8.1 The ways to space. The whole universe beyond Earth sits ready and waiting as our story setting. There is only one problem: How do we get there?
We will suggest a number of ways to move to and around in space. After a brief summary of each, we will push every one to its limit (and perhaps a little beyond).
The dozens of different systems for moving to and in space can be divided into three main types:
Category A, Rocket Spaceships. These achieve their motion via the expulsion of material that they carry along with them. Usually, but not always, the energy to expel the reaction mass comes from that reaction mass itself, by burning or through nuclear reactions. As an alternative, the energy to move reaction mass at high speed comes from another energy source, either on the rocket or elsewhere.
Category B, Rocketless Spaceships, which do not carry their own reaction mass. These ships must derive their motive force from some external agent.
Within each of the two major divisions we will find considerable variation. Category A includes:
* Chemical rockets.
* Mass drivers.
* Ion rockets.
* Nuclear reactor rockets.
* Pulsed fission rockets.
* Pulsed fusion rockets.
* Antimatter rockets.
* Photon rockets.
Category B includes:
* Gravity swingbys.
* Solar sails.
* Laser beam propulsion.
* The Bussard ramjet.
We will also mention a trio of hybrid systems:
* Laser-powered rockets.
* The Ram Augmented Interstellar Rocket (RAIR).
* The vacuum energy drive.
Finally, in a Category C we examine special devices able to take people and cargo into orbit without using rockets. Category C includes:
* Beanstalks.
* Dynamic beanstalks.
* Space fountains.
* Launch loops.
As we will see, some of these are suited only for in-space operations, while others are a natural choice for launch operations.
8.2 Rocket spaceships. A chemical propulsion system is just a fancy term for generating propulsion by ordinary burning (the burning takes place so rapidly and violently that we may prefer to think of it as a controlled explosion). This is a tried-and-tested standby, and every ounce of material launched into space today has been done using chemical rockets; yet in some ways, the rocket looks like the worst choice of all.
To see why, imagine that you have developed a wonderful new form of rocket that provides a significant thrust for many hours, or days, or even weeks, at the cost of very little fuel (fuel used by a rocket is termed "reaction mass," since the rocket is propelled forward as a reaction to the expelled fuel traveling backward).
In science fiction, and also in actual space travel, an acceleration equal to that produced by gravity on the surface of the Earth is called one gee. As we saw in Chapter 7, this is about 9.8 m/sec2. Accelerations are then specified in multiples of this. For example, three gees, during the Shuttle's ascent to orbit, means that the astronauts will experience an acceleration of 29.4 m/sec2, and no further explanation needs to be given. We will use the same convention.
Suppose, then, that the thrust of our new rocket engine is enough to generate an acceleration of half a gee. As we will see when we consider ways of moving around once we are in space, a half gee acceleration provides easy access to the whole solar systemonce we have managed to get away from Earth.
We place our new rocket upright on the launch pad and switch on the engine. Reaction mass is expelled downward to provide an upward thrust. What happens next?
Absolutely nothing. When the total upward thrust is less than the weight of the rocket, the whole thing will simply sit there. Earth's gravity provides an effective "downward thrust" equal to the rocket's total weight, and unless the upward thrust provided by the propellant's ejection exceeds that weight, the rocket will not move one inch. We can fire our engine for hours or weeks or years, but we will not achieve any movement at all. (Even trained engineers can sometimes miss this basic point. In 1995 I received a proposal from a Canadian engineer for a launch system using an ion engine that could produce an acceleration of only a tiny fraction of a gee.)
Things are slightly better when the thrust of the rocket's engines is a little bigger than the total weight. Suppose that the thrust is 1.01 times the initial weight. The rocket will move upward, but agonizingly slowly. At first, the acceleration will only be one hundredth of a gee. The rocket will accelerate faster as it ascends, since it no longer lifts the weight of fuel already expelled. But it will still provide a puny acceleration. You can get to orbit that way, but it will take a long time. And for all that time, while you move slowly upward, your rocket is wasting thrust. Almost all the fuel being expelled as reaction mass is simply going to counteract the downward acceleration provided by the Earth itself.
Now it is clear why astronauts are trained to accept high accelerations. The faster that the rocket can burn its fuel, the higher the thrust will be, the higher the useful thrust (more than gravity's pull) will be, and the less the fuel wasted in reaching orbit. Once we are in orbit, fuel is no longer needed to fight Earth's gravity. Hence the old maxim of spaceflight, once you are in orbit you are halfway to anywhere.
Notice the basic difference between flight to space, and flight in space. Any rocket engine that provides less than one gee of acceleration cannot take us up to orbit. Once we are in space, however, and in some orbit, there is no lower limit below which an acceleration is useful. Any acceleration, no matter how small, can be used to transfer between any two orbits, though it may take a while to accomplish the move.
You may object to one of the assumptions made in this analysis. Why place the rocket vertically? Suppose instead that we placed the rocket horizontally, on a long, smooth railroad track. Then any acceleration, no matter how small, will speed up the rocket, since horizontal motion does not fight against Earth's gravity. If we keep increasing in speed, eventually we will be moving so fast that the rocket would have to be held down on the track, otherwise centrifugal force would make it rise. If we could reach a speed of eight kilometers a second before releasing from the track, the rocket would be going fast enough to take it up to orbit.
This is good science and good engineeringbut not for launch from Earth. The atmosphere is the spoiler, making the method impractical because of air resistance and heating. For the Moon, however, with its negligible atmosphere, the method will be perfectly fine. It was suggested long ago as a good lunar launch technique.
First, however, we have to be on the Moon, or out in space. That takes us back to where we started. We conclude as many have concluded: beginning the exploration of space from the surface of the Earth may be our only option, but given a choice we would not start from here.
Unfortunately, we are here. Back to our launch problem, leaving from the surface of the Earth.
8.3 Measures of performance. We need some way to evaluate rocket propulsion systems, so that we can say, "Of these types of rocket-propelled systems, Type A has greater potential than Type B."
With chemical fuels such as kerosene or liquid hydrogen, it is natural to look for guidance from the way in which we compare fuels here on Earth. An easy general measure happens to be available: the number of kilocalories produced when we burn a gram of the fuel. For instance, good coal will yield about 7 kilocalories a gram, gasoline about 11.5 kilocalories a gram. Based on that measure alone, we would expect to prefer gasoline to coal as an energy source, and indeed, the coal-powered spaceship does not feature largely in science fiction.
However, for rocket propulsion the heat generated by burning the fuel is not quite the measure that we want. The right variable is the specific impulse of a given type of fuel, and it measures the thrust that the fuel can generate. Specific impulse, usually written as SI, is the length of time that one pound of fuel can produce a thrust of one pound weight. SI is normally measured in seconds. Since weight depends on the value of surface gravity, and since surface gravity depends where you are on Earth (it is more at the poles than at the equator) this may seem like a rather poor definition. It came into use in the 1920s and 1930s, when people doing practical experiments with rockets found it a lot easier to measure the force that a rocket engine was developing on a stand than to measure the speed of the expelled gases that formed the rocket's exhaust. That speed is a better measure, and it is termed the effective jet velocity, or EJV. We say effective velocity rather than actual velocity, because if the reaction mass is not expelled in the desired direction (opposite to the spacecraft's motion) the EJV will be reduced. Thus EJV measures both the potential thrust of a fuel, and also the efficiency of engine design.
We will use mainly EJV. However, SI is still a widely-used measure for comparing different rocket fuels, so it is worth knowing about. To convert from SI values (in seconds) to EJV (in kilometers per second), simply multiply by 0.0098.
Naturally, neither SI nor EJV is a useful measure in propulsion systems that do not employ reaction mass. However, they are supremely important factors in near-future practical spaceflight, because the ratio of final spacecraft mass (payload) to initial mass (payload plus fuel) depends exponentially on the EJV.
