US3033117A - Propellent charge - Google Patents

Description

May 8, 1962 L. G. BONNER PROPELLENT CHARGE Filed June 17, 1948 FIG. 4

y/nan 6I Bonner FlC INVENTOR.

AGENT.

United States Patent OA 3,033,117 PROPELLENT CHARGE Lyman G. Bonner, Cumberland, Md., assignor to Hercule Powder Company, Wilmington, Del., a corporation of Delaware` Filed June 17, 1948, Ser. No. 33,582 5 Claims. (Cl. 102-98) This invention relates to propellent charges and more particularly to neutral burning propellent charges for rockets, thrust units and other gasfproducing devices.

In the prior art, rocket propellent granulations have been primarily confined to singly perforated cylinders, star perforated grains and outside restricted end burning grains. The singly perforated cylindrical grain is undesirable in that the chamber wall of the rocket motor is exposed to the hot combustion gases. lf this grain is restricted on its outer surface, it becomesrprogressive in burning and gives a highly fluctuating thrust. The star perforated grain does not expose the chamber wall to the hot combustion gases but does give undesirably low loading densities, especially in short grains. Because of its configuration, this grain burns very rapidly and is therefore useful only for charges of comparatively short burning times. Conversely,` end burning grains restricted on their outer surfaces are useful only when charges are desired which have comparatively long burning times. Furthermore, to obtain the mass rate of gas discharge essential to even moderate thrust levels with end burning grains it is necessary to employ grains of undesirably larger diameter. It is not feasible to hold the grain diameter low and employ propellants of higher burning rate because the faster burning and therefore hotter burning charges have undesirable temperature ycoeicients.

More recently a charge has been developed which comprises an outside restricted, perforated cylindrical grain and an unrestricted solid cylindrical grain loaded in tandem. By proper adjustment of the length of each of these grains, the regression in the surface of the solid grain can be made to compensate exactly for the progression of the perforated grain during all stages of burning. While this charge is substantially neutral in burning surface and does give a desirable mass rate of gaseous discharge, it still does not overcome the difficulties of low loading density and exposing chamber walls to combustion gases. Furthermore, this charge is necessarily bulky and it is diicult to support the solid grain centrally in the chamber during burning and thus prevent a shift in center of gravity. In addition, little control can be exercised `over port area, this fact contributing to the low loading density.

In accordance with the present invention, neutral burning propellent charges of high loading density may be produced which have a wide latitude in burning time, permit a wide choice of powder web and surface, give a satisfactory mass rate of gaseous discharge and which afford complete chamber wall protection during burning.

Generally described, this invention comprises a perforated outer propellent member, the outer surfaces of which are protected to prevent combustion, and having an inner propellent member disposed Within the perforation of the outer member in spaced relationship to form a `combustion area therebetween. The inner core is proportioned so that during burning its regression in burning surface substantially compensates for the progression in burning Surface of the outer member. If the ends of both members are protected from combustion, exact neutrality `of burning surface may be obtained in charges of any Ilength. However, if the length of the entire charge is great in comparison with` its cross-sectional area, the ends need not be restricted since the surface lost by longitudinal burning will be a negligible fraction of the total 3,@33Jl7 Patented May 8, 1S62 HCC burning surface. Highest loading densities are obtained when the inner member is the samelength as the outer member and its radius is the same as the thickness of the outer member.

FIG. l is a cross-sectional view of a propellent charge made according to one embodiment of this invention.

FIG. 2 is a diagrammatic View of one end of a charge prepared in accordance with a further embodiment of the invention.

FIG. 3 is an elevational end view of the trapping assembly employed in FIG. l.

FIG. 4 is a sectional side view of the trapping assembly of FIGS. 1 and 3.

FIG. 5 is a sectional side view of an embodiment of a composite inhibitor plate for the ends of the charge in accordance with the invention.

FIG. 6 is a fragmentary sectional view of a loaded charge in accordance with the further embodiment of the invention.

In FIG. l a solid cylindrical propellent core 10 is centrally located in a cylindrically perforated propellent cylinder 11, which is restricted from burning on its outer diameter by a jacket of cellulose acetate 12. The ends of the perforated propellent cylinder 11 are restricted from burning by annular rings of cellulose acetate 13 while the ends of the central core l0 are likewise restricted by centrally orifced annular cellulose acetate discs 14. A cylindrical port l5 is left between the outside diameter of the core 10 and inside diameter of the perforated cylinder 11 and is of suicient cross-sectional area to allow proper venting of combustion gases. The core is secured in its central position throughout burning and the entire charge held in place by a suitable trapping system 30 described in detail in connection with FIGS. 3 and 4. The entire charge is contained in snug, slidable relationship within a combustion chamber 1 of a rocket-actuated device (not shown). Threadedly engaged with the chamber 1 is a discharge nozzle 2. The nozzle 2 holds the trapping assembly 30 in position when screwed into place following loadingof the charge. A similar trapping assembly is placed at the other end of the charge and held in place in similar fashion by the member attached thereto such as a warhead or a second discharge nozzle if the device is to be employed as an optional thrust unit.

