missile simulation, rail launch, using nastran

8/9/2019 Missile Simulation, Rail Launch, Using NASTRAN

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Rail Launch Missile Simulationusing MSC.Nastran Software

Author: Peter Zeman

Paper: 2001-34

COMPANY:

MBDA Inc.5701 Lindero Canyon Road,Suite 4-100

Westlake Village,CA 91362

USA

Phone: (818) 991-0300 (x 248)

Fax: (818) [email protected]

ABSTRACT:

The purpose of this paper is to present a dynamic simulation of a rail launchedmissile as it is fired from its rail. Missile loading conditions during a launch

change rapidly based on the missiles position on that rail. The missile's timespent on the rail is a fraction of a second and the simulation must take intoaccount the missile's dynamic characteristics. Rail stiffness is also position

dependent and the gaps and friction between the missile hangers and rail requiremodeling.

The nonlinear finite element transient analysis is capable of simulating themissile's dynamic conditions during its travel along the rail. The outcome of the

analysis has multiple usage. The missile's movement on the rail, in real time, isobservable and the missile's velocity, acceleration, roll angle and roll rate can be

plotted. For structural purposes, the missile's component stresses can be plottedand also valuable missile shoe reaction forces can be determined at different

locations on the rail.

The sample problem described in this paper does not represent any specific

missile design. However, the presented analysis technique was successfullyutilized and verified in the Brimstone missile project at Alenia Marconi Systems.


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INTRODUCTION:

Rail launched missiles can be divided into three basic categories:

a) Ground launched missiles (no aerodynamic load during the launch)

b) Helicopter launched missiles (aerodynamic load is low during the launch)c) Fast Jet launched missiles (aerodynamic and inertia load can be

very high during the launch)

It is usually a high aerodynamic and inertia load acting in the lateral axis, which

introduces high reaction forces between the missile shoes (hangers) and thelaunch rail. These high reaction forces can cause structural damage to the

missile components or the rail. They can also prevent a successful missilelaunch by slowing or even stopping the missile on the rail and thus causing amissile hang fire.


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MISSILE/RAIL SKETCH:

MISSILE SHOE REACTIONS:

Rail, Cross - section

Missile Shoe

Missile

Lateral Load

F1

Reaction, -F1

Reaction, -R1

Reaction, +R1

A relatively small lateral load

(F1) generates high shoe

reaction forces (-R1 and +R1).


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PROBLEM DEFINITION:

During a successful launch:

The missile rapidly accelerates on the rail.

The missile usually does not remain on the rail longer than 150

milliseconds. The missile shoe reaction forces change rapidly during the launch based

on the missiles rail position. The main contributors to these changes arevarying aerodynamic missile loads and rail stiffness for various positions

along the rail.

Other considerations requiring modeling:

The missiles shoes fit loosely in the rail (some free play exists).

Friction exists between the shoes and the rail.

The analysis simulates:

a) Dynamic conditions on the rail (Nonlinear, transient analysis).b) Friction and gaps between shoes and rail (Slide lines with friction option).

c) Calculation of missile load for each position on the rail (Function).d) Calculation of rocket motor thrust as a time function (Function).

ANALYSIS OBJECTIVES:

a) Determination of shoe reaction forces during the launch.

b) Determination of missile acceleration, velocity and roll rate on the rail.c) Determination of missile tip off roll rated) Obtaining missile and rail stress plots for various positions on the rail.

FINITE ELEMENT MODEL DESCRIPTION:

Major Features:

a) The model has to be dynamically representative. The missile/rail mass and

stiffness must be correctly simulated.b) Missiles shoe/rail interface has to be representative. Slide line elementsmust be placed in the shoe/rail contact areas. Friction between the missile

shoes and rail has to be considered.c) The model size should be moderate (sample problem includes approximately

8 000 dof). The nonlinear transient analysis is an iterative analysis, whichrequires numerous time steps. Large models require long run times!


