Because of this, MagBeam imparts rapid acceleration to the departing spacecraft, on the order of 1 ms-2 [7]. This may not sound very large—the equivalent of a car taking about twenty-five seconds to reach sixty mph—but it's far higher than other ion drive systems. By comparison, the solar-powered ion drive on DS-1 provided acceleration about ten thousand times smaller. Proposed nuclear-powered craft, such as VASIMR [9], do about ten times better than solar powered systems, but still accelerate a thousand times slower than MagBeam.
A logical question might be “How does the beam remain targeted on the vessel?” With such high accelerations, the distance of the vessel from the platform gets very large very quickly—reaching as much as 150,000 km for the longest anticipated accelerations. This isn't expected to be a problem. The plasma beam is like a stroke of lightning. Lightning is a very coherent beam of plasma that seeks a region of high conductivity to transfer excess energy from a cloud to the ground: that's why smart people don't fly kites in thunderstorms. Our magnetized beam behaves in the same way.
The payload out in space acts as a lightning rod, and the plasma beam is essentially the same as an electric current, with the magnetic field it drags along acting similarly to a wire. Space itself is pretty much empty, and therefore of low conductivity. When the beam is turned on, it makes an electrical contact with the payload near the station, and then it's just like paying out a transmission line as the vessel moves away. The beam stretches out between the two regions of high conductivity (the platform and the payload) until it's turned off at the platform. Electricity always seeks the path of least resistance, and so the plasma flows along the conductive path traced out by the retreating vessel, confined by its inherent magnetic field.
Once the spacecraft has been accelerated, the beam is turned off, and the craft coasts toward its destination at very high speed—as much as 20kms-1 for a 50-day trip to Mars. This is faster than any human vehicle has ever traveled, including the Voyager probe with its multiple gravity assists over several years. On arrival, of course, the MagBeam vessel has to brake, or it would shoot past its destination and never be seen again. The MagBeam system uses another platform on the opposite end of the journey to provide the thrust to brake.
Braking is more difficult than acceleration, because there's no conductive path between the platform and the payload for the plasma beam to follow. So, the platform at the destination would have to be activated when the spacecraft was already quite close to it, then be run at higher power than during acceleration to get in the same amount of energy in a shorter time. However, once the connection is made, the vessel uses the same thrust system it used for acceleration to brake by simply deflecting the ionized gas ahead of itself instead of behind.
Advantages and Risks of
the MagBeam System
One useful way to think of the MagBeam system is to compare it to an electric train. Huge generators and transformers are needed to produce the electricity to power such a train, but none of this equipment is carried on the train itself. If it were, the train would be so heavy that it would hardly be able to move. Instead, the electricity is transmitted to it through the electric rail—just as it's transmitted through space in MagBeam—and can not only power the train to much higher speeds, but allow it to carry more cargo in place of the generators.
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Figure 3: Schematic of MagBeam propulsion for a fifty-day mission to Mars.
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In a conventional system, where the propulsion system and propellant are all carried on the payload they are propelling, the acceleration potential for the payload is reduced because of the mass of the propulsion system (as much as 80% by mass of the Deep Space-1 vessel consisted of propellant, propulsion system, and power supply [10]). The MagBeam system transport spacecraft will be much lighter than the platform that carries its propulsion system [7], and so can be accelerated much more rapidly and to much greater speeds than any spacecraft that carries its own propulsion system on board.
The amount of time the platform needs to beam energy to the craft is quite short—as little as five minutes to transfer a ten-ton payload from suborbital altitudes to a low earth orbit, or thirty minutes to propel it to geostationary or escape velocity, according to Professor Winglee's calculations [1]. For maximum flexibility, the system uses two platforms: one in a low Earth orbit (LEO), and one in a much higher orbit. Payloads are raised to sub-orbital altitudes using standard launch methods (such as those used for SpaceShipOne), and then the LEO platform takes over. The LEO platform propels the payload into low Earth or geostationary orbit, or even to escape velocity for a trip to the Moon. For longer missions requiring higher speeds, such as a trip to Mars, the higher orbital platform is used to provide additional acceleration. (See Figure 3.) Longer interaction times are necessary to achieve the higher speeds for interplanetary travel. To achieve these, a higher orbiting platform is required, so that the Earth doesn't get in the way during the interaction.
In addition to the fast accelerations and high final speeds MagBeam offers, the savings in terms of fuel are also significant. Professor Winglee predicts that for MagBeam to send a ten-ton payload to Mars in just fifty days, only a four-hour interaction time and a total of seven tons of propellant are required. Carrying out the same mission using today's conventional rockets would require an enormous 18,000 tons of fuel. The potential of MagBeam is obvious.
Currently, space missions operate using a “one power plant, one rocket, one mission” paradigm, where all the expensive propulsion hardware on a deep space vessel is lost after one mission. Not only does it cost money to keep building new power plants for successive missions, but it costs on the order of $10,000 per kilogram just to get them into orbit. The MagBeam system, by using an orbiting propulsion platform with an effectively infinite lifetime, and propelling a succession of small, reusable, unpowered or low-powered craft, would reduce hardware costs radically in the long term.
