Subsonic and Supersonic Antiship Missiles: An Hfectiveness and

Utility

Comparison

F

I

G

U

R

E

2.

Ship-La unched Range Requirement

Source:

1990-91

Combat Fleets

of

the World

World

Wide S S M Dep loyme nt Excluding

US.

and SS-N-19, then a range req uirement o f 185 km for our surface- launched generic ASM appe ars reasonable.

THE RANGE TRADEOFF

Havi ng arrived at a range requirement o f 18 5 km for sur- face l aunch , we can proceed to compare the performance of several ASMs. We

w l l

use the classic Breguet equati on (Corning, 1970 and many others) to calculate the maxi- mum range of our candidate ASMs: R

=

~.~(L/D)(V/C)(LOG~O(W~/W~))

Here, L/D is the missile lift-to-drag ratio,

V

is the velocit y i n knots, C is the specific fuel consump tion (SF C) in lbllbihr, and wlIw2 is the ratio

o f

launch weight to weight at fuel exhaustion. We can furthe r define a typical subsonic tur bojet -powered AS M to me et the longest range requirement: LID

=

2.5 V

=

530 knots (Mach 0. 8)

C

=

1.5 l bi l bl hr w l h 2

=

1.13

Substituting in the above equation we obtain

R

=

108

nm

(200

km). T he weight ratio chos en, wlI w2

=

1.13,

is easily obtained in a small, lightweight airframe. Now, let

us

increase ASM speed to Mach

2.0

(V

=

1322 knots). Various propulsion options can be used, but, for our illus- tration, we choos e the nea res t term ca ndi dat e-a li qui d fueled ramjet (an advanced turbojet would give better re- sults, a rocket motor wou ld giv e considera bly worse re- sults). For reasons that

w l l

be discussed later, it is desir- able to fly the ASM mission at low altitude,

i f

possible. At low altitude, the supersonic ASM might have LID

=

0.5

and C

=

3

lbl lb/ hr. I f we ass um e the same w liw 2

a s

the subsonic ASM (e.g. same structural weight, fuel weight, warhead, etc.), then we obtain: R

=

2.3(0.5)(1322/3)(LOG10(1.13))

=

26.8

nm

(50 km) Thus a Mach

2.0

supersonic missil e

i s

onl y twenty-five percent as range effici ent as a sub sonic missile

o f

the same size, weight, and payload. Here then

i s

the supersonic desig ner’s d ilemm a. I f th e loadout con stra ints all ow, the size and weight could be increased and payload (warhead weight) decreased, resulting in a greater wliw2. Super- sonic ramjet designs in the 300-900 kg class can achieve, after boost,

a

wl/w2 of about

1.3.

Using this number, we obtain:

R

=

2.3(0.5)(1322/3)(LOG10(1.3))

=

58

nm

(107

km) I f we w ere c ompletely uncon straine d i n size and weight, we could potentially achieve our goal with an enormous

missile.

This was the tradi tiona l Sovi et approach. The

S S -

N-22, for example, is re po rte d to fl y a lo w altitude, Mach 2.5 trajectory to a range

of

about 110 km. To obtain this performance, the Soviets required a missile 9-10

m

long and weighing 3500 -400 0 kg Simila rly, the air-launched ASM-MSS integral rocket-ramjet shown at M osAero show ’92 achiev es a lo w altitud e range o f 150 km a t Mach

2.1,

but is 9.7 m long and weighs 4,500 kg at launch (Leno- rovitz, 1992). It is therefore apparent that radical tech- nological improvements are needed to meet the range re- quirement at low altitude with supersonic speed. T he alterna tive is to specify a h i- lo w prof il e i.e ., high altitude trajectory for the supersonic ASM employing either a terminal dive on the target or a terminal low altitude run-in as shown in Figure

3 .

The advanta ge here is that LID might increase to 1 .5 whil e keeping C constant. This yields a factor o f th re e incre ase in range (to 320 km) for the design above

in

an “all high” trajectory The loadout constraints will govern the feasibility

o f

meeting range re quirem ents with a giv en trajectory I f it mus t be launchabl e from ce rtain ships, the AS M migh t be limited to five to

six

m ete rs in length. We cannot cover al l

F

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3.

Typical Supersonic and Subsonic ASM Traj ectorie s to Achieve Range Requirement

NAVAL ENGINEERS JOURNAL

January

1997