High-Performance Computing and Simulation for Advanced 

Armament Propulsion 

by Michael J. Nusca 

ARL-TR-3215 June 2004 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-TR-3215 June 2004 

High-Performance Computing and Simulation for Advanced 

Armament Propulsion 

Michael J. Nusca 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited.
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High-Performance Computing and Simulation for Advanced Armament Propulsion 

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14. ABSTRACT 

The U.S. Army is exploring a variety of armament propulsion options for indirect- and direct-fire weapons to be employed in the

legacy force and for the Future Combat System (FCS). Development of propelling charges for these weapons requires state-ofthe-art physics-based modeling codes. The gun propulsion-modeling environment has traditionally been one in which 

independently developed and upgraded computer codes were used to analyze different propulsion techniques, with no single 

maintainable code able to evaluate several candidate systems. This situation rendered comparison of ballistic performance for 

various weapon options cumbersome and inconclusive. In direct response to this situation, the U.S. Army Research Laboratory 

(ARL) has developed a “next-generation,” computer scaleable, three-dimensional, multiphase, computational fluid dynamics 

code for armament propulsion (interior ballistics) modeling. ARL’s NGEN3 code represents the sole Department of Defense 

(DOD) computer tool that is able to simulate the highly complex physics associated with indirect- and direct-fire guns. NGEN3 

code development and application to the FCS was a 2001–2002 DOD high-performance computing challenge project and was 

being greatly advanced by priority access to the DOD Major Shared Resource Centers. 

15. SUBJECT TERMS

gun charges, solid propellant, multiphase flow, computer simulation 

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19a. NAME OF RESPONSIBLE PERSON

Michael J. Nusca 

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iii

Contents 

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Complex Physics Requiring HPC 2

3. The Emergence of a New Computing Tool for the U.S. Army 3

3.1 Generalized Governing Equations...................................................................................4 

3.2 Boundary Conditions for Propelling Charges .................................................................5 

3.3 Coupled Eulerian/Lagrangian Solution Schemes............................................................5 

4. Recent Accomplishments 6

4.1 Description of the MACS................................................................................................6 

4.2 Results for a Typical Modular Charge ............................................................................7 

4.3 Description of HLD Charges and ETC Technology for the FCS....................................9 

4.4 Results for a Typical Direct-Fire Gun Chamber ...........................................................11 

5. Summary 12

6. References 14

Distribution List 15 
iv

List of Figures 

Figure 1. Schematics of gun charge configurations: (a) loose charge, (b) packaged charge in 

single module, and (c) packaged charge in two modules. Ignition source indicated by (*).....8 

Figure 2. Computed results for packaged charge (two modules): (a, b) time = 1-ms, (c, d) 

time = 3-ms, (e, f) time = 8-ms, and (g, h) time = 11-ms gas velocity vectors along with 

module boundaries (a, c, e, and g) and propellant temperature contours (b, d, f, and h) 

from 294 K (white) to 440 K (black). Computed results for packaged charge (2 

modules): (i) time = 13-ms and (j) time = 21-ms gas velocity vectors along with module 

boundaries................................................................................................................................10 

Figure 3. Computed results for (a) packaged charge (two modules): projectile velocity, 

breech pressure, and pressure difference (breech pressure minus pressure at chamber end) 

vs. time and (b) computed results for one module...................................................................11 

Figure 4. Computed results for 120-mm HLD charge (SP disks and grains): (a) porosity 

contours (black is dense material) at initial time, (b) porosity contours at 6 ms, and (c) 

propellant temperature contours (black is ignited propellant at 440 K) at 6 ms......................12 
v

Acknowledgments 

The U.S. Army Research Laboratory (ARL) mission program in multidimensional modeling, 

directed by Dr. M. Nusca, has primarily supported development of the NGEN3 code. NGEN3 

code development has also been supported by contracts between ARL and Paul Gough 

Associates of Portsmouth, NH. Computer resources at ARL, Naval Oceanographic, and 

Engineer Research and Development Center Major Shared Resource Centers (MSRCs), and the 

Space and Missile Defense Command distributed center were provided via support from the 

Department of Defense High Performance Computing (HPC) Modernization Office under the 

HPC Challenge program and are gratefully acknowledged. Support from the ARL-MSRC 

Scientific Visualization Team (R. Angelini and M. Bolstad) is also gratefully acknowledged. 

