naval engineers journal volume 109 issue 1 1997 j. f. mceachron -- subsonic and supersonic antiship...

ABSTRACT The paper reviews the advantages and disadvantages of supersonic speed for the antiship missile application. Specifically, the supersonic speed benefits of reduced defensive reaction time and relaxed navigation accuracy are contrasted against limitations of range, payload, cost, electronic counter counter-measures performance, and signature. The paper concludes that, for the immediate future, subsonic advantages are likely to continue to outweigh the two major supersonic benefits. This view is reflected in the current lack of development being applied to supersonic antiship missiles.

Subsonic and Supersonic Antis h i p M i ssi les: An Effectiveness and Utility Comparison

Introduction

ver the last twenty years, maritime forces around the world have deployed a wide variety of antiship missiles (ASMs). These missiles have periodically incorporated new technologies in order to main- tain an effectiveness advantage over evolving defensive systems.

Interest is growing in the application of new propulsion technologies for efficient supersonic ASMs in the 1990s and beyond. It is therefore appropriate and timely to evaluate the overall effectiveness and utility payoff for supersonic ASM speed.

With the exception of several Soviet missiles and dual purpose surface-to-air missiles (SAMs) such as Standard SM-1/2, current ASMs fly subsonic trajectories which enable maximum standoff range w i t h size and weight constraints and give adequate time for guidance processing to maximize target acquisition. Studies of advanced antiship missiles employing either subsonic or supersonic flight profiles have been performed by missile manufacturers, to examine the relative merit of these two flight regimes for the antiship mission. This paper will compare the effectiveness of subsonic and supersonic antiship missiles and present a rationale for selection between the two.

Requirements Before designing a missile to perform a mission, requirements for that mission are organized in a systems engineering flowdown matrix. An example of such a flowdown is shown in Figure 1. Here, the requirements in each performance area are related to some top-level effectiveness criterion:

Effectiveness Criterion-The overall performance goal for the ASM system. For example, we might specify that the ASM must be able to achieve a 0.5 probability of disabling the surface-to-surface missile capability (SSM kill) on a given target moving at 30 knots with a single shot from 60 nm range. Loadout-This is a requirement for how many ASMs must be carried on which launch platforms and be compatible with which launch equipment. The loadout requirements set a number of constraints on missile design, such as maximum length, diameter, and weight. Availability-This defines the percentage of time that the ASM system will be ready to launch when needed. It is another family of constraints that may be driven by factors such as depot maintenance schedule, logistics, storage/ pre-launch reliability, ease of repair, and response time during combat. Launch Platform Survivability-Survival of the launch platfonin is essential and will set the ASM standoff range requirement. In-Flight Reliability (Pr)-The probability that the ASM will function properly after launch. Target Acquisition (Pacq)-The probability that the ASM will acquire the intended target.

N A V A L E N G I N E E R S J O U R N A L January 1997 57

Subsonic and Supersonic Antiship Missiles: An Effectiveness and W i t y Comparison

Anti-Ship Missile 1 Effectiveness Criteria. 1

j.lllllilllj..II.I)Yl El Survivabilit

+Example: Hit 30

F I G U R E 1. System Engineering Performance Flowdown

rn Missile Survivability (Ps)-The probability that the ASM, given target acquisition, will survive the target hard kill defenses and reach the target with its ordnance section intact. Probability of Hit (Ph)-This is the probability that the ASM, having acquired the target and evaded defenses, will actually impact the target structure. For the purposes of this discussion, survivability against electronic countermeasures is treated as part of the probability of hit.

rn Lethality (Pk)-This is the probability that the ASM, having hit the target, will achieve the desired level of damage. If we treat the loadout and availability as constraints and

design our ASM to fly far enough to ensure platform sur-

Abbreviations and Acronyms Used in This Paper AOA = Angle of attack AOU = Area of uncertainty AP = Missile presented area ASM ClWS = Close-in weapons system EMCON = Emission control IR = Infrared LID = Lift-to-drag ratio mr = milliradian OTH = Over-the-horizon Pacq Ph Pk

Pkss Pr = Missile in-flight reliability Ps RCS = Radar cross section RF = Radio frequency SAM = Surface-to-air missile (system) SFC = Specific fuel consumption sr SSM

STOT I*m = micron

= Antiship missiles (used to describe own-forces)

= Probability of target acquisition by ASM = Probability of an ASM impacting the target = Probability of kill (ASM killing ship or SAM killing

= Single shot probability of kill

= Probability of survival (vs hard kill defenses)

ASM)

= Steradian (a measure of solid angle) = Surface-to-surface missile (used to denote a threat

= Simultaneous time on target ASM)

vivability, then we can relate the rest of the performance factors to our top-level requirement by

Effectiveness = Pr x Pacq x Ps x Ph x Pk

where Pr is the in-flight reliability, Pacq is the probability of acquisition, Ph is the probability of hit, Pk is the probability of kill, and effectiveness is the top-level requirement (0.5 probability of SSM kill in the example above). The important thing to note from this is the ASM must offer balanced performance in order to be effective: reliability and probability of hit are just as important as survivability and lethality. Two additional program (as opposed to performance) requirements, cost and schedule, must also be considered when designing an ASM.

In the following sections, generic subsonic and supersonic antiship missiles will be compared on the basis of the following nine requirements areas:

Launch Platform Survivability Loadout CostlSchedule Target Acquisition Missile Survivability Terminal Hit Probability Lethality Availability Reliability

The Design Realities: Weight, Size, and Cost LAUNCH PLATFORM SURVIVABILITY- THE RANGE REQUIREMENT A key driver in the subsonic vs. supersonic comparison is the range requirement. Our generic ASM must, as a minimum, provide enough range to allow the launch platform to remain outside the envelope of target ship defenses. For a surface platform, the range requirement might be set by the need to stand off from threat surface-to-surface missiles (SSMs-we will use this nomenclature for the threat antiship missiles to avoid confusion with the generic ASM we are designing). For an airborne platform, the range requirement might be set by the need to stand off from SAMs or possibly carrier-based air defense intercep- tors. This paper will focus on the surface-launch mission.

SSMs which potentially could threaten surface platforms have ranges of 10 km to more than 450 km. The SSMs which have the longest ranges, such as Tomahawk Antiship Missile (TASM), SS-N-12, and SS-N-19, are only carried by the largest combatants of the U.S. and Russian navies. There were over 6,000 SSMs deployed on surface combatants in 1994, not counting those in the U.S. Navy. Figure 2 shows the cumulative number of SSMs as a function of SSM range (Prezelin, 1993 and Freidman, 1991). If we design to standoff from all but the SS-N-18

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Subsonic and Supersonic Antiship Missiles: An Hfectiveness and Utility Comparison

F I G U R E 2. Ship-Launched Range Requirement

Source: 1990-91 Combat Fleets of the World World Wide SSM Deployment Excluding US.

and SS-N-19, then a range requirement of 185 km for our surface-launched generic ASM appears reasonable.