Explicitly, the relationship is MI/MP=e(V/EJV), where MI=initial total mass of spacecraft plus fuel, MP=final payload mass, and V=final spacecraft velocity. This is often termed the Fundamental Equation of Rocketry. It is true only at nonrelativistic speeds, so some day we may have to change our definition of "Fundamental."
To see the importance of this equation, suppose that a mission has been designed in which the initial mass of payload plus fuel is 10,000 times the final payload. That is a prohibitively high value for most missions, and the design is useless. But if the EJV of the mission could somehow be doubled, the initial payload-plus-fuel mass would become only 100 (the square root of 10,000) times the final payload. And if it could somehow be doubled again, the payload would increase to one-tenth (the square root of 1/100) of the initial total mass. The secret to high-performance missions lies in high values of the EJV.
Very good. But what kind of values of the EJV might we expect in the best rocket system?
The chemical rockets that we can make with present technology, using a liquid hydrogen/liquid oxygen (LOX) mix, produce an EJV rather more than 4 kilometers per second. LOX plus kerosene is less good, with an EJV of about 2.6. Potassium perchlorate plus a petroleum product (a solid fuel rocket) has an EJV of about 2. Liquid hydrogen with liquid fluorinea tricky mixture to handle, with unpleasant combustion productshas an EJV as high as 4.5. That is probably the limit for today's chemical fuel rockets. To do better than this we would have to go to such exotic fuels as monomolecular hydrogen, which is highly unstable and dangerous.
As we noted in Chapter 5, the maximum performance for chemical fuels occurs when the energy released is perfectly converted to kinetic energy. The theoretical values obtained there were 5.6 kms/sec for the H-O16 mix, and 5.95 kms/sec for H-O14. If you write a story and your chemical-fuel rocket has an EJV of 50 (i.e. an SI of 5,100), you'll have to provide a pretty good explanation of how you did it; otherwise, you are writing not science fiction but pure fantasy.
Luckily for the writer, the chemical rocket is not the only one available; it merely happens to be the only type used so far for launches. That promises to be true for some time in the future. Even NASA's "new" development, the single-stage-to-orbit reusable rocket, will be a chemical rocket.
8.4 Mass drivers. The mass driver consists of a long helical spiral of wire with a hollow center (a solenoid). Pulsed magnetic fields are used to propel each payload along the solenoid, accelerating it until it reaches the end of the solenoid and flies off at high speed.
Mass drivers are usually thought of as launch devices, throwing payloads to space using electromagnetic forces. However, suppose that we invert our thinking. A mass driver in free space will itself be given an equal push by the material that is expelled (Newton's Third Law: Action and reaction are equal and opposite). If we regard the expelled material as reaction mass, then the long solenoid itself is part of the spacecraft, and it will be driven along in space with the rest of the payload.
Practical tests suggest that ejecting a series of small objects using the mass driver can give an EJV to the mass driver itself of up to 8 kms/second. This is almost double the EJV that can be achieved with chemical rockets; however, note that the energy to power the mass driver must be provided externally, for instance as electricity generated using nuclear or solar power. The mass of such power-generation equipment will diminish the mass driver's performance as a propulsion system. Mass drivers do not offer a solution to the problem of reaching orbit from the surface of the Earth. In addition, solar power is fine, close to the Sun, but it would be a major problem out at the edge of the solar system. Available solar energy falls off as the inverse square of the distance from the Sun. We will encounter the same problem later, with other systems.
The good news is that working mass drivers have been built. They are not just theoretical ideas.
8.5 Ion rockets are similar in a sense to mass drivers, in that the reaction mass is accelerated electromagnetically, and then expelled. In this case, however, the reaction mass consists of charged atoms or molecules, and the acceleration is provided by an electric field. The technique is the same as that used in the linear accelerators employed in particle physics work here on Earth. Very large linear accelerators, miles in length, have already been built; for example, the Stanford Linear Accelerator (SLAC) has an acceleration chamber two miles long.
SLAC is powered using conventional electric supplies. For use in space, the power supply for ion rockets can be solar or nuclear (or externally provided; see the discussion of laser power later in this chapter). As was the case with mass drivers, provision of that power supply must diminish system performance.
Prototype ion rockets have been flown in space. They offer a drive that can be operated for long periods of time, and thus they are attractive for long missions. Practical tests suggest that they can produce an EJV of up to 70 kms/second, far higher than the EJV of either chemical rockets or mass drivers. However, because the onboard equipment to produce the ion beam is bulky, these are low-thrust devices providing accelerations of a few micro-gees. In order to achieve final velocities of many kilometers per second, ion rockets must be operated for long periods of time. They are not launch devices.
8.6 Nuclear reactor rockets use a nuclear reactor to heat the reaction mass, which is then funneled to expel itself at high temperatures and at high velocities.
Systems with a solid core to the reactor achieve working temperatures up to about 2,500deg.C, and an EJV of up to 9.5 kms/second. Experimental versions were built in the early 1970's. Work on the most developed form, known as NERVA, was abandoned in 1973, because of concern about spaceborne nuclear reactors. A solid core reactor rocket with hydrogen as reaction mass has an EJV more than double the best chemical fuel rocket, but the nuclear power plant itself has substantial mass. This reduces the acceleration to less than a tenth of what can be achieved with chemical fuels.
A liquid core reactor potentially offers higher performance, with a working temperature of up to 5,000deg.C and an EJV of up to 25 kms/second. Gaseous core reactors can do even better, operating up to 20,000deg.C and producing an EJV of 65 kms/second. However, such nuclear reactor rockets have never been produced, so any statements on capability are subject to question and practical proof.
I believe we could go to orbit with a liquid core nuclear-powered rocket, safely and more efficiently than with a chemical rocket. However, I think it will be some time before we are allowed to. The suspicion of nuclear launchor, indeed, all things nuclearis too strong.
8.7 Pulsed fission rockets form the first of the "advanced systems" that we will consider; advanced, in the sense that we have never built one, and doing so might lead to all sorts of technological headaches; and also advanced in the sense that such rockets, if built, could take us all over the solar systemand out of it.
The idea for the pulsed fission rocket may sound both primitive and alarming. A series of atomic bombs (first design) or hydrogen bombs (later designs) are exploded behind the spacecraft, which is protected by a massive "pusher plate." This plate serves both to absorb the momentum provided by the explosions, and also to shield the payload from the radioactive blasts.
The pulsed fission rocket was proposed by Stanislaw Ulam in 1955. The idea, later known as Project Orion, appeared practical and could have been built. However, the effort was abandoned in 1965, a casualty of the 1963 Nuclear Test Ban Treaty. Project Orion called for full-scale atomic explosions, and the treaty made it impossible to test the idea. The EJV is excellent, up to 100 kms/second, but the mass of the pusher plate may limit practical accelerations to a few centimeters/sec2 (less than a hundredth of a gee). This is no good for a launch system, but it will achieve respectable velocities over long periods. An acceleration of 1 cm/sec2 (just over a thousandth of a gee) for one year produces an end speed of 310 kms/second. Note that, even at this speed, Neptune is still more than six months travel time away.
8.8 Pulsed fusion. The pulsed fission rocket of Project Orion has two big disadvantages. First, the nuclear explosions are full-scale nuclear blasts, each one equivalent in energy release to thousands or even millions of tons of conventional explosives. Second, the massive "pusher plate" is useful as a protection against the blasts and as an absorber of momentum, but it greatly decreases the acceleration of the ship and the system efficiency.
