"Engines for the Cosmos," Feature Article, January 2003

engines for the cosmos

Exploration of deep space requires systems of propulsion that can go the distance.

by Aloysius I. Reisz and Stephen L. Rodgers

Galactic forces spiral across the cosmos, fueled by nuclear fission and fusion, and atoms in plasmatic states churn in the constraints of gravitational forces and magnetic fields. In their wanderings, the galaxies spew light, radiation, atomic, and subatomic particles through the universe.

Throughout the ages of man, minds have wondered about visions of journeying through the stars. If humans and human devices from Earth are to go beyond the moon and journey into deep space, they must use the forces of the cosmos, such as ions, electrons, and energies generated from the manipulation of subatomic and atomic particles. Forms of electromagnetic waves, perhaps in the light or radio frequencies, must control deep space engines. We won't get far from Earth on our accustomed hydrocarbon fuels.

Deep Space Propulsion

Rocket propulsion to explore deep space has different requirements than launch propulsion on Earth, where high thrust is required to escape the planet's gravitational pull. Vehicles in deep space are only faintly affected by Earth's gravity. Consequently, high thrust engines are not required.

To explore the outer planets in a reasonable time, engines must generate either high exhaust velocity or high specific impulse. Specific impulse, abbreviated as I_sp, is a measure in seconds of propulsion system efficiency in converting fuel energy into momentum. Chemical propulsion can provide high thrust, but is limited in specific impulse (fewer than 500 seconds). The reason is that chemical propellants carry within their chemical composition all of the energy that can be generated, and current technology is about at the limit of the amount of energy that can be put into a chemical bond.

To achieve higher specific impulses, we must look to other energy sources, including fission and fusion. With the very high energies from these sources, subatomic particles of light gases can be manipulated to efficiently generate low thrust energy by using electron guns, electrical fields, magnetic fields, electric currents, lasers, radio waves, or combinations thereof.

Resistive heaters simulate the thermodynamics of a fission reactor, without using nuclear materials, in an experiment at NASA Marshall's Propulsion Research Center.

This idea was shared by several early 20th-century
chemical rocketeers. In 1947, Wernher von Braun asked Ernst Stuhlinger to research the concept of electric propulsion as written in Herman Oberth's book, Possibilities of Space Flight, published in 1929 in Berlin. As Stuhlinger recalls, von Braun said, "I wouldn't be a bit surprised if we flew to Mars electrically."

Stuhlinger immersed himself in studying electric propulsion possibilities and, in August 1954, presented to the International Astronautical Congress in Vienna a paper titled "Possibilities of Electrical Space Ship Propulsion."

In 1952, von Braun had presented his first plan for a journey to Mars. This plan using chemical propulsion required 5,320,000 metric tons of fuel for 10 spaceships and the assemblage of 37,200 tonnes in Earth orbit. Stuhlinger's plan for a Mars journey with electric propulsion required putting only 2,788 tonnes into Earth orbit.

Von Braun wrote in 1952, "The small thrust [from electric propulsion] is effective for missions to the more distant parts of the solar system."

Stuhlinger threw himself into the development of electric ion propulsion. According to Stuhlinger, he and some technicians from a California company, working under a contract with NASA's Marshall Space Flight Center in the early '60s, demonstrated the operation of a low-thrust (0.1 lb.), high specific impulse ion propulsion engine in a vacuum chamber simulating outer space. This was the first firing of a non-chemical rocket of the order that is likely to take man beyond the moon and into deep space.

By 1962, the nuclear-electric propulsion research work was transferred to NASA's Lewis Research Center in Cleveland—now known as the John H. Glenn Research Center at Lewis Field—as the Marshall Center turned its focus to the mission of developing the Saturn rockets that would overtake Soviet space superiority and land Americans on the moon.

The United States recognized early the benefit that nuclear propulsion could provide for interplanetary exploration and ran an extensive research and development program devoted to it. The bulk of United States efforts came to a close in the early 1970s, but the Russians continued development and have flown more than 30 nuclear reactor-powered systems, while the United States has flown but one, in the mid-'60s, the Snap-10A.

