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MEC463 - Advanced Propulsion > A.A.B Engines - SCRAMJET > Flashcards

A.A.B Engines - SCRAMJET Flashcards

1
Q

Similarities and differences of Ramjet and Scramjet

A

Similarities:
- Thermodynamically similar

Difference:

  • Speed of combustion
  • Scramjet reduces static temperature to rise more heat addition
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2
Q

List components of scramjet

A
  1. Forebody
  2. Internal inlet
  3. Isolator
  4. Combustor
  5. Internal nozzle
  6. Aft body
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3
Q

List 3 applications of Scramjet

A
  1. Long-range high-speed weapons
  2. Long-range high-speed recon/strike
  3. Prompt TSTO (Two stage to orbit)
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4
Q

Give 3 limits of hypersonic travel

A
  1. Vehicle structure limit: dynamic pressure too high
  2. Combustor blowout limit
  3. Thermal management limit: aerodynamic overheating
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5
Q

What is the disadvantage of having to use entire aircraft body to compress the air for scramjet aircraft

A
  1. Boundary layer at intake 40% of flow
  2. Interdependent flow fields
  3. High speeds - high heating - cooling requirements
  4. Highly sensitive system
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6
Q

List 5 regimes for scramjet

A
  1. Perfect Gas
  2. Two temperature ideal gas
  3. Dissociated gas
  4. Ionised gas
  5. Radiation-dominated
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7
Q

Describe Perfect gas regime

A

The lower border of this region is around

Mach 5, where ramjets become inefficient, and the upper border around Mach 10-12.

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8
Q

Describe Two temperature ideal gas regime

A

This is a subset of the perfect gas regime, where the gas can be considered chemically perfect, but the rotational and vibrational temperatures of the gas must be considered separately, leading to two temperature models.

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9
Q

Describe Dissociated gas regime

A

In this regime, diatomic or polyatomic gases (the gases found in most atmospheres) begin to dissociate as they come into contact with the bow shock generated by the body.

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10
Q

Describe ionised gas regime

A

The ionised electron population of the stagnated flow becomes significant, and the electrons must be modelled separately. Often the electron temperature is handled separately from the temperature of the remaining gas components. This region occurs for freestream flow velocities around 10–12 km/s.

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11
Q

Describe radiation-dominated regime

A

Above around 12 km/s, the heat transfer to a vehicle changes from being conductively dominated to radiatively dominated.

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12
Q

What is the challenge with supersonic combustion

A

Challenge is the short mixing time

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13
Q

What are the 3 fuel choices for scramjets

A
  1. Hyrdrogen:
    - rapid burning, high mass specific energy content, short ignition delay time
    - low density, larger vehicle required
    - good diffusivity
  2. Hydrocarbon
    - storage/density good
    - long ignition time delay
  3. High reactivity fuels
    - invention of new fuels
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14
Q

List 5 ways of adding/mixing fuel

A
  1. Fuel air mixing (simply add two streams together)
  2. Fuel injection
  3. Strut injection
  4. Ramp injectors
  5. Cavity injectors
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15
Q

Describe fuel air mixing design

A

If velocities of two streams are different, shear layer is generated causing lateral transportation of momentum from fast to slow streams

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16
Q

Describe fuel injection

A
  • Jet fuel acts as a cylinder in the flow field
  • normal shock set up upstream causes separation and subsonic wake
  • this wake adds to flame holding
  • reduced efficiencies due to pressure losses however
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17
Q

Describe strut injection

A
  • fuel feed arm
  • precompression system
  • used in arrays
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18
Q

Describe ramp injectors

A
  • vortex enhances mixing
  • injection location important
  • stagnation region near loading edge improves ignition
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19
Q

Describe cavity injectors

A
  • generate acoustic oscillations
  • characterised by L/D

if L/D < 1 then transverse waves

if L/D > 1 then shear layer attaches to bottom wall, generates longitudinal waves, helps flame holding

20
Q

What does SABRE stand for

A

Synergetic Air-Breathing Rocket Engine

21
Q

Difference between SABRE design and Scramjet

A
  • SABRE combustion much easier to control (uses cryogenic fuel)
  • Oxygen is also heavy and using it as fuel is desirable
22
Q

