A rocket engine is a type of jet engine that uses only stored rocket propellant mass for forming its high-speed propulsive jet. Rocket engines are reaction engines, obtaining thrust in accordance with Newton’s third law. It is a non-air breathing engine. Most rocket engines are internal combustion engines, although non-combusting forms also exist. Vehicles propelled by rocket engines are commonly called rockets. Since they need no external material to form their jet, rocket engines can perform in a vacuum and thus can be used to propel spacecraft and ballistic missiles.
Working principle of a rocket engine:
A working principle of a rocket engine involves 4 processes.
1.Injection: Liquid-fuelled rockets force separate fuel and oxidizer components into the combustion chamber, where they mix and burn. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. Both liquid and hybrid rockets use injectors to introduce the propellant into the chamber.
2. Combustion: The fuel gets combusted in the combustion chamber. The combustion chamber is typically just a cylinder which also includes a flame holder in rare cases. The length of the chamber is typically in the range of 25–60 inches and is calculated by the formula.
V=volume of the chamber
A=Area of the throat
The hot gas produced in the combustion chamber is permitted to escape through a diverging expansion section. When sufficient pressure is provided to the nozzle, the nozzle chokes and a supersonic jet is formed, dramatically accelerating the gas, converting most of the thermal energy into kinetic energy. Exhaust speeds vary, depending on the expansion ratio the nozzle is designed for, but exhaust speeds as high as ten times the speed of sound in air at sea level.About half of the rocket engine’s thrust comes from the unbalanced pressures inside the combustion chamber, and The rest comes from the pressures acting on the inside of the nozzle. As the gas expands, the pressure against the nozzle’s walls forces the rocket engine in one direction while accelerating the gas in the other.
The exit static pressure of the exhaust jet depends on the chamber pressure and the ratio of exit to throat area of the nozzle. As exit pressure varies from the ambient (atmospheric) pressure, a choked nozzle is said to be
- under-expanded (exit pressure greater than ambient),
- perfectly expanded (exit pressure equals ambient),
- over-expanded (exit pressure less than ambient; shock diamonds form outside the nozzle),
- Grossly over-expanded (a shock wave forms inside the nozzle extension)
Net Thrust: This is an equation for calculating the net thrust of a rocket engine.