1. Subject Name: AIRCRAFT PROPULSION Subject Code: 10AE55
Prepared By : DEEPA M S
Department : AERONAUTICAL
Date : 15/11/2014
Unit 7 – Ramjet Propulsion
2/23/2019

2. CONTENTS Ramjet Propulsion: Operating principle
Sub critical, critical and supercritical operation
Combustion in ramjet engine
Ramjet performance
Sample ramjet design calculations
Introduction to scramjet
Preliminary concepts in supersonic
combustion – Integral ram- rocket
2/23/2019

3. Lect-38
Ram j et engines
Ram j et is the sim plest of all the airbreathing engines.
I t consists of a diffuser, com bustion cham ber and a nozzle.
Ram j ets are m ost efficient when operated at supersonic speeds.
When air is decelerated from a high Mach num ber to a low subsonic Mach num ber, it
results in substantial increase in pressure and tem perature.
Hence ram j ets do not need com pressors and consequently no turbines as well.

4. Ram j et engines Schem at ic of typical ram jet engine Lect-38
Diffuser
Com bustion cham ber
Nozzle
Flam e holders
Supersonic com pression
Subsonic com pression
Schem at ic of typical ram jet engine

5. Ram j et engines a- 2: isentropic
Lect-38
Ram j et engines 4
a- 2: isentropic
com pression in the intake 2- 4: com bustion at
constant pressure
4- 7: I sentropic expansion through the nozzle T 2 7 a s
I deal ram jet cycle on a T- s diagram

6. Lect-38
Ram j et engines
I n an ideal ram jet cycle, there are no irreversibilities considered.
Therefore there are no pressure drops or
efficiencies of the com ponents com prising a ram jet.
I f we assum e com plete expansion in the nozzle, Pa= P7= Pe
We shall use the isentropic relations to determ ine the variation of pressure and tem perature in the intake and nozzle.

7. Ram j et engines T0 a  T02 T0 e  T06  T04 P07  P0 a Te u  T04 u
Lect-38
Ram j et engines
I ntake :
T0 a
 1  γ  1 M2
 T02
T0 e  T06
 1  γ  1 M
 T04 2
Also, e Ta 2 Ta
Ta T6 2 Ta 
γ / ( γ 1)
γ / ( γ 1)
Sim ilarly, P0 a  1  
γ  1 
and, P07  1   
γ  1 M M 2
2    
P 2  P 2 e a e
P07  P0 a
From the above equations,
Pe Pa
and therefore, M  M, or u  ae u 
Te u 
T04 u
e e a Ta
T02

8. h02 cp T02 Ram j et engines Com bustor :
Lect-38
Ram j et engines
Com bustor :
Ener gy balance across the com bustor gives, m h02  m f Q  (m  m f )h04
h02
 fQ  (1  f )h04
cp T02
 fQ  (1  f )cp T04
( cp T04 / cp T0 a )  1
therefore, f 
(Q / cp T0 a )  ( cp T04 / cp T0 a )

9. T04 T04 1  γ  1 M2 T0 a m Ram j et engines Thrust developed is :
Lect-38
Ram j et engines
Thrust developed is :
F  m (1  f )u  u or F / m
 (1  f )u  u e e
T04 T04 1
We know that, u  u
 u e
T0 a Ta
1  γ  1 M2 2
The thrust equation can be re  written as 
1 / 2  
 1 F 
γ  1 M2  
 M γRT (1  f )
T / T 1  a 04 a  m 2    
and TSFC  m f  f
F F / m

10. Ram j et engines  P02 P0a  P04 P02  P07 P04 P0a
Lect-38
Ram j et engines
A real or actual ram jet cycle will have irreversibilities like pressure drop and efficiencies of intake, com bustor and nozzle.
 P02
Pr essure recovery of the intake, π , d
P0a
 P04
Pr essure loss in the com bustor, π b
P02
 P07
Stagnation pressure ratio in the nozzle, π n
P04
Overall pressure ratio, P07
 π π π
d b n
P0a

11. Ram j et engines P02 P04 a- 2: Com pression in the intake
Lect-38
Ram j et engines
P02
P04 4
a- 2: Com pression in the intake
2- 4: Com bustion at constant pressure
4- 7: Expansion through the nozzle T 2 P7 7 Pa a s
Real ram jet cycle on a T- s diagram

