Commander Mcllvaine is a submarine officer assigned to U.S. Joint Forces Command. He enters the PCO pipeline for USS OHIO (GOW) SSBN 726 this January.
A common refrain among professionals, be it sports or warfare, is that execution of fundamentals is the key to success. The Submarine Force can better answer the needs of the warfighting CINCs by the application of this concept to the arena of sea control, and specifically sea denial, through the development of a half stow length torpedo.
Sea control has been recognized since the time of Mahan as a critical warfare capability. Given the amount of goods that are delivered over the oceans, and more importantly from a warfighting perspective, the importance to logistics delivery that the seas represent, sea control is critical. Conversely, sea denial is a critical warfare capability. We should not assume that we can control the sea near an enemy’s shores, nor should we assume that we would be able to control the airspace over the enemy sea lanes. There exist means for an enemy to limit our ability to operate close in, particularly if we are dealing with a country with a large number of ports and a large coastline. If we truly need to stop the enemy’s ability to move warfighting material over the oceans, the ability to execute the sea denial mission becomes critical. Given that this is a critical warfighting capability, the ability of our Submarine Force to operate for extended times in areas fills this need.
Submarine warfighting endurance is a function of several items in the submarine mission profile. These include length of transit, time on station, and the number of weapons that a submarine torpedo room holds. Many discussions as to the right number of submarines have taken place over the last several years. These include the QDR, the Joint Staff study, and by the leadership of the submarine community. Over the long term, the only way the CINC’s requirements will be met is to increase the build rate of attack submarines. In the mean time, can the current force of submarines improve the level of support provided to the warfighting CINCs.
It is clear that in a sea denial role, the limiting component for the Submarine Force is going to be magazine size. For the U.S. Submarine Force, ship fuel endurance is obviously not an issue, and with the ability of the ships to support themselves with food stores for over 60 days, it becomes clear that limiting component in a wartime environment will be ordnance endurance. By improving the ordnance endurance of our current submarines, wartime submarine requirement shortfalls can be addressed.
The Submarine Force can provide more bang per platform by developing a torpedo that is one half the length of the current Mk 48 torpedo, allowing two torpedoes per stow. The design criteria of the Sturgeon class submarine torpedo room was to be able to fit two Mk 37 torpedoes in each stow. The Seawolf class submarines basically identified torpedo room size as the requirement, and the rest of the ship was built around it. The Submarine Force can effectively double the torpedo weapons loadout of Los Angeles and Virginia class ships through the development of a half length torpedo.
Would it make tactical sense to build a modern half length torpedo in order to provide more weapons for submarine skippers? This can be resolved through analysis of the targets submarines are expected to counter and the tactical environment in which those weapons would be employed.
The Mk 48 torpedo was initially designed to go against a submarine threat that could be detected, tracked, and classified at significant ranges. Our ability to successfully classify targets as submarines allowed us to target without requiring a visual identification-which obviously is challenging against a submerged submarine! Additionally, the titanium double hulled submarines of Russia which were the design target required a large warhead (650 pounds) to ensure a platform kill. An analysis of targets in the sea denial role allows you to design a weapon with granted much reduced capabilities, but still providing enough capability to give the submarine CO the flexibility in ordnance to greatly increase the effectiveness of his torpedo room.
Targeting is a process that has identifiable steps-a common mantra in the Submarine Force is detect, track, and classify-these each being components of the process of targeting another platforms. It is wasteful in terms of ordnance volume to have a weapon whose range is much longer than you can target. Conversely, it clearly increases risk to have a weapon that requires the submarine to approach more closely than is necessary to identify the target. It should be noted, however, that this is precisely the situation that American submarines fought under in World War II. Is it realistic to assume that U.S. submarines will be give carte blanche to attack surface targets without identification?
This will depend on whether or not we can positively classify the target without visual identification. If so, then shooting a torpedo at a target you can’t see is acceptable, and range at launch is not driven by a rules of engagement requirement. There still is some benefit to be gained by a half length torpedo, though, since the submarine skipper can make employment choices in order to improve his torpedo room utilization.
