IL170599A - Single-piston recoilless propulsion system - Google Patents

Description

170599/2 170599 f?'Ti I 453522 ηικ JH7O D57 Π5ΡΓΠ vfrh ηΠ7 ΓΌΊ57» SINGLE-PISTON RECOILLESS PROPULSION SYSTEM SINGLE-PISTON PROPULSION SYSTEM FIELD AND BACKGROUND OF THE INVENTION The present invention relates to propulsion systems and, in particular, it concerns a single piston propulsion unit suitable for launching a projectile from a launch tube.

The propelling device of a shoulder-launched projectile generates the forces required to accelerate the projectile to a velocity consistent with accuracy requirements. For unguided projectiles fired even at short ranges, a reasonable accuracy necessitates a launch velocity in the range of 100 - 200 meters/sec. Further requirements are a result of the method of operation. Namely, for shoulder-fired weapons there is a strict requirement for low level of recoil, which invariably results in design in which the momentum of the forward-moving projectile is balanced by the momentum of a rearward moving mass (such as gases, liquid, solid particles or a combination thereof). In order to avoid injury to the gunner, the rearward-moving mass must egress the propelling device before tube exit. This requirement in turn dictates a high burning rate of the propellant in the device, which inevitably involves high operating pressure and high mass-flux of rearward ejection. Such conditions normally result (for example when using solid propellant rocket motors as boosters) in a high level of noise (Sound Pressure Level) and in a considerable back-blast, which creates hazards to personnel and material behind the launcher. In addition, if combustion products are conveyed backwards, toxicity of the combustion products has to be considered. Back-blast, noise and toxicity are of special concern in Fire-from-Enclosure conditions, which are characteristic for urban warfare scenarios. Specifically, a typical propulsion unit contains a propellant charge in the shape of a powder or a grain within a combustion chamber. For a proper launch, the charge must be burned efficiently and at a steady, fast rate. In many applications, the projectile is launched from a tube which can be supported on a mount or by an individual.

At launch, the burning propellant charge generates exhaust gases. Accordingly, handling of the exhaust gases is an ongoing concern. If exhaust gas exits the back end of the launch tube, disadvantages include the creation of a potentially lethal zone behind the launcher caused by the shock waves, the presence of turbulent hot toxic gases, the generation of considerable sound and pressure levels, and the discharge of flash and smoke. These disadvantages therefore generally prevent the use of such launchers in confined or closed spaces, or in covert operations. In special, in order to reduce Sound Pressure Level at tube exit, working with closed systems has been suggested as an attractive solution. There are two basic types of closed propulsion systems known in the art. The first is the "double piston" launch system, in which the combustion occurs between two pistons within a launcher, one moving forward and the other one moving backward. The front piston impinges on the projectile, thus accelerating it in the forward direction, while the aft piston impinges on a counter-mass which is ultimately ejected at the rear end of the tube. The main difficulty with such concept is that it is necessary to arrest the pistons at the end of their motion in order to prevent their ejection from the tube. The two pistons could be arrested by mechanical limiters at the tube ends or by an interconnecting element installed between them. In both cases the requirement to arrest the two pistons results in a significant weight penalty.

The second type of closed propulsion system, to which the present invention belongs, is the "single piston" propulsion system. In this type of system, a propulsion unit containing both the propellant charge and the counter-mass is attached to the warhead. The propellant charge when combusted generates high pressure combustion products which impinge on a piston. The piston pushes the counter-mass backwards ejecting it from the propulsion unit. The projectile including the warhead and the propulsion unit is thus propelled in the forward direction.

Examples of single piston propulsion systems may be found in a number of publications, including US Patent No. 4,244,293 to Grosswendt et al. and US Patents Nos. 5,952,601 and 6,543,329 to Sanford et al. However the existing patents fail to disclose several important issues which are crucial for a solution which would meet the requirements as delineated above. Specifically, these propulsion systems typically have high residual gas pressure within the unit at the end of the piston stroke. This pressure is typically vented through peripheral vents or, in the case of Grosswendt, is employed to give additional thrust through a rear nozzle formed through the piston. This high-pressure gas release, occurring very soon after ejection of the countermass, carries with is many of the aforementioned problems of high noise level and potential hazard to personnel and property. Furthermore, where the piston itself is to be retained within the propulsion unit at the end of its stroke, the piston is typically stopped by an obstruction (such as constriction 4c of Grosswendt) against which it strikes percussively at the end of its motion. As there is no provision for arresting the piston in an axi-symmetrical position, thrust misalignment is generated when gases are released through a rear nozzle formed through the piston. The percussive impact of the piston gives a potentially damaging jolt to the projectile, and may also generate an undesirable loud noise.