The pulsed fusion rocket potentially overcomes both these problems. Each fusion explosion can be a small one, involving only a gram or so of matter. The fusion process is initiated by a high-intensity laser or a relativistic electron beam focused on small spheres of nuclear fuel. The resulting inward-traveling shock wave creates temperatures and pressures at which fusion can occur. If the right nuclear fuels are used, all the fusion products can be charged particles. Their subsequent movement can therefore be controlled with electromagnetic fields, so that they do not impinge on the payload or the walls of the drive chamber.
An analysis of a pulsed fusion rocket mission was performed in the late 1970s by the British Interplanetary Society. Known as Project Daedalus, it was a design for a one-way trip to Barnard's Star, 5.9 light-years from the Sun. Small spheres of deuterium (D) and helium-3 (He3) were used as fusion fuels. (Deuterium is "heavy" hydrogen, 1H2, with a neutron as well as a proton in the nucleus; helium-3, 2He3, is "light" helium, missing a neutron in its nucleus.)
The D-He3 reaction yields as fusion products a helium nucleus and a proton, both of which carry electric charges and can thus be manipulated by magnetic fields. The estimated EJV for Project Daedalus was 10,000 kms/second, leading to a fifty-year travel time for the 5.9 light-year journey. The mass at launch from solar orbit was 50,000 tons, the final mass was 1,000 tons, and the terminal velocity for the spacecraft was one-eighth of the speed of light.
The design was a technical tour de force, but the complications and caveats are significant. First, controlled pellet fusion of the type envisaged has never been demonstrated. The D-He3 fusion reaction in the fuel pellets proceeds rapidly only at extreme temperatures, and while other fusion reactions, such as deuterium-tritium, take place at a sixth of this temperature, they produce uncharged neutrons as fusion products and the direction of travel of these uncharged particles cannot easily be controlled.
Third, and perhaps the biggest problem of all, the nuclear fuels needed are not available. Deuterium is plentiful enough, at one part in 6,000 in ordinary hydrogen. But He3 is very rare on Earth. The total U.S. supply is only a few thousand liters. The Daedalus design calls for 30,000 tons of the stuff, far more than could be found anywhere on Earth. The only place in the solar system where He3 exists in enormous quantities is in the atmospheres of the gas-giant planets, Jupiter and Saturn and Uranus and Neptune.
Project Daedalus proposed the use of a complicated twenty-year mining operation in the atmosphere of Jupiter, to be conducted by automated factories floating in the Jovian atmosphere. The construction of the spacecraft itself would be carried out near Jupiter. I took over their method in my novel Cold as Ice, but I assumed that the moons of Jupiter had already been colonized by humans. Accessand management oversightwas easier, and in fact the necessary helium mining formed only a minor element of the book.
8.9 Antimatter rockets. To every particle in nature there corresponds an antiparticle. Matter constructed from these antiparticles is termed antimatter, or mirrormatter. For example, antihydrogen consists of a positron moving about an antiproton, whereas normal hydrogen is an electron moving about a proton.
When matter and antimatter meet, they annihilate each other. They therefore represent a vast source of potential energy.
If electrons and positrons meet, the result is high-energy gamma rays, and no particles. If protons and antiprotons meet, the result is an average of three charged pions and two uncharged pions, with the charged pions carrying 60 percent of the total energy. Neutral pions decay to form high-energy gamma rays in less than a thousand trillionth of a second. Charged pions last a lot longer, relatively speaking, decaying to the elementary particle known as a muon in 26 nanoseconds. Muons decay in their turn to electrons and neutrinos, lasting on average 2.2 microseconds before they do so.
These are short times, but relativity helps here. The charged pions created in this process are traveling fast, at over ninety percent of the speed of light, and thus the effect of relativistic time dilation is to increase their lifetime from 26 nanoseconds to 70 nanoseconds. This is more than long enough to control the movement of the charged pions with magnetic fields. Similarly, the rapidly-moving muons that appear as decay products last on average 6.2 microseconds rather than 2.2 microseconds, before they in turn decay. They too can be controlled through the use of magnetic fields.
Antimatter is a highly concentrated method of storing energy. The total energy produced by a milligram of antimatter when it meets and annihilates a milligram of ordinary matter is equal to that of twenty tons of liquid hydrogen/LOX fuel. It is therefore ideal for use on interstellar missions, where energy per unit weight is of paramount importance in fuels.
The most economical way of using such a potent fuel is not to take it "neat," but to dilute the antimatter with a large amount of ordinary matter. Matter/antimatter annihilation then serves to heat up ordinary matter, which is expelled as reaction mass. In this case, both the high-energy gamma rays and the pions serve to heat the reaction mass; and by choosing the antimatter/matter ratio, many different missions can be served with a single engine design. A highly dilute matter/antimatter engine also has excellent potential for interplanetary missions.
Given all these useful properties of antimatter, why are we waiting? Well, one question remains: How do we get our hands on some of this stuff?
That leads us to one of the major mysteries of physics and cosmology. There is as much reason for antimatter to exist as for ordinary matter to exist. Logically, the universe should contain equal amounts of each. In practice, however, antimatter is very rarely found in nature. Positrons and antiprotons occur occasionally in cosmic rays, but if we discount the highly unlikely possibility that some of the remote galaxies are all antimatter, then the universe is ordinary matter to an overwhelming extent.
One product of the recent inflationary models of the early universe is a possible explanation of the reason why there is so little antimatter. This, however, is of little use to us. We need antimatter now, and in substantial quantities, if we are to use matter-antimatter annihilation to take us to the stars.
Since antimatter is not available in nature, we will have to make our own. And this is possible. One by-product of the big particle accelerators at Fermilab in Illinois, at IHEP in Novosibirsk in the Soviet Union, and at CERN in Switzerland, is a supply of antiprotons and positrons. The antiprotons can be captured, slowed down, and stored in magnetic storage rings. Anti-hydrogen can be produced, by allowing the antiprotons to capture positrons. Antimatter can be stored in electromagnetic ion traps, and safely transported in such containers.
We are not talking about large quantities of antimatter with today's production methods. Storage rings have held up to a trillion antiprotons, but that is still a very small mass (about a trillionth of a gram). And antimatter takes a lot of energy to produce. The energy we will get from the antimatter will not be more than 1/10,000th of the energy that we put into making it. However, the concentrated energy of the end product makes this a unique fuel for propulsion.
The EJV of a matter/antimatter engine depends on the matter-to-antimatter ratio, and it can be selected to match the needs of particular missions. However, for interstellar travel we can safely assume that we want the biggest value of the EJV that we can get. This will occur when we use a 1:1 ratio of matter to antimatter, and direct the charged pions (and their decay products, the muons) with magnetic control of their final emission direction. Since the charged pions contain 60 percent of the proton-antiproton annihilation energy, and since the uncharged pions and the gamma rays will be emitted in all directions equally, we find the maximum EJV to be 180,000 kms/second. With such an EJV, and a ratio of initial mass to final mass of 3:1, the terminal velocity of the mission will be almost two-thirds of the speed of light. We are in a realm of velocities where relativistic effects have a big effect on shipboard travel times.
8.10 Photon rockets. This takes the matter-antimatter rocket to its ultimate form. It represents the final word in rocket spaceships that employ known physics.
If we could completely annihilate matter, so that it appeared as pure radiation (and was heading in the right direction, as a collimated beam), the EJV would be the speed of light, about 300,000 kilometers per second.
This is the highest EJV possible. It implies perfect magnetic control and redirection of all charged pions, plus the control of all uncharged pions and gamma rays and of all decay products such as electrons and neutrinos. Every particle produced in matter-antimatter annihilation ultimately decays to radiation, or to electrons and positrons that can then annihilate each other to give pure radiation. All this radiation must be emitted in a direction exactly opposite to the spacecraft's motion.
