GB2200915A - An insulating composition for producing an insulating layer on a rocket solid propellant charge - Google Patents

Abstract

Insulating compositions comprise, as low-smoke filler, a melamine salt of the formula <IMAGE> in which x and y are whole numbers and HA is an acid, preferably phosphoric, sulphuric, boric and/or oxalic acid.

Description

An Insulating Composition for Producing an Insulating Layer on a Rocket Solid Propellant Charge This invention relates to an insulating composition for producing an insulating layer on a rocket solid propellant charge which contains a hardenable binding agent and a filler.

It is known in respect of rocket solid propellant charges that in order to effect burn-off only from the desired burning surfaces, other surfaces which are not to take place in the burn-off have to be provided with a well adhered insulating layer. Such an insulating layer also imparts certain protection against mechanical stress to the propellant present in the combustion chamber. In the case of front burners, i.e. propellant charges insulated at the jacket surface and the head-side surface, the insulating layer serves above all also for protecting the combustion chamber against the hot gases arising upon burn-off of the propellant.

In the case of guided, particularly wire-guided, tactical rockets it is necessary to keep the development of smoke in the exhaust-gas jet of the rocket as small as possible. One reason this is important is that development of smoke, will make the flightpath of the rocket detectable, thereby possibly betraying its firing position.

Another reason is that such rockets are frequently equipped with an incandescent radiator at the tail, i.e. a pyrotechnical charge which generates IR radiation and is detected with controlled apparatus by the firer and the detectability thereof is made more difficult by the presence of smoke in the exhaust-gas jet.

Thus, the development of smoke in a flying body guided by sight should always be sufficiently low that the controllability of this flying body is ensured in every flight phase. In this respect, end-burner propellant charges are most commonly used and in this type of charge the permanently enflamed cylindrical part of the insulating layer can contribute considerably to the development of smoke.

To achieve low-smoke burn-off it is known to use melamine as a filler of the insulating layer (See German specification DE-AS 25 24 843). When melamine decomposes it absorbs -energy absorption, thus cooling the binding agent, and colourless, cooling reaction products, mainly water, ammonia and carbon dioxide, are formed. Melamine thus represses smoke formation and enhances the heat stability of the insulating layer.

To produce the insulating layer, the insulating composition is poured or pressed into a narrow gap between a casting mould and the external surface of a propellant charge which is arranged in the casting mould. It has become apparent that melamine influences the hardening speed of binding agents, particularly polyurethane binding agents, and reduces the pot time of the castable insulating composition in a non-reproducible manner. Moreover, the cooling effect achievable by melamine leaves something to be desired.The last mentioned disadvantage could be reduced by increasing the melamine content but limits are placed on using too high a proportion of filler by the associated deterioration of the processing properties of the insulating composition, such as the castability, in which respect, as mentioned, the pot time of the castable composition is reduced with increasing content of melamine.

The object of the invention is to provide a filler for the insulating layer of rocket solid propellant charges which is similar to melamine with respect to the poverty of smoke, but does not influence the pot time of the binding agent and brings about a marked increase in the cooling effect.

This is achieved, in accordance with the invention, by using, as filler, a melamine salt of the formula

in which x and y are in each case a whole number and HA is an acid.

HA may be a monobasic or polybasic inorganic or organic acid, but is preferably phosphoric acid (H3P04), sulphuric acid (H2S04), boric acid (H3B03) or oxalic acid (C2H204).

As a result of the use of these melamine salts as fillers, an undisturbed hardening reaction of the binding agent of the insulating composition is achieved as well as very high heat stability and low-smoke burn-off of the solid propellant charge.

The fact that in accordance with the composition of the invention the pot time of the insulating composition is not adversely influenced would seem to be traceable back to the fact that the melamine salt does not intervene in the hardening reaction of the binding agent, in contrast to melamine in basic form, which accelerates the hardening reaction of the binding agent, but in a non-reproducible manner, e.g. as a function of the particle size of the respective batch of melamine.The considerably better cooling effect of the filler in accordance with the invention would seem to be due to the fact that, in contrast to melamine, which at higher temperatures sublimates undecomposed and thereby disappears from the binding agent matrix (sublimation being less energyexpensive than chemical decomposition (pyrolysis) into small molecules), the melamine salts used in accordance with the invention are not subject to sublimation.

