Document created: 22 October 03
Air University Review, September-October 1973
Charles S. Epstein
We are living in a speed-oriented culture. Whenever we see a shiny, sleek new automobile, boat, or aircraft, the first question is apt to be, “How fast will it go?” We tend to associate maximum speed with all tactical operations, whether they be dogfighting, intercepting enemy aircraft, or air-to-ground weapon delivery. The news media amplify this tendency by releases such as “The F-4 Phantom II is capable of carrying 16,000 pounds of weapons at 1600 miles per hour.”
Those who realize that the F-4 can indeed carry 16,000 pounds of bombs or go 1600 mph—but not simultaneously—are at least aware of some of the severe operating limitations imposed on today’s high-speed tactical aircraft when they are carrying external stores. However, there is a vast lack of understanding as to why these limitations are imposed and how they affect tactical operations.
Inevitably, when fighter pilots have exhausted their stories of heroic deeds, they turn to serious discussion of mutual frustrations and their drive to enhance their chances of survival. Many of these pilots believe that if they had only been able to go faster—supersonic, preferably—they would have been much safer and could have done a better job at the same time. These comments are even more interesting in light of the fact that today’s pilots are saturated with the number and types of actions they must perform in the extremely short time available in a bombing run. Going faster would decrease even further—drastically in the case of supersonic delivery—the time available to the pilot for target detection and identification, lineup of the sight, and positioning of the aircraft during run-in. How do we explain this apparent paradox?
First of all, to understand this situation fully, we must know something of the nature of the air war in Southeast Asia. Army, Air Force, and Navy pilots attacking North Vietnam were subjected to the most intense and highly sophisticated air defense network ever encountered in warfare. Yet, despite the degree of sophistication, the vast majority of U.S. aircraft lost over North Vietnam were shot down by small arms and antiaircraft guns, most of which were not even controlled by radar. The surface-to-air missile (SAM) was a very ineffective weapon in terms of number of kills per weapon fired. Our pilots learned early how to stay low or maneuver to avoid the SAM’S. These very actions, however, forced us to operate in the environment that makes the ground antiaircraft (AA) guns so effective.
This, then, is where the first and foremost need for supersonic delivery became apparent. Anything that could reduce the effectiveness of the enemy guns would greatly enhance survival of the attacking pilots. Flying low (to avoid SAM’S) at supersonic speeds would impose almost impossible tracking rates on these gunners.
It now becomes important to distinguish between supersonic carriage and supersonic delivery. Attacking aircraft must penetrate to the target as well as attack it. Avoiding SAM’S and AA fire is important in both phases. However, in Southeast Asia, the majority of our aircraft losses were incurred within a very few miles of the target area. This was in part because the enemy knew generally from which direction we would be most likely to attack, and they concentrated guns in certain areas.
From this, it follows that, while it is important from a survivability standpoint to achieve a supersonic capability for carriage of the weapons to the target, it is much more important to achieve a capability, in the target area, to deliver the weapons supersonically. This latter capability can be a limited one in that it is not needed for long periods of time.
It is not my intent in this article to explore in further detail the justification for a supersonic delivery capability. (I fully recognize the arguments that any new general air war would probably be fought differently than in the past or that other weapons could be developed to attack targets more efficiently while standing off far enough to enhance survival.) I believe, however, that the Vietnam war experience and political reality require that a supersonic capability be developed. General William W. Momyer, Commander, Tactical Air Command, once said, at a Tactical Fighter Symposium:
I think the day is past when we can expect to have the strike force penetrating at a slower speed than the protecting fighters. If one believes that air superiority will require deep penetration of enemy defenses, strike forces to destroy the enemy air force on the ground and in the air, and limited time in the target area, I think one would place speed as the most important consideration.
Let us look at the limitations imposed on present-day fighters by the addition of externally carried weapons, usually on multiple ejector racks (MER’s) or triple ejector racks (TER’s).
Every present-day jet fighter has a maximum operating speed (VH) that is achievable only while carrying no external stores. When such stores are carried, the “allowable” speed then becomes much less—sometimes less than half the clean aircraft speed. It is important to understand that the “allowable” speed is usually imposed by the store. This imposed limitation may be a flutter limit for the particular aircraft/store combination, a structural limit on the store itself or on the aircraft because of the store being carried, or it may just be an arbitrary limit because no one knows what loads or temperature limits the store can endure. Sad to say, it is generally the latter. Almost no work has been done to investigate whether stores can survive supersonic speeds or to see if specific aircraft/store combinations can be safely flown above 1.0 Mach (1.0M).
