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Aerospace Propulsion Ramjet Aerospace Engineering Department

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Aerospace Propulsion


Aerospace Engineering Department

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Main Principle of the Ramjet 

Basic design

•The air inlet/diffuser admits air to the engine, reduces air velocity and develops ram pressure.

•The combustor adds heat and mass to the compressed air by burning a fuel. The nozzle converts some of the thermal energy of the hot combustion products to kinetic energy to produce thrust.

•Compression is given by the vehicle speed (bad performance at low speed, auxiliary bosster needed to reach interesting performances).

•No moving parts, flexibility in geometrical design.

•High thrust per unit frontal area.

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Ideal Ramjet Cycle

• 21554654-SESA2005LN2006-Propulsion.pdf

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• 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


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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)

– RAMJET.docx

• 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






















































4 111


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Specific Thrust

• Figure 5.9 from Hill and Peterson: 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

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• Figure 5.10 from Hill and Peterson: 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

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Ramjet engine integration

• Inlet GeometrySlowing down the incoming air to about Mach 0.4—0.5 is the primary purpose of an inlet system to keep the tip speed of compressor blades below sonic.

• The installedperformance of a jet engine greatly depends upon the air-inlet system. • The type and geometry of the inlet and inlet duct determine the pressure loss and distortion of the air supplied

to the engine, which will affect installed thrust and fuel consumption. • Roughly, 1% reduction in inlet pressure recovery πinlet will reduce thrust by ~ 1.3%.

• Also, the inlet’s external geometry including the cowl and boundary-layer diverter will greatly influence the aircraft drag.

• There are basically four types of inlets.

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Basic Types of Inlet• NACA flush

• Pitot

• Conical

• 2-D ramp

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NACA Flush Inlet

• Used by several early jet aircraft but is rarely seen today because of its poor pressure recovery (large losses).

• However, the NACA inlet tends to reduce aircraft wetted area and weight if the engine is in the fuselage.

• The NACA inlet is regularly used for applications in which pressure recovery is less important, such as the intakes for cooling air or for auxiliary power units.

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Pitot inlet• It is simply a forward-facing hole (also called normal shock inletin supersonic flight).

• It works very well subsonically and fairly well at low supersonic speeds.

• The cowl lip radius has a major influence upon engine performance and aircraft drag.

• A large lip radius tends to minimize distortion and accommodate additional air required for takeoff , especially at high angles of attack and sideslip.

• However, it will produce shock-separated flow outside the inlet as speed of sound is approached which greatly increases drag. Hence, supersonic jet cowl lip are nearly sharp.

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Spike and 2-D Ramp Inlets

• Better performance than the normal shock inlet at higher supersonic speeds .• Supersonic flow over cone (spike) or wedge (2D-ramp) .• Spike inlets are typically lighter and have slightly better pressure recovery but with higher cowl drag and

more complicated variable geometry mechanisms. • Ramp inlets are used up to Mach 2, while spike inlets are used beyond that.• Pressure recovery through a shock depends on the strength of the shock.• N-S: (M0= 2 → M1= 0.57, p1/p0 = 72%) –(M0= 1.1 → M1= 0.91, p1/p0 = 99.9%)• An oblique shock does not reduce the air speed all the way to subsonic. • Final transition from super to subsonic speed occurs through a normal shock.• Speed reduction and pressure recovery depends on the wedge or cone angle.

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• 1. O-S: (δ= 10º, M0= 2 → σ= 39º, M1= 1.66, p1/p0 = 98.6%).

• 2. N-S: (M1= 1.66 → M2= 0.65, p2/p1 = 87.2%). Then, p2/p0 = 87.2 ×98.6% = 86%

• The greater the number of oblique shocks, the better the pressure recovery.

