A.A.B Engines - RAMJET

Question 1

What are most common performance metrics used in Ramjet engines

Answer 1

Specific thrust

Specific fuel consumption

Question 2

What do performance metrics specific thrust and SFC not account for

Answer 2

Don’t account for on-board storage of oxidants

Question 3

Equation for Specific Impulse

Answer 3

Is = F / m g(o)

Question 4

List applications of Ramjets

Answer 4

1. Missiles and UAVs (hybrid system with rockets to start)
2. Compared to pure rocket, a hybrid can triple the range of missile
3. Can make smaller for required range

Question 5

List difference between characteristics of gas turbine and design of ramjet

Answer 5

Gas turbine:

1. Turbine drives compressor at low velocity
2. Performance decreases at high velocity
3. Need to design for high temperature creep

Ramjet:

1. At high velocity ram compression alone is enough
2. Does not work at static or low velocity conditions
3. Ramjets have no turbo machinery (easier design)

Question 6

List 3 components of Ramjet

Answer 6

1. Diffuser
2. Combustion chamber
3. Exhaust nozzle

Question 7

List method of Ramjet propulsion

Answer 7

1. air decelerated and compressed in the diffuser
2. air mixed with fuel and burned in combustion chamber
3. exhaust gas expelled through nozzle by the high pressure in the diffuser

(comb. cham. constantpressure)

Question 8

Advantages and Disadvantages of Ramjet propulsion

Answer 8

Adv:

1. Very simple
2. Low mechanical stresses
3. Will operate at high mach no.

Dis:

1. Inability to self-start
2. Performance highly sensitive to design of diffuser

Question 9

Can you draw:

1. Ramjet
2. Scramjet
3. Thermallly chocked ram accelerator
4. Supersonic combustion ram accelerator

Answer 9

Pics

Question 10

What are stagnation conditions

Answer 10

The properties of the flow when brought to rest in some manner

Question 11

Equation for stagnation Temp

Answer 11

To = T + u^2 / 2Cp

Question 12

Equation for stagnation Pressure

Answer 12

Po / P = (To / T)^γ/(γ-1)

Question 13

Key assumption in ideal ramjet

Answer 13

Full expansion in the nozzle and 100% efficiency. No pressure losses

Question 14

What happens in a real ramjet

Answer 14

1. Pressure losses in diffuser, combustion chamber and nozzle
2. Compression and expansion are not isentropic

Question 15

Why do Ramjets struggle at speed M>5

Answer 15

The combustor inlet temperature is very high and it’s difficult to burn any fuel

Question 16

Give equation for shockwave angle and Mach number

Answer 16

u = sin-1(1/M)
M = 1 / sin(u)

Question 17

When does a shockwave occur

Answer 17

Only occurs at supersonic speeds in a decelerating flow

Question 18

What happens to static pressure/temp, stagnation pressure and entropy across a shockwave

Answer 18

1. Static pressure and temperature increase across shockwave

2. Stagnation pressure and entropy decrease

Question 19

Why can a shockwave form on an aerofoil even if Ma = 0.7

Answer 19

Velocity of top of aerofoil is accelerated and can reach supersonic speeds even though main flow is subsonic

Question 20

What will a shockwave do to boundary layer

Answer 20

Shockwave can thicken the boundary layer and can cause vortex shedding
(Large loss of stagnation pressure)

Question 21

How does heat transfer to a gas affect it at subsonic and supersonic flow

Answer 21

Subsonic flow:
Heat transfer will increase velocity to M=1

Supersonic flow:
Heat transfer will decrease velocity to M=1

Question 22

When is air seen as compressible

Answer 22

Compressible when M>0.3

Question 23

Give characteristics of normal shock wave

Answer 23

1. Adiabatic but non-isentropic
2. Downstream of wave M < 1
3. Density, static pressure and temp increase
4. Velocity and total pressure decrease
5. Total temp stays constant

Question 24

When is normal shock wave attached and when is it detached

Answer 24

Attached:
Pointy object

Detached:
Rounder object

Question 25

How can decelerating supersonic flow be done more efficiently than normal shock waves

Answer 25

Using oblique shocks

Question 26

For oblique shocks, what happens if Θ > Θmax

Answer 26

• No oblique shock possible

- curved detached shock wave is formed instead

Question 27

What happens to Θmax as M1 increases

Oblique shock

Answer 27

Θmax increases as M1 increases

As M1 –> inf, Θmax —> 45.5. (for gamma =1.4)

Question 28

For oblique shocks, what happens if Θ < Θmax

Answer 28

• There are 2 straight oblique solutions:
  1. Weak solution
  For small β
  M2 > 1

2. Strong solution
   For large β
   M2 < 1

Note: nature favours weak solution

Question 29

How should swept wings be designed to reduce wing drag in supersonic aircraft

Answer 29

Wings must be inside the ‘Mach cone’ i.e. angle must be smaller than u

If bigger, than drag increased

Question 30

How is lift produced on supersonic airfoil

Answer 30

When AoA > 0, the lower shockwave is stronger than the upper. This slows down airflow below the wing and static pressure is increased. This produces lift upwards.