Liquid propellant gun

This invention provides a liquid propellant gun embodying a first species of this invention wherein the inner differential area piston is controlled by a variable damping mechanism, and both pistons have respective cross-sectional areas coupled to the pumping chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to guns utilizing liquid propellant and a 
differential piston to provide regenerative injection of the propellant 
into the combustion chamber after an initial ignition of propellant in the 
combustion chamber. 
2. Prior Art 
An extensive summary of the prior art appears in the patent applications of 
M. Bulman, Ser. No. 840,074, R. A. Algera, Ser. No. 840,075, and R. E. 
Mayer, Ser. No. 840,104, all filed Oct. 6, 1977. More recent exemplars of 
the prior art are the patents of R. E. Mayer, U.S. Pat. No. 4,341,147, 
issued July 27, 1982; I. K. Magoon, U.S. Pat. No. 4,523,507, issued June 
18, 1985; R. E. Mayer et al, U.S. Pat. No. 4,523,508, issued June 18, 
1985; and I. K. Magoon, U.S. Pat. No. 4,586,422, issued May 6, 1986. 
These more recent exemplars of Mayer and Magoon show a stationary central 
control rod aft of the combustion chamber which cooperates with an outer 
annular injection piston to pump liquid propellant from a mutually defined 
storage chamber, through a mutually defined injection annular orifice, to 
the combustion chamber. 
The exemplar of Mayer, U.S. Pat. No. 4,341,147, shows both a stationary 
central control rod and a moveable central piston which controls the flow 
of propellant through a series of holes in the shaft of a T-shaped 
propellant injection piston as a function of the relative displacement of 
the inner and outer elements. 
The applications of Bulman and Mayer show a stationary gun barrel with an 
outer annular piston and an annular, piston-like, fill valve, both 
substantially forward of the combustion chamber. The piston pumps liquid 
propellant from a storage chamber defined by the piston and the valve into 
the combustion chamber. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a liquid propellant gun wherein 
the mass rate of flow of liquid propellant can be repetitively, 
selectively, and continuously varied throughout the interval of time of 
firing a single shot. 
Another object of this invention is to provide a liquid propellant gun 
wherein the mass rate of flow of liquid propellant can be selectively 
varied from shot to shot. 
The ability to continuously vary the mass rate of flow provides control of 
the combustion gas pressure in the combustion chamber and the gun barrel 
aft of the projectile during the interior ballistic period of the gun 
cycle and thereby provides control over the acceleration and the exit 
velocity of the projectile. This control permits the use in the same gun 
of projectiles of respective different weights, of different sensitivities 
to acceleration, and of different desired trajectories. 
A feature of this invention is the provision of a liquid propellant gun 
wherein (i) the mass rate of flow of the liquid propellant into the 
combustion chamber is a function of the cross-sectional area of the 
injection orifice, and (ii) said area is a function of the differential 
displacement of two differential area pistons, and (iii) the displacement 
of each of said pistons are a function, inter alia, of the gas pressure in 
the combustion chamber.

DESCRIPTION OF THE INVENTION 
The several species of the invention each have two differential area 
pistons which jointly pump propellant under the control of a programmed 
mechanism to provide a programmed injection of propellant into the 
combustion chamber. Thereby, a large propellant pumping rate may be 
programmed by a control means applied to a small volume of control fluid. 
In each species, after initiation of combustion in the combustion chamber, 
the displacement of one differential area piston, which is herein called 
the controlled piston, is a function of (i) the gas pressure in the 
combustion chamber, (ii) the propellant liquid pressure in the pumping 
chamber, and (iii) the displacement of the other differential area piston, 
which is herein called the controlling piston. The displacement of the 
controlling piston is a function of (i) the gas pressure in the combustion 
chamber, (ii) the liquid pressure in the damping mechanism, and in 
addition (iii) the liquid pressure in the propellant pumping chamber for 
species shown in FIGS. 1, 2 and 5. The cross-sectional area of the 
injection orifice, which orifice is an annulus defined by the relative 
displacement of the respective heads of the two pistons, is a function of 
said relative displacement, which is the output function of a servo loop. 
The basic principle of operation in achieving controlled injection lies in 
making the respective ratios of the differential areas of each of the 
pistons different and providing a programmed resistance to the shaft of 
the controlling piston. The ratios are chosen such that the ratio of the 
contolling piston is greater than the ratio of the controlled piston, 
i.e.: 
Eq. 1: 
[Combustion chamber face area/Pumping chamber face area] of the controlling 
piston is greater than 
[Combustion chamber face area/Pumping chamber face area] of the controlled 
piston. 
The pumping chamber and the combustion chamber can be considered to be 
plenum chambers with respective uniform pressures acting on all surfaces 
in each chamber at a given instant of time. 
