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Sukhoi Flanker
Analysis [Click for
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S-300 SAM
System Analysis [Click for
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First
published in Asia
Pacific Defence Reporter, 2005
by
Dr Carlo Kopp
Text,
Line Art ©
2005, 2007 Carlo Kopp
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Su-35
Flanker E (KnAAPO).
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Reports indicating that the
Russian Lyulka/Rybinsk Al-41F supersonic cruise engine is entering Low
Rate Initial Production, and the recent Defense Acquisition Board
decision to transition the supercruising F-22A Raptor to full rate
production, illustrate the highly competitive nature of the fighter
business. Supersonic cruise will change many aspects of how air
superiority is won, and the future question in assessing fighters will
inevitably be 'does it have supercruise?'
Supersonic cruise or supercruise is a term which refers to the ability
of a combat aircraft to sustain supersonic flight without using
afterburning thrust.
Combat aircraft have had supersonic capability since the 1950s,
exploiting afterburners to effectively multiply available thrust and
thus overcome the drag rise characteristic of transonic and supersonic
flight, as well as improving climb, turn and acceleration performance.
The additional thrust advantage of the afterburner comes at a
prohibitive price - fuel burn is multiplied severalfold as fuel is
injected into the tailpipe and combusted. A byproduct of afterburner
use is a dramatic increase in the aircraft's heat signature, the engine
plumes becoming effectively an infrared beacon which can be detected
and tracked from dozens of miles away.
In practical terms conventional gas turbine engines afford only a
transient supersonic capability, one which must be used very carefully
as it can expend thousands of pounds of fuel in minutes, and advertise
the fighter's presence and energy state from tactically very useful
distances.
Having the ability to sustain supersonic speeds without these drawbacks
affords numerous advantages in combat. The first of these is that
entering an engagement the supersonic fighter has a reserve of kinetic
energy which a subsonic opponent does not have. As a result the
supersonic fighter can often dictate the terms of the engagement.
More importantly, sustained supersonic speed presents genuine problems
in engagement kinematics for an opposing conventional fighter. Even in
Beyond Visual Range (BVR) combat, air to air missiles have kinematic
limitations. To effect a kill a fighter must position itself so the
target falls into a 'no escape zone' for the missile type being used.
Unless this precondition is met, the missile will likely run out of
energy and be unable to engage the target.
In classical intercept geometries, fighters are typically vectored into
a head to head closing geometry upon which the player with the earliest
firing opportunity, whether afforded by longer radar/missile range, or
supporting networking capability, has the advantage. Where both
fighters have matched conventional kinematic capabilities, the game
well and truly revolves around incremental advantages in missile
capability, or situational awareness, provided by onboard or offboard
sensors.
This delicate balance, and the advantages yielded by incremental
imbalances in missile and sensor technology, will collapse once one of
the fighters has the capacity to sustain supersonic speeds. As a
result, even modest heading changes by the supersonic fighter, when
positioning for the engagement, will force the conventional fighter to
go into afterburner early, and typically will create enough separation
to ruin the conventional fighter's missile shot geometry. In effect,
conventional fighters flying against fighters with sustained supersonic
capability usually do not get good opportunities for BVR missile shots.
Only a very significant advantage in the kinematic performance of the
missiles carried by conventional fighters can offset the advantages
held by the player with sustained supersonic capability.
The classical case study is the obsolete Cold War MiG-25 Foxbat, used
in reconnaissance, interceptor and defence suppression roles. Capable
of sustaining Mach 1.8 class speeds even with external stores, the
aircraft proved extremely difficult to engage during the 1991 Desert
Storm campaign - the only case study we have of an air battle involving
BVR combat and top tier conventional fighters such as the F-15C and
F-14 series. While the US and Israelis have successfully killed Foxbats
on numerous occasions, invariably this involved carefully setting up an
engagement against a target behaving predictably.
The reality is that the situational awareness advantages afforded by
modern ISR and networking capabilities only work where the fighters
using them have kinematic parity with their opponents. Once the
opposing fighter has a significant kinematic advantage, the tables may
well be turned. Given that most modern fighter fleet operators have
AEW&C capabilities, or are acquiring AEW&C capabilities, the
line of argument which presents AEW&C and networking as an air
combat panacea is little more than nonsense. The issue of long range
Russian 'counter-AWACS' missiles such as the R-172 and R-37 is a
consideration in its own right. AEW&C and networking are becoming a
common prerequisite, driving the capability contest yet again into
other areas - and supersonic cruise will be the next arena in the
global competition for air superiority.
Achieving genuine supersonic cruise capability hinges on two
technological prerequisites. The first is having a powerplant which
develops enough dry thrust at altitude to offset supersonic airframe
drag. The second is having an airframe design built for low supersonic
drag. Unless both conditions are met, supersonic cruise capability is
not achievable.
The airframe issues dictate a wing design typically with 45 degrees or
more of leading edge sweep, and suitable fuselage area ruling.
