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A fundamental change is underway in naval aviation; the large carrier concept is giving way, in part at least, to small carriers embarking relatively small, supersonic fighter/attack aircraft which possess vertical or short take-ojf and landing capability.
-L he recently initiated Sea Control Ship program, predicated on the concept of a small aviation ship embarking helicopters and high-performance vertical take-off and landing VTOL aircraft, represents a radical departure from the naval aviation concepts in effect for the last quarter century. This program has given rise to many questions revolving around the basic issue of whether the concept truly represents progress in our naval aviation capabilities and provides a stepping-stone for further evolution in this direction, or whether it is simply an experiment or "pet” concept of a few currently at the helm, or a capitulation to the "cost is everything” approach to defense. The speed with which this program has been pursued, the emphasis on cost, the general lack of detailed information, the administrative shortcuts which have been taken, and the technical and operational unknowns associated with this program and related VTOL aircraft programs provide ample cause for doubt or concern by the conservatively inclined. It is the intention of this article to summarize some of the considerations, both operational and technical, which underlie and otherwise relate to the small carrier/VTOL aircraft concept and its potential for further evolution.
The basic concept of the small carrier is not new. During World War II, at peak strength, the U. S. Navy operated more than 100 carriers of all types, most of them small "jeep carriers,” CVEs, and CVLs. However, even at the relatively slow landing approach speeds (less than 100 knots) of propeller aircraft, recovery aboard small carriers was a hazardous undertaking. With the coming of jet aircraft, landing approach speeds increased substantially. Operational losses on recoveries at sea, which were always high, rose exponentially as a function of approach speed. As a result, it became necessary to provide a larger, (angled) more stable deck for recoveries as well as improved catapults for take-off. This, together with the absence of a significant threat at sea with resultant concentration on the mission of projecting naval power over enemy land masses, contributed to the increase in carrier size over the years. Projecting naval power over enemy land masses has attendant requirements for relatively large aircraft capable of long range with a large ordnance load. It also requires long steaming range and time on station for the carrier. Both carrier and aircraft sizes have increased and, thus, the aircraft complement-per-carrier has changed very little since World War II.
The possibility of using the emerging VTOL technology to obtain both high aircraft performance and small carrier basing has been considered frequently, if not continuously, over the years. Until recently, however, there had been no real impetus toward this concept. All that has changed; the dramatic rise of the Soviet
naval threat seems to have provided that impetus.
Force Concept. The Soviet Union, recognizing U. S. dependence on the sea in pursuit of a policy of cooperative defense with overseas partners, has primarily structured its naval forces for impairing America’s use of the seas as an operational base and a logistic link. Basically, this places our forces on a tactical defensive, at least at the outset of such a conflict, with all the inherent disadvantages of the defender. While a decisive engagement of surface naval forces could well occur early in the conflict, history suggests that other threat elements, submarines and land-based aircraft, are not as vulnerable to early decisive defeat and may continue to be a threat throughout the conflict.
The Soviet naval threat is highly oriented to air systems. The cruise missile constitutes a major new air threat to naval forces and commercial shipping, whether it be air-, surface-, subsurface-, or shore- launched. In particular, the emergence of the cruise missile as the primary weapon of attack submarines has greatly increased the area of operation of these forces, their potential for control of sea areas and sea lanes, and the difficulties of successfully countering their threat. Antiship cruise missile systems are now incorporated in 22 major Soviet surface units, in 65 submarines, and in 160 patrol craft. Also, Soviet naval aviation has over 300 land-based, medium-strike bombers to serve as launch platforms. Moreover, the increasing availability to satellite countries of small craft and shore batteries employing these missiles extends this threat to peripheral areas and lesser conflict situations.
The first and obvious reaction to this Soviet threat has been a greater emphasis on the mission and forces for sea control relative to other Navy missions. Less obvious perhaps, is an apparent trend toward greater dispersion as a basic concept for future naval forces. The primacy of the air threat appears to be the major motivation toward this concept. There are, however, other advanced threat elements which undoubtedly contribute to this trend, including satellite surveillance systems and ballistic and orbital attack vehicles. Dispersion in this context does not simply mean spreading formations over greater ocean areas, though this type of dispersion will probably increase as a result of the increase in range of the submarine-launched cruise missile over the torpedo and in response to greater in-flight performance of air threat elements. Dispersion also implies a tendency toward smaller, more numerous ships to complicate the enemy targeting problem and to dilute his offensive resources. Another type of dispersion entails distribution of various force capabilities to a number of more or less multiple-purpose ships to provide some assurance of operational continuity of the force in the face of losses. Finally, dispersion in
time is offered by naval craft incorporating new lift and propulsion principles that provide marked speed S- improvements over displacement vessels.