THE RANGE TRADEOFF Having arrived at a range requirement of 185 km for surface launch, we can proceed to compare the performance of several ASMs. We will use the classic Breguet equation (Corning, 1970 and many others) to calculate the maximum 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

velocity in knots, C is the specific fuel consumption (SFC) in lbllbihr, and wlIw2 is the ratio of launch weight to weight at fuel exhaustion. We can further define a typical subsonic turbojet-powered ASM to meet the longest range requirement:

LID = 2.5 V = 530 knots (Mach 0.8) C = 1.5 lbilblhr w l h 2 = 1.13

Substituting in the above equation we obtain R = 108 nm (200 km). The weight ratio chosen, wlIw2 = 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 illustration, we choose the nearest term candidate-a liquid fueled ramjet (an advanced turbojet would give better results, a rocket motor would give considerably worse results). For reasons that will be discussed later, it is desirable to fly the ASM mission at low altitude, if possible. At low altitude, the supersonic ASM might have LID = 0.5 and C = 3 lbllb/hr. If we assume the same wliw2 as 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 missile is only twenty-five percent as range efficient as a subsonic missile of the same size, weight, and payload. Here then is the supersonic designers dilemma. If the loadout constraints allow, 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)

If we were completely unconstrained in size and weight, we could potentially achieve our goal with an enormous missile. This was the traditional Soviet approach. The SS- N-22, for example, is reported to fly a low 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 3500-4000 kg! Similarly, the air-launched ASM-MSS integral rocket-ramjet shown at MosAeroshow 92 achieves a low altitude range of 150 km at 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 technological improvements are needed to meet the range requirement at low altitude with supersonic speed.

The alternative is to specify a hi-low profile 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 advantage here is that LID might increase to 1.5 while keeping C constant. This yields a factor of three increase in range (to 320 km) for the design above in an all high trajectory

The loadout constraints will govern the feasibility of meeting range requirements with a given trajectory If it must be launchable from certain ships, the ASM might be limited to five to six meters in length. We cannot cover all

F I G U R E 3. Typical Supersonic and Subsonic ASM Trajectories to Achieve Range Requirement

N A V A L E N G I N E E R S J O U R N A L January 1997 59

Subsonic and Supersonic Antiship Missiles: An Effectiveness and Utility Comparison

possible loadout constraints here, but it should be clear that loadout is a major design driver in any comparison between subsonic and supersonic ASMs.

THE COST/SCHEDULE TRADEOFF Two additional factors which should be considered are cost and schedule. We have seen that a supersonic ASM is likely to be substantially larger and heavier than a subsonic ASM. For missile systems of similar technology and complexity, unit cost can be estimated as a function of weight. Knowing the weight ratio (wllw2) above does not directly give us the weight-both large and small missiles could conceivably have the same wllw2. However, we can get an idea of the weight differences by comparing typical surface launched point designs:

Subsonic Design-680 kg Supersonic Design #1-930 kg Supersonic Design #2-1520 kg

These weights include the required rocket boosters. Even though the supersonic designs are considerably heavier, the subsonic design still offers superior low altitude range.

A valid cost comparison as a function of weight requires the missiles to incorporate the same kind of functionality (e.g. seeker and computer, but no data link) and to have similar subsystem technology Comparing a subsonic turbojet-powered missile to a supersonic ramjet-powered missile is not strictly appropriate (because of different functionality), but can be done to a first order approxi- mation. Based on extensive industry experience, the cost of missiles with similar functionality and technology can be scaled by the difference in weight raised to the 0.76 power. Using this relationship, we predict the following difference in cost:

Design Normalized Weight Normalized Cost Subsonic 1.0 1.0 Supersonic #1 1.37 1.27 Supersonic #2 2.2 1.82

These numbers are only approximate and apply to point designs, but they suggest that a medium range supersonic ASM is going to cost twenty-seven to eighty-two percent more than a comparable subsonic ASM. This assumes the two missile types are in the same stage of production maturity

With the exception of several Soviet missiles, all of which have high weight and large volume, no supersonic ASMs are in production today If our comparison is between a relatively mature existing or modified subsonic missile and a new supersonic design, then substantial additional differences in cost arise due to production maturity For example, let us assume, based on the data above, that the supersonic design is fifty percent more costly than the subsonic design for similar maturity At some future production date, say 1998, the first supersonic missile is

rolled off the assembly line. At the same time, the 2,000th subsonic missile is rolled off a competing assembly lie. By using a learning curve methodology, we can estimate the cost and schedule tradeoffs to expect.

In aerospace applications, manufacturing improvement curves (Gibson, 1981 and many others) are often used to determine expected recurring costs over a production run. The average cost of a production run of X missiles is given bY

cost = (Z)(X)-

where Z is the cost of the first production unit and n = 0.2 (equivalent to a typical eighty-seven percent learning curve). The kind of unit cost we see today for a generic 2,000th subsonic turbojet-powered ASM is about $1 million for production rates of several hundred units per year (production rates of fifty or less will add substantial fixed overhead cost to the numbers shown here). We can therefore expect that the 2,000th supersonic ASM will cost $1.5 million. Using the equation above, we estimate thz first unit cost for the supersonic ASM to be $6.9 Million. The first 100 missiles will have an average cost of $2.7 million, the second 100 missiles-$2.1 million, and so forth. Thus, if we want an ASM in 1998, we will likely have to pay twice as much for a new supersonic design as an existinglmodified subsonic design. If we can slip our schedule until 2000 supersonic ASMs had been produced, then we might buy one for $1.5 million.

In comparing supersonic to subsonic ASMs, the basic design realities may be summarized as follows: 1. For a given range, flight profile, and payload, a super-

sonic ASM will be significantly larger and heavier than a subsonic ASM.

2. Supersonic ASMs of reasonable sizelweight must fly a high altitude trajectory to meet the 185 km standoff range requirement.

3. Supersonic ASMs will be significantly more expensive than subsonic ASMs, especially if the subsonic ASMs are further down the production learning curve.

With these design realities in mind, we should now ask the question, Why would we want a supersonic ASM? Two areas of system effectiveness are often evaluated to answer this question: target acquisition and survivability.

Target Acquisition An autonomous ASM must be able to fly out to the expected target location and find the target w i t h an area of uncertainty (AOU) around the aimpoint. The size of the AOU is determined by three primary factors: (1) random motion of the target during ASM flyout; (2) ASM navigational errors incurred during flyout; (3) targeting errors, including target motion between time of measurement and time of launch (time late).

