Physics and Technology – Technology – Rockets
A rocket engine functions according to the reaction principle, just as the jet engine of an aircraft does.
However, rockets require far greater thrust than jet aircrafts and must also be able to function in airless outer space.
Rockets work according to the reaction principle, whereby the propulsion unit ejects a mass-for example, in the form of combustion gas-and rocket and expulsion mass repel each other, pushing the rocket forward. This force is referred to as thrust, which increases with exhaust speed and fuel throughput: that is the fuel mass used per second.
A substantial difference, however, is that an aircraft engine uses atmospheric oxygen to burn fuel, while a rocket must carry oxygen with it for the chemical propellant to function in airless outer space. There are ways to create thrust. Chemical propellant engines use solid or liquid fuels, together with an oxidant such as liquid oxygen or an oxy- gen-containing chemical compound.
In starting liquid-fuel rockets, fuel and oxidant are pumped into the combustion chamber and ignited. The exhaust gases generated escape at high speeds through the jet. Liquid hydrogen, hydrocarbons, or hydrazine, among others, are used as rocket fuel. With solid-fuel rockets, fuel and oxidant are mixed as solids in the combustion chamber.
Hybrid propellants usually combine a solid fuel with a liquid oxidant.
Cooling procedure
The combustion of rocket fuel creates extreme heat; therefore, the combustion chamber and the jet must be cooled. In large engines, liquid fuel is pumped through tubes alongside the combustion chamber and jet as coolant.
Another method is to apply an ablative coating of material that conducts heat poorly and is gradually worn away. In solid-fuel propulsion units, the mixing ratio of fuel to oxidant along the periphery is specifically adjusted so that the burning temperature is low in that area.
Electric propulsion
Electric rocket engines use an ionizing gas or a metal that can readily be vaporized. This is accelerated by electric or magnetic forces and subsequently expelled. Since this type of engine does not produce sufficient thrust, it cannot be used for launches from Earth. Instead, electric engines are sometimes used for position adjustments of satellites or for operating space probes.
THE STAGES PRINCIPLE
To propel a significant payload. such as a satellite, into Earth orbit, a process of staging must be employed. In using this principle, each rocket unit (stage) is built with its own engine and fuel tanks.
The stages are cast off once their fuel has been used up. at which point the next stage is ignited. This has the effect of reducing the weight of the rocket, meaning that it gets faster acceleration from the thrust. The stage engines can be arranged one on top of another or side by side.