The Challenge of the Planets, Part Two: High Energy

JPL's nuclear-electric "Space Cruiser" could in theory reach Pluto from Earth orbit in slightly more than three years. Image credit: NASA/JPL.

President John F. Kennedy did not call only for a piloted lunar landing by 1970 in his 25 May 1961 "Urgent National Needs" speech before a joint session of the U.S. Congress. Among other things, he sought new money to expand Federal research into nuclear rocketry, which, he explained, might one day enable Americans to reach "the very ends of the solar system."

Today we know that Americans can reach the "ends" of the Solar System without resorting to nuclear rockets. When President Kennedy gave his speech, however, it was widely assumed that "high-energy" propulsion — which for most researchers meant nuclear rockets — would be desirable for round-trip journeys to Mars and Venus and an outright necessity for voyages beyond those next-door worlds.

In his speech, President Kennedy referred specifically to the joint NASA-Atomic Energy Commission (AEC) ROVER nuclear-thermal rocket program. As the term implies, a nuclear-thermal rocket employs a nuclear reactor to heat a propellant (typically liquid hydrogen) and expel it through a nozzle to generate thrust.

ROVER had begun under U.S. Air Force/AEC auspices in 1955. AEC and the Air Force selected the Kiwi reactor design for nuclear-thermal rocket ground testing in 1957, then the latter relinquished its role in ROVER to the newly created NASA in 1958. As President Kennedy gave his speech, U.S. aerospace companies competed for the contract to build NERVA, the first flight-capable nuclear-thermal rocket engine.

Nuclear-thermal propulsion is not the only form of nuclear-powered high-energy propulsion. Another is nuclear-electric propulsion, which can take many forms. This post examines only the form known widely as ion drive.

An ion thruster electrically charges a propellant and expels it at nearly the speed of light using an electric or magnetic field. Because doing these things requires a large amount of electricity, only a small amount of propellant can be ionized and expelled. This means in turn that an ion thruster permits only very gradual acceleration; one can, however, in theory operate an ion thruster for months or years, enabling it to push a spacecraft to high velocities.

American rocket pioneer Robert Goddard first wrote of electric rocket propulsion in his laboratory notebooks in 1906. By 1916, he conducted experiments with “electrified jets.” He described his work in some detail in a report in 1920.

Interest remained minimal, but picked up in the 1940s. The list of ion-drive experimenters and theorists reads like a "Who's Who" of early space research: L. Shepherd and A. V. Cleaver in Britain, L. Spitzer and H. Tsien in the United States, and E. Sanger in West Germany all contributed to the development of ion drive before 1955.

In 1954, Ernst Stuhlinger, a member of the German rocket team the U.S. Army brought to the United States at the end of the Second World War, began small-scale research into ion-drive spacecraft while developing missiles for the Army Ballistic Missile Agency (ABMA) at Redstone Arsenal in Huntsville, Alabama. His first design, poetically nicknamed the "cosmic butterfly," relied on banks of dish-shaped solar concentrators for electricity, but he soon switched to nuclear-electric designs. These had a reactor heating a working fluid which drove an electricity-generating turbine. The fluid then circulated through a radiator to shed waste heat before returning to the reactor to repeat the cycle.

Stuhlinger became a NASA employee in 1960 when the ABMA team at Redstone Arsenal became the nucleus for Marshall Space Flight Center (MSFC). In March 1962, barely 10 months after Kennedy's speech, the American Rocket Society hosted its second Electric Propulsion Conference in Berkeley, California. Stuhlinger was conference chairman. About 500 engineers heard 74 technical papers on a wide range of electric-propulsion topics, making it perhaps the largest professional gathering ever devoted solely to electric propulsion.

Among the papers were several on ion propulsion research at the Jet Propulsion Laboratory (JPL) in Pasadena, California. JPL had formed its electric-propulsion group in 1959 and commenced in-depth studies the following year.

One JPL study team compared different forms of "high-energy" propulsion to determine which, if any, could perform 15 robotic space missions of interest to scientists. The missions were: flybys of Venus, Mars, Mercury, Jupiter, Saturn, and Pluto; Venus, Mars, Mercury, Jupiter, and Saturn orbiters; a probe in solar orbit at about 10% of the Earth-Sun distance of 93 million miles; and "extra-ecliptic" missions to orbits tilted 15°, 30°, and 45° with respect to the plane of the ecliptic. In keeping with their robotic payloads, all were one-way missions.

The five-person JPL comparison study team found that a three-stage, seven-million-pound chemical-propellant Nova rocket capable of placing 300,000 pounds of hardware — including a hefty chemical-propellant Earth-orbit departure stage — into 300-mile-high Earth orbit with a meaningful scientific instrument payload could achieve just eight of the 15 missions: specifically, the Venus, Mars, Mercury, Jupiter, and Saturn flybys; the Venus and Mars orbiters; and the 15° extra-ecliptic mission. A chemical/nuclear-thermal hybrid comprising a Saturn S-I first stage, a 79,000-pound Kiwi-derived nuclear-thermal second stage, and a 79,000-pound Kiwi-derived nuclear-thermal stage with interplanetary payload could carry out the Nova missions plus the 30° extra-ecliptic mission.

