GB2354311A

GB2354311A - Rocket motor igniter

Info

Publication number: GB2354311A
Application number: GB9917350A
Authority: GB
Inventors: Malcolm Stewart Place; Richard Martin Hall; John Hall
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1999-07-26
Filing date: 1999-07-26
Publication date: 2001-03-21
Also published as: WO2001014705A1; GB9917350D0; AU6580500A

Abstract

An igniter for igniting the propellant of a rocket motor, especially of the radial or cigarette burn type, having at least one sheet of vapour deposited pyrotechnic material (18) locatable adjacent the surface of the propellant to be ignited, and a means (26) for igniting the vapour deposited pyrotechnic material, e.g. by direct electrical or laser ignition. The pyrotechnic material may comprise magnesium on a PTFE film.

Description

2354311 Rocket Motor Igniter

This invention relates to the field of rocket motor igniters, in particular to rocket motors having solid propellant in a radial or cigarette bum configuration.

Rocket motors having solid propellant are well known and are either of the cigarette bum type in which the propellant is formed as a block towards the front of the motor cavity, possibly with a plenum chamber at the rear. The propellant is ignited at the base and bums steadily through the propellant in the manner of a cigarette. Alternatively, the motor may be of the radial bum type in which the propellant has a central cavity and the inside surface of the propellant is ignited and bums steadily outwards. In both cases to achieve desired start up characteristics and ensure consistent burning of the propellant of the motor, it is necessary to ignite all the relevant surface of the propellant at the same time in a reliable and consistent manner.

Standard igniters for such rocket motors are the so called 'gas stove' igniters. These are generally located towards the rear of the motor chamber and comprise a pyrotechnic composition and means of ignition. When the motor is to be ignited the pyrotechnic composition is ignited releasing a high temperature gas which in turn ignites the rocket propellant.

With the cigarette bum type motor the igniter is arranged to produce flames over all the surface as equally as possible. However, there will inevitably be areas of the base of the propellant which receive a greater ignition stimulus than others, especially as there must be gaps in the stove arrangement for the propellant gasses produced to reach the rocket nozzle. This can therefore lead to unequal ignition which can lead to uneven burning which can reduce the motor efficiency, possibly affecting the accuracy, or in extreme cases lead to catastrophic failure.

The situation is even worse for the radial bum arrangement as the flame produced must be enough to ensure ignition of the propellant at the surface of the central cavity but must not be too strong as to bum away the propellant at any one zone.

2 Conventional arrangements further involve excess weight and reduced volume in the rocket motor. The housings are generally made from metal and have no part in the working of the rocket motor beyond ignition. Thus once the rocket motor has been ignited the conventional igniter is dead weight which must be carried. Further, the igniter apparatus reduces the volume available for the rocket payload or extra propellant.

In order to ensure the correct ignition of the rocket motor with conventional arrangements it is also necessary to release a substantial amount of high temperature gas. This then creates an increased pressure within the rocket motor. Furthermore once ignition begins the propellant will also begin to bum producing gas. All of this occurs when there is the smallest volume available in the rocket motor cavity. The pressure generated in the rocket motor during the ignition phase is therefore much greater than the actual working pressure of the rocket motor. The rocket casing must be designed to withstand this pressure which increases the Cost and weight of rocket casings with again a consequent reduction of the amount of propellant and/or payload which can be carried.

Furthermore, the use of a conventional arrangement requires the central cavity of radial bum type rocket motors to be kept substantially clear to allow the flame to ignite the whole internal surface of the propellant. This leaves such rocket motor vulnerable to fragment attack as fragment impact, even if not sufficient to cause ignition or detonation of the propellant, may cause propellant to spall from the surface adjacent to the internal cavity and be propelled into the propellant on the other side of the cavity. The effect of energetic impact of propellant upon propellant such as this often causes ignition or even detonation.

It is therefore an object of the present invention to mitigate at least some of the aforementioned disadvantages.

Thus according to the present invention there is provided an igniter for igniting the propellant of a rocket motor comprising at least one sheet of vapour deposited pyrotechnic material adapted so as to be locatable adjacent substantially all of a surface of the propellant which is available for ignition and a means for igniting the vapour deposited pyrotechnic material.

