Study on the Multi-Component Test Technology of Solid Rocket Motors

ZHU Ziwen,QU Youyang,HAN Chenglin

Xi’an Aerospace Power Research Institute,Xi’an 710025

Abstract:This paper presents a calculation method,and composition of a five-component force test device,to resolve the requirement for solid rocket motor multi-component force test.It compares the difference between two classical algorithms,and summarizes the structure and practical usage of the solid rocket motor multi-component force using soft connection structure measurement.The decoupling method of test data is described,and a reasonable research method for solid rocket motor multi-component force is also given in this paper.

Key words:solid rocket motor,multi-component force,vertical test

1 INTRODUCTION

At present,the axial direction component of thrust is generally measured in the test of a solid rocket motor,while other components are ignored.So,the result of thrust measurement is only the main thrust component of thrust vector,not the net thrust vector.It is necessary to measure the multi-component force of a solid rocket motor in order to have accurate thrust control.The axial main thrust and slight lateral force of solid rocket motor can be measured at the same time,which means a multi-component force measurement.We can obtain the net real thrust vector through multi-component force measurement.

Through research on the general technologies such as the design of a five-component force test device for a solid rocket motor,the decoupling algorithm of a five-component force test,the calibration method of a test system,this paper proposes a new multi-component force measurement method,which can improve the accuracy of test data and meet the requirements of future solid rocket motor research and key technology research and testing.

2 RESEARCH DIRECTION SELECTION

At present,the multi-component force measurement of the solid rocket motor is mainly divided into two types,one is horizontal and the other is vertical.Because the horizontal test has become more mature,most multi-component force measurements of the solid rocket motors are horizontal.However,in the horizontal multi-component force test,the measurement of lateral force will be affected by many factors such as the superposition of gravity,the change of mass,and the displacement of mass center,which makes it inaccurate.At the same time,as the magnitude affected by these factors is close to the magnitude of lateral force,it is difficult to separate them from the lateral force,hence impacting the measurement accuracy.

The United States started earlier in the field of solid rocket motor multi-component force measurement,and now its technology has become mature.As early as in the 1970s,the six-component force test of a solid rocket motor was recorded in a NASA report.

In order to improve the measurement accuracy as much as possible,the vertical multi-component force measurement method is adopted in this paper.Compared with the horizontal measurement method,the vertical measurement takes into account the effect of gravity on the main thrust.However,due to the fact the main thrust is far greater than the lateral force,the effect of gravity will not have a significant impact on the measurement results.In addition,there are few interference factors in the measurement of lateral force through the vertical dimension.At present,the lateral sensor will measure the torque generated by the hard connection during the measurement process in other vertical multi-component force measurements,which is difficult to separate from the measurement results.Moreover,the axial extension and radial expansion of the solid rocket motor will still affect the output of the sensor through the traditional vertical measurement.In this paper,a new measurement method is proposed based on the vertical measurement,in order to solve these issues through the improvement of test devices.

3 FIVE-COMPONENT FORCE MEASUREMENT METHOD

3.1 Theoretical Hypothesis

In order to simplify the measurement model and avoid the coupling of sensor output force and torque,this paper uses the rigid equilibrium method,appropriately arranging several constraints.It limits the five degrees of freedom of the engine (three degrees of freedom of movement and two degrees of freedom of rotation),thus establishing a static equilibrium state,based on the following assumptions:

1) The solid rocket motor is an axisymmetric rotating body;

2) The main thrust and lateral force of the solid rocket motor always pass through its axis,that is,the rolling torque of the engine is 0;

3) The barycenter of the solid rocket motor is always on the central axis;

4) The transverse thrust of the solid rocket motor is less than the main thrust.

3.2 Mechanical Model

At present,there are two main measurement methods of multi-component force.One is assuming that the measurement model is an ideal hinge,and the other is using single force measurement planes to solve the other force measurement plane.The following is a comparison between the two methods.

As shown in Figure 1,it is considered that the center point of the flexible part in the solid rocket motor force measuring component is the origin

O

,and the mechanical model can be simplified as the ideal hinge with point

O

as the fulcrum.It is specified that the pressure output of the sensor is positive and the tension output of the sensor is negative.The positive direction of the moment is specified by the right-hand screw rule.So,establishing a rectangular coordinate system in the

O

-

XYZ

plane.We take the intersection of the

F

,

F

theoretical axes as

O

,the intersection of the

F

,

F

theoretical axes as

O

,the theoretical axis of

F

is

X

axis,the theoretical axis of

F

is

Z

axis,and the theoretical axis of

F

is

Y

axis,the direction of installed sensor is in the positive direction of each coordinate axis.

Figure 1 Force analysis diagram of motor

According to method I:

In Equations (1) and (2),

L

is the distance between

O

and

O

;

L

is the distance between

O

and

O

;

G

is the gravity of the solid rocket motor itself;

F

-

F

are the force measured by the sensor;

F

,

F

,

F

are the projection of resultant force on each coordinate system;

M

,

M,

M

are the projection of the resultant moment on each coordinate system.

According to scheme 2:

L

is the distance between the upper and lower lateral force component planes,that is the distance between

O

and

O

;

L

is the distance between the solid rocket motor mass center and the origin

O

of the coordinate system.

A hypothetical model is used in order to compare the difference between method 1 and method 2.

It is considered that the the exit plane of nozzle is the point where the lateral force is generated.Assuming the lateral force in

X

direction is 10000 N,then method 1 can simplify the test model to the theoretical model in Figure 2.

Figure 2 Hypothesis model of scenario 1

It is considered that the the exit plane of nozzle is the point where the lateral force is generated.Assuming the lateral force in

X

direction is 10000 N,then method 2 can simplify the test model to the theoretical model in Figure 3 .