Application of the NGEN code to the 155-mm Modular Artillery Charge Systems propelling 

charge has been supported by Project Manager-Crusader/Munitions (POC: Mr. J. Rutkowski) at 

the U.S. Army’s Armament Research, Development, and Engineering Center (POC: 

Mr. P. Hui). 
vi

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

The U.S. Army is exploring a variety of armament propulsion options for indirect- and direct-fire 

weapons (guns) for the legacy force and the Future Combat System (FCS). As it transforms, the 

U.S. Army has identified requirements for hypervelocity projectile launch systems for strategic 

U.S. Army missions. Among these systems are those that use solid propellant—granular form 

loaded in modules (indirect-fire) or disk and strip form for high loading density (HLD) cartridges 

(direct fire)—augmented by electrothermal-chemical (ETC) technology. Two such armament 

propulsion systems are the U.S. Army’s Modular Artillery Charge System (MACS) and HLD 

charges for the FCS. The MACS is being developed for indirect fire cannon on current 155-mm 

systems (e.g., M109A6 Paladin and M198 Towed Howitzer). The efficiency of the MACS 

charge is dependent on proper flamespreading through the propellant modules—a process that 

has been repeatedly demonstrated in gun firings, successfully photographed using the U.S. Army 

Research Laboratory (ARL) 155-mm ballistics simulator, and numerically modeled using the 

ARL NGEN3 code. The FCS requires weapons systems exhibiting increased range, accuracy, 

and highly repeatable projectile launch performance. One of the technologies under 

investigation to achieve these goals is the ETC concept, in which electrically generated plasma is 

injected into the gun chamber in order to efficiently and repeatedly ignite the high-energy and 

HLD solid propellant charge. 

As modular and HLD propelling charges are being developed, optimized, and ultimately mated 

to systems such as indirect fire cannon and the continually evolving FCS, there is a critical need 

to have a single, validated, maintainable computer code based on state-of-the-art computational 

fluid dynamics (CFD), as an evaluation and performance analysis tool. It has long been 

recognized that the availability of such a tool would provide the U.S. Army with the unique 

capability to simulate current and emerging gun propulsion systems using computer simulations. 

These simulations would serve to both streamline testing and aid in the optimization of weapon 

performance. Indeed, such a tool would dovetail nicely with the U.S. Army’s initiative in the 

creation of national high-performance computing (HPC) facilities. However, the gun 

propulsion-modeling environment has historically been one in which separate codes (some onedimensional [1-D], some two-dimensional [2-D]) are used, with no single multidimensional code 

able to address the truly three-dimensional (3-D) details of all of these weapons systems. This 

unfortunate situation renders comparison of ballistic performance cumbersome and inconclusive. 

In contrast, the multiphase continuum equations that represent the physics of gun propulsion 

comprise a set of general equations universally applicable to all solid propellant armament 

propulsion systems. 

In direct response to this situation, ARL began a development program ~8 years ago to 

revolutionize the U.S. Army’s ability to use HPC to simulate propelling charges. The current 
2

author at ARL, with consultation from noted industry/academic experts, has worked on the 

project. The result is the U.S. Army’s “Next Generation” computer scaleable 3-D multiphase 

CFD code for armament propulsion modeling. The ARL’s NGEN3 code represents the sole 

Department of Defense (DOD) computer tool that is able to simulate the highly complex physics 

associated with indirect and direct fire guns. NGEN3 code development and application to the 

FCS was a 2001–2002 DOD HPC Challenge Project and was being exercised regularly with 

priority access to the DOD Major Shared Resource Centers (MSRCs). 

2. Complex Physics Requiring HPC 

A brief description of the physics of armament propulsion demonstrates the nature of this 

computing challenge and the necessity of state-of-the-art computer tools for use by the Army 

weapon system designers. 