The MagBeam system would require a large initial investment to build the platforms and associated infrastructure (an estimated $10 billion at present to launch a platform capable of sending that ten-ton payload to Mars in 50 days [7]), but repeated missions would bring vast long-term savings over more conventional propulsion systems. This is partly due to the far smaller propellant requirement and partly due to the reusable nature of the orbiting platforms.
Having platforms at both ends of proposed journeys is a key element of the MagBeam system. Unfortunately, while the system would provide drastically reduced trip times once in place, currently no way exists to get the destination platforms into place quickly. To propel a spacecraft, the MagBeam platform will need to be between 20 and 100 times more massive than the craft it pushes [7]. Positioning this much mass around Mars will still require longer trip times, using conventional propulsion methods. If we continue the analogy with a rail system, this is equivalent to the time and cost of laying the tracks and building the stations.
However, adding MagBeam capability to facilities already in orbit could be relatively simple. For instance, if an orbital station already existed around Mars in support of manned exploration, a MagBeam platform could be attached and powered, making fast trip times possible. The equipment to produce the plasma beam is relatively light—the great bulk of the mass comes from the batteries or other power supplies needed to operate it. For example, to carry out the four-hour acceleration required for the 50-day trip to Mars, the platform would require about 3,000 tons of batteries to power its 300-megawatt thruster. This seems like a lot, and indeed getting an equivalent mass to Mars by conventional means to carry out decelerations would be expensive. But once in place, these battery systems could be recharged indefinitely via solar power, so the cost would be a one-off capital investment. Better still, even conservative estimates suggest that improving battery efficiencies in the next decade could bring the weight requirement down by as much as a third [1].
If MagBeam has a downside, it co
mes from the high speeds that the spacecraft obtain.
“You're traveling at high speed,” Winglee told us, “so anything at high speed is going to be more dangerous per se than lower speed. On the other hand, if you go out at low speed, you end up with a whole lot of biological dangers. So we're swapping biological hazards for physical hazards. If you make a mistake at 20 kms-1 vs. your standard 7 kms-1, it's going to make a bigger change. Would it still be safe? We try to keep it as safe as possible insofar as you guide the system and make course corrections, but once you're up to speed, you're up to speed, and you have to wait to the other side to make course corrections because the speeds are so high."
Implications for the Future
The validity of the MagBeam idea has been proved in the laboratory, and plans are afoot to expand the testing to larger scales. At the NIAC conference in March 2005, Professor Winglee expressed his hope for a space-borne test as soon as 2009. Even though a MagBeam commuter system doesn't yet exist, we can still speculate on what it might mean to the manned and unmanned exploration of the solar system, and to robotic flights to nearby stars.
We've seen a pattern throughout history of the expansion of civilizations as transportation to new lands becomes cheaper and easier. Humans first migrated to North America because the land bridge between Alaska and Siberia made it possible. When the land bridge was no longer available, Europeans took to the seas and sailed to the new world, where expansion took place first on foot, then by covered wagon, and eventually by rail. During the Great Depression, automobiles made possible the huge migrations of Midwesterners to California, where people searched for new opportunities. Today, many people on the eastern seaboard use commuter trains to get from their homes to their jobs, covering distances daily that would have awed the early colonists. This would be impossible without inexpensive, dependable transportation.
MagBeam should not be seen exclusively as a method for reducing trip times to distant locations in our solar system. A platform in a low Earth orbit could be used to modify the orbits of satellites or space stations, and also to boost payloads headed for the Moon—all far more cheaply than currently possible. For example, a platform on the Moon could be used for returning rock sample containers to Earth far more economically than dragging chemical propellant and rockets to the Moon for the purpose. A platform in Earth orbit would decelerate the Moon samples, and insert them for re-entry. When combined with inexpensive orbital launch techniques, the possibilities for an expanded presence on and commercial development of the Moon become financially much more attractive, both for businesses and for tourists.
Although the need for deceleration platforms means that the MagBeam system would not allow us to reach distant destinations any sooner than existing methods, it would allow for much cheaper, more rapid repeat journeys. Instead of being hugely expensive single endeavors as they are now, trips to Mars or the outer planets could easily become much more commonplace. For example, the unmanned Mars Exploration Rover Mission took six and a half months to arrive on Mars [11], while a MagBeam system, once implemented, could have a crew there and back in just 96 days, including eleven days on the surface. (See Figure 4.)
Successful colonization of distant places like Mars hinges on an easy system of re-supply until such time as the colony can become self-sufficient. With our present technology, we can only launch missions to Mars about once every two years, when the planets are in the proper alignment. However, with the kind of acceleration MagBeam could impart, we would no longer have to wait for those brief windows of opportunity to send people and supplies. Although trips at times when the Earth and Mars are not aligned would take longer than the 50-day trip described by Professor Winglee, they could still be made in relatively short amounts of time and at very low cost compared to those fueled by other means.