The target motion contribution to AOU can be modeled by assuming a target moving away from the aimpoint in a

60 January 1997 NAVAL E N G I N E E R S J O U R N A L

Subson ic and Supersonic Antiship Missiles: An Effectiveness and Utility Comparison

random direction. This gives a circular AOU with radius equal to the flight time of the ASM multiplied by the speed of the target. It is easy to see that supersonic speed will decrease this contribution to the AOU sigmficantly when compared to subsonic vehicles. For example, if a 30 knot target is attacked from a range of 110 km, a Mach 0.8 vehicle will have a flight time of 408 seconds, while a Mach 2.0 vehicle will have a flight time of 163 seconds. This results in a maximum cross range error of 6.3 km and 2.5 km for subsonic and supersonic, respectively. Errors in downrange will also occur, but the potential for the ASM to search in a bearing only mode makes downrange errors less important for the purposes of the illustration here.

In the absence of GPS or other position update, the navigational errors for an autonomous ASM are also a function of time of flight, and, for a given type of midcourse guidance system, supersonic speed will reduce this contribution to AOU as well. This is a statistically quantified error considered to be random and normally distributed. A low grade midcourse guidanceinavigation system might have a one sigma error buildup of 35 km per hour in cross range. For the flight times above, the subsonic vehicle would have a navigational cross range error of 4 km. The supersonic vehicle would have an error of only 1.6 km.

The targeting error is the difference between the target's true location and the ASM aimpoint at launch. This error is also statistically quantified with both a bias and a normally distributed random component. It is the most difficult part of AOU to quantify. For engagements within the horizon, sensors on the launch platform can typically locate the target with minimal errors. In over-the-horizon (OTH) engagements, the errors can be many kilometers in both bias and standard deviation: a one sigma standard deviation of 3.7 km is not unrealistic. Supersonic speed does nothing to compensate for targeting error.

How sigmficant are these errors relative to the comparison of subsonic and supersonic ASMs? The answer is that it depends upon seeker detection range and scan characteristics. This can be best illustrated by postulating the characteristics of two ASM radio frequency (RF) missile seekers which differ only in their power output. We will use the radar range equation (Frieden, 1985 and many others) to show the resulting difference in detection range and the significance of that range for accommodating re- alistic AOUs (we will ignore low altitude and sea surface effects for this illustration). The seeker range versus a one square meter target is given by:

R = (A/B)**0.25 where A = (Pt)(Gt)(Gr)(Lt)(Lr)(A**2)

and

B = (7.94E - 8)(S/N)(Br)(NF - 1)

Pt is the peak power (watts), Gt is the transmitter gain, Gr is the receiver gain, Lt is the transmitter losses, Lr is the receiver losses, X is the wavelength, SIN is the signal-

to-noise ratio required for detection, Br is the receiver noise bandwidth (MHz), and NF is the noise factor. The characteristics shown below may be assumed for a generic Ku band pulse radar seeker:

Pt = 5,000 watts (seeker A) or 40,000 watts (seeker B)

Gt = Gr = 31 dB, Lt = - 2 dB, Lr = -7 dB, A = 0.02 m (15 Ghz)

Br = 2 MHZ (lipulse width, where pulse width = 0.5 microsec)

SIN = 15 dB, and NF = 10 dB

Substituting into the range equation above, we obtain

R = 3.0 km (seeker A) and R = 5.2 km (seeker B) vs 1 square meter. A typical corvette-sized target might have a radar cross section of 1,000 square meters. Against this target, R(A) = 11.9 km and R(B) = 20.6 km.

If we assume a 30 knot target, a one sigma midcourse guidance error of 35 Whr, and a one sigma targeting error of 3.7 km, we can calculate the growth in AOU size as a function of range and speed. Since the AOU size has a statistical nature, we choose two sigma errors such that ninety-five percent of the potential cases are enclosed in our AOU. Figure 4 shows the result. The cross-range extent at ASM launch is Z 4 km due to targeting and grows to over 19 km at 185 km range for the subsonic missile. The supersonic missile's corresponding errors only grow to 13.5 km at the same range.

Our generic seekers can detect a 1,000 square meter target at crossranges of 0.71 of R(A) and R(B) given above. Figure 5 shows the seeker detection capability overlaid on the AOU size graph. Seeker A can cover the

18 - 16 - 1 4 -

E

0 20 40 MI MI 100 120 140 160 180 200 Target Range - Km

F I G U R E 4. ASM Single Pass Target Acquisition Requirements

2 Sigma NavigationlTargeting Errors 30 Knot Random Target Motion

NAVAL ENGINEERS JOURNAL January 1997 61


24 I / I -

cross Ran- Coverage - Seeker B -

-

- CmssRange

Search Allnude

*-185 Km Range Requirement

I I

4 -

2 -

0 0 20 40 60 80 100 120 140 160 180 200 220 240 260

Range - Km

F I G U R E 5. ASM Seeker Search Capabilities

1,000 mz Target

subsonic missiles AOU out to 72 km and the supersonic missiles AOU to 135 km. If our technology cannot provide a seeker better than A, then we clearly need supersonic speed to provide one-pass target acquisition at long ranges. Seeker B, however, can cover the subsonic missiles AOU out to 190 km, beyond the standoff range requirements computed in previous sections of this paper. If we can build seeker B, then supersonic speed is unnecessary strictly for the purposes of aiding target acquisition.

Survivability Survivability is one performance area which is considered a key advantage for a supersonic ASM when compared to a subsonic ASM. Increased speed reduces the reaction time available to a defender and allows greater terminal maneuverability In the following paragraphs, we will address the survivability tradeoffs between subsonic and supersonic flight for the antiship mission against defenses consisting of radar guided SAMs, infrared guided SAMs, and point defense guns.

RADAR GUIDED SAMs Radar guided SAMs provide the longest range protection of surface ships and are typically associated with sophis- ticated fire control systems mounted on larger combatants. A notional engagement sequence is shown in Figure 6. The ASM is first detected and identified as a potential target by an acquisition radar. This radar transfers or hands-off the ASM position to a fire-control radar which often performs the missile guidance or illumination function. Once the fire-control radar has the ASM in track, a

SAM is launched and is guided to the target by command from the radar or by a semi-active seeker in the SAM homing on an illumination signal from the radar. The SAM fuze senses the proximity of or hits the target and deto- nates the SAM warhead. Significant reaction time delays are incurred between acquisition-track, and track-and launch. In addition, all SAMs have a minimum range inside of which they cannot reliably intercept a target. Obviously, the detection range of the SAM radars, reaction times, dead zone distance, SAM flyout velocity, accuracy, fuzing capability, and warhead size all affect the ultimate effectiveness of the SAM. Supersonic speed compresses the defense timeline, potentially resulting in fewer SAM intercept opportunities.

