Faget also believed that the "overall planning of a total spaceflight program should be based on a logical series of steps." Mercury and Gemini would provide basic experience in living and working in space, paving the way for Apollo, which would, Faget explained, "have the first real mission." After that, NASA should build an Earth-orbiting space station and possibly a lunar base. 6
For Faget, a piloted Mars flyby mission in the 1970s was a deviation from the model von Braun popularized in the 1950s, which placed the first Mars expedition a century or more in the future. Faget avoided mentioning, however, that he had already been compelled to rationalize Kennedy's politically motivated drive for the Moon. Going by von Braun's logical blueprint, piloted lunar flight should have been postponed until after the Earth-orbiting space station was in place.
For the EMPIRE study, three contractors studied piloted flyby and "capture" (orbiter) expeditions to Mars and Venus. Aeronutronic studied flybys 7 ; Lockheed looked at flybys and, briefly, orbiters 8 ; and General Dynamics focused on orbiter missions. 9 Aeronutronic's study summed up EMPIRE's three goals:
- Establish a requirement for the Nova rocket development program.
- Provide inputs to the joint AEC-NASA nuclear rocket program, which had been established in 1960 and included a flight test program over which Marshall had technical direction.
- Explore advanced operational concepts necessary for flyby and orbiter missions. 10
The first two goals were contradictory as far as spacecraft weight minimization was concerned. Seeking justification for a new large rocket provided little incentive for weight minimization, while one of the great attractions of nuclear-thermal rockets was their increased efficiency over chemical rockets, which helped minimize weight. The contractors' tendency not to tightly control spacecraft weight assisted them with crew risk minimization. For example, all three contractors saw fit to include in their EMPIRE designs heavy spacecraft structures for generating artificial gravity.
Lockheed identified two main Mars flyby trajectory classes, which it nicknamed "hot" and "cool." In the former, the piloted flyby spacecraft would drop inside Earth's orbit (in some launch windows Venus flyby occurred), reach its farthest point from the Sun (aphelion) as it flew by Mars, and return to Earth about 18 months after launch. In the latter, the flyby spacecraft would fly out from Earth's orbit, pass Mars about 3 months after launch, reach aphelion in the Asteroid Belt beyond Mars, and return to Earth about 22 months after launch.
The Aeronutronic team opted for a "hot" trajectory. They assumed a Nova rocket capable of lifting 250 tons to Earth orbit. For comparison, the largest planned Saturn rocket, the Saturn C-5 (as the Saturn V was known at this time) was expected to launch around 100 tons. One Nova rocket would thus be able to launch the entire 187.5-ton Aeronutronic flyby spacecraft into Earth orbit.
Aeronutronic's "design point mission" had the flyby spacecraft leaving Earth orbit between 19 July 1970 and 16 August 1970, using a two-stage nuclear-thermal propulsion system. Aeronutronic's design retained the empty second-stage hydrogen propellant tanks to help shield the command center in the ship's core against radiation and meteoroids. Two cylindrical crew compartments would deploy from the core on booms; then the ship would rotate to provide artificial gravity. An AEC-developed radioisotope power source would deploy on a boom behind the ship. At the end of the flight the crew would board a lifting body Earth-return vehicle and separate from the ship. A two-stage retrorocket package would slow the lifting body to a safe Earth atmosphere reentry speed while the abandoned flyby ship sailed by Earth into orbit around the Sun.
Lockheed also emphasized a rotating design for its EMPIRE spacecraft. In the company's report, the flyby crew rode into orbit on a Saturn C-5 in an Apollo Command and Service Module (CSM) perched atop a folded, lightweight flyby spacecraft. A nuclear upper stage would put the CSM and flyby ship on course for Mars. The CSM would then separate and the flyby spacecraft would automatically unfold two long booms from either side of a hub. The CSM would dock at the end of one boom to act as counterweight for a cylindrical habitation module at the end of the other boom. When the ship rotated, the CSM and habitation module would experience acceleration the crew would feel as gravity.
The weightless hub at the center of rotation would contain chemical rockets for course correction propulsion, a radiation shelter, automated probes, and a dish-shaped solar power system. At Mars, the crew would stop the spacecraft's rotation and release the probes. At journey's end, the crew would separate from the flyby craft in the CSM, fire its rocket engine to slow down, discard its cylindrical Service Module (SM), and reenter the Earth's atmosphere in the conical Command Module (CM). The abandoned flyby craft would fly past Earth into solar orbit. Lockheed's report mentioned briefly how a Mars orbiter mission might investigate the Martian moons Phobos and Deimos. 11
The General Dynamics report was by far the most voluminous and detailed of the three EMPIRE entries, reflecting a real passion for Mars exploration on the part of Krafft Ehricke, its principal author. Ehricke commanded tanks in Hitler's attack on Moscow before joining von Braun's rocket team at Peenemünde. He came to the U.S. in 1945 with the rest of the von Braun team but left in 1953 to take a job at General Dynamics in San Diego, California. There he was instrumental in Atlas missile and Centaur upper-stage development. In the late 1950s he became involved in General Dynamics advanced planning.