Finally, a critical issue for all single-piston propulsion systems is weight. Since the propulsion system forms an integral part of the projectile during launch, and typically also after completion of its propulsion function, it is vital that the weight be kept to the minimum possible. This would encourage the use of lightweight composite materials where the majority of the load-bearing stress is carried by a filament winding. However, given the very rapid burning rates preferred for these applications, optimal systems typically require peak pressures of in excess of 1000 bar for which composite materials alone are not generally suitable. Most existing single piston propulsion systems employ all metallic pressure vessels which are relatively heavy for a given pressure rating.

There is therefore a need for a single-piston propulsion unit which would avoid percussive impact of the piston at the end of its range of motion, which would reduce pressurized gas release to a minimum, and which would employ a hybrid combination of metallic materials and composite materials to withstand high maximum operating pressures where necessary and to reduce pressure vessel weight in regions with lower pressure loading.

SUMMARY OF THE INVENTION The present invention is a single piston propulsion unit suitable for launching a projectile from a launch tube.

According to the teachings of the present invention there is provided, a single piston propulsion unit suitable for launching a projectile from a launch tube, the propulsion unit comprising: (a) a pressure vessel having a substantially cylindrical inner surface of a given internal diameter, the pressure vessel being sealed at a first end and terminating at a second end in a tapered portion, the tapered portion having an inwardly tapered conical surface extending along a length of at least half the internal diameter; (b) a gas generator including a quantity of combustible propellant deployed within the first end of the pressure vessel; (c) a piston deployed within the pressure vessel adjacent to the gas generator and in sliding engagement with the substantially cylindrical inner surface, the piston being formed primarily of a ductile material; and (d) a momentum transfer medium deployed within the pressure vessel between the piston and the second end, such that, on initiation of the gas generator, gas pressure forces the piston along the pressure vessel, thereby ejecting the momentum transfer medium in a first direction and propelling the propulsion unit in an opposite direction, and such that, on reaching the tapered portion, the piston undergoes plastic deformation through sliding contact with the inwardly tapered conical surface, thereby achieving non-percussive braking of the piston relative to the pressure vessel.

According to a further feature of the present invention, the pressure vessel is formed with a plurality of exhaust vent openings, the exhaust vent openings being located near the tapered portion such that gas from the gas generator cannot escape through the exhaust vent openings until the piston has at least partially entered the tapered portion.

According to a further feature of the present invention, the gas generator generates a given maximum gas pressure within the pressure vessel, and wherein the gas generator and the exhaust vent openings are designed such that gas is released from the exhaust openings after a pressure within the pressure vessel has dropped below one fifth of the maximum gas pressure.

According to a further feature of the present invention, the tapered portion terminates in an outlet nozzle having an internal diameter of between 60 and 90 percent of the internal diameter of the cylindrical portion.

According to a further feature of the present invention, the piston is formed primarily from ductile steel. According to an alternative feature of the present invention, the piston is formed primarily from aluminum. According to a still further alternative feature of the present invention, the piston is formed primarily from titanium.

According to a further feature of the present invention, a length of the pressure vessel is between 8 and 12 times the internal diameter.

According to a further feature of the present invention, the momentum transfer medium is a gel.

According to a further feature of the present invention, the pressure vessel includes: (a) a first portion proximal to the first end having a wall formed substantially exclusively of a metallic material with a first minimum wall thickness; (b) a second portion including the tapered portion having a wall formed substantially exclusively of a metallic material with a second minimum wall thickness; and (c) an intermediate portion between the first portion and the second portion, the intermediate portion including a metallic material with a minimum wall thickness less than both the first and the second minimum wall thicknesses, the intermediate portion further including a reinforcement layer of filament winding around the metallic material.

According to a further feature of the present invention, the reinforcement layer of filament winding is wound in a hoop orientation.

According to a further feature of the present invention, the gas generator is configured to complete combustion during no more than the first 20 percent of a range of motion of the piston.

According to a further feature of the present invention, the gas generator is configured to complete combustion during no more than the first 15 percent of a range of motion of the piston.

According to a further feature of the present invention, the gas generator includes at least a first propellant having a web thickness no greater than about 0.3 millimeters.

According to a further feature of the present invention, there is also provided a tail fin assembly including a plurality of wrap-around fins deployed around an external surface of the tapered portion.

