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In the event of a major war, it is generally assumed that substantial numbers of Soviet SSBNs, general-purpose submarines and surface combatants will be committed to hiding-in or defending ocean areas adjacent to the USSR — under the cover of land-based air and surface warships. In light of the intense threats against U.S. surface forces operating near the periphery of the USSR at the start of war, the burden of initiating an early offensive against Soviet naval power is likely to fall to u.s. and allied SSNs.

However, in conducting a campaign against one or more high-priority classes of naval targets deployed in or near Soviet home waters, U.S. SSNs also must face and overcome an ASW threat of uncertain strength. In conducting a general offensive against all classes of Soviet submarines, u.s. SSNs must avoid or escape prosecution by mines, other supporting Soviet submarines, surface combatants and sea-and landbased ASW aircraft, cued by overhead and underwater surveillance systems. In a focused offensive against Soviet SSBNs, U.S. SSNs might properly regard all other Soviet submarines as secondary targets and part of the defense.

The purpose of this article is to quantitatively explore certain aspects of a submarine campaign in defended waters and briefly consider their planning implications — so as to raise some issues and stimulate discussion of this complex and important problem.

The exchange ratio is defined as the expected number of enemy submarines destroyed per SSN lost in fighting an unlimited number of engagements of a specified type. It is an important measure of the combat potential of an SSN. However, in fighting a number of enemy submarines protected by defending forces, a certain number of SSNs might be lost in encounters with the defense. These unproductive losses of SSNs to the defense mean that the attrition of target submarines will be less than that predicted, for sub vs exchange ratios.

Consequently, when fighting an undersea campaign in defended waters, the effective exchange ratio — will be less than the exchange ratio, absent defenses, as usually defined. How much less will depend upon the strength of the defense.

The unique feature of a submarine campaign in the presence of a continuously acting ASW defense is that as the campaign proceeds and target submarines are found and destroyed, the density of targets in the theater will decline. As a result, the time between engagements with target submarines will tend to increase, on average. Since the continuously acting defense has a longer time to work between target engagements, the probability of an SSN encounter with some element of the defense, instead of a target submarine, will steadily rise throughout the course of the campaign.

This increase in the relative strength of the defense is most pronounced after a substantial fraction of primary targets has been destroyed. Indeed, in the limiting case in which all target submarines have been destroyed, surviving SSNs that are unaware oi’ the status of the campaign can only encounter elements of the defense.

Simply stated, each unit in a composite ASW defense can engage and destroy a searching SSN at a certain rate, characteristic of the interaction between that unit and an.SSN. The sum of these lethal rates of engagement, from all ASW units participating in the defense, sets the overall rate at which an SSN will be destroyed by the defense.

Similarly, a searching SSN will engage its primary submarine targets at a certain rate that is proportional to the number of such targets present in the theater. Hence, the rate of engagement with target submarines will fall during the course of a campaign as they are found and destroyed.

At any time during the campaign, the relative rate of engagement between the defense and target equals the odds that an SSN will next engage an element of the defense, instead of a primary submarine target. Hence, the probability of a lethal encounter with the defense, versus a target submarine, provides a useful measure of the strength of the enemy defense.

If the composition of the enemy’s defense remains constant throughout the campaign, then the odds of encountering the defense, rather than a target submarine, must increase as the campaign proceeds, while the enemy submarine population declines.

The aim in the rest of this article is to suggest answers to the following questions about an SSN campaign in defended waters — where there is an assumed SSN exchange in the absence of defense and an initial probability of a lethal encounter with the defense at the start of a campaign:

  1. Compared to the SSN exchange ratio in the absence of a defense, what is the effective exchange ratio for the entire campaign and what degree of attrition can X SSNs expect to inflict on Y target submarines — protected by ASW defenses of different strengths?
  2. Is campaign effectiveness influenced more by the exchange ratio or by the SSN’s ability to avoid the ASW defenses?
  3. Is it advantageous to overcommit a larger force of SSNs and then withdraw them all as soon as a predetermined number of SSNs have been lost?
  4. What advantage is gained by localization of target submarines in the pre-war period and then attacking these at the outbreak of war, before the ASW defense can have an effect?