12. c T / c T   1  2   γ  1  M2  γ  1  Pe 
Lect-38
Ram j et engines
The exhaust Mach num ber can be related to the inlet Mach num ber as
( γ 1) / γ  
 2   γ  1  P  M2
  1  M 2  
π π π a 
 1    e
 γ  1 
d b n 2 
Pe 
I n the above exp ression, if
πd  πb  πn
 1 and
Pe  Pa , then Me  M  I deal ram j et cycle. We can det er m in e, f , by energy balance,
c T / c T   1
f 
( η Q / c T )  c
pg 04 pa 02
T / c T 
b pa 02
pg 04
pa 02

13. 1  M  T07  T04 m m Ram j et engines  1  M  , T T Te γRT04
Lect-38
Ram j et engines
T07  T04
 1  M 
γ1 2 , e T T 2 7 e Te
γRT04
ue  Me γRTe  Me γRT
 M
1  M  04 e T
γ1 2 04 2 e
The specific thrust and SFC can be calculated
in a m anner sim ilar to that adopted for the ideal ram j et.
F A
 (1  f )u  u  f e
(Pe  Pa ) e m m
and TSFC 
F / m

14. Lect-38
Ram j et engines F T
Specific thrust TSFC 04 m f
T04
T / m
Mach num ber
Variation of specific thrust and TSFC with Mach num ber

15. Pulsej et engines Pulsej et is a very sim ple engine like a ram jet.
Lect-38
Pulsej et engines
Pulsej et is a very sim ple engine like a ram jet.
Com prises of an intake, com bustion
cham ber and an acoustically resonating exhaust pipe.
Com bustion occurs in pulses resulting in a pulsating thrust.
Tw o types of pulsejet engines: valved and valveless engines.
Pulse Detonation Engines ( PDE) is being evolved conceptually.

16. Pulsej et engines Schem at ic of typical pulsejet engine a 2 3 7
Lect-38
Pulsej et engines a 2 3 7
Tail pipe
Com bustion cham ber
Fuel supply
Valves
Schem at ic of typical pulsejet engine

17. M2  M2   T01  03  Pulsej et engines I ntake :  γ  1 
Lect-38
Pulsej et engines
I ntake :
γ / ( γ 1)
 γ  1 
P  P  P 1 
M2  
01 0 a a  2 
γ  1 
Sim ilarly, T
 T  T 1 
M2  
01 0 a a  2
For an ideal pulsejet, P02  P01 and T02
 T01
Com bustor :
Com bustion takes place at constant volum e ( ideal cycle).
 T 
Therefore, P03  P02
 03 
 T02 

18. cp T03  cp T02  Pulsej et engines m  m c T
Lect-38
Pulsej et engines
Energy balance across the com bustion cham ber :
m  m c T
 m c T  m Q
f p 03 p 02 f
cp T03  cp T02
Sim plifyin g, f 
Q  cp T03
Tailpipe :
( γ 1) / γ
Assu m in g, P  P , T03 
  P  03   
7 a T7
 P7 
The exhaust velocity is calculated as : 
( γ 1) / γ 
  P
u   
2c T 1  a    
e p 03 
 P03 
The thrust and TSFC can therefore be calculated.

19. Pulsej et engines a- 2: Stagnation pressure loss 2- 3: Non 2- 3
Lect-38
Pulsej et engines 3
a- 2: Stagnation pressure loss
2- 3: Non 2- 3
3- 7: Non- isentropic process T 2 7 a s
Real pulsejet cycle on a T- s diagram

20. Pulsej et engines  P02 P0a  P04 P02  P07 P04 P0a
Lect-38
Pulsej et engines
A real or actual pulsejet cycle will have irreversibilities like pressure drop and efficiencies of intake, com bustor and
nozzle.
 P02
Pr essure recovery of the intake, π , d
P0a
 P04
Pr essure loss in the com bustor, π b
P02
 P07
Stagnation pressure ratio in the tailpipe, π n
P04
Overall pressure ratio, P07
 π π π
d b n
P0a

21. Lect-38
Pulsej et engines
Besides this com bustion process m ay have an efficiency associated with it .
c T  c T
f  p 03 p 02
ηbQ  cp T03
The thrust and fuel consum ption will be affected as a result of the irreversibilities.

22. Lect 37 Ram jet The ram jet operates on the sam e cycle as a turbojet.
As per the nam e all the com pression in the cycle is m ade to occur in the intake diffuser, producing a significant r ise in static
pressure.
Fuel is burnt in this high pressure air and the m ixture is then expanded to am bient static pressure through a nozzle system .

23. Lect 37
R a m j e t
Supersonic Flow
Subsonic Flow

24. Lect 37
The ram jet engine produces power by increasing the m om entum of the working fluid by induction of energy by com bustion of fuel, so that the
m om entum of the exhaust jet exceeds that of the incom ing air, on a continuous basis.
I n contrast to the other air- breathing engines, the working cycle is accom plished without additional com ponents of com pression and expansion, and also without any need for enclosed com bustion.
Ram jet engine is m echanically the least
com plicated air- breathing jet engine for thrust production for flying vehicles.