On a Single Tank of Xenon

In June 1996, a prototype xenon ion engine built at Lewis began long-duration tests in a vacuum chamber at the Jet Propulsion Laboratory in Pasadena, Calif., and recorded more than 8,000 hours of operation. On Oct. 24, 1998, Deep Space 1 was launched, the spacecraft that reached the comet Borrelly in September 2001.

In electric ion propulsion systems, the electric energy is deposited into the propellant flowing into the engine. Whereas chemical propulsion systems use heat to eject combusting propellants, ion propulsion systems have an electrical field that ejects propellant ions into space, thereby transferring momentum to the spacecraft.

An artist's conception of a high power plasma propulsion device using a fusion energy source; the large radiators are needed to dI_spose of excess heat.

Cesium or xenon gas is injected with ions from an electron gun. The electric field from the voltage on a pair of metal grids extracts the charged ions and expels them into space. A cathode located toward the end of the engine injects electrons into the charged exhaust so that the spacecraft body does not build up a negative charge. Deep Space 1's ion drive is powered by solar panels, but could be powered by a nuclear reactor for more powerful and deeper space missions. The electrical grids physically limit the flow of the ion beam and therefore the power generated.

In the Deep Space 1 ion engine, electrons are emitted from a hollow tube cathode and enter a magnetic-ringed chamber, where they strike xenon atoms. This impact knocks away a xenon electron, causing the xenon to become ionized. As the ionized gas flows to the rear of the engine, it encounters a 1,280-volt electrical field from a pair of metal grids that forces the xenon ions to shoot from the engine body at speeds of 100,000 km/h, about 60,000 mph. This engine develops 1/50th of a pound of thrust, much less than chemical rockets, but with its high specific impulse, it can journey in space for years.

Ion thrusters emit beams of positive ions and have high I_sp values. Ion thrusters are still powering the Deep Space 1 probe.

Hall Effect in Space

The Russians have built Hall-effect thrusters and flown them on more than 100 satellites since the early '70s. The Russian Hall-effect thruster engines also have an electric field that ejects high-temperature charged xenon gas particles.

The electric field is not created by electrical grids as in the ion engines of Deep Space 1, but by a ring of magnets around the peri- meter of the chamber, with a magnetic core rod running axially down the center so as to generate a radial magnetic field. This radial magnetic field causes the xenon electrons to circle the chamber interior, thereby inducing an axial electrical field without grids and a Hall current that ejects the charged particles out into space.

The Hall current derives from the differing behavior of electrons and heavier ions in field-induced spiral pathways. The Hall-effect flow of particles out into space is not impeded by grids, but the engines are less efficient than ion engines because electrical energy is not injected into the gas as in the ion engine. The energy comes only from the gas.

The Russian thruster SPT 140 is a 5-kW engine with a thrust of 250 mN, exhaust velocity of 22.5 km/s and 57 percent efficiency.

Deep Space Energy Sources

Electric propulsion devices require an energy source and an electric generation method in order to operate. Thermodynamic conversion methods include photovoltaic, thermoelectric, thermionic, and electrochemical reactions, and the Brayton, Rankine, and Stirling cycles. Typically, these systems will provide power for the entire spacecraft as well as for the propulsion device.

Batteries, photovoltaic cells, isotope thermoelectric generation units, and fuel cells have been used in space flight operation. So far, the power output capacity for operational systems has been low, in the tens of watts. Much higher power levels will be needed for space travel to the outer planets in reasonable time frames or for manned space travel.

Although it's a long way from ready, fusion power may possibly see its first practical use in space, rather than in Earth-bound power plants.

The energy sources considered most practical for spacecraft applications are chemical (batteries or fuel cells), solar, isotope radiation from decay, and nuclear fission. Chemical fuel cells can supply up to 1 kW of power for a few weeks. Solar cells have supplied most of the power for longer-duration space missions. Power levels from solar cells have upper limits of around 15 kW at 8 to 10 watts per square foot. Efficiency is about 8 to 12 percent.

Nuclear processes show the most potential for providing the large amounts of power needed for timely deep space missions or for powering large spacecraft.

Nuclear fission processes offer the potential of much higher power generation for propulsion. Nuclear reactors provide energy that can be used to directly heat a working fluid (typically hydrogen) and provide thrust through an expansion similar to other thermal rockets. The performance limits of this system are dictated by material properties and are about 3,000°C, producing a specific impulse between 500 and 800 seconds.

