At the Seventh International Astronautical Congress, held in Rome in September 1956, Italian aviation and rocketry pioneer Gaetano Crocco described a manned space mission in which a spacecraft would conduct a reconnaissance flyby of Mars, swing past Venus to bend its course toward Earth, and, one year to the day after departing Earth orbit, reenter Earth's atmosphere. After Earth-orbit departure, the spacecraft would need no additional propulsion. Crocco told the assembled delegates that an opportunity to commence such a mission would next occur in June 1971.

A little less than six years later, in May 1962, the Future Projects Office (FPO) at NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, awarded manned Mars mission study contracts worth $250,000 each to General Dynamics, Lockheed, and the Aeronutronic Division of Ford Motor Company. General Dynamics was instructed to study Mars orbital missions, Lockheed to look at Mars flyby and orbital missions, and Aeronutronic to study dual-planet (Mars-Venus) flybys. The combined study effort was known as EMPIRE, an evocative (if somewhat tortured) acronym that stood for Early Manned Planetary-Interplanetary Roundtrip Expeditions.

EMPIRE took place against the backdrop of the Apollo lunar program. One year before its start, in a speech before a special joint session of Congress, President John F. Kennedy had put NASA on course for the moon. He had given the U.S. civilian space agency, which had been founded less three years earlier, until the end of the 1960s to achieve his goal. It was hoped, however, that an American could land on the moon as early as 1967, during Kennedy's second term in office.

As EMPIRE began, NASA had nearly completed the contentious 14-month process of choosing the fastest, most reliable, and cheapest way of placing men on the moon. The Lunar Orbit Rendezvous mode, which would rely on MSFC's Saturn C-5 rocket, was selected in July 1962, before EMPIRE reached its conclusion. C-5 was soon renamed Saturn V.

MSFC was fertile ground for NASA's first major manned planetary mission study. The Huntsville Center's director was Wernher von Braun (image at top of post), a famous advocate of manned flight to the moon and Mars. Von Braun's efforts in the 1950s to popularize spaceflight had helped to prime the American public for the 1960s Space Race with the Soviet Union.

The Saturn family of rockets was expected to lead to larger Nova and Supernova rockets by the end of the 1960s. In the NASA illustration above, the three-stage C-1 and four-stage C-5 are anemic predecessors to the Apollo Saturn IB and Saturn V designs. The Apollo-Saturn rockets adopted features of the proposed (but never flown) Nova depicted; in particular, the Nova rocket's 22-foot-diameter third stage.

EMPIRE was, however, not merely the whim of a long-time moon and Mars fan; it was also part of a strategy to protect and promote MSFC. The Huntsville Center's specialty was rocket development. Its engineers hoped to develop rockets larger than the Saturn V, which they dubbed Nova and Supernova. Von Braun knew that, unless a post-Apollo program was in place as MSFC's Apollo responsibilities wound down in 1966 and 1967, his center would suffer deep staff and funding cuts, not develop new big rockets.

Besides calling for a man on the moon, Kennedy's May 1961 speech had requested new funding for nuclear propulsion. MSFC was involved in joint NASA/Atomic Energy Commission (AEC) nuclear rocket development through the Reactor In Flight Test (RIFT) program. The precise form the RIFT mission would take changed rapidly; for a time, it aimed to launch a 33-foot-diameter rocket stage with a NERVA nuclear engine on a Saturn V in 1967. Another goal of EMPIRE was, thus, to create a justification for NERVA and RIFT.

In the introduction to their December 1962 EMPIRE final report, Aeronutronic's engineers praised NASA for its "forward thinking approach." They added that

[b]y attacking the areas of interest at this early date[,] it will be possible to obtain a clearer picture of the requirements for early manned planetary and interplanetary flight. Thus the nation's resources, and the NASA and other United States space programs[,] can be oriented toward long range goals at an early date. [EMPIRE] represents an unusually early attack on this type of analysis.

Aeronutronic looked at two interplanetary trajectories for its dual-planet manned flyby mission. The first was dubbed the Crocco trajectory because it was based on the scenario in Crocco's 1956 paper. The second was the symmetric trajectory, which would see the flyby spacecraft swing around the Sun one-and-a-half times. It would fly past Mars halfway through its mission and would cross the orbit of Venus once before the Mars flyby and once after, flying past the planet during one of the orbit crossings.