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Modeling Hints:

a) A convenient combination of element types was selected to obtain a highlyrepresentative model, which was still of moderate size.

b) Slide line elements have to be defined in X -Y plane of assigned coordination

system which is defined in the slide line element property. While the defaultcoordinate system was proper for vertical slide line elements, a new

coordinate system, which was rotated by 90 degrees, was assigned tohorizontal slide line element property.

c) Slide line elements are gradually added to the model. The model is run andcorrect functions of the added slide line element are then verified.

d) Slide line master nodes are located on the rail. There is just one slave node

for each slide line element, which is associated with the missile shoe.e) Slide line elements do not have gap force measuring capability, hence the

shoe load can not be determined from the slide line element. This deficiencyis overcome by adding two extra elements for each slide line element. In the

nonlinear transient analysis the output can be written for DOF Spring Elementforce, hence the shoe reaction force can be determined if this element isincorporated in the model. The extra 5 DOF are added by rigid element(s) as

shown on the picture below.

f) The aerodynamic load acting on the missile changes with the missile position

on the rail. The load change is simulated by two independent MSC.Nastranfunctions. Function No 2 is time dependent, and function No 3 is associated

with a Nonlinear Force load, which is position dependent. The nonlinear,position dependent force is superimposed on the time dependent force.

g) The analysis is gradually run with various coefficient of frictions, typically 0.0,

0.15 and 0.25. All these output sets are stored in the same database. Thisenables the plotting of shoe reaction forces, missile acceleration and velocity

on the same plot for various coefficient of frictions.

SLIDE LINE SLAVE NODE

RIGID ELEMENT MASTER NODE

DOF SPRING ELEMENT (1 DOF) and

RIGID ELEMENT (REMAINING 5 DOF)

SHOE ELEMENTS (STIFF BEAMS)


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Missile Finite Element Model:

Rail finite element model:

Rail Finite Element Model

Front Shoe Detail

FWD

Rail Top View

Front Shoe Intermediate Shoe Aft Shoe

Rail Bottom View

Constraints location


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Missile/Rail Finite Element Model:

Constraints

Shoe Detail

DOF SPRING ELEMENT ANDRIGID ELEMENT 5 DOF

DOF SPRING ELEMENT:

The DOF Spring Element isinserted between the missile

shoe beam element and the SlideLine Element. The purpose of

this DOF Spring Element is tomeasure the reaction forcebetween the missile shoe and the

rail. The DOF Spring Element isrelatively stiff and does notreduce the actual stiffness

between the shoe and the rail.


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Slide Lines:

There are nine slide line elements in this model. Each shoe contact with the rail

is a separate slide line element.

Slide Line Parameters (Specified in Slide Line Element Property):

a) Stiffness Factor = 1.0

b) Width = 5.0c) Coefficient of Friction = 0.0, 0.15 and 0.25d) Non sliding Friction Stiffness = 1000

e) Unsymmetrical Penetrationf) One slave node for each slide element

g) Slide line coordinating system definition (X-Y plane, correct orientation).

ANALYSIS:

Software: MSC.Nastran for Windows, version 4.6.Nonlinear Transient Analysis Parameters:

a) Convergence Tolerance: Displacement = 0.01, Load = 0.01, Work = 1e-5

b) Solution: Full Newton - Raphsonc) Time increment: 0.0005 sec, 650 steps, Max. iterations = 25d) Output: ALL, Every Step,

e) Overall Damping: 0.08 (Critical Damping = 0.04) (F= 10 Hz)f) Analysis: Nonlinear Transient , Advanced options,

Output requested for DOF Spring Force

(9 elements), Displacement for all model,Acceleration and Velocity for Node 979.

Front and Intermediate Shoe Slide Line Elements

LH Side, (Property 11)

Front and Intermediate Shoe Slide Line Elements

RH Side, (Property 9)


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Load acting on the missile:

ROCKET MOTOR THRUST

(associated with Function 1)

Aerodynamic and Inertia Load


LOAD ACTING ON THE MISSILE

on linear Force


The missile load isgradually increased for the

first 50 ms. Afterwards, thiscomponent of the missile

load is steady, in this

sample case.

The rocket motor thruststarts at 100 ms. At this

time the missile is loadedand it can be considered

steady. The thrustprofile is simplified in this

sample case, but reflectsthe general rocket motor

thrust shape.