Suppose these store limits were erased. What could a typical fighter aircraft do just on the basis of power available? If one were to overlay the clean aircraft performance flight envelope with those of several different configurations including certain external stores, it would become apparent, from a thrust-minus-drag standpoint, that there are loadings that could be used supersonically—if all the store/aircraft limitations could be ignored. One point stands out: the possible envelope where stores are carried on multiple racks protrudes only slightly into the supersonic regime. Even then, the lowest altitude at which speeds of even 1.10M are attainable is about 20,000 feet. This altitude factor will severely limit the possible choice of weapons to use supersonically, since many cannot be efficiently delivered at such high altitudes. In general, it can be said that “iron bombs,” whether guided or unguided, are the only unpowered weapons that can be delivered at these altitudes, subsonically or supersonically. Weapons such as dispensers, firebombs, and rocket pods are generally used at low altitudes, although some cluster bombs, because they fall away from the aircraft like a bomb before opening, can be adapted to high-altitude release by the use of delay-opening timers.
Another useful point may be made by comparing atypical aircraft’s performance envelope using maximum afterburner power with one using only military (maximum continuous) power. It becomes apparent immediately that to go supersonic, even without stores, military power must be exceeded. With stores attached, the power requirements go up drastically, and so does the fuel flow. Few present-day fighters can operate very long with afterburner power and still have a fuel reserve for return from the target. A practical time for most aircraft of this type would be something less than ten minutes. Fuel then becomes a very limiting factor.
Previous comparisons considered only one-g straight and level flight. When maneuvering flight at other than one-g is considered, the possible aircraft performance envelope shrinks drastically until, at three-g, for example, the envelope is less than one-half that possible at one-g. The altitude penalty required to maintain level flight in a 3-g maneuver is also very large. This means that en route to the target the aircraft is extremely unresponsive during evasive maneuvers and vulnerable if jumped by enemy interceptors until the ordnance load is jettisoned, and by then it may be too late.
Stores carried externally on the aircraft wing, some distance from the aircraft longitudinal or roll axis, also penalize the aircraft in roll performance. The roll rate reduction, coupled with the very restrictive g envelope available, can literally make some aircraft sitting ducks, unable to take any meaningful evasive action.
On analyzing the limitations discussed, we find that multiple carriage and high drag impose the most severe restrictions. Summarizing all their effects, we can say that, to achieve the best, usable supersonic delivery envelope, iron bombs should be carried singly on pylons. This configuration minimizes drag and fuel required, maximizes the possible maneuver envelope, and provides a weapon that can be employed supersonically at both low and high altitudes. Since not all targets can be attacked efficiently with iron bombs, whether they be guided or unguided, any attempt to achieve a supersonic attack capability should be centered at first on those targets which are compatible with bombs.
It should be obvious from the preceding discussion that achieving a true supersonic carriage and delivery capability for today’s operational fighter aircraft will be an extremely difficult problem. Many technological barriers must be crossed, and a drastic change in store carriage methods may be required. Even though supersonic carriage of stores is important, however, supersonic delivery is vastly more important. A short supersonic dash capability, to be used in the target area only, is within reach on today’s aircraft without any significant changes in the state of the art. To attain this capability, the following steps should be taken.
For the particular aircraft selected, typical mission profiles should be computed, using only those targets deemed suitable for attack at supersonic speeds. To do this, the complete mission must be planned, using specific weapons loaded on the aircraft in certain configurations. Among the important factors to be considered are fuel, time, airspeed, attack mode (level, dive, toss), and type and number of weapons to be released to “kill” the particular target. Not every target is of the type that can be attacked efficiently at supersonic speeds. For example, close air support targets or targets of opportunity are difficult to attack supersonically because of the short time available for target detection and identification, as well as attack. Targets such as these should not be prime candidates for developing the interim capability. On the other hand, deep interdiction targets such as dams, power plants, factories, etc., which are likely to be defended fairly heavily by the enemy, are good candidates for supersonic attack.