• Theoretical optimal is the isentropic rampinlet (infinite O-S) with 100% pressure recovery, which works properly only at its design Mach number, so rarely used

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Flight Range of Ramjet Propelled VehiclesConsumption of air-breathing engines (Pratt & Whitney)

The hypersonic funnel (Mc Donnell Douglas)

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

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SOME DETAILS ON BOMARC MISSILE• Flight testing started in 1952

– First launch from Cape Canaveral in September of 1952• Bomarc A became fully operational in 1959

– Numerous deployments from Florida to Maine defended U.S. eastern sea board• Booster on Bomarc A was source of problems

– Fuel was too corrosive to store in missile, so fueling took place immediately before launch (increasing time to launch)

– Fueling process was also quite hazardous, involving three steps (white fuming nitric acid, analine-furfuryl alcohol, and kerosene)

• New model that utilizes a solid fuel booster– Bomarc B became operational in 1961, and featured a safer solid fuel booster

and more powerful sustainers• Boeing built 700 Bomarc missiles between 1957 and 1964, and Bomarc in active

service until 1972• Length 46 ft. 9 in, Wingspan 18 ft. 2 in, Speed Mach 2.8, Range 250 miles,

Ceiling 65,000 ft, Cost: $ 1,154,000 per shot• Propulsion:

– One Aerojet General LR59-AG-13 liquid rocket– Two Marquardt RJ43-MA-3 ramjets videoplayback_29.FLV

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

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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

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RAMJET POWERED MISSILESOrbital Sciences GQM-163 Coyote: Ducted rocket/ramjet engine, Flight speed up to Mach 2.8 at seal-level

Hercules MK 70 rocket booster

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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

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From Ramjet to Scramjet (Supersonic Combustion Ramjet or SCRJ)

• Beyond Mach = 5 (hypersonic domain), ramjet is less and less efficient. Increasing of air stagnation temperature and pressure tends to limit the performance and to increase the thermal and mechanical loads on the combustion chamber walls

• To bypass these issues, the solution is to maintain the flow supersonic from the air inlet to the engine exit and to achieve the combustion in the supersonic flow

• A geometrical throat is therefore no longer needed to accelerate the flow and produce the thrust ; transition from subsonic to supersonic flow can also be achieved, without geometry variation, by heat addition

• Two variants of scramjet :

– pure scramjet

– dual mode ramjet allowing transition from subsonic to

supersonic combustion

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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 challengingvideoplayback_23.FLV

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Boeing X-51 WaveRider Scramjet

• videoplayback_30.FLV

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Scientific Issues of Ramjet• Air intakes : the design is critical (efficiency on the whole flight range, sensitivity

to flow distorsions, participation to instabilities)

• Combustion chamber

– combustion efficiency

– lean and rich stability limits

– wall cooling

– operating instabilities

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Combustion Efficiency and Stability Limits• Combustion efficiency and stability limits are depending on several parameters : fuel,

equivalence ratio, air stagnation pressure and temperature

• Experimental approach through tests : expensive

• The ONERA's approach, the research ramjet :

– a modular design reproducing the main features of an actual ramjet

– the capability to get a detailed characterization of the reactive flow by using the most advanced optical diagnostics (LDV, LIF, PLIF…)

– a tool for validation the 3D turbulent reactive two phase flow codes (MBDA-ONERA)

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Combustion Chamber Wall Cooling• Passive thermal protections

– different materials (silicone based) are available

• Active thermal protection

– a portion of the air flow entering is bypassed to protect the case and is reinjected in the combustion chamber through perforations

– compatibility with booster integration demonstrated ; concept rather interesting for combustion stability

– solution limited to flight Mach number less than 4

• Some successful developments of all composite cases, application to ASMP-A

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Operating Instabilities• Different types of instability :

– overall instability involving the air intake(s) : low/medium frequency (100 to 300 Hz) ; generally cured by improving the air intake(s)

– combustion chamber acoustical instability : from medium to high frequency ; the highest frequencies (tangential modes) are the most dangerous, inducing an accelerated consumption of the thermal protections

• Mainly faced on LFRJ ; some instabilities on DR connected to specific solid propellant combustion phenomena

• Numerical prediction of ramjet instabilities is not yet achieved. Different solutions are however known to reduce or suppress the instability levels, like :

– geometrical devices : baffles, local caps

– modification of the injection devices or controlled distribution of the fuel

• A compromise between performance and instability level is generally sought

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• 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

• Next step: We desire an engine that develops static thrust

– Put in a device to mechanically compress air (compressor)

– Put in a device to power compressor (turbine)

• Solution: Turbojet engine

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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

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Conclusion• Ramjet technology is now mature

– the main applications are military

– turboramjet has a potential for high supersonic or hypersonic transportation

• Scramjet technology still needs heavy developments

– military applications are considered

– a potential for future RLV

• Future of ramjet/scramjet technology implies coupling of different thermodynamic cycles (hybrid powerplant systems)