THERMAL INSULATORS FOR ROCKET MOTORS

Abstract: A thermal insulator for a rocket motor body (3) comprises a member adapted to be fitted as a sleeve on the outer surface of the body the member (3) comprising a composite of cork (7) having on its outer surface a layer of fibre reinforced polymeric material (9) wherein a substantial proportion of the fibre reinforcement comprises fibres of low-conducting material.

Description: The present invention relates to thermal insulators for rocket motors. Rocket motor designs of the type which are of interest today have existed since the late 1940's. During the intervening years the design, development and engineering of motor hardware components have progressed appreciably contributing to the continually improving efficiency of associated missiles. Two fundamental chemical parameters have imposed temperature limitations on motors. Firstly, the burning rates of propellants used in solid propellant rocket motors are related to the propellant temperature and in most instances the control of the burning rate, and consequently, control of the missile, becomes difficult above 60-80°C, the exact temperature depending on the propellant in use. Secondly, conventional organic polymers, on which many of the adhesives and rubbers used in rocket motor hardware are based, typically start to lose strength at about 100°C and frequently commence to decompose near 200°C. (Advanced polymers are slowly becoming available which, although of lower strength, may be expected to raise these temperatures but only by a limited amount, say to 200°C and 400°C respectively). With a few exceptions the above two fundamental parameters have not inhibited the design of rocket motors to date. However, an era is approaching when aerodynamic heating (aeroheat) is likely to become increasingly severe. This is inevitable as the speeds of aircraft increase resulting in higher aeroheat temperatures due both to the carriage by those aircraft in captive flight of underslung missiles and to the increasing speed required in missile free flight in order both to attack other high speed aircraft and to avoid interception. It is very unlikely that propellant technology will change significantly, hence it will be necessary to incorporate a thermal barrier to protect the propellant from aeroheat. This could be on the outer or the inner surface of the motor body and certainly in the former and probably in the latter instance will incur mass and volume penalties. It will also be necessary to modify or protect any organic polymer based hardware materials. If modified, a mass penalty due to their lower strength may be incurred and, if protected, a thermal barrier on the outer surface of the body, as for protection of the propellant, will be required. Desirably, any protection added also complies with the current military requirements for insensitive munitions, i.e. munitions which are not operationally activated without control in the event for example of bump, shock, vibration or in a fuel fire in storage or under bullet or shrapnel attack. Furthermore such protection desirably has suitable mechanical properties in typical environmental conditions e.g. over a wide range of environmental temperatures and humidities and other atmospheric conditions e.g. wind, rain, hail etc. It is therefore appropriate to provide as a thermal insulator to protect against aeroheat a material which is specially selected to provide an optimum combination of properties, whereby there is no substantially adverse effect on the cost, weight and volume or performance of rocket motor designs incorporating such a material. US Patent No.3440820 describes a thermal protection system for protecting missiles, such as solid propellant rocket motors from the effect of aeroheating in which an insulating shield surrounds the rocket motor case but is separated therefrom by a gap which can either be evacuated or through which a fluid can be circulated. Such an arrangement is not integral with the rocket motor case and is complicated to manufacture. German Patent No.3509286 refers generally to the use of plastics and cork for providing external insulation for explosives charges to protect against heat from fires, but such materials on their own would be unsuitable for aeroheat protection of rocket motor cases. According to the present invention there is provided a rocket motor with a thermal insulator, the rocket motor comprising a rocket motor body, and the thermal insulator comprising a member adapted to be fitted as a sleeve on the outer surface of the rocket motor body and containing cork as a thermal insulator, characterised in that the member is a composite comprising an inner layer of cork having on its outer surface an outer layer of fibre reinforced polymeric material and in that the fibre reinforcement comprises a substantial proportion of fibres of low-conducting material and the fibre reinforced layer is formed from a resin impregnated cloth formed around the cork layer. By a 'low conducting' material is meant a material having a thermal conductivity lower than that of carbon. Since carbon can have typically (at 0°C) a thermal conductivity of 100 to 5000 units where 1 unit is the thermal conductivity of less than 100 times that of air at O°C. By a 'substantial proportion' is meant at least 50 per cent, desirably at least 90 per cent, e.g. at least 95 per cent, of the volume occupied by the fibres. For example, substantially all of the fibres may be low-conducting. Where fibres which are not low-conducting are also included, these may be carbon fibres. Thus up to 50 per cent, but desirably not more than 10 per cent, by volume of the fibres present may be carbon fibres. The polymeric matrix of the fibre reinforced polymeric material may comprise a chemically cured, eg. cold setting or thermosetting, or thermoplastic resin selected from those well known in the art. For example, a thermosetting materials epoxy, polyester and resins made by the Friedel Crafts process are suitable. As thermoplastic materials polysulphone, polyethersulphon are suitable examples. The reinforcing fibres of the fibre reinforced polymeric material may for example be selected from glass, silica, polyaramid, nylon or polyolefin. Preferably, the fibre reinforced polymeric material incorporated in the thermal insulator in the thermally insulated rocket motor according to the present invention is formed from a flexible resin impregnated cloth which may be wrapped around a hollow body, for example, of cork. The composite may be formed by providing a wrapping around the cork and then treating the material both to polymerise and to bond it to the cork In a known manner. For example, where the cloth is an uncured epoxy resin impregnated cloth the resin of the cloth upon curing at room temperature or by heating to a suitable curing temperature, eg. typically 50°C to 180°, will provide a suitable unitary structure by bonding to the cork. Although cork and fibre reinforced polymeric materials are known per se as thermal insulators their use in the form of a composite as in the present invention for application to the outside surface of rocket motor bodies to protect against aeroheat is unusual because the degree of heating and the mechanical stresses involved are different from those encountered in prior art applications of these materials. Cork provides an efficient, inexpensive thermal insulator of low weight which is suitably stiff and provides a suitable surface to enable another layer to be bonded to the outer surface thereof. The fibre reinforced polymeric material provides good mechanical properties and this together with the cork unexpectedly provides a composite having a suitable combination of properties for use in the protection of rocket motor bodies i.e. suitable rocket motor insulation and mechanical strength over a wide range of environmental conditions at low cost and low weight and volume. Such a material also complies with the requirements of current military insensitive munitions programmes. A further material, which may he the same as or different from that on the outer surface, may also be applied on the inner surface of the insulator according to the present invention. The insulator according to the present invention may comprise a hollow substantially cylindrical shape, although it may have a shape adapted to fit over protrusion on the outside of the rocket motor body. The insulator used in the thermally insulated rocket motor according to the present invention may be used with rocket motor bodies of various known materials, eg. metals such as steel or aluminium alloys or composites such as containing glass, carbon and/or polyaramid fibre reinforced polymeric materials or laminate of metallic and non-metallic materials. In the insulator used in the thermally insulated rocket motor according to the present invention, the composite may typically be from 0.5 to 10mm thick, especially from 1 to 4mm thick. The cork layer may for example be from 0.2 to 5mm thick eg. to 1 to 4mm thick and the fibre reinforced layer formed thereon may be from 0.2 to 2mm thick. Preferably the thickness of the cork layer is equal to or greater than that of the fibre reinforced layer. An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing, Figure 1 which is a cross-sectional end view of a rocket motor body including an insulator applied thereto. In Figure 1 the rocket motor shown comprises a solid propellant 1 inside a body 3 having a conventional internal insulator 2 separating the two. The body 3 has an outer thermal insulator 5 formed as a composite sleeve thereon comprising a layer 7 of cork having on its outer surface a suitably cured glass fibre reinforced epoxy resin 9. The insulator 5 is formed from a 3mm thick sheet of cork rolled to form the layer 7 having bonded thereon an approximately 0.3mm thick glass fibre reinforced epoxy layer 9 produced using a cloth provided by Marglass (Registered Trade Mark) having a fibre density per unit surface area of 127gm⁻². A composite such as that described for the insulator 5 has been found to provide suitable protection to the rocket motor body 3 and its contents including the propellant 1 at an external temperature of 170°C, a typical aeroheat temperature. Using a composite comprising 1mm cork having a 1mm thick glass fibre reinforced epoxy resin thereon it has been found that the time for the body 3 to reach a temperature of 70°C is approximately five times that experienced with the uninsulated (on its outer surface) body 3. Testing has indicated that the outer thermal insulator 5 is capable of withstanding environmental conditions of temperature, typically -40 degrees Celsius to +60 degrees Celsius and humidity, typically up to 90% relative humidity, for periods of many weeks without detriment. The insulator 5 also withstands typical environmental weather conditions such as wind, rain and hail. It has also been shown that the thermal insulator 5 does not cause the rocket motor to have inferior insensitive munition properties.

