MAE 5360: Hypersonic Airbreathing Engines
Ramjet Overview
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk
Ramjet Operation
Fuel
injectors
• Ramjet has no moving parts
• Achieves compression of intake air by forward speed of vehicle
• Air entering the intake of a supersonic aircraft is slowed by aerodynamic
diffusion created by the inlet and diffuser to low velocities
• Expansion of hot gases after fuel injection and combustion accelerates
exhaust air to a velocity higher than that at inlet and creates positive thrust
Key Results: Ramjet
U

T
 M o  e  1
m o ao
 Uo 
RTe M e Te

RTo M o To
Ue M e

Uo M o


 Tt 4

 4

T



 Mo
 1  M o t b  1  M o 
 1
m o ao
 Tt 3

 0

m f h  m o c p Tt 4  Tt 3 
m f  m o
I sp 
c pTo
h
T
m f g
TSFC 
m f
T
TU 0
 overall 
m f h
 4   0 
• Begin with non-dimensional thrust
equation, or specific thrust
• Ratio of exit to inlet velocity
expressed as ratio of Mach numbers
and static temperatures. Recall that
for a Ramjet Me=M0
• Ramjet specific thrust depends on
temperature ratio across burner, tb
– Compare with H&P EQ. (5.27)
• Energy balance across burner
• Expression for fuel flow rate for
certain temperature rise of incoming
mass flow and fuel energy, h
• Useful propulsion metrics
– Specific impulse, thrust specific
fuel consumption, and overall
efficiency
Thrust and TSFC Performance Summary
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Ramjet performance parameters vs. flight Mach number
Specific thrust has peak value for set Tmax and Ta
Specific thrust increases as maximum allowable combustor exit temperature increases
Specific fuel consumption decreases with increasing flight Mach number
Thrust per unit Mass and Efficiency Summary
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Ramjet performance parameters vs. flight Mach number
Specific thrust has peak value for set Tmax and Ta. Peak is around Mach 2.5
Propulsive, thermal and overall efficiencies increase continually with increasing Flight Mach number
Temperature Dependence of Specific Heats
Example:
• Enthalpy often approximated as
h(T)=CpT
• In combustion chemistry, enthalpy
must take into account variable
specific heats, h(T)=Cp(T)T
• If Cp(T) can be fit with quadratic,
solution for flame temperature for
certain classes of problems f < 1
and T < 1250 K leads to closed
form solutions
• For higher order fits or f > 1
and/or T > 1250 K, iterative
closure schemes are required for
solution of flame temperature
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Also will discuss a definition of
enthalpy that accounts for chemical
bonds
1st law concepts defining heat of
reaction, heating values, etc.
Normal Shock Total Pressure Loss
1
0.9
0.8
M2, P02/P01
0.7
0.6
0.5
0.4
0.3
0.2
Downstream Mach Number, M2
Total Pressure Ratio, P02/P01
0.1
0
1
1.5
2
2.5
3
3.5
Upstream Mach Number, M1
4
4.5
5
Example: Supersonic
Propulsion System
• Engine thrust increases
with higher incoming
total pressure which
enables higher pressure
increase across
compressor
• Modern compressors
desire entrance Mach
numbers of around 0.5 to
0.8, so flow must be
decelerated from
supersonic flight speed
• Process is accomplished
much more efficiently
(less total pressure loss)
by using series of
multiple oblique shocks,
rather than a single
normal shock wave
• As M1 ↑ p02/p01 ↓ very rapidly
• Total pressure is indicator of how much useful work can be done by a flow
– Higher p0 → more useful work extracted from flow
• Loss of total pressure are measure of efficiency of flow process
Ramjet Powered Missile
Boeing/MARC CIM-10A BOMARC A Surface-to-Air Missile
Aerojet General LR59-AG-13 liquid rocket; Two Marquardt RJ43-MA-3 ramjets
HyFly Ramjet Concept
http://www.globalsecurity.org/military/systems/munitions/hyfly.htm
• Hypersonic Flight Demonstration Program
• Cruise Flight Mach Number ~ 6
• Range 600 nm (1111 km)
HyFly Ramjet Concept
http://www.designation-systems.net/dusrm/app4/hyfly.html
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HyFly program was initiated in 2002 by DARPA (Defense Advanced Research Projects Agency) and
U.S. Navy's ONR (Office of Naval Research) to develop and test a demonstrator for a hypersonic Mach
6+ ramjet-powered cruise missile
Prime contractor for HyFly missile is Boeing, Aerojet builds sustainer engine
Air-launched from F-15E and accelerated to ramjet ignition speed by solid-propellant rocket booster
Engine runs on conventional liquid hydrocarbon fuel (JP-10)
– Much easier to handle than cryogenic fuels (LH2) used on other hypersonic scramjet vehicles
Sustainer engine of HyFly is a dual-combustion ramjet (DCR) (very complex)
– Two different air inlet systems
• Operate as a "conventional" ramjet with subsonic combustion
• Operate at hypersonic speeds as a scramjet
First scramjet engine (hybrid or otherwise) to demonstrate operability with LH2 fuel
Ramjet Missile Concept
Orbital Sciences GQM-163 Coyote: Ducted rocket/ramjet engine, Flight speed up to Mach 2.8 at seal-level
http://www.orbital.com/
Hercules MK 70 rocket booster
Russian P-700 Granit Long-range Anti-ship Missile
(SS-N-19 ‘Shipwreck’)
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Launched by two solid-fuel boosters
before sustained flight with ramjet
Maximum speed believed ~ Mach 2.25
Range is estimated at 550 to 625 km
Weight: 7,000 kg, Length: 10 m,
Diameter: 0.85 m
Altitude up to 65,000 ft
J58 SR-71 Engine: Ramjet/Turbojet Hybrid Engine
http://aerostories.free.fr/technique/J58/J58_01/page8.html
Ramjet/Turbojet Hybrid Engine Operating Modes
Ramjet vs. Scramjet
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Large temp rise associated with deceleration from high speed to M~0.3 for combustion
Solution for increased flight speed: decelerate to ‘lower’ supersonic speeds in combustor
Combustion very difficult (flame support) in a high speed flow
Vehicle cooling requirements become very challenging
X-51 Scramjet
Interstellar Ramjet: ‘Hydrogen Breathing Engine’
• In this concept, interstellar hydrogen is scooped to provide propellant mass
– Hydrogen is ionized and then collected by an electromagnetic field
• Onset of ramjet operation is at a velocity of about 4% speed of light
• Typically, interstellar ramjets are very large systems
• A ramjet sized for a 45-year manned mission to Alpha Centauri would have a ram
intake 650 km in diameter and weigh 3000 metric tons including payload
http://www.daviddarling.info/encyclopedia/I/interstellar_ramjet.html
Summary
• Ramjet develops no static thrust
• Relies on ‘ram’ compression of air
– Requires high speed flight
• Performance depends on increase in stagnation temperature across burner
(combustor)
• Efficiencies (thermal, propulsive, and overall) increase with increasing flight Mach
number
Homework Assignment 1: Ramjet Analysis