According to a further feature of the present invention, the piston is formed with an elongated cylindrical sliding surface in contact with the substantially cylindrical inner surface of the pressure vessel, the cylindrical sliding surface having an axial length not less than 20 percent of the internal diameter of the pressure vessel.

According to a further feature of the present invention, the cylindrical sliding surface has an axial length of between 30 percent and 50 percent of the internal diameter of the pressure vessel.

According to a further feature of the present invention, the piston is formed with a concave surface facing the momentum transfer medium.

There is also provided according to the teachings of the present invention, a single piston propulsion unit for launching a projectile from a launch tube, the propulsion unit comprising: (a) a pressure vessel having a substantially cylindrical inner surface of a given internal diameter, the pressure vessel being sealed at a first end and terminating at a second end in an outlet nozzle; (b) a gas generator including a quantity of combustible propellant deployed within the first end of the pressure vessel; (c) a piston deployed within the pressure vessel adjacent to the gas generator and in sliding engagement with the substantially cylindrical inner surface; and (d) a momentum transfer medium deployed within the pressure vessel between the piston and the second end, such that, on initiation of the gas generator, gas pressure forces the piston along the pressure vessel, thereby ejecting the momentum transfer medium in a first direction and propelling the propulsion unit in an opposite direction, and wherein the gas generator is configured to complete combustion during no more than the first 20 percent of a range of motion of the piston.

According to a further feature of the present invention, the gas generator is configured to complete combustion during no more than the first 15 percent of a range of motion of the piston.

According to a further feature of the present invention, the gas generator includes at least a first propellant having a web thickness no greater than about 0.3 millimeters.

According to a further feature of the present invention, the pressure vessel is formed with a plurality of exhaust vent openings, the exhaust vent openings being located near the second end such that gas from the gas generator cannot escape through the exhaust vent openings until the piston has passed the exhaust vent openings.

According to a further feature of the present invention, the gas generator generates a given maximum gas pressure within the pressure vessel, and wherein the gas generator and the exhaust vent openings are designed such that gas is released from the exhaust openings after a pressure within the pressure vessel has dropped below one fifth of the maximum gas pressure.

There is also provided according to the teachings of the present invention, a single piston propulsion unit for launching a projectile from a launch tube, the propulsion unit comprising: (a) a pressure vessel having a substantially cylindrical inner surface of a given internal diameter, the pressure vessel being sealed at a first end and terminating at a second end in an outlet nozzle, the pressure vessel including: (i) a first portion proximal to the first end having a wall formed substantially exclusively of a metallic material with a first minimum wall thickness, (ii) a second portion proximal to the second end having a wall formed substantially exclusively of a metallic material with a second minimum wall thickness, and (iii) an intermediate portion between the first portion and the second portion, the intermediate portion including a metallic material with a minimum wall thickness less than both the first and the second minimum wall thicknesses, the intermediate portion further including a reinforcement layer of filament winding around the metallic material; (b) a gas generator including a quantity of combustible propellant deployed within the first end of the pressure vessel; (c) a piston deployed within the pressure vessel adjacent to the gas generator and in sliding engagement with the substantially cylindrical inner surface; and (d) a momentum transfer medium deployed within the pressure vessel between the piston and the second end, such that, on initiation of the gas generator, gas pressure forces the piston along the pressure vessel, thereby ejecting the momentum transfer medium from the nozzle in a first direction and propelling the propulsion unit in an opposite direction. 170599/2 BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIGS. 1A and IB are schematic longitudinal-section illustrations of a tube-launched projectile including a single-piston propulsion unit, constructed and operative according to the teachings of the present invention, shown prior to and immediately after launch, respectively; FIG. 2 is a longitudinal-section view of the propulsion unit of Figure 1A; FIGS. 3A-3D are schematic cross-sectional views similar to Figure 2 showing a sequence of motion of the piston along the pressure vessel; FIG. 4A is a longitudinal-section view of a first implementation of a gel canister for storing a gel momentum transfer medium within the propulsion unit of Figure 2; FIG. 4B is a transverse cross-sectional view of the canister of Figures 2 and 4A; FIG. 4C is an enlarged view of a part of a closure configuration of the canister of Figure 4A; FIG. 5A is a longitudinal cross-sectional view of a second implementation of a gel canister for storing a gel momentum transfer medium within the propulsion unit of Figure 2; FIG. 5B is a transverse cross-sectional view of the canister of Figure 5A;FIG. 5C is an enlarged view of a part of a closure configuration of the canister of Figure 5 A; FIG. 6 is a graph illustrating schematically values of gas pressure within the propulsion unit of Figure 2 as a function of time after initiation according to the teachings of the present invention; FIG. 7 A is an enlarged view of a tapered portion of the pressure vessel from the propulsion unit of Figure 2 showing a first preferred implementation of a set of exhaust vent openings; and FIG. 7B is an enlarged view of a tapered portion of the pressure vessel from the propulsion unit of Figure 2 showing a second preferred implementation of a set of exhaust vent openings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a single piston propulsion unit suitable for launching a projectile from a launch tube.

