Electric propulsion systems have been developed over many years and have already flown on unmanned spacecraft. There is a fundamental difference between chemical and electrical propulsion systems. With a chemical system the energy required to accelerate the propellant out of the rocket nozzle is obtained from burning the fuel/oxidizer combination. The speed we can impart to the vehicle is fundamentally limited by the amount of energy contained in the chemical propellants. However, an electric propulsion system is separately powered, inasmuch as the energy to accelerate the
propellant comes from a separate source, and so in principle is unlimited. As the name implies, this separate source is electricity, and this can be generated using sunlight (solar panels) or nuclear energy.
A common form of electric propulsion, called the ion engine, is shown in Figure 10.10, which depicts a simplified cross section through a typical ion engine. The engine is a squat cylindrical chamber, a bit like a paint can, into which the propellant is injected. In the ion engines that are used in small unmanned spacecraft, the diameter of the cylinder is around 10 cm (4 inches) in size, and the usual propellant is an inert gas, such as argon or xenon. As well as the propellant, electrons are also injected into the chamber from a heated hollow tube, called a hollow cathode. Surrounding the cylindrical surface of the chamber are solenoids—basically electromagnets—that produce a magnetic field within the chamber, causing the electrons to corkscrew around the magnetic field lines (see Figure 6.7 in Chapter 6). In this way, the likelihood of collisions between the electrons and the propellant atoms is increased, and these collisions cause ionization of the propellant. In other words, electrons are stripped from the propellant atoms, causing them to acquire a positive electric charge. These positively charged propellant atoms are called ions. (There are a variety of ways of producing the population of ions in the chamber other than the use of a hollow cathode described here.) At the exit of the device, there are metal grids to which a voltage is applied to accelerate the ions out of the device, forming a high-speed ion beam. Finally, to prevent the spacecraft acquiring a huge negative electric charge, electrons are squirted downstream into the ion beam by a beam neutralizer, to allow the electrons to recombine with the propellant ions.
The resulting exhaust velocity of the device is typically on the order of 50 km/sec (30 miles/sec)—about 10 times higher than that of a highperformance chemical system—but the achievable mass flow rate is much smaller. As a consequence, a typical ion engine has a large specific impulse (which is good), but a low thrust level (which is not so good). Recalling the discussion about specific impulse in Chapter 5, this means that, all other things being equal, the ion engine will produce about 10 times more AV (speed change) for a given mass of fuel than a chemical system. However, the low thrust means that it will take a long time to do so. Fortunately ion engines can operate for thousands of hours, so the tiny accelerations that they produce can build up large A Vs, but one has to be patient. It is a 0 to 60 mph in 2 weeks kind of performance! For an ion engine powered by solar panels, with an input of around 1 kWof electrical power, the level of thrust is quite tiny—on the order of 1/20th of a Newton!
To date, this kind of system has been used on unmanned spacecraft for things like orbit control of spacecraft in Earth orbit, or for missions to the Moon and near-Earth asteroids. But the question is, Can the system be scaled up to produce a useful means of propelling large manned spacecraft? The obvious route is to use a nuclear reactor power system to increase the power levels to the hundreds of kW level. Research into the feasibility of such NEP systems has been underway for many years, but such a system is yet to be flown in space. Using some simple calculations, we can scale up the 1 kW system mentioned above to a power input of, say, 500 kW. The configuration of such an ion drive is speculative, but we can envisage this power input supplying maybe five ion engine units each 60 cm in diameter. A quick calculation gives a thrust level of around 15 N, which (if we recall our informal definition of a Newton) is a force equivalent to the weight of about 15 small apples. Although this is more useful than the 1/20th of a Newton we had before, even so it is questionable whether an ion drive can be scaled to provide a propulsion system for manned spacecraft, which tend to be large (of the order of 100 metric tonnes in mass). We will come back to this in our summary later.
Another issue that affects the dynamics of nuclear powered propulsion systems is the mass of the power plant. The nuclear reactor is likely to be of a significant mass, which will add to the already burgeoning mass of a manned spacecraft. This is hard to estimate, but the mass of a 500 kW reactor, for example, might be around 3 metric tonnes.
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