A solid propellant gun system consists of a reaction chamber connected to a gun tube through 

which a projectile is guided once propelled by pressurization of the chamber. Chamber 

pressurization is accomplished by placing a solid propellant (SP) charge in the chamber and 

igniting it by various means. Current SP charges are generally complex structures consisting of 

hundreds or even thousands of distinct regularly formed (e.g., spherical and cylindrical) grains, 

which may be loaded in either regular or random arrangements. In addition to small-scale 

voidage between grains (i.e., porosity), many charges also contain large-scale voidage (i.e., 

ullage), which surrounds the entire charge (such as when the charge does not fill the entire 

chamber volume) or separates distinct subcharges (i.e., increments or modules) that together 

comprise the whole charge. The addition of energy to the chamber, usually near the gun breech, 

or rearmost end of the chamber, and in some cases through a tube extending along the centerline 

of the chamber, ignites the SP. In general, all of the grains are not ignited simultaneously, but an 

ignition flame spreads from the breech to the projectile base. 

The burning of the SP transforms chemical energy into heat as hot gases evolve from the surface 

of each grain of propellant. Initially the projectile resists movement allowing the pressure in the 

chamber to climb rapidly. Because the burn rate of the propellant is proportional to the pressure, 

hot gases are produced at an accelerated rate until peak pressure is reached in the chamber. 

Movement of the projectile down the gun tube, usually slight before peak pressure and much 

more significant afterwards, causes the chamber volume to increase and generates rarefaction 

waves that lower the pressure and thus the burn rate of the propellant. Upon ignition and 

burning, the gas dynamic flowfield in the gun chamber takes on a highly complex structure that 

includes the dynamics of propellant motion and combustion and various gas dynamic flow 

phenomena such as turbulent mixing, highly transient pressure waves, steep gradients in porosity 

and temperature, nonideal thermodynamics, and gas generation. 
3

As a direct result of the complexity of the interior ballistic event, previously described, numerical 

simulation of the multiphase gas dynamics and propellant combustion in a gun chamber is 

critically important to gun charge design. In some cases, a new charge design is not performing 

as planned and numerical simulations are used to elucidate the important physics involved during 

flamespreading in the SP and augment experimental data gathering during gun firings. These 

efforts commonly lead to subtle charge design changes that increase the efficiency of the new 

design. In other cases, a new charge design can generate pressure waves of dangerously high 

levels that inappropriately stress the gun system. A central goal in computer modeling of the 

interior ballistics event is to predict the maximum pressure in the chamber and the projectile's 

gun tube muzzle velocity. 

For some SP charges, successful modeling can be accomplished using 1-D, single-phase, 

“lumped parameter” models, which are based on the assumption that grains and the products of 

combustion constitute a well-stirred mixture. However, it has long been recognized that many 

charges, especially those involving multiple increments and complex distributions of ullage—the 

effects of which are nonuniformities in certain interior ballistic events (e.g., ignition) can only be 

simulated on the basis of a multidimensional multiphase flow model. In an ETC gun, the plasma 

injection event and the resulting plasma convection, diffusion and participation in ignition and 

flamespreading, through the SP, is also most accurately simulated using a multidimensional 

multiphase flow model. 

Multidimensional, multiphase flow modeling of a single armament launch scenario proceeds 

from propellant and projectile loading (initial conditions) to propellant consumption and 

projectile launch. Each detailed simulation necessarily requires large amounts of computer 

memory (10–50 Gb) and time (10–130 CPU hr). As a result, the HPC resources made available 

by the DOD are being utilized by ARL when employing the NGEN3 code, as discussed in the 

following section. 

3. The Emergence of a New Computing Tool for the U.S. Army 

As the U.S. Army transforms, there is a renewed interest in armament propulsion systems 

wherein the projectile is of greater complexity and larger size, with an afterbody that extends into 

the gun chamber and a solid propellant charge that is contained in a smaller volume, and is 

therefore necessarily of high energy and HLD. Design and optimization of such systems 

requires a “next generation” of computer modeling tools. The ARL NGEN3 code is a direct 

response to this need, having been in development, testing, and validation for the last eight years. 

In this section, the truly unique features of the NGEN3 code are reviewed including the first use 

of a coupled Eulerian/Lagrangian solution scheme in multiphase CFD propulsion codes. This 

original modeling feature enables the NGEN3 code to resolve system details not attainable from 

other models. 
4

The NGEN3 code has been documented elsewhere (1–3) The governing equations, state 

equations, and constitutive relations have been detailed in other papers (1, 4, 5). The 2-D version 

of the code (physical and numerical models) is briefly described herein. Extensions to 3-D are 

described elsewhere (2). 