The principles and operation of propulsion units according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, Figures 1A and IB illustrate schematically a preferred context for use of the propulsion unit of the present invention, namely, as part of a tube-launched projectile. Thus, Figures 1A and IB show a projectile launcher system, generally designated 10, constructed and operative according to the teachings of the present invention, including a projectile 12 that includes a warhead 14 and a propulsion unit 16, deployed within a launch tube 17. As better visible in Figure 2, propulsion unit 16 includes a pressure vessel 18 having a substantially cylindrical inner surface of a given internal diameter D, and which is sealed at a first end 20 and terminates at a second end at an outlet nozzle 22. A gas generator 24 is deployed within first end 20 of pressure vessel 18. A piston 30 is deployed within pressure vessel 18 adjacent to gas generator 24 and in sliding engagement with the substantially cylindrical inner surface of pressure vessel 18. Finally, a momentum transfer medium 32, preferably in the form of a gel, is deployed within pressure vessel 18 between piston 30 and outlet nozzle 22. On initiation of gas generator 24, the pressure of generated gases drives piston 30 along pressure vessel, thereby ejecting momentum transfer medium 32 in a first direction (to the right as shown in Figure IB) and propelling the propulsion unit 16, and the associated projectile 12, in an opposite direction (to the left as shown in Figure IB).

Propulsion unit 16 of the present invention has particular advantages over conventional single-piston propulsion units such as those of the prior art described above due primarily to three sets of distinctive features, each of which is believed to be patentable in its own right, but which combine in synergy to provide a particularly advantageous combination. A first set of these features relate to the braking configuration for preventing egress of piston 30 from the unit which minimizes percussive impact of the piston at the end of its range of motion. A second set of features relate to the use of a particularly short propellant burn time so as to complete combustion of the propellant at an early phase of the piston motion. This minimizes the residual pressure within the unit at the end of the propulsion stroke which must be vented to the atmosphere. It also provides synergy with the piston braking configuration by avoiding significant pressure loading of the piston during braking. A third set of features relate to the use of a hybrid pressure vessel structure for the propulsion unit wherein solid metallic walls are used for regions exposed to the most extreme working conditions while a thinner metallic wall reinforced by a filament-wound composite material layer are used for other regions, with the fiber-reinforced portion typically making up the majority of the wall surface area of the unit. The transition between the solid metallic regions and the fiber-reinforced section is preferably achieved using a gradual ramp of increasing thickness of the metallic layer and decreasing thickness of the composite reinforcing layer. This hybrid structure results in profound weight reduction compared to solid metallic vessels of similar ratings while avoiding the need to compromise on performance. These features will now be addressed in order.

Turning now to the piston braking aspect of the present invention, it is particularly preferred that piston 30 is formed primarily, and typically entirely, from a ductile material. The term "ductile" in this context is used to refer to materials which can undergo significant (higher than 20%) dynamic plastic deformation in a controlled and predictable manner without fracture or other mechanical failure. Preferred examples of ductile materials suitable for piston 30 include, but are not limited to, stainless steel and titanium. The actual choice is ultimately a result of trade-offs including ductility, strength and density as parameters. Parenthetically, piston 30 is referred to herein as being made entirely of ductile metallic material, even if it carries certain additional components with it, such as a seal 31 as shown.

A preferred implementation for the shape of piston 30 is shown in Figure 2. In order to provide stability of the piston during operation, the piston preferably has an elongated cylindrical sliding surface in contact with the wall of the pressure vessel. The axial length of the cylindrical sliding surface is preferably at least about 20 percent of the internal diameter of the vessel, and more preferably in the range of 30-50 percent of the diameter. The surface of the piston facing the momentum transfer medium 32 is preferably concave. The concave surface is preferably supported on at least one side by a number of radially extending reinforcement ribs.

To provide the required breaking effect, pressure vessel 18 is formed with a tapered portion 34 which has an inwardly tapered conical surface. Unlike the retaining rings or wedges of the prior art, tapered portion 34 extends along a length L which is at least half the internal diameter D of the cylindrical part of vessel 18, and more typically between 60 and 80 percent of the diameter. As a result of this tapered structure and the ductility of piston 30, on reaching the tapered portion, piston 30 undergoes plastic deformation through sliding contact with the inwardly tapered conical surface as illustrated in the sequence of Figures 3B, 3C and 3D, thereby achieving non-percussive braking of the piston relative to pressure vessel 18.

