AN AIRBORNE UNDERSEAS WEAPON SYSTEM

Because of the limited range of both surface and subsurface Naval ASW systems, it would appear that the tactical effectiveness of the Navy ASW mission could be greatly enhanced by a weapon system which consisted of vertical take off and landing (VTOL) transport type “mother” aircraft which would be able to deliver to any given ASW area a combined capability for detection, localization, classification and kill. A small 100 knot deep-diving (6,000 fl} winged twoman submarine (which would be light enough for airborne delivery yet would contain suitable detection and armament equipment) could be flown out to a deployment area where it would be launched into the ocean by the mother plane. At this point, the submarine could elect to either perform a short patrol of up to 2 days or to immediately begin neutralizing action against a possible enemy in that area. In either case, the mother aircraft would be avaiJable for pick-up at any time at a predetermined rendezvous. The mother aircraft would also have an all-weather pick-up and launch capability combined with comprehensive long-range communications as well as an air-underwater communication link with its underwater vehicle.

The VTOL Airborne Delivery Vehicle
This type of aircraft is being developed (as represented by the V-22 OSPREY) and would be capable of lifting a 12,000 pound payload over a range of 1,000 miles at a cruise speed of 350 knots. The description of this vehicle need not be duplicated here.

The Winged Pulse-Power Submarine
Studies show that it is possible to build a two-man winged submarine with a detection and attack capability within the weight and size restrictions imposed by a VTOL carrier aircraft. Past experience has indicated that its two-man crew carried within a five foot diameter pressure sphere is the minimum possible for the vehicle to satisfactorily perform its mission.

These requirements establish the following approximate boundary conditions for the Pulse-Power Submarine Length: 20 feet Width: 5 feet Weight: 12,000 lbs. max. at launch, 10,000 lbs at recovery Speed: 100 knots Depth: 6,000 feet Endurance: 2 days In addition, it is desirable to have: a) Positive buoyancy at all times b) A weapon’s payload of underwater rockets c) Sensitive detection and communications capabilities.

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Because of the very wide speed range and the flexible mission capabilities of this vehicle, an additional auxiliary power-plant is indicated to provide electric power for lowspeed secondary propulsion and the electronic gear.

The Sub’s Propulsion
Minimum drag considerations for high-speed underwater flight dictate a body of revolution type hull with a maximum length to diameter ratio in the range of 7 to 8. Using the known shape of the SSN 585 SKIPJACK class submarine, operating in fully turbulent flow, the power required for the Pulse-Power Submarine is shown in Figure 2 over its operating speed range. Obviously, it is desirable to avoid powerabsorbing cavitation effects and, accordingly an operational envelope can be derived within which the Pulse Power Submarine can safely maneuver free of cavitation.

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It is apparent from Figure 2 and the power equation, that, assuming completely attached flow, the power required for any given speed is directly proportional to the wetted area of the vehicle. Since the extreme depth requirement dictates a series of maximum strength/weight ratio spherical shells for enclosing the electronic gear, crew, and auxiliary powerplant, a significant saving in overall length and wetted area is accomplished when all these functions are enclosed within three spherical shells as shown in Fig. 3.

The short tail of this configuration will normally lead to a high pressure drag addition to the wetted area friction drag. However, a suitably chosen powerplant will create a positive pressure gradient on the aft portion of the hull such that the water flow remains attached at all points, thus removing the pressure drag and allowing a large reduction in power. The solid curve of Figure 2 describes the power required of this shape with its propulsion driven boundary layer control.

Results of the mass flow entrainment possibilities of pulsed ejectors are sufficiently encouraging (with 100% entrainment attained) to enable their use as an underwater propulsion device with unique characteristics. The very large mass flow entrainment achieved by the pulsed ejectors allows this device – in a suitable array – to create the desired pressure gradient for efficient attachment of the aft-end water flow. In addition the pulsed propulsion system is a rugged lightweight thrust producer which will require no lubrication and can be built to take the heavy loads of high-speed underwater flight.

A very important characteristic of this pulsed propulsion system is that it can operate over a range of frequencies with high amplitudes of sound energy. This might well be used as an active sonar transducer for acoustic tracking and homing during the high-speed attack phase of the mission. In this manner the submarine can avoid carrying a bulky and powerabsorbing active sonar transducer.

Since the high speed of this vehicle will only be used for a short period of time, a sufficient quantity of lithium waterreactive fuel can be carried to run the propulsion system with complete control including throttling, shut down and restart capabilities. Because this is an open cycle system with water containing the oxidizer, the propulsion system operates independently of external pressures and will function equally well at any depth as long as cavitation is avoided.

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Figure 3 shows the arrangement of the pulsed ejectors at the submarine’s stem to provide a positive pressure gradient and prevent turbulent separation.

Secondary Propulsion
In order to provide noiseless, long endurance auxiliary propulsion for extended underwater reconnaissance where hovering and 2-3 kt. speeds would be required, a closed Rankine cycle steam turbine system is employed. Using the heat released from an annular surface burner fueled by a lithium-water reaction, this cycle provides a simple lightweight powerplant. Since the ocean is a virtually infinite heat sink, the spent steam can be efficiently condensed through radiators mounted in the stub wings to a quite low temperature; the deeper and colder the water, the more efficient will the cycle become.

The steam turbine drives a high-frequency alternator which, in tum, supplies constant frequency high voltage power to a solid-state voltage regulator for the electronic system and also to a small water-cooled variable frequency propeller drive motor which will allow a 0-12 knot speed range with a maximum power output of about 30 horsepower (note, separation drag will be high at these low speeds with the pulsed propulsion off, but this is of little consequence since the power level is quite low).

