1969), the first piloted lunar landing.

Apollo 11 is best understood in an engineering context: it was a cautious end-to-end test of the Apollo system with a single two-and-a-half hour moonwalk and only limited science objectives. Apollo 12 (November 1969) demonstrated the pin-point landing capability required for pre-mission geologic traverse planning by setting down near a known point on the moon: specifically, the Surveyor III automated soft-lander, which had landed in April 1967. It also saw a pair of moonwalks lasting nearly four hours each and deployment of the first Apollo Lunar Scientific Experiment Package (ALSEP).

Apollo 13 (April 1970) suffered a crippling explosion midway to the moon, scrubbing its lunar landing, but its crew's safe return to Earth demonstrated the Apollo system's maturity and the Apollo team's experience. Apollo 14 (January-February 1971) included two science-focused moonwalks, each lasting more than four-and-a-half hours. They included a strenuous 1.3-kilometer trek through the hummocky ejecta blanket surrounding 300-meter-wide Cone Crater.

Apollo 15 (July-August 1971), Apollo 16 (April 1972), and Apollo 17 (December 1972), designated "J" missions, featured a host of evolutionary improvements. Beefed-up LMs permitted surface stay times of up to three days at complex and challenging landing sites, larger returned lunar samples, and more complex ALSEPs. Space suit improvements and the Lunar Roving Vehicle enabled geologic traverses ranging over kilometers of the lunar surface. Each "J" mission CSM included a suite of sensors which its pilot could turn toward the moon while his crewmates explored the surface.

As early as 1962, engineers foretold two evolutionary paths for Apollo space technology after it had accomplished President John F. Kennedy's goal of a man on the moon. The engineers were in part guided by President Lyndon Baines Johnson's 1964 declaration that NASA's space program after the moon landing should be based on Apollo hardware. One path would see moon missions continue more or less indefinitely, growing ever more capable and culminating in a permanent lunar base in the 1980s. Alternately, NASA might repurpose Apollo hardware for an evolutionary space station program in Earth orbit.

The space station path appeared pedestrian compared to the lunar path, yet it offered greater potential for long-term future exploration. This was because it promised to prepare astronauts and spacecraft for long-duration missions beyond the moon. In 1965-1966, NASA advance planners envisioned a series of Earth-orbiting space workshops based on the Apollo LM and the Saturn IB rocket S-IVB stage. Apollo CSMs would ferry up to six astronauts at a time to the workshops for progressively longer stays.

Some planners thought that NASA should jump straight from the early space workshops to nuclear-propulsion piloted Mars landing missions, but others called for a continuation of the evolutionary approach. If these conservative engineers had had their way, the mid-1970s would have seen a new-design space station climb to Earth orbit on top of an improved Saturn V rocket. Derived from Apollo hardware and new technology tested on board the orbiting workshops, it would have constituted a prototype interplanetary mission module (image at top of post). A crew might have lived on board it for almost two years to help to prepare NASA for its first piloted interplanetary voyage.

In keeping with the evolutionary approach, that first piloted voyage beyond the moon might have been a Mars flyby with no landing. It might have commenced as early as the late 1975, when a minimum-energy opportunity to launch a Mars flyby would take place. As they raced past Mars in early 1976, the flyby astronauts would have released automated probes and operated a suite of sensors. They would have reached their greatest distance from the Sun in the Asteroid Belt. As their Sun-centered elliptical orbit brought them back to Earth's vicinity in 1977, they would have separated in an Apollo CSM-derived Earth-return spacecraft, fired its engine to slow to a safe reentry speed, and reentered Earth's atmosphere in its conical capsule.

In addition to observing Mars, the astronauts would have continued the effort, begun during Gemini flights and continued on the Earth-orbiting workshops and prototype interplanetary mission module, to determine whether piloted spaceflights lasting years were medically feasible. The flyby crew might have found, for example, that artificial gravity is a must in interplanetary space. Their results would have shaped the next mission in spaceflight evolution, which might have taken the form of a piloted Mars orbiter in the spirit of Apollo 8 and Apollo 10, or, if the space agency felt sufficiently confident in its abilities, an orbital mission with a short piloted Mars surface excursion in the spirit of Apollo 11.

