US Navy Adjusts to Aging Aircraft Issues with “Stump-54”

16 June 2006

In June, the US Navy awarded Lockheed Martin a one year, $3 million contract for the High Altitude Anti-Submarine Warfare Weapons Concept (HAAWC) Program, which is to demonstrate successful delivery of the Mk-54 hybrid lightweight torpedo from a P-3C aircraft operating at approximately 20,000 feet. The demonstration is to include a high-altitude launch, after which the modified torpedo will glide to its normal launch altitude (approximately 500 feet above sea level), jettison its aerodynamic mod kit, and make a successful water entry.

The flight surfaces mod are part of the LongShot kit, a relatively low-cost, self-contained wing adaptor kit providing range extension and self-contained guidance to several existing air-to-surface weapons such as mines, “iron” bombs, laser-guided bombs, and tactical munitions dispensers. Longshot is completely self-contained and requires no airplane airframe modifications. It has its own flight control computer, GPS nav system, and power supply, requiring only a control interface with the tactical coordinator and pilot stations aboard the P-3C. After a stand-off launch from approximately 20,000 feet, the “Stump-54” would glide down to about 300 feet altitude at which point the glide kit would be jettisoned, and the Mk-54 would enter the water and seek out the sub. The Kettering “Bug” Aerial Torpedo of World War I used a similar attack profile, although it was used over land to surprise the enemy and not very reliably. The “Stump-54” is more likely to use JDAM-like components. The Navy notes that this new weapon configuration will be used against both surface and shallow-water sub-surface targets, although how a P-3 flying at 20,000 feet will be able to locate and track a submarine to attack criteria remains undisclosed.

For the past three decades, the P-3’s primary anti-submarine warfare weapon has been the Mk-46 torpedo, a two-speed, reciprocating external combustion, 518-pound weapon with a range of up to 12,000 feet at speeds approaching 45K at depths ranging from 60 to 4500 feet. The Mk-54 is heavier at about 700 pounds but only a little larger in size than the Mk-46, and thus suitable for carry/launch from the P-3’s eight bomb bay and ten wing weapons stations. The Mk-54 combines the Mk-50 search and homing system with the propulsion system of the Mk-46 torpedo. The MK-54 was developed by Raytheon under the US Navy Lightweight Hybrid Torpedo (LHT) program but, due to Mk-50 shortages and higher than budgeted costs, the Mk-54 was designed to replace both Mk-46 and Mk-50.

Launch of the Mk-46 is most commonly accomplished at altitudes as low as 500 feet upon the aircraft’s having tracked the submarine successfully enough to have developed a track and attack locus (usually within 500 yards of the target’s DR track). However, such prolonged flight in low-level, salt-spray turbulence, combined with the 2G, 60 degree angle of bank required for a successful Magnetic Anomaly Detection pattern, is very punishing on aircraft and has led to an alarming rate of airframe fatigue and corrosion combined with the US Navy having to have flown the P-3 fleet far past its design life.

In The past several years the Navy was horrified to discover during fatigue tests that some P-3Cs suffered stress cracks under as little as +2.9G although engineering specs had called for an ultimate (no damage) momentary load factor of +4.5Gs when the aircraft were built. This forced the Navy into the impossible position of trying to create a “low-stress” working environment for a combat airplane. In addition to the flight loading problem is the fact that P-3s now fly into tactical scenarios in which anti-aircraft fire becomes a real threat – a situation that did exist for the P-3 from small arms fire during the Vietnam War but then ceased during the blue water ASW of the 1970s and 1980s. This latter problem is the one cited by the Navy in its just awarded HAAWC contract, although the former is more likely to be the driving force.

The US Navy currently has about 228 P-3Cs, including several dozen in various stages of rebuild and/or maintenance. The Navy maintains that the P-3Cs will be replaced with the P-8A, beginning in 2012, and plans to build 108 of these new airplanes by 2019, to be augmented with 50 dedicated Broad Area Maritime Surveillance Unmanned Aerial Vehicles. However, as money grows ever tighter in a 21st century dominated by asymmetric land combat and as the Navy once again sacrifices anti-submarine warfare and anti-mine warfare to larger and flashier projects, the outlook for 108 P-8s is dim. Perhaps 50 will be built, with the BAMS project scrapped in favor of using a generic UAV on an as-available basis. In the meanwhile, the Navy does the best it can with available resources.

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