In FIG. 2 the ends of the outer cylindrical member 20,

its surface inhibiting material 21, and the core 22 are ber and size of the connecting segments as Well as ther thickness of the entire plate may be varied according to the size and weight of the components of the charge. Furthermore, the inhibitor plate may be made to incorporate recesses or projections to engage the trapping and/ or supporting systems of the various rocket motors or other thrust devices.

In FIGS. 3 and 4 is shown a simple trapping means which comprises a flat, outer, annular metallic ring 31 connected to a concentrically disposed, pointed, metallic stud 32 by four thin radially spaced metallic spokes 33. VThe pointed stud passes through the orifice of the inhibitor disc 14 and when the nozzle 2 is screwed into place, the point of the stud 32 is forced slightly into-the end of the solid propellent core 10. Together with a similar trapping assembly disposed at the other end of the charge, the core 10 is maintained in its concentric location within the outer propellent member 11. The trapping assemblies are preferably made from steel or other metal capable of withstanding the temperature of the combustion gases without distortion. The inhibitor disc 14 need not be orificed if well bonded to the end of core It) since it has been found that adequate support may be obtained if the point of the stud projects only into the inhibitor disc.

In FIG. 5 is shown a further embodiment of a composite inhibitor plate made from cellulose acetate and disposed in operative relationship with a further embodiment of a trapping system which may be employed in accordance with the invention. An annular outer ring 50 is connected to the concentrically disposed disc 51 by means of radially disposed segments as shown at 24 in FIG. 2. An annular recess 52 is formed at the periphery of the outer ring 50. The trapping system 53 comprises an outer ring 54 of rectangular cross section. A concentrically disposed, pointed stud 55 having an annular flange 56 is joined to the edge of the ring 54 by evenly spaced spokes 57. The ring 54 rests in recess 52 of the inhibitor plate, the pointed stud 55 passes through the central orifice of disc 51, and the flange 56 rests against the disc 51 when, as shown in FIG. l, the discharge nozzle or warhead is screwed into place. As in the trapping assembly of FIGS. l, 3, and 4, the point of the stud need not project into the core of the charge but may simply engage the disc 51. If the disc 51 is orificed, the stud should lill the orifice and thus prevent the combustion gases from igniting the end of the core 1t) behind the inhibitor disc.

In FIG. 6, a charge in accordance with the invention consisting of an outer member 60 and an inner core 61 is disposed in tight, slidable fit within `a steel combustion chamber 62. The wall of the combustion chamber acts as the inhibiting means and prevents combustion on the outer surface of the member 60.

Having now described the nature and purpose of the present invention, the following example is given:

Propellent charges were prepared from the following ingredients: v

Percent Nitrocellulose (13.15% N) 58.5 Nitroglycerin 22.5 Ethyl centralite 8.0 Dinitrotoluene 2.5 Triacetin 8.5 Lead Stearate (added) 0.5

The charges were designed to give a V200-pound thrust for 4 seconds and have the following specifications:

Length of perforated cylinder inches 23.50 Length of solid core do 23.50 Outer diameter of perforated cylinder do 5.27 Inner diameter of perforated `cylinder do 2.87 Outer diameter of solid cylindrical core do 2.40 Ratio of port area to nozzle throat area 52.5

The outer surfaces of the perforated cylinders of these grains were restricted longitudinally with a cellulose acetate jacket. To achieve perfect neutrality of burning surface it was necessary to also restrict the ends of both the outer cylinder and the core but in this particular grain, highly saisfactory results were obtained by restricting only the ends of the solid core with cellulose acetate discs 1/s inch thick. These discs were cemented into place with acetone as the cementing medium. Satisfactory neutrality of burning was also obtained when the ends of yboth the perforated cylinder and solid core were left unrestricted since in this particular granulation the length is so great in relation to cross-sectional area that the surface lost from endwise burning of both perforated cylinder and core is a nearly negligible `fraction of the total surface. In static tests, these charges give a minimum of thrust fluctuation and excellent neutral burning pressuretime and thrust-time records Showing sharp ends of burning. Propellant charges of this granulation have been extensively tested in glider thrust units, which are symmetrical rockets designed to give optional direction of thrust, with highly satisfactory results.

It has been found that only very simple trapping systems are necessary when charges are prepared in accordance with this invention. This is due to the fact that because the grain is restricted on its outer surface, the chamber may be fitted closely around the periphery of the outer cylinder and is supported by the chamber Wall. Since little or no powder silvering is encountered with the charges of this invention the primary function of the trapping system is to center the solid core and hold it in place. This can be accomplished by a simple trap at each end of the grain.

Because the charge is restricted from burning on its outer surface, the ilow of hot combustion gases over the inner wall of the chamber tubing is eliminated. This makes possible the use, for chamber construction, of materials such as aluminum alloys which have excellent strength to weight ratios at ordinary temperatures but which lose strength rapidly at elevated temperatures.

FIt is possible to obtain very high loading densities in charges prepared in accordance with this invention. For a given powder web and weight, grain length and outside diameter may be adjusted to give the minimum acceptable port area which results in the maximum loading density. Conversely, for a given outside diameter and length, the web and weight may be adjusted to produce the same effect.