A 1500-kilowatt ion system starting from Earth orbit could achieve all 15 missions. The JPL team told the Berkeley meeting that an unspecified chemical-propellant booster rocket would launch the 45,000-pound ion system into a 300-mile-high orbit as a unit. There the reactor and ion thrusters would activate and the slow-accelerating ion system would begin gradually to gain speed and climb toward Earth-escape and its required interplanetary trajectory.

For several of the missions to more distant targets — for example, the Saturn flyby — the ion system had enough time to accelerate so that it could reach its goal hundreds of days ahead of the Nova and chemical/nuclear-thermal hybrid systems. It could also provide its instrument payload and long-range telecommunications system with ample electricity, boosting data return. A smaller ion system (600-kilowatts, 20,000 pounds) that could be launched atop NASA's planned Saturn C-1 booster rocket could accomplish all but the extra-ecliptic 45° mission.

Missiles & Rockets magazine devoted a two-page article to the JPL comparison study. It headlined its report "Electric Tops for High-Energy Trips," which must have been gratifying for many long-time ion-drive supporters.

Many technical problems remained, however. The five JPL engineers who performed the comparison study optimistically assumed that for every kilowatt of electricity its 1500-kilowatt system applied to generating thrust, only 13 pounds of hardware — reactor, turbo-generator, radiator, structure, wiring — would be required. In 1962, a ratio of about 70 pounds of hardware per kilowatt of thrust with a maximum generating capacity of only 30 kilowatts was considered much more realistic.

They also assumed that its electricity-generating system and its ion-drive system could operate more or less indefinitely despite the presence of moving parts operating at high temperatures. The whirling turbo-generator, for example, would for some missions need to operate non-stop at a temperature of about 2000° Fahrenheit for years. A one-year operating time was considered a bold aspiration in 1962.

The five engineers did not specify the precise form their ion-drive spacecraft would take, but it would probably have resembled the design depicted at the top of this post. A trio of JPL engineers produced it during the 1960-1962 period, while the five-person JPL team conducted its comparison study.

The automated, 20,000-pound "space cruiser," as the three engineers dubbed their creation, would include a radiator surface area of roughly 2000 square feet, making it a large target for micrometeoroid strikes. In 1962, little was yet known of the quantity of micrometeoroids in interplanetary space, so no one could judge accurately the likelihood that such a radiator might be punctured, nor the mass required for effective puncture-resistant radiator tubes, redundant cooling loops, or "make-up" cooling fluid.

The five-person team only briefly mentioned the potentially profound effects of ion-drive power and propulsion systems on other spacecraft systems. The turbo-generator, for example, would impart torque to the spacecraft, creating a requirement for a spin-nulling attitude-control system — for example, a momentum wheel and chemical-propellant thrusters (the momentum wheel is visible near the center of the truss in the image above).

The turbine, flow of coolant through the radiator, and momentum wheel would, it was expected, cause vibration that could interfere with scientific instruments. In addition, ion drive systems would of necessity generate powerful magnetic and electric fields that might make difficult many desirable scientific measurements.

The space cruiser engineers sought to reduce radiation effects by placing its reactor at its front (upper right in the illustration above) and its science instruments at its rear. Unfortunately, this put the instruments among the space cruiser's ion thrusters, where intense electric and magnetic fields would occur.

The space cruiser designers looked at a thermionic power system that would use electrons from its reactor to produce electricity directly and would include neither moving parts nor high-temperature systems. They did not favor it because it was new technology. In addition, the thermionic system's nuclear reactor would need cooling fluid, a circulating pump, and a radiator, so in terms of vibration and micrometeoroid damage would offer only a little improvement over the better-understood turbo-generator design.

Close on the heels of the ARS Electric Propulsion Conference in Berkeley, NASA Headquarters opted to concentrate electric propulsion research at the NASA Lewis Research Center in Cleveland, Ohio. The move was probably intended to eliminate costly redundant research programs and keep JPL and MSFC focused on their Apollo Program-related tasks. Research did not stop entirely at NASA MSFC and JPL, however. Stuhlinger, for example, continued to produce designs for piloted ion-drive spacecraft.

Ironically, while the nearly 500 electric-propulsion engineers met near San Francisco, a young mathematician working alone near Los Angeles was busy eliminating any immediate need for ion drive or any other kind of high-energy propulsion system for planetary exploration. The third part of this three-part series of posts will examine his work and its profound impacts on planetary exploration.

Sources

"Electric Tops for High Energy Trips," Missiles & Rockets, 2 April 1962, pp. 34-35.

Nuclear Electric Spacecraft for Unmanned Planetary and Interplanetary Missions, JPL Technical Report No. 32-281, D. Spencer, L. Jaffe, J. Lucas, O. Merrill, and J. Shafer, Jet Propulsion Laboratory, 25 April 1962.

The Electric Space Cruiser for High-Energy Missions, JPL Technical Report No. 32-404, R. Beale, E. Speiser, and J. Womack, Jet Propulsion Laboratory, 8 June 1963.

"Electric Spacecraft – Progress 1962," D. Langmuir, Astronautics, June 1962, pp. 20-25.

More Information

The Challenge of the Planets, Part One: Ports of Call

The Challenge of the Planets, Part Three: Gravity