MAE 4243 Oral Exam Prep

Ramjet Advantages

High specific thrust at design Mach

Highly efficient at Mach 3-5

No Moving Parts

Ramjet Disadvantages

Has specific operating range

Scramjet Advantages

High specific thrust at hyper sonic, mach 5+

Efficient at hypersonic, more than ramjet

No moving parts

Scramjet Disadvantages

Has specific operating range

Turbojet Advantages

Highest specific thrust

High thrust for given airflow

Large operation envelope

Turbojet Disadvantages

Highest TSFC

Lowest propulsive efficiency

Turbofan Advantages (Low Bi-pass Ratio)

High Specific Thrust

Descent propulsive efficiency

Turbofan Advantages (High Bi-pass Ratio)

Low TSFC

High Propulsive efficiency

Turbofan Disadvantages (Low Bi-pass Ratio)

High TSFC

Turbofan Disadvantages (High Bi-pass Ratio)

Low specific thrust

Needs greater Airflow

Turboprop advantages

Highest propulsive efficiency

Lowest TSFC

Turboprop disadvantages

Lowest specific thrust

Only viable at low subsonic

Complex gearboxes, mechanical losses

Turboshaft Advantages

Highest propulsive efficiency

Lowest TSFC

Turboshaft disadvantages

Lowest specific thrust

Only viable at low subsonic

Complex gearboxes, mechanical losses

What is the purpose of the inlet/diffuser?

to capture incoming air, slow/condition it, and deliver a more uniform usable flow to the compressor.

What is the purpose of the fan?

To add energy to a large mass of air and increase its pressure. In a turbofan, much of this air becomes bypass flow that produces thrust.

What is the purpose of the compressor?

To add shaft work to the flow, increasing pressure and temperature before combustion.

What is the purpose of the combustor?

To add fuel energy to the flow, causing a large increase in temperature and enthalpy.

What is the purpose of the turbine?

To extract energy from the hot gas and provide shaft power to the compressor, fan, or other components.

What is the purpose of the afterburner?

To add additional fuel downstream of the turbine and increase exhaust temperature and thrust.

What is the purpose of a mixer?

To combine the bypass and core flow streams before the exhaust nozzle.

What is the purpose of the exhaust nozzle?

To convert pressure/enthalpy energy into kinetic energy and accelerate the exhaust flow to produce thrust.

Phase 1-2

Inlet

Phase 2-3

Fan/Compressor

Phase 3-4

Combustor/Afterburner

Phase 6-7

Combustor/Afterburner

Phase 4-5

Turbine

Phase 5-6

Mixer

Phase 7-8-9

Exhaust Nozzle

Inlet (diffuser) conservation

As kinetic energy decreases, enthalpy increases.

Fan/compressor conservation

Work goes in; enthalpy, temperature, and pressure increase.

Combustor/afterburner conservation

Heat goes in; enthalpy and temperature increase.

Turbine conservation

Work comes out; enthalpy, temperature, and pressure decrease.

Nozzle conservation

Enthalpy decreases and kinetic energy increases.

Name the highlighted section

Inlet

Name the highlighted section

Fan/Compressor

Name the highlighted section

Combustor

Name the highlighted section

Turbine

Name the highlighted section

After Burner

Name the highlighted section

Exhaust Nozzle

Name the highlighted section

Inlet

Name the highlighted section

Fan/Compressor

Name the highlighted section

Combustor/Afterburner

Name the highlighted section

Turbine

Name the highlighted section

Mixer

Name the highlighted section

Exhaust Nozzle

Rocket Advantage

Can operate without outside air because it carries both fuel and oxidizer; can operate at very high altitude and in space.

Rocket Disadvantage

Must carry its own oxidizer, which increases propellant mass and consumption.

What is thrust?

Force produced by the engine due to the change in momentum of the flow plus any pressure thrust.

What is specific thrust?

Thrust produced per unit air mass flow.

What is TSFC?

Fuel consumed per unit thrust.

What is specific impulse?

Thrust produced relative to propellant weight flow.

What do propulsion efficiencies measure?

How effectively the engine converts energy into useful thrust or propulsive power.

How are TSFC and engine efficiency related?

They are inverse trends. High TSFC means lower engine efficiency; low TSFC means higher engine efficiency.

General specific thrust trend by engine type (highest and lowest)

Turbojet has the highest specific thrust. As bypass ratio increases, specific thrust decreases. Turboprop/turboshaft have the lowest specific thrust.

