To give the gas its very high speed, its component atoms are each stripped of an electron—ionized—in the thruster. This diffuse plasma (charged gas) is the heart of an ion propulsion system. The ions are then accelerated down a lightweight cylinder with a set of magnets and electrical grids to control the ions’ direction. The ions that depart the engine form a diffuse beam, much faster than the thermal output of a chemical rocket. The faster the propellant leaves, the less weight you have to carry along for a flight.
Benefits of Ion Propulsion
Two types of missions have been studied extensively by the NASA Office of Exploration for possible application of ion drives. These programs involve many flights to the moon and Mars over several years. Their ultimate goal is to help eventually construct permanent settlements from which humans can range and explore, seeking the knowledge of new worlds.
Consider a manned mission to Mars. NASA mission planners have suggested sending the food, science payloads, and propellants needed at the Red Planet by astronauts aboard cargo vehicles, sent well ahead of the vehicle carrying people. Since the manned craft would be lightened, a faster flight would be possible, which would minimize the crew’s exposure to solar flares and other space hazards. Once both vehicles were in orbit around Mars, they would rendezvous and transfer the food and other items.
In one design, the cargo to be taken to Mars would be 180 metric tons. But with fuel added, the total mass of the Mars cargo vehicle, using oxygen and hydrogen propellants, would be 590 tons! With ion propulsion that total would only be 295 tons.
So a Mars mission using ion propulsion for the cargo vehicles could weigh as little as half a comparable mission using oxygen and hydrogen propellants. Also, the Mars craft could be placed in Earth orbit more quickly because fewer flights would be needed to launch the vehicle components, especially propellant. (In this mission, solar sails would have a slight edge in efficiency, but might be slower. In any event both would be far cheaper than chemical rockets.)
Flights to lunar orbit could also significantly benefit from alternative propulsion, and here ion drives come out even ahead of sails.
The total mass that must be delivered for a lunar base is large: 300 to 1,000 tons for Phase One alone in one NASA scenario. Each piece could be transported from the ground to Earth orbit by conventional rocket, and then to lunar orbit by an ion propulsion vehicle at a mass savings of 50 percent over oxygen and hydrogen propulsion. Solar sails, on the other hand, would have little room to maneuver on the way to the moon. One thing that would strongly affect the sail is the Earth’s atmosphere. Because of its large surface area, the sail would have to be deployed at a high altitude. If it were unfurled below 1,000 to 2,000 kilometers altitude (625 to 1,250 statute miles), the sail would begin to re-enter the atmosphere. But an ion propulsion system could be deployed at 500 kilometers (313 statute miles). A lower starting orbit would mean either smaller launch vehicles to put the payload up or more payload launched by the same rocket from Earth. The sail’s very large area also could make it difficult to maneuver close to Earth—and it would have to maneuver every orbit to keep the best angle to the sun.
Solar-powered ion propulsion at very high power levels (many tens of megawatts) has the same problems as a sail because of the large collectors required. However, the power level used for most missions would be modest. For these reasons, ion drives may edge out sails for use in Earth-lunar space.
Other missions proposed by the Jet Propulsion Laboratory include extensive exploration of all aspects of the solar system, from the tenuous dust trails surrounding comets to the multihued clouds wreathing the outer planets. Like solar sails, ion propulsion could provide great benefits to all of these explorations. But there is one extra advantage ion systems would bring to such missions: added power for science experiments. After the spacecraft arrived at its destination, the power source used to propel the ions wouldn’t be needed for that purpose anymore. Whether it was solar or nuclear, the source could then provide electricity to advanced science instruments, such as powerful radars. These could probe the dense atmospheres of outer planets, determine the subsurface structure of Mars and the moon, and provide better communications with Earth. By using this power other than solely for propulsion, the entire spacecraft would become more capable and more cost-effective.
