Everything about Reaction Engine totally explained
A
reaction engine is an engine which provides propulsion by expelling
reaction mass, in accordance with
Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there's an equal, but opposite, reaction force".
Examples include both
jet engines and
rocket engines, and more uncommon variations such as
Hall effect thrusters,
ion drives and
mass drivers.
Thrust
The force generated by a reaction engine is in accordance with Newton's second law:
» . Thus the latter shouldn't be too large.
On the other hand if the exhaust velocity can be made to vary so that at each instant it's equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space and has no kinetic energy; and the propulsive efficiency is 100% all the energy ends up in the vehicle (in principle such a drive would be 100% efficient, in practice there would be thermal losses from within the drive system and residual heat in the exhaust). However in most cases this uses an impractical quantity of propellant, but is a useful theoretical consideration.
Some drives (such as
VASIMR or
Electrodeless plasma thruster ) actually can significantly vary their exhaust velocity. This can help reduce propellant usage and improve acceleration at different stages of the flight. However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed. Proposed ion and plasma drives usually have exhaust velocities enormously higher than that ideal (in the case of VASIMR the lowest quoted speed is around 15000 m/s compared to a mission delta-v from high Earth orbit to Mars of about
4000m/s).
For a mission, for example, when launching from or landing on a planet, the effects of gravitational attraction and any atmospheric drag must be overcome by using fuel. It is typical to combine the effects of these and other effects into an effective mission
delta-v. For example a launch mission to low Earth orbit requires about 9.3-10 km/s delta-v. These mission delta-vs are typically numerically integrated on a computer.
Cycle efficiency
All reaction engines lose some energy- mostly as heat.
Different reaction engines have different efficiencies and losses. For example rocket engines can be up to 60-70% energy efficient in terms of accelerating the propellant- the rest is lost as heat primarily in the exhaust, but also a small amount lost as thermal radiation.
Types of reaction engines
Airbreathing
solid exhaust
Further Information
Get more info on 'Reaction Engine'.
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