In January 1966, United Aircraft Research Laboratories engineer R. R. Titus unveiled a proposal for a new step in spaceflight evolution. He dubbed it FLEM, which stood for "Flyby-Landing Excursion Mode." FLEM missions would, Titus wrote, occur naturally in the evolutionary sequence between piloted Mars flybys and piloted Mars orbiters. FLEM might even have become the basis for an early brief manned Mars landing.

Titus explained that, in the "standard stopover mode," all major maneuvers would involve the entire Mars spacecraft. This meant that it would need a large mass of propellants, which in turn meant that many expensive heavy-lift rockets would be required to launch the spacecraft, its propellants, and its propellant tanks into Earth orbit for assembly. Propellant mass would vary greatly from one Earth-Mars transfer opportunity to the next because Mars has a decidedly elliptical orbit. Because of this, the Mars spacecraft and the sequence of launches needed to boost its components and propellants into Earth orbit would have to be redesigned for each standard stopover Mars mission.

The United Aircraft engineer added that errors or malfunctions during the standard stopover's "high-risk" Mars capture and escape maneuvers could yield "complete mission failure" because the entire ship would be affected. Because the Mars spacecraft would be very massive already, it would be difficult and costly to include extra propellants to enable a mission abort.

He noted that required propellant mass might be reduced and made more equal over multiple transfer opportunities if the spacecraft skimmed through Mars's atmosphere to slow down so that the planet's gravity could capture it into orbit (that is, if it performed aerocapture). If, however, artificial gravity were found to be necessary for crew health, then packing an artificial-gravity system behind an aerocapture heat shield would probably prove infeasible.

Titus explained that his FLEM concept, in addition to being a natural evolutionary extension of piloted Mars flybys, would address many of the standard stopover mode's inherent problems. He envisioned a two-part chemical-propulsion FLEM spacecraft with a total mass low enough that it could reach Earth orbit on two Saturn V rockets. Assembly would thus be limited to one docking between the two Saturn V payloads.

Photographer: Sage Sohier

Title: "Couple With Tattoos, Sandwich, New Hampshire" Year: 2004 Series name: Perfectible Worlds Flyby-Landing Excursion Module (FLEM) mission schematic. 1. The two-part FLEM spacecraft departs Earth orbit. 2. The excursion module and parent space separate. A rocket burn occurs to enable the excursion module to reach Mars ahead of the parent spacecraft. 3. The excursion module captures into Mars orbit using rockets or aerocapture. A period of Mars exploration, possibly including a brief piloted Mars landing, ensues. 4. The excursion module departs Mars to rendezvous and dock with the parent spacecraft. 5a. The parent spacecraft ignites rocket motors to perform a powered Mars flyby that bends its course toward Earth. 5b. The excursion module performs rendezvous and docking with the parent spacecraft. After crew transfer, it is cast off. 6. The parent spacecraft returns to Earth. A small capsule bearing the crew separates, reenters the atmosphere, and lands. Note: The smiley Sun is on the original drawing, proving perhaps that FLEM's author had a sense of humor. Image: United Aircraft Research Laboratories/David S. F. Portree.

One part of the FLEM spacecraft, the parent spacecraft, would not capture into Mars orbit. It might include a spinning artificial gravity system. The other part, the excursion module, would capture into Mars orbit using chemical rockets or, perhaps, by skimming through Mars's atmosphere behind an aerocapture heat shield.

Titus noted that Earth-Mars transfer opportunities that required less propulsion for Earth departure would arrive at Mars moving quickly, while opportunities that required more propulsion for Earth departure would arrive at Mars moving slowly. In the former instance, the excursion module would need a large quantity of propellants to slow down enough for Mars's gravity to capture it into orbit, so would need to be the more massive of the two FLEM spacecraft. Because of this, the lower-mass parent spacecraft would ignite its rocket motors to slow down so that the excursion module could reach Mars first. In the latter case, the excursion module would not need a large mass of propellants to capture into Mars orbit, making it the less massive of the two FLEM spacecraft. It would thus speed up to reach Mars ahead of the more massive parent spacecraft.