The angle of the inward taper of tapered portion 34 is preferably in the range of 5-20 degrees relative to the axis of the cylindrical portion of vessel 18.

The combination of the conical angle and the length of tapered portion 34 are preferably chosen in order to ensure that outlet nozzle 22 has an internal diameter of between 60 and 90 percent of internal diameter D of the cylindrical portion. This avoids excessive resistance to ejection of momentum transfer medium 32.

Turning now to the short propellant burn time feature of the present invention, it is a particularly preferred feature according to this aspect of the present invention that gas generator 24 is configured to complete combustion during no more than the first 20 percent of a range of motion of the piston, and more preferably, during no more than the first 15 percent of a range of motion of the piston.

The significance of this feature may be appreciated with reference to Figures 3A-3D and 6. Referring first to Figure 6, this illustrates the variation of gas pressure within pressure vessel 18 as a function of time after initiation of the gas generator. After an initial build up time tram to reach an actuating pressure of typically no more than about 50 bar, the gas generator drives the pressure very rapidly up to its maximum value Pmax which is preferably in the range of 1000-2000 bar, and typically around 1300 bar. The force threshold for starting the piston movement is preferably set to be low by design, thereby avoiding "violent" action in a shoulder-fired weapon, maintaining simplicity of the mechanical design and low volume requirements. The total burn time to reach the maximum pressure is preferably less than about 2 milliseconds and occurs while the piston 30 is still in the initial stages (i.e., less than 20%, and typically 10-15%) of its motion. From this point on, gas generator 24 has effectively finished combustion and does not add significantly to the pressure. As a result, while piston 30 continues to be driven along the vessel and eject momentum transfer medium 32 from the unit, the pressure within vessel 18 falls as the gas undergoes adiabatic expansion. By the end of ίρ^, as the piston 30 reaches tapered portion 34 (Figures 3B and 3C), the pressure has preferably dropped by a factor of at least 5 (i.e., less than 20% of Pmax), and more preferably, by a factor of 10 (i.e., no more than about 10% of Pmax). The total power stroke tpush is preferably no more than about 5 milliseconds. The piston then undergoes plastic deformation during the braking stage (Figures 3C and 3D) as described above for a period t^p, typically an additional few milliseconds, until it reaches its final resting position relative to pressure vessel 18 as illustrated in Figure 3D. As it nears the end of its motion, the piston preferably passes, and thereby exposes to the residual gas pressure, a plurality of exhaust vent openings 36 located near the outlet nozzle end of the vessel. These relatively small exhaust vent openings allow equalization of the residual gas pressure with the atmosphere which takes place over the period ase which may extend for a period of up to a couple of seconds. Since the gas pressure has already dropped to a small fraction of the Pmax before exhaust vent openings are exposed, the gas release no longer generates all the problems of blast damage and noise associated with the higher pressure gas release of the prior art.

Parenthetically, it should be noted that, while the residual gas pressure released from exhaust vent openings 36 is relatively low, the directional gas flow via these openings may still be used to contribute to the dynamics of the projectile flight. Thus, by way of a first example, in Figure 7A, openings 36 are deployed to generate roll of the projectile. In an alternative version illustrated in Figure 7B, a rearward inclination of exhaust vent openings 36' additionally, or alternatively, provides additional forward thrust for the projectile during release of the residual gas pressure.

In structural terms, referring back to figure 2, gas generator 24 includes an initiator 25 for igniting a primer propellant 26 located in a first canister 27. By way of one non-limiting example, the initiator may be of the TBI (Through-Bulkhead Igniter) type, or another type of initiator with appropriate safety features. Primer propellant 26 in turn ignites a quantity of a main propellant 28 in a second canister 29. The main propellant 28 is retained within canister 29 by a retaining cap 38 and is held in a predefined spatial relation to piston 30 by a positioning pin 40, thereby defining a space between the gas generator and the piston which helps to achieve the desired level of peak pressure.

The entire gas generator structure is mounted within first end 20 of the pressure vessel, which is preferably implemented as a high strength cover made of suitable material, such as 250 Maraging steel, preferably sealing to the main part of pressure vessel 18 with a seal 21. This part screws into the main tube of pressure vessel 18, which employs a metal body which may be made primarily of slightly less hard materials, such as 4340 Steel.

