1. FLUID MECHANICS: DERIVATION OF THRUST EQUATION
Chemical
Energy
Thermal
Energy
Kinetic
Energy
Flow through engine is conventionally called THRUST
Composed of net change in momentum of inlet and exit air
Fluid that passes around engine is conventionally called DRAG

2. THERMODYANMICS: BRAYTON CYCLE MODEL
1-2: Inlet, Compressor and/or Fan: Adiabatic compression with spinning blade rows
2-3: Combustor: Constant pressure heat addition
3-4: Turbine and Nozzle: Adiabatic expansion
Take work out of flow to drive compressor
Remaining work to accelerate fluid for jet propulsion
Thermal efficiency of Brayton Cycle, hth=1-T1/T2
Function of temperature or pressure ratio across inlet and compressor

3. EFFICIENCY SUMMARY Overall Efficiency What you get / What you pay for
Propulsive Power / Fuel Power
Propulsive Power = TUo
Fuel Power = (fuel mass flow rate) x (fuel energy per unit mass)
Thermal Efficiency
Rate of production of propulsive kinetic energy / fuel power
This is cycle efficiency
Propulsive Efficiency
Propulsive Power / Rate of production of propulsive kinetic energy, or
Power to airplane / Power in Jet

4. PROPULSIVE EFFICIENCY AND SPECIFIC THRUST AS A FUNCTION OF EXHAUST VELOCITY
Conflict

5. COMMERCIAL AND MILITARY ENGINES (APPROX. SAME THRUST, APPROX
COMMERCIAL AND MILITARY ENGINES (APPROX. SAME THRUST, APPROX. CORRECT RELATIVE SIZES)
GE CFM56 for Boeing 737 T~30,000 lbf, a ~ 5
Demand higher efficiency
Fly at lower speed (subsonic, M∞ ~ 0.85)
Engine has large inlet area
Engine has lower specific thrust
Ue/Uo → 1 and hprop ↑
Demand high T/W
Fly at high speed
Engine has small inlet area (low drag, low radar cross-section)
Engine has high specific thrust
Ue/Uo ↑ and hprop ↓
P&W 119 for F- 22, T~35,000 lbf, a ~ 0.3

6. EXAMPLE: SPECIFIC IMPULSE
PW4000 Turbofan
SSME
Space Shuttle Main Engine
T ~ 2,100,000 N (vacuum)
LH2 flow rate ~ 70 kg/s
LOX flow rate ~ 425 kg/s
Isp ~ 430 seconds
Airbus A , A , Boeing , /300, MD-11
T ~ 250,000 N
TSFC ~ 17 g/kN s ~ 1.7x10-5 kg/Ns
Fuel mass flow ~ 4.25 kg/s
Isp ~ 6,000 seconds