3 Vapour deposited pyrotechnic materials provide cheap and reliable means of ignition. The pyrotechnic materials are consumed during ignition so may be located adjacent all of the surface of the propellant which is to be ignited without affecting the plenum chamber or flow of exhaust gases in the rocket motor. The proximity of the pyrotechnic material to the propellant results in reliable and consistent ignition of the propellant when the pyrotechnic material is ignited. Further, the ignition of the vapour deposited pyrotechnic material is very rapid resulting in ignition of the whole surface of the propellant at approximately the same time.

Conveniently, the means of igniting the vapour deposited pyrotechnic material comprises two metallic contacts in contact with the vapour deposited pyrotechnic material and connected to a firing circuit capable of generating a firing current between the contacts. The vapour deposited pyrotechnic material may be ignited by direct electrical methods. The firing circuit is connected to the metallic contacts and can create a suitable potential difference across the contacts. This causes a current to flow in the vapour deposited pyrotechnic material and localised resistive heating generated within the pyrotechnic material causes ignition. The ignition rapidly spreads throughout the whole of the material generating a large amount of heat, which ignites the solid propellant of the rocket motor.

Electrical ignition provides an easy and reliable method of igniting the vapour deposited pyrotechnic material. Further, as substantially all of the vapour deposited pyrotechnic material is consumed on ignition there is very little of the igniter remaining after the propellant has been ignited. Thus there is very little parasitic weight and very little volume taken up, even before ignition. Also, the vapour deposited pyrotechnic material is quite insensitive to impact or fragment attack and if the metallic contacts were short circuited or earthed when 'safe' i.e. the rocket was not armed, the chance of accidental ignition is minimal, even in strong rf fields.

The metallic contacts are preferably vapour deposited on top of the pyrotechnic material. This ensures an intimate, strong and durable contact between the metallic contacts and the pyrotechnic material. The contacts are connected to the firing circuit by 4 any convenient means, such as soldering. Soldering in a simple and inexpensive way of making a contact and gives a strong and robust contact with good conduction properties.

The contacts may take the form of two pads located close to one another in any convenient location on the pyrotechnic material. The pads may be about 1 micron thick as this gives a good contact and base to which to attach the firing circuit but without having an effect as a heat sink. Copper is a good material for the pads due to its good conduction properties and ease of deposition and is easy to solder onto although it will be apparent to the person skilled in the art that other materials would Ruiction as well.

The vapour deposited pyrotechnic material is preferably sealed in a cover of polytetrafluoroethylene (PTFE) or similar non permeable sheet material. This will protect the pyrotechnic material from moisture, dirt and the like. Further, as the igniter may be in situ in a rocket motor for a long time a non permeable cover will protect the pyrotechnic material from noxious substances which may leach from the propellant and which would otherwise degrade the performance of the igniter. The metallic contacts may be also sealed within the cover and connected to the firing circuit by insulated wires thus providing a totally sealed system. When the cover is PTFE it can be consumed in ignition and may even add to the heat generated by the pyrotechnic material. Also where PTFE is used magnesium could be coated on the inner surface to add to the heat of reaction.

Other means for igniting the vapour deposited pyrotechnic material could be used, for example a laser ignition system comprising a firing circuit controlling a means for directing laser light onto the vapour deposited pyrotechnic material. Preferably a source such as a laser diode is used and directed either directly or via wave guides onto the vapour deposited pyrotechnic material. When activated the laser energy will cause localised heating of the vapour deposited pyrotechnic material leading to ignition of the material.

A laser ignition system could be mounted onto the rocket launcher and would mean that again the rocket motor igniter contributed very little weight to the rocket motor.

Conveniently the sheet of vapour deposited pyrotechnic material may comprise a substrate film of an oxidising halogenated polymeric material having vapour deposited thereon at least one layer of an oxidisable metallic material in at least one location on the surface of the substrate, the polymeric and metallic materials being conjointly capable of reacting together exothermically on ignition, the thickness and composition of the metallic material being such as to ensure reliable and consistent lateral progression of the exothermic reaction.

Such vapour deposited pyrotechnic material is described in European Patent EP0,463,022, the content of which is incorporated by reference herein. The material is relatively easy and cheap to produce and can be ignited by applying a suitable voltage across the metallic contacts. The reaction rate of the material is such that for a small sheet of material, such as the area of a propellant surface to be ignited in a rocket motor, the ignition is virtually instantaneous. Due to the reactive nature of the material, once ignited a large amount of heat is produced which is sufficient to ignite the propellant in close proximity. Further the pressure spike generated by the material is not large compared with the conventional ignition systems. In use the pressure within the rocket motor during ignition is only slightly higher than that of the standard operating pressure of the rocket motor. This results in a more even bum and means that the rocket casing can be designed for a pressure comparable to the operating pressure which reduces the need for weightier and more expensive casings. In addition this soft ignition characteristic is unlikely to damage the propellant grain as occasionally happens with convention systems and could lead to catastrophically high bum rates.

