In this section we will examine soM-propellant, liquid-propellant, hybrid-propellant, and multistage rockets. There are three main types of chemical propellant rocket. There are those that use solid fuels combined with a solid oxidizer in a single matrix, the so-called solid rockets. Then there are those that use separate liquid fuels and oxidizers, the so-called liquid rockets. And there are those that use a solid fuel with a liquid oxidizer (or vice versa), a design that gives a solid fuel rocket some of the benefits of a liquid rocket. These are the hybrid rockets. All rockets require onboard supplies of both a fuel and an oxidizer.
Solid propellant rockets use a single rubbery compound bonded to the interior of the rocket casing itself. The inside of this solid "grain" is hollowed out in a specific shape to provide a chamber for combustion to take place at a certain rate and in a certain way. The grain contains both the fuel and the oxidizer mixed together, so that combustion can be sustained for as long as the propellant lasts. Once a solid rocket is ignited, there is no shutting it off; there are no valves and there is no other method of squelching the high energy combustion. The inside of the grain, along its entire length, serves as the combustion chamber, so that the propellant canister and the combustion chamber are one and the same.
One of the advantages of a solid-propellant rocket is that it can be stored for very long periods of time before it is used. Another advantage is the very simple design, which makes it easy to produce, possible to refurbish with a new rubber matrix, and very reliable. These items are so reliable, in fact, that the explosive bolts holding the Space Shuttle to the pad are blown just before the solid rocket boosters are ignited. Imagine what would happen if the solids did not ignite. In that case, the Shuttle would tip over and blow up, with no hope of rescuing the crew.
The major drawback to solid rockets is the fact that they cannot be turned off, throttled down, or powered back up. Although they can be refurbished, which involves repacking the case with new propellant, the cost and complexity of doing so are large compared to liquid refueling operations.
Liquid-propellant rockets (Fig. 4.3) use a variety of fuels and oxidizers, kept in isolated tanks until they reach the combustion chamber. Some of these are cryogenic, requiring extremely low temperatures and tank insulation - liquid hydrogen and oxygen are two examples. Some are hypergolic, igniting spontaneously on contact. Monomethyl hydrazine (MMH) and nitrogen tetroxide (N2O4) are examples of hypergolic propellants.
Some liquid propellants are very toxic; others are extremely corrosive. And some are ordinary fuels, such as gasoline or kerosene. The V-2 rocket used alcohol made from potatoes. The great advantage in liquid rockets is that they can be turned off or on, and throttled to provide a variable thrust. High specific impulse rockets often use cryogenic propellants during launch, while in-space reaction control system thrusters tend to rely on hypergolic propellants. The major disadvantage in liquid rocket engines is their complexity. The propellants have to be highly pressurized before being admitted to the thrust chamber, and valves have to work in extremes of temperature and pressure; nozzles have to be cooled. Above all, the engine must not malfunction during operation, because then the "controlled explosion" of the propellants could potentially become an uncontrolled conflagration. Helium tanks are sometimes used to pressurize and deliver propellants to the thrust chamber, avoiding the use of temperamental turbopumps. This increases the reliability of the liquid engine during space missions, but it also increases the weight of the vehicle.
A third class of rocket engine is the hybrid-propellant rocket. This is the type used by SpaceShipOne to win the Ansari X-prize. A hybrid rocket uses either a solid fuel and a liquid oxidizer, or vice versa. By using a liquid together with a solid, the engine can be throttled like a liquid-propellant rocket, but retains some of the simplicity and reliability of the solid rocket motor design. Like both solid and liquid rockets, hybrid engines can be dangerous if not handled carefully.
A multistage rocket is simply a series of smaller rockets stacked together. Only the tip requires an aerodynamic point, turning the lower stages into cylindrical sections. By using each stage in succession, and dropping off expended stages, the remaining stack of rockets can be accelerated to much greater velocities. The staging concept turns a regular rocket into a modular missile, with a greatly increased effective mass ratio, greater range, and higher speed. Orbital spaceflight is now within reach. As we will see in the next section, the staging concept is not limited to launch vehicles alone, but is used in modular spaceflight operations as well.
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