A detailed discussion of the capabilities of present and projected ASM threat defense systems is beyond the scope of this paper, but some important tradeoffs can be illustrated using a hypothetical RF-guided SAM system. We may define the system to be as follows:

Acquisition radar range = 100 nm vs 1 square meter target

Track radar range = 50 nm vs 1 square meter target

Time Delays = 10 or 20 seconds acquisition to track 10 or 20 seconds track to SAM launch

Sufficient to reach edge of dead zone in 6 seconds, Mach 3 average over longer ranges 5 meters miss distance (one sigma) 18 meters

Dead Zone = 2 nm SAM velocity =

Accuracy =

Radar antenna height =

Figure 7 illustrates some observations which can im- mediately be made. The radar horizon against an ASM flying at an altitude of 5 meters is 26.7 km. This is the ships first detection opportunity even if the ASM was launched at the example standoff range requirement of 185 km. If the ASM flies a high altitude trajectory, as our ramjet-powered supersonic missile must do to achieve the standoff range, then first detection could take place soon after launch at ranges exceeding 160 km. Our hypothetical SAM could achieve four shoot-look-shoot opportunities against a Mach 2 ASM on this trajectory prior to the ASM reaching the SAM dead zone.

While the probability of kill (Pk) of each SAM intercept is complicated to estimate, even a small Pk, when com- pounded by multiple intercepts, lowers the overall ASM probability of survival significantly Figure 8 shows the compounding effect of multiple intercepts. Flying faster, say Mach 3, will reduce but not eliminate intercept oppor- tunitie s .


Subsonic and Supersonic Anliship Missiles: An Effecliveness and Ulilily Comparison

I Track I Delay

I Subsonic C

Handover I - Delay I

SAM Min. Range :

(Limited by (Limited by : EW Handover Horizon or ASM Delay or ASM RCS)

RCS)

I I I I I I I I I I ! I Target Ship Downrange

EW Detect

Track Track Handover Delay Delay

ASM Early

Warning Radar /

(Limited by EW Handover Horizon or ASM Delay or ASM RCS)

RCS) I I I I I I I I I I I I 1

Target Ship Downrange

F I G U R E 6.

RF Guided SAMs

SubsonidSupersonic Engagement Scenario

One solution to this problem is to reduce the radar cross-section (RCS) of the ASM enough to completely avoid intercepts. The tradeoff between speed and signature required to completely deny intercept is shown in Figure 9. The requirements for defeating either the acquisition radar or the track radar are given for the twenty second reaction time and improved ten second reaction times. Defeating the acquisition radar is not a reliable means of defeating the SAM because even if the missile were totally invisible at acquisition frequencies, the seeker emissions would likely be detected by electronic support measures (ESM) soon after turn-on. The tracking radar function, in contrast, is required to prosecute the intercept.

To defeat the trackmg radar of our hypothetical SAM, the Mach 2 ASM would have to have an RCS of -27 dBsm (decibels relative to 1 square meter). If the reaction time was 10 seconds, the requirement drops to -34 dBsm (for comparison, the median I-band RCS of a typical missile-like shape is about 0 dBsm, a pigeon is* - 20 dBsm and a locust is between - 30 and - 40 dBsm [Nathanson, 19911). The requirements for a subsonic ASM are less than - 40 dBsm. Thus, high altitude defense penetration

requires extreme stealth, extreme speed, or some combination of the two.

The speed and RCS requirements to deny intercept at low altitude, shown in Figure 10, are somewhat less severe. Since detection is limited to the horizon, no matter how high the RCS, there is an ASM speed above which SAM intercepts are completely denied, regardless of signature. For our hypothetical SAM with baseline twenty second time delays, an ASM flying from over the horizon directly to the defending ship at greater than Mach 1.5 would never be intercepted. Here, supersonic speed shows a clear advantage. Unfortunately, the requirement is very sensitive to the assumed reaction times. An improved SAM system with ten second reaction time would force the ASM speed requirement above Mach 2.6. Again, we see that to achieve high confidence of survival based on intercept denial, extremely high speed, low RCS, or a combination is needed.

Having discussed this tradeoff, it is important to rec- ognize that intercept denial is not the only criterion for setting survivability requirements. Another criterion which could be used is low SAM single-shot probability of kill (Pkss) given intercept. The Pkss of semi-active RF

NAVAL E N G I N E E R S J O U R N A L January 1997 63

Subsonic and Supersonic Antiship Missiles: An EIIectiveness and Utility Comparison

1 .o

- 0.6

5 1

0.6 .- - E n a' 0.4 a3 .- L - 2 5 0.2 0

0.0

of ASM Kill (Pkss)

I I I I I I I I I 0 2 4 6 8 10

Number of SAM Intercepts

F I G U R E 8. Multiple SAM Intercepts Substantially Reduce Survivability

Any Significant Pkss Will Make It Impossible To Meet The Effectiveness Requirement

100,000

10,000

2 5 1,000 E

Q

1 Q U a = 100 ? .I-

10

0

SAMs against low altitude targets is inherently limited by the capability of the SAM seekers to detect and track in clutter. The Pkss may also be degraded by maneuver- induced large miss distance or by low altitude-induced fuze pre-function on the sea-surface. Quantifying the first two effects is beyond the scope of this paper, but we will examine the fuze pre-function effect in more detail.

Most radar guided SAMs use radio frequency (RF) fuzes which operate like a tiny radar. These fuzes do not work well at very low altitudes because they have difficulty in discriminating between the real ASM target and the sea surface. The fuze pre-function geometry is shown in Fig- ure 11. The ASM flies a fixed altitude above the sea surface and is intercepted by a SAM with a normal miss distance distribution. The SAM RF fuze beam extends outward at an angle. The fuze is triggered when a "target" is detected inside the absolute cutoff range. If this target is really the sea-surface, then the SAM warhead is deto- nated before reachmg the ASM.

Using this geometry and a hypothetical fuze, the probability of fuzing on the ASM can be determined. We select the following representative parameters for our example:

-

0 - First Potential Detection

Subsonic Low ~. -,-** ,--""Altitude Trajectory

180 200 0 20 40 60 80 100 120 140 160

Range From Target - Km

F I G U R E Z 0 - First Potential Detection

Detection Scenario-RF Guided SAMs

64 January 1997 N A V A L E N G I N E E R S J O U R N A L


Fuze cone half-angle = SAM miss distance = SAM dive angle = Fuze range cutoff =

Figure 12 shows the resulting probability of fuzing on the ASM. In our example, ASMs flying at altitudes less than 5 meters nearly always defeat the SAM fuze. The fuze operates reliably against ASMs flying substantially higher than true sea-skimming. Translating this fuzing example into Pkss is a complex process, but for our purposes here we may assume that fuze function means a kill and pre-function means no kill. If we are generous in assuming that supersonic ASMs can fly as low as subsonic ASMs, then flight altitudes less than 5 meters result in high survivability against radar mided (and fuzed) SAMs regardless of speed.