Interplanetary trajectory types Ford Aeronutronic considered for its Mars-Venus flyby mission. Image: Ford Aeronutronic/NASA

Aeronutronic engineers found that Crocco had been mistaken when he wrote that an opportunity to start an Earth-Mars-Venus-Earth voyage would occur in June 1971. They determined that a dual-planet Crocco mission could depart Earth in mid-to-late August 1971. A symmetric opportunity would occur sooner: between 16 July and 19 August 1970.

The Crocco trajectory would require more propulsive energy than the symmetric trajectory, Aeronutronic found. The amount of energy required would depend on at what time during the launch opportunity the spacecraft would leave Earth orbit, but Aeronutronic selected single representative values for preliminary design purposes. Assuming a departure from a 300-kilometer-high circular Earth orbit, a Crocco spacecraft launched in August 1971 would need to increase its speed by 11.95 kilometers per second to achieve the desired dual-planet flyby trajectory. A symmetric mission launched in the July-August 1970 window would, by contrast, need a velocity increase of only 5.3 kilometers per second.

Low departure velocity meant reduced propellant requirements. In short, a symmetric mission would have a much lower mass than a Crocco mission. A low-mass spacecraft would need fewer launches to place its components and propellants into Earth orbit. Fewer launches would mean lower mission cost and less chance that a launch vehicle would fail and destroy its payload, placing the assembly campaign in Earth orbit in jeopardy. It would also mean less complex orbital assembly (or perhaps no assembly at all, if a Nova rocket were used), further trimming mission risk.

Aeronutronic found that no dual-planet Crocco mission could last only 365 days; for design purposes, it assumed that a mission launched in the August 1971 opportunity would last for 396 days. Though longer than Crocco had claimed, this was much less than the 611-day representative duration Aeronutronic selected for the July-August 1970 symmetric opportunity. Aeronutronic's engineers determined that radiation and meteoroid shielding and life support masses, which would increase with trip time, would not approach the mass of the propellant required to launch the Crocco mission out of Earth orbit; they thus accepted the symmetric mission's greater duration.

The symmetric mission would have a low Earth-departure speed, but would pay for it by having a high Earth-atmosphere reentry speed. As a general rule, the faster a spacecraft is moving as it plummets into Earth's atmosphere, the more reentry heating it will undergo. Greater heating calls for more massive thermal control systems; basically, active cooling employing coolant and radiators and a heat shield that ablates (that is, chars and erodes away, carrying away heat). The reentering spacecraft might also use a braking rocket to slow itself before reentry.

The Aeronutronic engineers did not consider the difference between the Crocco and symmetric Earth-atmosphere reentry speeds to be significant. For a symmetric mission launched between July 16 and August 19, 1970, reentry speed would top out at 15.8 kilometers per second; the corresponding figure for the August 1971 Crocco opportunity would be 13.5 kilometers per second. The propellant mass and Earth-to-orbit launch vehicles saved as a result of the Crocco mission's somewhat slower Earth-atmosphere reentry speed were minimal compared with the mass and launchers saved by the symmetric mission's dramatically reduced Earth-departure velocity.

Dual-planet flyby spacecraft size comparisons. Note comparison with Mercury-Atlas rocket and spacecraft (lower right). Mercury was the only piloted U.S. spacecraft that had flown at the time Ford Aeronutronic completed its study. Image: Ford Aeronutronic/NASA

Ford Aeronutronic's nuclear Crocco spacecraft. Image: Ford Aeronutronic/NASA

Aeronutronic calculated that, based on the mission design values it had selected, the nuclear Crocco dual-planet flyby spacecraft would have a fully assembled, fully fueled mass of 1121.5 tons, while an equivalent nuclear symmetric spacecraft would have a mass of only 188 tons. This would place the latter within the planned payload capacity of a single Nova rocket, eliminating assembly in Earth orbit entirely. The Aeronutronic team did not bother to determine the mass of a chemical Crocco spacecraft because it knew that it would be even greater than that of the nuclear Crocco spacecraft. They calculated, however, that the chemical symmetric spacecraft would have a mass of 350.5 tons (nearly twice that of the nuclear symmetric spacecraft) or 929.5 tons (nearly as much as the nuclear Crocco spacecraft) depending on the thrust capability of its rocket motor. Aeronutronic thus selected the nuclear symmetric mission for more detailed study.