A far more likely scenario is that submarine skippers will need to perform an identification of the target. This is particularly true for non-warship surface targets. Submarines by their nature and our methods of water space management are likely to be able to be identified as enemy based on the fact that we detect them. If rules of engagement require a visual identification for surface targets, as is likely the case, then the limiting component of the submarine skippers attack profile is not weapon endurance but rather range and height of eye concerns to support an acceptable visual identification. The skipper does not have to be able to identify necessarily what it is, but should be able to say what it is not. Simple height of eye calculations for submarines and masthead height implies that even large merchants will require range closure to 20,000 yards as a m1mmum. Based on real world experience, this is probably conservative. Even if a visual identification is required, we need to be sure that submarines are not placed at risk by closing range too far.
Again-we must return to the threat. As a counter to submarine based sea denial, the enemy with ASW assets is likely to resort to convoys in order to concentrate those assets. This improves his probability of detection, but conversely allows the submarine CO to probably forego visual identification, since the act of convoying can be reasonably assumed to reflect a warfighting basis. The use of a longer range weapon to target the enemy’s ships in this instance is warranted.
So if we go back to first principles and ask what it is we try to target, in the sea denial role it is clear that a half length torpedo would give submarine skippers the advantage of increased warfighting endurance by simply having more weapons, but only if the weapon capabilities match the target set. What would be the minimum required capabilities of this torpedo? To answer that effectively, determine what specific targets the weapon will attack. Since weapons load out will become the limiting item for the submarine CO, the CO will not want to employ a larger weapon than required, if he has a choice.
Target: Merchant Vessels
This is not the limiting target set by any means. Merchants probably can’t detect the incoming weapon. They essentially alert on explosion. Typical merchants have a top speed of about 20 knots, and typical cruising speeds of 17 knots. Depending on the size of the ship, sinking may take more than one shot. Given the limited ability of merchants to perform basic damage control, this is unlikely. The worst case scenario for this target set would be a stem aspect target at high speed. A more likely scenario is a beam target at high speed. This assumes that the submarine will be able to detect, track, and classify the target prior to it driving to a stem aspect. Even if this is not the case, the speed and endurance capabilities of U.S. submarines will let the CO control range and aspect.
Target: Diesel submarines
This is a more limiting target. To be conservative, we must assume diesel submarines probably will detect the incoming weapon. Worst case assumption in this case is an alert on launch. If diesel is at speed, it is probably detectable, trackable, and U.S. skipper can work to control aspect or simply use a Mk 48. The diesel submarine as a target warrants it. The worst case scenario for diesel submarines is driven not by target geometry, but by ability to detect the target submarine. The worst case assumption in that situation is a dead in the water target. The appropriate assumption then is that the target has a dead astern aspect, and will start opening range immediately. Open source data for Kilo Class submarines gives their top speed as 17 knots, and the Type 212/214 submarines a top speed of 20 knots.
This is also a limiting target, and it should be realized that while a half length torpedo might not be a one shot kill, the ability to have more weapons is advantageous. As the Russians say about nuclear weapons “Quantity has its own quality.” That said, if presented with the opportunity for a one shot kill against a threat warship, most CO’s would probably use the Mk 48, so exclude these from analysis. Even so, it is safe to say from recent real world examples that the 300 pound warhead of a half length torpedo would still provide at least a mission kill against most warships.
Given the above targets sets of merchants and diesel submarines, what are acceptable minimum performance parameters needed? The characteristics that are critical are speed, warhead, endurance, and brains.
To provide adequate range closure and provide room for growth in top end diesel submarine speed, a 30 knot torpedo should be sufficient. If we reduced the available power by one half, we do not get a fifty percent reduction in speed. Basic fluid dynamics would estimate a loss of thirty percent of top end speed. Reducing the engine size and power output may also allow you to reduce the level of noise generated by the torpedo, reducing chances for target alertment. It may be that the power plant volume cannot be reduced by one half. While this area is clearly the one where the experts need to weigh in, it is worth exploring.