The passive defense capabilities of the dispersed force •ty concept when coupled with operational procedures for
se low radiation and deception appear to offer a significant
k- defense contribution at relatively nominal cost and with
e’ a favorable dollar exchange ratio. Indeed, it may be
ie the only economically feasible defense against the threat
-1' of the ballistic/orbital attack vehicle. Yet, it is against
-A the primary air threat that this force concept seems to
ef offer the most promise as compared to the present force
re concept. Because of the more favorable reaction time
Vj factors associated with dispersed aviation elements, the
active defense capability of the force can provide initial iif intercept of the threat at a greater distance, establish ih a defense in greater depth through multi-phase engage- g. ment, and achieve significant economies over the cur- c- rent fleet air defense concepts.
se The conventional fleet air defense concept, which as heretofore has been essentially limited in application
:S. to strike forces, is predicated on fighter aircraft and
:s, missile systems of various ranges. The longest-range
:if element, the fighter aircraft, is employed in two phases.
o- The first phase consists of orbiting combat air patrol
:S, (CAP) fighters continuously on station at the outer
as limits of the formation. The second phase of the de-
fC fense consists of deck-launched interceptors (DLI)
a- maintained in a deck alert status on large carriers near
re the center of the formation. The orbiting CAP fighters,
Jt which are intended to provide quick reaction time and
additional depth to the defense, are normally operating at at some fuel state less than full as a result of traveling
eS from the carrier to the CAP station (150 miles or more)
s$ and loitering on station. As a result, dash speed and
e? range are limited as compared to a deck alert inter-
\C ceptor. More important, however, is the fact that many
i' more fighters are required to maintain an airborne
;f station as compared to a deck alert station. Because
;i- of the combined effects of lesser sector coverage and
iS the disparity in fighter force level requirements between
r> orbiting stations and deck alert stations, many more
a- conventional fighters are required for employment on
■){ orbiting CAP stations to provide the same intercept
jc sector coverage capability as would be the case of VTOL
s- fighters employed on deck alert. Various attempts to
it improve the capability of the CAP fighter have included
o use of airborne tankers and larger more complex aircraft
iS designs, such as the F-i4.
d The dispersed force concept also appears to offer S' advantages in subsequent phases of the defense over ;S a conventional fleet air defense. The conventional fleet >s air defense establishes a vital area near the force center ,f which is protected primarily by surface-to-air missiles.
The second phase of the defense, the deck-launched interceptor, is based on board large carriers near the center of the vital area, but it must traverse its radius to reach the primary interceptor engagement area which lies outside the vital area. This transit time is dead time not suffered by interceptors dispersed on the threat side of the formation. The concept of a defense vital area is somewhat eroded by the dispersed force concept but it will probably remain in some form for the foreseeable future for those high value units remaining in the force. Further, on protective missions, the protected elements (merchant convoys, replenishment group, landing forces in transit) would constitute a vital portion of the defense formation.
Force Evolution. Recent events indicate that the U. S. Navy has already made a substantial start toward implementing a dispersed force concept. The shift toward smaller, less expensive vessels is not only exemplified by the Sea Control Ship (SCS), but also by the Patrol Frigate (PF), Hydrofoils (PHM) and Surface Effect Ships (SES). The latter two classes provide a significant performance increase over displacement-type vessels. The Surface Effects Ship, planned at an initial class-size of 2,000 tons, is of sufficient size to embark a small air complement. It constitutes yet another potential means of dispersed basing of fleet aviation.
On the other side of the coin, large carrier force levels have been approximately halved in the last several years. Of the remaining carriers, the Midway (CVA-41) and prior classes are nearly 30 years old and are nearing the end of their service life. The CVAN-70 is the only new large carrier construction which is currently programmed. Thus, the reduction in the near future to a force level of 12 large carriers is a strong possibility.
The presently announced Sea Control Ship (SCS) program encompasses eight ships of new construction. There is also the possibility of conversions of landing force helicopter carriers (LPH) of the Iwo Jima (LPH-2) class.1 The new construction will provide a ship of approximately 15,000 tons gross weight capable of embarking 17 aircraft entirely on the hangar deck. Construction cost will be less than $100 million per ship. By comparison, the Nimitz-class attack carrier, at approximately 91,000 tons, is six times larger and, at a construction cost of approximately $1 billion, is ten times more expensive. The ship tonnage-per-embarked- aircraft is not much different between them, making the SCS ship construction cost per embarked aircraft roughly half that of the Nimitz-class CVAN. This relatively low cost of the SCS per embarked aircraft results
n
The Future of Navy VTOL Systems 27
‘In addition to the USS Guam (LPH-9) being used in tests. The delivery of LHAs commencing in 1973 may render some LPHs excess to landing force needs.