Aeronutronic envisioned a 156-foot-long, 33-foot-wide symmetric dual-planet flyby spacecraft with a single 18,300-pound NERVA engine capable of generating 50,000 pounds of thrust. A tungsten shield at the front of the drum-shaped NERVA reactor would create a radiation shadow that would encompass the crew modules during Earth-orbit departure. The engine would need to operate for nearly 48 minutes to boost the spacecraft's speed by 5.3 kilometers per second and place it onto its symmetric trajectory. Aeronutronic identified the lengthy burn time as a possible show-stopper; available reactor materials, it explained, could not withstand such prolonged operation. Increasing thrust would reduce required burn time, but the company was not confident that a nuclear engine with more than 50,000 pounds of thrust could be developed in time for the July-August 1970 symmetric launch opportunity.

Ford Aeronutronic's chemical symmetric spacecraft designs. Image: Ford Aeronutronic/NASA

The nuclear symmetric spacecraft became Ford Aeronutronic's dual-planet flyby mission baseline design. Image: Ford Aeronutronic/NASA

Two tank clusters would supply liquid hydrogen propellant to the NERVA engine. The first-stage cluster, which would contain 56.2 tons of propellant, would comprise a core propellant tank to which the NERVA engine would be mounted and six detachable "perimeter" tanks. After expending the first-stage propellant and discarding the perimeter tanks, the spacecraft would have a mass of 127.7 tons.

Eight second-stage tanks would then supply a total of 38.3 tons of liquid hydrogen to the NERVA engine. After it expended the second-stage propellant, the NERVA engine would detach along with the first-stage core tank.

The spacecraft would then have a mass of 76.7 tons and would measure about 78 feet long. The empty second-stage tanks, with a total mass of 5.2 tons, would be retained to shield the spacecraft core from meteoroids. From aft to front, the cylindrical core would comprise twin SNAP-8 nuclear power sources; the spacecraft's navigational stable platform; a small weightless experiment area; a 20-ton command center/solar flare radiation shelter clad in 50 centimeters of polyethylene plastic and surrounded by four tanks holding 10.9 tons of chemical trajectory-correction propellants; small rocket motors for trajectory-correction burns; a two-stage reentry braking propulsion module ("retro-pack"); and the Earth-atmosphere reentry vehicle.

Earth-orbit departure completed, the six-man crew would reconfigure their spacecraft for the interplanetary voyage. The twin cylindrical living modules, each with an empty mass of 4.5 tons, would extend on hollow telescoping arms, and one SNAP-8 nuclear power source would deploy a radiator panel and begin producing electricity (the other SNAP-8 would be held in reserve in case the first failed). Sixteen-meter-diameter dish antennas would unfurl from the aft end of each living module to ensure continuous radio communication with Earth. Aeronutronic noted that, when the spacecraft was farthest from Earth, one-way radio signal trip time would reach 22 minutes. The crew would then spin their spacecraft about its long axis at a rate of three revolutions per minute to create acceleration in the living modules, which they would feel as gravity.

Based on experience with nuclear submarine crews, Aeronutronic allocated 750 cubic feet of living space to each astronaut, except inside the command center/radiation shelter, where it judged that 50 cubic feet per man would suffice. The twin living modules and command center/shelter would each have an independent life support system designed to recycle all air and water. The company estimated that life support mass would total 10.9 tons, of which food would make up nearly five tons.

The six-man crew would follow a complex schedule designed to combat boredom while providing adequate rest and recreation. Except for sleep, crew activities would occur in two-hour blocks. The crew would include a Commanding Officer, an Executive Officer, a Flight Surgeon, and three astronauts identified simply as "crew members." All six would take it in turns to serve as duty officer in the command center/shelter, maintenance & repair crewmember, and scientific activity crewmember. The Commanding Officer and Flight Surgeon would not, however, take part in hazardous repairs (for example, those involving spacewalks).

Symmetric Mars-Venus piloted flyby trajectory: 1 = Earth launch; 2 = Venus orbit crossing (possible flyby); 2* = second Venus orbit crossing (possible flyby); 3 = Mars orbit crossing; 3* = second Mars orbit crossing; 4 = Mars flyby/Earth position during Mars flyby; 5 = Earth return. Image: Ford Aeronutronic/NASA

A symmetric dual-planet flyby mission departing Earth in the 16 July-19 August 1970 opportunity would fly first past Venus between 97 and 102 days after launch from Earth orbit. Departure at the start of the launch opportunity would yield the shortest trip time on each leg of the interplanetary voyage (and shortest total trip time - 611 days); departure at the end of the opportunity would yield the longest trip time on each leg (and longest total - 631 days).

Earth-departure near the start of the opportunity would yield a Venus flyby distance of about 4890 miles; the corresponding figure for the end of the opportunity would be 7520 miles. As the spacecraft approached Venus, the crew would fire the trajectory-correction motors, changing their speed by about 650 feet per second to ensure an on-target flyby. The spacecraft would carry enough course-correction propellants to change its velocity by a total of 2000 feet per second.

