GB1605239A - Smokeless propellant systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO SMOKELESS PROPELLANT SYSTEMS (71) I,THESECRETARY OFSTATE FOR DEFENCE, London do hereby declare the invention,for which I pray that a patent may be granted to me, and the method by which it is performed, to be particularly described in and by the following statement: The invention relates to propellant charges for rocket motors and specifically to propellant charges surrounded by an inhibiting coating.

In rocket motors, especially those employing an end-burning propellant charge, the propellant is encased, on those faces where combustion is not desired, by an inhibiting coating. In motors required to be substantially smoke-free, typically employing double-base propellants, this inhibiting coating may constitute a major source of smoke emission.

It has now been found that very low levels of smoke emission from rocket motors may be achieved by using an inhibiting coating comprising the formaldehyde polymer, polyacetal, either alone or in combination with other oxygen-containing polymers.

According to the invention, therefore, a solid propellant charge has an inhibiting coating which contains between 30% and 100% by weight of polyoxymethylene and has an overall carbon to oxygen atomic ratio of not more than 2 to 1. Preferably the carbon to oxygen ratio is not-more than 1.2 to 1.

Polyoxymethylene, also known, and herinafter reffered to, as polyacetal, has the basic structure:

In practice, however, commercial forms will normally also contain small quantities of end-stopping groups and copolymers, antioxidants and mildy basic additives to remove traces of acid. It gives extremely little smoke, either as soot or chemical mists, when exposed to intense flame, even in the absence of oxygen or air. This is probably related to the structure and the low carbon to oxygen ratio (C: O= 1: 1) which avoids the formation of soot and to the direct decomposition to the "clean" gases carbon monoxide and hydrogen without the formation of potentially mistproducing intermediates.

Polyacetal is readily injection moulded into a pre-formed "beaker" to be filled with propellant and has low water and nitroglycerine absorption making it highly suitable as a propellant coating. However, the surface of the unmodified moulded material will not readily form with most propellants a bond of sufficient strength for effective inhibition unless the surface of the polyacetal is prepared, preferably by surface modification.

According to one preferred aspect of the invention therefore, the inhibiting coating, especially when consisting of substantially unmodified polyacetal, is subjected to a surface treatment to improve its capacity to bond to the propellant charge. Suitable surface treatments include radio-frequency etching in argon or acid etching, for example the satinising process generally described in UK Patent Specification No 950,703, using a dispersion of an inert, finely-divided solid, such as a silica, in a solution or dispersion in a liquid vehicle of a strong etching agent which is capable of etching the thermoplastic rapidly at ordinary temperatures and pressures. Suitable etching agents include phosphoric, sulphuric and hydrochloric acids, hexafluoroacetone sesquihydrate, chlorosilanes, silyl peroxide and especially p-toluene sulphonic acid.The liquid vehicle may be water or an organic solvent especially a chlorinated hydrocarbon.

The surface treated polyacetal inhibitor may be bonded to the propellant with a conventional adhesive such as an epoxide or a phenol-formaldehyde/polyvinyl formal adhesive. A pre-formed propellant charge may be coated with adhesive and inserted into the preformed inhibiting coating. Preferably, however, the adhesive is applied to the heated interior surface of the preformed inhibiting coating and cured. The propellant is then cast and cured within the preformed coating to form the bond.

Alternatively, the bonding of the polyacetal to the propellant may be improved by incorporating various polymers, normally oxygen containing polar polymers into the inhibiting coating. Such polymers also tend to flexibilise the inhibiting coating, thereby reducing the bond strength necessary to avoid failure under the stress of firing.

Virtually all commercial polymers (except nitrate polyhydroxy structures, such as cellulose nitrate) have much higher carbon to oxygen atomic ratios than polyacetal and many are liable to produce dense white mists on rapid pyrolysis. The aim for low smoke inhibition is to achieve the highest possible practical loading of polyacetal in any combination of suitable polymers whilst retaining the necessary requirements of compatability with propellant, physical properties (such as flexibility), ease of fabrication etc, conferred by the other polymers.

Any polymer used in conjunction with polyacetal must itself have a reasonably favourable C: 0 ratio or the low smoke benefit of the polyacetal may be lost. The higher the C : O ratio of the other polymer, the higher the loading of polyacetal that will be required to achieve desired low smoke levels. The overall value of the C: 0 atom ratio for the inhibiting coating should be not more than 2 to 1 and should preferably not exceed 1.2 to 1.