from the absence of catapults and arresting gear, the austerity of the ship in terms of on-board military subsystems, a lower design speed, smaller size of embarked aircraft, and possibly is due in part to the adaptability of this size ship to series construction. The cost margin between the two carrier concepts is more than adequate to cover the additional cost of a VTOL over a conventional take-off and landing (CTOL) aircraft complement. In one study, the cost of a Navy VTOL fighter over an equal performance CTOL fighter of the early 1980s, technology level was found to be approximately 20%. (Somewhat lower margins could have been achieved if greater advantage were taken of unique VTOL characteristics to reduce wing aspect ratio and landing gear design criteria, and to incorporate engine- out recovery capabilities on board the carrier rather than in the aircraft.) Thus, even with the low density aircraft complement represented by hangar deck spotting only, the small VTOL carrier concept appears, from a cost standpoint, to compete favorably with the conventional carrier concept. If the greater defensive effectiveness of the force is taken into account, it can be reasoned that the Sea Control Ship program as announced to date is likely to be an initial installment in a broad new naval aviation concept.
If one accepts this thesis, a small carrier or Sea Control Ship program can be projected in simplified form based on certain assumptions, e.g., small carriers of approximately 15,000 tons full load displacement, no large carrier construction after CVAN-70, and construction at the mean tonnage rate applicable to large carriers for the Forrestal class onward. The result would provide a construction rate of 2.78 per year, with a stable force level of 83 small carriers ultimately being achieved on a 30-year service basis. The initial class
would probably be used exclusively in Sea Control Forces while subsequent classes would be expected to be used more extensively in Strike Forces as well. Thus, the term "Sea Control Ship,” while applicable to the first class of this type carrier, is probably not accurate when it is applied to subsequent classes or the broad generic type.
Force levels achieved by the latter 1980s under such a small-carrier program would permit a Strike Force composition and disposition similar to that depicted in Figure 1. Note that it contains two large carriers, centrally located, and 10 small carriers disposed uniformly in two rings for equal all-around defense. A Sea Control Force composition and disposition in this time period might be somewhat as depicted in simplified form in Figure 2. Note that there are no large carriers and that missile cruisers, frigates or destroyers are provided for close-in air defense. Although a significant difference in size of the two types of forces is indicated, the major differences in composition, disposition, and tactics which heretofore existed between them are expected to be much less in the future. Instead, there appears to be a trend toward constituting all tactical surface forces so as to possess substantial capabilities for anti-air warfare (AAW), surface combat, and antisubmarine warfare (ASW). The mix of these capabilities, of course, would be tailored to the mission emphasis of the force and its area of operation. As a corollary to the above, it is reasonable to expect that these various capabilities will tend to blend and merge into a single force operational system into which the individual systems are closely integrated. The changing nature of the threat, particularly the cruise missilearmed submarine, has necessitated that AAW and ASVf operation be closely integrated at all levels.
The Future of Navy VTOL Systems 29
I
(RPVs) The strike capabilities of vessels can be expected to be further enhanced through more advanced, longer-range missile systems.
System Performance and Operational Characteristics. The VTOL aircraft to replace the Harrier will be the first high-performance VTOL aircraft designed specifically for naval service. In all likelihood, it will be the first operational VTOL aircraft in the world to have a significant supersonic capability. As such, it will probably be considerably different from the Harrier in its technology, its operational characteristics, and its impact on the associated small carrier.
As always, the determination of system characteristics ultimately is dependent to a considerable extent on
The capabilities of all three major classes of carrier aviation are expected to be represented on board small carriers, e.g., fighter, attack, antisubmarine warfare. They will also need miscellaneous specialized supporting aircraft such as airborne early warning, utility, and reconnaissance. It is visualized that these small carriers will employ a mixed aircraft complement which will vary as to force assignment, area of operation, and specific mission of the individual ship. Conventional helicopters will perform the ASW and miscellaneous tasks from these ships until they are eventually replaced by intermediate performance VTOL aircraft. The AV-8A Harrier will be available at first to fulfill the fighter and attack role of these ships. This model is now being used in tests and, on the basis of current programs, it is the only high performance VTOL aircraft which will be available for initial Sea Control Ship deployments. The Harrier, however, was designed for short- range, close air support of ground forces and its speed and range are limited for performing Navy missions.
Its replacement is expected to enter service in the early o to mid-1980s. It is to this aircraft and the related class
s, of small carriers that the balance of this article is de-
ic voted.
