As NASA and other space agencies continue humanity’s interplanetary reconnaissance, one thing is becoming very clear: on balance, the solar system is a rather soggy place. Water, mostly in the form of ice, lurks practically everywhere we look. There are water deposits on the Moon, on Mars, and even in the cold, shadowed floors of deep polar craters on sun-broiled Mercury. Water exists in even greater abundance further out from the sun, constituting much of the crust for a wealth of dwarf planets, moons, and asteroids and even occasionally forming subsurface oceans.
Planetary scientists speak often and with great eloquence about how all this water boosts the possibility of alien life right in our solar system; much less discussed is how it boosts the possibility of carrying human life far beyond Earth. Water will be a cornerstone of our existence everywhere we go, of course, perhaps in more ways than you realize. The killer app for all that extraterrestrial water isn’t just beverages and baths—it’s also rocket fuel.
Water already serves as a fuel for rockets, by way of its chemical constituents, hydrogen and oxygen. Today, the highest performance rocket engines burn liquid hydrogen and oxygen to create a very hot exhaust of pure water that propels them through space. Such rockets are very complex and expensive, requiring cryogenic tanks to prevent the liquid hydrogen and oxygen from boiling away. They are also very mature technologies, performing at the outer edge of what is possible for chemical propulsion. In cost and scope, they offer limited room for growth. Fortunately, there is another way to fuel rockets using water, one that requires no cryogenic storage and that has huge possibilities for further development. Plain, old water, combined with electric propulsion, offers many advantages that chemical rockets simply can’t match.
An electric water engine produces thrust by using electricity to accelerate water vapor at high velocities through an exhaust nozzle. That electricity can be produced essentially for free via sunlight soaked up by solar arrays on a spacecraft. From there, it can be used to produce microwaves or other electromagnetic forces that heat and accelerate the water vapor. A wide variety of electric propulsion technologies can be used in this form of rocket.
To understand why electric water rockets offer revolutionary prospects for space travel, consider the challenge facing a designer of a 500-day round-trip voyage to the Martian moons. To get there and back, every kilogram of mass—the crew, their life support and their spacecraft—needs many, many kilograms of rocket fuel. Thus mission planners ruthlessly reduce mass, often through compromising the robustness of life-support systems and minimizing the amount of potable water and food. Cut too much mass, and you jeopardize your crew. Cut too little, and you’ll be too heavy to ever go anywhere. For a 500-day mission to Mars, each astronaut may only have about 5 tons of water with which to drink, bathe and clean. A very short shower might be a weekly luxury.
A water-fueled electric rocket can change the rules of this game, and transform the economics of spaceflight, because it allows most of a crew’s water reserves to also serve double-duty as propellant.
This transformation of a large deadweight into working mass offers many advantages. Water for propellant would mean a huge reserve of water for the crew, greatly improving safety margins. For instance, if there was a problem with the spacecraft’s oxygen supplies, a small fraction of its extensive water reservoir could be electrolyzed to replenish the oxygen for many years, allowing time for a deep-space rescue or delayed return to Earth. Further, water could also bring another benefit, being held in tanks around the crew quarters to act as a shield against cosmic radiation. Even though all that water would be heavy, it would confer such significant mass savings overall that it could result in missions with a fraction of the mass of their chemical-rocket counterparts, dramatically reducing the high cost of launching supplies into Earth orbit.
Eventually, we envision that water ice from off-world locales around the solar system could be mined and used to refuel deep-space electric-water vessels, creating “interplanetary water holes” that would foster further development of space-based economies and infrastructures. Ceres, with its enormous water reservoir and low gravity, is an especially interesting destination.
This all may sound too good to be true, but a wealth of data confirms the fundamental promise of electric water rockets. Tests with a few possible engine types, such as microwave electrothermal thrusters, electrodeless Lorentz force thrusters and helicon double layer thrusters, show that they can produce exhaust velocities two to ten times that of the best chemical rockets while using water or waste gases as propellant. Those higher exhaust velocities translate into greater fuel efficiencies, and thus even larger cost savings for any interplanetary trip. However, while these engines have high exhaust velocities, they produce little thrust. This makes them unsuitable for launches directly from Earth to orbit, but they are perfect for long space voyages as the great distances involved allow the slow build up of velocity to astonishing speeds.
Developing electric water rockets would be only one small step in what is needed for humanity’s further expansion into outer space; what’s needed next is a giant leap, one that takes us from throwaway rockets to fully reusable interplanetary spacecraft. Reducing the mass of a spacecraft will only provide minimal cost savings if that spacecraft is only used once then thrown away. Very few of us would be flying to tourist resorts if our ticket price included building the airplane as well! Using a vehicle many times is a key to low ticket costs, and yet the wasteful paradigm of single-use spacecraft has held sway over human spaceflight for its entire history. (NASA’s vaunted Space Shuttles were, alas, only partially reusable, requiring extensive and expensive post-flight refurbishment that counteracted any possible cost savings.).
To forge a path forward, we can look to relevant examples from the past. At the dawn of the space age, many experts predicted that sturdy, enduring spaceships, such as Ernst Stuhlinger’s Cosmic Butterfly, would someday voyage between planets like any seagoing vessel. They would always stay in space just as ships stay in the sea, to be used many times before being scrapped or recycled, with crew and passengers ferried to and from by smaller craft. It was that system that allowed the Pilgrims to charter an aging mercantile ship, the Mayflower, to sail them to the New World, in 1620. Centuries later, stagecoaches were the great movers of people and goods across the American continent before the advent of railroads.
In honor of those pragmatic times, we propose the development of a “spacecoach,” our name for a fully reusable water-propelled solar electric interplanetary spacecraft. The synergies of using water and electric engines, as well as the reusability of the spacecraft, suggest that this approach should be considerably less expensive and thus more enabling than the current government plans for extending humanity’s off-world presence. We believe that this is a viable approach that could open up spaceflight to nations and institutions that could never afford today’s limited and exceedingly expensive disposable options.
While the idea of a reusable interplanetary spacecraft may seem like a huge technological leap, something that’s decades away, the component technologies needed to build spacecoaches already exist. Solar power, electric propulsion, and inflatable structures are well understood technologies that are in active development and are space ready. Through the development of elegant, inexpensive spacecoaches to harness the solar system’s abundance of solar power and water, perhaps a future group like those 17th-century pilgrims could gain the chance to begin again and found an off-world colony.
Brian McConnell is an author, engineer and technology entrepreneur based in San Francisco. Alex Tolley is a retired engineer, university lecturer and MOOC designer based in the Central Valley.For more information about the spacecoach concept, or their upcoming book from Springer Verlag, readers can visit www.spacecoach.org