If the best chemical rocket with a fuel-to-payload ratio of 10,000:1 could be replaced with a photon rocket, the mission would be 99.99 percent payload; the fuel would be a negligible part of the total mass. Having said that, we must also say that we have no idea how to make a photon rocket. It could exist, according to today's physics; but it is quite beyond today's technology.
8.11 Space travel without reaction mass. The central problem of the rocket spacecraft is easy to identify. For low to moderate EJV's (which we will define as less than 100 kms/seconda value that would make any of today's rocket engineers ecstatic) most of the reaction mass does not go to accelerate the payload. It goes to accelerate the rest of the fuel. This is particularly true in the early stages of the mission, when the rocket may be accelerating a thousand tons of fuel to deliver ten tons of payload. All systems carrying their reaction mass along with them suffer this enormous intrinsic disadvantage. It seems plausible, then, that systems which do not employ reaction mass at all may be the key to successful space travel. We now consider:
* Gravity swingbys.
* Solar sails.
* Laser beam propulsion.
* The Bussard ramjet.
Also, we will touch on three hybrid systems:
* Laser-powered rockets.
* The Ram Augmented Interstellar Rocket (RAIR).
* The vacuum energy drive.
8.12 Gravity swingbys. There is one form of velocity increase that needs neither onboard rockets nor an external propulsion source. In fact, it can hardly be called a propulsion system in the usual sense of the word. If a spacecraft flies close to a planet it can, under the right circumstances, obtain a velocity boost from the planet's gravitational field. This technique is used routinely in interplanetary missions. It was used to get the Galileo spacecraft to Jupiter, and to permit Pioneer 10 and 11 and Voyager 1 and 2 to escape the solar system. Jupiter, with a mass 318 times that of Earth, can give a velocity kick of up to 30 kms/second to a passing spacecraft. So far as the spaceship is concerned, there will be no feeling of onboard acceleration as the speed increases. An observer on the ship experiences free fall, even while accelerating relative to the Sun.
If onboard fuel is available to produce a velocity change, another type of swingby can do even better. This involves a close approach to the Sun, rather than to one of the planets. The trick is to swoop in close to the solar surface and apply all available thrust near perihelion, the point of closest approach.
Suppose that your ship has a small velocity far from the Sun. Allow it to drop toward the Sun, so that it comes close enough almost to graze the solar surface. When it is at its closest, use your onboard fuel to give a 10 kms/second kick in speed; then your ship will move away and leave the solar system completely, with a terminal velocity far from the Sun of 110 kms/second.
The question that inevitably arises with such a boost at perihelion is, where did that "extra" energy come from? If the velocity boost had been given without swooping in close to the Sun, the ship would have left the solar system at 10 kms/second. Simply by arranging that the same boost be given near the Sun, the ship leaves at 110 kms/second. And yet the Sun seems to have done no work. The solar energy has not decreased at all. It sounds impossible, something for nothing.
The answer to this puzzle is a simple one, but it leaves many people worried. It is based on the fact that kinetic energy changes as the square of velocity, and the argument runs as follows: The Sun increases the speed of the spacecraft during its run towards the solar surface, so that our ship, at rest far from Sol, will be moving at 600 kms/second as it sweeps past the solar photosphere. The kinetic energy of a body with velocity V is V2/2 per unit mass, so for an object moving at 600 kms/second, a 10 kms/second velocity boost increases the kinetic energy per unit mass by (6102-6002)/2=6,050 units. If the same velocity boost had been used to change the speed from 0 to 10 kms/second, the change in kinetic energy per unit mass would have been only 50 units. Thus by applying our speed boost at the right moment, when the velocity is already high, we increase the energy change by a factor of 6,050/50=121, which is equivalent to a factor of 11 (the square root of 121) in final speed. Our 10 kms/second boost has been transformed to a 110 kms/second boost.
All that the Sun has done to the spaceship is to change the speed relative to the Sun at which the velocity boost is applied. The fact that kinetic energy goes as the square of velocity does the rest.
If this still seems to be getting something for nothing, in a way it is. Certainly, no penalty is paid for the increased velocityexcept for the possible danger of sweeping in so close to the Sun's surface. And the closer that one can come to the center of gravitational attraction when applying a velocity boost, the more gratifying the result.
Let us push the limits. One cannot go close to the Sun's center without hitting the solar surface, but an approach to within 20 kilometers of the center of a neutron star of solar mass would convert a 10 kms/second velocity boost provided at the right moment to a final departure speed from the neutron star of over 1,500 kms/second. An impressive gain, though the tidal forces derived from a gravitational field of over 10,000,000 gees might leave the ship's passengers a little the worse for wear.
Suppose one were to perform the swingby with a speed much greater than that obtained by falling from rest? Would the gain in velocity be greater? Unfortunately, it works the other way round. The gain in speed is maximum if you fall in with zero velocity from a long way away. In the case of Sol, the biggest boost you can obtain from your 10 kms/second velocity kick is an extra 100 kms/second. That's not fast enough to take us to Alpha Centauri in a hurry. A speed of 110 kms/second implies a travel time of 11,800 years.
8.13 Solar sails. If gravity swingbys of the Sun or Jupiter can't take us to the stars fast enough, can anything else? The Sun is a continuous source of a possible propulsive force, namely, solar radiation pressure. Why not build a large sail to accelerate a spacecraft by simple photon and emitted particle pressure?
We know from our own experience that sunlight pressure is a small forcewe don't have to "lean into the sun" to stay upright. Thus a sail of large area will be needed, and since the pressure has to accelerate the sail as well as the payload, we must use a sail of very low mass per unit area.
The thinnest, lightest sail that we can probably make today is a hexagonal mesh with a mass of about 0.1 grams/square meter. Assuming that the payload masses much less than the sail itself, a ship would accelerate away from Earth orbit to interstellar regions at 0.01 gees.
This acceleration diminishes farther from the Sun, since radiation pressure per unit area falls off as the inverse square of the distance. Even so, a solar sail starting at 0.01 gees at Earth orbit will be out past Neptune in one year, 5 billion kilometers away from the Sun and traveling at 170 kms/second. Travel time to Alpha Centauri would be 7,500 years. Light pressure from the target star could be used to slow the sail in the second half of the flight.
8.14 Laser beam propulsion. If the acceleration of a solar sail did not decrease with distance from the Sun, the sail we considered in the last section would have traveled ten times as far in one year, and would be moving at 3,100 kms/second. This prompts the question, can we provide a constant force on a sail, and hence a constant acceleration, by somehow creating a tightly focused beam of radiation that does not fall off with distance?
Such a focused beam is provided by a laser, and this idea has been explored extensively by Robert Forward in both fact and fiction (see, for example, his Flight of the Dragonfly, aka Rocheworld; Forward, 1990). In his design, a laser beam is generated using the energy of a large solar power satellite near the orbit of Mercury. This is sent to a transmitter lens, hanging stationary out between Saturn and Uranus. This lens is of Fresnel ring type, 1,000 kilometers across, with a mass of 560,000 tons. It can send a laser beam 44 light-years without significant beam spreading, and a circular lightsail with a mass of 80,000 tons and a payload of 3,000 tons can be accelerated at that distance at 0.3 gees. That is enough to move the sail at half the speed of light in 1.6 years.
Forward also offers an ingenious way of stopping the sail at its destination. The circular sail is constructed in discrete rings, like an archery target. As the whole sail approaches its destination, one inner circle, 320 kilometers across and equal in area to one-tenth of the original sail, is separated from the outer ring. Reflected laser light from the outer ring serves to slow and halt the inner portion at the destination star, while the outer ring flies on past, still accelerating. When exploration of the target stellar system is complete, an inner part of the inner ring, 100 kilometers across and equal in area to one-tenth of the whole inner ring, is separated from the rest. This "bull's-eye" is now accelerated back towards the Sun, using reflected laser beam pressure from the outer part of the original inner ring. The travel time to Alpha Centauri, including slowing-down and stopping when we arrive, is 8.6 years (Earth time) and 7 years (shipboard time). Note that we have reached speeds where relativistic effects make a significant difference to perceived travel times.