:C Before turning to the specifics of new generation,
d high-performance Navy VTOL aircraft and associated
carriers, it is appropriate to round out the visualization h of the 1980s naval tactical surface forces. A number
:C of current developments are predicated on more diver-
d sified force capabilities embarked on a broad range of
s, ships. Force strike capabilities are being improved and
i' diversified by the addition of missile systems capable A of embarkation an board escort ships. These include is dual mode SAM/SSM systems, such as the Standard mis- i- sile, and anti-ship cruise missile systems, such as the \C Harpoon. The aviation capability of escort vessels is r$ similarly being upgraded by the Light Airborne Multi- y Purpose System (LAMPS). This system provides a sur- ;S veillance and weapons launch platform which will con
s' siderably extend capabilities over those of hull-mounted n systems. The capabilities for defensive operations are
v being dispersed in a similar fashion as evidenced by
g a trend toward providing some AAW and ASW capabil-
ai ity to all units. Many developments are directed toward
t, improving command and control capabilities, especially
■,c for the time-critical defensive operations. In this cate-
n gory are developments in force-wide systems for sur-
a veillance, identification, situation evaluation, opera-
if tional commitment, information/data transfer, and
<c related functions.
1 c Looking beyond current developments, conjecture
g suggests the possibility of greater use of aviation em- r- barked on escorts and small craft. This would include
fl higher performance aircraft and unmanned aircraft.
30 U. S. Naval Institute Proceedings, September 1973
subjective evaluations of applicable operational, technical, and resource considerations. Nonetheless, it is possible to bound the problem so as to provide an approximation of the end-system characteristics.
Since early indications point to only one high performance VTOL end-system for this period, it follows that the aircraft, like its first operational generation predecessor, will perform both fighter and attack functions. Such an approach localizes the technical risk associated with the first high-performance VTOL end- system development undertaken in this country. It also minimizes nonrecurring costs, provides operational flexibility in force complements, and recognizes that operational inventory levels at this point in force evolution may not warrant differentiation of fighter and attack systems. A more basic consideration, however, is the attack weapons trend. This trend is away from large payloads of lower-accuracy weapons and toward lesser payloads of high accuracy guided weapons, particularly for the sea control attack mission. As a result, the disparity between fighter and attack mission requirements and their design impact would appear to be less for this time period than the disparity which currently exists. In all other respects, the performance requirements of the fighter role are either equal to or more severe than those of the attack role and are the determinants of system characteristics.
While the performance requirements of both offensive and defensive fighter missions are both stringent, they are different in nature. The offensive mission is characterized by a relatively long radius flown at sub-
sonic speed with a short mid-mission combat segment involving high maneuverability. The defensive interceptor mission involves high speeds for relatively short ranges and does not involve a sustained close combat segment per se. As previously indicated, the combat air patrol loiter mission would not be applicable to a VTOL fighter employed in a dispersed force context.
The defensive fighter intercept mission is fundamental and underlies the entire VTOL small-carrier concept. Here the problem is the classic velocity vector intercept in which area of coverage and distance at which initial intercept takes place is directly proportional to interceptor speed. The interceptor’s average outbound speed is intimately related to range on supersonic fighters. On all but the very short-range intercepts, the distance-to-intercept and available fuel determine the speed at which intercept can be accomplished.
Figure 3 indicates the size of aircraft necessary to achieve given intercept capabilities within the speed range of subsonic to Mach 2.2 at altitude. These curves suggest that the marginal increase in capability begins to fall off rapidly at the higher size and performance levels. This is somewhat misleading in that the ability to attack the launch vehicle, rather than simply its missile after launch, places a premium on additional capability. If speed capability were increased substantially beyond Mach 2.2, the size of the aircraft would begin to increase rapidly because of structural heating effects. To achieve Mach 3.0 would necessitate a weight increase of 30% or more over the Mach 2.2 design depending on the specific design conditions adopted. This does not even take into account the effect on missile stores and the probable need for internal carriage. Furthermore, such a speed regime would compound the technical risk of the program.
Within the performance range short of severe structural heating, aircraft life-cycle costs increase at a slower rate than performance. When the cost basis is expanded to a total system basis by including missile and ship costs, the differences are almost halved through the dilution effect. On this basis, the increase in intercept capability achieved at higher performance levels appears to be well worth the price. Hence, the Harrier replacement should probably perform well into the supersonic range but short of severe structural heating effect, i.e., from the lower to mid-Mach 2 range.
