Starflight
A Matter of Perception
by Dave Plassman
A perennial view among science fiction readers is that travel to the stars depends upon faster-than-light velocities, wormholes in space or some form of dimension-hopping. “If we can never travel faster than light, voyages must always take too long.” Even at a few percent of Light Speed, energy requirements of interstellar voyages are enormous. (We presuppose evidently that with FTL, energy requirements will somehow drop?) What constitutes star travel, travel within reasonable time spans and over significant volumes of Galaxy or Universe, are essentially subjective matters and whether interstellar projects are ever accomplished, may depend as much upon psychology and sociology as physics or propulsion engineering.
Before examining the issues of why we might want to travel between the stars, let’s have a look at what the costs of interstellar travel at Minimally Relativistic velocities really are, so far as we can see today.
We tend to think of interstellar flight in terms of rockets which carry their power and propellant with them like an airplane or oceangoing ship. We most often think in terms of fusion energy, the combination of hydrogen atoms to form helium and a lot of energy. Fusion powers our sun but isn’t really quite what we need for starships. The problem is that four hydrogen fusing to form one helium atom, has a maximum theoretical exhaust velocity of 11.6 percent light speed. if a rocket using this sort of fuel at 100% efficiency, wishes to fly at an average speed of one tenth light speed, it will need to carry 33 times as much fuel as the ship weighs empty. Since our starships will likely be in the thousand ton range at least, if crews will be carried aboard, we’re talking about very large structures. Besides this, we don’t know how to fuse and control 4-8ydrogen fusion. The types we almost know how to use, are much less energy-productive.
Of course other proposals have been set forward for propelling starships than simple rockets. Probably about the most energy-efficient way of getting a starship to a reasonable fraction of light speed is to employ a source of both power and propellant which remains in our solar system. I am partially indebted to the science fiction writer and propulsion systems engineer G. David Nordley for the concept of The Matter Beam. If a continuous stream of projectiles or matter chunks is fired from a super-fast catapult, at perhaps a third of the speed of light, A ship could use this highly energetic stream of mass to “sail” to the stars. As the matter chunks, each weighing perhaps a few milligrams, drew close to the starship, they would be zapped by lasers perhaps and their kinetic energy absorbed in a magnetic field which would push the starship along. If a 1000 ton starship is to accelerate over 20 years, to 20-pc of light speed, (for an average voyage velocity of 10-pc Can.) it must accelerate at about one hundredth of a gravity. (one percent of the force normal to earth’s surface.) to provide such an acceleration a matter beam travelling at 28 percent of light speed would perform most efficiently from the standpoint of energy usage. The beam would deliver a mass of two and a quarter grams each second to push the spacecraft. Though this is a seemingly small amount of mass, the energy needed to accelerate it to it’s requisite velocity of over 84 thousand meters per second or 188 million miles per hour, requires at least eight (picawatts). A kilowatt is about the average amount of energy needed to power a home immoderate weather. A thousand of these make a megawatt. A million megawatts make a picawatt. If solar cells were used to generate electricity at 10-% efficiency, eight picawatts would require a square of land 177 miles on a side, 20 million acres in all; probably on the surface of the moon or distributed across several big asteroids.
So far though, we’ve just gotten the starship up to 20-pc of light speed. How is it supposed to get back down to planetary velocities when the destination is reached? A matter beam or as Mr. Nordley terms it, a “pusher beam” can actually provide deceleration as well as acceleration, if we employ a special sort of engine.
Let’s imagine a vulume of space containing a small amount of matter and held in the grasp of a very powerful magnetic field. Let’s say that a smaller amount of matter enters our confined space at extremely high velocity. The high speed matter gives up it’s energy to the matter originally in the confined space. The matter within our magnetic field becomes very hot indeed and can be directed by means of the magnetic field in any direction of thrust we desire, even in a direction opposite to the high velocity matter beam. The mathematics are a bit complex, but if the same amount of energy we used to push the starship were directed into a beam meant for deceleration, we’d use much less mass but the beam would travel at a higher velocity, perhaps two-thirds of the speed of light. By turning kinetic motion energy first into heat energy, then into thrust, we could decelerate our spaceship to zero velocity at the end of the trip.
Decelerating a starship, though powered by our matter beam, would still require some fuel. To use as little energy as possible, we’d want about four times as much fuel as we have empty rocket. This means that we either need to design a starship weighing only 1000 tons or we need to start with a launch mass of 394 thousand tons and our matter beam must be about four times as powerful. This means we’d need a solar cell covered area of land about 350 miles on a side, Washington State with a lot of Oregon thrown in. Will we ever aspire to power projects of this magnitude?
Before trying to answer this question let us take a look at the consequences of a 43-year-long flight to Alpha-Centauri. Specifically, would the people who would best land on a virgin planet be the same persons who would make the trip?
Sociology; The people who enter upon a space voyage of forty years or more must be special folk indeed. To live in a confined space for half a human life span is a challenge to any discipline however rigorous.. Persons qualified to enter upon such an expedition must surely be near 30 years of age before being ready to depart, and must be well past conventional retirement age on arrival at their destination.
