Breaching by Line Charge Analogue
This appendix is a reprint of cleared material excerpted from a 1994 Naval Studies Board classified report, Mine Countermeasures Technology.1
Bold Approach Alternative
“Overwhelming force” may be the only alternative if the operational imperatives require rapid breaching, with surprise or in emergency, of a sophisticated, robust, highly effective combination of mines and barriers in the very shallow water and surf zone. The task group has assumed that existing and planned MCM technologies will assist the safe transport of troops and equipment to the water depths at the deep-water end of the 2,000-yd approach lane. In the operational scenarios assumed, this 2,000-yd path must be 165 yd wide from the deep-water end to about 10-ft depth and 50 yd wide from about 10-ft depth onto and over the mined areas of the beach. The total length of the 50-yd wide section through the surf zone and on the beach is estimated as about 250 yd, of which 100 yd are underwater.
Surf and Beach Zone
For the surf and beach section of the assault lane, from 10 ft of water and onto the beach, where a high density of mines and obstacles is likely, the task group proposes that aircraft deliver a row of large, bottom-penetrating bombs that explode below ground level under the water bottom, and under the beach, to eject many of the mines and obstacles, and to form a channel deep enough that an LCAC could ride on water over the remaining
mines and obstacles, without contact, to beyond the mine zone on the beach. The task group proposes that this wave of overwhelming force be followed with guinea pig barges as a second layer of mine countermeasures. The barges would be sunk or, if floating, stopped at the end of the channel to form a causeway for the landing force. The two tactics should result in a more robust system and increase confidence in the effectiveness of this bold approach.
To effectively excavate a channel of sufficient width and depth, the bombs have to be big enough to have a large crater radius and to penetrate sufficient depth. There is much more information on cratering on land than underwater. However, tests, e.g., by Davis and Rooke (1968)2 and analyzed by O’Keeffe and Young (1984)3 indicate that burial of a conventional explosive under a sand or mud bottom in shallow water can significantly increase the crater diameter compared to that from an explosion on the bottom. O’Keeffe and Young indicate that an explosive of W pounds (equivalent of TNT) should be buried to a depth of about W0.33 ft below the bottom for maximum cratering, which for 10,000 lb of TNT equivalent explosive would be 21 ft. Young and O’Keeffe’s data plots and other work on cratering,4 done on land, indicate that the crater radius near the maximum may not be very sensitive to the exact depth of burial. Also, the crater diameter appears to be weakly dependent on the seabed material (except for rocky bottom) and on water depths, for this size of explosive, in a range between 10 and about 3 ft. O’Keeffe and Young suggest that the radius (Rc) of the crater at optimum depth of explosive burial in a soft bottom (about W0.33, in this case 21 ft) would be given by Rc=4.4 W0.33. Thus, a 10,000-lb explosive would produce a crater 95 ft in radius or 64 yd wide in the section of the lane from 10 ft to perhaps 3 ft of water. For lesser water depths and up to the (assumed sand) beach edge, the cratering phenomenology changes, leading to a gradual decrease of crater radius for a 10,000-lb TNT-equivalent explosive to about 65 ft on a wet sand beach, with a corresponding optimum explosive burial depth of about 40 ft, and to about 55 ft in completely dry sand for about the same depth of burial. The crater depths are greater up the beach than underwater.5
Work done at the Army’s Waterways Experiment Station and the Atomic Energy Commission’s project PLOWSHARE included use of row charges, buried in the bottom underwater, ranging from pounds to tons TNT equivalent, and detonated nearly simultaneously to approximate a line charge, to excavate boat channels.6 Thus, PLOWSHARE’s subproject TUGBOAT7 excavated a boat channel and harbor in a coral bottom at
Kawaihae, Hawaii, by detonation of a row of large charges buried beneath the bottom. Using 10-ton charges (of explosives of yields slightly greater than TNT) simultaneously detonated and buried about 34 ft below the bottom, the channel was 250 ft wide at maximum (not all the explosives went off) with a relatively flat bottom about 12 ft below the original depth. While based on data not including coral explosions, for 10 tons TNT equivalent, O’Keeffe and Young’s formula, above, gives a 239-ft width for a burial depth of 27 ft below the bottom.
Project PLOWSHARE’s work on row charges on land also indicated than an excavated channel with fairly uniform, smooth sides can be achieved if the explosive spacing is about 30 percent greater than the individual crater radius.8 However, the TUGBOAT charges were spaced more closely, about one crater radius apart. The task group also used a spacing equal to the estimated crater radius and estimated that the row charges would have to be detonated to within about 0.01 s simultaneity to act effectively like a single-line charge.