3.1 Generalized Governing Equations 

A modeling code capable of treating a wide variety of gun propulsion systems must be based on 

a solution of the conservation equations for a multiphase, turbulent reacting flow. Two phases 

are considered: a continuous phase (multicomponent mixture of gases and liquids in local 

mechanical equilibrium) and a discrete phase (aggregate of particles made up of solid/liquid 

propellant reactants and combustion products as well as wear-reducing additives and pieces of 

the propellant container). The continuous phase is characterized by single local values for the 

dependent variables: density (ρ), velocity (u), pressure (p), temperature (T), shear stress (τ), and 

internal energy (e). It is assumed to comprise NC species (e.g., air, plasma, and solid propellant 

gaseous products) with local values of the mass fraction, Yi, and molar fraction, Xi. Each 

component, i, is characterized by a diffusion velocity, vi, relative to u, the mass weighted 

average velocity of each of the components. The discrete phase consists of Nd components (e.g., 

unburned/burning propellant), each characterized by values for the dependent variables, number 

density (nd), volume (Vd), and surface area (Sd). 

On a sufficiently small scale of resolution in both space and time, the continuous phase is 

represented by the balance equations for a multicomponent mixture cast in conservation form 

(1, 4, 5). These equations describe the conservation of mass (global and for each species), 

momentum, and energy. For example, the balance equation for the energy is 

– 0

2 2

2 2

=















 • + 















+ ∇ • +

































+

∂

∂

ρ ρ u u σ q

u

e

u

e

t , (1) 

where the stress tensor is σ = –p I + τ, with I as the unit tensor. Heat flux, q, includes radiative 

transport as well as mass and thermal diffusion. The presence of a dispersed phase makes the 

governing equations computationally intractable. In order to achieve more reasonable computer 

times, a macroscopic representation of the governing equations is adopted based on properties of 

the flow over length scales whose size is large compared to the length scale of the individual 

particles. A detailed derivation of the macroscopic balance equations is presented elsewhere (4). 

Mass and momentum are conserved in the discrete phase. A porosity variable (α) is defined to 

map variables from the discrete to the continuous phase 

= ∑ ∑ =

Nd di

i

N

i

di di Vdi α 1- α 1- n , (2) 

where αdi denotes the volume fraction of the i-th discrete phase consisting of ndi particles per unit 

volume, and the volume of a particle, Vdi. No provision is made for transformation of one 
5

discrete phase into another. If the solid phase is considered to be incompressible (or nearly so, 

such that the solid phase is not thermally excited), the energy equation is not required. The 

macroscopic balance equations require a number of constitutive laws for closure. These include 

laws governing the molecular and turbulent transport terms, the equation of state for continuous 

and discrete phases, chemical reaction rates, intergranular stresses, and interphase transfer of 

energy, momentum, and mass. In addition, the morphology of the discrete phases is required and 

is typically expressed as a surface area per unit volume of each particle. 

3.2 Boundary Conditions for Propelling Charges 

Boundary conditions for the governing equations are divided into external and internal types. 

The external boundaries, i.e., those outside the numerical domain, usually include the gun breech 

face, the gun tube wall, and the base of the projectile. On these surfaces, conditions are derived 

that prescribe tangential slip of both phases. Flux conditions can be prescribed for reactive walls 

(e.g., a center core igniter tube or the plasma moderator tube). If wall boundary layers are to be 

resolved on inert surfaces, no-slip conditions are prescribed for the continuum phase. A flux 

condition is used for surfaces where propellant or plasma is injected. Internal boundaries (i.e., 

those within the numerical domain) usually separate two distinct regions and require a coupling 

formulation. A type of internal boundary is used in the MACS charge simulation. Boundary 

cells are placed around each module that are impermeable to flow until a prescribed pressure 

loading is attained. Once this threshold is exceeded and a prescribed rupture delay time interval 

has elapsed, flux into and out of the charge is unimpeded (partial flow is permitted when the 

pressure criteria is satisfied, but the time interval has not elapsed). A rigid body analysis is 

enforced up to the instant of container burst. In addition, the separation of a container end wall 

(or cap) due to overpressure can be modeled without relaxation of the rigid body treatment for 

the remainder of the container. The temperature of the cases, initially at an ambient value, is 

raised via convective heat transfer from the surrounding hot gases (generated by components that 

are ignited). The cases ignite producing hot gases in accordance with specified material 

properties. 

