The Rocket

Just as the airplane relies on laws of flight physics that are fundamentally different from the laws governing the buoyant flight of balloons and airships, so the rocket makes use of unique physical laws in its own operation. These laws were first written down by the English physicist Sir Isaac Newton, and are basic to the flight of all spacecraft:

1. An object in motion or at rest remains so unless influenced by an external force.

2. A force imposed on an object causes an acceleration proportional to its mass.

3. For every force or action, there is a counter-force or reaction.

We will examine these laws in reverse order, since this permits a more logical discussion. The third law explains how rockets work, and especially why they work in the vacuum of space. In a nutshell, the velocity of the exhaust gases exiting the rocket engine builds up a momentum, causing the rocket to accelerate in the opposite direction. The greater the momentum - mass times velocity - of the rocket exhaust, the faster the rocket-powered vehicle will go. A toy balloon released after inflation is a good example of the rocket reaction. When a rocket operates inside Earth's (or any planet's) atmosphere, the ambient pressure conditions affect the operation of the engine, usually resulting in less-than-efficient operation. Rockets are therefore happiest in the vacuum of space, where they can operate at optimum performance.

The second law tells rocket scientists how fast a spacecraft or launch vehicle will speed up when the engines are fired. A heavy vehicle, fully fueled, will obviously accelerate very slowly, while a lighter one will accelerate much more rapidly under similar thrust. The less mass a rocket has to accelerate, the easier it is to speed that rocket up. And speed is everything in spaceflight. This naturally intuitive concept explains why all launch vehicles shed pieces of themselves on the way into orbit. For every stage that is dropped off, the remaining rocket has a significantly easier job getting the rest of the vehicle up to the required speed.

The first law is what allows a spaceship to coast, in the absence of air drag, almost all the way from its launch point to its ultimate destination without firing its rocket engines (Fig. 3.4). It is also what allows orbiting spaceships, stations, satellites, and probes to freely operate for extended periods. All orbits may be described in terms of free-fall trajectories.

Rockets do not need to push against anything for their operation. In fact, they work better in the vacuum of space than they do in the atmosphere, as described above. Just how do rockets work, though? Why do they always seem to lift off vertically?

Rockets are the ultimate in controlled combustion heat engines. They work by taking the potential energy stored in a fuel, burning that with an oxidizer in a specific

Fig. 3.4 Artist's concept of Orion spacecraft with attached Lunar Surface Access Module approaching Luna (courtesy NASA)

way, and turning that energy into the potential energy of altitude and the kinetic energy of high velocity. This explains how the Space Shuttle, for example, starts out with a huge external tank filled with more than 1/ million pounds of liquid propellants and two enormous solid propellant boosters, and 8/ min later winds up in a low Earth orbit at a speed of 17,500 mph, having burned virtually all of its propellant.

Fuel and oxidizer are injected into the thrust chamber of a rocket engine, either through the operation of high-pressure turbopumps or by means of an inert gas pressurization system. The high pressures are necessary because of the voracious appetites of rocket engines. As combustion takes place, the hot gases can escape in one direction only, which is out the rear through a specially designed supersonic nozzle. The nozzle is designed to accelerate the exhaust gases with maximum efficiency, because it is the acceleration of the gases that imparts the required momentum to the vehicle itself in the opposite direction.

Unlike airplanes, most rockets depart vertically from the ground. Aside from the obvious reason that the destination is straight up, there are other considerations that compel this approach. In contrast to other vehicles, rockets carry everything they will need with them. This includes the oxygen they will need to sustain combustion of the fuel in their engines. The onboard propellants - fuel and oxidizer - serve the dual purposes of providing the energy and the working mass for the thrust required. Because there is no air in space, all the working mass must be brought along, and it is convenient that the fuel and oxidizer fill this role. Therefore, launch vehicles have no need of the atmosphere at all. In fact, it represents a barrier to be crossed as quickly as possible. Too much speed in the atmosphere can result in dynamic pressures that can tear a vehicle apart. Drag losses also increase the required "delta-V"

to reach orbit. So the quicker a rocket can punch through the atmosphere - before it has built up too much speed - and reach the rocket's real element of space, the better. Further, vertical launch permits a rocket to be designed in a structurally simple way, because the acceleration loads are mainly longitudinal - along the length of the stack. Finally, rockets have no requirement for landing gear or wings, because they never return to Earth. This further simplifies the design of these vertical, nonreusable tubes. These factors make rockets both lightweight and structurally sound for their one-time missions. For all these reasons, rockets have always been launched straight up.

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