25. Lect 37
This high pressure gas is expanded through a nozzle, converting a low subsonic flow in the com bustion cham ber - - to a supersonic jet.
The m ixture of air and burnt fuel is exhausted through a con v er g en t - d iv er g en t ( C- D) n ozzle.
Exit pressure ( Pe) is sam e as or nearly sam e as the am bient pressure. Thus, the chocking pressure ( Pc) at the throat is higher than the am bient pressure
( Pa) .
However at low supersonic flights the exhaust m ay be sonic through a con v er g en t n ozzle.
At f lig h t Mach 5 an d ab ov e the unit becom es a
Su p er son ic Com b u st ion Ram j et ( SCRAMJET) in which the com bustion is done in supersonic flow.

26. Lect 37
Specific Thrust

27. Lect 37

28. Lect 37
Flam e Holders

29. Lect 37
Germ an V- 2 Bom ber 10

30. Lect 37
Ram jet Pow ered Supersonic Aircraft

31. Lect 37
Pulsejet
Sim ilar to ram jet in sim plicity, but operating on different principle, is pulsejet, used in Germ an V- 1.
Compressed Air

32. Lect 37

33. Lect 37 Air is drawn into the system through a set of
v alv es, and fuel is sprayed into the incom ing air.
Com bustion occurs and pressure is built up in the closed com bustion region, closing the inlet valves and then accelerating the colum n of gas in the
tailpipe outward.
The escape of gases in exhaust perm it s the
com bustion gases to expand, and the inertia of the out m oving colum n of gases leaving the system lowers the pressure in the com bustion cham ber, allowing a fresh charge to enter through the inlet
valve and repeat the cycle.

34. Lect 37 To start this process it is necessary to in it iat e air
f low through the duct, often with the help of a high pressure air source.
Once started and injected with fuel, the device is self- sustaining, requiring no further spark.
The fuel flow is steady, and ignition is accom plished by the residual flam e.
The frequency of the pulses determ ines the thrust and depends upon the volum e of the com bustion
region and the length of the tail pipe.
The pulsejet, usable at subsonic speeds, in it s
present state of developm ent is inferior to turbojet in over- all propulsive efficiency.

35. Lect 37
Pulsejets - operation
A spark plug init iates the com bustion process inside the com bustion cham ber when the inlet valves are closed
Com bustion occurs in an enclosed
cham ber and is approxim ately a constant volum e process
Com bustion is nearly an explosion in that enclosed volum e and raises the pressure and tem perature to high values

36. Lect 37
Pulsejets - operation
The high pressure and tem perature forces the gases to flow out of the tail pipe and
nozzle
Evacuation of the com bustion cham ber results in pressure drop – that opens the spring loaded inlet valve and air com es in from the intake
The spring loaded inlet valves are
norm ally closed and open only when the pressure difference across it is attained.

37. Lect 37
Pulsejet - operation

38. Lect 37
Germ an Heinkel Aircraft

39. Lect 37
Modern pulsejet powered aircraft 20

40. Lect 40
Ram jet Engine Therm odynam ic cycle

41. Perform ance Param et ers
Lect 40
Perform ance Param et ers
Th r u st : Ram jet engine thrust is defined as the net change in total m om entum as the working fluid passes through the engine.
The general expression for thrust,
F = ṁ( C4 - C1 ) +
Ae.( Pe- Pa)
where Pe and Pa are the engine exit and am bient static pressures respectively

42.   Ve  pe 1   pa  F  .Va .A1  m.  1  Ae pa  Va 
Lect 40
The general thrust equation m ay be m odified to
include fuel m ass flow and then m ay be rearranged by substituting the m ass flow term from the
continuity condition,
  Ve
 pe 1 
 pa 
and
F  .Va .A1  m. 
1  Ae pa  Va 
m  1 f
Where, , f= fuel/ air ratio,
A1= Area of free stream air entering the engine

43. aVa  pa  m F  V    A pa  p  1  V . m 1  . .    Va A1
Lect 40
Specific thrust m ay be written as : F  V   
A pa  p  1
 V . m e 1
 . e . e  
aVa  pa   a m Va A1 
For a reasonable positive value of specific thrust to be achieved,
i) Either Ve> Va i.e. substantial acceleration
through the engine needs to be accom plished,
ii) or pe> pa
i.e a substantial pressure ( static)
residual ( at exit face) inside the engine
are required to be achieved.

