Tests using seaplanes for antisubmarine warfare in the mid-1950s highlighted problems in take-off and landing in heavy seas. Here, a Martin Marlin P5M-2 negotiates calmer seas; the sonar hoist test rig pictured approximates the AN/AQS-6 (XN-2) sonar transducer, photos of which apparently are lost. The entire seaplane ASW project went down the tube when the Navy canceled the follow-on aircraft—the four-jet Martin P6M Seamaster.
In June 1992, U.S. Navy representatives met in Washington with members of the Russian Beriev Design Bureau, developers of the jet-powered A-40 Albatross amphibian, to learn more about this unusual, advanced aircraft and to explore Navy interest in modern antisubmarine warfare and patrol seaplanes. One of the subjects discussed was the acoustic equipment on the Russian aircraft and the advantages of such equipment. The primary advantage claimed was that the aircraft could remain on station, waterborne, for more than 24 hours.
News of this discussion carried me back some 35 years to a project I call the “84,000-Pound Sonobuoy.”
In June 1957, I was ordered to Air Development Squadron One (VX-1) as a project pilot/engineer on the P5M Marlin. The squadron used the Marlin as a platform for the operational test and evaluation of ASW equipment: radars, electronic intercept receivers, magnetic anomaly detectors, and sonobuoys.
The flying boat was an integral part of naval aviation in those days. The flying boat/seaplane tender team often could be based much closer to areas of interest and thus provide better coverage than could contemporary land planes. The Navy had 12 P5M squadrons and three large seaplane tenders. The landing ship, dock, Ashland (LSD-1) and the submarine Guavina (AOSS-362) had also performed tending experiments. The four-jet Martin P6M Seamaster was in development, and the large seaplane tender Albemarle (AV-5) was being converted specifically for Seamaster tending. A competition was in progress to follow Marlin with a new flying boat capable of routine, open-ocean operation.
With the growth of the Soviet submarine threat after World War II and the development of Soviet submarine-launched nuclear missiles that could threaten the United States, the U.S. Navy explored every conceivable physical phenomena useful in the detection, localization, tracking, and destruction of enemy submarines. Sonics seemed to be the only reliable detection system, despite the promise shown by infrared detection, exhaust emission detectors, plankton detectors, blue-green lasers, and Japanese “C.”
Therefore, sonobuoys were given directional capability, small explosive charges were used with passive sonobuoys to make a crude active system called “Julie,” and helicopters and airships were equipped with dipping and towed active sonars. Explosive charges as sound sources were also used with these sonars to make the “Gilda” system. All of these schemes had a low search rate, which limited their operational utility, particularly in the search phase. In the quest for higher search rates, the concept of a truly open-ocean seaplane as a platform for a large active/passive sonar was born. The Office of the Chief of Naval Operations issued Operational Requirement Number AS-040502, which required future ASW seaplanes to land, take off, and maneuver in the open sea.
No truly open-ocean seaplane had ever been built. To be operationally useful, such an aircraft would have to make routine, day and night landings and take-offs in Sea State Five—a very rough sea, indeed. Two possible approaches to the basic design concept of the aircraft had been advanced: the use of load alleviating devices, such as hydrofoils or hydroskis or, since the impact load is proportional to the square of the impact velocity, by achieving low landing and take-off speeds. In the design competition for this aircraft, Grumman proposed a four-turboprop aircraft with a hydroski; Martin, a four-reciprocating-engine craft with slow speeds achieved by boundary layer control; and Convair, a three-engine, parasol-winged machine reminiscent of the Dornier 24. The Navy wisely selected the low-speed approach, and Convair won the competition. The winning design, designated P6Y, was to be capable of landing and take-off at 35 knots’ air speed. The Navy ordered two XP6Y-Is (Buno 147206 and 147207). Since the need was critical, the Navy also began the simultaneous development of the seaplane and its sonar.