Moreover, weapons must be carried internally or in a semi-conformal or
conformal arrangment, to avoid a supersonic drag penalty. Pylon mounted
missiles are not the preferred strategy. To date, airframe aerodynamics
have not been the obstacle in the supercruise game. Engine capabilities
have remained the principal obstacle.
A turbofan engine designed for supersonic cruise will be characterised
by a much higher turbine inlet temperature than contemporary
'conventional' fighter engines. It is this operating cycle which
permits the engine to sustain higher dry thrust ratings at high
altitudes. This has also proven to be the primary obstacle to date in
building supercruise engines, as it requires advanced materials and
advanced turbine cooling techniques.
The first service to recognise the importance of supercruise was the US
Air Force, which incorporated supercruise into the early requirements
definition of the Advanced Tactical Fighter (ATF) program, which
eventually coalesced into today's F/A-22A Raptor. An extensive and
expensive engine technology research and development effort led to the
design of the Pratt and Whitney F119-PW-100 engine which powers the
F-22A. Delivering around 35,000 lbf of afterburning thrust, the
F119-PW-100 is the most powerful fighter engine manufactured in the
Western world. The simplest qualitative measure of the F119-PW-100's
performance is that this engine has a dry thrust performance envelope
matching the afterburning thrust envelope of the F100-PW-100 series
engines fitted to the F-15C/E and many F-16 variants.
As a result the F-22A is the only production fighter in existence with
a genuine supersonic cruise capability and the enormous kinematic
advantages this affords in combat. This analyst had the opportunity to
discuss the practical aspects of supercruise capability with one of the
F-22A test pilots some years ago. Not only were chase fighters unable
to keep up, but in mock intercepts flown by F-16Cs and F-15Cs against
development F-22A airframes, even modest 20 degree heading changes
caused the teen series fighters to abort their intercepts, having
burned their fuel down to bingo levels.
A
derated AL-41F supersonic cruise engine has been trialled in an Su-27S
since 2004.
Russia's Supercruise Program
The Soviets recognised the threat presented by the emerging ATF with
its supercruise capability and initiated the development of a fifth
generation supercruise engine in 1985. The program was awarded to the
Lyulka bureau, under the leadership of Viktor Mikhailovich Chepkin. The
designation Al-41F or Izdyeliye (Article) 20 was adopted.
The design aims for this engine were ambitious - to meet or exceed the
performance of the US F119 family of engines. To that effect the Al-41F
was to achieve a thrust to weight ratio in excess of 10:1, over the 8:1
ratio characteristic of the Al-31F series developed for the Su-27/T-10
fighters. The engine was to have a reduced number of stages and
increased pressure ratio, like the F119. EU sources claim that a
turbine inlet temperature increase of 250 degrees C was sought over the
Al-31F, pushing the value up to 1600 degrees C. A Full Authority
Digital Engine Control (FADEC) was ultimately sought, with the engine
controls tightly integrated with the flight controls of the fighter
using it.
The first flight testing of prototypes was performed at the Gromov
Institute in 1990, using a Tu-16LL Badger testbed. Subsequently a
MiG-25PD was retrofitted with one of its R-15-300 engines replaced with
a Al-41F prototype. Over the subsequent years thirty test flights were
performed, before funding constraints slowed the effort down.
Development tests demonstrated static afterburning thrust performance
claimed to be at 39,600 lbf (176 kN) with 45,000 lbf (200 kN) a design
target. These figures exceed cited thrust ratings for the US F119 and
F135 family engines.
TBO was reported to be an issue with demonstrator engines, with latter
development effort being concentrated on achieving similar engine life
to the Al-31F. Derating the engine would address this to a large
extent, as not every application demands the full performance potential
of the Al-41F.
The Al-41F was the designated engine for the Su-32/34 Fullback, now
entering Low Rate Initial Production (LRIP) for the Russian Air Force.
All seven Fullback demonstrators were flown with earlier Al-31F engines
due to the then unavailability of pre-production Al-41Fs. The Fullback,
designed to fill the niche between the F-15E and F-111, is heavier than
the Su-27/30 and required the Al-41F to meet performance targets - the
10-13% better SFC of the Al-41F was also a benefit for this strike
fighter.
MiG
MFI Prototype (RuMoD).
AF-41F
in MFI (All
images via FAS)
The Al-41F did not debut until the 1998 rollout of the stillborn MiG
I.42/I.44 MFI (Mnogo-Funktsioniyy Istrebitel') multirole fighter
demonstrator - the most notable exposure during this event were
photographs showing the ceramic coating inside the Al-41F nozzles. The
MFI, designed as a supercruising challenger to the F/A-22A, is claimed
to supercruise at Mach 1.8 to 1.9 using a pair of Al-41Fs. The MFI
prototype first flew in early 2000. Russian and EU sources claim the
MFI's Al-41F engines have 3D thrust vectoring capability. Current
indications are that the MFI will remain a demonstrator, with the
Sukhoi bureau winning a 2002 tender to develop Russia's new fifth
generation fighter.