captive flight envelope determination
Once the specific aircraft, weapons, loading configurations, and attack modes have been identified, the maximum possible operating envelope can be determined. If this proves to be too restrictive, the configuration should not be explored further. If, however, the performance envelope does show promise, an allowable captive flight envelope should be determined. This allowable envelope should be the result of investigation or analysis of the configuration from the standpoint of flutter, structural loads, stability and control, and aerodynamic heating. To determine the allowable envelope for that particular aircraft/store combination, flutter or stability flights utilizing a specially instrumented aircraft may be required. Additionally, ground structural tests of the store or the store/aircraft structure may be required. None of these tests is beyond the capabilities that exist today.
Aerodynamic heating limitations. By far the most severe restriction preventing an expanded supersonic captive envelope comes from the aerodynamic heating effect. Almost all present-day bombs and fuzes have, as their explosive charge, some form of TNT, usually Tritonal or H-6, which melts at 178°F, although most engineers conservatively use 160-165°F. When this explosive melts, it becomes unstable and very dangerous. To determine at what point the TNT in a bomb melts, two things must be known: the total temperature level to which the bomb is being subjected and the length of time it is left at that temperature. At the present time it is virtually impossible to predict heating levels for a specific aircraft configuration using a specific weapon. Because of this difficulty, it is convenient—and conservative—to compute the maximum aerodynamic ram air temperature rise, which would be experienced on the extreme front end of the bomb. This temperature of the weapon’s stagnation point is called adiabatic wall temperature (TAW). The advantage of using TAW is that it is easy to compute for a given flight condition and that it is by definition the absolute highest temperature level to which the weapon can possibly be raised at that flight condition. It is conservative in that the weapon cannot possibly be subjected to that temperature over its entire exterior surface. Using TAW gives only the maximum temperature experienced for continuous operation at a particular flight condition. Obviously, the bomb explosive will not melt instantaneously, so some time must also be specified. Cook-off tests of bombs, in which the live bomb is immersed in an extremely hot jet-fuel fire, have been run by both the Navy and Air Force. Nearly all bombs will last 5 minutes before cook-off, even though the flame temperature is about 1600°F.
If one were to plot, on a Mach-number/altitude graph, lines of constant equivalent airspeed and lines of constant TAW, it would be apparent that the 650 Knots Equivalent Airspeed (KEAS) line is parallel and close to the 175°F TAW line until about l.4M, at which point the 650 KEAS line bends away rapidly, with a corresponding rise in TAW. From this, then, it can be said that, for any aircraft or weapon combination, 650 KEAS up to 1.4 Mach can be maintained for less than 5 minutes without danger of explosive melting. This limit, while conservative, is considerably better than the limits in current use by most fighter aircraft, and, more important, it is a safe, reliable, and quickly determined limit that can be applied to today’s aircraft and weapons without a great deal of analysis and test.
weapon separation envelope determination
Once the allowable captive flight envelopes for the particular aircraft/store configurations have been established, the maximum safe separation envelopes for the stores can be developed.
One of the first steps in determination of the separation envelope is a wind-tunnel test. Assuming that the wind-tunnel tests show that an acceptable safe separation envelope for the stores may be possible, flight tests to confirm this may begin. In the Armament Laboratory at Eglin AFB, we use a technique called photo-grammetry in our flight testing, to keep actual flights to an absolute minimum. This technique essentially gathers quantitative store angular and linear displacement data during store separation and, by computer reduction, processes it into a form that can be compared directly to the wind-tunnel data. Good correlation allows flight testing to be reduced because flight safety hazards are minimized. During the flight testing, weapon ballistic trajectory determinations should also be made so that accurate bombing tables or ballistic computer inputs may be generated. This task is generally done at Eglin by tracking the weapons after release with high-speed ground-based cameras and Contraves cinetheodolites. The data so gathered are processed by a computer program to generate the necessary tables.
current Air Force efforts
The entire process described above to achieve an interim supersonic delivery capability is now being accomplished by the Air Force Armament Laboratory (AFATL). We have obtained from TAC several specific weapon-loading configurations on the F-4 and the F-111 aircraft, which, TAC feels, are most likely to be used in attacking specific targets supersonically. Using Armament Development and Test Center (ADTC) aircraft, we will perform the flight tests necessary to certify these specific configurations for operational use. This entire project, because of our heavy in-house involvement in the wind tunnel, engineering analysis, flight testing, and data reduction phases, is budgeted for less than $500,000.