Claims: A rocket motor with a thermal insulator, the rocket motor comprising a rocket motor body (3), and the thermal insulator comprising a member (5) adapted to be fitted as a sleeve on the outer surface of the rocket motor body and containing cork as a thermal insulator, characterised in that the member is a composite comprising an inner layer of cork (7) having on its outer surface an outer layer (9) of fibre reinforced polymeric material and in that the fibre reinforcement comprises a substantial proportion of fibres of low-conducting material and the fibre reinforced layer is formed from a resin impregnated cloth formed around the cork layer. A rocket motor as claimed in claim 1 and wherein the polymeric matrix of the fibre reinforced polymeric material comprises a chemically cured resin. A rocket motor as claimed in claim 1 and wherein the polymeric matrix of the fibre reinforced material comprises a thermoplastic resin. A rocket motor as claimed in claim 1, and wherein the reinforcing fibres of the fibre reinforced polymeric matrix of the fibre reinforced polymeric material are selected from glass, silica, polyaramid, nylon and polyolefin. A rocket motor as claimed in claim 1 and wherein the fibre reinforced composite has been formed by providing a wrapping around the cork and then treating the polymeric material to bond this to the cork. A rocket motor as claimed in claim 5 wherein the said cloth is an uncured epoxy resin impregnated glass fibre cloth the resin of the cloth being curable by heating to a suitable curing temperature in the range 50 degrees centigrade to 180 degrees centigrade to impregnate the cellular structure of the cork and provide a suitable unitary structure bonding thereto. A rocket motor as claimed in any of the preceding claims and wherein a further material is applied on the inner surface of the insulator. A rocket motor as claimed in any of the preceding claims and wherein the insulator is from 0.5 to 10mm thick. A rocket motor as claimed in claim 8 and wherein the cork layer is from 0.2 to 5mm thick and the fibre reinforced layer formed thereon is from 0.2 to 2mm thick.

Inventors: CHASE, MICHAEL JOHN
Applicants: ROYAL ORDNANCE PLC
Application year: 1989
Main classification: F02K9
IPC codes: F02K9, F42B15