This particular vapour deposited material is also amenable to electrical and laser ignition methods and is particularly amenable to having metallic contacts vapour deposited on the surface thereof as the metallic surface offers a good surface upon which to receive deposited contacts. The material may also be ignited by conventional igniters such as an electric match.

Preferably the polymeric material is PTFE due to its high potential oxidant energy content. The metallic material may be magnesium due to its reactive and energetic nature, especially when used with PTFE. However, both PTFE and magnesium are generally non toxic materials which are not especially harmful to the environment.

6 According to a second aspect of the invention there is provided a rocket motor comprising propellant and an igniter as described above.

In a third aspect of the invention there is provided a rocket motor with a cigarette bum propellant configuration further comprising an igniter comprising at least one sheet of vapour deposited pyrotechnic material disposed adjacent to the rearmost surface of the propellant and a means for igniting the propellant.

The at least one sheet of pyrotechnic material is substantially the same shape as the base of the propellant. This ensures that the base of the propellant is ignited in a consistent way across the whole surface to ensure consistent burning. As the pyrotechnic material is mostly consumed during ignition, the igniter does not interfere with any plenum chamber or flow of propellant gases. Further, should the rocket motor contain any supports for the propellant, e.g. to maintain a plenum chamber, the vapour deposited pyrotechnic material may be located on the propellant side of the supports ensuring that any such supports do not interfere with ignition over the whole surface of the base of the propellant.

According to a further aspect of the invention there is provided a rocket motor with a radial bum propellant configuration wherein the propellant defines at least one cavity, further comprising an igniter comprising a roll of vapour deposited pyrotechnic material disposed adjacent to an inner surface of the propellant bounding said or each cavity and a means for igniting the vapour deposited pyrotechnic material.

Again the ignition is very rapid across the whole sheet of the vapour deposited pyrotechnic material and so the whole internal surface of the propellant will be ignited at the same time again ensuring consistent burning.

Using vapour deposited pyrotechnic material in this way also removes the need for any cavity to be kept empty. The pyrotechnic material is adjacent to the propellant and so ignition is not necessarily affected by other material which may be present in the cavity. Thus the rocket motor may also comprise a barrier means for preventing spalling from the internal surface. Such means could include a roll of suitable material inserted into the cavity or a foam insert. This would reduce the chance of fragments spalling off the inner surface of the propellant and stop any such fragments from impacting energetically on the 7 other side of the cavity. The barrier means could be formed from a lightweight material, such as cardboard or combustible foam, and be adapted such as not to interfere with the operation of the rocket motor or could be such as to be ignitable by the vapour deposited pyrotechnic material so as to be consumed during ignition of the rocket motor. Further examples and embodiments of the invention will now be described with reference to the following drawings of which; Figure 1 shows a rocket motor of the radial bum type having an igniter according to the present invention.

Figure 2 shows an alternative embodiment of an igniter according to the present invention, Figure 3 shows a cross section of the vapour deposited pyrotechnic material of the igniter of figure 2, Figure 4 shows a cigarette bum type rocket motor having an igniter according to the present invention, Referring to figure 1 a rocket, generally indicated 2, has a motor cavity defined by the rocket casing 4 and forward wall 6. At the rear of the motor cavity is a nozzle 8. Contained within the cavity is a block of propellant 10 arranged in a cigarette bum configuration with a rearward plenum chamber 12. Adjacent the rear surface of the propellant 10 and covering the whole of the surface is a sheet of vapour deposited pyrotechnic material 14. Fibre optic guides 16 may be attached to the rocket casing 4 and could either be connected with a laser source (not shown) within the rocket or communicate with a laser source mounted on the launcher. The fibre optics guides 16 are directed to focus the light 8 from the laser source onto the surface of the vapour deposited pyrotechnic material to cause localised heating of the vapour deposited pyrotechnic material resulting in ignition.