All the foregoing discussion was centered around achieving an interim supersonic delivery capability with today’s aircraft and today’s technology. A substantial improvement in capability can be achieved quickly within the state of the art by making certain rationalizations, such as that used for aerodynamic heating. Devices such as this will enable us to cut down some of the large gap between what the clean aircraft is capable of achieving and what we allow today, in terms of delivery envelopes for stores. Closing this gap, however, requires significant advances, both in technology and in the methods we now employ for carriage and release of weapons.
There are basically three areas of technology in which advancement is required before we can attain a true supersonic capability for weapons: (1) aerodynamic heating, (2) store airload prediction, and (3) store and rack static-strength determination.
Aerodynamic heating. If we are going to fly to the aircraft limits, externally mounted stores are going to have to withstand sink temperatures of 300°F or above. Now, before we rush out and try to develop some system of protecting the bomb from the high temperatures or develop new explosives that will withstand these extreme temperatures, we should first know to what temperatures the bombs—and the explosives inside—will really be subjected in flight. Unfortunately, testing to make this determination is the real problem.
As an aircraft reaches transonic speeds, shock waves begin to form on various parts of the aircraft and stores. As the aircraft speed increases, these shock waves change shape, position, and intensity. Some of the shocks impinge upon other parts of the aircraft or stores, and heat flows rapidly down the shock to the part impinged upon. Several years ago Navy flight tests on externally carried missiles measured heat transfer coefficients of up to ten times the ambient in the region of the shock wave impingement. This means that “hot spots” are being formed. Since the sweepback (or Mach angle) and position of the shock vary directly with Mach number, these hot spots are not constant, either in position or in level of temperature. Since a particular store carried externally, particularly on a MER or TER, may be impinged upon by several shock waves simultaneously, and since these impingements may move around drastically with varying speeds, it becomes virtually impossible to predict temperature levels on the store surface or heat flux rates through the store to the explosive.
Flight testing becomes the only practical method of determining how hot the explosive is getting. But how do we test? Where do we install the heat sensors? The number of sensors and whether to locate them inside or outside the store become the difficult questions. To find out truly what effect temperature/time is having on the explosive, every store on every bomb rack position, on each pylon, for each configuration, and on each aircraft type must be tested. The number of flight tests then becomes phenomenal. In addition, we obviously don’t want to test with live explosive bombs. What inert filler simulant we use then becomes a problem. The simulant, to give us realistic values of heat-level buildup, must simulate closely the heat transfer characteristics of the real explosive. Finding such a simulant becomes, in itself, a major problem. There are different schools of thought on testing methods, on what type instrumentation should be used, and also on the number of test points required per test. I can offer no solution to these differences, but I strongly believe that a representative flight test should be undertaken using a specific aircraft and store configuration as soon as possible. This test would not solve all the problems, but it should give a data base from which a decision could be made as to whether flight testing for aerodynamic heating is practical and cost-effective. Furthermore, it would give insight into what methodology should be used, if a more definitive flight test were attempted.
Store airload prediction. One of the primary points to be determined prior to carrying a particular store supersonically is the effect on the aircraft structure caused by the store being carried in some specific configuration (pylon, bomb rack, MER, etc.). There are only three basic techniques available to determine this effect: theoretical calculations, wind-tunnel measurement, and flight test with instrumented aircraft. Instrumented aircraft flight test is by far the most expensive and should be used only when necessary. The instrumented aircraft is generally used to confirm previously predicted airloads rather than to explore new areas.
