Stuhlinger's ships would each include a nuclear reactor producing 115 megawatts of heat. The reactor would heat a working fluid which would drive a turbine; the turbine in turn would drive a generator to supply 40 megawatts of electricity to two electric-propulsion thrusters. To reject the heat it retained after leaving the turbine, the working fluid would circulate through radiator panels with a total area of 4,300 square meters before returning to the reactor. The ship would move through space with its radiator panels edge-on to the Sun. Radiator tubes would be designed to be individually closed off to prevent a meteoroid puncture from releasing all of the ship's working fluid into space.
Each flat, diamond-shaped ship would weigh 360 tons when it switched on its electric thrusters in Earth orbit at the start of the Mars voyage—a little more than half as much as the NASA Lewis nuclear-thermal Mars ship. Of this, 190 tons (for the "B" ships) or 120 tons (for the "A" ships) would be cesium propellant. As already indicated, the price of low spacecraft weight was low acceleration—Stuhlinger's fleet would need 56 days to spiral up and out of Earth orbit; then, after a 146-day Earth-Mars transfer, it would require 21 days to spiral down to low-Mars orbit.
Stuhlinger's ships would rotate 1.3 times per minute to produce acceleration equal to one-tenth of Earth's gravity in the crew cabin. The reactor, located at the opposite end of the ship from the crew cabin, would act as an artificial gravity counterweight. Thus, the separation needed to keep the crew away from the reactor would also serve to increase spin radius.
Engineers designing artificial gravity systems must endeavor to make the spin radius as long as possible. This is because an artificial gravity system with a short spin radius must rotate more rapidly than one with a long spin radius to generate the same level of acceleration, which the crew feels as gravity. A short-radius, fast-spinning rotating system produces pronounced coriolis effects. For example, water leaving a faucet curves noticeably. Similarly, a person moving toward or away from the center of such a rotating system tends to veer sideways. Turning the head tends to produce nausea. In addition, a troublesome gravity gradient occurs vertically along the body—the head experiences less acceleration than the feet.
Stuhlinger's electric thrusters would be mounted at the ship's center of rotation on stalks. These would rotate against the ship's spin to remain pointed in the required direction. In addition to aiding the crew, Stuhlinger noted, artificial gravity would prevent gas pockets from forming in the working fluid. 20
Stuhlinger's design included a 50-ton, graphite-clad radiation shelter (about 15 percent of the entire weight of the ship) in the ship's crew compartment. Drinking water, propellant, oxygen cylinders, and equipment would be arranged around the shelter to provide additional shielding. The 2.8-meter-diameter, 1.9-meter-high shelter would hold a three-person ship's complement comfortably and would protect the entire 15-person expedition complement in an emergency. The crew would live in the shelter for 20 days during the outbound Van Allen belt crossing.
The Moon Intervenes
Stuhlinger wrote that it "is generally accepted that a manned expedition to . . . Mars will be carried out soon after such an ambitious project becomes technically feasible . . . [it is] the natural follow-on project to be undertaken after the lunar program." 21 Mars planners took Kennedy at his word when he said that reaching the Moon was "important for the long-range exploration of space."
On 11 July 1962, NASA announced that it had selected Lunar Orbit Rendezvous (LOR) over EOR and Direct Ascent as the Apollo mission mode. Attention had turned from EOR and Direct Ascent to LOR early in 1962. LOR, a concept zealously promoted by NASA Langley Research Center engineer John Houbolt, promised the lowest lunar spacecraft weight. This enabled a lunar expedition with only a single Saturn rocket launch, making LOR the fastest, cheapest way of meeting Kennedy's end-of-decade deadline. 22
In LOR, the lunar spacecraft—which consists of a small lander and a command ship—blasts off directly from Earth with no Earth-orbital assembly. The lander lands on the Moon, leaving the large command ship in lunar orbit. Surface exploration completed, the lander blasts off from the Moon and returns to the orbiting command ship. Spacecraft weight is reduced because only the small, light-weight lander must burn propellant to land and lift off.
It should be noted that the NASA Lewis and Stuhlinger Mars plans used the same general approach for the same reason. Landing the entire massive ship on Mars and launching it back to Earth would require impossible amounts of propellant or an impossibly small interplanetary vehicle. The standard NASA Mars plan can thus be dubbed Mars Orbit Rendezvous (MOR).
The LOR decision impacted post-Apollo ambitions. The reduction in lunar expedition mass promised by LOR removed the need for a post-Saturn Nova rocket, as well as the need to learn how to assemble large modular vehicles in Earth orbit. It thus reduced Apollo's utility as a technological stepping stone to Mars. The need to create a new justification for big rockets influenced Marshall's decision to start a new Mars study in early summer 1962. As will be seen in the next chapter, this study, known as EMPIRE, kicked off the most intense period of piloted Mars mission planning in NASA's history.