60 degrees 5 meters (1 sigma) 10 degrees 20 meters

INFRARED GUIDED SAMs Infrared (IR) guided SAMs are deployed on small combatants and are much shorter range than radar guided SAMs. They typically employ electro optical fuzes which function at low altitude. The small detection range of an optical fuze is acceptable on an IR guided SAM because of inherently higher terminal accuracy than RF guided SAMs. An IR SAM system is cued by a long range sensor on the defending ship, and is physically pointed toward the incoming ASM to attempt seeker lock-on. The SAM is launched when the received signal from the target be- comes large enough. The IR guided SAM has a dead zone, but it is much smaller than for RF guided SAMs.

The survivability of subsonic and supersonic ASMs against IR guided SAMs is driven by Merences in ASM signature. Figure 13 shows the variation of IR radiant

Time Delays

-1 0

-20

n

u) E rn ;E? -30 cn 0 U

-40

-50 0 1 2 3 4 5 6

ASM Speed (Mach Number)

F I G U R E 9. ASM SpeedlRCS Requirements for High Altitude Attack

Intercept Denial Criterion



+

- Mach

-1 0

-20

E m 'El -30 cn 0 U

UJ

1

-40

-50

Horizon - Limits

Mach 2.6

1.5

F I G U R E 10. ASM SpeedlRCS Requirement for Low Altitude Attack

Intercept Denial Criterion Horizon Defined by 60 m Radar Antenna and 5m ASM Attitude

I Miss Distance Distribution

Range Cufofl

Sea Surface

F I G U R E 11. Low Altitude SAM Fuzing Geometry

RF Fuze Causes Predetonation on Sea Surface

intensity in the 3-5 micron band as a function of speed for a generic 38 cm diameter missile-shaped metal body at low altitude. The 3-5 micron operating band is representative of common IR seeker designs. A subsonic missile at a nominal 4-5 degrees angle of attack (AOA) has an IR signature of 0.5-0.7 wattskteradian (this number is most applicable to a turbojet powered missile-a subsonic rocket powered missile has a much higher signature due to the rocket plume). By comparison, a Mach 2 missile at a nominal 1 degree AOA has a signature of 20 wattsisr. If the supersonic missile were to employ high-g maneuvers to aid survivability, then AOA would increase, leading to increased IR signature. For example, in order to pull a 10 g sustained maneuver requires 8 degrees AOA, and/or the required lift, thus increasing the missile presented area and the corresponding IR signature to 50 wattsisr.


Subsonic and Supersonic Antiship Missiles: An Effectiveness and Utirity Comparison

1 .o

z 2 0.8 .- F

2

0

2 0.6 u.

8 0.4 6

2

C .- 1 0 0.2 LL

0.0 0 5 10 15 20 25 30

ASM Cruise Altitude - meters

F I G U R E 12. SAM Fuzing Limits SAM P, In Low Altitude ASM Intercepts

Fuze Range Cut Off = 20 rn SAM Dive Angle = lo" SAM Miss Distance (1 Sigma) = 5 rn Fuze Cone Half Angle = 60"

We can relate these signatures to survivability by com- puting the seeker lock-on range. This is given by solving for R in

J = (H)(S/N)(R2)/(T) and T = e-aR

where J is the contrast signature in wattsisr, H is the seeker sensitivity in wattsicm', SAV is the signal-to-noise ratio required for lock-on, R is the range in meters, T is the atmospheric transmission (a non-dimensional fraction), and a is the extinction coefficient. The contrast signature is obtained by subtracting the background radiance from the target signal received at the seeker. Calculating this background can be very complex as it involves time of dax sun position relative to target and sensor, sensor liie- of-sight (e.g. target against sky or sea background), sea state, cloud cover, and a host of other factors. For our example here, we will use data from Wolf and Zissis (1985) for daylight and ocean background (sea-skimmer ASMs will be seen against ocean background). By multiplying the background radiance area by the presented area of the ASM (say 26 cm in diameter and 457 cm long at 5 degrees AOA), we obtain a background radiance of about 0.5 wattslsr.

We can now define the other parameters in the lock-on equation above:

H = 1E-12 watts/cm2 S/N = 10 a = -0.000345 m-l

The seeker sensitivity shown is representative of an advanced design. The extinction coefficient-a-corre- sponds to an attenuation of 1.5 dBkm (the weather in the North Atlantic allows 1.5 dB/km or less 90% of the time on a yearly average basis). We also define the kinematic limitations of a generic IR guided SAM:

10,000

5 1,000 2 o! 9?

B g 10

d

.- U

100 B I

m U .-

1

0.1 I I I I I I 0 30 60 90 120 150 180

Vehicle Aspect Angle - deg

F I G U R E 13.

Generic Missile Shape in Low Altitude Flight Body Aero Heating and Hot Nozzle 3-5 krn Band

Estimated ASM IR Signatures

Minimum range (dead zone) = Time to flyout to edge of dead zone = Time from lock-on to missile away =

Substituting into the equation above, we obtain the IR seeker lock-on range as a function of contrast signature. The contrast signature for the subsonic turbojet-powered missile is near zero, since the background radiance is about the same as the target radiance. Even if the background is ignored, the lock-on range would only be 1.9 km. The Mach 2 missile, with its 19.5 wlsr contrast signature (20 wlsr from Figure 13 less 0.5 wlsr background radiance), gives the IR seeker a lock-on range of 5.5 km. If the supersonic missile is maneuvering (contrast signature = 49.5 wisr), lock-on range would be 6.8 km.

As with RF guided SAMs, there is a relationship between the ASM IR signature and speed required to deny an intercept opportunity to the IR SAM. This relationship, shown in Figure 14, can be derived from the IR seeker lock-on equation and the SAM kinematic characteristics given above. Under these assumptions, a Mach 2 ASM must have an IR signature less than 10 wisr to avoid intercept. Since the Mach 2 ASM's signature is likely to be 20-50 w/sr, it could be intercepted by the SAM. A Mach 0.8 ASM must have a signature less than 1 wlsr to avoid intercept and typically has a lower nose-on signature than that even if background effects are ignored. Thus, a subsonic ASM is more survivable against an IR SAM threat than a supersonic ASM.