If a store is fairly large, dense (heavy), and carried singly (one per pylon), the effect it has on aircraft structure can be predicted with some accuracy either by purely theoretical means or by several wind-tunnel measuring techniques. If several stores of different types (such as bomb and fuel tanks, or napalm and bombs) are carried at the same time, even though they are still carried singly on separate pylons, the problem becomes more difficult. Even in this case, however, store airloads and their effect may be predicted fairly accurately. The real problem arise when stores are carried on multiple bomb racks (MER’s or TER’s), generally in combination with other stores on adjacent pylons. In this case, nearly all theoretical prediction methods break down badly. From those that do not we get only approximations. Wind-tunnel methods, for the most part, will give only total loads, such as all six bombs plus the MER plus the pylon. Some experimenters have been able to isolate the effect of the loads of all the bombs plus the rack on the pylon, or of just the aft or forward three bombs plus the rack on the pylon. To my knowledge, only one wind-tunnel group in the country has been able to measure with any accuracy the airloads on individual stores of a MER in a wind tunnel, primarily because of the stringent requirement of subminiaturizing the store balance assembly.
In the past, most aircraft contractors, and government agencies as well, concerned themselves only with the total effect that a group of stores had on the aircraft structure. The bomb racks, both the MER/TER and the one in the pylon to which the MER or TER is itself attached, are usually supplied to the contractor. The contractor generally asks the government to furnish the strength characteristics of these racks, and to his dismay he finds that none exists—nothing, that is, except the design specifications for the racks. The type of qualification testing required for bomb racks has generally been of little or no benefit to the aircraft structures engineer. Faced with this problem, the usual practice in the past has been for the aircraft contractor to assume that the stores themselves and the racks can withstand all the loads imposed. They have concerned themselves only with assuring that the basic aircraft structure will not fail. Some contractors, unwilling to accept this method entirely, have performed static and other structural tests on the racks and even a few stores. The data have for the most part not been made available for general use, so the problem continues to be either ignored or retested with every new aircraft.
To carry stores supersonically, we must know the airload acting on each store separately, even if carried in multiples on a MER. This information is vital to insure that local structural components (racks, pylons, etc.) are not overstressed, in addition to knowing the total effect which the whole group of stores has on the basic aircraft structure. Furthermore, we should be able to predict these store airloads accurately and without highly complex calculations or testing. AFATL has just begun work on a funded project to develop an empirical store airloads prediction technique which is intended to be readily usable.
Store and rack static strength. The fact that static strength capabilities for most bomb racks either are not known or the data are not generally available applies also to most of today’s commonly used stores. Classically, the munition designer has never worried about the static strength of a general purpose iron bomb. It is made of extremely heavy, dense steel. However, many of the attachments to these bombs, such as fins, fuzes, fuze drive assemblies, guidance and control units, etc., are not made of such sturdy material. Other stores, such as dispensers, firebombs, and fuel tanks are made from various thicknesses of sheet metal. The munition designer starts with an assumed set of maximum loads, to which he designs his store. If his store has high margins of safety when these assumed loads are imposed, no further calculations or static strength tests are generally made. Static strength data (tested to ultimate loads or destruction) are available today on very few of the stores or racks in actual use. To cloud the picture further, the data that are available are only as good as the assumed loads, unless the store was tested to destruction. We are currently experiencing a problem at AFATL that clearly illustrates the point. A standard 750-pound finned firebomb, which can be and has been safely carried on several aircraft to speeds of 600 knots calibrated airspeed (KCAS) is now failing below 500 KCAS on another aircraft. We did not detect this failure until we began flight test.
Obviously we need to know the actual failure loads and complete static strength capabilities of the stores we use before we attempt to fly with them, either subsonic ally or supersonically. We must develop a standardized method of testing stores that will yield the data necessary to predict safe carriage of the store. This method should then become a mandatory part of all store development programs. In addition, a project should be established to test many of the stores already in use, especially those we expect to have in inventory for some time, and any other store for which we can foresee some application of supersonic delivery.
None of these technological barriers, taken singly, appears to present a problem that will require technology significantly beyond today’s state of the art. Aerodynamic heating, however, does pose another interesting problem. Even if we are able to develop a testing method and determine heating levels, it may prove to be too costly to be used on an everyday basis. Also, even if we solve all three barrier problems, we are still left with external carriage of stores, which itself imposes severe performance, fuel, and maneuvering restrictions on today’s aircraft, particularly if we use today’s stores and store-carriage equipment.
