I.  Introduction to Propulsion

A. Goal:  Create a Force to Propel a Vehicle

Two options:

  1. Take mass stored in a vehicle and throw it backwards (rocket propulsion). Use the reaction force to propel the vehicle.
Propellant ---> burn ---> expand through nozzle
(chem. energy)   (thermal energy)   (kinetic energy & momentum)

 

Q1 (PDF)
  1. Seize mass from the surroundings and set the mass in motion backwards. Use the reaction force to propel vehicle (air-breathing propulsion).
Continuously: a) Draw in air.
  b) Compress it.
  c) Add fuel and burn (convert chemical energy to thermal energy).
  d) Expand through a turbine to drive compressor (extract work).
  e.1) Then expand in a nozzle to convert thermal energy to kinetic energy & momentum (turbojet).
  e.2) Or expand in a second turbine (extract work), use this to drive a shaft for a fan (turbofan), or a propeller (turboshaft). The fan or propeller impart k.e. & mom. to the air.

 

*Remember:

Overall goal:  take  at Vo (flight speed), throw it out at Vo + DV

Q2 (PDF)

Gas Turbine Engine Schematic

Figure 1.1 Schematics of typical military gas turbine engine: J57 turbojet with afterburning.

 

Turboprop Engine
                 

Figure 1.2 A typical high bypass-ratio turbofan (Adapted from Pratt & Whitney).

For more examples of real world powerplants, refer to Hill, P. and C. Peterson. Thermodynamics of Propulsion. 2nd Ed. Addison-Wesley, 1991.

 

B. Performance Parameters

The two performance parameters of greatest interest for a propulsion system are the force it produces (thrust, T), and the overall efficiency with which it uses energy to produce this force (hoverall).  We will begin by looking at the production of thrust using the integral form of the momentum theorem.  In the second lecture we will discuss the efficiency of propulsion systems.

 

C.   Propulsion is a systems endeavor

There are a multitude of other factors which a propulsion engineer must take into account when designing a device including weight, cost, manufacturability, safety, environmental effects, etc.  Thus propulsion is truly a systems endeavor, requiring knowledge of a variety of disciplines:
Fluids  +  thermo  + structures  +  dynamics  +  controls  +  chemistry  +  acoustics + …

We will focus mostly on these two disciplines in the Unified propulsion lectures.

 

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