Function 2, Aero

and Inertia Load

Function 1, Rocket

Motor Thrus t

Function 3, Nonlinear

Force

The nonlinear force is themissile load component,

which changes with the

missile position on the rail.


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ANALYSIS RESULTS:

Missile movement on the rail, Coefficient of Friction = 0.15

Comments:

During the first 100 ms, the load is gradually applied to the missile(load function 2). After 100 ms the rocket motor thrust starts to move the missile

on the rail (load function 1). The missile starts to move on the rail as shown

above. During successful launch the missile accelerates rapidly.

CAPTIVE CARRY POSITION

FRONT SHOE LEAVING THE RAIL

INTERMEDIATE and AFT SHOES LEAVING THE RAIL

MISSILE FREE FLIGHT


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Missile Acceleration on the rail, Coefficient of Friction = 0.0, 0.15 and 0.25

Missile Velocity on the Rail, Coefficient of Friction = 0.0, 0.15 and 0.25

Time [sec]

ROCKET

MOTORFIRES

Missile Acceleration [mm/sec^2]

=0.0

=0.15

=0.25

Time [sec]

= 0.25

= 0.15

= 0.0

Missile Velocity [mm/sec]

Missile velocity at

the end of the rail


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Missile Roll Rate, Coefficient of Friction = 0.15

Missile/Rail Stress Plot Cross-section:

Time [sec]

Roll Rate [Rad/sec]

Rocket Motor

Fires

Missile

Leaves

The Rail

Missile

Tip-off

Roll Rate

Rail Stress - Detail


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Missile Shoe Reaction Forces:

Deformed Shape, End of the Rail

DEFORMED SHAPE

DEFORMED SCALE 1:1

In this image, the missile isleaving the rail. The

intermediate and aft shoes arestill engaged in the rail.

The rail is noticeably deformed.The missile roll and yaw anglesare also evident.

Rocket MotorFires

Time [sec]

Shoe Reactions [N]

Front Shoe Reactions

Intermediate Shoe Reactions

Aft Shoe Reactions

=0.0 =0.15

Shoe reactions change rapidly with the missile position on the rail. Aft shoe

reaction achieves its maximum magnitude just before the missile leaves the rail.


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DISCUSSION:

Shoe reactions are the most valuable outcome of the analysis. With this rail

design the aft shoe reactions achieve a very high magnitude at the end of the rail

position. This is caused by relatively small torsional rail end stiffness incomparison with mid section rail stiffness.

CONCLUSION:

MSC.Nastran nonlinear transient analysis enables the simulation of dynamicphenomena like a missile launch from a rail under extreme load conditions and

with high coefficients of friction. Valuable shoe reaction forces can also bedetermined with this simulation. Alternatively, measurement of reaction forces is

difficult to obtain during a missile launch and conventional analysis provides onlyapproximate results. Missile acceleration on the rail obtained through

MSC.Nastran analysis has been verified by measured acceleration obtainedduring trials.

The MSC.Nastran analysis methodology is also capable of determining frictionalforces between the rail and the missiles shoes. Frictional forces contribute toacceleration anomalies and rocking that occurs as the missile progresses along

the rail.

BENEFITS TO MSC.SOFTWARE USERS:

a) Slide lines can be used even if there is some sliding in the plane

perpendicular to the slide line plane. The slide line has a width.b) Slide lines are very stable and analysis converges quickly.

c) If friction is defined in slide line element property, the analysis convergesmore slowly, but still converges.

d) Functions used in MSC.Nastran software are very useful. While the rocket

motor thrust was described by the simple time function, the missileaerodynamic load was calculated for various positions on the rail. This is an

extremely useful feature of the MSC.Nastran interface modeler. This featurecan be utilized for aerodynamic load calculations, which changes withgeometrical parameters during analysis, such as the missile position on the

rail.e) Contact forces cannot be obtained directly during the nonlinear transient

analysis. Slide lines do not provide this option yet. However, a combinationof DOF spring and rigid elements, properly added to the slide line element (asdescribed in the body of this article) enables the user to obtain these forces,

as was demonstrated in this paper.

missile simulation, rail launch, using nastran

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