A further advantageous feature of the fillers used in accordance with the invention is that, despite their salt character, they possess extremely low watersolubility and hygroscopicity, such that the insulating layer, both upon production and upon use, is practically insensitive to air moisture. This means that the mechanical properties of the insulating layer in accordance with the invention are, even after fairly long storage, not reduced by efflorescense, swelling or the like. To give particular examples, the water solubility at 200C of melamine phosphate (melamine)2 H3P04 amount to 0.2g/100g and of melamine sulphate (melamine)2 . H2S04 amounts to 0.16g/100g.

Additionally, the decomposition temperatures of the preferred melamine salts used in accordance with the invention are relatively favourable, e.g. th & decomposition temperature of melamine phosphate (melamine)2 . H3P04 is higher than 340 C, that of melamine sulphate (melamine)2 H2P04 is about 380 C and that of melamine borate melamine H3B03 is greater than 360 C. This means that the fillers used in accordance with the invention decompose only when the binding agent of the insulating layer is pyrolised by the hot propellant waste gases. In other words, the strength of the insulating layer is not reduced by premature decomposition of the filler. On the other hand, the decomposition temperatures of the melamine salts are not so high that the binding agent pyrolises and vaporises before causing decomposition of the melamine salts in the insulating layer.

The melamine salts used in accordance with the invention are preferably contained in the insulating composition in a proportion of 20 up to 60r by weight.

Known binding agents, particularly binding agents on a polyurethane or polyester basis, are used as the hardenable binding agents. Apart from the melamine salt(s), the insulating composition in accordance with the invention can contain still other fillers, e.g. oxamide.

Upon use of melamine phosphate and melamine borate as filler it has additionally become apparent that upon decomposition of the filler a viscous smelt forms, which adheres well to the enflamed insulation and further represses the decomposition thereof.

The following examples serve to explain the invention further.

EXAMPLE 1 38.20 parts by weigbt of a low-viscous polyether which contains hydroxyl groups with on average three hydroxyl groups per molecule (Desmophene 550 U), 0.05 parts by weight of phenyl mercury oleate and.40.00 parts by weight of melamine borate are put together in this sequence, mixed and degassed in a vaccum. Then 21.75 parts by weight of hexamethylene diisocyanate- 1,6 are added to this premix and the mixture is again degassed in a vacuum.

Then a double-based propellant. block is arranged centrally in a casting mould and the annular gap between the propellant block and the casting mould is filled with the aforesaid readily castable insulating composition.

After that the insulating composition is allowed to harden possibly with slight heating.

EXAMPLES 2, 3 AND 4 Example 1 is repeated, apart from the fact that, instead of melamine borate (melamine . H3B03), 40.0 parts by weight of melamine phosphate (melamine)2 . H3P04 or, respectively, melamine sulphate (melamine)2 . H2S04- or, respectively, melamine oxalate melamine . C2H204 are used.

Claims (7)

1. An insulating composition for producing an insulating layer on a rocket solid propellant charge, which contains a hardenable binding agent and a filler, characterised in that the filler consists of or includes a melamine salt of the formula

in which x and y are in each case a whole number and HA is an acid.

2. An insulating composition as claimed in claim 1 wherein x is 1 or 2 and L is 1.

3. An insulating composition as claimed in claim 1 or 2 wherein HA is phosphoric acid (H3P04), sulphuric acid (H2S04), boric acid (H3B03) and/or oxalic acid (C2H204).

4. An insulating composition as claimed in claim 3 wherein the filler is melamine phosphate of the structure (melamine)2 . H3P04, melamine sulphate of the structure (melamine)2 . H2S04, melamine borate of the structure melamine . H3B03 and/or melamine oxalate of the structure melamine . C2H204.

5. An insulating composition as claimed in any preceding claim which includes 20 up to 60% by weight of the melamine salt.

6. An insulating composition as claimed in any preceding claim which includes other fillers in addition to the melamine salt.

7. An insulating composition for producing an insulating layer on a rocket solid propellant charge substantially as hereinbefore described.