Ehricke's team looked at piloted Mars orbiter missions. These would permit long-term study of the planet from close at hand, thus answering critics who complained that piloted flybys would spend too little time near Mars. General Dynamics' 450-day Mars orbiter mission was set to launch in March 1975.
Modularized Mars ships would travel in "convoys" made up of at least one crew ship and two automated service ships. Ship systems would be "standardized as much as practical" so that the crew ship could cannibalize the service ships for replacement parts. If a meteoroid perforated a propellant tank, for example, the crew would be able to replace it with an identical tank from a service ship. The ships would carry small "tugboat" spacecraft for moving propellant tanks and other bulky spares. 12 This approach—providing many spares—helped minimize risk to crew, but would dramatically boost overall expedition weight.
General Dynamics described many possible ship configurations; what follows was typical. The company allotted a nuclear propulsion stage for each major maneuver. After performing its assigned maneuver, the stage would be cast off. Ehricke's team estimated that nuclear engine flight testing would have to occur between May 1968 and April 1970 to support a March 1975 expedition. The M-1 engine system would perform Maneuver-1 of the Mars expedition, escape from Earth orbit (hence its designation). The M-2 engine system would slow the ship so Mars' gravity could capture it into Mars orbit, and M-3 would launch the spacecraft out of Mars orbit toward Earth. The M-4 engine system would slow the ship at Earth at the expedition's end.
Attached to the front of the M-4 stage would be the 10-foot-diameter, 75-foot-long spine module, or "neck," which served two functions: in addition to separating the astronauts from the nuclear engines to minimize crew radiation exposure, it would place distance between the crew and the ship's center of gravity, making the artificial gravity spin radius longer.
General Dynamics opted arbitrarily for providing artificial gravity equal to 25 percent of Earth's surface gravity and estimated that five rotations per minute was the upper limit for crew comfort. As engine systems were cast off, however, the ship's center of rotation would shift forward. For example, before the M-1 maneuver it would be at the aft end of the M-2 engine system, 420 feet from the ship's nose, and at the start of the M-2 maneuver it would be at the front of the M-2 system, 265 feet from the nose. As the ship grew progressively shorter, the spin radius would decrease, forcing faster rotation to maintain the same artificial gravity level. The report proposed joining the aft end of the crew vehicle to the end of a service vehicle during return to Earth, after the M-3 engine system was cast off, in order to place the center of rotation at the joint between the two vehicles and permit an acceptable rotation rate.
The General Dynamics crew ship design included the Life Support Section (LSS) for the eight-person crew. The LSS, which would be tested attached to an Earth-orbital space station beginning in November 1968, again illustrated the intense modularity of the General Dynamics design. The 10-foot-diameter central section would be attached to the front of the spine module and would house the repair shop, food storage, and radiation-shielded Command Module (not to be confused with the Apollo CM). The Command Module would serve double duty as the ship's radiation shelter and "last redoubt" if all other habitable modules were destroyed. Crewmembers would sleep in the Command Module's lower level to reduce their overall radiation exposure. The top level would serve as the crew ship's bridge and the "blockhouse" from which the service vehicles would be remote-controlled.
Two-level, 10-foot-diameter Mission Modules would cluster around the central section to provide additional living space. Individual levels could be sealed off if penetrated by meteoroids, and entire Mission Modules could be cast off if the crew had to reduce spacecraft mass to permit return to Earth—for example, if a large amount of propellant were lost and could not be replaced from the service vehicles. The LSS would also include the Earth Entry Module, an Apollo CM-style conical capsule. In addition to carrying the astronauts through Earth's atmosphere at voyage's end, it would serve as emergency abort vehicle during the M-1 maneuver. The service vehicles would each carry a spare Earth Entry Module.
On the service ships, a hangar for robot probes would replace the LSS. Unlike the Lockheed and Aeronutronic reports, the General Dynamics report treated its automated Mars probes in some detail. They would include the Returner Mars sample collector, a Mars Lander based on technology developed for NASA's planned Surveyor lunar soft-landing probes, Deimos Probe (Deipro) and Phobos Probe (Phopro) Mars moon hard landers based on technology developed for NASA's Ranger lunar probes, the Mars Environmental Satellite (Marens) orbiter, and Floater balloons. 13
General Dynamics' EMPIRE statement of work specified that it should study piloted Mars-orbital missions; however, enthusiastic Ehricke could not resist inserting an option to carry a small piloted Mars lander. A piloted Mars orbiter must, after all, enter and depart Mars orbit, thus performing all the major maneuvers required of a Mars Orbit Rendezvous landing mission except the landing itself. The Mars Excursion Vehicle lander, which would be based on the automated Returner, would be carried in a service vehicle probe hangar. It would support two people for seven days on Mars. 14 Ehricke's team proposed that a crew test it on the Moon in November 1972.
To get its ships into Earth orbit, Ehricke's team invoked a very large post-Saturn heavy-lift rocket capable of launching 500 tons. Two of these giants would be able to place parts for one ship into orbit so that only one rendezvous and docking would be required to complete assembly. By contrast, if the Saturn C-5 were used, eight launches and seven rendezvous and docking maneuvers would be needed to launch and assemble each General Dynamics Mars ship. The Ehricke team targeted post-Saturn vehicle development to commence in July 1965; the giant rocket would be declared operational in August 1973.