These questions require quantitative answers, derived fronJ an undersea campaign that allows for the possibility of SSN attrition by a continuously acting defense.

A series of tables follow which show the predictions for a submarine campaign in defended waters, where the u.s. submarines enjoy a 5:1 exchange ratio. Specifically, the primary targets for a u.s. campaign are assumed to be 40 Soviet SSBNs deployed in Arctic seas. In this focused campaign, all other Soviet submarines are regarded as units of the defense — to be avoided — in addition to mines, surface combatants, sea-and land-based ASW aircraft and surveillance systems, in various combinations. The lethal encounters with the defense at the start of the campaign are then varied while u.s. losses are limited to a specific number of SSNs.

Question 1: Effective Exchange Ratio

As a base case, suppose that 10 BLUE SSNs with tbe exchange ratio of 5:1, hunt for 40 RED unprotected SSBNs. Table 1 summarizes the expected results of this campaign.

View full article for table data

On average, the foroe of 10 SSNs can destroy all ~0 SSBNs, for the price of 8 SSNs.

Now suppose that RED defends the operations area with various types of ASW systems. The net effect is to raise the probability of a lethal SSN encounter with the defense — .05 at the start of the campaign. Instead of zero, 1-chance-in-20 is used, the expected results of this campaign can be shown. See Table 2.

View full article for table data

Table 2 shows that, by confronting SSNs with a modest initial risk of a lethal encounter with the defense, RED can noticeably improve his effectiveness. Instead of losing all 40 SSBNs and destroying 8 SSNs, the addition of a low level.of defense preserves 9 SSBNs, destroys all 10 SSNs -4 by the defense — and reduces the exchange ratio from 5:1 in the absense of defense to an effective value of 3.1:1.

Although the chances of an encounter with a continuously acting defense might seem small at the start of a campaign, thi~ initial risk steadily grows as target SSBNs are destroyed. The cumulative effect of this increasing risk of an encounter with the defense is the reason for the, perhaps surprising, effectiveness of a seemingly low level of defense. This effect can be seen more easily by raising the strength of the defense.

Table 3 shows the results of campaigns by 10 and 20 SSNs, where the chances of engaging the defense, rather than an SSBN, are 1-in-10 at the start of the campaign.

View full article for table data

Against this stronger defense, 10 SSNs are able to destroy only 25 of 40 SSBNs (as compared to 31 of 40 SSBNs in the case of Table 2) — for an effective exchange ratio of 2.5:1. This is half the exchange ratio when there is no defense. If 20 SSNs are committed to this campaign, then 36 of 40 SSBNs could be destroyed, but at an effective exchange ratio of 1.8:1.

Table 3 also reveals that by using 10 more SSNs, 11 more SSBNs are killed but 10 more SSNs are lost. Consequently, against moderately strong defenses, the cost of attempting to destroy a large fraction of an enemy force is likely to be high.

Finally, Table 4 shows the results of again doubling the probability of a defense encounter at the start of the campaign from 1-chance-in-10 to 1-chance-in-5.

View full article for table data

At this level of defense, the effective exchange ratios have fallen well below 2:1 and the exchange ratios for incr·ements of force beyond the first 10 SSNs are 1:1 or less.

Question 2 : Defense Avoidance

The expected numbers of SSBN’s destroyed in a campaign in defended waters can be increased by improving either : (1) the SSN’s ability to avoid or survive attacks by enemy ASW systems or, (2) by improving the exchange ratjo against SSBNs. But it becomes readily apparent that defense avoidance is far preferable to improving exchange ratios against undefended targets. This is particularly true for “closely” protected SSBNs — and even more so if the weapons employed are not covert. If the SSN exchange ratio was improved 50~ to 7.5:1 (under Table 4 conditions) a calculation would show that 10 SSNs could destroy 19 SSBNs -only two more than for a 5:1 exchange, and only an 11% improvement.