The operation may be understood by considering a ramp function of 
combustion gas pressure to be applied within the combustion chamber while 
the pumping chamber is full of liquid propellant, and while both pistons 
are at rest with their respective heads in mutual contact, thereby closing 
the injection orifice. 
The ramp function of combustion gas pressure acts on the respective 
combustion chamber faces of both pistons and causes them each to be 
accelerated aftwardly compressing the propellant and increasing the liquid 
propellant pressure. When the propellant pressure in the pumping chamber 
reaches a value which satisfies equation (2) the net force acting on the 
controlled piston is zero and its acceleration is zero. 
Eq. 2: 
[Combustion chamber pressure.times.combustion chamber face area] of the 
controlled piston minus [Pumping chamber pressure.times.pumping chamber 
face area] of the controlled piston equals zero. 
Meanwhile the controlling piston continues to accelerate aftwardly with a 
net force acting toward the breech since by virtue of the inequality 
stated by Eq. 1: 
Eq. 3: 
[Combustion chamber pressure.times.combustion chamber face area] of the 
controlling piston minus [Pumping chamber pressure.times.pumping chamber 
face area] of the controlling piston is greater than zero. 
Consequently, the two pistons tend to move aftwardly at different 
respective velocities, thereby increasing the gap between their respective 
heads, i.e. increasing the cross-sectional area of the injection orifice. 
There is a tendency for the controlling piston to try to pump the liquid 
pressure in the pumping chamber to a value above that required for force 
balance of the controlled piston (Eq. 2). But such an over-pressure 
produces deceleration of the velocity of the controlled piston which 
further increases the cross-sectional area of the injection orifice, which 
reduces the hydraulic flow resistance of the liquid in the pumping 
chamber, which results in an actual instantaneous liquid pressure 
somewhere between the values determined at force balance by the two 
different ratios of area of the respective pistons. This system is thus a 
closed loop hydraulic servo system. 
If now a hydraulic resistance is applied to an aft face portion of the 
controlling piston such that the pressure developed against the aft face 
of said shaft is a function, for example, of the velocity of the 
controlling piston, the controlling piston will continue to accelerate 
until it reaches a velocity which provides sufficient hydraulic resistance 
(pressure.times.aft face area) to balance the driving force of the 
pressure of the combustion gas on the forward face of the controlling 
piston: 
Eq. 4: 
[Combustion chamber pressure.times.combustion chamber face area] of the 
controlling piston minus [Propellant pumping chamber 
pressure.times.propellant pumping chamber face area] of the controlling 
piston minus [Damping chamber pressure.times.damping chamber face area] of 
the controlling piston equals zero. 
Then, a steady state operating condition is achieved, assuming constant 
combustion chamber pressure, in which both pistons are in force balance at 
zero acceleration and moving aftwardly at the same velocity with the 
cross-sectional area of the injection orifice determined by the difference 
in relative positions of the controlled piston and the controlling piston. 
If one of the parameters considered fixed in the analysis above varies 
from the assumed condition, the steady state operating condition will 
shift to accomodate the new parameter for force balance. Under transient 
conditions, inertial forces must be taken into account to determine the 
instantaneous acceleration of the pistons, but the result is that the 
velocity of the controlled piston tends to be "served" to a force balance 
to follow and to approximate the velocity of the controlling piston as 
steady state velocity is approached. 
In order to select and to provide a desired profile of mass-rate of flow of 
injected liquid propellant for a selected shot, the hydraulic resistance 
applied to the shaft of the controlling piston can be programmed as a 
function of several possible parameters. The hydraulic resistance can be a 
function of the cross-sectional area of an orifice in the hydraulic 
control circuit, which area can be a function of the position of the 
controlling piston, or the temperature or pressure of the liquid 
propellant in the pumping chamber. If the viscosity of the hydraulic 
control fluid is not sensitive to temperature, then, notwithstanding that 
the viscosity of the liquid propellant may be sensitive to temperature, 
the pressure-time curve of the combustion chamber can be made more 
insensitive to temperature variations. Those temperature variations may be 
either as a result of the firing schedule (i.e. burstfiring) or the 
ambient temperature. 
The hydraulic resistance may be increased significantly towards the end of 
the aftward stroke of the controlling piston, to bring both pistons to a 
relative soft stop at the end of their respective strokes. 
The gun shown in FIG. 1 includes a receiver 10 having a longitudinally 
extending cavity 12 therein, whose forward end receives a gun barrel 14 
and whose aft end receives a breech obturator 16. The barrel and the 
obturator are each releasably secured to the receiver by conventional 
means, here shown as threads. The obturator may be advanced into the 
cavity 12 more or less as desired to vary its internal open volume or it 
may remain fixed and the initial chamber volume allowed to vary as the 
charge is varied. A projectile 17 may be inserted into the projectile 
chamber 18 of the gun barrel 14, which barrel may have a conventional 
forcing cone 20 and rifling 22. Projectiles may be sequentially fed and 
chambered by an appropriate loading mechanism which is not shown here; but 
see, for example, U.S. Pat. No. 4,244,270, issued to D. P. Tassie on Jan. 