Citing a November 1961 report planetary astronomer Gérard de Vaucouleurs had prepared for the U.S. Air Force, Aeronutronic allotted 1000 pounds for scientific equipment for studying Venus and Mars. The company declined, however, to define a scientific payload for its spacecraft, arguing that the EMPIRE mission's science objectives at Mars and Venus would be shaped by results from NASA robotic spacecraft launched between 1962 and 1968.

The Mars flyby would occur between 191 and 199 days after the Venus flyby, at the mid-point of the symmetric mission. During Mars approach, the crew would perform a second trajectory-correction burn, changing their spacecraft's speed by 330 feet per second. Mars flyby distance would range from about 2740 miles for an Earth departure at the beginning of the July-August 1970 opportunity to 4220 miles for a departure at its end.

The nuclear symmetric spacecraft would undergo configuration changes as it carried out its mission, casting off parts that were no longer useful and deploying power, communications, and artificial-gravity systems. The spacecraft would spend most of the flight in the configuration labeled "on-orbit spacecraft." Image: Ford Aeronutronic/NASA

An EMPIRE ground rule was that the contractors should use Apollo hardware and techniques wherever feasible, though this was not strictly enforced. The conical Apollo Command Module (CM) with its bowl-shaped ablative heat shield was considered a prime candidate for use as EMPIRE's Earth-reentry vehicle, but Aeronutronic rejected it. The company opted instead for a 14.75-ton lifting-body with a pointed nose and a two-stage chemical-propellant retro-pack. The company had determined that the lifting-body shape was more tolerant of reentry errors - that is, that it would be less likely to burn up or skip off the atmosphere - than the Apollo capsule. The lifting-body would, Aeronutronic noted, also offer a broader choice of land landing or sea splashdown sites because of its improved ability to maneuver in the atmosphere.

Landing/splashdown site flexibility would become especially useful if an abort during Earth departure became necessary. Aeronutronic found that the retro-pack, intended to slow the reentry vehicle as it neared Earth at the end of the mission, could return the crew to Earth no later than 16.7 hours after Earth-orbital launch provided that an abort was initiated before the flyby spacecraft passed beyond about 12,000 miles from Earth.

If, however, the mission unfolded as planned, the astronauts would return to Earth between 312 and 343 days after passing Mars. Prior to reentry, they would perform the final trajectory-correction burn of the mission, changing their spacecraft's speed by about 360 feet per second. Shortly thereafter, the astronauts would strap into the Earth-reentry lifting-body and fire small solid-propellant rocket motors to separate it from the flyby spacecraft. The abandoned flyby spacecraft would subsequently swing past Earth and enter orbit around the Sun.

After properly orienting the Earth-reentry vehicle, the crew would ignite the two stages of its nose-mounted retro-pack in succession, steering the lifting-body toward its reentry corridor and reducing its reentry velocity by 2.8 kilometers per second to 13 kilometers per second. The stages would detach in turn as they expended their propellants.

Ford Aeronutronic's preferred Earth-return reentry vehicle configuration. Image: Ford Aeronutronic/NASA

During reentry, the hottest part of the vehicle would be its small pointed nose, which Aeronutronic expected would be actively cooled to prevent melting. A blowtorch-like plume of superheated plasma would trail from the nose beneath the lifting-body, but would not contact its concave underside. The astronauts, who would recline facing away from the nose, would experience deceleration equal to 10 Earth gravities, which they would feel across their backs. After deceleration and descent into the lower atmosphere, the lifting-body would deploy parachutes and descend more or less vertically.

Aeronutronic provided a detailed development schedule and cost estimate for its nuclear symmetric dual-planet flyby mission. The company placed the mission's cost at $12.6 billion, or about half the projected Apollo Program cost. Cost savings would be due in large part to experience gained and hardware developed during Apollo. Peak funding year, with expenditures totaling about $3.5 billion, would be 1966.

Aeronutronic estimated that development of a 50,000-pound-thrust NERVA engine capable operating continuously for 60 minutes would need to begin on 1 January 1963 (that is, 10 days after the company completed its final report for MSFC FPO), as would development of the Earth-reentry lifting body. NERVA engines would be tested at the Nuclear Rocket Development Station (NRDS) at Jackass Flats, Nevada, beginning in mid-1965. Earth-reentry vehicles would be drop-tested over the dry lake bed at Edwards Air Force Base in California beginning in mid-1966 and would be flight-tested unmanned and manned using Saturn C-1 boosters beginning a year after that.

