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Aviation History
1958
1958 - 0231.PDF
21 February 1958 CROSS-SECTIONAL AREA MISSILES^ * Figs. 3a and 3b. Can- berra all - weather fighter project, and application of area rule (see Col. 2 below). CANOPY. With semi-active weapons (e.g., some versions of Falcon andSparrow), in which the larger airborne radar transmits the signal and the weapon receives the echo from the target, it is only neces-sary for the fighter not to turn away so rapidly after firing its weapon that its airborne radar loses effective sight of the target. But with unguided weapons (e.g., guns or rockets), homingweapons (e.g., infra-red weapons like the D.H. Firestreak) or fully active weapons with their own radar transmitter (e.g., VickersRed Dean), the fighter can turn away as sharply as it likes. Unguided weapons follow an approximately straight path for onlya very short distance, as they follow a ballistic path with appreci- able aerodynamic dispersion. To ensure vital hits on the targetit is usually necessary to aim a large number of missiles in a rapid burst at short range, but this situation can be improved con-siderably by using more powerful rockets and warheads. Weapon Guidance will also reduce the number of missilesrequired to destroy a target. The navigation system of these guided weapons is usually such that, after launching, the missileturns rapidly on to the collision course which should meet the target. Unless the target manoeuvres rapidly, this course is almoststraight, but the missile should always pass very close to the target if a number of conditions are satisfied:— (a) The guidance system is properly designed and aims continuouslynear the vital area of the target, e.g., an infra-red system homing on major heat-sources in the central area.(b) The missile is able to respond rapidly so as to reduce the terminal errors. This can always be obtained with a small missile and a properlydesigned control system. (c) The missile has sufficient manoeuvrability to turn on the pathdictated by initial aiming errors or by evasive manoeuvres of the target. The wing size required to satisfy this is a small fraction ofthat required to bring the complete fighter or ground-launched missile up to the same altitude.(d) The missile has sufficient performarce to reach the target. Even if the target is at very high altitude, the size of the rocket motorrequired to meet this case is again a small fraction of that required to bring the complete fighter or ground launched missile up to the samealtitude. (e) All components of the missile should function reliably through-out its flight. Due to the shorter flight under its own power and simpler conditions, this requirement is much more easily met than on ' ground-launched weapons.Nevertheless, (e) is still a difficult condition to satisfy, and there is always a chance that the proximity or contact fuse will not setoff the warhead at sufficiently close range to destroy the target. More than one guided weapon must therefore be carried if we areto have an effective fighter weapon sytstem. An additional safe- guard is to supplement the guided weapons by other weapons,which would cenainly be adequate to finish off a crippled target or to allow attacks on different targets. Until the practical servicereliability of guided weapons has been thoroughly assessed, the safest policy is to carry as many different armaments as possibleor to make them quickly interchangeable on the ground. This policy will always be necessary to some degree to cater for possibletarget countermeasures. Ground-launched Weapons may be considered to supplementthe fighter. Even with fighter-weapon systems designed as dis- cussed above there is obviously some chance of occasionalbombers slipping past the first wave of interception. This chance is clearly reduced by having sufficient fighters in the defensivesystem to ensure that the enemy is always outnumbered. This requirement is easier to satisfy if the fighter has sufficient rangeto reinforce the battle from some less-threatened sector. 241 Where special ground targets are of sufficient importance towarrant expenditure on localized defences, it will pay to ring these with ground-launched weapons. These are now sufficientlyadvanced in development to supersede the manned rocket fighter as originally used in Germany for this purpose. (The value ofrocket boost for high-altitude interceptions is, however, discussed later in this article.) A shortcoming of these weapons is thatthey cannot join the battle very far from their base. They are therefore of restricted use against the bomber which can dropglide-bombs or powered bombs at a considerable distance from the target, or against the enemy's radar-jamming aircraft lurkingat long range. It is on these and similar grounds that, so long as there arebombers or other aircraft to intercept, the case for the modern fighter will remain. The best division of expenditure betweenfighters and guided weapons depends on geography and the nature of the threat, but it is still possible to visualize situationswhere the case for the fighter remains nearly 100 per cent. PERFORMANCE PHILOSOPHY. As soon as we can relyon small airborne missiles to complete the interception under all conditions, then we can accept a complete change in philosophyas to the performance of the fighter. It will no longer be necessary for the fighter to have speed and altitude supremacy over allpossible targets, but only sufficient overall performance and manoeuvrability to get into position to use its airborne radar andthen to launch its missiles. Subsonic fighter experience provides an illustration of thefuture problems of intercepting bombers with similar performance to the fighter. This has required the fitting of sophisticated radarand missiles, and these have been of such dimensions as to require aircraft of Canberra size to make a useful weapon system(Fig. 3a). Fixed forward-firing guns are of value only if the fighter hassufficient speed to allow it to manoeuvre quickly into the pursuit position, closing up from the stern of the target. Similar require-ments apply to many of the present forms of guided weapons, as the main advantage of these should come in terms of lethality onlyafter they have been launched from the correct position. The fighter must therefore continue to have a large speed superiorityuntil even more sophisticated weapons have been brought into service, and these will still require the fighter to have an approachspeed that is a high percentage of that of the bomber. Since bomber speeds have generally increased to near the speed ofsound, with the prospect of more at supersonic speeds, this means that future fighters must have substantial supersonic performance,and that this requirement will continue. This needs very careful definition, and certainly requires economical solution of most ofthe problems associated with the so-called "sound barrier." It means that aircraft like the Canberra are no longer of interest asfighters, even though they continue to be of value in many other roles.The "Sound Barrier" has proved a true barrier to many modern fighters, and this can be explained by reference to the AreaRule. At transonic speeds one merely needs to measure up the cross-sectional areas of the aircraft and plot these as shown forthe Canberra in Fig. 3b. This simple form of Area Rule suggests that the wave drag at near M= 1 will be similar to that of a body ofrevolution having cross-sectional areas equal to the total shown. Even a body of smooth shape will have an appreciable drag riseup to M= 1, but the large bulges representing the unswept wings, engine nacelles, missiles and tail surfaces give the Canberra aprohibitive drag-rise for efficient supersonic flight. Moreover, it is of little value to try to boost such an aircraft with rockets, as notonly is the fuel consumption prohibitive but the buffeting, wing dropping and other trim changes make an aircraft disobeying theArea Rule of little value at transonic speeds. It is theoretically possible to reduce the transonic drag of the Canberra by theaddition of fuselage bulges fore and aft of the wing, similar in principle to that tried on current fighters. This gives a smallreduction of drag in practice, but it is doubtful whether the com- plication or weight penalty of the fairings is justified. The direct method of reducing these problems is to cut downthe cross-sectional areas, e.g., by using wing and tail surfaces of less span and thickness. This has been done in a number ofcurrent aircraft, but unless the full implications of the transonic Area Rule are used (e.g., very large sweepback) the net resultis only to eliminate most of the useful space in the aircraft and to give thrust and drag curves which at best resemble those ofFig. 4. Even though these result in appreciable supersonic per- formance, there will be insufficient margin for practical inter-ception. Supersonic Performance Margins are now essential for afighter, as it is of little use to reduce supersonic drag to the point where it is only slightly less than the thrust from all the engines(plus reheat) that can be crammed into the aircraft. Although this can give Mach numbers as high as 2 or more on paper, thesecan be demonstrated only by flights in unrepresentative condi- tions. Reheat behaves very much like a ramjet at these speeds and
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