The question of providing offensive fighter capabilities in the aircraft has several ramifications. If this follow-on generation of high performance VTOL aircraft and associated small carriers is limited to employment in Sea Control forces, as is contemplated for the first generation system, there would be little, if any, requirement for an offensive capability. If, however, one subscribes to employment of second generation systems it1
The Future of Navy VTOL Systems 31
Strike Forces as well, some requirement for offensive capability is visualized. Nevertheless, in any case, there would be some large carriers of modern vintage in these forces through the turn of the century. Their CTOL fighter complement would be expected to constitute the primary offensive capability while the VTOL fighters aboard small carriers fulfilled the primary role of defensive operations. Of course, for purposes of flexibility, each type of fighter should probably have some capability for complementing the other. Thus, at least for the second generation, the degree of offensive capability built into the VTOL fighter is likely to be some lesser level than its CTOL counterpart.
A supersonic fighter designed for the defensive mission has an inherent degree of offensive capability. The power required to achieve high acceleration and dash speed for intercept also contributes to maneuverability[1] in offensive close combat. The fuel required to provide relatively short range intercept dashes will yield much greater subsonic cruise range on offensive missions. Further, additional radius can be achieved without significant design impact by initiating offensive missions in a design overload condition with additional fuel carried externally. The problem arises when degrees of offensive capabilities are imposed on the design over and above those inherent from defensive capabilities. These involve high aspect ratio wings for more efficient cruising, lower wing-loading for close combat maneuverability, and additional maneuver power beyond that required to achieve maximum design dash speed. The latter feature in particular has a growth effect on design.[2] It may be that the new generation of close combat missiles will enable the VTOL fighter aircraft to achieve a respectable close combat capability, at least in a supplementary role, at a relatively modest level of combat power.
Any level-attitude VTOL aircraft lends itself to operating in the short takeoff and landing (STOL) mode. The short takeoff mode provides additional payload capability provided the design load factor is degraded. We can expect VTO mode to be used on quick reaction missions involving high acceleration, early combat phase, or adverse weather and sea conditions. Short takeoff (STO) mode would be favored for preplanned missions such as attack missions involving long range, relatively low cruise speeds, low or deferred combat maneuver requirements, and favorable weather and sea conditions. All landings would normally be vertical. The amount of additional capability which can be achieved in a 300-foot STO overload mode over a VTO
design condition with respect to a typical Mach 2.2 design is represented by a takeoff gross weight increase of almost 20%. This impressive increase in useful load capability is primarily a result of degrading the load factor. Slightly less than one-third of this difference is because of the greater lift efficiency of STO over VTO. Instead of designing for VTO at normal load factor, it would be possible to design alternately for a 300-foot STO at normal load factor or, at the other extreme, VTO at overload load factor.[3] These would result in changes in takeoff gross weight of about 6% each from the base design conditions. From an operational standpoint, the STO design criterion would provide no overload capability operating from a small ship. The VTO overload criterion would provide this capability without weather and sea restrictions. Overload capabilities need to be applied at least in part to fuel if range is not to be degraded. In the era of higher accuracy weapons, it is expected that overload capabilities would habitually be devoted entirely to fuel.
Aircraft Design Considerations. The basic problem in VTOL design is the divergent requirements of operation in two fundamentally different regimes of flight, e.g., hover and forward flight. These design considerations involve size and complexity over and above CTOL design which are usually referred to as the "VTOL penalty.” This is not to say that there is no "CTOL penalty,” particularly in carrier aircraft, and that some of these are not offsets against the VTOL penalty. In this category are wing aspect ratio, low speed lift coefficient, and structural facilities for catapult and arrestment.
VTOL design approaches are normally classified in terms of propulsion system type. A useful parameter for general classification is disc loading, defined as thrust-per-unit area of the thruster. The high discloading of jet propulsion is required to obtain high forward speeds. However, within the realm of jet forward flight propulsion systems, there remains the choice of disc loading in the hover regime. The higher hover disc-loadings tend to be low in installed weight, volume, and technical risk. However, they are comparatively inefficient and entail higher fuel weights. On the other hand, lower hover disc-loadings are attractive from an efficiency standpoint but, when combined with high forward flight disc-loading in hybrid systems, entail substantial installed weight, volume, cost, complexity, and higher technical risk.5 In VTOL design, efficiency and simplicity tend to be opposing consid-
4Normal structural load factor for a fighter is 7.33g at combat weight; overload structural load factor is 4.0 at takeoff weight.
