Page 11 - Curriculum Visions Dynamic Book

Engines and motors
Launchers use either solid propellants or liquid propellants (fuels) in their propulsion systems. The term rocket motor is used if the propulsion system burns solid fuel and rocket engine if it burns liquid fuel.
Solid fuels produce more thrust, but the rate of fuel usage cannot be closely controlled; liquid fuels produce less thrust, but are controllable in that the amount of fuel can be changed, just as we might change how far we press down on the gas pedal of a car.
Since both types of fuel have their own advantages and disadvantages, both are usually used. Solid fuels often power early launcher stages, for example, getting the launcher off the ground. Liquid fuels propel the last phases of ascent
and aid maneuvering.
The rate at which a fuel of any kind will burn
depends on how quickly oxygen reaches it (as you notice when you increase the draft to a fire). In this role oxygen is called an oxidizer. The amount of oxygen in air is small, and the concentration of oxygen decreases as
you go up through the atmosphere. In addition, if you simply suck air in from the surroundings, you have little control over how the fuel burns. So, the best solution is not to rely on air at all and use oxygen (as, for example, in a liquefied gas).
Kerosene is a common liquid fuel, stored in one tank strapped to the launcher, with liquid oxygen in a second tank. Early launchers used this system. Other chemicals include hydrazine as fuel and nitrogen tetroxide, instead of oxygen, as the oxidizer. These chemicals ignite spontaneously when they mix, avoiding the need for an igniter as with kerosene and oxygen. Engines that ignite spontaneously are called hypergolic engines.
There are alternatives to liquid and solid fuels. One important one is hydrogen. Hydrogen is a gas. In its gaseous form it would take up a huge volume (think of the airships of the past), so it has to be compressed to liquefy before use.
corrosive substance Something that chemically eats away something else.
oxidizer The substance in a reaction that removes electrons from and thereby oxidizes (burns) another substance.
propulsion system The motors or rockets and their tanks designed to give a launcher or space vehicle the thrust
it needs.
rocket engine A propulsion system that burns liquid fuel such as liquid hydrogen.
rocket motor A propulsion system that burns solid fuel such as hydrazine.
11
The weight equation
To launch a space vehicle requires
an enormous fuel-to-payload weight ratio. In the Apollo mission a huge Saturn V rocket was used to launch the small command module and Moon lander. The equation remains true. You can see it here, for example, with the Space Shuttle (figures below are approximate).
Orbiter empty:
External tank empty:
Two solid rocket boosters empty:
Fuel
External tank:
Solid rocket boosters: Total fuel:
75,000 kg 35,000 kg
170,000 kg
700,000 kg 1,000,000 kg 1,700,000 kg
Total weight at launch,
including tanks, orbiter,
and its payload: about 2,000,000 kg
Fuel alone weighs over 20 times the weight of the payload. Fuel plus tanks weigh nearly 30 times the payload.