While detailed design of the aircraft proceeded, the Bureau of Aeronautics awarded a contract to the Martin Company for modification of two P5M-2 aircraft. This allowed for evaluation of the sonar seaplane concept before committing to series production of the P6Y. The P5M-2s were to serve as test beds for the installation and development of a prototype seaplane sonar. Under subcontract to Martin, the Raytheon Manufacturing Company designed and built the AN/AQS- 6(XN-2) Seaplane Sonar in accordance with Martin design objectives and technical data requirements. The evaluation of the seaplane sonar concept was assigned to VX-1. From its Key West base, the squadron could work against actual submarine targets in relatively benign sea conditions. A background in seaplane design and operational experience flying the JRM Mars and the PBM Mariner, earned me the designation of Seaplane Sonar Project Pilot. Lieutenant Reynolds Beckwith, an acoustics postgraduate, was Sonar Engineer.
With no P5M experience, I spent the rest of 1957 checking out in the P5 and flying routine equipment tests. In late 1957, Commander Richardson Phelps and I flew to the Martin plant in Baltimore to take delivery of the first sonar test bed aircraft.
The sonar P5M was fitted with Curtiss Electric propellers from a B-29. While waterborne, their pitch could be adjusted by special toggle switches for the precise thrust control necessary for “hovering” over the sonar in order not to tilt the sonar transducer. Since the engines were running without the cooling of forward motion, sea water pumped through special heat exchangers cooled the engine oil. The sonar transducer weighed 1,200 pounds, was about ten feet tall, and when stowed within the aircraft had a diameter of about three feet. When in the water the array was expanded to a 52-inch diameter. The transducer entered the water through a hole in the bottom just aft of the main step. The hole was the bottom of a standpipe; the top had no cover, since it was well above the waterline. Because the hole was aft of the step and therefore not in an area of dynamic pressure, the water level did not rise in the standpipe during taxi or planing.
Detection and localization were and still are only part of the ASW equation. The test-bed P5Ms were also equipped with variable incidence torpedo launchers that would fire submarine-homing torpedoes while the aircraft was on the water. Although these launchers performed satisfactorily during Martin Company tests in the swell-less Chesapeake Bay, I had my doubts that they would be very useful in the open sea.
We checked out the aircraft and the sonar in the Bay and found that although the electrical/mechanical systems functioned satisfactorily, the sonar itself was not completely debugged and was really not ready for operational testing. Since this was more a developmental than an operational acceptance project, however, it was decided that the debugging could best be done at Key West. So we took delivery and headed south.
Debugging, as usual, took much longer than expected. Starting in January 1958 preliminary work involved mostly practicing open-sea landings and take-offs, operation of the sonar mechanical systems, and water hover. After determining the availability of the target submarine (which was in great demand for other testing), we had to schedule a safety/rescue boat and hope that the aircraft and the sonar would be ready. Then we would reserve an operating area and schedule the test about a week in advance. On the morning of a scheduled test, we would pray for “just right” weather for the very weather-sensitive P5M. After all, it was only a testbed for the sonar and was never designed for routine, safe operation in the open sea.
To ensure the safety of the aircraft and its crew, the commanding officer of VX-1, Captain Waller C. Moore, provided the project pilots a guidance principle that open- sea landings and take-offs should be made only under perfect conditions.
Because getting the test aircraft and the sonar to the open-sea operating area was the key to the test program, I reviewed all the literature I could find on open-ocean seaplane operations, perhaps the most valuable being the booklet on ditching at sea by Captain D. R. McDiarmid, U.S. Coast Guard. I also talked to many pilots who had made open sea landings or who claimed rough-water experience.
The basic physics involved dictated that impact force be proportional to the vehicle mass times the square of the impact velocity. The pilot could keep mass low by flying at as light a weight as possible, which had the additional advantage of minimizing stall speed; I could minimize velocity by touch-down technique.
The velocity that produced impact forces was the vector sum of the aircraft’s horizontal and vertical motion and the horizontal and vertical motion of the ocean surface. The pilot could minimize the aircraft’s vectors by Hying a slow, flat approach, and the ocean’s by the proper selection of a landing spot and heading relative to the moving swell system. Following Captain McDiarmid’s advice, I tried to land parallel to the main swell. Studies of aircraft performance data revealed an appreciable spread between the power-on and the power-off stalling speed.