5See G. G. O’Rourke, "Lightweights and Heavyweights,” Proceedings, February 1973, pp. 26-33.
erations. For the Navy high-performance VTOL mission, considerations of simplicity may override those of hover efficiency since the hover regime is probably the least demanding of any conceivable military VTOL regime. This mission is characterized by one takeoff and landing per mission, no mid-mission hover, all hover at sea level, no takeoff and landing obstacles, wind over deck can be generated, no serious surface erosion problem, and no foreign object ingestion problem.
Jet VTOL may be classified in descending order of hover disc-loading as all-jet systems, lift fan systems, jet ejector systems, and stopped/stowed rotor systems. Most jet VTOL development has taken place in all-jet systems of which there have been 10 flight article development programs in the Free World. In the other hover discloading classes there has been one development each of the lift fan type (XV-5A fan-in-wing) and the jet ejector type (XV-4A Hummingbird), of which the latter was disappointing in performance. There has been some interest in stowed/stopped rotor lift jet VTOL systems but no flight articles have been developed to date apparently due to their high complexity levels and related cost impact.
The all-jet VTOL systems may be further classified by the manner in which thrust is generated in the two regimes. In the unified propulsion system (lift/cruise engine) approach, such as the Harrier, the same power plant is used for both hover and forward flight. This
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approach represents the limit from a simplicity standpoint. At the other end of the spectrum is the independent propulsion systems (lift plus cruise engines) class in which separate power plants are used for hover and forward flight. This design approach was used in two early programs but the advent of lightweight deflecting nozzles for cruise engines and more advanced control systems has made it obsolete and of academic interest only. In between these two concepts is the combined propulsion system (lift plus lift/cruise engine) in which one power plant provides forward flight propulsion and some of the lift thrust. The remaining lift thrust is provided by lift engines. This is the design approach used in the German VAK-191B and the Dormer DO-31E. Also, this was the approach used in the U. S.-German VTOL strike/fighter program which was terminated after considerable development effort in the latter 1960s. This program left a legacy of high performance VTOL components including the XJ-99 lift engine development and a number of deflecting nozzle developments.
For all but the all-jet unified propulsion system, the VTOL penalty as manifested in the design approaches discussed above is rather obvioqs. The penalty is reflected in extra components for generating lift thrust including engines, nozzles, inlets, ducts, fans, and ejectors. In the case of the unified propulsion system, the penalty is not reflected so much in extra components but in larger components (engines, inlets, nozzles) to meet the thrust requirement of VTOL which is over and above forward flight requirements. To the extent they are oversize for the remainder of the mission, these components constitute dead weight in the same sense as an idle lift component does in other design approaches. But the penalty in this design approach does not stop there. The oversize power plant operates for most of the mission at an inefficient output level, thus fuel consumption is high. Also, the inlet size and shape and engine location for balance purposes create a high drag design which limits both speed and range. Thus, the VTOL penalty in the unified propulsion system approach is very high indeed.6
Figure 4 shows the results of a series of parametric design sizing studies of both supersonic and subsonic designs. The supersonic (Mach 2.2) designs employed a 350-nautical mile offensive mission with 1,000 feet per second specific excess power for combat.7 The subsonic designs were for a 150-nautical mile defensive
6 See G. G. O’Rourke, "Wondrous World of VSTOL,” Proceedings, November 1972. pp. 33-41.
7 At 10,000 feet, 1 g, Specific excess power =
(thrust-drag \
---- 1 velocity.
weight I
The Future of Navy VTOL Systems 33
mission. Four propulsion system design approaches were used in each case, e.g., all-jet unified (lift/cruise), combined (lift + lift/cruise), and independent (lift + cruise) types, plus a lift-fan type. The abscissa in this chart is the percentage of required lift thrust which is (or, in the case of lift fans, could be) developed from the cruise engines. In all but the unified propulsion system, the cruise engines are sized by mid-mission requirements. The differing severity of these requirements causes the combined and augmented flow type propulsion systems’ cruise engine lift contribution to be relatively greater for the supersonic than the subsonic designs. As might be expected, the designs which provide 100% of lift thrust in their cruise engines are relatively heavy for the reasons indicated above. On the other end of the spectrum, designs which do not use installed cruise engine power in the lift mode tend to be heavier because of the additional lift engines required. Note that the supersonic lift fan design is considerably heavier than the lift plus lift/cruise design whereas this difference is less in the case of subsonic design. The result apparently reflects the greater sensitivity of the supersonic design to the additional volume entailed in the lift fan system.
The offensive mission of the supersonic design has a two-minute close combat phase. The figure seems to suggest that an increase in gross weight of some 5,300 pounds would provide the simplicity of a unified propulsion system and increased power for close combat as well. While it is true that the engine size in the unified propulsion system design has the capability of providing a combat specific excess power level of over 1,400 feet per second or a combat thrust-to-weight ratio of over 1.55:1, the design does not incorporate the additional fuel required to operate the engine at this combat power level or the aircraft growth effects entailed by carrying this additional fuel. If the design were to incorporate combat use of full engine power, the takeoff gross weight would increase an additional 6,200 pounds.