What, then, can we do to enhance the capabilities of our already existing fleet of aircraft? Obviously we must reduce drag while carrying stores, thereby increasing performance and lowering power and fuel requirements. We must also increase the available maneuver envelopes of the aircraft. Finally, we must develop weapons that can be used at supersonic speeds and be accurate enough to hit the target. If we were designing a new aircraft, we would have several different options available to do these things. When we start with our existing aircraft fleet, however, it becomes a problem of tailoring a specific method to a particular type of aircraft. What works on one aircraft may not on another.
supersonic weapons separation technology
One obvious solution to lowering drag and increasing performance is to carry the bombs internally in a bomb bay. Most existing supersonic fighter aircraft, however, do not have either a bomb bay or space for one. The F-111 does have a bay in which presently only two bombs can be carried. The idea of bluff (blunt-nosed) bombs, which has been around for about twenty years, offers several distinct advantages. First, the bluff bomb, being short and dense, can be packaged more efficiently in a bomb bay because there are no large, cumbersome fins to take up space. Also, because the shape has a very low lift curve slope, this bomb can be released at very high speeds with little or no tendency to “float” or “fly” back into the aircraft. Finally, because it has extremely high drag, its trajectory is more vertical and much shorter than that of a pointed, low-drag bomb. This shorter trajectory allows a pilot more time during a bombing run to identify and lock on a target before the bomb must be released to hit it.
These ideas form the basis for the Supersonic Weapons Separation Technology Program now in progress at AFATL. With a kit designed by Convair Aerospace, we take an ordinary 750-pound M117 bomb case (minus the fins), turn it around backwards, and install nose and tail caps, thereby converting this low-drag bomb to the bluff shape. We have installed three additional bomb racks in the F-111 bomb bay, so that now a total of five bluff bombs can be carried. This is possible because the bluff bomb is only about 52 inches long, whereas the standard bomb, with its tail fin, is about 90 inches long. Studies have shown that, if desired, seven of these bombs can be carried in the existing bomb bay with essentially no modification to the aircraft structure except the installation of the additional racks. Carrying these bombs internally can add substantially to the aircraft’s combat radius because of the drag reduction. Also, because the bombs are all carried inside the fuselage, the roll rate and acceleration (g) envelope are the same as for the “clean” or empty aircraft.
To date, bluff bombs of two aerodynamic configurations have been released from the F-111. We are currently dropping up to five bombs per mission at low altitudes, in single and ripple mode (down to 50 milliseconds) at speeds up to 1.3M. We have dropped the initial bomb configuration already at high altitude at 1.3M. These initial tests showed us that the bomb needed an increase in both dynamic and static stability. A second, more stable configuration was then developed. It is this configuration we are now testing. When this phase of test is completed, we plan to extend the separation envelope out to 2.0M.
Should these tests prove successful, a true supersonic capability for both carriage and release of conventional bombs will have been attained. As a matter of interest, the bombs, while in the bomb bay, are kept at temperatures less than 160°F by the aircraft environmental control system. After release, the bomb drag is so high that the bomb speed is reduced below 1.0M in a matter of seconds, thereby preventing any significant temperature rise. Every bomb dropped is tracked with cinetheodolites to determine its ballistic trajectory characteristics, and the separation trajectory of each is compared against the predicted trajectory.
The objective of this project is not just to develop a specific bomb that can be carried and delivered supersonically from the F-111. In fact, the primary objective is technology-oriented: to provide basic data on bluff bomb aerodynamics and ballistic performance and to investigate the feasibility of packing bombs densely in a bomb bay. The project, if successful, should provide a great deal of basic data that will be valuable in new aircraft design as well as application to aircraft now in development, such as the B-l.
Putting bombs inside the F-111 to enhance its performance and lower its drag was a relatively simple undertaking because the F-111 already has a weapons bay. But what can we do to improve the F-4? There is no bomb bay and no room to put one. After several years of independent study by both the Air Force and the Navy, the two services have now embarked on a joint feasibility/development program involving the F-4, called “conformal carriage.” The Boeing Company, Seattle, has fabricated a large, thin pallet that fits over the entire bottom of the F-4 fuselage. This pallet houses up to 12 bomb-ejector racks, is only 5 to 6 inches deep, and weighs about 1000 pounds. It will carry, in various arrays, 12 MK-82 (500-pound) bombs, or cluster bombs such as Rockeye II, and 9 of the bluff bombs.