3.3 Coupled Eulerian/Lagrangian Solution Schemes 

The balance equations constitute a system of coupled partial differential equations that can be 

solved using a finite-difference technique. The derivatives in these equations are converted to 

algebraic expressions that pertain to a rectangular mesh of discrete, ordered points distributed 

about the geometry. Nonorthogonal geometric surfaces are handled using partially occluded 

cells (i.e., partially within geometric surfaces). Values of the dependent variables are determined 

at the cell centers using an iterative numerical method in space and time. An explicit method is 

chosen because the resolution of boundary layers (either on chamber walls or particle surfaces) is 

not considered. The numerical time step is limited by characteristic cell dimension and the 

fastest wave speed (Courant condition), except near the mesh boundaries where numerical 

stability is ensured by a Method-of-Characteristics based technique (5). The Courant condition 
6

is not overly restrictive for the present class of gas dynamic problems because the need for wave 

tracking is apparent, and thereby integration using Courant numbers larger than unity is not 

desirable. 

Particularly unique and original to the NGEN3 code are the Continuum Flow Solver (CFS) and 

the Large Particle Integrator (LPI). The spatial values of the dependent variables at each time 

step are determined by a numerical integration method, denoting the CFS, which treats the 

continuous phase and certain of the discrete phases in an Eulerian fashion. The Flux-Corrected 

Transport scheme (1) is a suitable basis for the CFS because the method is explicit and has been 

shown to adapt easily to HPC systems. The discrete phases are treated by a Lagrangian 

formulation, denoting the LPI, which tracks the particles explicitly and smoothes discontinuities 

associated with boundaries between propellants, yielding a continuous distribution of porosity 

over the entire domain. The manner of coupling between the CFS and the LPI is through the 

attribution of properties (e.g., porosity). The size of the grid as well as the number of Lagrangian 

particles is user prescribed. For the simulations of novel solid propellant configurations, such as 

slabs (disks) stacked axially along the chamber centerline and/or thin annular concentric layers 

(wraps), the NGEN3 code takes a macroscopic approach. These solid propellant media are 

modeled using Lagrange particles that regress, produce combustion product gases, and respond 

to gas dynamic and physical forces. Individual grains, sticks, slabs, and wrap layers are not 

resolved; rather, each medium is distributed within a specified region in the gun chamber. The 

constitutive laws that describe interphase drag, form-function, etc., assigned to these various 

media, determine preferred gas flow paths through the media and responses of the media to 

forces. 

4. Recent Accomplishments 

4.1 Description of the MACS 

The U.S. Army’s MACS is the propulsion system for all fielded 155-mm guns, including the 

M109A6 Paladin and the M198 Towed Howitzer. The MACS charges are designed to both 

significantly increase muzzle energy and to be suitable for application in automated loading 

systems. As a result of the modular charge design there is a physical barrier (i.e., the case) at the 

interface between two adjacent charge increments. Because direct ignition transfer is therefore 

impeded, charge ignition may not proceed in the order in which the charges are loaded into the 

gun chamber without a highly effective ignition system. These system-specific details (such as 

the propellant module cases and igniters) must be modeled as part of the overall numerical 

simulation and cannot be accurately treated with less than a multidimensional CFD code (i.e., 

1-D “lumped-parameter” models cannot properly model the MACS). The efficiency of the 

MACS charge is highly dependent on proper flame spreading through the modules, which can 

only be modeled using the NGEN3 code because ignition and flame spreading in the MACS 

charge are largely three-dimensional. Data from experimental pressure taps located on the gun 
7

chamber wall and photographs/movies of the MACS in the ARL’s clear tube simulator do not 

provide enough detail to diagnose the performance of the MACS charge. 

The NGEN3 code was used to analyze the M231 and M232 charges that have been recently 

developed and type classified at the U.S. Army’s Armament Research, Development, and 

Engineering Center in a typical 155-mm cannon. Three significant accomplishments using the 

NGEN3 code are as follows: 

1. NGEN3 simulations for the M231, low-zone MACS charge, displayed the importance of 

nonsequential center core charge ignition, module movement and case ignition/burnthrough, 2-D and 3-D propellant flame spreading behavior within the modules, module cap 

separation, and nonsequential module bursting (1). 

