MAE 5360: Hypersonic Airbreathing Engines

Ramjet Overview

Mechanical and Aerospace Engineering Department

Florida Institute of Technology

D. R. Kirk

Ramjet Operation

• 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

Fuelinjectors

Key Results: Ramjet• 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, b

– 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

hm

TUT

mTSFC

gm

TI

h

Tcmm

TTcmhm

MMT

TM

am

T

T

T

M

M

RT

RT

M

M

U

U

U

UM

am

T

foverall

f

fsp

opof

ttpof

obot

to

oo

o

e

o

e

o

e

o

e

o

e

o

eo

oo

0

04

34

0

4

3

4 111

1

Thrust and TSFC Performance Summary

• 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

• 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 HeatsExample:• 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 < 1 and T < 1250 K leads to closed form solutions

• For higher order fits or > 1 and/or T > 1250 K, iterative closure schemes are required for solution of flame temperature

• 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

• 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

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 1.5 2 2.5 3 3.5 4 4.5 5

Upstream Mach Number, M1

M2,

P02

/P01

Downstream Mach Number, M2

Total Pressure Ratio, P02/P01

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

Ramjet Powered Missile

Boeing/MARC CIM-10A BOMARC A Surface-to-Air MissileAerojet General LR59-AG-13 liquid rocket; Two Marquardt RJ43-MA-3 ramjets

HyFly Ramjet Concept

• Hypersonic Flight Demonstration Program • Cruise Flight Mach Number ~ 6• Range 600 nm (1111 km)

http://www.globalsecurity.org/military/systems/munitions/hyfly.htm

HyFly Ramjet Concepthttp://www.designation-systems.net/dusrm/app4/hyfly.html

• 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 ConceptOrbital 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’)

• 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

• 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