GB2250960A - Float for an air-launched sonobuoy - Google Patents

Abstract

A sonobuoy suitable for launch from an airborne vehicle and comprising a parachute (2) for retarding the descent of the sonobuoy and provided at its crown with a vent (6), an inflatable flotation chamber (4) connected to the parachute in an airtight manner over the vent such that air pressure generated inside the parachute during descent causes the chamber to inflate, and means for preventing substantial ingress of water into the chamber upon impact with the water surface. The means may comprise a perforated flexible tube (5) within the chamber, sealed at one of its ends into the chamber wall and the chamber (4) being twisted around the tube when assembled together with the parachute (2) and the sonobuoy casing (1), so that on release and deployment the flexible tube (5) is initially untwisted and will permit flow of air into the chamber (4) whilst on inflation of the chamber the greater resilience of the chamber causes the chamber to untwist and the tube to become twisted so preventing further ingress of air and of water into the chamber (4) on impact with the surface. <IMAGE>

Description

FLOAT FOR AN AIR-LAUNCHED SONOBUOY This invention relates to floats for non-buoyant bodies and in particular to flotation devices for sonobuoys which are required to deploy telemetry antennas above the water surface. A sonobuoy, which may be launched from a ship or an aircraft, consists of acoustic sensors which are suspended under the surface of the water from a float and it is convenient to attach antennas, from which information is transmitted to airborne or shipborne receivers, to the float which is intended to provide sufficient directional stability (normally in a vertical direction) to the antenna.

It is common for non-buoyant sonobuoys to use a gas bottle-inflated float or buoyancy chamber to enable the telemetry antenna to remain above the sea or water surface during operation. This necessitates the inclusion within the sonobuoy of a gas bottle, an initiation device (often containing pyrotechnics) and a float which can consist of an expandable flotation chamber. In operation, when the sonobuoy strikes the water a sea battery is activated to fire a gas bottle. The gas then inflates the chamber which supports the antenna vertically above the surface of the water.

These additional items are heavy and bulky and the gas bottle and initiation device can present an explosion hazard under certain circumstances. Furthermore, in the case of aircraft-launched sonobuoys.

because of volume restrictions imposed by the limited size of a sonobuoy, it is often not possible to provide a parachute system which is effective in controlling oscillation of the sonobuoy during its descent after release from the aircraft. This can result in high aspect angles at water entry which in turn can impart very high rotational forces to the sonobuoy, resulting in damage to the internal sensors or to electronic systems.

The present invention is aimed at providing a aircraft-launched sonobuoy having a float which is able to inflate during descent after release from the aircraft and after impact with water can maintain an antenna associated with the sonobuoy at a pre-determined height above the water surface.

This invention accordingly consists of a sonobuoy suitable for launch from an airborne vehicle and comprising: a parachute for retarding the descent of the sonobuoy and provided at its crown with a vent; an inflatable flotation chamber connected to the parachute in an air-tight manner over the vent such that air pressure generated inside the parachute during descent causes the chamber to inflate; and means for preventing substantial ingress of water into the chamber upon impact with the water surface.

The flotation chamber may consist of a flexible bag sealed to or formed as an integral part of the parachute at its crown and which can be folded together with the parachute and packed with the sonobuoy before release.

The means for preventing substantial ingress of water into the chamber may comprise a flexible tube within the chamber, sealed at one of its ends into the chamber wall and open at its other end where it is sealed into the periphery of the vent, the tube being perforated towards its closed end. Preferably the chamber has more torsional stiffness when inflated than the tube and is twisted around the tube when assembled together with the parachute and the sonobuoy so that on release and deployment the flexible tube is initially untwisted and will permit flow of air into the chamber. On inflation of the chamber however the greater stiffness of the chamber causes the chamber to untwist and the tube to become twisted so preventing further,ingress of air and of water on impact with the surface.

The invention also provides a method of assembly of a air-launched sonobuoy comprising sealing one end of a flexible, perforated tube to the inside of a flotation chamber which is in the form of a flexible bag, stiffer when inflated than the tube and twisted round the tube, sealably attaching the other end of the tube and sealing the opening of the chamber to a vent at the crown of a parachute, and attaching said parachute to the sonobuoy whereby upon launch of the sonobuoy air enters the bag through the tube until the bag becomes sufficiently inflated for its stiffness to cause the tube to twist and prevent.further fluid ingress or egress.

By way of example, one embodiment of the invention will now be described with reference to the drawings, of which: Figure 1 shows schematically an aircraft-launched sonobuoy constructed in accordance with the invention in its form immediately after release and before inflation of the float; Figure 2 shows the same sonobuoy in its form shortly after the instant shown in Figure 1, with the float in an inflated form; and Figure 3 shows the same sonobuoy suspended in water.

With reference to Figure 1, a sonobuoy assembly consists of a cylindrical casing 1 whose descent after launch from an aircraft is retarded by a cruciform parachute 2 made of a non-porous nylon material and attached by threads 3 to the casing, the parachute 3 being initially deployed by a drogue (not shown). A flotation bag 4 made of polyurethane-coated rips top nylon, which in form is generally conical with an upwardly-pointing apex, is sealed to the crown of the parachute 2. The bag contains a flexible tube 5 sealed at its upper end to the apex of the bag and at its lower end sealed to the parachute material around a vent 6.

During manufacture of the assembly the parachute, flotation bag and drogue are folded together at one end of the sonobuoy casing with the bag twisted around the tube so that on initial deployment the tube is straight and under the action of the greater pressure at the crown of the parachute air is able to pass into the bag through orifices 7 close to the top end of the tube.

As the bag 4 inflates, however, it takes up its natural shape and as it becomes stiffer than the tube 5 as it unwinds, the tube becomes twisted.