2. The NGEN3 code was used to uncover a heretofore-unknown physical phenomenon for the 

M232 high-zone MACS charge that only occurs during charge fallback in an elevated gun 

tube (6). NGEN3 simulations of these charges proved that a prevailing gas dynamic 

process, when the modular charges are positioned adjacent to the gun chamber breech face, 

would cause the rigid charge modules to be displaced forward. This displacement 

mitigates any pressure wave formation that would have occurred had the charges remained 

on the breech face during flame spreading and rupture. Laboratory fixture tests, conducted 

at ARL, validated the code. Subsequently, the NGEN3 simulations for the M232 were 

used to plan range testing of the XM297 cannon. These gun tests confirmed that M232 

increments at zones 4–6, positioned adjacent to the wedge face, support predictions by the 

NGEN3 code; the ballistic cycle is virtually unchanged, except for slight differences in 

timing. 

3. NGEN3 code simulations for the M232 charge uncovered a heretofore-unknown physical 

phenomenon involving the importance of case combustion (7). It was found that case 

combustion is clearly a significant contributor to chamber pressure and a significant factor 

in the generation of pressure differentials in the gun chamber. NGEN3 simulations indicate 

that the prediction of pressure differentials that are similar to those measured in the gun 

firings are not possible without first achieving good agreement with measured pressure data 

early in the ballistic cycle and including case combustion effects. 

Unfortunately, graphical results of the simulations leading to these accomplishments cannot be 

provided in this public-release report. As both a substitute and as a means of demonstrating the 

NGEN3 code, this section does include several illustrative examples of both MACS-type 

charges. These results represent a unique examination of the MACS charge not achievable 

without the NGEN3 code and heretofore not available to the charge design community. 

4.2 Results for a Typical Modular Charge 

Figure 1a shows a schematic of the gun chamber filled with propellant. Because the 

configuration is axisymmetric, only one radial plane is displayed that extends from the chamber 
8

Figure 1. Schematics of gun charge configurations: (a) loose charge, 

(b) packaged charge in single module, and (c) packaged 

charge in two modules. Ignition source indicated by (*). 

centerline (y = 0) to the outer wall (sloped from 8.4 to 8.1 cm). The axial extent, from the breech 

to the projectile base, is 106 cm (the gun tube extends beyond this point for an additional 

594 cm). The projectile mass is 44 kg and resists movement for the first 5 cm of travel. The 

granular propellant is cylindrical. The porosity of the propellant bed is 0.63. 

Figures 1b and 1c show alternate arrangements in which the granular propellant is packaged in 

either one or two cylindrical modules. These modules are assumed to be aligned along the 

centerline of the chamber and to consist of a main charge section (porosity of 0.63) and a center 

core that is filled with ball propellant with properties that are distinct from the granular 

propellant (porosity of 0.4). The module is sealed with walls that are rigid but permeable, 

admitting flow (or “leaking”) only after a significant gas pressure differential has been reached. 

Subsequently, these walls are allowed to break or “burst” after a fixed time interval has elapsed 

since over pressurizing. A rigid body analysis is enforced up to the instant of container burst. 

These walls can resist substantial pressure loading except for the radial wall separating the main 

charge from the center core (i.e., an internal wall) and the axial wall that represents a module cap 

that is used to fill the modules. This cap is located at the module end closest to the chamber 

breech. However, breakage of the internal wall or the module cap does not relax the rigid body 
9

treatment of the remaining module components. In each case, the charge is ignited at the breech 

end, near the chamber centerline over 2 cm of length, 3 cm in radius, and during the first 6 ms. 

Figure 2 summarizes the computed results for the packaged charge of two modules (figure 1c). 

The breech-end igniter starts an ignition wave or flame that quickly opens and ignites the first 

pressure waves, traveling though the radial ullage and into the region near the projectile base, 

over-pressurizing and bursting the second center core at its far axial end (figure 2e). As a result, 

main charge flame spreading in the second module begins nearest to the projectile base 

(figure 2h). By 11 ms, the main charge of the first module is fully ignited, causing increased 

pressure that bursts the end cap, causing high-velocity flow against the breech face (figure 2g) 

and a sudden forward movement of the first module. This movement increases the pressure 

against the end cap of the second module and causes it to burst (figure 2i). These events cause 

the second module to move forward and impact the projectile base (figure 2j). 