How does SABRE cool incoming oxygen

A

Uses the cryogenic fuel on-board to cool incoming air
Then burns air and fuel in a rocket engine combustion chamber
designed to be reusable

23
Q

What were the problems with HOTOL design

A

HOTOL - Horizontal take-off and landing

  • engine at back and heavy hyrdaulics required for balance
  • frost in heat exchanger
  • adjustable intake required
24
Q

List advantages of SYLON design over HOTOL

A
  • cooling inlet air means higher density

- lower temp means lighter alloys can be applied

25
Q

When do SABRE engines transition from air breathing to rocket

A

At Mach 5.5

then use rockets to get up to orbital velocity M = 25

26
Q

Can you draw SABRE engine system

A

PIC

27
Q

What is special about expansion deflection nozzle and why

A
  • advanced rocket nozzle that achieves altitude compensation
  • has a component at the beginning part of the bell that pushes airflow around it and along nozzle walls
  • creates closed wake in a vaccuum
28
Q

List 5 applications of pulse jets

A
  1. Target drone aircraft
  2. Flying model aircraft
  3. Fog generators
  4. Industrial drying
  5. Home heating equipment
29
Q

1 big disadvantage of pulse jets

A

Very noisy

30
Q

What does PDE stand for

A

Pulse Detonation Engine

31
Q

Difference between pulsejet and PDE

A

PDE - detonation

Pulsejet - deflagration

32
Q

Advantage of PDE over pulsejet

A
  1. Pressure increases significantly during combustion

2. PDE initiates supersonic combustion

33
Q

Describe detonation

A
  • supersonic wave which propagates through shock compression of the fuel/air mixture
  • shock wave heats gas
  • ignites chemical reaction / large energy release
  • energy pushes shock wave into unreacted gas, which is self-sustaining
34
Q

Describe deflagration

A
  • subsonic wave propagates by heat conduction

- mass diffusion from the hot burnt products of the chemical reaction and the cold gas mixture ahead

35
Q

Describe difference between normal shock and detonation

A

Normal shock:
- downstream velocity always subsonic

Detonation:
- downstream velocity always M = 1

36
Q

In Rankine-Hugoniot curve describe 5 regions

A 
I - strong detonation M2<1
II - weak detonation M2 > 1
III - weak deflagration M2 < 1
IV - strong deflagration M2 > 1
V - Impossible detonation
37
Q

On Rankine-Hugoniot curve what are points D and E known as

A

D - upper CJ point

E - lower CJ point

38
Q

In most cases how does detonation occur

A

DDT
Deflagration Detonation Transition

  • ignition energy has to be very high to trigger detonation directly
  • detonation speed is function of pressure difference across shock wave and initial density
39
Q

When are PDE’s better than Ramjets

A

For Mn = 1.35

40
Q

How much faster is detonation than deflagration

A

1000x faster

41
Q

Describe process of detonation

A
  1. Combustible material in pipe
  2. Ignite one end, get laminar flame
  3. flame front wrinkles increases area and burning rate
  4. wrinkling generates weak pressure pulses and turbulence ahead of the flame which preheat gas
  5. as flame speeds up, pressure pulses increase. They coalesce - more preheat until auto-ignition temp reached local explosion
  6. Rapid expansion causes shock wave
  7. If turbulence induced, can shorten this process
42
Q

In theory which is more efficient detonation and deflagration

A

Detonation is in theory more efficient

43
Q

PDE is described by which cycle

A

Humphrey cycle

44
Q

Advantages of PDE

A
  1. No moving parts
  2. High thermodynamic efficiency
  3. Operating in high Mach no.s
  4. Simplicity/flexibility of geometrical config.
  5. Easy integration to vehicle
  6. Low cost
45
Q

Challenges of PDE

A
  1. Detonation initiation
  2. Air inlet design
  3. Coupling with external flow
  4. Design optimisation
  5. Very noisy
46
Q

Describe idea of RDE - Rotating Detonation Engine

None in production

A

Engine where one or more detonations continuously travel around an annular channel
(Would be superior to PDE)

47
Q

Advantage of a RDE

A
  1. High intensity of reaction
  2. Self-pressurisation
  3. Rapid heat release

MEC463 - Advanced Propulsion (10 decks)

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