Chapter 3: EMPIRE and After
Manned exploration of Mars is the key mission in interplanetary space flight. Man must play a key role in the exploration of Mars because the planet is relatively complex, remote, and less amenable to exploration by unmanned probes than is the Moon ... serious interest in the Manned Mars Mission is springing up . . . with many planning studies being performed by several study teams within [NASA] and within industry . . . . Perhaps the most important result emerging from the present studies is the indication that the Manned Mars Mission can be performed in the relatively near future with equipment and techniques that will for the most part be brought into operation by the Apollo Project . . . the Manned Mars Mission is rapidly taking shape as the direct follow-on to the Apollo Project. (Robert Sohn, 1964) 1
EMPIRE
Ernst Stuhlinger's Research Projects Division was the smaller of two advanced planning groups in ABMA. The larger, under Heinz Koelle, became the Marshall Space Flight Center's Future Projects Office. Until 1962, Koelle's group focused primarily on lunar programs—Koelle was, for example, principal author of the U.S. Army's 1959 Project Horizon study, which planned a lunar fort by 1967. Koelle's deputy, Harry Ruppe, also supervised a limited number of Mars studies. Ruppe had come from Germany to join the von Braun team in Huntsville in 1957.
In the 1962-1963 period, however, the Future Projects Office spearheaded NASA's Mars planning efforts. As discussed in the last chapter, Marshall's primary focus was on launch vehicles. Advanced planning became important at Marshall in part because of the long lead times associated with developing new rockets. Marshall director von Braun foresaw a time in the mid-1960s when his center might become idle if no goals requiring large boosters were defined for the 1970s. As T. A. Heppenheimer wrote in his 1999 book The Space Shuttle Decision,
The development of the Saturn V set the pace for the entire Apollo program. This Moon rocket, however, would have to reach an advanced state of reliability before it could be used to carry astronauts. The Marshall staff also was responsible for development of the smaller Saturn IB that could put a piloted Apollo spacecraft through its paces in Earth orbit. Because both rockets would have to largely complete their development before Apollo could hit its stride, von Braun knew that his Center would pass its peak of activity and would shrink in size at a relatively early date. He would face large layoffs even while other NASA Centers would still be actively preparing for the first mission to the Moon. 2
Mars was an obvious target for Marshall's advanced planning. Von Braun was predisposed toward Mars exploration, and landing astronauts on Mars provided ample scope for his Center to build new large boosters. The timing, however, was not good. The Moon would, if all went well, be reached by 1970—but NASA would certainly not be ready to land astronauts on Mars so soon. For one thing, planners needed more data on the Martian environment before they could design landers, space suits, and other surface systems. What Marshall needed was some kind of short-term interim program that answered questions about Mars while still providing scope for new rocket development.
A 1956 paper by Italian astronomer Gaetano Crocco, presented at the Seventh International Astronautical Federation Congress in Rome, offered a possible way out of Marshall's dilemma. 3 Crocco demonstrated that a spacecraft could, in theory, fly from Earth to Mars, perform a reconnaissance Mars flyby, and return to Earth. The spacecraft would fire its rocket only to leave Earth—it would coast for the remainder of the flight. The Mars flyby mission would require less than half as much energy—hence propellant—as a minimum-energy Mars stopover (orbital or landing) expedition. This meant a correspondingly reduced spacecraft weight. Total trip time for a Crocco-type Mars flyby was about one year; for the type of mission von Braun employed in The Mars Project (1953), trip time was about three years.
Flyby astronauts would be like tourists on a tour bus, seeing the sights from a distance in passing but not getting off. Crocco wrote that they would use "a telescope of moderate aperture . . . to reveal and distinguish natural [features] of the planet . . . ." He found, however, that Mars' gravity would deflect the flyby spacecraft's course so it missed Earth on the return leg if it flew closer to Mars than about 800,000 miles. Such a distant flyby would, of course, "frustrate the exploration scope of the trip."
To permit a close flyby without using propellant, Crocco proposed that the close Mars flyby be followed by a Venus flyby to bend the craft's course toward Earth. The Venus flyby would be an exploration bonus, Crocco wrote, allowing the crew to glimpse "the riddle which is concealed by her thick atmosphere." Crocco calculated that an opportunity to begin an Earth-Mars-Venus-Earth flight would occur in June 1971. 4
From a vantage point at the start of the twenty-first century, a piloted planetary flyby seems a strange notion, yet in the 1960s NASA gave nearly as much attention to piloted Mars flybys as it did to piloted Mars landings. Piloted Mars flybys are now viewed from the perspective of more than three decades of successful automated flyby missions (as well as orbiters and landers). Of the nine planets in the solar system, only Pluto has not been subjected to flyby examination by machines. Robots can do flybys, so why entail the expense and risk to crew of piloted flybys?
Indeed, there were critics at the time the Future Projects Office launched its Early Manned Planetary-Interplanetary Roundtrip Expeditions (EMPIRE) piloted flyby/orbiter study. For example, Maxime Faget, principal designer of the Mercury capsule, coauthored an article in February 1963 which pointed out that a piloted Mars flyby would "demand the least [propulsive] energy . . . but will also have the least scientific value" because of the short period spent near Mars. He added that data on Mars gathered through a piloted flyby would be "in many ways no better than those which might be obtained with a properly operating, rather sophisticated unmanned probe." 5
The key phrase in Faget's criticism is, of course, "properly operating." When the Future Projects Office launched EMPIRE in May-June 1962, robot probes did not yet possess a respectable performance record. The Mariner 2 probe carried out the first successful flyby exploration of another planet (Venus) in December 1962, midway through the EMPIRE study, but the other major U.S. automated effort, the Ranger lunar program, was off to a shaky start. That series did not enjoy its first success until Ranger 7 in July 1964. The first successful Mars flyby did not occur until a year after that. In fact, one of the early justifications for piloted flybys was that the astronauts could act as caretakers for a cargo of automated probes to keep them healthy until just before they had to be released at the target planet.