13, 1981. The gun barrel 14, the cavity 12, and the obturator 16 are shown 
as mutually coaxial, i.e., "in-line," on the longitudinal axis 24 of the 
gun. 
An outer, differential area, controlled piston 26 has an aft tubular body 
28 which rides within an annular cavity defined by the inner wall of the 
cavity 12 and the outer wall 32 of a reduced diameter forward portion 34 
of the obturator 16. The piston 26 has a forward annular head 36 having a 
forward annular face 38 of relatively large cross-sectional area, an aft 
annular face 40 of relatively small cross-sectional area and a conical 
opening 42. 
An inner, differential area, controlling piston 44 has an aft cylindrical 
body 46 which rides within a cylindrical cavity 48 having a side wall 50 
and a base wall 52 in the obturator 16. The body 46 has an aft face 47 and 
carries an annular seal 54 which seals against the side wall 50 to close, 
with aft face 47, the cavity 48. The piston 44 has a forward 
frusto-conical head 56 having a forward circular face 58 of relatively 
large cross-sectional area, an aft annular face 60 of relatively small 
cross-sectional area, and a conical side wall 62 which mates with the 
conical opening 42. The combustion chamber 63 is defined by the aft face 
of the projectile 17, the piston forward faces 58 and 38, and the inner 
wall of the cavity 12. 
The obturator 16 also includes an internal cylindrical cavity 64 having a 
sidewall 66, a forward wall 68 and a base wall 70. A longitudinal bore 72 
extends between the faces 52 and 68. A piston 74 is disposed within the 
cavity 64 and has an annular seal 76 which seals against the side wall 66 
to divide the cavity 64 into a forward portion 64F and an aft portion 64A. 
A rod 77 is fixed to and between the controlling piston 44 and the piston 
74 and passes through the bore 72. A seal 78 is fixed in the bore 72 and 
seals against the rod 77. 
A control valve mechanism, here shown as two valves 80a and 80b, is also 
respectively connected to and between the faces 52 and 68 to permit the 
flow of hydraulic fluid between the cavity 48 and the cavity 64F. The 
orifice area in each control valve may be variably controlled through a 
respective control passageway 82a and 82b so as to variably limit the mass 
rate of flow of hydraulic fluid between the cavities 48 and 64F. The 
control valve may be pressure controlled, spring return, where the 
pressure in the control passageway is controlled by a cam operated spool 
valve assembly as shown in U.S. Pat. No. 3,763,739 issued to D. P. Tassie 
on Oct. 9, 1973. 
The cavity 84, defined by the two pistons 26, 44 and the obturator 16, 
serves as the liquid propellant reservoir or propellant pumping chamber; 
and is filled through a passageway 86 having a checkvalve 88, both in the 
obturator 16. The volume of this reservoir 84 is determined by length that 
the obturator 16 has been set into the cavity 12 of the receiver 10. 
Alternatively, a lesser volume can be determined by limiting the joint 
forward travel of the two pistons to less than full forward. 
A latch mechanism to hold the outer piston 26, and with it, the inner 
piston 44, fully seated in its aft disposition on the forward portion 34 
of the obturator 16, may include an annular notch 90 in the outer piston 
26 and a pressure controlled, spring return detent 92 having a control 
passageway 94, whose pressure may be controlled by a cam operated spool 
valve assembly. 
An annular cavity 100 may be provided around the outer piston 26 which may 
be prefilled with hydraulic fluid via a passageway 102 with a check valve 
104 to provide hydraulic support to the annular wall of the piston during 
firing. This cavity may also receive additives, if desired, to be passed 
into the combustion chamber 63 during the aftward stroke of the outer 
piston 26. Alternatively, the cavity 100 may be omitted. 
A source 110 of initial combustion gas is coupled by a passageway 112, 
which may have a check valve 114, into the combustion chamber 63 to 
provide an initial supply of gas under pressure in the combustion chamber 
to initiate the aftward stroke of the controlling piston 44, to apply 
pressure to the liquid propellant in the pumping chamber 84 and thereafter 
to open the injection orifice defined by the conical surfaces 42 and 62. 
This source may be an electrically fired primer, which is replaced as each 
projectile is chambered; or it may be an electrically fired liquid 
propellant initiator, or it may be an adiabatic igniter as shown in U.S. 
Pat. No. 4,231,282 issued to E. Ashley on Nov. 4, 1980. 