Warhead size can be reduced by approximately one half. Open source information puts the Mk 48 torpedo warhead at 650 pounds. Based on pictures of the effects of a Mk 48 torpedo (http:www.navy.gov.au/3 _photo/sinkexvid.htm) and recent real world experiences, three hundred pounds detonated beneath the keel should be enough for at least a mission kill. One hundred pounds, as in the Mk 46 and the cancelled Mk 50 torpedoes, is probably not enough. Six hundred and fifty pounds is overkill for most of the targets that this weapon would be targeted against. Three hundred pounds should be more than enough for a mission kill even against warships. It would likely result in a hull kill against any merchant or diesel submarine. Reducing warhead size lets you reduce torpedo length proportionately.
Required endurance for the weapon is tied directly to top end speed. A faster torpedo will not have to run as far. If the top speed of the torpedo is less then the top speed of the target, unless the target alerts too late, no amount of endurance will result in a hit. The following statements are just time I speed math based derivations. 20,000 yard endurance was selected in order to allow the submarine CO to ensure that he can see the length of run of his torpedo. The following assumptions are then needed:
1. A target takes about three minutes to reach top speed.
2. The torpedo reaches top speed in thirty seconds.
3. Weapon impacts on first intercept (no reattack required)
Given those assumptions:
- A 30 knot torpedo can chase down a stern aspect 20 knot diesel submarine from DIW in under 20,000 yards, assuming the target alerts on launch with a launch range of 7,000 yards.
- A 30 knot torpedo can chase down a stern aspect 25 knot diesel submarine from DIW in under 20,000 yards assuming the target alerts on launch, though the range at launch is prohibitive at 3,900 yards.
- A 30 knot torpedo with 20,000 yard endurance can intercept a 17 knot beam aspect merchant with a range at launch of 16,000 yards.
- A 30 knot torpedo with 20,000 yard endurance can intercept a 17 knot stem aspect merchant with a range at launch of 8,500 yards.
In each of these cases, if the CO can safely launch from a closer range or better control aspect of the target at launch, the weapon has more fuel left at intercept and will have more fuel remaining for reattack.
Keep in mind that detection range may be very limited for DIW diesel submarines. In this case the benefit of the half length torpedo is that much clearer, since limited endurance becomes less limiting, and having more weapons becomes more attractive. This is reinforced by the experience of the British Royal Navy in the Falklands war, when they experienced high ASW weapon usage rates. The half length torpedo is probably not an effective weapon against another country’s nuclear submarines, since the torpedo has little if any speed advantage. It could be effectively used as a break contact counter fire weapon, however.
As far as the brains of the weapon are concerned, there may or may not be much to be gained here. Some finite amount of volume will be required for the sensors, and while Moore’s law may reduce the volume of the brains behind the sensors, unless that volume is large to start with it does not result in much gain.
Can it be done? Should it be done?
The answer to the first question is probably best left to torpedo designers and engineers, although the above analysis indicates it is possible. Should it be done is a much harder question to answer. Torpedo shooters surely desire the added number of weapons that a half length torpedo allows, provided they can be tactically effective. Most attack submarines on deployment today carry far more Tomahawks than torpedoes, because that is what they shoot. In a major theater war, torpedoes will become more important. Given the large usage rate of ASW employment experienced by the British Navy in the Falkland’s war, the advantages of the half length torpedo become even more clear. The development of this weapon would compete with other programs within the Navy and the Department of Defense, each of which is seen as vitally important to somebody. It may be that this simply does not make the cut. Finally, this could be used as basic platform for modular UUV development. A careful consideration of utility will have to be done if UUV’s start to take away from precious weapons stows, panicularly when we are called on to fight. If we cannot stow two UUV’s per stow, they will quickly limit our ability to carry weapons of war to the fight. Even the limited number of torpedoes currently carried could be doubled by a half length torpedo.
The engineering challenges are not mind boggling-the scaling of the power plant and the sensors and brains of the weapon are probably the most difficult issues. This is an effective way to improve the warfighting endurance of American submarines.