The charges of this invention may be fabricated by extrusion or casting. If prepared by extrusion, it is preferred to extrude the outer cylinder .and central core separately. The outer surafce of the perforated cylinder and the ends of the perforated cylinder and solid core may then be restricted with cellulose acetate or other nonflammable material as desired. Extrusion is preferred in grains of small diameter but when grains of diameter of six inches or more are desired the extrusion process becomes undesirable both because of the massive and expensive equipment necessary and the hazards involved.

The charges may be cast safely and easily by utilizing the apparatus and process disclosed in the copending application of Gordon W. McCurdy, Serial No. 28,218, led May 20, 1948. According to this process, the casting powder is introduced into a cylindrical cellulose acetate casting container of proper size, equipped with a hollow cylindrical core which forms the port of the charge and to which the casting will not adhere. The casting powder may be tightly packed into the container by tamping or vibration. Pressure is then exerted on the powder at the top of the container and a suitable casting liquid is admitted to the bottom of the container through a manifold base assembly. This base assembly is so constructed that the incoming liquid is evenly distributed over the cross-sectional area at the base of the container and any air entrained in the casting liquid system is trapped. After the liquid has risen through and covered the casting powder, the casting liquid system is disconnected and the entire apparatus is stored at a suitable temperature until cured.

After curing, the casting container with its contents is removed from the base assembly. The cellulose acetate casting container is preferably utilized as the restrictive covering for the outside surface of the outer cylinder. However, in case the casting container employed is constructed of a material which is undesirable as an inhibiting material, the casting may be removed from the casting container after curing and restricted with cellulose acetate or other suitable material as desired. If desired, the ends of both cylinder and core may be restricted by cementing or otherwise bonding annular cellulose acetate rings to the ends of the outer cylinder and cellulose acetate discs to the ends of the core. However, in order to enable shipment,y storage and loading of the two commay be coated with a thin film of silicone grease or other suitable lubricant prior to introduction of the casting powder. Unce the `form has been extracted another composite inhibitor plate is then bonded to the opposite ends. Once the charge is loaded, the radial connecting segments between the inner and outer portions of the inhibitor plate are consumed by initial combustion and the trapping and supporting systems then maintain the core in spaced relationship with the outer propellent member during burning.

Alternatively, the outer cylinder and core may be cast separately, or the outer cylinder may be cast and the core extruded. The two components may then be assembled and maintained in spaced relationship by use of the composite inhibitor plate. However, casting the entire charge simultaneously is desirable in order to insure t. that the components possess substantially identical burning characteristics.

In certain applications, having the hot combustion gases contact the chamber walls at the end of burning may not be disadvantageous. It may not be necessary, therefore, to inhibit the peripheral surfaces of the outer member of the charge with an independent means. In that ease, the gas-producing device-may be so designed that the chamber wall serves as the inhibiting means.

The size of the cylindrical port left between the inner member and the outer member depends on the particular granulation and on the particular rocket motor, thrust unit, or gas-producing device into which the chargev is to be loaded. Based on empirical knowledge, the most satisfactory results are obtained when 2.5 is taken as the limiting ratio of port area to nozzle throat area.

Propellant charges produced in accordance with this invention have many advantages. Charges may be designed with a wide choice of web and surface while maintaining a high loading density. Since the chamber walls can at all times be protected from hot combustion gases, lighter and less refractory materials can be used in construction of the chamber. Furthermore, the

charges may be made absolutely neutral in burning surface or can be made regressive or progressive to the degree desired. It is therefore to be understood that the example given is presented for illustrative purposes and that the invention is not limited by the charge described therein but only by the scope of the appended claims.

What I claim and desire to protect by Letters Patent is:

1. A substantially neutral burning propellant charge comprising a cylindrical outer member of combustible explosive composition having a centrally disposed cylindrical perforation longitudinally thereof, means disposed about the curved outer surface of said outer member which protects said surface from combustion, and a solid cylindrical inner member of combustible explosive composition disposed within the perforation in spaced relationship to form an annular combustion zone between the members, the radius of said inner member being equal to the thickness of the outer member.

2. The charge of claim il wherein the surfaces which are protected to prevent combustion are so protected by means associated with the device containing the charge.

3. The charge of claim 1 wherein the surfaces which are protected to prevent combustion are so protected by means suitably bonded tothe charge itself.

4. The charge of claim 1 wherein the end surfaces are protected to prevent combustion by means bonded to the charge, said means being units of flame-resistant material comprising an outer annular ring joined to a concentrically disposed disc and being adapted to maintain the inner memberk in spaced relationship to the outer member prior to combustion.

5. A propellent charge in accordance with claim l wherein the inner and outer propellent members are of equal length.

References Cited in the le of this patent UNITED STATES PATENTS 622,777 McGahie Apr. 11, 1899 654,471 Maxim July 24, 1900 1,018,312 Gherassimoi Feb. 20, 1912 FOREIGN PATENTS 26,430 Great Britain 1907 502,560 France Feb. 24, 1920 516,865 Great Britain Ian. 12, 1940

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