A simplified version of the hydraulic damping control of FIG. 1 is shown in 
FIG. 5. Here the damping, variable area, orifice 80', equivalent to 80a or 
80b, is defined by an annular opening 120 formed in the breech obturator 
16' and a contoured stem 122 which extends aftwardly from the controlling 
piston 44'. The diameter of the stem adjacent the opening 120 determines 
the area of the orifice. As here shown, the orifice area will be minimized 
towards the end of the aftward stroke of the controlling piston 44' to 
bring both it and the controlled piston 26' to a soft stop into rear 
dwell. 
FIG. 1 shows one mechanism for refilling liquid propellant into the pumping 
chamber. When the controlled piston 26 reaches the end of its aft stroke 
its notch 90 is captured by the latch 92 to hold that piston 26, and 
thereby the controlling piston 44, in aft dwell. When liquid propellant is 
admitted through the passageway 86 the latch 92 is released, or overcome 
by propellant loading pressure, to move both pistons together, with the 
injection orifice closed, forwardly to the end of their forward stroke. 
Captured gas under pressure in the cavity 64A may be utilized to insure 
that the controlling piston 44 moves with the controlled piston 26 to keep 
the injection orifice closed. 
Alternatively, it may be desired to permit an initial leak of a controlled 
volume of liquid propellant at the start of the forward stroke, so that 
subsequently such liquid propellant then in the combustion chamber, may be 
ignited by an electric spark or laser to provide combustion gas to start 
the firing cycle. 
FIG. 1 also shows a rib 140 extending radially from the forward face 58 of 
the controlling piston 44. This rib is maintained in a substantially fixed 
position relative to the injection orifice throughout the firing stroke 
and serves to disperse or break up the flowing sheet of the propellant 
throughout the combustion chamber. 
In many usages, as for example in an artillery weapon, it is necessary not 
only to vary the propellant injection rate, and thereby the combustion gas 
generation rate, but also to vary the total propellant charge. As shown in 
FIG. 1, the obturator 16 may be advanced more or less into the receiver 10 
to decrease or to increase the volume of the pumping chamber 84 and 
thereby the volume of propellant admitted into the pumping chamber. 
FIG. 2 shows the outer differential area piston 200 serving as the 
controlling piston, and the inner differential area piston 202 serving as 
the controlled piston. In this species the motion of the controlling 
piston 200 is controlled by a variable hydraulic control circuit 204 
coupling annular cavities 206 and 208. The annular stem 210 of the piston 
200 travels within the cavity 208 during the aft stroke of the piston and 
must displace the hydraulic fluid from the cavity 208 through the control 
circuit 204. A helical compression spring 212 is disposed on the stem of 
the piston 202 to hold this piston 202 against the piston 200 during the 
propellant loading process to maintain the injection orifice closed. A 
deflector such as rib 214 extending radially from the combustion chamber 
face of the controlling piston 200 may be used to serve as a break up 
device for the sheet of liquid propellane injected through the injection 
orifice. 
FIG. 3 shows a species which is similar to that of FIG. 1, except that the 
controlling inner piston 300 has a combustion chamber circular face 302 
and a damping chamber annular face 304, but not any face on the pumping 
chamber 306. Thus, the second term of equation 4 is zero with respect to 
this species. 
FIG. 4 shows a species which is similar to that of FIG. 2 except that the 
controlling outer piston 400 has a combustion chamber annular face 402 and 
a damping chamber annular face 404, but not any face on the pumping 
chamber 406. Thus, the second term of equation 4 is zero with respect to 
this species. 
Thus in the species of FIGS. 3 and 4, the motion of the controlling piston 
is independent of the pressure fluctuations in the liquid propellant in 
the propellant pumping chamber. 
FIG. 6 shows another version of the hydraulic damping control of FIG. 1. 
Here the variable area damping orifice 352, equivalent to 80a, and 80b, is 
defined by a series of orifices connected to a series of orifices 353 by a 
slot 342 located by front 340F and aftward 340A portions of cylindrical 
stem 340 which extends aftwardly from the controlling piston 330. The 
cylindrical stem 340 slides into a cylindrical bore 341 provided in the 
breech obturator 346. As the stem 340 slides into the cylindrical bore 
341, the slot 342 also moves and connects the variable area damper 
orifices 352 to the variable area orifices 353. The variable area orifices 
352 are connected to cavity 350 by the passageway 351 and the variable 
area orifices 353 are connected by the passageway 354. Here as shown, the 
damper orifice area will be minimized towards the end of the aftward 
stroke of the controlling piston 330 to bring both it and the controlled 
piston 331 to a soft stop into rear dwell. The damper orifice area can be 
varied by opening or closing the orifices 352. A passageway 343 is 
provided to fill hydraulic fluid into the damper cavity 350.