Could we build such a ship, assuming an all-out worldwide effort?
Not yet. The physics is fine, but the engineering would defeat us. The power requirement of the laser is thousands of times greater than the total electrical production of all the nations on Earth. The space construction capability is also generations ahead of what can reasonably be projected for the next half century. We are not likely to go to the stars this way. Something better will surely come along before we are ready to do it. I feel this way about some other ideas, discussed later in this chapter.
8.15 The Bussard Ramjet. This is a concept introduced by Robert Bussard in 1960. It was employed in one of science fiction's classic tales of deep space and time, Poul Anderson's Tau Zero (Anderson, 1970).
In the Bussard ramjet, a "scoop" in front of the spaceship funnels interstellar matter into a long hollow cylinder that comprises a fusion reactor. The material collected by the scoop undergoes nuclear fusion, and the reaction products are emitted at high temperature and velocity from the end of the cylinder opposite to the scoop, to propel the spacecraft. The higher the ship's speed, the greater the rate of supply of fuel, and thus the greater the ship's acceleration. It is a wonderfully attractive idea, since it allows us to use reaction mass without carrying it with us. There is interstellar matter everywhere, even in the "emptiest" reaches of open space.
Now let us look at the "engineering details."
First, it will be necessary to fuse the fuel on the fly, rather than forcing it to accelerate until its speed matches the speed of the ship. Otherwise, the drag of the collected fuel will slow the ship's progress. Such a continuous fusion process calls for a very unusual reactor, long enough and operating at pressures and temperatures high enough to permit fusion while the collected interstellar matter is streaming through the chamber.
Second, interstellar matter is about two-thirds hydrogen, one-third helium, and negligible proportions of other elements. The fusion of helium is a complex process that calls for three helium nuclei to interact and form a carbon nucleus. Thus the principal fusion reaction of the Bussard ramjet will be proton-proton fusion. Such fusion is hindered by the charge of each proton, which repels them away from each other. Thus pressures and temperatures in the fusion chamber must be extremely high to overcome that mutual repulsion.
Third, there is only about one atom of interstellar matter in every cubic meter of space. Thus, the scoop will have to be many thousands of kilometers across if hydrogen is to be supplied in enough quantity to keep a fusion reaction going. It is impractical to construct a material scoop of such a size, so we will be looking at some form of magnetic fields.
Unfortunately, the hydrogen of interstellar space is mainly neutral hydrogen, i.e., a proton with an electron moving around it. Since we need a charged material in order to be able to collect it electromagnetically, some method must first be found to ionize the hydrogen. This can be done using lasers, beaming radiation at a carefully selected wavelength ahead of the ramjet. It is not clear that a laser can be built that requires less energy than is provided by the fusion process. It is also not clear that materials exist strong enough to permit construction of a magnetic scoop with the necessary field strengths.
The Bussard ramjet is a beautiful concept. Use it in stories by all means. However, I am skeptical that a working model will be built any time within the next couple of centuries, or perhaps ever.
8.16 Hybrids. For completeness, we will also mention three other systems. One has an onboard energy source and uses external reaction mass, the other two have onboard reaction mass and use external energy.
8.17 Laser-powered rockets. These rockets carry reaction mass, but that mass does not produce the energy for its own heating and acceleration. Instead, the energy is provided by a power laser, which can be a considerable distance from the target spaceship.
This concept was originally proposed by Arthur Kantrowitz as a technique for spacecraft launch. It is attractive for interplanetary missions, although for laser power to be available at interstellar distances it is necessary to build a massive in-space power laser system.
The requirement for onboard storage of reaction mass is also huge. Even when all of this has been done, the EJV does not exceed maybe 200 kms/second. This system sounds fine for launches, less good for in-space use. Although we never named it as such, this is what Jerry Pournelle and I used as the launch system in our novel Higher Education (Sheffield and Pournelle, 1996).
Note that laser power could be used equally well to provide the energy for other propulsion systems, such as the ion drive. This removes the bulky onboard equipment that otherwise severely limits ship acceleration.
8.18 Ram Augmented Interstellar Rocket (RAIR). The RAIR employs a Bussard ramscoop to collect interstellar matter. However, instead of fusing such matter as it flashes past the ship, in the RAIR an onboard fusion reactor is used to heat the collected hydrogen and helium, which then exits the RAIR cylinder at high speed.
Certainly, this eliminates one of the central problems of the Bussard ramjetnamely, that of fusing hydrogen quickly and efficiently. It also allows us to make use of interstellar helium. However, the other problems of the Bussard ramjet still exist. One little-mentioned problem with both the RAIR and the original Bussard ramjet is the need to reach a certain speed before the fusion process can begin, since below that speed there will not be enough material delivered to the fusion system. The acceleration to reach that minimum velocity is itself beyond today's capabilities.
8.19 The vacuum energy drive. The most powerful theories in physics today are quantum theory and the theories of special and general relativity. Unfortunately, those theories are not totally consistent with each other. If we calculate the energy associated with an absence of matterthe "vacuum state"we do not, as common sense would suggest, get zero. Instead, quantum theory assigns a specific energy value to a vacuum.
In classical thinking, one could argue that the zero point of energy is arbitrary, so we could simply start measuring energies from the vacuum energy value. However, if we accept general relativity that option is denied to us. Energy, of any form, produces spacetime curvature, and we are therefore not allowed to redefine the origin of the energy scale. Once this is accepted, the energy of the vacuum cannot be talked out of existence. It is real, and when we calculate it we get a large positive value per unit volume.
How large?
Richard Feynman addressed the question of the vacuum energy value and computed an estimate for the equivalent mass per unit volume. The estimate came out as two billion tons per cubic centimeter. The energy in two billion tons of matter is more than enough to boil all Earth's oceans.
Is there any possibility that the vacuum energy could be tapped for useful purposes? Robert Forward has proposed a mechanism, based upon a real physical phenomenon known as the Casimir Effect. I think it would work, but the energy produced is small. The well-publicized mechanisms of others, such as Harold Puthoff, for extracting vacuum energy leave me totally unpersuaded.
Science fiction that admits it is science fiction is another matter. According to Arthur Clarke, I was the first person to employ the idea of the vacuum energy drive in fictional form, in the story "All the Colors of the Vacuum" (Sheffield, 1981). Clarke employed one in The Songs of Distant Earth (Clarke, 1986). Not surprisingly, there was a certain amount of hand-waving on both Clarke's part and mine as to how the vacuum energy drive was implemented. If the ship can obtain energy from the vacuum, and mass and energy are equivalent, why can't the ship get the reaction mass, too? How does the ship avoid being slowed when it takes on energy, which has an equivalent mass that is presumably at rest? If the vacuum energy is the energy of the ground state, to what new state does the vacuum go, after energy is extracted?
Good questions. Look on them as an opportunity. There must be good science-fictional answers to go with them.
8.20 Launch without rockets. The launch of a rocketany rocketis certainly an impressive sight and sound. All that noise, all that energy, thousands of tons of fuel going up in smoke (literally) in a few minutes.
But does it have to be that way? Let us invoke a classical result from mathematics: The work done carrying a test particle around a closed curve in a fixed potential field is zero.
Around the Earth there is, to good accuracy, a fixed potential field. A spaceship that goes up to orbit and comes back down to the same place is following a closed curve. Conclusion: we ought to be able to send a test particle (such as a spacecraft, which on the scale of the whole Earth is no more than a particle) to orbit and back, without doing any work.