Power: The Other Big Piece of the System
While we’re on the subject, power is a crucial part of any ion propulsion system. (This is one major distinction from solar sails, which get both their power and “fuel” from sunshine.) Both nuclear and solar power sources are possible. Any space power source must be efficient and lightweight.
Electronics must then take the power from the source and provide it at the right voltage and current to the engines. Together, the power source and electronics might make up more than 70 percent of the ion spacecraft’s weight (without propellants).
If the ion system uses nuclear power, a long boom between the power source and the rest of the spacecraft would be required to keep high-level radiation away from the crew and instruments. Or an ion-drive spacecraft might use solar power, concentrating sunlight to provide thousands of kilowatts. The solar panels might have an area of 10,000 square meters, larger than two football fields, but smaller than a sail used to carry the same payload.
Rebel Technology?
It has been asserted that space development calls for “rebel” technologies, to allow wide access to spaceflight. These technologies would provide not just big governments but corporations and even citizens the freedom to get into and out of Earth orbit inexpensively. Propulsion and power will certainly be two of the most important technologies enabling this “rebellion,” toward the settlement and industrialization of the planets. Making these widely available, and user-friendly, will be a great and exciting challenge.
This revolution will have to be a friendly one. The initial investment, proving the spaceworthiness of ion-propulsion spacecraft, must be financed by governments. But later, as with solar sails, the technology could be made available at significantly lower cost. Private users could then avail themselves of ion propulsion’s benefits. As the short story by Charles Sheffield demonstrates, some of these benefits may be astounding and even fun.
The Bright Future
There are few ideas or experiences which match the wonder of spaceflight. Exploration of the solar system thus far has been among humanity’s finest achievement. Future exploration will require new technologies, developed by NASA and other innovative organizations. Just as it has been unwise to ignore the potential of solar sails, it would seem silly not to explore the great possibilities offered by ion propulsion as well. This is an area in which a little friendly competition can only help us all.
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Bryan Palaszewski has recently joined the NASA Lewis Research Center’s Space Propulsion Technology Division. Prior to joining NASA, he was a member of the technical staff at the NASA Jet Propulsion Laboratory for six years. During that time, he led many diverse studies of advanced propulsion systems, including chemical propulsion and especially electric propulsion.
In our next story, Charles Sheffield illustrates some of the possibilities opened up by ion drives. And if the theme (a speed race in outer space) strikes you as familiar, well, aren’t some ideas so good they deserve another look?
Grand Tour
by Charles Sheffield
Tomas Lili had won the Stage, square if not fair, and now he was wearing the biggest, sweatiest grin you have ever seen. Tomorrow he would also wear the yellow jersey on the next-to-last Stage of the Tour.
Ernie Muldoon had come second. In one monstrous last effort of deceleration, I had almost squeaked in front of him at the docking, and hit the buffer right on the maximum allowable speed of five kilometers an hour. We had been given the same time, and now we were collapsed over our handlebars. I couldn’t tell about Ernie, but I felt as though I were dying. For the last two hours I had been pedaling with a growing cramp in my lef
t thigh, and for the final ten minutes it was as though I had been working the whole bike one-legged.
After five minutes’ rest I had recovered enough to move and speak. I unbuckled my harness, cracked the seals, and climbed slowly out of my bike. As usual at the end of a Stage, my legs felt as though they had never been designed for walking. I did a couple of deep knee-bends in the half-G field, then straightened up and staggered over to Muldoon. He had also flipped back the top of his bike and was slowly levering himself free.
“Tomas was lucky,” I said. “And he cheated!”
Muldoon looked at me with eyes sunk back in his head. He was even more dehydrated than I was. “Old Persian proverb,” he said. “Luck is infatuated with the efficient.”
“You don’t think he cheated?”
“No. And he wasn’t lucky, he was smart. He bent the rules, but he can’t get called on it. Therefore, he didn’t cheat. He was just a bit smarter than the rest of us. Admit it, Trace, you’d have done it too if you’d thought of it.”