Eventually the tube twists sufficiently to prevent further air passing into the bag, and the sonobuoy continues to fall with the bag inflated to its full extent, as illustrated in Figure 2.

When the assembly strikes the water surface 8 (see Figure 3), hydrophones associated with the sonobuoy are released and suspend from the casing on cables 9 whilst the casing continues to retain batteries and electronic components. The parachute 2 collapses but the flotation bag 4 retains its buoyancy since no air or water can pass through the constriction 10 caused by the twisting of the flexible tube 5.

The actual method of assembly of the tube/bag unit is optional; thus, whether or not the bag is initially formed as an integral part of the parachute, the tube may be firstly sealed at one end or the other. The tube may be twisted before sealing at its other end to the bag (or to the parachute crown) - in which case the unit needs to be untwisted before folding together with the parachute prior to packing - or the bag may be twisted round the tube before sealing. It is in all cases necessary however for the relative twist to be sufficient for the twisting of the tube brought about by the straightening out of the bag to prevent further flow of fluid (air or water) through the tube.

In an alternative form of the invention, the perforations 7 towards the top end of the tube are sufficiently small for air to pass readily through them before impact with the water and hence ensure that the bag 4 is sufficiently inflated, but not large enough for much water to pass from the tube to the bag in the instant between the initial impact with the water surface and the tube being brought into the vertical position under the weight of the sonobuoy.

In either embodiment, flow of air out from the bag, for example in heavy seas, can be reduced by providing a non-return valve across the vent 6.

In the absence of the tube 5 very little water would pass into the bag 4 upon impact provided that the impact were symmetrical but if the parachute and bag strike the surface at an angle it is possible for a significant amount of water to enter the bag even if, for example, a non-return valve is provided across the vent 6. As a result, the depth at which the hydrophone is suspended and the height above water of an antenna (not shown) supported on the bag 4 would be dependent on the angle of impact and hence unpredictable.

Although existing designs are able therefore to give consistent buoyancy in good sea-state conditions, the present invention enables entry of water into the flotation bag to be greatly reduced even in rough sea states so that a buoyancy and hence antenna signal level - and hence usefulness of the system - is less dependent on sea conditions.

The invention is not only applicable to floats for sonobuoys. It may with similar advantages be used with floats for small, temporary buoys or for rescue beacons, either where the float supports a transmitter or where the float incorporates a visual marker and it is important that the float rides as high in the water as possible, ie it ships as little water as possible upon impact with the water surface or in subsequent movement as a result of wave action.

In the latter context, deployment of the float before impact with the surface is not essential. If the invention is to be used, for example, in connection with a rescue beacon to be deployed in the event of an emergency, the parachute can be dispensed with and the flotation bag can be inflated, through a flexible, perforated tube around which the bag is initially wrapped as before, by means of a gas bottle since the space and weight penalties in such use are less significant than in sonobuoy applications.

Claims (9)

1. A sonobuoy suitable for launch from an airborne vehicle and comprising: a parachute (2) for retarding the descent of the sonobuoy and provided at its crown with a vent (6); an inflatable flotation chamber (4) connected to the parachute (2) in an air-tight manner over the vent (6) such that air pressure generated inside the parachute (2) during descent causes the chamber (4) to inflate; and means for preventing substantial ingress of water into the chamber (4) upon impact with the water surface.

2. A sonobuoy as claimed in Claim 1 in which the flotation chamber (4) consists of a flexible bag sealed to or formed as an integral part of the parachute (2) at its crown and which can be folded together with the parachute and packed with the sonobuoy before release.

3. A sonobuoy as claimed in Claim 2 in which the bag (4) when inflated and deployed is generally conical in form with its apex pointing upwards.

4. A sonobuoy as claimed in Claim 2 or Claim 3 in which the means for preventing substantial ingress of water into the chamber (4) comprises a flexible tube (5) within the chamber1 sealed at one of its ends into the chamber wall and open at its other end where it is sealed into the periphery of the vent (6), the tube being perforated towards its closed end.

5. A sonobuoy as claimed in Claim 4 in which the perforations (7) in the tube (5) are sufficiently small for air to pass readily through them before impact with the water and hence ensure that the chamber (4) is sufficiently inflated, but not large enough for much water to pass from the tube to the chamber in the instant between the initial impact with the water surface and the tube being brought into the vertical position under the weight of the sonobuoy.

6. A sonobuoy as claimed in Claim 4 or Claim 5 in which the chamber (4) has more torsional stiffness when inflated than the flexible tube (5) and is twisted around the tube when assembled together with the parachute (2) and the sonobuoy, whereby on release and deployment the flexible tube (5) is initially untwisted and will permit flow of air into the chamber (4) whilst on inflation of the chamber the greater stiffness of the chamber causes the chamber to untwist and the tube to become twisted so preventing further ingress of air and of water on impact with the surface.

7. A sonobuoy as claimed in any preceding claim including a non-return valve across the vent (6) of the parachute (2) for permitting the ingress of fluid into the flotation chamber.

8. A method of assembly of a air-launched sonobuoy comprising: sealing one end of a flexible, perforated tube (5) to the inside of a flotation chamber which is in the form of a flexible bag (4), stiffer when inflated than the tube; twisting the bag rqund the tube; seal ably attaching the other the end of the tube and sealing the opening of the bag to a vent (6) at the crown of a parachute; and attaching said parachute to the sonobuoy; whereby upon launch of the sonobuoy air can enter the bag through the tube until the bag becomes sufficiently inflated for its stiffness to cause the tube to twist and prevent further fluid ingress or egress.

9. An aircraft-launched sonobuoy substantially as hereinbefore described with reference to the drawing.