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Chapter: Mechanical - Gas Dynamics and Jet Propulsion - Space Propulsion

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Overall rocket engine performance and Effective Speed Ratio

Rocket technology can combine very high thrust (meganewtons), very high exhaust speeds (around 10 times the speed of sound in air at sea level) and very high thrust/weight ratios (>100) simultaneously as well as being able to operate outside the atmosphere, and while permitting the use of low pressure and hence lightweight tanks and structure.

Overall rocket engine performance

 

Rocket technology can combine very high thrust (meganewtons), very high exhaust speeds (around 10 times the speed of sound in air at sea level) and very high thrust/weight ratios (>100) simultaneously as well as being able to operate outside the atmosphere, and while permitting the use of low pressure and hence lightweight tanks and structure.

 

Rockets can be further optimised to even more extreme performance along one or more of these axes at the expense of the others.

 

Specific impulse:

 

The most important metric for the efficiency of a rocket engine is impulse per unit of propellant, this is called specific impulse (usually written Isp). This is either measured as a speed (the effective exhaust velocity Ve in metres/second or ft/s) or as a time (seconds).

 

An engine that gives a large specific impulse is normally highly desirable. The specific impulse that can be achieved is primarily a function of the propellant mix (and ultimately would limit the specific impulse), but practical limits on chamber pressures and the nozzle expansion ratios reduce the performance that can be achieved. Space flight: Spaceflight is the act of travelling into or through outer space. Spaceflight can occur with spacecraft which may, or may not, have humans on board. Examples of human spaceflight include the Russian Soyuz program, the U.S. Space shuttle program, as well as the ongoing International Space Station. Examples of unmanned spaceflight include space probes which leave Earth's orbit, as well as satellites in orbit around Earth, such as communication satellites.


Effective Speed Ratio


Thrust Specific Fuel Consumption (TSFC)

 

The fuel consumption rate per unit thrust is known as Thrust Specific Fuel Consumption

 


Specific Impulse (Isp )

 

The thrust developed  per unit weight  flow rate is known as specific impulse


propulsive efficiency:


At the outlet of the engine , the power is



Thermal Efficiency

 

It is defined as the ratio of power output of the engine to the power input to the engine.


Where,                 

m       Mass of air fuel mixture         

c j      Velocity of  jet     

u   Flight velocity

                   fuel   

          m f     Mass of               

C V  Calorific value of   fuel

If efficiency of combustion      is considered ,


Overall Efficiency

 

It is defined as the ratio of  propulsive  power to the power input to the engine.

 




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