In order to achieve the rapid burn rate referred to above, at least main propellant 28 preferably has a web thickness no greater than about 0.3 millimeters, and typically in the range of 0.1-0.3 millimeters. The parameters are preferably chosen to result in a burn rate in the range of roughly 1-6 cm/second (depending on the pressure).

Turning now to the hybrid pressure vessel feature of the present invention, it is a particularly preferred feature according to this aspect of the present invention that pressure vessel 18 includes at least a first portion 42 proximal to the first end a wall formed substantially exclusively of a metallic material 44 with a first minimum wall thickness followed by a composite material portion 46. Composite material portion 46 includes a metallic material, typically formed as a continuation of the metallic material 44 of first portion 42, with a minimum wall thickness less than the first minimum wall thickness overlaid by a reinforcement layer 48 of filament winding around the metallic material. Preferably, composite material portion 46 again phases out towards tapered portion 34 such that a tail portion 50, including tapered portion 34, also has a wall formed substantially exclusively of metallic material 44 with a minimum wall thickness greater than that of the composite material portion 46.

It should be noted that pressure vessel 18 is shown in the figures with a central portion cut out in order to show the features of the end portions clearly. In practical implementations, a length of pressure vessel 18 is preferably between 8 and 12 times the internal diameter. In a preferred implementation, the composite material reinforced portion 46 extends along a majority of the length of pressure vessel 18, thereby allowing the use of relatively thin metallic walls and thus greatly reducing the total weight of the propulsion unit. At the same time, the use of thicker metallic walls for first portion 42, and preferably also for tail portion 50, provides extra reinforcement for these critical regions with relatively small extra contribution to the overall weight. While in principle the entire pressure vessel might be manufactured of composite material, there are several drawbacks and technologically considerations which make such an implementation less attractive. Firstly, a vessel formed entirely from composite materials without a structural metallic layer would require complex multi-directional filament windings in order to provide the necessary strength in all directions, thereby rendering the vessel expensive. In contrast, in the preferred implementation illustrated here, the filament windings are preferably deployed only in the "hoop" direction, i.e., in planes roughly perpendicular to a central longitudinal axis of the vessel. This provides the necessary reinforcement in the main radial direction of strain in the vessel wall. The relatively smaller axial component of strain is preferably borne primarily by the inner metallic wall. A further consideration discouraging implementations using composite materials alone is that it is uncertain that the section of the vessel which functions to arrest the piston would withstand the impact loads. Instead, a metallic vessel with composite reinforcement by filaments wound in the hoop direction according to the teachings of the present invention is believed to be an optimal solution, as it combines the convenience of manufacturing a metallic vessel with that of unidirectional filament winding. In respect to a metallic vessel it is 170593/2 easy to machine the arresting section with the necessary profile and to provide additional features, such as vent holes and fin assembly lugs. A metallic vessel also provides optimal sealing capabilities.

Referring now again briefly to Figures 1A-2, it should be noted that the tapered portion 34 of the propulsion unit, (most clearly visible in Figure 2), provides added advantages in that it makes available space for mounting of a tail fin assembly including a plurality of wrap-around (or otherwise folding) fins 52 deployed around an external surface of the tapered portion. These are shown schematically in Figure 1A in their folded position and in Figure IB deployed after launch. The use of wrap-around or otherwise externally folded fins avoids the intrusion of recessed fin structures into the allocable contour of the projectile structure.

Turning now to other preferred features of the propulsion unit of the present invention, as mentioned earlier, momentum transfer medium 32 is preferably implemented as a gel. Various suitable gel compositions, and corresponding structures for storing and deploying the gel, are addressed in greater detail in a co-assigned patent application filed contemporaneously with this application and entitled "Counter-Mass for a Recoilless Gun". Accordingly, the gel composition and corresponding structures will be addressed here only briefly in a small number of non-limiting implementations. The gel characteristics and the divergence of the exit flow preferably result in fast expansion, disintegration and dispersal of the gel, thus generating only minimal back-blast. In order to minimize variations of gel characteristics over a wide range of operating temperatures, elastic and plastic gelling agents 170599/2 are preferably introduced as additives to the gel. The gelling agents added to the gel make it a non-Newtonian material with an elastic branch. Thus, a most preferred implementation of a gel for this propulsion system is a non-Newtonian, shear-thinning, visco-plastic gel which remains fluidic in wide temperature range of -40 oC - 70 oC. The following is one specific example of a gel providing the preferred properties.