Let's do it, in several different ways.
8.21 The beanstalk. Suppose we have a space station in geostationary orbit, i.e. an equatorial orbit with period exactly 24 hours. A satellite in such an orbit hovers always over the same point on the Earth's equator. Such orbits are already occupied by communications satellites and some weather satellites.
Now suppose a strong loop of cable runs all the way down to the surface of Earth from the space station. The cable must be long as well as strong, since geostationary orbit is more than 35,000 kilometers above the surface. We defer the question as to how we install such a thing. (A geostationary satellite has a period of 24 hours, and hovers above a fixed point on the equator. A geosynchronous satellite simply has a period of 24 hours, but can be inclined to the equator and reach to any latitude.)
Attach a massive object (say, a new communications satellite) to the cable down on the surface. Operate an electric motor, winding the cable with the attached payload up to the station. We will have to do work to accomplish this, lifting the payload against the downward gravitational pull of the Earth. We do not, however, have to lift the cable, since the weight of the descending portion of the loop will exactly balance the weight of the ascending portion.
Also, suppose that we arrange things so that, at the same time as we raise the payload up from the surface, we lower an equal mass (say, an old, worn-out communications satellite) back down to the surface of the Earth. We will have to restrain that mass, to stop it from falling. We can use the force produced by the downward pull to drive a generator, which in turn provides the power to raise the payload. The only net energy needed is to overcome losses due to friction, and to allow for the imperfect efficiency of our motors and generators that convert electrical energy to gravitational energy and back.
The device we describe has been given various names. Arthur Clarke, in The Fountains of Paradise (Clarke, 1979), termed it a space elevator. I, in The Web Between the Worlds (Sheffield, 1979), called it a beanstalk. Other names include skyhook, heavenly funicular, anchored satellite, and orbital tower.
The basic idea is very simple. There are, however, some interesting "engineering details."
First, a cable can't simply run down from a position at geosynchronous height. Its own mass, acted on by gravity, would pull it down to Earth. Thus there must be a compensating mass out beyond geosynchronous orbit. That's easy enough; it can be another length of cable, or if we prefer it a massive ballast weight such as a captured asteroid.
Second, if we string a cable from geostationary orbit to Earth it makes no sense for it to be of uniform cross section. The cable needs to support only the length of itself that lies below it at any height. Thus the cable should be thickest at geosynchronous height, and taper to thinner cross sections all the way down to the ground.
What shape should the tapering cable be? In practice, any useful cable will have to be strong enough to stand the added weight of the payload and the lift system, but let us first determine the shape of a cable that supports no more than its own weight. This is a problem in static forces, with the solution (skip the next half page if you are allergic to equations):
A(r)=A(R).exp (K.f(r/R).d/TR)
In this equation, A(r) is the area of the cable at distance r from the center of the Earth, A(R) is the area at distance R of geosynchronous orbit, K is the Earth's gravitational constant, d is the density of cable material, T is the cable's tensile strength, and f is the function defined by:
f(x)=(3/2-1/x-x2/2)
The form of the equation for A(r) is crucial. First, note that the taper factor of the cable, which we define as A(r)/A(R), depends only on the ratio of cable tensile strength to cable density, T/d, rather than actual tensile strength or density. Thus we should make a beanstalk from materials that are not only strong, but light. Moreover, the taper factor depends exponentially on T/d. If a cable originally had a taper factor from geosynchronous orbit to Earth of 100, and if we could somehow double the strength-to-density ratio, the taper factor would be reduced to 10. If we could double the strength-to-density again, the taper factor would go down to 3.162 (the square root of 10). Thus the strength-to-density ratio of the material used for the cable is enormously important. We note here the presence of the exponential form in this situation, just as we observed it in the problem of rocket propulsion.
We have glossed over an important point. Certainly, we know the shape of the cable. But is there any material with a large enough strength-to-density ratio? After all, at an absolute minimum, the cable has to support 35,770 kilometers of itself. The problem is not quite as bad as it sounds, since the Earth's gravitational field diminishes as we go higher. If we define the "support length" of a material as the length of uniform cross section able to be supported in a one-gee gravitational field, it turns out that the support length needed for the beanstalk cable is 4,940 kilometers. Since the actual cable can and should be tapered, a support length of 4,940 kilometers will be a good deal more than we need. On the other hand, we must hang a transportation system onto the central cable, so there has to be more strength than required for the cable alone.
Is there anything strong enough to be used as a cable for a beanstalk? The support lengths of various materials are given in TABLE 8.1 (p. 227).
The conclusion is obvious: today, no material is strong enough to form the cable of a beanstalk from geostationary orbit to the surface of the Earth.
However, we are interested in science fiction, and the absolute limits of what might be possible. Let us recall Chapter 5, and the factors that determine the limits to material strength. Examining TABLE 5.1 (p. 122), we see that solid hydrogen would do nicely for a beanstalk cable. The support length is about twice what we need. It would have a taper factor of 1.6 from geosynchronous orbit to Earth. A cable one centimeter across at the lower end would mass 30,000 tons and be able to lift payloads of 1,600 tons to orbit.
Unfortunately, solid metallic hydrogen is not yet available as a construction material. It has been made as a dense crystalline solid at room temperature, but at half a million atmospheres pressure. We need to have faith in progress. There are materials available, today, with support lengths ten times that of anything available a century ago.
Beanstalks are easier for some other planets. TABLE 8.2 (p. 228) shows what they look like around the solar system, assuming the hydrogen cable as our construction material.
Mars is especially nice. The altitude of a stationary orbit is only half that of the Earth. We can make a beanstalk there from currently available materials. The support length is 973 kilometers, and graphite whiskers comfortably exceed that.
Naturally, the load-bearing cable is not the whole story. It is no more than the central element of a beanstalk that will carry materials to and from orbit. The rest of the system consists of a linear synchronous motor attached to the load-bearing cable. It will drive payloads up and down. Some of the power expended lifting a load is recovered when we lower a similar load back down to Earth. The fraction depends on the efficiency of conversion from mechanical to electrical energy.
So far we have said nothing about actual construction methods. It is best to build a beanstalk from the top down. An abundant supply of suitable materials (perhaps a relocated carbonaceous asteroid) is placed in geostationary orbit. The load-bearing cable is formed and simultaneously extruded upward and downward, so that the total up and down forces are in balance. Anything higher than geosynchronous altitude exerts a net outward force, everything below geosynchronous orbit exerts a net inward force. All forces are tensions, rather than compressions. This is in contrast to what we may term the "Tower of Babel" approach, in which we build up from the surface of Earth and all the forces are compressions.
After extruding 35,770 kilometers of cable downward from geostationary orbit, and considerably more upward, the lower end at last reaches the Earth's equator. There it is tethered, and the drive train added. The beanstalk is ready for use as a method for taking payloads to geosynchronous orbit and beyond. A journey from the surface to geosynchronous height, at the relatively modest speed of 300 kilometers an hour, will take five days. That is a lot slower than a rocket, but the trip should be far more restful.
The system has another use. If a mass is sent all the way out to the end of the cable and then released, it will fly away from Earth. An object released from 100,000 kilometers out has enough speed to be thrown to any part of the solar system. The energy for this, incidentally, is free. It comes from the Earth itself. We do not have to worry about the possible effects of that energy depletion. The total rotational energy of the Earth is only one-thousandth of the planet's gravitational self-energy, but that is still an incredibly big number.
The converse problem needs to be considered: What about the effects of the Earth on the beanstalk?
Earthquakes sound nasty. However, if the beanstalk is tethered by a mass that forms part of its own lower end, the situation will be stable as long as the force at that point remains "down." This will be true unless something were to blow the whole Earth apart, in which case we might expect to have other things to worry about.