For the time being at least, the Navy is pursuing several design approaches to high performance VTOL. Earliest in terms of service application is the Harrier which is the only high-performance VTOL aircraft which has achieved operational status to date. All indications point to some version of the Harrier as the first operational generation of Navy high performance VTOL aircraft. The Navy is also initiating further study of an all-jet combined propulsion system design. However, unlike the German VAK-191B, a new Navy design of this type would probably be of the longitudinally nonsymmetrical engine arrangement type to facilitate use of afterburning cycles and high-performance nozzles, and to obtain better high speed drag
characteristics. A final lower hover disc loading approach is represented by the jet augmented wing prototype program recently initiated by the Navy and designated the XFV-12A. This approach is generally conceded to entail relatively high technical risk and substantial development lead time. The limited data available does not permit comprehensive evaluation of this design approach at this time. Indeed, the primary reason for the prototype program seems to be the development of these engineering data.
Ship Design Considerations. The first generation small carrier, the Sea Control Ship, reflects many of the design considerations for a VTOL complement. On the other hand, it does not necessarily reflect the requirements for any potential follow-on employment of small carriers in Strike Forces. Neither does it necessarily interface with subsequent generation VTOL aircraft, especially high performance VTOL aircraft, as might be the case in later classes of small carriers.
In basic form, the small carrier of the future will probably remain much like the Sea Control Ship, i.e., a single-hull displacement type vessel of approximately 15,000 tons full load displacement, although a catamaran type is also a possible approach for subsequent classes. While the Surface Effects Ship will most likely come into the picture as a VTOL carrier, its very small size suggests that its air complement will not be capable of sustained operations. Hence, the SES role appears to be complementary to that of the small carrier, much like the air capability embarked aboard escort vessels. The single hull displacement vessel offers relatively low construction costs and volume for internal spotting of aircraft. The size seems to be that which the Navy considers to be a reasonable balance between the desire on the one hand to obtain sufficiently low unit cost and adequate numbers to effectively implement a dispersed force concept and, on the other hand, to have adequate size for sustained multi-mission operations of its complement. The full flight deck coupled with hangar-deck-only spotting provides a capability for simultaneous multi-mode operations, short take-off (STO) operations, and VTOL operations in the mid-ship area in high seas to minimize deck motion. Since VTOL aircraft are virtually immune to wave-off, an angled deck is not required.
VTOL aircraft, unlike helicopters, are unable to adjust the thrust vector direction with respect to the plane of the landing gear to avoid tilt-induced translation. Further, the characteristic high fuel consumption of high performance designs in thrust-supported flight makes it undesirable to hover extensively until the ship rolls level. Accordingly, some means of roll stabilization of the ship is desirable. In the first class Sea Control Ship, this sytem will be of the passive anti-roll-tank
type. Ship pitching is much less of a problem because the deck angle is much less than that generated in roll, the period is longer, and because the longitudinal axis of the ship and aircraft would normally be aligned for touchdown. Deck slam owing to pitching and heave should not be a serious problem because of the ability to use the mid-ship area where ship vertical motion is reduced and because VTOL aircraft are characteristically responsive to control in the vertical direction even with designs which exhibit negative buoyancy in the hover.
The efflux characteristics of high performance VTOL aircraft which employ high disc loading in the hover mode are of some concern in ship design. In the Harrier, efflux temperature is relatively low and creates no particular problem. In the next generation of aircraft, however, much higher temperature cycles will be sought so as to achieve maximum performance at minimum size and cost. The resulting efflux temperature could approach 3,500° Fahrenheit vice some 750° Fahrenheit for the Harrier. Obviously, such temperatures, if not provided for in ship design, could have severe implications on ship structure, safety of deck personnel, other aircraft in the vicinity, and on the generating aircraft itself. The interface between the VTOL aircraft and the carrier, is, of course, subject to tradeoff and may result in some restriction in efflux temperature for VTOL designs as well as ship design provisions to accommodate higher temperatures.[4] A number of the latter have been considered in one form or another including watercooled deck areas, grated deck areas vented overboard, VTOL platforms suspended over the
side or raised above the deck, and deck coatings. In addition, high efflux temperatures in the STO mode may require use of blast shields to protect deck personnel and other aircraft. With the possible exception of blast shields, the initial class of Sea Control Ships will not have features such as these, thus creating a potential need for subsequent modification.