In uniform media, the number of row explosives required per unit length (N/L) and acting as an equivalent line charge increases approximately as the square of the excavation radius desired, R. However, the excavated medium changes with water depth and up the beach require the analogue of a tapered line charge. In 10 ft of water, the excavation radius of an optimally buried 10,000 lb of TNT is estimated as about 95 ft, and, in wet sand the radius is about 65 ft. Therefore, to achieve a 75-ft radius up to the beach, the number of bombs per unit length must be gradually increased to the beach edge and up the beach. Three such bombs, accurately placed, should be sufficient for a 50×100 yd channel up to the beach edge. For practical reasons, however, it may be desirable to space the bombs evenly. Thus, it was conservatively estimated that four accurately placed penetrator bombs, spaced about 60 ft apart beginning about 60 ft from the waterline, each containing 10,000-lb TNT equivalent, could excavate a 50-yd-wide channel through a 100-yd surf zone to the beach zone. Three penetrator bombs containing 20,000-lb TNT equivalent could also be sufficient. If it is desired to continue the excavated channel up the gradually drier beach, because the crater radius in sand decreases to about 55 ft, for a 150-yd-long channel, about six to eight additional 10,000-lb explosives would be required.9
The explosives could remove many mines and obstacles from the excavated channel. Most mines that are used in the surf zone are activated by magnetic fields, local pressure, or tilt rods. The shockwave and movement associated with the explosion are expected to inactivate or trigger many of these mines. Those that are not triggered and removed from the channel may be buried under displaced sediment, which may complicate later removal. Also, the washback of the water may return some of the mines and obstacles into the assault lane. However, these would be at a greater depth than they originally were in the surf zone, and deeper in the channel up the beach, so that the guinea pig and LCAC could ride over them on the water level extending to the end of the channel without contact with tilt-rod or pressure mines. Mines may be neutralized by the shock wave, or actuated by their motion or the magnetic fields due to the water motion. Tests will have to be conducted to determine the probability of exploding or deactivating the mines that are moved, the distribution of mines and obstacles after the explosion, and the residual threat of these mines to the invasion force.
The end of the cratered trench is not expected to have as much ejecta as the sides if previous work with row charges is applicable to the surf and beach zone.10 Further, part of the return flow of water should wash up the channel, smoothing the slope of the end crater.11 If excavation is mainly in the surf zone, the crater’s edge should be close to the level of the original beach. Before smoothing, the average end-crater slope at the end of the channel up the beach zone is estimated as about 20°. It may be necessary to use smaller explosions or a high-volume water jet to get a small enough slope of the end crater for the LCAC or invasion vehicles. The guinea pig causeway may also offer a partial solution to this problem.
The explosives could be placed with seabed penetrator weapons that contain 2,000 or 10,000 lb of high explosives. These large conventional explosives can be transported to the target area by B-52 aircraft, C-130s. A-6s, if these are still available, could carry five 2,000-lb, low-drag (so each A-6 can drop all of them simultaneously) bombs, that would be filled with modern explosives that have twice the explosive power per pound of TNT. To act as one 10,000-lb explosive in the surf zone, these 2,000-lb bombs would have to be dropped in a cluster with terminal positions within about 30 ft of each other, use minetype fuzes adjusted for simultaneous detonation and configured to penetrate to approximately the optimum depth for a 10,000-lb bomb. The centers of the adjacent clusters, if they are equally spaced, would also be about 60 ft apart in the surf zone and up on the beach. Because of the timing feature, each A-6 could deliver its bombs independently but to a preset location….
Status of Supporting Technology
The “Tallboy” and “Grand Slam” penetrator bombs…, which weighed 12,000 lb and 22,000 lb with 5,000 lb and 9,400 lb of TNT, respectively, were build by the British and used in World War II.12 The U.S. Air Force (USAF) also built about 100 GPT-10s of the 12,000-lb variety toward the end of World War II and modified B-29s and B 17s to carry them, but none were actually used. In Desert Storm, there was renewed interest in these weapons to attack deep, hard targets. None were extant in the United States but the USAF found that several bomb cases in Britain could have been made available. Subsequently, the USAF (Eglin Air Force Base) has funded a study of penetrators for future hard target-related contingencies, including the 5- and 10-ton variety.13 Bombs of this size and construction can be filled with modern explosives to approximately double the equivalent TNT load. In tests in World War II, the 12,000-lb bomb could penetrate 47 ft of sand if dropped from 30,000 ft. The 2,000-lb bombs, if configured to penetrate 20 ft
into the sea bottom under 10 ft of water, may also be able to penetrate to 40-ft depth in sand on the beach.
