The Challenge of the Planets, Part Two: High Energy

JPL's nuclear-electric "Space Cruiser" could reach Pluto in slightly more than three years.

JPL’s nuclear-electric “Space Cruiser” could reach Pluto from Earth orbit in slightly more than three years. NASA/Jet Propulsion Laboratory

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 rockets, which, he explained, might one day enable Americans to reach to “the very ends of the solar system.”

Today we know that Americans can reach the “ends” of the Solar System without resort to nuclear propulsion (though a radioisotope system is handy for generating electricity in the dark beyond Jupiter, where solar arrays become impractical). 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 a 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 (USAF)/AEC auspices in 1955. USAF/AEC selected the Kiwi reactor design for nuclear-thermal rocket ground testing in 1957 – a major step forward for the U.S. nuclear rocket program – and USAF relinquished its role the program to 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 was not the only form of nuclear-powered high-energy propulsion. Another was 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 charging propellant and generating electric or magnetic fields require a great deal 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 despite the speed at which propellant leaves it; 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 propulsion in his notebooks in 1906. By 1916 be had begun experiments with “electrified jets.” Interest faded in the 1920s and resumed 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 before 1955.

In 1954, Ernst Stuhlinger, a member of the team of German rocketeers 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 designs while working to develop missiles for the Army Ballistic Missile Agency (ABMA) at Redstone Arsenal in Huntsville, Alabama. His first design relied on solar concentrators for electricity, but he soon switched to nuclear-electric designs. In these, a reactor heated 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 with the creation of the Marshall Space Flight Center (MSFC) out of ABMA. 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-related topics, making it perhaps the largest professional gathering ever devoted solely to electric propulsion.

Among the papers was one reporting results of ion propulsion studies performed at the Jet Propulsion Laboratory (JPL) in Pasadena, California. JPL formed its electric-propulsion group in 1959 and commenced in-depth studies the following year.

One JPL study 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: Venus, Mars, Mercury, Jupiter, Saturn, and Pluto flybys; Venus, Mars, Mercury, Jupiter, and Saturn orbiters; a solar probe in 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 six-person JPL team calculated that a three-stage, seven-million-pound chemical-propellant Nova rocket capable of placing 300,000 pounds of hardware – mainly a massive chemical-propellant Earth-orbit departure stage – into 300-mile-high Earth orbit could with a meaningful scientific instrument payload 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 rocket/spacecraft 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/interplanetary payload could carry out the Nova missions plus the 30° extra-ecliptic mission. By contrast, a 1500-kilowatt, 45,000-pound ion system starting from a 300-mile-high Earth orbit could achieve all 15 missions.

In several instances involving more distant interplanetary targets – for example, the Saturn flyby – the slow-accelerating ion system could reach its target hundreds of days ahead of the Nova and chemical/nuclear-thermal hybrid systems. It could also provide its instrument payloads and long-range telecommunications system with ample electrical power, boosting data return. A smaller system (600-kilowatts, 20,000 pounds) that could be launched atop the planned Saturn C-1 rocket could accomplish all but the extra-ecliptic 45° mission.

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

In 1962, NASA Headquarters opted to concentrate electric propulsion research at the Lewis Research Center in Cleveland, Ohio. Research did not stop entirely at NASA MSFC and JPL, however. Stuhlinger, for example, continued to produce designs for piloted ion-drive spacecraft as late as 1966.

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


“Electric Tops for High Energy Trips,” Missiles and Rockets, 2 April 1962, pp. 34-35.

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

“The Development of Nuclear Rocket Propulsion in the United States,” W. House, Journal of the British Interplanetary Society, March-April 1964, pp. 306-318.

Ion Propulsion for Space Flight, E. Stuhlinger, McGraw-Hill Book Company, New York, 1964, pp. 1-11.

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.

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