Even the bold adventurers who sailed with Magellan, Drake and Cartiers would have mutinied at the prospect of a voyage lasting 20 years let alone 40 or more. Yet the sort who sailed with the early explorers of the New World are precisely the sort we’d want for exploring an entire new planet. Certainly we could select a crew picked for cooperation and interrelation and plan to raise the next generation as Explorers. Here however, a very interesting point arises. If we plan to raise a generation for planet-fall which differs from the character of those embarking on the voyage, does it really matter if the Explorers are only one generation in the future, or twenty?
The Slow Route to the stars. If most of earth’s population is unlikely to journey out of the solar system any time soon, (we’ve gotten some idea of the expense involved even for a small mission) does it really matter how long it takes? What if we were content to reach Alpha Centauri in four hundred years rather than forty? Let’s consider a spacecraft weighing 100,000 metric tons–a pretty respectable payload. Let’s say we’re interested in accelerating it to one percent of light speed, about 6.7 Million miles per hour. Since the trip will be a long one, we can take 50 years to reach this speed. We can accomplish this using a matter beam travelling nine and a half million miles per hour and packing 810 (gigawatts) power. As a megawatt is 1000 kilowatts, a Gigawatt is 1000 megawatts. This means that for about two and a half percent of the energy budget for the 1stajth light speed voyage, we can launch 100 times as much ship for a 1 percent light speed mission. This is partly because we don’t budget fuel for deceleration, but mostly because using such slow matter beam velocities, go a great deal toward saving energy. For one Big, Slow Starship, we’d need only a bit over 30 square miles of solar cells, still a huge project, but with self-reproducing robot manufacturers/assemblers, it could be accomplished perhaps in a few months and on any of numerous lunar, asteroid or moonlet locales within the nearer solar system.
At a velocity of only 6.7 million miles per hour, engine deuterium-fuelled fusion engines might perform very adequately, but I think our slow-voyaging starbound colony might prefer a matter beam all the same. To see why, consider that the matter beam for this project will deliver 90 grams or about three ounces of matter each second. that’s nearly 17 tons per day or over 6,000 tons per year. While the faster starships will need to direct the matter from their pusher beams into the space ahead of the craft in order to screen against meteoroids and stray hydrogen molecules resident in the interstellar depths; the slower ships could use the material for manufacture or new construction. (The ship could Grow or metamorphose during transit, especially after the initial 50-year acceleration has been accomplished.)
Metaphysics; Why though, would anyone wish to embark upon even a potentially Growable starship if it would still take four centuries to reach any known landfall? We don’t know. Admittedly the Slow Starship may be one of those solutions awaiting a need but much will depend on what else happens in the next couple of centuries on Earth and the nearer space environs. I’d assume that a primary function of any Generation Ship, one meant to travel over multiple human lifespans, would be astronomical. Even if we build the smaller, faster starships, the Big Slows will afford the best opportunity to get really large astronomical observatories out beyond the Sun’s influence and nearer to the rest of the universe. What this will really mean, we can’t say, but I’d bet much could be learned about gravity as well, out beyond the fringes of the Solar system which would be unavailable anywhere near a star and it’s planets.
While we’re at it, what other forces and energies might we want to look at, well beyond Earth and it’s influences? Again, it’s impossible to know for sure, but we can guess. Though telepathy has been the subject of much research and even some space-based research. How far would telepathic influences extend (if indeed they exist at all?) Perhaps we shall not know much about the physics of mental influence until we can get away from the background noise of Humanity. Out among the stars would be a place to investigate such matters with only a few hundred minds rather than several billion.
A trillion or more miles from Old Sol would also be perhaps the first locale from which to conduct true human research. The preponderance of scientific inquiry has depended upon separating influences and studying one factor or phenomenon independently from others. Even on the Moon or Mars, a group of human beings are within easy radio reach of the teeming masses of Earth. A group 100 Billion miles from earth is an inviolable week away from any effect from Earth. A trillion miles away, means nearly nine weeks of radio delay. This far away from Home, we might start thinking about building truly new types of human interactions and ways of living. One can mine all of the utopian literature of Earth plus the depths of imagination for reasons to venture via the Slow Route toward the stars.
Biology: If a ship accelerates away from the solar system under about two ten-thousandths of an Earth Gravity, it will reach a velocity of 134 thousand miles per hour in a years time and will be about a half billion miles beyond it’s starting point. A year later it will be going twice as fast and be four times further out. Smaller, faster ships might well keep up some regular transportation between The Earth and other planets and the starship. Many people would be glad to visit a largish space station outbound from the Sun, if there was a chance of returning home again. As the ship pulled further and further away from the Solar System however, any attempt to catch it would demand craft which would approach the Fast Starship themselves for performance. After a few years therefore, the Slow Version starship would be pretty much a point of no return–unless human body and brain proves up to the task.
Energy Minimum Road to Outer Space 