44. m f f sfc   F F / m a Sp ecif ic f u el con su m p t ion :
Lect 40
Sp ecif ic f u el con su m p t ion :
The efficiency of an engine is expressed by it s
specific fuel consum ption, which is defined generally under a specified operating condition, as :
m f
sfc   f
F F / m a

45. Q f   F.Va f .m a .Q f Ef f icien cy
where ,
Lect 40
Ef f icien cy
The thermal efficiency of an engine represents the fraction of heat released in the com bustion process that is converted to work ( thrust work), and is a
useful param eter for com paring various engine designs under standard operating conditions.
F.Va
  th
f .m a .Q f
Q f
where ,
= heating value of fuel, k J/ kg

46. Lect 40
Design of a Ram jet
Design of a ram jet engine and perform ance prediction involves estim ation of pressures,
tem peratures, velocities and flow areas at the crit ical stations through the engine.
Even though various analytical CFD techniques, including those incorporating reactive flow, have now com e into use, it is st ill practical to start with an one- dim ensional ( constant flow properties
across any passage area at any station) fluid flow theories.
CFD techniques require a first cut geom etry.
Deviations from the one- dim ensional flow m ay be corrected for with em pirical correlations.

47. V 2 V 2 h   h  2 2 I n all the m ethods perfect gas law
Lect 40
I n all the m ethods perfect gas law
p = ρRT , is valid
as the equation of state for both fresh air and the com busted gas.
The energy equation m ay be expressed as :
V 2 V 2
h 
a b
 h 
+ Q a b
2 2
between any two points a and b. And Q is the heat energy added I n between the two points.

48. Lect 40
For a perfect gas undergoing an adiabatic process,
this m ay, thus, be written as :
V 2 V 2
c T 
a b
 c T   c T
p a
p b p 0
2 2
where T0 is the t ot al t em p er at u r e of the gas at that station.

49.  V c dT  2 So that at any station ,
Lect 40
So that at any station , 2 V T0
c dT   p 2 T
The exactness of the estim ation of velocity depends on i) the m ethod of estim ation of cp and on
( ii) the m ethod of estim ation or m easurem ent of the average tem perature at that station.

50. Lect 40
( 1 ) V a r y i n g sp e ci f i c h e a t m e t h o d :
I n the m ost exact m ethod of com puting energy
content of a gas inside a ram jet engine, the energy equation is used by considering specific heat ( at constant pressure) as :
cp  k0  k1T  k2T  k3T
and enthalpy change, h  cpdT
2 3
which are then used with averaged tem perature, T obtained either by analytical or em pirical or
experim ental m ethod.

51. Lect 40
( 2 ) A v e r a g e sp e ci f i c m e t h o d :
As a f ir st ap p r ox im at ion, the specific heat is assum ed to be an average value, which is constant across a
process or a part of a process : 2 V T0
c dT  c (T  T )  
p p 0 2 T
Where, cp = average specific heat at
a station

52. be such that specific heat ratio = 1.4 in air before
Lect 40
( 3 ) A r b i t r a r y a n d / o r co n st a n t sp e ci f i c h e a t m e t h o d :
I n these m ethods the specific heat values are considered to be constant, first arbitrarily held to
be such that specific heat ratio = 1.4 in air before
the com bustion and, = 1.3 after the com bustion process.
I n a further sim plificat ion for quick first cut
estim ation = 1.4, held constant across the entire
engine, m ay be used for the engine perform ance prediction.

53. Lect 40 Com p ar at iv e d iscu ssion f or d esig n of an en g in e
Com parison of these m ethods has shown that in a subsonic flow arbitrary specific heat m ethod is useful for engineering approxim ations.
But at supersonic flow conditions only the first two m ethods should be used for results within acceptable lim it s.
Thus in a ram jet ( or scram jet), where m ajor
portion of the flow is supersonic, last two m ethods can provide only approxim ate estim ates.
More accurate estim ates shall require use of the first m ethod in a scram jet.

54. Lect 40
Under ‘design point’ flow condition accurate analysis is desirable to arrive at or optim ize the engine internal flow path geom etry.
On the other hand “ off- design point” flow analysis is carried out with engine geom etry already available.
But a num ber of flow param eters m ay be
unknown variables, requiring sim pler approach at the init ial stages of analysis, to be followed later on with m ore r igorous estim ations ( e.g. CFD).

55. Lect 40
Per form ance variation of ram jet engine

56. Lect 40
Per form ance variation of ram jet engine

57. Lect 40
Per form ance variation of ram jet engine

58. Lect 40 Ty pical Ram jet Engine ( with Dual com bustion
– Sub and Supersonic Com bustion)

59. Lect 40
Scram jet Pow ered Vehicle