Sr-71 Blackbird

Knowledge

What is the most popular method of propulsion?

What devices use chemical propulsion?

What is the purpose of chemical propulsion?

What does a rocket need to work?

What is better liquids or solid?

Comprhension

1) The Basics

A) Propulsion

B) The necessity for chemical propulsion

C) Effectiveness of chemical propulsion

2) Solid propellants

A) Advantages

B) Disadvantages

3) Liquid propellants

A) Advantages

B) Disadvantages

4) The process

A) Where are they kept before burned

B) How is the chemical push

C) What does the cemical come out of

Application

Analysis

SOLID PROPELLANTS

The idea behind a simple solid-fuel rocket is straightforward. What you want to do is create something that burns very quickly but does not explode. As you are probably aware, gunpowder explodes.

A solid-fuel rocket immediately before and after ignition

On the left you see the rocket before ignition. The solid fuel is shown in green. It is cylindrical, with a tube drilled down the middle. When you light the fuel, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small model rocket engine or in a tiny bottle rocket the burn might last a second or less. In a Space Shuttle SRB containing over a million pounds of fuel, the burn lasts about two minutes.

Solid-fuel rocket engines have three important advantages:

* Simplicity

* Low cost

* Safety

They also have two disadvantages:

* Thrust cannot be controlled.

* Once ignited, the engine cannot be stopped or restarted.

The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks (like missiles), or for booster systems. When you need to be able to control the engine, you must use a liquid propellant system.

LIQUID PROPELLANTS

The basic idea is simple. In most liquid-propellant rocket engines, a fuel and an oxidizer (for example, gasoline and liquid oxygen) are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then they leave the engine. The following highly simplified diagram shows you the basic components.

One of the big problems in a liquid propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to cool them. The pumps have to generate extremely high pressures in order to overcome the pressure that the burning fuel creates in the combustion chamber. The main engines in the Space Shuttle actually use two pumping stages and burn fuel to drive the second stage pumps.

All kinds of fuel combinations get used in liquid propellant rocket engines. For example:

* Liquid hydrogen and liquid oxygen - used in the Space Shuttle main engines

* Gasoline and liquid oxygen - used in Goddard's early rockets

* Kerosene and liquid oxygen - used on the first stage of the large Saturn V boosters in the Apollo program

* Alcohol and liquid oxygen - used in the German V2 rockets

* Nitrogen tetroxide/monomethyl hydrazine - used in the Cassini engines

Synthesis

Liquid solid

Spacecraft propulsion is used to change the velocity of spacecraft, Delta V. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. Most spacecraft today are propelled by heating the reaction mass and allowing it to flow out the back of the vehicle. This sort of engine is called a rocket engine. All current spacecraft use chemical rocket for launch, though some have used air-breathing engines on their first stage. Most satellites have simple reliable chemical rockets or resistojet rockets to keep their station, although some use momentum wheels for attitude control. Newer geo-orbiting spacecraft are starting to use electric propulsion for north-south station keeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters with some success.