Therefore, in the light winds of the test area, the swell system governed landing heading. The flight path to the landing was a flat, power semistall, with touch-down at a near-zero rate of descent. Because under these conditions the aircraft was in a very nose-high attitude, I waited until I could feel the sternpost dragging the water before committing to the landing. When I was satisfied that all conditions were right, I chopped the throttles, resulting in an immediate power-off stall. At this point, I would put both hands on the yoke and my trusty (and trusting) copilot. Lieutenant George W. East, would apply full reverse power. Both hands went on the yoke, because when the aircraft stalled and the forebody fell into the water, the resultant hydrodynamic lift caused a rather violent pitch-up. Failing to control this pitch-up by rapid and positive down elevator would cause the aircraft to be thrown out of the water in a stalled condition, and then fall back in an uncontrolled and dangerous motion. With this pitch-up under control, the landing was firm, but smooth and short.
Once we were on the water, we set up for sonar testing; our plane captain, Len Riccio, inspected the aircraft for damage, Dean Buchanan sent a “safety on the water” report, George put the propellers in toggle control and activated the seawater engine cooling system, and Chief Sonarman Norman Nicholson lowered the transducer to maximum depth to obtain a bathythermogram. We then positioned the sonar for the best performance predicted by RayPath diagrams, and the target submarine opened the range on a predetermined pattern for active and passive tracking. After a lengthy debugging the sonar was working well for that era, particularly in the passive mode—so well, in fact, that shipping in the area interfered with our data collection. Screw noise from an over-the- horizon tanker could ruin a day’s tests.
At the completion of the day’s testing, or if the weather began to deteriorate, our ordnanceman. Bill Churchwell, rigged the four jet-assisted take-off (JATO) bottles, and we would don crash helmets, strap in tightly, and prepare for an open-sea take-off, the ultimate test of a seaplane pilot’s skill.
The open-sea take-off was a much more exciting and dangerous operation than the landing. When landing, the aircraft was under complete control until the instant of touch-down. If the touch-down pitch-up was controlled, reverse power brought it to a quick and safe stop. In the take-off, the aircraft had to be accelerated from zero to flying speed. Within the low speeds of this regime, the aerodynamic controls were not very effective, and the acceleration from being a displacement vessel, through transition "hump speed,” to planing and flying put the aircraft through a lengthy contact with the unpredictable sea. Especially near take-off speed, when the aircraft was still below stalling speed and with full power applied, contacting a wave at the wrong angle could throw the aircraft into the air, with a severe impact when it fell back and with any subsequent bounces leading to even more severe impacts.
The general take-off direction, like the landing, was parallel to the main swell, usually with the wind broad on the bow. I would add power slowly, keeping the aircraft just below hump speed. This gave a good feel for the sea-surface motion and its interaction with the aircraft at speeds low enough to preclude any damage. If I was uncomfortable with the feel of things, I would cut the power and alter the take-off heading. When I was satisfied with the situation, I would apply full power and turn throttle control over to George. As the aircraft came up on the step at about 35 knots, I would drop take-off flaps. When stabilized on the step and upon reaching about 50 knots, I would fire all four JATO bottles. As the aircraft accelerated quickly to flying speed, I would try to keep the hull angle of attack with the sea surface fairly constant. If this could be done, the airplane flew off at about 75 knots, and we headed for the barn.
The danger came home when one airplane was damaged on an attempted open-sea take-off flown by an experienced alternate project pilot. Although the aircraft had to be flown to Norfolk for inspection and repair, this incident did not delay testing, since I delivered the second sonar test bed to VX-1 the next day.