The pressure history of the events described in figure 2 is shown in figure 3a. For this case of 

two modules, the breech pressure is slowly increasing and is nearly coincident with projectile 

base pressure (zero pressure differential) until ~10 ms. Subsequently, the base pressure increases 

rapidly (negative 1.5-MPa pressure differential) in response to pressure waves caused by the 

initial disintegration of the first module and the igniting of the second. Between 11 and 18 ms, a 

series of three pressure waves (repeating positive, +1 MPa, and negative, –1 MPa, differentials) 

is caused by the movement and breakup of the modules. Another negative pressure differential 

(–2.5 MPa) occurs at 21 ms due to the impact of the second module upon the projectile base. 

Negative pressure differentials (i.e., pressure at the projectile base is larger than that at the 

chamber breech) are potentially dangerous to the structure of the gun chamber and the projectile 

payload. Such waves are generally absent from unconfined (i.e., nonpackaged) propellant 

charges. After this time, the now-loose propellant is free to move within the chamber, and these 

waves subside (note positive differential after 22 ms in figure 3a). Even though the case of a 

single module would seem to be less complex, the pressure history for this case (figure 3b) 

shows a larger negative pressure differential that is fortunately short lived. Detailed results from 

the NGEN3 code (omitted in this report due to space limitations) show similar events as reported 

in figure 2. 

4.3 Description of HLD Charges and ETC Technology for the FCS 

The FCS requires weapons systems exhibiting increased range, accuracy, and highly repeatable 

projectile launch performance. Several technologies are under investigation in order to achieve 

these goals. HLD charges consist of SP cast in several media types (e.g., grains, sticks, and 

disks) combined into a single charge. These combinations along with small-scale gaps between 

SP media and large-scale ullage around the projectile after body create special concerns for 

efficient ignition and flame spreading in the HLD charge. A promising ignition technology for 

HLD charges is the ETC concept. In the ETC concept, energy, which is stored either in batteries 

or in a rotating device, is converted on demand into an electrically generated plasma (resulting 

from the ablation of polyethylene material in a capillary) that is injected into the chamber of a 

gun. This plasma energy is used to ignite the chemical charge as well as to enhance gun 
10

Figure 2. Computed results for packaged charge (two modules): (a, b) time = 1-ms, (c, d) time = 3-ms, (e, f) time 

= 8-ms, and (g, h) time = 11-ms gas velocity vectors along with module boundaries (a, c, e, and g) and 

propellant temperature contours (b, d, f, and h) from 294 K (white) to 440 K (black). Computed results 

for packaged charge (2 modules): (i) time = 13-ms and (j) time = 21-ms gas velocity vectors along with 

module boundaries. 
11

Figure 3. Computed results for (a) packaged charge (two modules): projectile velocity, breech pressure, and 

pressure difference (breech pressure minus pressure at chamber end) vs. time and (b) computed results 

for one module. 

performance by taking advantage of a number of unique plasma characteristics. For example, a 

low-density plasma jet can efficiently ignite charges of HLD, can control propellant mass 

generation rates, can reduce propellant charge temperature sensitivity (i.e., the variation of gun 

performance with changing ambient temperature), and can shorten ignition delay (i.e., the time 

interval between firing of the igniter and ignition of the propellant). Plasma igniters also 

eliminate the conventional chemical igniter and can thus enhance the safety aspects of the overall 

gun propulsion system. All of these observations have a significant effect on the ballistics of 

ETC gun systems and can lead to a useful improvement in gun performance. 

Unfortunately, graphical results of the specific FCS simulations cannot be provided in this 

public-release report. As both a substitute and as a means of demonstrating the NGEN3 code, 

this section does include an illustrative example of HLD charges. 