“Maybe.”
“Maybe, schmaybe. Come on. I’ve been cramped in this bike for too long. Let’s beat the crowd to the showers.”
He was right; the others were streaming in now, one every few seconds. As we left the docking area a whole bunch zoomed in together in practically a blanket finish. I saw five riders from Adidas, so close I was sure they’d been slipstreaming for a sixth member of their team. That was outside the rules, and they were bound to be caught. Five years ago, slipstreaming had been worth doing. Today, it was marginal. The teams did it anyway—because the man who benefited from the slipstreaming was not the one doing anything illegal. The rider who had given the momentum boost would be disqualified, but that would be some no-hoper in the team. Illogical? Sure. The Tour had a crazy set of rules in the first place, and as more and more riders became part of the big teams, the rules became harder to apply. Ernie Muldoon and I were two of the last independents racing the Grand Tour. Ernie, because he was famous before the team idea caught on; me, because I was stubborn enough to want to win on my own.
Tomas was already sitting in the cafeteria as we walked through it, surrounded by the microphones and cameras. He was enjoying himself. I felt angry for a moment, then decided that it was fair enough. We waved to the media and went on to the showers. Let Tomas have his day of glory. He was so far down in the overall ratings that there was no way he could be the outright Tour du Système winner, even if he won tomorrow’s and the final Stages by big margins.
Ernie Muldoon thought that the overall Grand Tour winner was going to be old and wily Ernest Muldoon, who had already won the Tour du Système an unprecedented five times; and I thought it was going to be me, Tracy Collins, already identified in the media coverage as the Young Challenger; or maybe, as Ernest put it, the Young Pretender. Which made him, as I pointed out, the Old Pretender.
I had modeled my whole approach to the Tour on Ernie Muldoon, and now it was paying off. This was only my third year, but unless I was disqualified I was certain to be in the top five. My cumulative time for all the Stages actually placed me in the top three, but I hate to count them little chickens too soon.
The shower facilities were as crummy as we’ve grown to expect. You’ve got one of the premier athletic events of the Solar System, with coverage Earthwide and Moonwide, and still the showers at the end of each Stage are primitive. No blown air, no suction, no spin. All you get is soap, not-warm-enough water, drying cloths. It must be because we don’t attract top video coverage. People are interested in us, but what sort of TV program can you build out of an event where each Stage runs anything up to thirty-six hours, and the competitors are just seen as little dots for most of the time? Maybe what the media need are a few deaths to spice things up, but so far the Tour has been lucky (or unlucky) that way.
Muldoon slapped me on the back as we were coming out of the shower area. “Three quarts of beer, three quarts of milk, thirty ounces of rare beef, and half a dozen potatoes from the original Owld Sod, and you’ll not even notice that leg of yours. Are you with me, lad?”
“I’m with you—but not this minute. Don’t you want to get a weather report first, for tomorrow?”
“A quick look, now. But I doubt if we’ll see anything special. The wind forecasts for tomorrow have all been quiet. It’s my bet we’ll see stronger winds for the final Stage. Maybe a big flare-up.”
Muldoon was casual, but he didn’t really fool me. He had told me, a dozen times, that the solar wind forecast was the most important piece of a rider’s knowledge—more relevant than local gravity anomalies or superaccurate trajectory calculations. We went over to the weather center and looked at the forty-eight-hour forecast. It was pretty calm. Unless there was a sudden and dramatic change, all the riders could get away with minimal radiation shielding.
That wasn’t always the case. Two years ago, the second half of the Tour had taken place when there was a massive solar flare. The solar wind of energetic charged particles had been up by a factor of a hundred, and every rider added another two hundred kilos of radiation shielding. If you think that doesn’t make a man groan, when every ounce of shielding has to be carried around with you like a snail carrying its shell—well, then you’ve never ridden the Tour.