Suitable polymers for use in conjunction with polyacetal in inhibiting coatings in accordance with the present invention include cellulose materials, typically conventional cellulose-based inhibiting materials such as ethyl cellulose and especially cellulose acetate.

According to a further aspect of the invention the inhibiting coating on the solid propellant charge comprises polyacetal containing a cellulosic material in a proportion of up to 70% by wt of the inhibiting coating.

The cellulosic material is preferably plasticised cellulose acetate and the cellulose acetate and plasticiser together preferably comprise about 40% to 50% of the inhibiting coating in order to achieve adequate bonding without undue smoke emission.

In addition to low smoke generation, the inhibiting coating should have low nitroglycerine absorption since absorbed nitroglycerine may seriously reduce the burn-through times of the inhibitor.

Nitroglycerine is readily absorbed from the propellant by the cellulose materials but the overall amount taken up by the inhibitor is limited by a high proportion of polyacetal in the mixture (50% to 60%). Additional protection against failure as a result of nitroglycerine absorption by the cellulose acetate part of the matrix may be provided by the inclusion of char catalysts which increase the resistance of the inhibitor to propellant flame. These catalysts induce char formation in the polyacetal/cellulose acetate mixture during pyrolysis and the char reduces the heat flux from the rocket flame gases to the inhibitor substrate.Lead salts of organic acids, lead oxides, basic lead carbonate (white lead) and antimony oxide have been found to be effective char catalysts at concentration of approximately 1 to 4% w/w typically 2%, of the inhibiting coating. The char produced may be chemically active and, after nitroglycerine uptake by the cellulose acetate, may have reduced efficiency in providing flame resistance. This residual activity should therefore preferably be poisoned by a further addition of small amounts (e.g.about 1% by weight) of a phosphate or borate. The ammoniumphosphatea have been found to be effective at about 1% w/w of the inhibiting coating.

For ease of fabrication the cellulose acetate should be plasticised and for minimum smoke this should be by a low-smoke plasticiser having a relatively high oxygen to carbon atom ratio, for example triacetin, triethyl citrate or acetyl triethyl citrate.

The mixed polymer material may be readily injection moulded to form a "beaker" as with polyacetal alone. (The material from injection moulding of polyacetal/cellulose acetate appears to be a two phase system of polyacetal fibres in a continuous cellulose acetate matrix).

The propellant may often be bonded directly to the moulded beaker, although in some cases, especially with inhibitors of high polyacetal content, the use of adhesive may be desirable.

Surface treatment will not, however, generally be necessary except with very high polyacetal loading.

Solid propellant charges with inhibiting coatings according to the invention may be of any known solid propellant type, but preferably the propellant system is a double-base propellant, containing both nitrocellulose and nitrate ester plasticiser since such propellants are themselves reatively smoke-free.

Processes for the production of inhibited propellant charges in accordance with the various aspects of the invention will now be described by way of example and the products compared with conventionally inhibited charges.

EXAMPLE 1 To assess the effects of various surface treatments on the bonding of polyacetal to propellant in the absence of adhesive, sample discs of polyacetal were surface treated by various means and then bonded to double-base propellant charged by pressing together discs of propellant and polyacetal, coated casting liquid (desensitised liquid organic nitrate ester) on the mating surfaces, and heating at 60"C for 6 days to cure the bond.Two typical doublebase propellants were tested: Propellant A had a low nitrocellulose content (43%) and TABLE 1