Nuclear electric propulsion schemes involve converting the heat from a reactor to electrical energy by way of a power conversion device. These devices may be dynamic, such as Brayton or Stirling engines, or thermoelectric. The electricity generated can then be used in conjunction with an electric propulsion thruster. The power available for propulsion is limited simply by the reactor size and efficiencies of each device. Reactor and power conversion technologies are available today to accomplish a space demonstration with only some thruster development needed to be able to take advantage of the power available from such a nuclear propulsion system.

Fusion propulsion may play a role for very large craft going into deep space. Fusion, which derives its energy from the fusing of two atomic nuclei, offers the advantage for deep space propulsion in that the plasma produced in the fusion reaction can be used directly for momentum transfer, and would not have to be converted to electrical energy for use. A magnetic nozzle would be used to control the plasma flow. Fusion propulsion devices are necessarily large (80 megatonnes and above) in order to achieve a scaling fusion.

Though we have over 40 years of fusion research and technology to draw on, significant breakthroughs in fusion system efficiencies must be accomplished before a fusion propulsion engine can be built. Fusion still remains a long way from practical use. Propulsion, having less strenuous economical requirements than terrestrial-based fusion for power, may be the first practical application for fusion devices.

Scientists peering into the cosmos talk about dark matter and dark energy. Scientists can't see dark forces, but they can see their apparent work. Force is the effect of energized matter. Matter we can see and feel is composed of atoms that have negatively charged electrons, positive protons, and neutrons with no charge at all. Antimatter is built in much the same way, except that its electrons are positive and its protons negative.

It's a long way to Jupiter, and fossil fuels aren't likely to get us there. As new technologies drive voyages of the future, they may do more than cross the reaches of space; they may improve our way of life on Earth, as well.

Many scientists believe that the universe was created with equal parts matter and antimatter. Recently, researchers have created antimatter atoms that last long enough for scientific study. When atoms of matter and antimatter collide, they obliterate each other, and their mass is converted to energy. Theorists foresee this power as being a source of rocket propulsion for deep space travel.

The Marshall Space Flight Center is conducting experiments leading to an antimatter trap, essentially a magnetic bottle that will contain the antiprotons in magnetic fields. The emphasis is on learning how to store and manipulate antimatter for eventual use in propulsion devices.

The first application for antimatter is likely to be in the medical field. Antiprotons can be handled as a molecular beam, which can be used with high resolution to treat deep cancers within the body. The molecular beam would not annihilate until the antiproton comes to rest, where it will destroy the surrounding tissue with no residual radioactive decay products. Antiprotons could also be used to produce short-lived radioisotopes that could be used in various medical diagnostic procedures. Marshall researchers are working with partners to develop antiproton handling technology to enable these applications.

Clean Energy, Light Gases

Engines being engineered for deep space missions are, out of necessity, fueled by clean energy from light gas atoms. The fuels are brought to certain physical states and subjected to electric or magnetic fields that accelerate and eject charged particles out of the engine, thereby giving momentum to the spacecraft. These new deep space engines will enable us to send missions to the far reaches of the solar system and beyond with exploratory instruments. Possibly even manned missions may follow. From these cosmic exploratory missions we will expand our knowledge of the universe we live in and our intellect.

Of even more direct use than exploring the universe, these engines potentially can be reconfigured to provide clean, cheap, abundant power on Earth.

Engineering systems that sent men to the moon during the 1960s were a catalyst for improving life systems on Earth. Hydrogen was proven to be a clean, efficient source of power. Fuel cells and photovoltaic cells were proved practical in the Apollo projects and have since been used increasingly in other life systems. Apollo began the popular use of computers and lasers.

So in developing engines to explore deep space, we have opportunities to develop new clean power systems on Earth.
To learn more about ourselves and our role in the universe, we must go deep into the cosmos. And to provide clean, abundant energy for life on Earth, we must engineer the use of the energy of atoms and stars. That is the engineering challenge of this new millennium.

Al Reisz is a Fellow of ASME and president of Reisz Engineers in Huntsville, Ala. Steve Rodgers is the manager of the Propulsion Research Center at NASA's Marshall Space Flight Center in Huntsville.