Of course, you don’t have to carry the shielding. That’s a rider’s choice. Four years ago, on the eleventh Stage of the Tour, Crazylegs Gerhart had done his own calculation of flare activity, and decided that the radiation level would drop nearly to zero a few minutes after the Stage began. When everybody else crawled away from Stage-start loaded down with extra shielding, Crazylegs zoomed off with a minimal load. He won the Stage by over two hours, but he just about glowed in the dark. The wind level hadn’t become low at all. He docked so hot with radiation that no one wanted to touch him, and he was penalized a hundred and fifty minutes for exceeding the permissible dosage per Stage by ninety-two rads. Worse than that, they dumped him in the hospital to flush him out. He missed the rest of the Tour.
Every rider had his own cookbook method for guessing the optimal shielding load, just as everyone had his own private trajectory program and his own preferred way of pacing the race. There were as many methods as there were riders in the Tour.
Muldoon and I made notes of the wind—we’d check again, last thing at night—and then went back to the cafeteria. A few of the media people were still there. Without looking as though we were avoiding them, we loaded our trays and went off to a quiet corner. We didn’t want the Newsies tonight. The next-to-last Stage was coming up tomorrow morning, and it was a toughie. We had to ride nearly twenty-five thousand kilometers, dropping in from synchronous station, where we had docked today, to the big Sports Central station in six-hour orbit.
Some people complain because we call it the Tour du Système when the only part of the solar system we travel is Earth-Moon space. But they’ve never ridden the Tour. When you have, the six-hundred-thousand-kilometer course seems quite long enough. And the standards of competition get tougher every year. All the original Stage records have been broken, then broken again. In a few years’ time it will be a million-kilometer Tour, and then we’ll zip way out past the moon before we start the inbound Stages.
Muldoon and I stuffed ourselves with food and drink—you can’t overfeed a Tour rider, no matter what you give him—then went off quietly to bed. Two more days, I told myself; then I’ll raise more hell than the devil’s salvage party.
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Next morning my first worry was my left thigh. It felt fine—as it ought to; I’d spent an hour last night rubbing a foul green embrocation into the muscles. I dressed and headed for breakfast, wanting to beat the rush again.
“Well, Tracy, me boyo.” It was Muldoon, appearing out of nowhere and walking by my side. “An’ are you still thinking ye have the divil’s own chanst of beating me?”
He can speak English as well as I can, but when he senses there are media people around he turns into the most dreadful blarney-waffling
stage Irishman you could find.
“Easily.” I nudged him in the ribs. “You’re a tough man, Muldoon, but your time has come. The bells will be pealing out this time for handsome young Tracy Collins, overall winner of the Grand Tour du Système.” (So maybe I respond to the media, too; I sounded confident, but Muldoon couldn’t see my fingers, crossed on the side away from him.)
“Not while there’s breath in this breast, me boy,” he said. “An’ ’tis time we was over an’ havin’ a word here with the grand Machiavellian Stage winner himself.”
Muldoon stopped by Tomas Lili’s table, where a couple of press who must have missed the Stage winner the previous night were sitting and interviewing. “Nice work, Tomas, me boy,” Muldoon said, patting the yellow jersey. “An’ where’d you be getting the idea of doin’ that what yer did?”
A couple of media people switched their recorders back on. Tomas shrugged. “From you, Ernesto, where we all get our ideas. You were the one who decided that it was easier—and legal—to switch the ion drive around on the bike at midpoint rather than fight all the angular momentum you’d already built up in your wheels if you tried to turn the bike through a hundred and eighty degrees. I just built from there.”
“Fair enough. But your trick won’t work more than once, Tomas. We’ll be ready.”
Tomas grinned. He had won a Stage, and that’s more than his Arianespace sponsors had expected of him. “What trick ever did work more than once, Ernesto? Once is enough.”
The media rats at the table looked puzzled, and now one of them turned to Ernest Muldoon. “I don’t understand you. What ‘trick’ is this you’re talking about?”
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