Surface Treatment of Polyacetal Bonding failure load Propellant A Propellant B Failure Mode KPa (psi) KPa (psi) Polyacetal Pet ether wipe 1030 (150) 1650 (240) Interfacial P/A Decrease by: Acetone - 2340 (340) " Trichlorethylene - 2475 (360) " Scrubbed with Teepol 1170 (170) 2755 (400) Polyacetal 1375 (200) 2755 (400) " Abraded Polyacetal Acid H2SO4/HNO3 1:1 v/v 10 sec dip 1375-(200- 2475 (360) " (conc.) (conc.) 1895 275) H2SO4/HNO3 2:1 v/v 10 sec dip - 3440 (500) " (conc.) (conc.) H2SO4/HNO3 2:1 v/v 10 sec dip - 2890 (420) " (conc.) (conc.) H2SO4/HNO3 3::1 v/v 10 sec dip 1375 (200) 3990 (580) " (conc.) (conc.) H2SO4/HNO3 3:1 v/v 10 sec dip 1790 (260) 3990 (580) " (conc.) (conc.) H2SO4/HNO3 1:1 v/v 10 sec dip 1375 (200) 5235 (760) " (Propellant A interfacial, (conc.) (conc.) (Propellant B, 60% A, 40% P/A H2SO4/HNO3 1::1 v/v 10 sec dip - 5925 (860) " Propellant A interfacial, (conc.) (conc.) (Propellant B, 60% A, 40%P/A Conc HNO320 sec dip 1550 (225) Interfacial Polyacetal Satinized 1995 (290) 6200 (900) Propellant A interfacial, Propellant B, 55% A, 45% P/A Polyacetal Radio 1720-(250- 4825-(700- Interfacial Polyacetal Frequency 2065 300) 5510 800) Polyacetal Sputter Etch Failure Mode P/A Denotes propellant to polyacetal interface A Denotes failure within the polyacetal but near the surface Notes:I As in example 3 "Teepol" is a Registered Trade Mark.

Propellant B had a high nitrocellulose content (54%). The bond strengths are shown in Table 1.

EXAMPLE 2 Whilst the above bond strengths may be sufficient for some purposes, higher bond strengths are often desirable. Hence similar tests were carried out applying various conventional adhesive coatings, used for bonding propellants to inhibitors, to polyacetal which had been radio-frequency etched in argon or satinised with a solution comprising 4150 ml tetrachloro-ethylene, 250 ml dioxan, 21 g finely divided silica and 21 g p-toluene sulphonic acid at 105 to 115 for 2 minutes followed by rinsing and drying. The resulting bond strengths to the same propellants A & B as in example 1 are given in Table 2.

EXAMPLE 3 A smokeless propellant charge having a polyacetal inhibiting coating was produced starting with a preformed injection moulded polyacetal beaker approximately 150mm diameter, 200mm long and of wall thickness 2.5mm having one end closed except for a central spigot hole of internal diameter 16mm.

The beaker was supported in a jig with the open end uppermost and the rim horizontal.

The spigot hole was plugged and the beaker filled up exactly to the rim within 30 seconds with a satinising liquid, comprising 4150 ml tetra-chloro ethylene, 205 ml dioxan, 21g finely divided silica (Cab-O-Sil; Registered Trade Mark) and 21g p-toluene sulphonic acid at 105 to 1 150C. The liquid was left for 1 minutes and was then drained off through the spigot hole within 30 seconds. Within 30 seconds of draining the beaker it was suspended by the spigot, in an oven at 100 # 2 C for 2 minutes # 5 seconds. The etching was then immediately arrested by plunging the beaker into warm (45 - 60 C) water followed by washing for hour.

The beaker was dried at 100 + 5 C for 15 minutes. Application of adhesive to the satinised surface could have commenced as soon as the beaker had cooled to less than 30 C TABLE2

Adhesive Coating t Bonding failure load KPa (psi) Propellant A Propellant B Untreated Polyacetal Redux/Formvar 1040(150) 4000 (580) Satinised Polyacetal Redux/Formvar 2100-2800 (300-400) 5959-7000 (850-1000) Formvar 1050-1750 (150-250) 2100-3850 (300-550) Formvar/MMU 1330(19(1);; 154(1(22(1) Epophen (N) 8 1260 (180) 1750 (250) Scothweld 1260-3115 (180-445) 2460-3150 (350-450) Epophen (N)8/Formvar 203(1-329(1(29(1-471)) 5530 (790) Epophen (N)8/Redux/Formvar 51811(74(1) Scothweld/Formvar 1190-2030) (17(129(1) 126(1-14(X) (18()-200) RF Etched Polyacetal Redux/Formvar | 2240 (320) | 7000 (1000) Formvar 245() (350) 5880 (84()) Epophen (N)8 119(1(17(I) 1050-3360 (150-480) Epophen (N)8/Formvar/MMU 262() (36()) 7280 (1040) * Redux - Vinyl phenolic adhesive, supplied bi CIBA Ltd Formvar- Polyvinyl formal, supplied by Shawinigan Co MMU - Methoxy methyl uron Epophen - Bisphenol A - type epoxide with aromatic amine hardener, supplied by Border Chemical Co Scotchweld - Epoxide adhesive, supplied by Minnesota Mining & Manufacturing Cp "Redux". "Formvar" and "Epophen" are Registered Trade Marks.

but usually about an hour intervened.