Although these flights were exciting and challenging, the chronology of the sonar tests is a tale of frustration. After reviewing my old report, I find that we scheduled some 91 test operations during 1958. Of these, 22 were not flown because of sea states too high for safe P5M operations, 9 because of aircraft problems, 1 because of a search-and-rescue mission, and 25 because the sonar was not ready. Of the 34 actual test flights, only 15 yielded good data against a submarine. Not until late August did the engineers solve most of the sonar’s development problems. With pulse width doubled, we started to get meaningful results. In September, the sonar began to live up to its design potential, and we achieved active ranges of 13.0 yards, explosive echo ranging ranges of 17,000 yards, and passive ranges against a snorkle of more than 30.0 yards. As we were preparing for maximum-range tests, we received word that the P6Y program had been canceled and that the Navy had no more need for seaplane sonar. We flew a few more flights in order to write a closeout report on the sonar and turned in our sonar P5Ms to be converted back to fleet configuration. Although the AQS-6 sonar showed tremendous potential for the era, it really had no viable platform after cancellation of the P6Y. It was too big and heavy for the helicopters of the time, and since it had to be stationary in the water, it was not suitable for airship deployment. I had hoped that it might work with hydrofoil patrol boats, and I pointed out its potential to the hydrofoil desk in the Bureau of Ships. Hydrofoil patrol boat development had its own problems, however, and the last time I saw the sonar transducer was in a storage yard at the Naval Air Development Center in the mid-1960s. The Navy subsequently provided technical data on the sonar to the Japanese, under a data exchange agreement for the development of their PS-1 flying boat.
I believe that the P6Y program was canceled because of the development of the sound surveillance system (SOSUS) and the Jezebel low-frequency analysis and ranging (LOFAR) technique. The former provided an area initial detection, and the latter gave aircraft a respectable ASW search rate through a simple sonobuoy. With these technological advances, the operational requirement for the open-ocean seaplane disappeared.
During the tests, a number of operational problems surfaced that would limit the effective tactical employment of seaplane sonar. The aircraft was very uncomfortable while waterborne: it tended to bob like a cork, and the crew was susceptible to seasickness. (Some later work was done with “spar buoy” floats to minimize this motion). Keeping the sea unit in the proper vertical position was nearly impossible for any length of time with manual “water hovering.” Some type of automatic hovering would be necessary for operational use. Attack of the target presented another challenge. With the sonar deployed, the aircraft was an 84,000-pound sonobuoy tethered to the transducer. To make an attack, it would have been necessary to break contact, retract the transducer, and taxi or fly to a torpedo-drop point. We found that to be extremely time-consuming, and taxiing rapidly in any normal sea state would have been extremely difficult. We never bothered to test the variable-incidence torpedo launchers for these reasons. We concluded that for the sonar seaplane to be an effective weapon system, it would have to operate in “leapfrogging” pairs, with the waterborne aircraft holding contact and its airborne partner making the attack. We made one test flight of this technique, and it worked well. But even so, the very thought of night open-sea landings and takeoffs, even in a 35-knot short take-off and landing aircraft, made my blood run cold.
Nevertheless, the seaplane sonar concept stayed alive in the West for a number of years. The Japanese Shin Meiwa PS-1 open-ocean seaplane, which first flew in 1967, was originally designed around a dipping sonar, although I suspect this was done more to sell an aircraft capable of rough-sea operation than for its ASW utility. Periodically, the idea surfaced in the United States to use the seaplane for a rapidly deployable towed array. As Commander, Oceanographic Systems Pacific, I participated in a number of discussions of the idea. As much as I loved seaplanes and believed in their military utility, tales of my experiences with the 84,000-pound sonobuoy often dampened the enthusiasm of proponents.
Although the seaplane sonar concept was an ASW cul-de-sac, in 1957 it was not a stupid idea. It was developed at a time of near-desperation in ASW, when every conceivable avenue to submarine detection had to be explored. It was a project from the era that predated Secretary of Defense Robert McNamara; two prototypes could be built, tested, and evaluated in an operational setting for less money than it now costs to generate mountains of paper “analyses.”
Captain Hoffman writes from his home in California. The retired naval aviator is a veteran of the Korean and Vietnam wars and served extensively in capacities related to seaplanes, patrol aircraft, and general antisubmarine warfare. In the business world, he has held positions at Martin Marietta Aerospace, Lockheed, and McDonnell Douglas Electronics Systems Company, where he retired in 1991.