Performance and stability wind-tunnel and flight tests have shown that the aircraft, with 12 bombs installed, is able to achieve over 90 percent of the clean-aircraft performance envelope. Subsonic and supersonic weapon separation flight tests were equally encouraging. All bombs separated cleanly and with little or no pitch excursion at speeds up to 1.6M.
The weapons carried are mounted tangentially to the lower pallet surface, held in place by the submerged ejector racks. When carrying high-drag bluff bombs, a fairing is placed in front of the forward bombs to reduce drag. When low-drag bombs are carried, no fairing is used.
This project, like the F-111 project, will demonstrate a true supersonic delivery capability for the F-4 aircraft. However, it too is primarily technology-oriented. Data from this test can be of great value in the design of several advanced fighter aircraft already in the concept formulation stage by both the Air Force and the Navy. Particular care is being given in this test to such problems as how the bombs will be loaded and fuzed and how the aircraft can be serviced, since the pallet covers most of the bottom of the fuselage. The results of these evaluations will assist immeasurably in any determination of whether the conformal carriage concept can be applied to existing aircraft and those now in development.
Currently, the modified F-4 aircraft is at the Naval Weapons Center, China Lake, California, where the store separation tests have just been completed. The gains in aircraft performance, stability, range, and store separation have matched or exceeded all wind-tunnel predictions. Because of its success, a follow-on joint Air Force/Navy development program is now being planned.
In the past several paragraphs, I have discussed projects designed to enhance, or improve, the performance capabilities of certain specific aircraft, the F-111 and the F-4. The Armament Laboratory is also developing a new series of warheads, to which can be attached several different nose cones, tail fins, guidance packages, or rocket motors. These attachments will permit a small number of basic warheads to do many jobs. The warheads are being sized so that the larger ones will be carried singly on an aircraft pylon, while the smaller one will lend itself to single carriage, or multiple carriage on a MER, in a bomb bay, or on a conformal carriage pallet. It may also be packaged densely in a low-drag wing-mounted pod or inside a cluster case.
This development project, while still retaining external carriage, is looking closely at aircraft performance and overall drag on many different aircraft. For those aircraft where internal or conformal carriage is not possible (or economically feasible), the modular weapons approach may offer a distinct improvement over current carriage capabilities.
In this article I have discussed some of the current limits placed on existing aircraft, the potential of these aircraft to achieve at least a partial supersonic delivery capability, some of the technological problems we face, and briefly outlined some current Air Force efforts to overcome these problems.
Again turning to a recent Tactical Fighter Symposium, I believe there are two highly appropriate quotations:
Development of munitions in the past has been a matter of hanging ordnance on an airframe after the airframe has been developed. The result has been degradation of performance inherent in the aircraft in our fighter force.
The failure to develop weapon systems is the principal reason for the existence of supersonic aircraft which become subsonic aircraft as soon as ordnance is hung. This shortcoming has complicated the problem of achieving a supersonic carriage and delivery capability since all aircraft are handicapped by wing-hung ordnance which penalizes aircraft performance.
I have endeavored to sidestep the emotion packed issue of whether or not there is really a requirement for supersonic delivery and whether pilot could hit the target if he had the capability. Rather, I have attempted to analyze what we could do with our existing aircraft quickly and what problems we face in the future and to suggest several alternate methods of achieving the goal both with current aircraft and with those of the future.
I believe strongly that we can no longer endure the limitation of having aircraft operating in arbitrary speed and maneuver envelopes which are substantially lower than the aircraft is capable of achieving. Ideally, aircraft and weapons should be designed together as a system. Only then can both be operating at peak efficiency. Barring that, and recognizing the existence of our large inventory of aircraft and weapons, we can do no less than work as hard and as fast as we can toward expanding the aircraft/store operating envelope to the maximum possible limit.
Air Force Armament Laboratory
Charles S. Epstein (B.S.A.E., Georgia Institute of Technology) is Chief, Compatibility Engineering Team, Air Force Armament Laboratory, Eglin AFB, Florida. He is a commander in the active Naval Reserve, having been a patrol plane pilot. He was a structures engineer with General Dynamics/Ft. Worth before entering Civil Service at Armament Development and Test Center in 1961 and has been in his present position since 1969.
The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.
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