Exceptionally, provided the beaker was kept in a dirt free atmosphere, the interval could be up to four days.

A duplex layer adhesive phenolic/polyvinyl formal was used since this was well tried in other inhibition systems. The phenol formaldehyde adhesive was applied as a solution of equal parts by wt. of vinyl phenolic (liquid Redux 775) and polyvinyl formal (Formvar 1595E) in a solvent mixture comprising cyclohexanone/ethylene dichloride (2:1 by vol), referred to as lacquer No.1. The viscosity of this laquer was adjusted so that the time taken for a 0.80mm dia steel ball to fall 100mm in a 25mm dia glass cylinder filled with a sample of the lacquer at 20 +2 C was between 5.1 and 5.9 seconds. While the beaker was being rotated horizontally about its axis, on suitable rollers, 80ml lacquer No 1 was poured into it. The speed of rotation must be fast enough to prevent spillage, typically 25 rpm.

Rotation was then slowed until the lacquer flowed to the rim but did not spill out. Lacquer was now spread with a camel hair brush onto the base and any other part of the inside of the beaker not already covered. The base was coated thinly to avoid globules forming during the ensuing drainage period. After 10 minutes total rolling time the beaker was inverted over another beaker so that the rims were in intimate contact and the inside surfaces continuous with each other. The lacquered beaker then drained into the lower one without leaving a build up of lacquer at the rim. This system was sealed except for the spigot hole of the upper beaker. After 30 minutes drainage the beaker was dried at room temperature for one hour (hanging by the spigot) and then at 60 C for 1 hour.

Up to four coats of lacquer No 1 was applied in this manner after which, storage at 60 C for 6 hours was necessary to reduce the solvent present to an acceptable level. A weight increase of at least 2.5g was necessary to ensure sufficient lacquer thickness.

To produce the Formvar coat 5g polyvinyl formal were dissolved in the same solvent mixture as lacquer No.1, to give 100ml of solution known as lacquer No..2. This was sprayed evenly onto the inside of the beaker rotating at 200 rpm taking 40-60 seconds for each coat. Two coats were applied in this way, with hr at room temperature and lh hr at 60 C in between. Then followed drying for hr at room temperature and 6 hrs at 60 C to reduce the solvent content to an acceptable level. The increase in weight due to this process was at least 1g.

The adhesives were cured by heating the beaker for 1 to 1 hrs at a maximum of 150 C and at atmospheric pressure. Minimum temperatures were 146 C during the earlier period.

If there had been any areas where the adhesive layer was too thick, the formaldehyde vapour formed during the curing schedule would have been likely to cause bubbling in the adhesive layer. This is not acceptable except for isolated bubbles not larger than a pin head, in the base area.

The double base propellant charge was then cast in situ by filling the beaker with casting powders for the propellants, first propellant A then propellant B (see Example 1), flooding with casting liquid and heating to 60"C for 6 days.

EXAMPLE 4 A propellant charge in accordance with the invention was prepared having an inhibiting coating comprising polyacetal and plasticised cellulose acetate.

Moulding granules of the inhibiting material were prepared by mixing the powered ingredients and plasticiers in a heavy duty mixer and granulating the mix in a conventional machine. The pre-granulation mixture was compounded with: (, w/w Polyacetal resin powder (PA) (Celcon Stabilised Flake; "Celcon" is a Registered Trade Mark) 54 Cellulose Acetate Powder (CA) (54% Acetylation) 24 Triacetin (TA) 19 Lead Citrate (fine powder) Ammonium dihydrogen orthophosphate (fine powder) I Minor variations in plasticiser to polymer ratios, for convenience of moulding or to accommodate different (low smoke) plasticisers, could however have been made without significantly affecting the functioning of the moulded product.

The moulding granules were injection moulded to form a beaker as described in Example 3 and this was then filled with propellants A and B as in Example 3, but without prior surface treatment.

The amounts of smoke generated by rocket motors using inhibited charges as described in Examples 3 & 4 were measured and compared with that produced by the same motor configurations employing other low smoke inhibiting materials currently available. The test vehicle had a configuration similar to that of a practical anti-tank guided missle except that all other materials within such a motor that might also produce smoke were rigorously excluded. The propellant charge was a solid end-burning grain about 150mm diameter, inhibited on the cylindrical and head-end surfaces by a 2.5mm thickness of the test inhibition. The double-base propellants used in the sustain phase (20-25 seconds - Propellant A) and the boost phase (5 seconds - Propellant B) of this test motor were virtually smokeless.