Far distant locations such as Jupiter or Neptune are currently not considered plausible destinations for manned missions because of the extensive travel time involved, and scientific missions taking more than ten years to produce data are rarely considered useful. However, if MagBeam can reduce the travel time to Jupiter from ten years to a year and half, travel to these places by humans becomes more enticing. And before humans go, an Earth-orbital MagBeam platform gives us the chance to launch larger and more frequent robotic science missions at vastly reduced cost, so we'll be better prepared when we do set forth.
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Figure 4 (image used courtesy of Robert M. Winglee):
Orbital schematic of 96 day Mars return trip.
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Another exciting application for MagBeam could be accelerating interstellar probes. MagBeam has the capability to accelerate robotic probes to much greater speeds than those attained by any craft to date. Additional speed could be obtained by using successive platforms at distant locations such as Mars and Jupiter to boost the probes’ speed as they head out of the solar system, greatly reducing trip times to the nearest stars. Any such trip would probably still take centuries, but the prospect is an exciting one nonetheless.
Like the commuter train, MagBeam offers humans the chance to expand the neighborhood and explore new places, yet still remain in close touch with our roots on Earth. Professor Winglee sums it up succinctly.
“Yep, you could do that. You could really have some fun."
References
1. R.M. Winglee, “Magnetised Beamed Plasma Propulsion (MagBeam),” paper presented at the Fellows Meeting of the NASA Institute for Advanced Concepts, March 2005
2. M. Martinez-Sanchez & J.E. Pollard, “Spacecraft Electric Propulsion—An Overview,” Journal of Propulsion and Power, 14, 5, 688-699 (1998)
3. J. Brophy et al, “The Status of Ion Propulsion Development and Implementation at JPL in 2003,” AIAA Joint Propulsion Conference, 39th AIAA/ ASME/SAE/ASEE joint propulsion conference, page(s) 4711 AIAA, 2003
4. M. Noca, “Next Generation Ion Engines: Mission Performances,” presented at NASA Jet Propulsion Laboratory International Electric Propulsion Conference 2003, March 17-21, 2003
5. R.H. Frisbee, “Advanced Propulsion for the XXIst Century,” AAIS/ICA international air and space symposium 2003, vol. 19, ISSU 2625-2778
6. O. Batischev & K. Molvig, “Kinetic Models for the VASIMR Thruster Helicon Plasma Source,” presented at 43rd Annual Meeting of American Physical Society Division of Plasma Physics, Oct 29 2001
7. Interview with Professor Robert M. Winglee, Dept. of Geophysics, University of Washington. Interview carried out by Kathy Ferguson on March 7, 2005
8. R.M. Winglee et al, “Laboratory Testing of the Mini-Magnetospheric Plasma Propulsion (M2P2) Prototype,” Proceedings of Space Technology and Applications International Forum (STAIF-2001)
9. I. Katz et al, “Technologies to Improve Ion Propulsion System Performance, Life and Efficiency for NEP,” presented at NASA Jet Propulsion Laboratory Workshop on Technology and System Options Towards Megawatt Level Electric Propulsion, June 9-10, 2003
10. J.R. Brophy et al, “Ion Propulsion System (NSTAR) DS1 Technology Validation Report,” Jet Propulsion Laboratory Available at nmp-techval-reports.jpl.nasa.gov/DS1/IPSIntegrated Report.pdf
11. Mars Rover Exploration Mission: Press Release 6 August 2003 etc. at marsrovers.jpl.nasa.gov/newsroom/pressreleases/pressreleases-2003.htm
About the Authors:
James Grayson is 25 and holds a Masters degree in Physics from the University of Bath, England, where he was awarded the annual Physics World prize for achievement as a student in 2002. He has also worked as a research scientist for the Engineering and Physical Sciences Research Council of the UK. He has written for online compilations, University newspapers, and the hell of it for several years, but this is his first professional fiction publication. He has no cats and lives in the rain somewhere in the south of England.
Kathy Ferguson has undergraduate degrees in Animal Sciences, Psychology. and Secondary Education. She also holds a Masters in Clinical Psychology. For the past
ten years, she has worked as a technical writer in the computer industry, transforming geek-speak into English suitable for machine translation to other languages. Prior to her writing career, she held jobs as varied as hotel maid, vetrinary technicial, and vocational rehabilitation councelor. Previous publications include articles, essays, and show reports in several dog magazines, chapters in computer manuals, and technical articles on the TechNet website. In the rare moments when she isn't at a computer, she trains her Doberman Pinscher in obediance, agility, and tracking for competitions around the Pacific Northwest. At the moment, she calls Cheney, Washington home.
Copyright © 2006 James Grayson & Kathy Fergusonn
[Back to Table of Contents]
* * *
Original Sin
by Richard A. Lovett
Any tool can be put to alternative uses, some of which are ... tempting.
Eight thousand meters isn't really all that far to run. But it can be a hell of a long way to race. Especially on a tough cross-country course like this one, where mud and hills turn the legs turn to jelly until, by the final circuit, you're running solely on willpower.
I knew. It was my second time through, feeling that I was on the ragged edge of collapse but continuing, nonetheless. I'd run the race of my life the first time. Now, I was doing it again, for Dylan.
Analog SFF, June 2006 Page 8
