This picture taken from LA times shows the pass of Shuttle Atlantic
on a recent mission to dock with the Russian space station.
As we saw in Chapter 3, in order for a satellite to remain in orbit, (circular or elliptic), it must be inserted into that orbit with a specific (tangential) velocity. In cases of circular orbits, this velocity depends only on the altitude. For example, for a satellite to have a circular orbit of 500 km above the earth, it should have a tangential velocity of 7630 m/s (see example in Chapter 3).
The objective of the rocket is then to take this satellite to 500 km altitude, in such a way that when this altitude is reached, it will have a tangential velocity of 7630 m/s, referred to as burnout velocity VBo.
This picture taken from LA times shows the pass of Shuttle Atlantic
on a recent mission to dock with the Russian space station.
A rocket launched from a launchpad takes off vertically to clear the launchpad and other structures. After that, through its guidance system, it turns to a "tilt angle" and proceeds on a curved path to desired altitude as shown in the figure above. Theoretically, the amount of fuel in the rocket is such that, usually, at this altitude, all of the fuel has been consumed and the rocket is at "burnout" state. Furthermore, the rocket (and the satellite) has the desired velocity, referred to as "orbital velocity". The satellite can then be inserted into orbit and, since it has the proper velocity for that orbit, it will remain in orbit.
The physics of rocket propulsion dictates that large amounts
of fuel must be burned in relatively short periods of time to
avoid the energy expenditure of carrying unburned fuel too high.
Therefore, rocket launch is inherently dangerous with the possibility
of explosions resulting from massive amounts of stored fuel.
This danger is further increased by the need for high energy propellants,
which produce large exhaust velocities in a rapid burning process.
but they generally provide very low thrust.
Rocket motors vary considerably in type and design but generally
it may be said that they all have at least three components.
A brief outline of these three components is given below.
There are various propellants which are used in rockets. In
order for the rocket to function in the vacuum of space, an oxidant
must be carried as well as the necessary fuel. Propellants may
be solid or liquid. Solid propellants are easier to store and
handle; but being solid, their burning rates cannot be controlled
nor can their combustion be "turned off". Liquid fuel,
however, provides controllable thrust since the rate of combustion
can be regulated by regulating the fuel-oxidant feed and the combustion
can be stopped, allowing for re-ignition later. On the other
hand, they are difficult to store. There are also some proposed
rocket engines which use ions (plasma) or nuclear fuels for propulsion,
Solid propellants may be separated into two principal groups: double-base propellants and composite propellants.
There are several kinds of double-base propellants, however, in general, most are gelatinized mixtures of nitroglycerin and nitrocellulose, to which certain stabilizers are added. The basic concept is to obtain a controlled combustion of nitroglycerin (which is also used to make dynamite) as opposed to its more natural unstable combustion (explosion). The reasonably high specific impulse, smoke-free burning characteristics with very low toxicity still make these propellants attractive, even though the manufacturing process is (as you might expect) very dangerous.
Composite propellants, on the other hand, use an oxidizer in
powdered form incorporated into some form of fuel; the latter
also acts as a binder. In its simplest form, this would involve
potassium perchlorate (KClO4) as the oxidizer and something
as simple as asphalt for the fuel. Today, some very sophisticated
(and secret) mixtures are used for both the oxidizer and fuel.
A monopropellant may be thought of as a double-base solid propellant in liquid form. That is, it is a substance which breaks down, releasing energy and needs no auxiliary oxidizer. These are not commonly used.
Bipropellants are by far the most widely used form of liquid
propellants. In these, the fuel and oxidizer are stored separately
and are injected into the combustion chamber where they combine,
releasing the energy needed for propulsion. The most common oxidizer
used is (surprise!) liquid oxygen, or LOX as it is known among
the cognizant, while the fuel can be as ordinary as kerosene or
hydrogen.
The effectiveness of a rocket fuel is measured by a parameter referred to as "specific impulse", Isp, which is defined as
The overall performance is measured by 
.
Some examples of Isp are
| gun powder | |
| kerosene + LOX** | |
| LH2 + LOX** | |
| H2 + Nuclear | |
| H + electrical arc |
There is nothing complicated about a combustion chamber. It
is nothing more than a volume in which the combustion or energy
liberation reaction takes place.
The thrust of a rocket is generated, as we saw earlier, by the difference in pressure between the exhausted gas and the ambient, but much more importantly, by the high speed exit of the combustion bases. This high exit speed is given to the gases by the exit nozzle.
To increase the speed of the combustion gases, a converging section is used initially. This means the gas is made to flow through a smaller and smaller flow cross-section. This technique works up to the point where the flow speed is equal to the local speed of sound in the gas. If further acceleration is required, now a diverging section is needed, i.e., the gas must now flow through a larger and larger cross-section. This is the reason for the bell-shape of most rocket motor exhausts - we are seeing the diverging section of the nozzle. The figure below shows a typical rocket nozzle.