The amount of smoke emitted by a rocket motor is greatly affected by the presence or absence of secondary combustion (flash; after burning) of the fuel-rich exhaust gases in the atmosphere. Organic smokes can be burned up in this flame but inorganic smokes cannot.

Although advantageous as far as organic smokes are concerned, secondary flame may interefere with guidance and at night might well be intolerable; smokeless without secondary combustion is therefore desirable.

The present invention aims at very low smoke with or without the secondary combustion and those inorganic components which are providing a useful function in the formulations are kept to a minimum.

Table 3 below compares the smoke outputs, in the presence or absence of secondary TABLE 3 Observed Level of Smoke (Arbitrary 0-1U Scale) Time Inhibition/Time from ignition (secs) 5 10 15 20 25 average 1 WITH EXHAUST FLAME Hypalen CL4851 2.8 1.5 1.4 1.2 1.2 1.6 HypalonCL898O 1.1 1.4 1.0 1.2 1.4 1.2 Polyacetal 0.5 0.5 í).4 0.5 0.2 0.4 Polyacetal-modified 0.7 0.5 0.7 0.8 (1.8 0.7 cellulose acetate 2 WITHOUT EXHAUST FLAME Hypalon CL4851 2.5 2.5 2.2 1.6 1.4 2.0 HypalonCB8980 1.5 2.1 1.7 1.8 1.9 1.8 Polyacetal 1.1 1.2 1.0 1.0 1.0 1.1 Polyacetal-modified 1.5 1.2 1.1 1.0 ().9 1.

cellulose acetate Average of several runs Present invention See Table 4 ("Hypalon" is a Registered Trade Mark) Thus inhibitors according to the present invention give smoke levels less than conventional inhibitors.

TABLE 4

Burn-throuh Composition! wt time (see) Char PA CA TA catalyst Other 35 33.7 29.3 | 2leadcitrate 0 47 45 28.5 24,5 2leadcitrate 0 46 55 23 2() 2 lead citrate 0 64 50 28.8 20 | 1 white lead 0.2Carbamite' | 40 25 48 25 2 lead citrate 0 50 40 38 | 20 2 lead citrate 0 48 55 27.8 15 2 lead citrate 0.2 Carbamite | 49 54 29,8 15 0 .2Carbamite | 39 55 27,8 15 2 white lead 0,2Carbamite 77 54 26.8 | 15 4whitelead 0.2Carbamitel 75 100 | 0 | 0 | 0 | 0 | 37 0 75 25 0 (1 27 0 74 25 l lead citrate 0 40 0 | 65 | 35 | 0 | 0 | 19 0 1 64 35 1 lead citrate () 41 Hypalon CL27X9 1 70 Hypalon CL8436 | 40 Hypalon CL4X51 24 Hypalon CL8980 | 23 dimethyl diphenyl urea Filled chlorosulphonated hydrocarbon rubbers (supplied by IMI Ltd) used as propellant inhibitors combustion of the exhaust, for the least smoky inhibition materials currently available.The conventional inhibition materials phthalate plasticised ethyl cellulose and cellulose acetate are much more smoky than the materials compared below and have not been included.

Moreover unmodified cellulose acetate compositions would have a strictly limited storage life in contact with most double-base compositions and ethyl cellulose would present bonding difficulties in the double-base propellant filling process.

EXAMPLE 5 To function effectively the charge inhibition must resist heat penetration from the hot propellant gases to the unburnt propellant for the designed operating time of the motor. A laboratory test for comparing the resistance of materials suitable for charge inhibition to high temperature flame, compares the times required for the rear face of a specimen 2.5 mm thick to reach a temperature of 1500C when a coal gas/oxygen flame is applied to the front face. " Burn-through times" for the proposed inhibitors with and without various char catalysts are compared with other kinds of inhibition currently available in Table 4.

EXAMPLE 6 Cellulose acetate inhibition fails to function after storage in contact with double base propellants because of inadequate resistance to flame nitroglycerine has been absorbed from the propellant. Whereas char-forming catalysts increase the flame resistance of cellulose acetate at ordinary pressures whether nitroglycerine is present or not, there is a risk that they may be less effective under conditions of high pressure and high nitroglycerine concentrations. This risk is removed by the addition of small amounts of ammonium phosphate which reduce the chemical reactivity of the char.