Example: 19.2 kg anhydrous CaC12 was mixed with 45.5kg of water. 325 g hydroxyethyl cellulose was dissolved in the resulting brine by stirring for 1 hour. The resulting gel had a density of 1.28 gr/ml and mediocre settling properties. It solidified well below 40 oC.

While the gel may be filled directly into the pressure vessel between the piston 30 and outlet nozzle 22, it is generally considered advantageous for handling and assembly of the projectiles for the gel to be stored and provided in a sealed canister 60 which forms a modular component for assembly within the pressure vessel. One such implementation of a gel canister 60 is illustrated in Figures 2 and 4A-4C. In this case, the canister 60 is a single compartment canister which is shaped to be inserted through the first end of pressure vessel 18 prior to insertion of piston 30 and gas generator 24 and sealing with the end cap 20. The canister 60 is preferably made of lightweight material such as polymers which are readily ejected from the unit by piston 30 without interfering with operation of the propulsion unit. A gel canister cap 62 (seen partially in detail in Figure 4C) is screwed onto gel canister 60 so as to close off the mouth of the gel canister 60 prior to actuation, thereby protecting the gel and preventing leakage, with screws 64 closing the volume after regulating overflow through a small orifice in the diagram. The gel canister cap 60 serves as an elastic diaphragm which accommodates the thermal expansion of the gel. Optionally, a supplementary outer cap 54 may temporarily protect the outlet nozzle 22 as illustrated in Figure 2.

Figures 5A-5C illustrate a further option for the gel canister in which the canister is internally compartmentalized by separators into a number of distinct volumes 68 in order to mitigate center-of-gravity shifts due to possible sedimentation over a long timescale. The compartments are held together by fastening strips 66. In this case, the compartments are shown lengthwise, i.e., along the length of the propulsion unit parallel to a central axis. As a result of the compartmentalization, even if sedimentation does occur within each separate compartment, the overall effect on the weight distribution is small. Here too, the canister is sealed by a cover 70 retained by screws 72. This arrangement enables another possible design of the ignition system, namely installing the gas generator initiator in the piston and igniting the said initiator by a pyrotechnic cord connecting the triggering system to the gas generator initiator with part of it extending through the centerline of the counter-mass in a void space between compartments.

It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims (23)

170599/2 WHAT IS CLAIMED IS:

1. A single piston propulsion unit suitable for launching a projectile from a launch tube, the propulsion unit comprising: (a) a pressure vessel having a substantially cylindrical inner surface of a given internal diameter, said pressure vessel being sealed at a first end and terminating at a second end in a tapered portion, said tapered portion having an inwardly tapered conical surface extending along a length of at least half said internal diameter; (b) a gas generator including a quantity of combustible propellant deployed within said first end of said pressure vessel; (c) a piston deployed within said pressure vessel adjacent to said gas generator and in sliding engagement with said substantially cylindrical inner surface, said piston being formed primarily of a ductile material; (d) a momentum transfer medium deployed within said pressure vessel between said piston and said second end, and (e) a plurality of exhaust vent openings located near said tapered portion so that gas from said gas generator cannot escape through said exhaust vent openings until said piston has at least partially entered said tapered portion , such that, on initiation of said gas generator, gas pressure forces said piston along said pressure vessel, thereby ejecting said momentum transfer medium in a first 170599/2 direction and propelling said propulsion unit in an opposite direction, and such that, on reaching said tapered portion, said piston undergoes plastic deformation through sliding contact with said inwardly tapered conical surface and exposes said vent holes to gas generated by said gas generator so as to enable the gas to escape from said pressure vessel through said exhaust vent openings .

2. The propulsion unit of claim 1, wherein said gas generator generates a given maximum gas pressure within said pressure vessel, and wherein said gas generator and said exhaust vent openings are designed such that gas is released from said exhaust openings after a pressure within said pressure vessel has dropped below one fifth of said maximum gas pressure.

3. The propulsion unit of claim 1, wherein said tapered portion terminates in an outlet nozzle having an internal diameter of between 60 and 90 percent of said internal diameter of said cylindrical portion.

4. The propulsion unit of claim 1, wherein said piston is formed primarily from ductile steel.

5. The propulsion unit of claim 1, wherein said piston is formed primarily from aluminum. 170599/2

6. The propulsion unit of claim 1, wherein said piston is formed primarily from titanium.

7. The propulsion unit of claim 1, wherein a length of said pressure vessel is between 8 and 12 times said internal diameter.