The vapour deposited pyrotechnic material 14 is that described in European Patent EP 0,463,022. This material is easy and cheap to produce and has reliable ignition over the whole of its surface generating a large amount of heat. A substrate of polytetrafluoroethylene (PTFE) film, about 25 trn thick is coated with magnesium on both sides, each side having a layer about 6 - 10 pm thick. The ratio of magnesium to PTFE is chosen so as to be near stoichiometric with a slight excess of magnesium. The material is produced by vapour depositing magnesium onto the PTFE by an ion plating process operating at a pressure of approximately 4 x 10' Torr and a relatively high acceptance angle. The magnesium layer therefore has a dark colour which improves its absorption properties and has an open microcrystalline structure which gives good laser ignition characteristics.

It will be readily appreciated by one skilled in the art that the manufacture of such materials could be easily given a high degree of automation with a roll of PTFE being coated on both sides in a deposition chamber and the appropriate shape for the igniter cut or stamped from the coated roll.

Upon initiation of the laser, energy is directed onto the pyrotechnic material 14. The size of the laser spot is such to illuminate only a few microcrystals of the magnesium layer. Due to the dark nature of the material a large amount of the energy is absorbed causing localised heating. As the structure of the magnesium is a disordered, microcrystalline structure thermal conduction is limited and therefore a localized reaction starts between the magnesium and the PTFE. This evolves a substantial amount of heat which causes the material around to react.

During ignition the vapour deposited pyrotechnic material 14 is consumed and due to the small amount of material used in the igniter, the amount of gas produced is minimal. Thus the pressure in the motor cavity is not much greater during ignition than the motor operating pressure. Rocket casing 4 can therefore be designed around the effective operating pressure.

9 Figure 2 shows an igniter according to an alternative embodiment of the invention. The igniter has a sheet of vapour deposited pyrotechnic material 18 sealed in a cover 20 of PTFE. The vapour deposited pyrotechnic material 18 is again material according to European Patent EP- 0,463,022 but has a structure as shown more clearly in figure 3. The vapour deposited pyrotechnic material comprises a film of PTFE 28 as the polymeric substrate, upon which magnesium 30 is coated on both sides. The PTFE 28 is approximately 25 Rm thick and each magnesium layer 30 is approximately 6 - 10 Rm thick. Again the ratio of magnesium to PTFE is chosen to have a slight stoichiometric excess of magnesium.

On top of one of the layers of magnesium 30 are vapour deposited two copper pads 22, although the skilled person will appreciate that other metallic materials could be used instead. Each pad is about 1 Rm thick and large enough to secure a connecting wire. The pads are typically about 2 mm apart. The copper pads 22 are connected to insulated wires 24 which are connected to firing unit 26. Wires 24 may be connected to pads 22 by any convenient means but soldering offers an easy, simple and cheap method of joining with a robust join and good conduction properties.

The copper pads 22 and contacts to the wires 24 are sealed with the PTFE cover 20 along with the sheet of vapour deposited pyrotechnic material. The vapour deposited pyrotechnic material and contacts are therefore protected from dirt and moisture etc. and the magnesium layers 30 are protected from atmospheric oxidation thus prolonging the effective life of the igniter. Further, the energetic materials, used as propellants in rocket motors contain noxious substances which may leach from the propellant over time and would otherwise degrade the performance of the igniter.

The cover may easily be formed by heat sealing PTFE around the vapour deposited pyrotechnic material although any suitable means for creating a cover could be used instead. It will also be apparent to one skilled in the art that other materials could be used for the cover which would offer the same protection and would be consumed during ignition.

The firing circuit 26 is capable of applying a firing current across the copper pads 22 in receipt of a firing signal. The firing circuit 26 may have a safety and arming means which short circuits or earths the copper pads 22 when in the safe position. When in armed mode receipt of a firing signal causes the firing circuit to generate a potential difference across the copper pads 22. A pulse of 12 V at 4 - 8 amps for approximately I second is sufficient to cause ignition of the pyrotechnic material 18. Once ignited the vapour deposited pyrotechnic material will rapidly react and the PTFE cover 20 will add to the reaction.

This igniter is suitable for use in the rocket motor shown in figure 4 where like components are given like numerals. Here the propellant 32 has an annular cross-section defining a cylindrical central cavity. The sheet of vapour deposited pyrotechnic material 18 is formed into a roll and lines the inner surface of the propellant 32. The pyrotechnic material may easily be inserted into the central cavity of the propellant as a loose roll and allowed to unwind until in contact with the propellant 32 where it may be fastened if required. The firing unit 26 is attached to the rocket casing 4 away from the propellant.