To appraise the effect of nitroglycerine uptake on the proposed inhibition, sheet samples 2.5mm thick were stored at an elevated temperature (60 C) in contact with propellant containing a high proportion of nitroglycerine (41%). After fifty days a total weight increase of 15% (assumed to be all nitroglycerine) has occurred and the laboratory flame test showed the aged material to have a greater resistance to flame than fresh, unexposed cellulose acetate (Table 5).

With unmodified cellulose acetate inhibition, rocket motor experience suggests that an overall concentration of nitroglycerine of 20% is likely to present a serious risk of failure. If a similar overall concentration is assumed to result in failure of the present polyacetal modified cellulose acetate, and the improvements imparted by lead compounds and ammonium phosphate are ignored pessimistic estimates of the storage life of the material may be obtained.Thus, considering storage under "UK ambient conditions" (nominally equivalent to 160C), the time for TABLES

Material (Nominal composition Weight prior to test) gain after time for 2.5mum propellant thick sample PA CA TA Char contact (seconds) catalyst (sus) 55 28 15 2 lead citrate 16 30 55 28 15 2 white lead 14 41 CONTROLS () 75 25 (I Unexposed 25 () 75 25 () 16(1) 19 ( I ) stored with propellant containing 34% nitroglycerine the polyacetal modified cellulose acetate (55% polyacetal) to acquire a failure concentration of nitroglycerine when stored in contact with propellant containing different concentrations of nitroglycerine is estimated as follows: Nitroglycerine in propellant: 27% 36% 41% Lowest Estimated 'life' of inhibition (years): Indefinite 8 5-6.

WHAT WE CLAIM IS; 1. A solid propellant charge having an inhibiting coating which contains between 30% and 100% by weight of polyoxymethylene and has an overall carbon to oxygen atomic ratio of not more than 2 to 1.

2. A solid propellant charge according to Claim 1, wherein the inhibiting coating has an overall carbon to oxygen atomic ratio of not more than 1.2 to 1.

3. A solid propellant charge according to either Claim 1 or Claim 2 wherein the inhibiting coating has been subjected to surface treatment to facilitate bonding to the propellant.

4. A solid propellant charge according to either Claim 1 or Claim 2, wherein the inhibiting coating has been subjected to surface treatment to facilitate bonding to the propellant.

4. A solid propellant charge according to Claim 3, wherein said surface treatment comprises acid etching.

5. A solid propellant charge according to Claim 4, wherein the acid etching is carried out with a dispersion of an inert, finely divided solid in a solution of dispersion of p-toluene sulphonic acid in a chlorinated hydrocarbon solvent.

6. A solid propellant charge according to Claim 3, wherein said surface treatment comprises radio-frequency etching.

7. A solid propellant charge according to either Claim 1 or Claim 2, wherein the inhibiting coating is bonded to the propellant by an adhesive.

8. A solid propellant charge according to Claim 7, wherein the adhesive is an epoxide ahhesive or a phenol-formaldehyde/polyvinyl formal adhesive.

9. A solid propellant charge according to any preceding claim, wherein the inhibiting coating contains at least one oxygen-containing polar polymer in addition to polyoxymethylene.

10. A solid propellant charge according to Claim 9. wherein said oxygen-containing polar polymer is a cellulosic material.

11. A solid propellant charge according to Claim 10, wherein the cellulosic material comprises up to 70% by weight of the inhibiting coating.

12. A solid propellant charge according to Claim 10 or Claim 11, wherein the oxygencontaining polar polymer is cellulose acetate.

13. A solid propellant charge according to Claim 12, wherein the inhibiting coating contains a plasticiser for the cellulose acetate.

14. A solid propellant charge according to Claim 13, wherein the plasticiser is triacetin, triethyl citrate, or acetyl triethyl citrate.

15. A solid propellant charge according to either Claim 13 or Claim 14, wherein the cellulose acetate and plasticiser together comprise 40% to 50% by weight of the inhibiting coating.

16. A solid propellant charge according to any one of Claims 11 tol5 wherein the inhibiting coating contains 1% to 4% by weight of a char catalyst which is a lead salt of an organic acid, lead oxide, basic lead carbonate or antimony oxide.