Weather will be no problem. The beanstalk presents so small a cross-sectional area compared with its strength that no imaginable storm can trouble it. The same is true for perturbations from the gravity of the Sun and Moon. Proper design will avoid any resonance effects, in which forces on the structure might coincide with natural forcing frequencies.
In fact, by far the biggest danger that we can conceive of is a man-made one: sabotage. A bomb exploding halfway up a beanstalk would create unimaginable havoc in both the upper and lower sections of the structure. The descent of a shattered beanstalk was described, in spectacular fashion, in Kim Stanley Robinson's Red Mars (Robinson, 1993). My only objection is that in the process the town of Sheffield, at the base of the beanstalk, was destroyed.
8.22 Theme and Variations. We now offer three variations on the basic beanstalk theme. None needs any form of propellant or uses any form of rocket, and all could, in principle, be built today.
The rotating beanstalk is the brainchild of John McCarthy and Hans Moravec, both at the time at Stanford University. Moravec and McCarthy termed the device a nonsynchronous skyhook, though I prefer rotating beanstalk. It is a strong cable, 8,500 kilometers long in one design, that rotates about its center of mass as the latter goes around the Earth in an orbit 4,250 kilometers above the surface. Each end dips into the atmosphere and back out about once an hour.
The easiest way to visualize this rotating structure is to imagine that it is one spoke of a great wheel that rolls around the Earth's equator. The end of the beanstalk touches down like the spoke of a wheel, vertically, with no movement relative to the ground. Payloads are attached to the end of the beanstalk at the moment when it touches the ground. However, you have to be quick. The end comes in at about 1.4 gees, then is up and away again at the same acceleration.
The great advantage of the rotating beanstalk is that it can be made with materials less strong than those needed for the "static" beanstalk. In fact, it would be possible to build one today with a taper factor of 12, using graphite whiskers in the main cable. There is of course no need for such a structure to be in orbit around the Earth. It could sit far out in space, providing a method to catch and launch spacecraft.
The dynamic beanstalk has also been called a space fountain and an Indian rope trick. It is another elegant use of momentum transfer.
Consider a continuous stream of objects (say, steel bullets) launched up the center of an evacuated vertical tube. The bullets are fired off faster than Earth's escape velocity, using an electromagnetic accelerator on the ground. As the bullets ascend, they will be slowed naturally by gravity. However, they will receive an additional deceleration through electromagnetic coupling with coils placed in the walls of the tube. As this happens, the bullets transfer momentum upward to the coils. This continues all the way up the tube.
At the top, which may be at any altitude, the bullets are slowed and brought to a halt by electromagnetic coupling. Then they are reversed in direction and allowed to drop down another parallel evacuated tube. As they fall they are accelerated downward by coils surrounding the tube. This again results in an upward transfer of momentum from bullets to coils.
At the bottom the bullets are slowed, caught, given a large upward velocity, and placed back in the original tube to be fired up again. We thus have a continuous stream of bullets, ascending and descending in a closed loop.
If we arrange the initial velocity and the bullets' rate of slowing correctly, the upward force at any height can be made to match the total downward gravitational force of tube, coils, and anything else we attach to them. The whole structure will stand in dynamic equilibrium, and we have no need for any super-strong materials.
The dynamic beanstalk can be made to any length, although there are advantages to extending it to geosynchronous height. Payloads raised to that point can be left in orbit without requiring any additional boost. However, a prototype could stretch upward just a few hundred kilometers, or even a few hundred meters. Seen from the outside there is no indication as to what is holding up the structure, hence the "Indian rope trick" label.
Note, however, that the word "dynamic" must be in the description, since this type of beanstalk calls for a continuous stream of bullets, with no time out for repair or maintenance. This is in contrast to our static or rotating beanstalks, which can stand on their own without the need for continuously operating drive elements.
8.23 The launch loop. As we have described the dynamic beanstalk, the main portions are vertical, with turnaround points at top and bottom. However, when the main portion is horizontal we have a launch loop.
Imagine a closed loop of evacuated tube through which runs a continuous, rapidly moving metal ribbon. The tube has one section that runs from west to east and is inclined at about 20 degrees to the horizontal. This leads to a 2,000-kilometer central section, 80 kilometers above the Earth's surface and also running west to east. A descending west-to-east third section leads back to the ground, and the fourth section is one at sea level that goes east to west and returns to meet the tube at the lower end of the first section.
The metal ribbon is 5 centimeters wide and only a couple of millimeters thick, but it travels at 12 kilometers a second. Since the orbital velocity at 80 kilometers height is only about 8 kilometers a second, the ribbon will experience a net outward force. This outward force supports the whole structure: ribbon, containing tube, and an electromagnetic launch system along the 2,000 kilometer upper portion of the loop. This upper part is the acceleration section, from which 5-ton payloads are launched into orbit. The whole structure requires about a gigawatt of power to maintain it. Hanging cables from the acceleration section balance the lateral forces produced by the acceleration of the payloads.
Although the launch loop and the dynamic beanstalk both employ materials moving through evacuated tubes, they differ in important ways. In the dynamic beanstalk the upward transfer of momentum is obtained using a decelerating and accelerating particle stream. By contrast, the launch loop contains a single loop of ribbon moving at constant speed and the upper section is maintained in position as a result of centrifugal forces.
8.24 Space colonies. I can imagine some readers at this point saying, all this talk of going to space and traveling in space, and no mention of space colonies except those on the surface of planets. There are hundreds and hundreds of stories about self-sufficient colonies in space.
There are indeed, and during the 1970s I read many of them with pleasure and even wrote some myself. One of the most fruitful ideas involved "L-5 colonies." "L-5" describes not a type of colony, but a place. In the late eighteenth century, the great French mathematician Joseph Louis Lagrange studied the problem of three bodies orbiting about each other. This is a special case of the general problem of N orbiting bodies, and as mentioned in the previous chapter, no exact solution is known for N greater than 2. Lagrange could not solve the general 3-body problem, but he could obtain useful results in a certain case, in which one of the bodies is very small and light compared with the other two. He found that there are five places where the third body could be placed, and the gravitational and centrifugal forces on it would exactly cancel. Three of those places, known as L-1, L-2, and L-3, lie on the line joining the centers of the two larger bodies. The other two, L-4 and L-5, are at the two points forming equilateral triangles with respect to the two large bodies, and lying in the plane defined by their motion about each other.
The L-1, L-2, and L-3 locations are unstable. Place a colony there, and it will tend to drift away. However, the L-4 and L-5 locations are stable. Place an object there, and it will remain. There are planetoids, known as the Trojan group, that sit in the L-4 and L-5 positions relative to Jupiter and the Sun.
The Earth-Moon system also has Lagrange points, which in the case of the L-4 and L-5 points are equidistant from Earth and Moon. In the 1970s, an inventive and charismatic Princeton physicist, Gerard O'Neill, proposed the L-5 location as an excellent place to put a space colony (L-4 would actually do just as well). The colonies that he designed were large rotating cylinders, effective gravity being provided by the centrifugal force of their rotation. Within the cylinder O'Neill imagined a complete and self-contained world, with its own water, air, soil, and plant and animal life. Supplies from Earth or Moon would be needed only rarely, to replace inevitable losses due to small leaks.
The idea was a huge success. In 1975 the L-5 Society was formed, to promote the further study and eventual building of such a colony.
What has happened since, and why? Gerard O'Neill is dead, and much of his vision died with him. The L-5 Society no longer exists. It merged with the National Space Institute to become the National Space Society, which now sees its role as the general promotion of space science and space applications.