A side issue related to infrared detection is long range cueing. We assumed in the analysis above that the IR SAM was cued to the target bearing by a scanning RF or IR sensor. The IR sensor, because it is passive, can operate

500 meters 2 seconds 4 seconds



0.01 0

F I G U R E 14. ASM SpeedllR Signature Requirements Intercept Denial Hypothetical IR Guided SAM

under EMCON (EMission CONtrol) doctrine when radars are not available. A supersonic missile flying a high altitude trajectory (necessary to achieve the range requirement), with its inherently high IR signature, will be detected at long range. The turbojet-powered subsonic missile will not be detected until seconds before impact. The high IR signature of supersonic missiles is a significant liability in the antiship mission which requires additional technology development to remedy

POINT DEFENSE GUN SYSTEMS The point defense gun system consists of a trackmg device or combination of devices (e.g. radar, IR, TV), a fire- control computer, and a high rate-of-fire cannon. The tracking device provides accurate target position (and sometimes velocity) to the fire-control computer, which points the gun. The projectiles may range in size from 20 mm to 40 mm and destroy the target antiship missile by direct hit, or, in the case of larger projectiles, by proximity detonation. Some gun systems have an autonomous acquisition capability while others rely on external ship sensors to provide the initial cueing of the trackmg sensor.

The survivability of ASMs against point defense guns is primarily affected by three factors: (a) the tracking accuracy of the gun pointing sensor, (b) the ballistic dispersion of the projectiles, and (c) the vulnerability of the ASM to a projectile given a hit. It is beyond the scope of this paper to address proximity fuzed projectiles and medium caliber guns (57 mm, 76 mm and larger may have some anti-ASM capability) in any detail-proximity fuzed projectiles have some of the limitations of proximity fuzed SAMs and medium caliber hit-to-kill guns generally do not have high enough rates-of-fire to obtain high confidence of a hit. If we h i t our discussion to small caliber (20-40 mm) guns with hit-to-kill projectiles, we can capture the driving factors in the well-known Carlton damage equation. This equation expresses the probability of any one projectile hitting a target as a function of the mean tracking system error at the target, the ballistic dispersion, and the presented area of the target.

We can define a generic gun system as follows:

Rate-of-fire = 6000 rounds/min Mean tracking system error = 2 milliradians (one

sigma)

68 January 1997 NAVAL ENGINEERS JOURNAL

Subsonic and Supersonic Antiship Missiles: An Hfecfiveness and Utility Comparison

Ballistic dispersion =

Open-fire range = 2500 meters Minimum range = 140 meters

We also define a generic ASM which is 36 cm in diameter and 457 cm long. The subsonic ASM cruises at 5 degrees AOA while a Mach 2 ASM cruises at 1 degree AOA. Assuming the ASM is flying directly at the gun system, the subsonic variant will have a presented area (Ap) of 0.23 square meters while the supersonic variant will have Ap of 0.13 square meters. Thus, if the two variants are identical in size as we have assumed here, then the supersonic missile has an immediate advantage of having only sixty percent of the subsonic Ap simply due to AOA (however, as we saw in our earlier discussion, for the same standoff range, the supersonic variant is inevit- ably larger).

Figure 15 shows the result of a simplistic gun analysis using the Carlton function. The Mach 0.8 missile would be expected to receive one hit by 800 meters range-to-go and eight hits into minimum range. The Mach 2 missile would receive one hit by 250 meters and two hits into minimum range. The supersonic missile has better performance because its speed limits the number of projectiles encountered.

Maneuvers might be used to attempt to further decrease the number of hits. A maneuver potentially decreases the pointing accuracy of the gun by changing the ASM position too rapidly for the gun fire-control computer to predict ahead. An aerodynamic maneuver, however, will increase the AOA and Ap and actually decrease survivability in cases where the maneuver is not heavily degrading the gun pointing error. The figure shows, for example, the

2 milliradians (one sigma)

8

7 - Mach 0.8 No Maneuver

6

e Ineffective Mach 2 109 Maneuver

P but No Increase in Gun System z 4 0

I 3

a Mach 2.0 No Maneuver

1

0 0 1000 2 m

z 4 0

I 3

a

Ineffective Mach 2 109 Maneuver but No Increase in Gun System Error

, Mach 2.0 No Maneuver - \ 1

0 0 1000 2 m

Range-to-Go - meters

F I G U R E 15. Supersonic ASM Speed Decreases Number of Gun Hits

2 mr Gun System Error 2 mr Ballistic Dispersion 6000 Rdh in Generic Gun System

effect of a Mach 2, 10 g maneuver on expected hits assuming the mean pointing error is unaffected. The result is more than double the number of expected hits. The maneuver must degrade the pointing error to above 3 milliradians just to recover the loss in survivability due to increased Ap. Whether this is possible or not depends on the sophistication of the gun fire-control algorithms and the timing of the maneuver.

The number of hits does not necessarily translate directly into survivability. Figure 16 shows the probability of gun kill (Pk) against our Mach 0.8 and Mach 2 generic ASMs assuming that one hit is a kill. The Pk against the subsonic missile reaches a high level at longer range than for the supersonic missile, but both are stiU killed. The one shot kill criterion may be more applicable to the supersonic missile than the subsonic for several reasons. First, the higher speed results in a higher projectile impact velocity, which ensures penetration of even armored warhead cases and coupling of more energy into the target. A 1000 meter/sec projectile, for example, impacts the Mach 2 vehicle with 78% more energy than the Mach 0.8 vehicle. Also, the Mach 2 air loads are six times as high as at Mach 0.8, resulting in greater vulnerability to projectile- induced aerodynamic damage.

The figure shows that a Mach 0.8 vehicle which could survive four hits would be equal in survivability with a Mach 2 vehicle which is vulnerable to a single hit. It should be remembered that ASM mission success is achieved by delivering an intact ordnance section to the target, regardless of what damage might have been taken by the rest of the missile in the process.

No matter how capable a defensive gun system may be, existing and projected types can only engage one target at a time. A Mach 0.8 ASM traverses the engagement zone postulated here in 9 seconds, the Mach 2 ASM in 3.6 seconds. The gun will need a substantial fraction of

I 1.0, ,

3000

I Range-lo-Go -meters F I G U R E 16. ASM Vulnerability Differences Affect Speed vs. Survivability Comparison

2 mr Gun System Error 2 mr Ballastic Dispersion 6000 Rd/Min Generic Gun System



this time to affect a single kill. In addition to the time required to engage each ASM in a multi-missile attack, the gun system takes time to switch targets-slew to the new bearing, track, lock-on, and fire-control solution. Two or more ASMs arriving at nearly the same time will vir- tually guarantee that at least one will penetrate the defense. The supersonic missile offers better performance in this regard, but both subsonic and supersonic will be effective.