17. A solid propellant charge according to any one of Claims 11 to 16 wherein the inhibiting coating contains about 1% by weight of a phosphate or borate.

18. A solid propellant charge according to Claim 1 substantially as hereinbefore described.

19. A solid propellant charge according to any one of the preceding claims in the form of a rocket motor.

20. A solid propellant charge according to Claim 1 substantially as hereinbefore described in either Example 3 or Example 4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (21)

**WARNING** start of CLMS field may overlap end of DESC **. TABLES Material (Nominal composition Weight prior to test) gain after time for 2.5mum propellant thick sample PA CA TA Char contact (seconds) catalyst (sus) 55 28 15 2 lead citrate 16 30 55 28 15 2 white lead 14 41 CONTROLS () 75 25 (I Unexposed 25 () 75 25 () 16(1) 19 ( I ) stored with propellant containing 34% nitroglycerine the polyacetal modified cellulose acetate (55% polyacetal) to acquire a failure concentration of nitroglycerine when stored in contact with propellant containing different concentrations of nitroglycerine is estimated as follows: Nitroglycerine in propellant: 27% 36% 41% Lowest Estimated 'life' of inhibition (years): Indefinite 8 5-6. WHAT WE CLAIM IS;

1. A solid propellant charge having an inhibiting coating which contains between 30% and 100% by weight of polyoxymethylene and has an overall carbon to oxygen atomic ratio of not more than 2 to 1.

2. A solid propellant charge according to Claim 1, wherein the inhibiting coating has an overall carbon to oxygen atomic ratio of not more than 1.2 to 1.

3. A solid propellant charge according to either Claim 1 or Claim 2 wherein the inhibiting coating has been subjected to surface treatment to facilitate bonding to the propellant.

4. A solid propellant charge according to either Claim 1 or Claim 2, wherein the inhibiting coating has been subjected to surface treatment to facilitate bonding to the propellant.

4. A solid propellant charge according to Claim 3, wherein said surface treatment comprises acid etching.

5. A solid propellant charge according to Claim 4, wherein the acid etching is carried out with a dispersion of an inert, finely divided solid in a solution of dispersion of p-toluene sulphonic acid in a chlorinated hydrocarbon solvent.

6. A solid propellant charge according to Claim 3, wherein said surface treatment comprises radio-frequency etching.

7. A solid propellant charge according to either Claim 1 or Claim 2, wherein the inhibiting coating is bonded to the propellant by an adhesive.

8. A solid propellant charge according to Claim 7, wherein the adhesive is an epoxide ahhesive or a phenol-formaldehyde/polyvinyl formal adhesive.

9. A solid propellant charge according to any preceding claim, wherein the inhibiting coating contains at least one oxygen-containing polar polymer in addition to polyoxymethylene.

10. A solid propellant charge according to Claim 9. wherein said oxygen-containing polar polymer is a cellulosic material.

11. A solid propellant charge according to Claim 10, wherein the cellulosic material comprises up to 70% by weight of the inhibiting coating.

12. A solid propellant charge according to Claim 10 or Claim 11, wherein the oxygencontaining polar polymer is cellulose acetate.

13. A solid propellant charge according to Claim 12, wherein the inhibiting coating contains a plasticiser for the cellulose acetate.

14. A solid propellant charge according to Claim 13, wherein the plasticiser is triacetin, triethyl citrate, or acetyl triethyl citrate.

15. A solid propellant charge according to either Claim 13 or Claim 14, wherein the cellulose acetate and plasticiser together comprise 40% to 50% by weight of the inhibiting coating.

16. A solid propellant charge according to any one of Claims 11 tol5 wherein the inhibiting coating contains 1% to 4% by weight of a char catalyst which is a lead salt of an organic acid, lead oxide, basic lead carbonate or antimony oxide.

17. A solid propellant charge according to any one of Claims 11 to 16 wherein the inhibiting coating contains about 1% by weight of a phosphate or borate.

18. A solid propellant charge according to Claim 1 substantially as hereinbefore described.

19. A solid propellant charge according to any one of the preceding claims in the form of a rocket motor.

20. A solid propellant charge according to Claim 1 substantially as hereinbefore described in either Example 3 or Example 4.

21. A process for the production of a

surface inhibited solid propellant charge substantially as hereinbefore described in Example 3 or Example 4.