4.4 Results for a Typical Direct-Fire Gun Chamber 

Figure 4 shows the computed porosity contours (white to black: open space to nearly solid 

material) and propellant temperature contours (white to black: ambient to 440 K) for a 120-mm 

HLD charge consisting of separate regions of disk and granular propellant. The propellant disks 

are stacked axially in the chamber; each disk has an inner radius that provides space for the 

igniter and the projectile after body and outer radius that is smaller than the radius of the 

chamber (shown from centerline to chamber wall). Figure 4a shows the initial condition of the 

charge (i.e., before the igniter is activated). Figures 4b and 4c show the condition after 6 ms 

(note that the projectile has moved into the gun tube and out of view). It can be noted that the 

granular propellant has been consumed and that the stack of disks has been pushed forward but is 

not fully ignited (figure 4c). This compressed region of disks (from 50 to 55 cm in figure 4b) 

lacks the interstitial gaps that have been closed, preventing convective heat transfer. When this 

simulation is repeated using an ETC igniter, plasma convection is accomplished between all 
12

Figure 4. Computed results for 120-mm HLD charge (SP disks 

and grains): (a) porosity contours (black is dense 

material) at initial time, (b) porosity contours at 6 ms, 

and (c) propellant temperature contours (black is 

ignited propellant at 440 K) at 6 ms. 

disks before significant disk compression, and as a result the entire stack of disk propellant is 

efficiently ignited (3). Thus, the low molecular weight plasma, generated by an ETC igniter, 

could circumvent this problem by establishing a convectively driven flame that propagates faster 

than the material compression wave in the disk propellant, thereby permitting an even ignition of 

the charge. A basic design tenet for using HLD charges was established, namely pressure waves 

in the chamber generated by the conventional igniter, when paired with the disk propellant 

charge, were avoided by using an ETC igniter. This conclusion, heretofore undocumented, is of 

critical importance to the design process for the FCS weapon. 

5. Summary 

In direct response to the immediate need for a reliable computer modeling tool to assist in the 

design and optimization of armament propulsion systems for both the legacy force and the FCS, 

ARL has developed a next generation computer-scaleable 3-D multiphase CFD code. ARL’s 

NGEN3 code represents the sole DOD computer tool that is able to simulate the highly complex 
13

physics associated with indirect and direct fire guns. The NGEN3 code, which incorporates 

general continuum equations along with auxiliary relations into a modular code structure, is 

transportable between computer architectures and is applicable to a wide variety of gun 

propulsion systems. Two such systems are the U.S. Army’s MACS and the FCS. NGEN3 code 

development and application to the MACS and the FCS is currently a DOD HPC Challenge 

Project. Significant progress has been made in optimizing both these systems—progress that 

was heretofore not attainable without a computer model and progress that has been greatly 

advanced by priority access to the DOD MSRC. 
14

6. References 

1. Nusca, M. J.; Gough, P. S. Numerical Model of Multiphase Flows Applied to Solid 

Propellant Combustion in Gun Systems. Proceedings of the 34th AIAA Joint Propulsion 

Conference, 13–15 July 1998; AIAA 98-3695. 

2. Nusca, M. Computational Fluid Dynamics Model of Multiphase Flows Applied to Solid 

Propellant Combustion in Guns. Proceedings of the 18th International Symposium on 

Ballistics, San Antonio, TX, 1999, pp 252–261. 

3. Nusca, M.; Conroy, P. Multiphase CFD Simulations of Solid Propellant Combustion in Gun 

Systems, Proceedings of the 2001 Users Group Meeting, sponsored by the DOD High 

Performance Computing Modernization Office, Biloxi, MS, 18–21 June 2001; see also 

Nusca, M.; Conroy, P. Progress in the Modeling of High-Loading Density Direct-Fire 

Charges Using the NGEN Multiphase (CFD Code); ARL-TR-2934; U.S. Army Research 

Laboratory: Aberdeen Proving Ground, MD, 2003. 

4. Gough, P. S. Modeling Two-Phase Flow in Guns, Interior Ballistics of Guns; Krier, H., Ed.; 

American Institute of Aeronautics and Astronautics: New York, NY, 1979. 

5. Gough, P. S.; Zwarts, F. J. Modeling Heterogeneous Two-Phase Reacting Flow. AIAA 

Journal, 1979, 17 (1), 17–25. 

6. Nusca, M. J. Investigation of Ignition and Flame Spreading Dynamics in the Modular 

Artillery Change System Using the NGEN Multiphase CFD Code. Proceedings of the 36th 

JANNAF Combustion Subcommittee Meeting; CPIA Publication 1999, 691 (3), 137–154. 

7. Nusca, M. J. Numerical Modeling of the Modular Artillery Change System Using the 

NGEN Multiphase CFD Code-Effects of Case Combustion. Proceedings of the 37th 

JANNAF Combustion Subcommittee Meeting; CPIA Publication 2000, 701 (2), 87–101. 
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