It is well known that current U.S. SSNs carry only on the order of twenty or so tube-launched weapons, with the improved 688s carrying up to an additional dozen weapons outside the pressure hull. Today, such a limited magazine capacity, comprising a mixture of Harpoons, Tomahawks, Mark 4& and perhaps other weapons, overtly limits the SSN’s effectiveness.
There are several reasons why a twenty-plus weapon loadout represents a severe handicap. Most important, the missions and effectiveness of the submarine have expanded vastly, and we can anticipate a Commensurate increase in weapon expenditure. For instance, the main mission of submarines in World War II was commerce destruction. At that time, a thirty-plus weapon load was evidently adequate, since few U.S. subs returned to base having expended all their weapons (when they worked).
In comparison to the U.S. fleet sub of fifty years ago, today’s SSN is not only faster and of unlimited endurance, it also has better sensors and fire control, and has access to satellite reconnaissance. Unlike the submarines of fifty years ago, today’s SSNs can find, trail, and attack a convoy at will, and maintain contact for an arbitrary length of time. An SSN attacking a surface action group or convoy might anticipate a running fight lasting days, with the attacking sub firing dozens of weapons. Among these, missiles are likely to be fired in salvos rather than singly, to saturate and confuse the defenses of the surface vessels. Other missions might also demand a heavy expenditure of weapons. Tomahawks are likely to be heavily used when possible in any land conflict where the opponent has a significant anti-air capability and might contest control of his littoral waters; under conditions where a plane and pilot would be seriously at risk, Tomahawks will be employed. Can anyone doubt we would have used hundreds of SLCMs against heavily defended North Vietnamese targets if they had been available?
The Iranians recently reminded the world of the effectiveness of a few (obsolete) mines in the right place, and the value of submarines as a covert dispenser of mines is well known. It would be surprising if any potential naval opponent believed our SSNs could not lay mines. The character of the SSN as a multimission platform is beginning to be appreciated, but its ability to do many things on a single voyage is jeopardized by a limited magazine capacity. Finally, the addition of twelve vertical launch tubes to the Improved 68& and the inclusion of space for a reported 50 weapons in the SEA WOLF design suggests the importance our own Navy attaches to this issue.
Unfortunately, the majority of our submarines will carry only a score of weapons until well into the next century, with little prospect of increase. For surface vessels, their magazine capacity is less of an issue, since with UNREP and VERTREP, they can rapidly transfer ammunition and other supplies, even while underway. For submarines, the story is different: shipping weapons is a ticklish business even in a calm harbor, and foolhardy under any other circumstances. The Soviets occasionally load weapons at sea, but their subs have notably greater freeboard than ours. Thus to reload, or to change weapons for a different mission, our submarines must return to a protected harbor or anchorage, perhaps with a submarine tender. In a future general war, this probably means returning to U.S. bases, since forward, improvised bases will almost certainly come under attack. Moreover, a submarine tender is difficult to hide from a satellite, and one with a few subs nestled alongside makes a soft, inviting target for precision-guided munitions. Yet if weapon expenditure rates are high, SSNs might spend a large portion of their time transiting to and from an operational area. For instance, an SSN (theoretically) tasked with a mission in the Sea of Okhotsk might spend two weeks getting there from Pearl Harbor, a few weeks performing that mission, and two weeks getting back, all the while under threat of attack. Although the mission is accomplished, the SSN is actually carrying the fight to the enemy only a fraction of its time underway. The German U-boats sunk in the Bay of Biscay in WW II en route to their operating areas in the North Atlantic are examples of submarines being interdicted well before they threaten their targets.
Thus our objective is to develop a safe, stealthy method for replenishing SSNs while at sea.
Ideally, the SSN would like to ship weapons while submerged, since it is stealthiest, least vulnerable, and best able to defend itself while submerged. The German WW II method of using “milch cows” for refueling and resupplying while on the surface is clearly unsatisfactory, since the U-boats were helpless when attacked during these evolutions. Underwater supply transfer might be possible if the supply ship were itself an SSN and the weapons were transferred by divers while both submarines were stopped and submerged. The supply submarine and the SSN rendezvous using their precision navigation systems at different depths to avoid collision: for instance, the SSN might be at 100 feet keel depth and the supply ship at 225 feeL The weapons are almost neutrally buoyant (or could be moved in containers that assured this), and at these depths wave effects are negligtble. The weapons are transferred by divers from the supply ship to the SSN through the latter’s torpedo tubes. The SSN’s torpedo tubes would have to be slightly modified for this purpose. The supply ship would probably be a modified Poseidon SSBN with its missile tubes replaced by weapons shipping gear and a decompression chamber. The decompression chamber is required to maximize diver efficiency at the depths required. The evolution would be easiest carried out with the SSN drifting passively, and the supply vessel obliged to maintain station; the supply vessel would require small thrusters, a precision depth keeping system, and a (probably optical) system for discerning the attitude and relative position of the SSN. Naturally, the trim and depth of both vessels will change slightly as weapons are loaded, or as a result of variations in current or salinity, and it will be the responsibility of the supply vessel to maintain a safe horizontal and vertical offset. The evolution is envisioned with the supply vessel at greater depth than the SSN, to minimize the depth change which the divers must undergo; probably about 80 feel However, it might be simpler and safer to maintain a satisfactory vertical offset with the SSN underneath the supply ship, since the SSN will tend to sink as it loads and the supply vessel will tend to rise as it unloads. Clearly the utility of this technique will depend on the speed and safety with which it can be done, which in tum is largely controlled by the ability to safely maintain a small depth offset.
The advantages of this approach are many. First, the entire evolution would be carried out underwater, with the submarines listening passively, and able to defend themselves. The resupply could conceivably be done anywhere, in any weather; underneath the polar ice cap might be a particularly good place. The resupply would be fast, since the weapons need not be shipped one at a time; a converted SSBN could carry however many divers were necessary. We note that while 100 feet is uncomfortably shallow, it is comparable to SSBN launch depths, and there is no signature at the surface; naturally, the rendezvous will be chosen to be away from sea lanes or probable ASW forces. Obviously, other supplies, including food, spare parts, and other expendables could be transferred. The SSN would not have to cut short a deployment or make a long, slow, transit back to a protected base.
Weapons transfer from a modified SSBN-640 class supply submarine to an SSN-637 class attack submarine, approximately to scale. The weapons are extracted by the divers from the hatch( es) at the rear of the SSBN’s missile compartment (cutaway), transferred to the SSN and shipped through the torpedo tubes. Note the decompression chamber also installed in the SSBN’s missile compartment.