However, a 50~ improvement in defense avoidance — by going from the .20% probability of loss to defenses of Table 4 to the .10% probability in Table 3 shows 8 more SSBNa destroyed for a 47% improvement in campaign effectiveness.

Thus, although the SSN exchange ratjo should be the best attainable, capabilities for defense avoidance have a higher priority when fighting in defended waters. It might also be true that there are more technical and operational possibilitie~ for improving SSN capabilities for defense avoidance of defenses as compared to increasing the exchange ratio. However, many relevant SSN subsystems contribute to both aspects of SSN performance.

Question 3: Overcommit and Withdraw

This question has an easy, and negative answer.

The larger the number of SSNs committed, the higher will be the force-wide rate or engagement with target SSBNs, but the rate at which the larger force of SSNs will encounter the defense will also increase in the same proportion. Thus, as shown by the Tables, an attack by 20 SSNs, followed by a withdrawal after the loss of 10 SSNs, would achieve the same result as an attack by 10 SSNs alone, but in a shorter period of time which might be an important consideration.

One additional option promises to increase the effectiveness of a campaign in  defended waters.

Question 4: Fast Start

It some number of target SSBNs can be localized or acquired and promptly attacked at the start of war, then the SSN force can gain an initial advantage before the ASW defense has an opportunity to be effective. In essence, a fast start enables the SSN force to destroy some initial number or SSBNs with maximum efficiency -and in a short time. In a parallel campaign started with no SSBNs vulnerable, after the destruction or the same number or SSBNs as in the initial case, the size of the SSN force will be smaller, because of its exposure to the defense.

However, against a weak defense the number or SSNs saved will not be so large as to make an appreciable difference in final campaign outcomes. On the other hand, against a strong defense the advantage of localization is greater. Fast Start will save a useful number or SSNs in the beginning. ~tit in continuing the campaign against a strong defense, these extra SSNs will exchange for SSBNs at generally less ratios, offsetting the initial advantage of the fast start.

Of course, the stronger the defense and the larger the number of SSBNs acquired, the greater the advantage or a fast start. Nevertheless, for force commitments likely to be of practical interest, this advantage does not appear to be great. This also means that the penalty against a fast start, in order to minimize the risk of compromising the mission by exposing SSNs to counter-detection in the pre-war period, should be small.

If SSBNs could be completely hidden from searching SSNs after war starts, a fast start becomes the only possible option for eventual attack.


When fighting in defended waters, SSNs should avoid the defense. However, in defended waters even seemingly small degradations in the SSN’s ability to avoid the defense translate into noticeable reductions in campaign effectiveness.

For instance, a 5:1 exchange ratio in the absence of defense might be reduced by oneMhalf, if the initial chances of encountering the defense, instead of a primary target, increase from zero to 1-in-10. And, the larger the number of SSNs committed, the greater the reduction in the exchange ratio. Such reductions in the exchange ratio must influence campaign planning by changing the relation between mission benefits and costs.

It is also difficult to overcome the effect of a modest level of defense by increasing the number of SSNs committed to a campaign.

The analysis also suggests that relatively small forces of SSNs, operating against weak or strong defenses, cannot substantially improve campaign effectiveness by a fast start against SSBNs. Viewed positively, this suggests that the SSN force need not be vulnerable in the preMwar period, for a small price in campaign effectiveness.

Unfortunately, the effect of SSN target classification capabilities on campaign effectiveness could not be examined. However, all measures that reduce erronious decisions which might reject valid targets or result in engaging elements of the defense, have significant value. Of course, peacetime estimates of exchange ratios, defense strengths and other relevant factor·s are uncertain at best, for both sides. Hence the attrition suffered by each side still would be known only probabilistjcally. Initially favorable situations could turn sour and unfavorable situations unexpectedly turn sweet.

Fighting an undersea campaign in defended waters js shrouded in uncertainties that should challenge SSN force planning at the levels of strategy, operations and tactics for a long tj n.e to come. Sound insight into the nature of such operations is a prerequisj te for· effective force development and employment plans.

Henry Young

Naval Submarine League

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