SUMMARY We may summarize the survivability discussion with the following observations: 1.

2.

3.

4.

5.

6.

- High altitude defense penetration requires either extreme stealth, extreme speed or a combination of the two. Very low altitude flight results in high survivability against RF guided SAMs regardless of ASM speed. A subsonic ASM will likely have higher survivability than the supersonic ASM against IR guided SAMs. The high IR signature of supersonic ASMs allows targets to be cued at long ranges during EMCON. Accurately directed point defense guns can defeat both subsonic and supersonic ASMs. Salvoes of subsonic and supersonic ASMs can both defeat point defense guns using simultaneous time-on- target.

The "Other" Factors Four additional system engineering performance areas should be considered in any comparison of subsonic and supersonic ASMs: (1) availability, (2) in-flight reliability, (3) terminal hit probability, and (4) lethality Differences between hypothetical ASMs in these areas are more difficult to quantify than survivability, but some observations can be made.

Availability as discussed here is the fraction of time an ASM would be ready to launch when needed. For two missiles of equal technological sophistication, the supersonic variant will be larger, heavier, have more partslexotic materials, and incorporate systems more resistant to high temperatures and g loads than a subsonic variant. If the supersonic variant is ramjet powered, as we have postulated in this paper, then a very large booster or multiple staged boosters are required to accelerate the missile to ramjet takeover speed. Careful design is required to produce a supersonic ASM which has comparable availability to a subsonic ASM. Reduced availability translates into reduced war-fighting capability or increased inventory requirements and depot costs.

In-flight reliability can also be related to the complexity of the missile system. A supersonic ASM will probably be more complex and thus inherently less reliable than a subsonic ASM. Acceleration, temperature, and vibration en-

vironments will also be more severe for the supersonic ASM. Reduced reliability must be weighed against potential advantages in other performance areas to determine if this liability for a supersonic missile can be tolerated.

The probability of hit (Ph) is driven by the ability of the ASM guidance system to find the true target in a countermeasures environment and to deliver the missile warhead to the target. Missile characteristics such as turning radius, seeker response to target signature transients, trajectory, and countermeasures processing capability all contribute to Ph. The turning radius of an air vehicle can be approximated by

R = V I A

where R is the turning radius in meters, V is the ASM speed in meters per second, and A is the acceleration in meterslsec'. A Mach 0.8 ASM which could pull 3 gs (29.4 m/sec2) would have a turning radius of 2500 meters. Supersonic ASMs generally can produce lateral high accelerations of 10-15 gs. However, for a Mach 2 ASM to have the same turning radius as a subsonic ASM, it must be able to pull almost 19 gs. This does not include any additional control margin that may be desirable to allow terminal evasive maneuvers. If substantially less maneuver capability is provided, Ph may be degraded because the missile will not be able to compensate for last-second seeker inputs due to either target signature transients or late target acquisition in countermeasures.

The general nature of this paper does not allow a detailed comparison of supersonic and subsonic ASM Ph in countermeasures, nor is such a comparison easy to make. We can, however, speak broadly about two classes of effects: (1) those effects which are driven by the speed of the ASM guidance computer, and (2) those effects which are driven by the time required for observable phenomena or critical engagement geometries to develop. The first class of effects should not be an issue in the comparison of supersonic and subsonic ASMs. An increase in speed from Mach 0.8 to Mach 2 is only a factor of 2.5. At the time of the writing of this paper, available processor speeds are doubling every eighteen months. Any new ASM (or ASM guidance set) could easily incorporate a processor which performs computations orders of magni- tude faster than anything currently deployed.

The second class of effects, discussed by Schleher (1986) and others, can potentially limit the capability of a supersonic ASM. A seduction chaff scenario is an example of such an effect. Figure 17 illustrates one possible engagement. Here, the target ship launches a single chaff cloud a short distance in a random direction. Depending on wind directiodspeed, target speed, and ASM range- to-go at chaff launch, the engagement may involve three possible cases from the ASM seeker's point of view: (1) The chaff cloud may be near the target-but resolvable by the seeker; (2) near the target-but unresolvable; or (3) completely out of the seeker field of view. The scenario


Subsonic and Supersonic Antiship Missiles: An //ectiveness and Utility Comparison

Time

Generic Anti-Ship T O Missile (ASM)

a \,

Chaff Cloud

Random Wind Direction Random Chaff Launch Direction

Time

T 1

YSM

Chaff Obscures Target Ship

F I G U R E 17. Seduction Chaff Scenario

may evolve from one of these cases to another over time. If the ASM can find the true target in one geometrx but not another, then supersonic speed may force the ASM to commit in an unfavorable situation where subsonic speed would allow time to wait.

This scenario can be modeled with a time-step computer program if a few simplifying assumptions can be made. We can describe the chaff as a circular cloud (ignore the vertical dimension) of radar scatterers with density varying from center to edge. The cloud is allowed to move and distort with the wind. A target ship can be described as a series of scatterers distributed along a he . For this example, the ASM is represented by a range-gated centroid seeker moving toward the target at subsonic or supersonic speed. The approach direction, target speedi orientation, wind speedldirection, chaff launch timeidirec- tion, and the RCS of the ship target and chaff cloud are all variables which affect the outcome of the engagement. We assume the chaff i s dispersed close to the target ship.

The seeker receives a radar return from both target and chaff (actually only from the part of the cloud and target inside the rangeiangle resolution of the seeker).

Time

T 2

Chaff Outside Seeker Field Of View

1 .o

0.8

- - 5 0.6 .g - a $ 0.4 P

0.2

0.0 , 0 5000 10000 I5000 20000 25000

ASM Range-to-Go at Chaff Launch (Meters)

F I G U R E 18. Scenario

ASMs With Generic Centroid Seekers RCS Ration (Chaff/Ship) = 10 Random Wind and Chaff Launch Direction

Effect of ASM Speed in Seduction Chaff



The seeker guides toward the centroid of the two signals. At some point in the ASM trajectory, the chaff and target will likely be more than one seeker beamwidth apart. The seeker will then settle on either the target or the chaff alone, depending on the engagement conditions.

Using this modeling approach, we can demonstrate a difference in probability of hit due to ASM speed. Figure 18 shows the probability of hit for a subsonic and a supersonic ASM with generic centroid type seekers as a function of chaff launch range. We use a chaff RCS ten times larger than the ship RCS. Although neither missile is par- ticularly effective under these assumptions, the extent of chaff launch ranges over which the ASM can be defeated is larger for the Mach 2 missile. If we assume that seeker turn-on occurs at 20 km and that chaff launch occurs within twenty seconds of reception of the ASM seeker transmission, then the ASM range to go would be about 14.5 km for the Mach 0.8 missile and 6.4 km for the Mach 2 missile. The Mach 0.8 missile allows enough time for the chaff to clear the target-resulting in Ph of over 0.9.