Also inserted into the central cavity of the propellant 32 is a generally cylindrical insert made from a combustible foam material. This insert 34 reduces the effect of spalling of propellant from the inner surface on fragment attack and stops any propellant that does spall from reaching the other side with high velocity. As there is no barrier between the vapour deposited pyrotechnic material 18 and the propellant 32 the effectiveness of the ignition is not diminished.

When the pyrotechnic material 18 is ignited very rapid ignition takes places across the whole of the roll, thus the top of the inner surface of the propellant is ignited at substantially the same time as the lower part ensuring an even bum with the desired propellant effect. On ignition the pyrotechnic material 18, and any PTFE cover, will combust and the majority of the material will be consumed during the reaction. The combustible foam material will also ignite and be consumed. Thus the operation of the rocket motor is unhindered and the weight of the igniter remaining after ignition is complete is very low.

I I It will be appreciated by one skilled in the art that other configurations of vapour deposited pyrotechnic material may be used without departing from the spirit of the invention.

Claims

1. An igniter for igniting the propellant of a rocket motor comprising at least one sheet of vapour deposited pyrotechnic material adapted so as to be locatable adjacent substantially all of a surface of the propellant which is available for ignition and a means for igniting the pyrotechnic material.

2. An igniter according to claim I wherein the means of igniting the vapour deposited pyrotechnic material comprises two metallic contacts in contact with the vapour deposited pyrotechnic material and connected to a firing circuit capable of generating a firing current between the contacts.

3. An igniter according to claim 2 wherein the metallic contacts are vapour deposited on top of the vapour deposited pyrotechnic material.

4. An igniter according to claim 2 or claim 3 wherein the metallic contacts comprise two pads vapour deposited close to one another on the surface of the vapour deposited pyrotechnic material.

5. An igniter according to claim 4 wherein the pads are formed from copper. 6. An igniter according to any preceding claim wherein the vapour deposited pyrotechnic material is sealed in a cover of PTFE.

7. An igniter according to claim I wherein the means for igniting the vapour deposited pyrotechnic material comprises a firing circuit controlling a means for directing laser light onto the vapour deposited pyrotechnic material.

13 8. An igniter according to claim 7 wherein the means for directing laser light onto the vapour deposited pyrotechnic material comprises a laser diode.

9. Am igniter according to any preceding claim wherein the vapour deposited pyrotechnic material comprises a substrate film of oxidising halogenated polymeric material having vapour deposited thereon at least one layer of an oxidisable metallic material in at least one location on the surface of the substrate, the polymeric and metallic materials being conjointly capable of reacting together exothermically on ignition, the thickness and composition of the metallic material being such as to ensure reliable and consistent lateral progression of the exothermic reaction.

10. An igniter according to claim 9 wherein the polymeric material is selected from PTFE or PCTFE.

11. An igniter according to claim 9 or claim 10 wherein the metallic material is magnesium.

12. Am igniter substantially as described hereiribefore with reference to figure 1 of the accompanying drawings.

13. An igniter as substantially hereinbefore described with reference to figures 2 to 4 of the accompanying drawings.

14. A rocket motor comprising propellant and an igniter according to claim 1. 15. A rocket motor with a cigarette bum propellant configuration and further comprising an igniter comprising at least one sheet of vapour deposited 14 pyrotechnic material disposed adjacent to the rearmost surface of the propellant and a means for igniting the vapour deposited pyrotechnic material.

16. A rocket motor according to claim 15 wherein the at least one sheet of vapour deposited pyrotechnic material is substantially the same shape as the rearmost surface of the propellant. 17. A rocket motor with a radial bum propellant configuration wherein the propellant defines at least one cavity, ftirther comprising an igniter comprising a roll of vapour deposited pyrotechnic material disposed adjacent to an inner surface of the propellant bounding said or each cavity and a means for igniting the vapour deposited pyrotechnic material. 18. A rocket motor according to claim 17 ftirther comprising a barrier means for preventing spalling of propellant from the inner surface thereof. 19. A rocket motor as claimed in claim 18 wherein the barrier means comprises a foam insert disposed within a cavity of the propellant. 20. A rocket motor as claimed in claim 18 wherein the barrier means comprises a roll of material disposed within a cavity of the propellant. 21. A rocket motor as claimed in any of claims 18 to 20 wherein the barrier means is formed from a flammable material and is adapted to be ignited by ignition of the propellant. 24. A rocket motor as substantially as hereinbefore described with reference to figure I of the accompanying drawings.

25. A rocket motor substantially as hereinbefore described with reference to figure 4 of the accompanying drawings.