In October 1998, Russian media reports disclosed that Rybinsk Motors
was nominated as the production plant for the Al-41F, with a 2 billion
Rouble investment program planned over a four year period. A report in
the AeroWorldNet journal described the use of composite casing for
weight savings, and advanced materials including boron-fibre composites
in the engine. The same report claimed that 27 ship sets of the
demonstrator Al-41F configuration were built by Saturn/Lyulka in their
Moscow facilities to date. Thrust to weight performance for the engine
was cited at 11:1, and sustained supercruise speeds of Mach 1.6 to 1.8
were claimed.
The first production type to be fitted with the Al-41F will almost
certainly be the Su-32/34 Fullback, and the recently initiated LRIP
build is likely to provide the revenue base for the current LRIP build
of the Al-41F at Rybinsk. The engine variant has been described as the
Al-41F1 which has engine mounts and tailpipe sized for the envelope of
the existing Al-31F used in Fullback demonstrators and production
Su-27/30.
What is clear at this stage is that Rybinsk/Lyulka now have a
powerplant in the thrust/weight class of the F119 and F135, with an
engine cycle suitable for supercruise, and a physical envelope
compatible with the existing Al-31F engine series. The implications of
this are far reaching.
Supercruising the Flanker
The ongoing budgetary woes of the Russian government present a very
unclear future for the planned fifth generation Flanker follow-on
design, and for the intended light multirole fighter, planned as a
competitor to the Joint Strike Fighter. Both of these designs are
unlikely to materialise as production items until 2020, assuming they
do not disappear down similar budgetary black holes to that seen with
the MFI.
What Russia does have now is a top tier fighter engine which is
competitive against the F119 in performance, and an investment bill
shared between the government, Lyulka/Saturn and Rybinsk, which
requires return on investment.
It is likely that the Sukhoi Su-32/34 Fullback will be exported to
China, and possibly India, but the production numbers of this capable
strike fighter remain unclear. While it offers significant performance
gains over the Su-30MK strike fighter, it is also a new design with
unique avionics, systems and airframe. In effect the greatest
competitor the Su-32/34 has in the market is the existing Su-30MK.
For Russia to recoup the investment made in the development of the
Al-41F, hundreds of engines will need to be exported for hard cash.
Indeed, reports that the Al-41F core is to be used for a commercial fan
indicate that pressure will exist to fund this further development, and
export sales of the Al-31F have to date been at the heart of
Lyulka/Saturn's success.
The clear candidate for high volume exports of the Al-41F near term is
the Sukhoi Flanker. With China now flying around 280 Flankers and
planning at least 380 airframes, India planning at least 180 airframes,
and Russia still operating several hundred, plus some regiments of
newer Su-35, the potential market for retrofit sales is considerable.
What the Al-41F would provide the Flanker with is unmatched
thrust/weight ratio in its class, and stores carriage permitting,
significant gains in supersonic persistence. It is no overstatement to
observe that an Al-41F equipped Flanker will provide F/A-22A class
kinematic performance, even if its supercruise endurance may be lower
due to less refined Mach 1.5 range aerodynamic design and external
stores carriage.
With typical combat weights around 46,000 lb, late model Flankers
equipped with a baseline Al-41F would have a staggering wet static
combat thrust to weight ratio of 1.7:1 or about 60% greater than
figures for evolved third generation Western fighters, or the new Joint
Strike Fighter. Even a derated Al-41F built for durability would
provide a decisive advantage to the Flanker in climb, acceleration and
sustained turn performance.
The principal issue in adapting the Flanker for supercruise will be
reduction of external stores drag. All Flanker variants carry stores on
wing pylons, inlet tunnel stations, and fuselage tunnel stations. A
design devised for the Fullback was a conformal stores pod which was
installed into the fuselage tunnel. The adaptation of this design for
internal or semiconformal carriage of the R-77 and R-73 series missiles
is not an unusually difficult task, although supersonic stores ejection
may present some clearance issues. A payload of six to eight AAMs in
such a conformal stores pod is feasible, with a clean wing, and well
matched to an air superiority role.
The advent of the Al-41F engine on the Flanker is arguably only a
matter of time now. Once this occurs, supercruising Flankers will have
a performance envelope not unlike the F/A-22A, putting them
kinematically well outside the reach of legacy teen series fighters,
the Eurocanards, and the Joint Strike Fighter.
If Australia does commit to the Joint Strike Fighter and or F/A-18E/F
rather than the F-22A, as an F/A-18A replacement, the certainty is now
that it will end up with an aircraft wholly uncompetitive in the brave
new world of supersonic cruise dominated air combat.
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Imagery Sources: RuMoD,
KnAAPO
Line Artwork: © 2005, 2007
Carlo Kopp |
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Artwork, graphic design and text © 2004, 2005, 2006, 2007 Carlo Kopp; Text © 2004, 2005, 2006, 2007 Peter Goon; All
rights reserved. |
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