General TSFC trend by engine type (highest and lowest)

Turbojet has high TSFC. Higher bypass ratio improves fuel efficiency and lowers TSFC. Turboprop/turboshaft have very low TSFC.

What happens when bypass ratio increases?

Fuel efficiency and propulsive efficiency improve, but specific thrust decreases.

What is an overexpanded nozzle?

Exit pressure is less than atmospheric pressure. Your review associates this with low altitude.

What is perfect expansion?

Exit pressure equals atmospheric pressure. This occurs at the design altitude.

What is an underexpanded nozzle?

Exit pressure is greater than atmospheric pressure. Your review associates this with high altitude.

Why does nozzle performance change with altitude?

Atmospheric pressure changes with altitude, changing how well the nozzle exit pressure

What happens through the compressor on a T-s diagram?

Temperature increases.

What happens through the combustor on a T-s diagram?

Temperature increases significantly as energy is added.

What happens through the turbine on a T-s diagram?

Temperature decreases as energy is extracted.

What is the purpose of a T-s diagram for a jet engine?

To visualize property trends through each engine component.

What happens in subsonic flow through decreasing area

Velocity increases.

What is choked flow at the throat?

Mach number equals 1.

What happens through a compressor?

Pressure and temperature increase.

What happens through a turbine?

Pressure and temperature decrease.

What happens across a shock?

Mach number decreases and total pressure decreases.

Why do we use simplifying assumptions?

They allow us to analyze complicated real engine equations more easily.

What is the disadvantage of simplifying assumptions?

The model becomes less representative of the real engine and results become less accurate.

How do you determine whether a propulsion answer is sensible?

Check expected engine trends, conservation laws, realistic magnitudes, signs, and efficiencies.

Compressor sanity check (what to look for)

Pressure, temperature, and enthalpy should increase.

Combustor sanity check (what to look for)

Temperature and enthalpy should increase.

Turbine sanity check (what to look for)

Pressure, temperature, and enthalpy should decrease.

Efficiency sanity check (what to look for)

Efficiency should be between 0 and 1.

Conservation sanity check (what to look for)

Mass and energy must satisfy conservation laws.

Name the highlighted line on the graph

Turbojet

Name the highlighted line on the graph

Low BPR Turbofan

Name the highlighted line on the graph

High BPR Turbofan

Name the highlighted line on the graph

Turboprop

Name the highlighted line on the graph

Turbojet

Name the highlighted line on the graph

Low BPR Turbofan

Name the highlighted line on the graph

High BPR Turbofan

Name the highlighted line on the graph

Turboprop

What is the air-breathing engine thrust equation?

$$F=\dot{m}_eV_e-\dot{m}_0V_0+(P_e-P_0)A_e$$

What is the thrust equation for a perfectly expanded nozzle?

$$F=\dot{m}_eV_e-\dot{m}_0V_0$$

What is the equation for specific thrust?

$$ST=\frac{F}{\dot{m}_0}$$

What is the equation for TSFC?

$$TSFC=\frac{\dot{m}_f}{F}$$

What is the equation for specific impulse?

$$I_{sp}=\frac{F}{\dot{m}_p g_0}$$

What is the general steady-flow First Law equation?

$$q-w=(h_{out}-h_{in})+\frac{V_{out}^2-V_{in}^2}{2}+g(z_{out}-z_{in})$$

What is the simple compressor work equation?

$$w_c=h_{out}-h_{in}=c_p(T_{out}-T_{in})$$

What is the simple turbine work equation?

$$w_t=h_{in}-h_{out}=c_p(T_{in}-T_{out})$$

What is the combustor fuel energy equation?

$$\dot{Q}_{in}=\dot{m}_f h_{PR}$$

What is the basic ideal turbojet power balance?

$$\dot{W}_t=\dot{W}_c$$

What is the basic ideal turbofan power balance?

$$\dot{W}_t=\dot{W}_c+\dot{W}_f$$

What is the compressor pressure ratio equation?

$$\pi_c=\frac{P_{t3}}{P_{t2}}$$

What is the fan pressure ratio equation?

$$\pi_f=\frac{P_{t13}}{P_{t2}}$$

What is the bypass ratio equation?

$$\alpha=\frac{\dot{m}_{bypass}}{\dot{m}_{core}}$$