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Aviation History
1957
1957 - 1894.PDF
982 FLIGHT RAMJET DEVELOPMENT Problems of Design and Testing Discussed in R.Ae.S. Lecture A LECTURE on ramjets, read to the Royal Aeronautical Society inLondon on December 12 by Mr. R. P. Probert, is summarized below; the sections dealing with ground and flight testing are reproduced almostin full, but the earlier part of the paper covering per for trance and design factors and intake operating problems has been abridged. Mr. Probeit isdeputy director of the engine-test facility at the National Gas Turbine Establishment, Pyestock. THE present time, when the prospect of high speed flight oftenseems to recede, not to approach [Air. Probert began] maywell be a good occasion to review the ramjet, an engine whose main use is at high flight speed. We owe the succinct name "ramjet"to the Americans and the original idea to the French. This paper is mostly concerned with British ideas, especially and naturallywith those conceived and prosecuted at the National Gas Turbine Establishment.Avoiding painful delusions about flying stovepipes, the ramjet consists of an intake or diffuser which converts to pressure some ofthe kinetic energy due to relative motion in flight and supplies the air inspired to a high-pressure combustion chamber, which in turndischarges hot gas rearwards through a propelling nozzle. Pressure ratio naturally increases with flight speed, presuming appropriateaerodynamic design; and overall efficiency, or thrust work per unit of fuel, can increase indefinitely with speed towards 100 per cent.Practical limits are set either by excessive ram temperature intro- ducing an alien cooling load in the normally cool part of the engine,or by inability to cool with hot ram air the normally hot parts of the engine at satisfactory flame temperatures. But these difficulties donot arise until a Mach number of 4 is exceeded, and less conven- tional possibilities remain thereafter.There have been many attempts to use the ramjet, often at a higher relative air speed than that of flight. For example, the RoyalAircraft Establishment quite early tried driving a propeller with tip ramjets and this has a modern counterpart in the ramjet tip-drive of helicopters, but with subsonic tip speeds the equivalent brake horsepower can only be obtained at 5 to 10 per cent efficiency,and such schemes depend upon the low weight of a simple engine giving relief from heavy fuel consumption for short endurance.During the 1939-45 war German invention was very active. Some serious, although inconclusive, work was done on subsonic engines,leading to trials in lorry dashes down an autobahn and, more use- fully, on a Dornier. Another idea was to increase the range of agun-fired shell by building a small ramjet around it and using ram pressure to expel a small liquid fuel charge. But the weight andsize increase was obviously considerable and the technique a diffi- cult one for the time, and the requirement was outdated before thedevice matured. We have our own record in Britain. One must admit the reduction of radiation drag by proper shaping of entryand exit ducting as prior art. More recognizable were attempts to give a temporary speed boost to wartime fighters by means of ram-jets. These schemes, too, were overtaken by events and, although ground development was physically successful, nothing directlyuseful emerged in time to meet a fleeting need. Only the guided weapon has as yet given the ramjet a real job todo. Here thrust deficiency at low speeds is immaterial and good economy, light weight and relative simplicity in the supersonicspeed band make it an attractive motor for all but the shorter ranges, in which the solid rocket will serve. Missile engines tend to be specialized. Their first job is toprovide some acceleration or, at least, to hold speed in a steep climb. They work at high ram pressures near the ground and areable to put in kinetic energy low down to help ascent at high altitudes, where ram pressure, mass flow and thrust all fait in rela-tion to weight. Thereafter they must maintain speed at lower thrust ratings against drag forces only. Their job and operatingconditions, therefore, differ from those of prospective aircraft engines where the ram pressures must be limited by aircraft stress factors and high longitudinal accelerations are unlikely, thoughsteady acceleration will be needed from the main engines. Since the war there has been widespread use of flight-test vehicles framedto provide a means of trying out ramjet propulsion techniques under representative conditions, and latterly, of course, some actualweapon engines. Now, individual missile or aircraft engines must be closely matched in size and operation to their special role ormay even be integrated structurally and aerodynamically into the systems they power. The ramjet is, therefore, a subject where awide variety of component techniques and operating phenomena deserve study because of the extreme range of possible requirementsand methods of application. The latent performance of a ramjet at any speed is set partly by ,pressure recovery, or internal aerodynamic efficiency, and chiefly by combustion temperature. Securing the performance dependsupon proper choice of critical flow areas at inlet, inlet throat, and nozzle throat, and so on, to give proper matching of components. It is possible to provide an engine which, simultaneously climb-ing and accelerating, provides roughly constant thrust over a large change of height. This latent acceleration and climb capability isa very handsome start to supersonic flight. If, now, at high speed the nozzle is closed and the temperature reduced (accompanied byclosure of the intake) one can drop down to a quarter of maximum thrust, with a decrease of some 40 per cent in specific fuel con-sumption. Clearly an engine with fully variable intake-capture and throat areas and variable exhaust throat-area is an advanced one,although not necessarily difficult, and mechanically simpler engines may be discerned. A design of fixed intake area with a varyingnozzle area can give good thrust and economy over the speed band and also efficient reduced thrust for cruising. Finally, there is ourpresent-day workhorse, and fixed-geometry engine, which earns our bread and butter.Without a variable nozzle it is impossible to match a fixed intake with a constant reasonably high maximum temperature over aspeed range. If the design is biased towards good thrust at high speed the intake area is large, and spill or blow-off drag must beincurred at low speed, with loss in thrust and economy. If the design is biased towards low speed, the high speed thrust andeconomy are cut off by limited airflow, and the internal pressure must fall below maximum values to maintain choking velocities atthe nozzle throat. In effect, shock and mixing losses are allowed to develop within the intake to reduce the pressure to the valuematching the nozzle flow coefficient. If the temperature is reduced at high speeds this process can go a stage further towards lowerthrust. There are, then, these three main categories of engine rangingfrom the evidently mechanically complicated to the apparently simple, with concurrent deterioration in thrust and performance.Short-range-weapon engines are typically in the third class. In extenuation of the missile engine it must be pleaded that the fuelweight consumed is comparable to engine weight; and weight due to mechanical complication rapidly eats up fuel saving, especiallysince the engine does most work at the lower speeds and altitudes where its design is relatively efficient. This type of engine is,therefore, entirely right for its purpose, but it is not necessarily the easiest to develop. The more complicated versatile enginesmay well turn out to be easier after all. In a fixed-geometry engine [continued Mr. Probert] workingover a range of speed and thrust, the space distribution of mass flow from the intake varies enormously and is, sadly, beyond ourability to account for analytically. The professional aerodynamicist restricts himself to the serious problems and cheerfully leaves thedistribution to be swept up and incinerated within the combustion chamber. Should the combustion system excel itself or, more likely,the control system lapse, thereby overdoing the temperature with reduction in mass flow, the intake reserves the right sometimes to Fig. 1 (below). Final combustion chamber; an early development scheme. Fig. 2 (right). A later stage in ramjet combustion chamber development. PYROTECHNIC IGNITION PILOT INTAKE SECONDARYFUEL JETS MAINFUEL JETS FUEL INJECTION SECONDARY FLAME SPREADERS
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