8. The propulsion unit of claim 1, wherein said momentum transfer medium is a gel.

9. The propulsion unit of claim 1, wherein said pressure vessel includes: (a) a first portion proximal to said first end having a wall formed substantially exclusively of a metallic material with a first minimum wall thickness; a second portion including said tapered portion having a wall formed substantially exclusively of a metallic material with a second minimum wall thickness; and an intermediate portion between said first portion and said second portion, said intermediate portion including a metallic material with a minimum wall thickness less than both said first and said second minimum wall thicknesses, said intermediate portion further 170599/2 including a reinforcement layer of filament winding around said metallic material.

10. The propulsion unit of claim 9, wherein said reinforcement layer of filament winding is wound in a hoop orientation.

11. The propulsion unit of claim 1, wherein said gas generator is configured to complete combustion during no more than the first 20 percent of a range of motion of said piston.

12. The propulsion unit of claim 1, wherein said gas generator is configured to complete combustion during no more than the first 15 percent of a range of motion of said piston.

13. The propulsion unit of claim 1, wherein said gas generator includes at least a first propellant having a web thickness no greater than about 0.3 millimeters.

14. The propulsion unit of claim 1, further comprising a tail fin assembly including a plurality of wrap-around fins deployed around an external surface of said tapered portion.

15. The propulsion unit of claim 1, wherein said piston is formed with an elongated cylindrical sliding surface in contact with said substantially cylindrical 170599/2 inner surface of said pressure vessel, said cylindrical sliding surface having an axial length not less than 20 percent of said internal diameter of said pressure vessel.

16. The propulsion unit of claim 15, wherein said cylindrical sliding surface has an axial length of between 30 percent and 50 percent of said internal diameter of said pressure vessel.

17. The propulsion unit of claim 1, wherein said piston is formed with a concave surface facing said momentum transfer medium.

18. A single piston propulsion unit for launching a projectile from a launch tube, the propulsion unit comprising: (a) a pressure vessel having: (b) a gas generator including a quantity of combustible propellant deployed within said first end of said pressure vessel; (c) a piston deployed within said pressure vessel adjacent to said gas generator and in sliding engagement with said substantially cylindrical inner surface; and 170599/2 (d) a momentum transfer medium deployed within said pressure vessel between said piston and said second end, such that, on initiation of said gas generator, gas pressure forces said piston along said pressure vessel, thereby ejecting said momentum transfer medium in a first direction and propelling said propulsion unit in an opposite direction, and wherein said gas generator is configured to complete combustion during no more than the first 20 percent of a range of motion of said piston

19. The propulsion unit of claim 18, wherein said gas generator is configured to complete combustion during no more than the first 15 percent of a range of motion of said piston.

20. The propulsion unit of claim 18, wherein said gas generator includes at least a first propellant having a web thickness no greater than about 0.3 millimeters.

21. The propulsion unit of claim 18, wherein said pressure vessel is formed with a plurality of exhaust vent openings, said exhaust vent openings being located near said second end such that gas from said gas generator cannot escape through said exhaust vent openings until said piston has passed said exhaust vent openings.

22. The propulsion unit of claim 21, wherein said gas generator generates a given maximum gas pressure within said pressure vessel, and wherein said gas generator and said exhaust vent openings are designed such that gas is released from said exhaust openings after a pressure within said pressure vessel has dropped below one fifth of said maximum gas pressure. 170599/2

23. A single piston propulsion unit for launching a projectile from a launch tube, the propulsion unit comprising: (a) a pressure vessel having a substantially cylindrical inner surface of a given internal diameter, said pressure vessel being sealed at a first end and terminating at a second end in an outlet nozzle, said pressure vessel including: i. a first portion proximal to said first end having a wall formed substantially exclusively of a metallic material with a first minimum wall thickness, ii. a second portion proximal to said second end having a wall formed substantially exclusively of a metallic material with a second minimum wall thickness, and iii. an intermediate portion between said first portion and said second portion, said intermediate portion including a metallic material with a minimum wall thickness less than both said first and said second minimum wall thicknesses, said intermediate portion further including a reinforcement layer of filament winding around said metallic material; (b) a gas generator including a quantity of combustible propellant deployed within said first end of said pressure vessel; 170599/2 (c) a piston deployed within said pressure vessel adjacent to said gas generator and in sliding engagement with said substantially cylindrical inner surface; and (d) a momentum transfer medium deployed within said pressure vessel between said piston and said second end, such that, on initiation of said gas generator, gas pressure forces said piston along said pressure vessel, thereby ejecting said momentum transfer medium from said nozzle in a first direction and propelling said propulsion unit in an opposite direction. Floor a a ot ns y treet Ramat Gan 52520 by: