GB2233745A

GB2233745A - Solid propellant blocks

Info

Publication number: GB2233745A
Application number: GB8426524A
Authority: GB
Inventors: Christian Desormeaux; Roger Mespoulede
Original assignee: Societe Nationale des Poudres et Explosifs
Current assignee: Societe Nationale des Poudres et Explosifs
Priority date: 1983-10-26
Filing date: 1984-10-19
Publication date: 1991-01-16
Anticipated expiration: 2004-10-19
Also published as: FR2640259A1; IT8468028A0; FR2640259B1; GB2233745B; IT1224139B; DE3438993A1; DE3438993C2

Abstract

A process for making a solid propellant block adapted for frontal combustion and incorporating longitudinally extending heat conductors, comprises (a) casting a first propellant composition to form a first part 2 of the block, the first part extending the intended length of the block but only occupying part of the cross-sectional area of the block, (b) placing some or all of the heat conductors 4 against the first part of the block, the conductors extending the intended length of the block and being in their desired positions in the finished block, and (c) casting a second propellant composition to form a second part 5 of the block, the mould for this casting being at least partially formed by that part of the first part of the block against which heat conductors have been laid, and where step (c) does not complete production of the desired finished block, repeating steps (b) and (c) until the desired finished block is obtained. <IMAGE>

Description

Process for making solid propellant blocks incorporating beat conductors,

and the blocks so obtained This invention is concerned with a process for making solid propellant blocks incorporating heat conductors which serve to increase the combustion rate of such blocks, and with the propellant blocks so obtained; such propellants are suitable for powering rockets, missiles and, generally, all vehicles propelled by solid propellants.

In rockets, missiles and, in general, all vehicles propelled by solid fuel, a fuel with high thrust characteristics is required. In certain types of vehicle, a high filling ratio is required and this leads to the use of frontal combustion propellant blocks and to reduce the diameter of these blocks without reducing the thrust characteristic, the block combustion rate must be increased. For this purpose, frontal combustion blocks are used, the combustion of which is controlled and accelerated by means of heat conductors.

It is known, in particular from French Patent 1 349 125 and U.S. Patent 3 509 822, that the apparent combustion rate of a propellant is considerably increased in relation to the intrinsic combustion rate if heat conductors are incorporated into the propellant. Heat conductors, generally in the form of metal wires or strips, modify the thermal field upstream of the flame face due to the difference in thermal conductivity between the propellant matrix and the heat conductor. This modification of the thermal field causes a modification of the combustion rate close to the conductor and causes the formation of a cone centered on the conductor. This cone creation increases the combustion surface and, thus, the quantity of propulsion gases.

Several methods have been described for the incorporation of such heat conductors. A first method comprises incorporating a number of randomly distributed short conducting wires into the propellant matrix. In a second method, a relatively smaller number of conducting wires are laid along the length of the block, perpendicularly to its end-faces. This latter method gives a greater and smoother apparent combustion rate.

Other studies, as described in French Patent 1 349 125 already referred to and in the article 'Unfluence of Long Metal Wires on Combustion of DoubleBase Propellants" by CHEN SHULING and LIFENGSHENG published in "Combustion and Flame" no. 45, pp. 213-2189 1982, have demonstrated the influence of the conductor diameter on the combustion rate; it has been found that the highest apparent combustion rate was achieved with wires having a diameter of about 0.15 mm.

However, production of such propellant blocks by casting the propellant in a mould in which the wires have been previously correctly positioned and are kept under slight tension, is very delicate, particularly with viscous propellant compositions. In fact, the pressure exerted by the propellant composition during casting frequently causes the wires to break so that variations occur in the combustion of the blocks obtained, this phenomenon being aggravated by the thin gauge of the wires. To avoid breaking the wires during casting of the propellant composition, the tension exerted on the wires 23 can be reduced, but this leads to blocks with wires that are not parallel to one another or to the length of the block, thus causing relatively large variations in the combustion rate of the block.

Moreover, the positioning and tensioning of the wires in the mould requires a relatively complex and bulky installation which interferes with the casting of the propellant composition.

We have now developed a process for making such propellant blocks having longitudinally extending heat conductors in which the difficulties mentioned above, particularly the breaking of the conductors during casting, are reduced or avoided.

According to the present invention, there is provided a process for making a solid propellant block adapted for frontal combustion and incorporating longitudinally extending heat conductors, which comprises (a) casting a first propellant composition to form a first part of the block, the first part extending the intended length of the block but only occupying part of the cross- sectional area of the block, (b) placing some or all of the heat conductors against the first part of the block, the conductors extending the intended length of the block and being in their desired positions in the finished block, and (c) casting a second propellant composition to form a second part of the block, the mould for this casting being at least partially formed by that part of the first part of the block against which heat conductors have been laid, and where step (c) does not complete production of the desired finished block, repeating steps (b) and (c) until the desired finished block is obtained.

It will be seen that the process according to the invention facilitates positioning of the heat conductors; since the heat conductors are positioned against the first part during the casting of the second part, they are subjected to only very slight pressures, thus virtually eliminating the risk of 5 breaking.

Another advantage of the process is that it enables single component or multiple component blocks to be readily produced, particularly double component ones, depending on whether the first and second propellant compositions are the same or different.

Propellant compositions are either cast if their viscosity is low or pressure moulded if their viscosity is high. Pressure moulding of the propellant composition is possible using the process according to the invention since the conductors are placed against a face during the pressure injection phase and are thus not subjected to forces which could break them.

The heat conductors are preferably metal wires, but metal strips can also be used; the conductors are, of course, formed of a material having a thermal conductivity coefficient which is higher than that of the propellant compositions of which the propellant block is formed. A suitable diameter for the metal wires is from 0.1 to 1 mm and the wires may, for example, be formed of silver or copper.

It is preferred that the conductors, when placed against the first part of the block, should be slightly tensioned, at least during casting of the second propellant composition. This may be done by fastening them to the end faces of the first part or by a tensioning means, such as a spring attached to one end of the conductors.

A preferred method of facilitating the 33 positioning and maintenance of the conductors against the first part is to form longitudinal grooves in the first part in which the conductors are located.

The first part of the block can be of any cross-sectional shape, but is preferably in the shape of a tube or bar. The tube or bar may be cylindrical: in this case, the conductors are placed on one or both walls of the tube or on the sides of the bar and the second propellant composition is cast around the tube or the bar or inside the tube. The tube or bar can also have a polygonal section, such as triangular, hexagonal, square or starshaped in preferred variants of the invention. In this case, the conductors are suitably positioned along the edges corresponding to the apices of the polygons and for this purpose, it is preferred to bevel the convex edges of the tube or bar to form longitudinal grooves to house the conductors.

The conductors can also, of course, be placed against the sides of such polygonal shapes, if desired.

The process according to the invention is thus very flexible since it enables blocks of very varied shapes to be produced and enables the conductors to be positioned very readily, in a previously calculated and defined pattern.

The present invention also comprises propellant blocks obtained by means of the process described, these blocks being characterized by the conductors being located at the boundary surface between successive propellant casts.

In order that the invention should be more fully understood, preferred embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a cross-section of a propellant block made by the process according to the invention, Figure 2 is a diagram showing the progression of the combustion front of a propellant block in the vicinity of a conductor wire, Figure 3 is a view of the rear face of a propellant block made by the process according to the invention, Figure 4 is a partial longitudinal sectional view along line IV-IV of Figure 3, and Figure 5 is a view of the rear face of another propellant block made by the process according to the invention.

Referring to Figure 1, a propellant block 1 is made by casting a first Dart 2 in the form of a thick walled tube having a triangular internal cross-section 10.

The propellant composition used to form the part 2 is a castable composition, such as a composite or homogenous propellant which is cast using an in situ or global casting method.

The part 2 of the block can either be cast directly in an inhibiting liner 3 or in a separate mould (not shown), its outer surface then being provided with an inhibiting coating at the end of the manufacturing process.

After setting of the propellant composition forming part 2, heat conductors 4 are placed inside the part 2 along the angles defined by the apices of the triangular cross-section 10 of the tube. The heat conductors 4 are preferably metal wires having a diameter of from 0.1 mm to 1 mm. The wires 4 are tensioned and are fixed by bonding or, preferably, by attaching the ends of the wires to the end faces of the block 1 or, even better, by using a tensioning device, such as a tension spring (not shown) attached to one end of the wire, the other end of the wire being connected to a fixed point.

A propellant composition is then cast into the tube or part 2 to form the second part 5 of the block.

As the conductor wires 4 are supported against the side of the first part 2, the pressure forces generated by casting of this propellant composition do not cause the wires to break; it is therefore possible to use propellant compositions with high or low viscosities and small diameter or fragile wires. The second part 5 of the block 1 is then simply allowed to set.

It will be understood that the propellant compositions used for the first part 2 and the second part 5 of the block 1 can be the same or different.

Figures 3, 4, and 5 show other production variants.

In the variant shown in Figures 3 and 4, the first part 2 is also tubeshaped, but the internal crosssection 10 of the tube, shown in chain lines, is star- shaped. Conductor wires 4 are located along the apices 6 of the star (in every second apex in the example shown).

The second part 5 of the block is made by casting a propellant composition inside the first part 2, the sides of the first part acting as the mould.

In the variant shown in Figure 5, the first part 2 is a cylindrical tube having an outside diameter less than that of the final block, and the conductor wires 4 are placed on the outer and inner faces of the tube. To provide accurate positioning of the wires 4, grooves 7 are formed on the outer and inner faces of the first part, for example by machining or casting. This variant thus enables two rows (circles) of wires 4 to be incorporated. The second part 5 is then cast in one shot inside and outside the first part 2, it being understood that for this purpose the tube 2 is placed inside a suitable dimensioned mould (not shown).

In an alternative method of making the block shown in Figure 3, a central core (first part) having a star-shaped cross-section of the shape designated 5 in Figure 3 is first made, the wires 4 are positioned along the apices of the star (every second apex), and the outer tubular part of the block (corresponding to 2 in Figure 3) is then cast. In this case, the apices of the first part in which the wires 4 are to be located are preferably bevelled to provide locating grooves similar to the grooves 7 shown in Figure 5.

In the case of all the variants shown in Figures 1, 3, 4 and 5, the second part 5 can be cast either by simple pouring in the case of relatively fluid propellant compositions or by pressure casting in the case of relatively high viscosity compositions.

As shown in Figure 4, it is preferred to provide the rear surface of the block with conical depressions 8 centered on the conductor wires 4. The provision of such depressions increases the initial combustion surface and thus enables the stabilized combustion rate to be reached very quickly.

As shown in Figure 2, the difference in thermal conductivity between the conductor wire 4 and the matrix 9 of the propellant modifies the profile of the combustion rate around the wire 4, thus generating the formation of a cone centered on the conductor wire. This phenomenon considerably-increases the block combustion area and the apparent combustion rate Va of the block, measured parallel to the block centre-line. The cone formation phenomenon stabilizes after a certain combustion time t 2, after which the cone thus formed, with an apex angle equal tocg,, moves at the apparent combustion rate of the block Va. Generation of cones 8 on the rear face of block 1 enables stabilization time t 2 of the combustion surface to be reduced, thus reducing the apparent combustion rate Va.

The time t 2 can also be reduced by other means, such as grooving of the end face of the block.

The apex anglecAof the cone enables the apparent combustion rate Va of a block to be calculated, using intrinsic combustion rate Vi of the propellant composition:

Va = vi (I) sin K Thus, the multiplying coefficient n of the combustion rate corresponding to the incorporation of conductor wires is given by formula II below:

n = 1 Va (I1) sincK vi The apparent rate Va is determined experimentally using the following formula:

Va e thickness burned t cr combustion time at steady rate This multiplying coefficient can also be calculated from the block combustion curves, particularly 35 from a point on the curve for combustion pressure against time, using formula (IV) below:

ni (IV) vi D 2 in which:

Vi is the intrinsic rate of the propellant at pressure p c D is the propergol flow coefficient is the density of the propellant is the mean combustion pressure d is the diameter of the nozzle neck D is the diameter of the block Blocks according to the variant shown in Figures 3 and 4 have been made by the process according to the invention with composite propellant compositions containing polybutadiene hydroxytelechelic (PBHT) binder and polyurethane binder and with heat conducting metal wires made of copper or silver. The results obtained upon firing these blocks are shown in the following 20 table.

p Test no. Nature Nature of Wire Firing n, calculated of binder wires dia. (mm) temp. (OC) using formulae (II, III) 1 PB11T Ag 0.2 -4o - 2 11 11 +20 3.5 3 TV 11 +6o - 4 Cu 11 +20 1.7 Polyure- Ag 0.3 -110 4.5 thane 6 Polyure- Cu 0.3 -110 4.8 thane 7 11 11 11 +20 5 8 PBHT Ag 0.2 +20 3.11 9 -110 3.22 -4o 3.42 11 +6o 3. A 12 +6o 2.99 13 Cu -4o 2.79 14 If +20 2.71 Test no. n calculated c/,= arc sin - 1 Remarks n using formula (IV) 1 2.95 20 0 2 2.9 20 0 3 3.05 19 0 4 1.6 39 0 4.4 13 0 6 4.9 11.80 0 7 5.2 11 8 3.36 17.3 0 block length 255 nun 9 3.43 17 0 11 3.43 17 0 block length 905 mm 11 3.30 17.60 block length 255 mm 12 3.05 19 0 block length 905 mm 13 2.74 21.4' block length 905 mm 14 2.65 22.2 0 11 The combustion pressures of these blocks were observed during firing and were noted as being virtually stable, which indicates that the wires had not been broken and were evenly distributed within the block.

The various tests carried out show that the combustion rate multiplying coefficients for propellant blocks made by the process according to the invention have the following mean values:

n = 3.5 for silver wires, dia.0.2mm and Pc 67 bars n = 2.65 for copper wires, dia.0.2mm and Pc 50 bars Blocks produced by means of the invention have also been subjected to heat treatment cycles without any damage.

Claims

Claims:

1. A process for making a solid propellant block adapted for frontal combustion and incorporating longitudinally extending heat conductors, which comprises (a) casting a first propellant composition to form a first part of the block, the first part extending the intended length of the block but only occupying part of the cross-sectional area of the block, (b) placing some or all of the heat conductors against the first part of the block, the conductors extending the intended length of the block and being in their desired positions in the finished block, and (c) casting a second propellant composition to form a second part of the block, the mould for this casting being at least partially formed by that part of the first part of the block against which heat conductors have been laid, and where step (c) does not complete production of the desired finished block, repeating steps (b) and (c) until the desired finished block is obtained.

2. A process according to claim 1, in which the heat conductors are wires and are tensioned when placed against the first part of the block.

3. A process according to claim 2, in which longitudinally extending grooves are formed in the exterior and/or interior surface of the first part of the block and the conductor wires are located in the grooves.

4. A process according to claim 2 or 3, in which the first part of the block is tubular.

5. A process according to claim 4, in which the internal cross-sectional shape of the tubular first part is polygonal and the conductor wires are located in apices of the polygon.

6. A process according to claim 5, in which the polygon is a triangle or a star-shaped polygon.

7. A process according to claim 2 or 3, in which the first part of the block is in the form of a solid bar or rod.

8. A process according to claim 7, in which the cross-sectional shape of the first part is polygonal and the conductor wires are located at apices of the polygon.

9. A process according to claim 8, in which the apices of the polygon are provided with longitudinally extending grooves in which the conductor wires are located.

10. A process according to any of claims 2 to 10, in which the conductor wires are tensioned by fixing their ends to the end faces of the first part of the block or by the use of tensioning means.

11. A process according to any of claims 2 to 10, in which the conductor wires have a diameter of from O.lmm to lmm.

12. A process according to any of claims 1 to 11, in which the first and second propellant compositions are the same.

13. A process according to any of claims 1 to 12, 1 in which the second propellant composition is cast under pressure.

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14. A process for making a solid propellant block adapted for frontal combustion and incorporating longitudinally extending heat conductors, substantially as herein described with reference to Figure 1, Figures 3 and 4, or Figure 5 of the accompanying drawings.

15. A solid propellant block adapted for frontal combustion and incorporating longitudinally extending heat conductors, when made by the process claimed in any of the preceding claims.

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