More important than either of these factors, however, is another one: economic justification. The prospect of a large self-sufficient space colony fades as soon as we ask who would pay for it, and why. Freeman Dyson (Dyson, 1979, Chapter 11) undertook an analysis of the cost of building O'Neill's "Island One" L-5 colony, comparing it with other pioneering efforts. He made his estimate not only in dollars, but in cost in man-years per family. He decided that the L-5 colony's per family cost would be hundreds of times greater than other successful efforts. He concluded "It must inevitably be a government project, with bureaucratic management, with national prestige at stake, and with occupational health and safety regulations rigidly enforced." All this was before the International Space Station, whose timid builders have proved Dyson exactly right: "The government can afford to waste money but it cannot afford to be responsible for a disaster."
The L-5 colony concept has appeal, and the technology to build the structure will surely become available. But it is hard to see any nation funding such an enterprise in the foreseeable future, and still harder to imagine that industrial groups would be interested.
The L-5 colonyregrettably, because it is such a neat ideais part of what I like to call false futures of the past, projections made using past knowledge that are invalidated by present knowledge.
I believe there will certainly be space colonies in the future. Write stories about them by all means. But don't make them rotating cylinders at the L-5 location. Those stories have already been written.
8.25 Solar power satellites. While in skeptical mode, let me say a few words about another concept of initial high appeal, the Solar Power Satellite. This was proposed in the 1960s by Peter Glaser, and like the L-5 colonies it had its heyday in the 1970s and early 1980s. Proponents of the idea believed (and believe) that it can help to solve Earth's energy problems.
A solar power satellite, usually written as SPS, has three main components. First, a large array of photoreceptors, kilometers across, in space. Each receptor captures sunlight and turns it to electricity. The most usual proposed location is in geosynchronous orbit, though some writers prefer the Earth-Moon L-4 location. The second component is a device that converts electricity to a beam of microwave radiation and directs it toward Earth. The third component is a large array on the surface of the Earth, usually known as a rectenna, that receives the microwave radiation and turns it into electricity for distribution nationally or internationally.
The SPS has some great virtues. It can be placed where the Sun is almost always visible, unlike a ground-based solar power collector. It taps a power source that will continue to be steadily available for billions of years. It contributes no pollution on Earth, nor does it generate the waste heat of other power production systems. It does not depend on the availability of fossil or nuclear fuels.
Of course, the SPS cannot be built without a powerful in-space manufacturing capability, something that is lacking today. We are having trouble putting modest structures, such as the International Space Station, into low orbit. It is likely that we will not be able to build an object as large as the proposed SPS for another century or more.
But when a century has passed, we are likely to have much better energy-raising methods, such as controlled fusion. Admittedly, progress on fusion has been slowwe have been promised it for fifty yearsbut it, or some other superior method, will surely come along. A fusion plant (or, for that matter, a fission plant) in orbit would have all the advantages of SPS, and none of the disadvantages. Sunlight is a highly diffuse energy source unless you get very close to the Sun. As we pointed out in Chapter 5, the history of energy use shows a move in the direction of more compact power sourcesoil is more intense and compact than water or wind, nuclear is more compact and intense than chemical. The other problem is that the Sun, unlike our future fusion reactors, was not designed to fit in with human energy uses and needs. I put the question the other way round: Why build a kilometers-wide array, delicate and cumbersome and vulnerable to micrometeor damage, when you can put the same power generating capacity into something as small as a school bus? Admittedly, we don't have controlled fusion yetbut we also can't build an SPS yet.
However, the real killer argument is not technological, but economic. Suppose you launch SPS to serve, say, the continent of Africa. You still have the problem, who will pay for the energy? Economists distinguish two kinds of demand: real demand: the need for food of starving people with money to buy it; and other demand: the need for food of starving people without money. Regrettably, much demand for energy is in nations with no resources to pay for it.
In spite of this economic disconnect, many people have suggested that an SPS would be great for providing energy to Africa, where energy costs are high. Suppose that you put SPS is geostationary orbit and beam down, say, 5 gigawatts. That's the power delivered by a pretty substantial fossil fuel station. Now, you could also generate that much energy by building a dam on the Congo River, where it drops sharply from Kinshasa to the Atlantic. So ask yourself which you would prefer if you were an African. Would you like SPS, providing power from a source over which you had no control at allyou couldn't even get to visit it. Or would you prefer a dam, which in spite of all its defects, sits on African soil and is at least in some sense under your control? SPS has to compete not only from an economic point of view, but from a social and political point of view.
I think it fails on all those counts. Like the L-5 colony, SPS is part of a false future. It is not surprising to find Gerard O'Neill arguing that the sale of electricity generated by an SPS at L-5 would pay for the colony in the breathtakingly short period of twenty-four years. When we want to do something, all our assumptions are optimistic.
There are still SPS advocates. A recent NASA study suggested that a 400 megawatt SPS could be built and launched for five billion dollars. Do I believe that number? Not in this world. We all know that paper studies often diverge widely from reality. NASA's original estimated cost to build the International Space Station was eight billion dollars. Over the years, the station has shrunk in size and the costs have risen to more than 30 billion dollars. Projects look a lot easier before you get down to doing them. Recall the euphoria for nuclear power plants in the 1940s, "electricity too cheap to meter." And that was for something we had a lot more experience with than the construction of monster space structures.
Certainly, we hope and expect that the cost of sending material to space will go down drastically in the next few generations. We also will become increasingly unwilling to pollute the Earth with our power generation. But frequent space launches have their own effects on the environment of the upper atmosphere. If there is ever an SPS, which I doubt, it will more likely make little use of Earth materials and depend on the prior existence of a large space infrastructure.
I feel sure that will comeeventually. By that time the idea of power generation plants near population centers will be as unacceptable as the Middle Ages habit of allowing the privy to drain into the well. However, I want to emphasize that our solutions to the problems of the future can be expected to work no better than two-hundred-year-old solutions to the problems of today. We can propose for our distant descendants our primitive technology as fixes for their problems. But I don't believe that they will listen.
TABLE 8.1
Strength of materials.
Material
|
Density
|
Tensile strength
|
Support length
|
|
(gms/cc)
|
(kgms/sq.cm.)
|
(kms)
|
Lead
|
11.4
|
200
|
0.18
|
Gold
|
19.3
|
1,400
|
0.73
|
Aluminum
|
2.7
|
2,000
|
7.40
|
Cast iron
|
7.8
|
3,500
|
4.50
|
Carbon steel
|
7.8
|
7,000
|
9.00
|
Manganese steel
|
7.8
|
16,000
|
21.00
|
Drawn steel wire
|
7.8
|
42,000
|
54.00
|
Kevlar
|
1.4
|
28,000
|
200.00
|
Iron whisker
|
7.8
|
126,000
|
161.00
|
Silicon whisker
|
3.2
|
210,000
|
660.00
|
Graphite whisker
|
2.0
|
210,000
|
1,050.00
|
TABLE 8.2
Beanstalks around the solar system.
Body
|
Radius of stationary
|
Taper factor
|
|
satellite orbit (kms)
|
|
Mercury
|
239,731
|
1.09
|
Venus
|
1,540,746
|
1.72
|
Earth
|
42,145
|
1.64
|
Luna
|
88,412
|
1.03
|
Mars
|
20,435
|
1.10
|
Jupiter
|
159,058
|
842.00
|
Callisto
|
63,679
|
1.02
|
Saturn
|
109,166
|
5.11
|
Titan
|
72,540
|
1.03
|
Uranus
|
60,415
|
2.90
|
Neptune
|
2,222
|
6.24
|
Pluto*
|
20,024
|
1.01
|
* Since Pluto's satellite, Charon, seems to be in synchronous orbit,
a beanstalk directly connecting the two bodies is feasible.