Borderlands of Science

by Charles Sheffield

  5,513.4

  360.14

  7.23

  0.75

  * The symbol R after the period indicates that the moon is in retrograde motion.

  TABLE 7.5

  Planets of other stars.

  Star

  Distance of planet

  from star

  (Earth to Sun=1)

  Minimum

  mass

  (Jupiter=1)

  Orbit

  Period

  (days)

  51 Pegasi

  0.05

  0.5

  4.3

  47 Ursae Majoris

  2.1

  2.4

  1,103

  70 Virginis

  variable

  6.6

  117

  55 Cancri

  0.11

  0.8

  14.76

  HD 114762

  variable

  10.0

  84

  Tau Bootis

  0.0047

  3.7

  3.3

  Upsilon Andromedae

  0.054

  0.6

  4.61

  Lalande 21185

  2.2

  0.9

  5.8 (yrs)

  HD 210277

  1.15

  1.36

  1.2 (yrs)

  CHAPTER 8

  Spaceflight

  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 system—once 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 engineering—but 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 fluorine—a tricky mixture to handle, with unpleasant combustion products—has 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 launch—or, indeed, all things nuclear—is 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 system—and 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.