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There are several other design features which, while not operationally essential, have been considered for potential application to small VTOL carriers to improve their capability on one way or another. One such is an automatic aircraft hold-down system similar to the French Harpoon system which couples with a deck grate to facilitate operation under adverse sea conditions. Others include a STO holdback to enable full engine runup before initiating deck roll, a barricade system to permit arrested conventional landings of aircraft which experience power or control system emergencies in flight, and various schemes for rapidly arming and servicing aircraft. None of these will be available on initial class Sea Control Ships. Their eventual appearance is subject to their operational worth which, at the present level of experience, is largely conjectural.
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There is some reason to believe that the logistical concept utilized with a small carrier dispersed force concept will be substantially different than at present Periodic retirement of the force to a replenishment area for simultaneous replenishment of carrier consumables is probably not feasible as a general practice. This method would take too long, require too many replenishment ships, and tend to destroy the operational integrity of the formation. Instead, continuous force replenishment on station in operating areas is expected to be the rule.
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34 U. S. Naval Institute Proceedings, September 1973
The specialized aviation supply and maintenance support concept may also have to undergo substantial change. It may not be feasible to have on board each small carrier the specially trained personnel, tools, test equipment, and parts required for all organization and intermediate maintenance of a mixed aircraft complc ment. In lieu of this approach, a concept which i* predicated on an aviation supply and maintenance ship is suggested. One such ship would support several small carriers with specialized supply and maintenance on 3 continuing basis. Figure 5 is an operation diagram of how this concept could operate. The supply and maintenance ship would be relatively centrally located in the force with respect to the small carriers it supports Utility type helicopters embarked on the supply and maintenance ship would transport special parts, specif maintenance personnel and their tools to small carrier* as needed to extend the carriers’ embarked supply and maintenance capability. In cases of substantial or long'
The Future of Navy VTOL Systems 35
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c& term maintenance work, the aircraft requiring it would
nt be evacuated to the supply and maintenance ship itself for this work either under its own power or by means 16 of the heavy lift helicopter embarked aboard the supply
fob and maintenance ship. The aviation supply and mainte- >jv nance ship itself is conceived as being similar to an
ijl LPD. Figure 6 shows a modified LPD with stern elevator,
rc( boat deck converted to a maintenance hangar deck, and
C& embarked utility and heavy lift helicopters.
Conclusions. Available evidence seems to indicate a ,<;£ substantial shift toward VTOL aircraft and small carriers jal in the naval aviation force structure. This shift is apparel1 ently part of a broader pattern for naval force evolution which, for lack of a better name, might be termed a qA "dispersed force concept.” The increasing air threat and |c the need for more effective means of countering it
i5 within reasonable costs constitute the impetus for this change. Although announced plans for small VTOL ^11 carrier systems encompass early employment in Sea ? Control Forces only, it is logical to expect their appear-
ot ance in Strike Forces in the 1980s to complement the
jr large carriers which remain in the force structure. jf> Current programs point toward the introduction of ($. a second generation high performance VTOL aircraft in ^ the early to mid-1980s. This aircraft should be relatively tfl small and, although its primary mission is visualized ,(j as being fleet air defense, it should also have some capability for offensive anti-air operations and surface a- attack, especially against targets at sea. Indicated per
formance is in the lower Mach 2 range. Such aircraft can be developed at reasonable size and cost.
A second generation high performance VTOL aircraft can be operationally compatible with small carrier basing. However, there is need for closer interface between aircraft and ship than with the first generation system. This is largely because of the efflux characteristics which will probably require some ship features to accommodate high temperature jet plumes.
In a force containing several small VTOL carriers, it will probably be advisable to echelon the VTOL supply and maintenance capability by means of a specialized logistic support ship. This concept is much like that employed by ground forces to provide logistical support to helicopters employed on a small unit widely dispersed basis.
Lieutenant Colonel Kuscwitt served in the Army during the period 1945 through 1965. He was designated an Army aviator in 1950 and his service included assignments in aviation, artillery, and general staff. Staff duty assignments included the Office of the Chief of R&D, the Office of the Deputy Chief of Staff for Logistics and Headquarters, Continental Army Command. Since retiring, he has been employed by Vought Systems Division, LTV Aerospace Corporation. He received his B.S. degree from the U. S. Military Academy in 1945 and his M.B.A. degree from the University of Alabama in I960.
[1]Sustained maneuverability involving maintaining high energy levels.
[2]This feature also has an effect on design choice as well as aircraft size
as subsequently discussed.
[4]Efflux temperatures for designs in Figure 4 were restricted to 2000°F in the hover mode.