Flyby spacecraft development would begin in early 1964, shortly before crew selection and the start of Nova development. Beginning in mid-1967, Saturn C-5 rockets would launch three symmetric dual-planet flyby spacecraft without Earth-reentry vehicles into Earth orbit for testing. A total of 13 Nova rockets would be required for ground and flight testing between mid-1966 and late 1969. Of these, the first four would be Nova development flights and the last three would launch complete manned symmetric dual-planet flyby spacecraft into Earth orbit for testing and crew training. The fourteenth Nova, designated N9, would launch the mission spacecraft and crew into 300-kilometer-high Earth orbit on 15 July 1970, the day before the symmetric launch opportunity was set to begin.

In a paper presented 10 months after Ford Aeronutronic delivered its final report to MSFC FPO, Franklin Dixon declared that development of an improved NERVA engine should have begun no later than July 1963 if it was to become available in time for the 16 July-19 August 1970 symmetric dual-planet flyby launch opportunity. Dixon had not participated in EMPIRE; on 1 July 1963, Ford had placed Aeronutronic under its Philco division, and Dixon had become Philco Aeronutronic's Manager for Advanced Space Systems.

Kiwi-B nuclear rocket engine. Image: Los Alamos National Laboratory

NERVA development lagged behind the schedule Dixon said was necessary in part because of Kiwi-B nuclear rocket engine failures in December 1961, September 1962, and November 1962. As the name implies, the Kiwi series engines were not meant for flight. In fact, though they each included a nozzle and blasted superhot hydrogen gas into the air, they were considered nuclear reactors, not nuclear rocket engines. The three failures were similar in nature: liquid hydrogen moving through the hot reactor core caused vibration which broke and eroded the reactor's uranium fuel rods. Hot gaseous hydrogen exhaust then blew uranium fragments out through the engine nozzle. The melting fragments sparkled as they sprayed into the open air.

The Kiwi-B failures set off a duel between the President's Science Advisory Council and the Bureau of the Budget on the one hand and NASA and the Atomic Energy Commission, championed by New Mexico Senator Clinton Anderson, on the other. Democrat Anderson's state contained Los Alamos National Laboratory, which led the AEC side of the nuclear rocket program. President Kennedy visited the NRDS in early December 1962 to size up the situation. On 12 December 1962, two weeks before Ford Aeronutronic completed its EMPIRE study, he indefinitely postponed RIFT. Citing fiscal restraint, Kennedy's successor, Lyndon Johnson, cancelled RIFT altogether in December 1963, and made NERVA a wholly ground-based research and development effort.

In December 1967, the NRX-A6 NERVA ground-test engine operated for 60 minutes without a hitch; that is, for longer than would have been required to boost Aeronutronic's dual-planet flyby spacecraft out of Earth orbit and onto its symmetric trajectory. President Richard Nixon cancelled NERVA in 1972 after program expenditures totaling $1.4 billion.

The Nova rocket experienced a less protracted and costly demise. By June 1964, von Braun called publicly for manned planetary flyby missions using Saturn V rockets and Apollo-derived hardware, thus acknowledging that the Lunar Orbit Rendezvous decision of July 1962 had all but doomed large rockets like Nova. Later in 1964, the Bureau of the Budget declared Nova rocket development to be of low priority, and called for NASA's post-Apollo program to be confined to Earth orbit and based on hardware developed for Apollo. The MSFC FPO would publish a feasibility study of an Apollo-derived piloted flyby mission in early 1965.

References:

EMPIRE: A Study of Early Manned Interplanetary Expeditions, NASA Contractor Report 51709, Aeronutronic Division, Ford Motor Company, 21 December 1962.

"The EMPIRE Dual Planet Flyby Mission," Franklin P. Dixon, Aeronutronic Division, Philco Corporation; paper presented in Palo Alto, California, at the Engineering Problems of Manned Interplanetary Exploration conference, 30 September-1 October 1963.

"EMPIRE: Early Manned Planetary-Interplanetary Roundtrip Expeditions Part I: Aeronutronic and General Dynamics Studies," Frederick I. Ordway III, Mitchell R. Sharpe, and Ronald C. Wakeford, Journal of the British Interplanetary Society, May 1993, pp. 179-190.

Humans to Mars: 50 Years of Mission Planning, 1950-2000, NASA Monographs in Aerospace History No. 21, NASA SP-2001-4521, David S. F. Portree, NASA History Office, February 2001 (