The Mach 2 missile in the same situation has a Ph of less than 0.5. A supersonic ASM must have countermeasure signal collection and processing which does not have to wait for interpretable geometries or phenomena.

Missile lethality, the probability that a certain level of damage will result from a hit, is the final factor in the ASM effectiveness equation. It is not significantly affected by the speed of the ASM. The Mach 2 ASM would offer potentially greater capability to penetrate armored structures, but structures sufficiently robust to degrade subsonic ASMs are not commonly deployed on modern combatants. However, as pointed out above, for a given loadout and range requirement, the subsonic ASM will almost always carry a larger warhead than the supersonic ASM.

Summary and Conclusions In the discussion of requirements at the beginning of this paper, we presented an effectiveness equation consisting of the terms in-flight reliability, probability of acquisition,

Effectiveness Factor

Availability

In-Flight Reliability

Probability of Acquisition 30 Knot Targets High Speed Targets (>40kn)

Probability of Survival RF SAMs RF SAMs W/Advanced Fuze IR SAMs Point Defense Guns

Clear Environment ECM Environment

Probability of Hit

Warhead Lethality (Probability of Damage Given a Hit)

cost

Overall

ASM Tvpe Subsonic

1++1 1++1

+ -

1+1 1+1

&

+

1++1 El

&I

- . Supersonic

+ Acceptable ++ Good 0 Preferred System

* Must Fly High to Meet Range Requirements

F I G U R E 19. Comparison of ASM Effectiveness Factors

Basis: Equal Loadout and Meet Range Requirement


Subsonic and Supersonic Antiship Missiles: An Elfectiveness and Ulility Comparison

probability of survival, probability of hit, and probability of damage given a hit. The equation relates each of these terms to a top-level performance requirement with the condition that platform survivability, availability, and loadout constraints have been met. The constraints address the standoff range required and how large (and complex) the ASM can be.

If we assume the constraints have been met, we could evaluate the effectiveness equation for subsonic and supersonic ASMs. Figure 19 presents a qualitative comparison of effectiveness factors. The subsonic ASM has higher availability and in-flight reliability, but the supersonic ASM is still acceptable. The probability of acquisition is really a tradeoff between ASM range, speed, and seeker range. In our example above, seeker B could cover the Mach 0.8 AOU (30 knot target) out to 190 km range. A seeker with Bs capability would give equal Pacq for subsonic and supersonic ASMs. Beyond 190 km, supersonic speed would be needed for one-pass acquisition. Thus, supersonic speed impacts Pacq for very long range ASMs and for very fast targets (although a subsonic missile could still acquire a very fast target at some reduced range).

The probability of survival is obviously dependent on what threats are present, but if the defendmg ship has RF guided SAMs and IR guided SAMs, then the subsonic ASM has an advantage: The subsonic and supersonic probability of survival is equal for low flying ASMs against RF SAMs-but the supersonic ASM must fly at least part of its trajectory at a higher, more vulnerable altitude in order to meet the range requirement. The supersonic ASM can clearly be shot down by an IR SAM. Survivability against point defense guns favors the supersonic ASM: The supersonic design has more potential to defeat a CIWS without STOT

The supersonic ASM has performance advantages which can greatly increase effectiveness over the subsonic ASM if balanced technologies are applied to overcome existing problems. To realize these advantages, a number of technologies must be carefully applied. First, a supersonic turbojet engine would make up much of the range deficiencies of the ramjet. Second, low signature technology in both RF and IR bands would be needed. The IR signature deficiency is of particular concern. The supersonic ASM must be engineered for higher seeker data rates and for tracking the target while maneuvering at very high lateral accelerations. It must have countermeasure processing with response time commensurate with supersonic speed. Subsystems must offer availability and reliability comparable to those in subsonic ASMs. Finally, the

cost must be managed such that sufficient numbers of missiles can be bought to address all the threats.

The subsonic ASM has a potential advantage in probability of hit due to complications arising in countermeasures environments. Warhead lethality (probability of damage given a hit) is generally going to be greater for subsonic ASMs simply because a larger warhead can be carried for a given loadout and range capability. Lastly, the subsonic ASM will always offer a considerable cost advantage. We have not precisely quantified each of these factors, but qualitatively the subsonic ASM offers higher overall utility for the next ten to fifteen years. This is especially true when cost and schedule considerations are taken into account.

ACKNOWLEDGMENT The author is indebted to Mr. Tom Reidy for his assistance in modeling the fuze and countermeasure examples cov- ered. + REFERENCES [l.] Prezelin, Bernard and A.D. Baker 111 (ed. 1, Combat

Fleets of the World 1993, Naval Institute Press, 1993. [2.] Corning, Gerald, Supersonic and Subsonic Airplane Design,

3rd Edition, Braun-Brumfield, Inc., 1970, (p. 2:40). [3.] Freidman, Norman, Naval Institute Guide to World Naval

Weapon Systems 1991-92, Naval Institute Press, 1991. [4.] Lenorovitz, Jeffrey M., Large Antiship Missile Powered

by RocketlRamjet Has 155 Mile Range, Aviation Week and Sgace Technology, August 24, 1992.

[5.] Gibson, D. C. Jr., Manufacturing Improvement Curves: Concept and Application, McDonnell Douglas Internal White Paper, 1981.

[6.] Frieden, David R. (ed.), Principles of Naval Weapons Sys- tems, Naval Institute Press, 1985, (pp. 71-75).

[7.] Nathanson, E E., Radar Design Principles, 2nd Edition, McGraw-Hill, Inc., 1991, (pp. 171-184).

[8.] Wolfe, WL. and G.J. Zissis (ed.), The Infrared Handbook, Revised Edition, Office of Naval Research, 1985, (p. 3-108).

[9.] Schleher, D. Curtis, Zntroduction to Electronic Warfare, Ar- tech House, 1986, (pp. 193-195).

James F. McEachron is currently the McDonnell Douglas Aerospace (MDA) test and evaluation process team leader for the Joint Air-to-Su&ce Standoff Missile Program. During his 16 years of experience an missile systems product definition, he has served as the operations analysis manager for the Harpoon Program, conducted force structure analysislstrategic planning for MDAs Washington Studies and Analysis Group, and pedormed survivability and effectiveness analyses for the SRAM II Program. Mx McEachron holds a B.S. degree in aerospace engineering from Georgia Institute of Ethnology.

NAVAL E N G I N E E R S JOURNAL January 1997 73

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