Titus calculated that separation 60 days ahead of the Mars flyby would enable the excursion module to reach the planet 16 days ahead of the parent spacecraft; separation 30 days before flyby would enable it to reach Mars while the parent spacecraft was nine days out. While it awaited arrival of its parent, the excursion module might remain in Mars orbit or all or part of it might land on Mars for a stay of several days.

FLEM, Titus noted, offered a "partial success capability" which, he opined, "may be very attractive." If the excursion module were lost, then the part of the crew remaining on board the parent spacecraft could still return safely to Earth. In addition, FLEM offered a simple (though admittedly incomplete) solution to the abort problem: if during pre-separation checkout the excursion module were found to be incapable of accomplishing its mission, then it would not undock, and the mission would become a simple Mars flyby.

Assuming that the mission took place as planned, the excursion module would ignite its rocket motors as the parent spacecraft passed Mars to depart Mars orbit and catch up with it. Following rendezvous, docking, and crew transfer, the excursion module would be cast off.

To squeeze even more benefit from FLEM, Titus proposed a variant of the standard ballistic flyby (that is, one in which the only major propulsive maneuver would occur at the start of the planetary mission, when the spacecraft departed Earth orbit). His "powered flyby" would include an optional maneuver near Mars that would dramatically reduce FLEM spacecraft mass during unfavorable Earth-Mars transfer opportunities, limit the wide swings in propellant mass required from one Earth-Mars transfer opportunity to the next, and slash total trip time. The maneuver would be optional in the sense that, if it could not occur, the FLEM spacecraft's Sun-centered orbit would still return it to Earth, though only after a longer trip. During return to Earth after a powered flyby, the FLEM spacecraft would pass as close to the Sun as the planet Mercury.

Titus determined that a powered-flyby maneuver in 1971 would have almost no effect on spacecraft mass at Earth-orbit departure - both the standard ballistic and powered-flyby FLEM spacecraft would have a mass of about 400,000 pounds - but would slash trip time from 510 to 430 days. The most dramatic improvement would occur in 1978, when the ballistic flyby FLEM spacecraft's mass would total nearly two million pounds and its mission would last 540 days. The powered-flyby FLEM spacecraft would have a mass of just 800,000 pounds at Earth-orbit departure and its mission would last only 455 days.

For a short time, Titus's FLEM concept exerted an unexpected influence on NASA piloted flyby studies taking place under the auspices of the Planetary Joint Action Group (JAG). The NASA Headquarters-led Planetary JAG, which met between 1965 and 1968, included representatives from Marshall Space Flight Center, Kennedy Space Center, the Manned Spacecraft Center, and advance planning contractor Bellcomm. The Planetary JAG's work will be described in detail in subsequent Beyond Apollo posts.

NASA abandoned its last vestige of the Apollo-based evolutionary model in February 1974, when the last crew of its only Apollo-derived space station, the Skylab Orbital Workshop, returned to Earth. The U.S. civilian space agency had come under new management in late 1968 after veteran NASA Administrator James Webb stepped aside and his deputy Thomas Paine took up the reins. When the new Administration of President Richard Nixon sought out NASA's vision of its post-Apollo future, Paine put forward a revolutionary Integrated Program Plan (IPP) that included multiple space stations, a moon base, and piloted nuclear-propulsion missions to Mars. The costly and complex IPP enjoyed almost no support, although one of its elements - the long-studied winged or lifting-body reusable Earth-to-Orbit Shuttle - gained Nixon's endorsement (with reservations) in January 1972.

Reference

"FLEM - Flyby-Landing Excursion Mode," AIAA Paper 66-36, R. R. Titus; paper presented at the 3rd AIAA Aerospace Sciences Meeting in New York, New York, 24-26 January 1966.

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