Terminal guidance for locating the explosives within about 30 ft of the desired point must be added. Differential GPS guidance that should be adequate is under development. It will be necessary to accurately establish the location of the water’s edge in GPS coordinates. To avoid misplaced explosions throwing mines and obstacles back on the lane to be cleared, it may be desirable to have a GPS-controlled “permissive-action link” negating detonation except in proper GPS coordinates. Also, dynamic controls to ensure the stability and flight dynamics of such a large weapon as the GP-T10, which may also be attractive for other applications, will require substantial development. However, the performance of the GP-T10 supports the feasibility of part of the concept.
A recent Lockheed study14 discusses bomblets dispersed from available dispensers to explode surface or slightly buried mines on the beach, assuming a kill radius equal to crater radius. For a 100- by 50-yd area with a 50-ft CEP (50 percent criterion), Lockheed estimates that about 50 SUU54 dispensers would be required. The explosive weight required to clear the beach area appears to be similar to that required for the penetrator weapon.
Other System Considerations
The geology of the target area in the beach and surf zones will determine the penetration of the 10,000-lb bombs and the radius of kill. No investigation was made of how severe this limitation will be for the beaches of potential interest. A system study should make that determination. Such a study should also look into ways to ensure near-optimum depths of penetration of the bombs.
A systems and cost trade-off between platforms and guided and unguided munitions for the VSW approach lane region must be conducted to find an optimum approach. The relative amounts of explosive required and the difficulties of achieving a uniform distribution in area bombing indicate that a high premium may exist for very accurate delivery.
Finally, the risk management is considerably improved by the addition of guinea-pig barges that lead the landing force through the assault zone to the beach after the overwhelming force option has cleared the lane of all obstacles and excavated and exploded many, if not all, of the mines. The barges could be crafts of opportunity, hardened for this mission, using modern damage-mitigation technology, and fitted with external and hardened propulsion systems, adapted with fore and aft platforms for easy transit of the amphibious force, and provided with a means to sink them if necessary when they reach the beach, where they become causeways to provide a predictable landing platform. However, all of this modification may justify the development and deployment of special-purpose guinea-pig causeways for the MCM application. A system-level cost-benefit trade study is needed to decide on the requirement for a special-purpose causeway. Without the overwhelming force as a first wave, the guinea-pig causeways would be vulnerable to the obstacles and could block the landing force. Without the guinea-pig causeways,
the effectiveness of overwhelming force could not be verified in the 2 hours allotted for the landing. The synergism between the two techniques seems compelling.
In summary, it is suggested that an approach using a row of deeply penetrating, large, simultaneously detonated explosive weapons delivered in a line-charge analog could quickly clear a channel through the surf zone and mined beach areas with surprise or in an emergency using aircraft. Uncertainties are the crater performance for different types of bottoms and beaches, the final location and state (exploded or unexploded) of the affected mines and of the obstacles, the degree of difficulty the amphibious landing force would have at the end of the channel, the means and accuracy of delivery, and the penetrator design and performance. The addition of guinea-pig causeways to follow the overwhelming force and lead the landing force to the beach provides a complementary MCM to proof test the channel, verify the MCM effectiveness, and build the confidence of the forces. An extension of the line-charge analog approach might clear bottom, buried, and moored mines up to about 40-ft depth and a massive countermining strike might be able to clear the remaining deeper section of the approach lane.
Support the exploration of the explosive excavation of a channel through the surf zone up the beach and in VSW by tests, calculations, and simulations15 on cratering by deeply buried rows of charges of different sizes and depths.
Conduct tests of the mobility of tracked vehicles out of the end crater on the beach and of the ability of small explosives and of water-cannon apparatus to reduce the end crater slope.
Determine the feasibility and accuracy of B-52 delivery of large, terminally-guided penetrator weapons, and the possibility of A-6 delivery of clusters of 2,000-lb bombs.
Support the development of appropriate delivery methods.
Study accuracy and cost effectivenes of mine-bomb placement by different platform/guidance combinations.
Conduct an engineering design study of penetrator weapon options and the aerodynamic controls necessary to obtain accurate placement of the explosive and of the fuze modifications for synchronized detonation and GPS-controlled permissive action links.
Assess the probable status and distribution of mines and obstacles in and near the crater.
Study the geology of beaches that are likely to be targets for invasion and determine the effect of the geology on the operation of the penetrators.
Determine the hydrodynamic configuration required to penetrate the bottom in the 10 to 30 ft of water and to sufficient depth near and on the beach.
Study the feasibility and synergism of the guinea-pig causeways following the overwhelming force and develop appropriate causeways or conversion kits to make causeways from craft of opportunity.
Investigate the lethality of detonation of explosive patterns on the bottom (with time-fuze controls) against expected types of bottom and moored mines in depth regions characteristic of the approach lane.