Thermodynamics and Propulsion

9.1 Goal: Create a Force to Propel a Vehicle

We have two options for creating a propulsion force:

1. Take mass stored in a vehicle and throw it backwards, using the reaction force to propel the vehicle. This is rocket propulsion. A photograph and schematic of a rocket engine are shown in Figure 9.1.

   $$\begin{subarray}{l}\textrm{Propellant} \textrm{(chem. energy)}\... ...m{expand through nozzle} \textrm{(kinetic energy and momentum)}\end{subarray}$$

   
   

   Figure 9.1: Typical liquid propellant rocket motor (Hill and Peterson, 1992).

   

2. Seize mass from the surroundings and set the mass in motion backwards, using the reaction force to propel the vehicle. This is air-breathing propulsion. Examples of air-breathing jet engines are shown in Figures 9.2, 9.3, and 9.4.

   Continuously:

   1. Draw in air.
   2. Compress it.
   3. Add fuel and burn (convert chemical to thermal energy).
   4. Expand through a turbine to drive the compressor (extract work).
   5. Then either
      1. expand in a nozzle to convert thermal energy to kinetic energy & momentum (turbojet), or
      2. 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 $\dot{m}$ at $V_0$ (flight speed), throw it out at $V_0 + \Delta V$ .

Figure 9.2: Schematics of typical military gas turbine engines. Top: turbojet with afterburning, bottom: GE F404 low bypass ratio turbofan with afterburning (Hill and Peterson, 1992).

Figure 9.3: Schematics of a PW PT6A-65, a typical turboprop (Hill and Peterson, 1992).

Figure 9.4: The RB211-535E4, a typical high bypass-ratio turbofan (Hill and Peterson, 1992).