Structure
Techniques exist for building strong light-weight solidpropellant rocket casings using filament wound reinforced plastics. Similarly light, high strength spherical shells can be produced for underwater operation at extreme depths. In particular a boron-based fiber with a composite tensile modulus of elasticity of approximately 33 x 106 psi and a composite specific gravity of something less than 2.0 provides a significant breakthrough for shell structure performance. A comparison with a solid shell of Alclad 75S aluminum, shows for a 6,000foot maximum depth capability and a five foot diameter sphere:

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Utilizing the optimum strength characteristics of the spherical shell, the hull is divided into the three basic pressure spheres:
#1 Sphere – Crew and control
#2 Sphere – Secondary powerplant
#3 Sphere – Electronics and communication

In order to minimize the weight still further, the outer hull is a non-pressure structure and the space between the three pressure spheres is filled with flexible fuel bags. In this manner, as fuel is consumed, the hull is flooded with sea water through a bow opening which also serves to pressurize the fuel tanks.

However, the hull filament wound impregnated plastic structure has sufficient strength to withstand the very high dynamic loads at 100 kts. ( = 28,400 psf) and can carry the control wings which are mounted on aluminum forgings encircling the crew sphere.

The pulsers, of aluminum alloy, have their thrust loads taken out through a sub-frame attached to the main structure around the crew sphere.

The weapon’s payload is carried in pods under the control wings. These rocket weapons would be solid-fuel propelled water-to-water rockets with a conventional warhead, and operate over a maximum range of 200 yards.

Stability and Control
The ability to hover and maintain station at very low speeds as well as the ability to fly at speeds up to 100 kts., dictates that wind tunnel tests will be required to fully establish the positioning and optimum shape of the stub wings and control surfaces.

The design provides for a maximum positive buoyant force of 2550 lbs. decreasing to 1350 lbs. as fuel is expended and the fuel cells are flooded with sea water.

Since positive buoyancy is maintained at all times, the stub wings are required to develop sufficient “negative” lift for the submarine to hold station at any given depth. With a wing area of 24 ft.2 a minimum speed of approximately 6 kts. is required to hold depth in the fully fueled condition. This requires approximately 5 SHP from the secondary propeller propulsion.

The control surfaces are capable of maneuvering the submarine under water in the same manner as a conventional fighter aircraft in the air.

Air-to-Undenvater Communications Link
The requirements of this weapon system are that the submarine be able to transmit information from any depth down to 6,000 feet to the air-sea interface from which it can be relayed either to the “mother” aircraft or to a surface ship.

By equipping this vehicle with sensitive passive sonar and echo ranging equipment it is possible to conduct an extended undersea reconnaissance — picking any thermal layer desired — to effectively pinpoint an enemy submarine. A laser operating in the blue or green spectrum should be quite effective for distances up to 2-3 miles.

The “mother” plane can drop a pattern of small floating laser-buoys (for receiving, decoding the underwater vehicle’s laser signal and then transmitting this information via radio frequency to surface vessels or the “mother” aircraft). The underwater submarine located near this laser buoy pattern could transmit vertically by laser beam to the nearest buoy, and have one of its many channels reflecting back from the buoy for an automatic lock-on to the buoy during the transmission time period.

Assuming a transmitting depth of 5,000 ft. and an attenuation of 10·2 DB per yard (see Figure 4), it is apparent that over a slant range of 6,380 ft., (i.e. the laser buoy is 2,000 ft laterally from the winged Pulse-Power Submarine) there is a power attenuation of only 17 DB. With the very high power density at one frequency radiating billions of times as much energy as an equivalent area of the sun’s surface, the laser would appear to be ideally suited for short range underwater communication.

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Launch and Recovery
The launch and subsequent recovery of the submarine requires a system that successfully overcomes several major problems, including:

1. Operations during high seas, requiring lift of as much as 100 ft.

2. Attachment of recovery harness to submarine during turbulent conditions of both sea and air.

3. Providing for lateral stability of submarine during launch and recovery.

4. Design for minimum weight.

The solution proposed provides the ability to operate in all weather conditions, while at the same time, requires a minimum amount of complex machinery and weight.

The submarine is lowered and raised by means of lifting cables attached to a recovery harness which is attached to the submarine. A telescoping stabilizer bar is attached between the lifting harness and the airplane, providing for lateral control and also limiting longitudinal swing. This stabilizer bar serves also to guide the lifting harness into position for attachment to the submarine during the recovery phase of the operation. Figure 5 shows the system partially extended for launch or recovery.

Lifting Harness
The harness is a rigid member which can be guided into position for recovery of the submarine. During the recovery operation a large 4-foot diameter wire loop is attached to the front of the harness. This loop will be guided by a crewman, through control of the stabilizer bar, until it connects with an extended hook on the submarine. U pan this contact the hook will be drawn into the submarine, or moved along it, moving the lifting harness into position for the lift. With the front of the rigid harness secured, the rear of the harness will move into position and with a similar but smaller cable ring attach the rear support cable to the dorsal fin. With the harness firmly attached front and rear, the lift can begin.

The stabilizer bar provides lateral stability to the harness at all times, with and without the submarine attached. Stability is extremely important, for yawing of a large mass suspended below the airplane would cause difficulty with the aircraft’s control and stability.

Second, the stabilizer bar, of aluminum tubing, consists of four to seven sleeves depending upon the location of the attachment point. Since the lower end of the bar is permanently attached to the lifting harness and will raise and lower with that harness, it is necessary to make this a hydraulically extensible or retractable boom. Locking sleeves grooved and keyed to prevent axial rotation prevent any unwanted movement of the lower end of the bar.

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Using the above outlined method, submarine launch and recovery appears quite feasible even under adverse conditions of sea and air.

Pulse-Power Submarine Specifications

Weights:

Fuel Available: 30 minutes@ 100 kts.
2 hours @ 60 kts.

Henry E. Payne III