It Is Rocket Science

The surprising thing about a rocket engine, as we have already intimated, is that the concept is not at all complicated. It does not have complex mechanisms like pistons going up and down as in an automobile engine. Instead, it comprises only three basic parts: a propellant feed system, a combustion chamber where the fuel is burned, and a nozzle out of which the resultant hot gases are exhausted. And that's it.

If we're thinking about rocket engines used on launch vehicles, then it is obvious that they need to be big. For example, all the components of a space shuttle sitting on the launch pad have a combined mass of about 2000 metric tonnes! The rocket engines—five of them in this case—have to produce enough thrust between them to match the vehicle weight, and then a bit more to move it vertically off the pad, as beautifully illustrated in Figure 5.1. Two types of rocket engine are used by the shuttle to gain orbit, the solid propellant motor and the liquid propellant motor, and these are the most commonly used systems on launchers.

The solid propellant motor (Fig. 5.2a) is a bit like a giant firework, inasmuch as you light it, and it burns to produce thrust until the solid propellant is exhausted. To burn the fuel in any rocket system, we need oxygen. In an airplane, the fuel is burned using oxygen taken from the atmosphere through the engine intakes. However, since there is no atmospheric oxygen available as the launcher approaches orbital altitudes, the rocket system has to take its own oxygen with it. For a solid propellant motor, the fuel and oxidizer are mixed together in a gooey substance, which is then poured into molds to set hard. Often the molds are shaped to produce sections of solid propellant like giant disks with a hole punched through the middle, which are then stacked in the rocket's cylindrical casing to produce the configuration shown in Figure 5.2b. The rocket is then lit by a pyrotechnic device, usually at the top of the rocket, and the propellant burns from the inside of the cylinder outward toward the metal casing, until depleted.

Winged orbiter

External tank (for liquid pro pe Hants)

Space Shuttle main engines (x3)

Solid propellant rocket boosters (x2)

Figure 5.1: The components that make up the space shuttle launch system. (Image courtesy of the National Aeronautics and Space Administration [NASA].)

Winged orbiter

Space Shuttle main engines (x3)

External tank (for liquid pro pe Hants)

Solid propellant rocket boosters (x2)

Figure 5.1: The components that make up the space shuttle launch system. (Image courtesy of the National Aeronautics and Space Administration [NASA].)

The space shuttle uses two large solid propellant rocket boosters for its first two minutes of flight, which use a combined fuel and oxidizer solid propellant. When the system was designed in the 1970s, a decision was made to use solid propellant boosters, mainly to constrain costs. Many of the rocket scientists involved were unhappy about their use on a system designed to carry people. The basis of this concern was the fact that solid rockets are less controllable than their liquid propellant counterparts, the main worry being that once a solid rocket is ignited, it cannot be stopped until the fuel is exhausted. The boosters used on the shuttle have a large thrust, and have to be ignited simultaneously at the moment of lift off. If one were to fail to ignite at that critical moment, the resulting asymmetry in the vehicle's thrust would be catastrophic.

The liquid propellant rocket motor (Fig. 5.3) is a little more complicated, requiring a propellant feed system, in addition to the combustion chamber

Pyrotechnic device to ignite propellant

Solid propellant (combined fuel and oxidiser)

Pyrotechnic device to ignite propellant

Solid propellant (combined fuel and oxidiser)

Nozzle

Propellant sections

Figure 5.2: (a) The elements comprising a typical solid propellant rocket, (b) The assembly of the solid propellant sections in the rocket casing.

Nozzle

Propellant sections

Figure 5.2: (a) The elements comprising a typical solid propellant rocket, (b) The assembly of the solid propellant sections in the rocket casing.

Figure 5.3: Schematic of the components of a liquid propellant rocket motor.

and rocket nozzle. In this case, the launcher carries its fuel and oxidizer in separate tanks, with a feed system—usually pumps—to shift the liquids into the combustion chamber, where they are ignited to produce a highly pressurized hot gas that exits through the nozzle of the engine to produce thrust. As we will see in a moment, the faster the exhaust gases exit the nozzle, the better. The internal cross section of the nozzle has a particular profile, first converging to form a throat and then diverging to form the familiar bell shape. The exhaust gases accelerate as they squeeze through the throat, typically reaching the local speed of sound. Thereafter, the gases continue to accelerate and expand in the divergent section, ensuring a high exit speed and ideally an exit pressure near to the ambient atmospheric pressure.

Since the propellant can be supplied to the combustion chamber in a controlled manner through the feed system, liquid propellant rockets are inherently more controllable, allowing the thrust level to be varied and the rocket to be turned on and off if required. There are a variety of fuel and oxidizer combinations, but commonly used ones are hydrogen/oxygen and kerosene/oxygen.

In addition to the two solid propellant boosters mentioned above, the space shuttle also requires the use of three liquid propellant rocket motors to acquire orbit; these are referred to as the space shuttle main engines (SSMEs). It is these liquid-powered rocket engines that continue to operate to take the vehicle to orbit, once the solid propellant boosters are depleted and fall away. The SSMEs use a combination of hydrogen and oxygen as fuel and oxidizer, respectively. These gases need to be cooled to very low temperatures to produce a cryogenic liquid, and these are stored in the large insulated external fuel tank prior to launch (see Fig. 5.1).

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