Figure 3 Hypothesis model of scenario 2

Through the above calculation,it can be seen that the two classical solutions are consistent in the five-component force test,which can be used as the calculation model to measure the multi-component force of the solid rocket motor.

4 FIVE-COMPONENT FORCE MEASURING DEVICE AND ITS COMPOSITION

The test device used in this paper was an integral,vertical multi-component force measurement test device which is composed of a fixed frame,a moving frame,a force measuring component and a safety limit device,as shown in Figure 4.The test device is connected with the solid rocket motor through a squirrel cage,which is connected with the vertical test device to complete the thrust transmission.The lower end is connected by a flange with all-direction flexible parts to ensure the necessary degree of freedom in all directions at the fulcrum.The thrust calibration is completed by scene calibration,which only calibrates sensor accuracy.

The thrust sensors are arranged on two measure planes by means of a soft link.The sensor layout of multi-component force is shown in Figure 5.Four sensors are arranged on one measure plane.So,the five-component force measuring device needs to be arranged with nine groups of sensors.However,due to the use of soft links,all sensors only read tension output without pressure output.The thrust direction is judged through the output values of two sensors in one direction.The issue that the lateral force of the traditional muti-component force test device absorbed by the flexible device is well solved,and the real lateral force can be obtained.

Figure 4 Schematic diagram of multi-component force

Figure 5 Sensor layout of multi-component force

The device connected with the solid rocket motor is a squirrel cage structure.The squirrel cage is a moving frame of the device,which is connected with the solid rocket motor through a skirt and is not in direct contact with the solid rocket motor at other positions.The squirrel cage resolves the problem which influences the solid rocket motor axial elongation and radial expansion on the test (the squirrel cage will not produce displacement because of solid rocket motor axial elongation and radial expansion),thus reduces the interference factors affecting the measurement of lateral force.

The lateral adjusting device is connected with the sensor and installed on the lateral fixing device of the fixed frame to complete the lateral fixing.By adjusting the lateral adjusting device,the solid rocket motor axis can be located in the center of the test device.During the whole process of calibration and force application,the soft connection always moves along its axis without rotation and friction.

The fixed frame structure consists of a base and a vertical frame.The main function of the base is to adjust a horizontal plane on which all other structures are established.The base frame can bear the main thrust of the engine.The vertical frame is used to bear the lateral force.It is connected with the base through locating pins and bolts,and all of them referenced together to ensure all benchmarks.

5 FIVE-COMPONENT FORCE VERIFICATION DEVICE AND VERIFICATION STEPS

In order to verify the force measurement accuracy and rationality of the test device,a five-component force verification device was designed for the test device.It was used to simulate the main thrust,space 45° angle and plane 45° angle thrust.A standard force was applied through a standard force source,recording the output value of each sensor to calculate a simulated force value.We could compare the simulated force value with the standard force to verify the rationality of the test device.

A cross beam made of square steel was used to simulate the main thrust and space 45° angular force.The axis alignment was completed by changing the position of the adjusting seat.Then the rationality of the mechanical model was verified by calibration.The schematic diagram of the calibration device is shown in Figure 6.

Figure 6 Schematic diagram of the calibration device

The simulation of the plane 45° was realized by installing a standard force application device at 45° of the higher force measuring plane.

Schematic diagram of installation and alignment structure of multi-component force test are shown in Figure 7.The installation alignment method and steps are as follows.

Figure 7 Schematic diagram of installation and alignment structure of multi-component force test

1) First installation of the base frame structure,adjustment of the mounting bolts to make the mounting plane of the main thrust sensor a horizontal plane,and then securing with bolts;

2) Installation of the main thrust alignment device;

3) Positioning the squirrel cage structure,and manually guiding it over the main thrust alignment device.Locking the safety limit device,removing the main thrust alignment device,and installing the main thrust force measuring device;

4) Using the lateral force alignment device to complete the alignment of the lateral structure,then removing the lateral force alignment device and installing the lateral force measuring device;

5) Installing the simulator;

6) Completing the alignment of the standard force application device with the axis of the test stand and simulation parts through changing the position of the adjusting seat,and then apply the standard force in the direction of main thrust,space 45° and plane 45° for verification.

6 DATA DECOUPLING

Under the test conditions,the loading steps of calibration test should be conducted independently.The purpose of static calibration of the multi-component force test system is to find the linear quantitative relationship between the output of the nine sensors and thrust,which can be expressed in matrix form as follow:

Assuming that

F

is the standard force which applied to the

j

direction,

x

only represents the sensor output in the

i

direction,and

k

represents the effect of the

j

direction force on the sensor output in the

i

direction.Matrix

K

is the calibration matrix,and

b

is the fixed value system error of the five-component force test system.

It is easy to establish that Equation (6) belongs to univariate linear regression.The working straight line of the five component force system can be obtained by linear fitting method.

where

B

represents the application of the calibration force in the

j

direction.The estimated value

B

obtained by the least square method,and the final estimated value of

B

can be obtained by averaging

B

:

Thus,the linear regression equation is:

According to Equations (6) (7) (8) (9),the measured value of the five-component force can be obtained by decoupling of the sensor output in each of the directions collected.The decoupling equation is:

It was proposed to use a graphical programming language to develop a general test platform based on the Windows operating system which can complete data decoupling.The specific programming and implementation mode was a major difficulty of the test,which needs further research and discussion.

7 CONCLUSION

Assuming that the main thrust and lateral force always pass through the solid rocket motor axis,the multi-component force measurement model of a solid rocket motor can be simplified,and the components in five directions can be easily separated.

The established model can reduce the influence on the multi-component force measurement,such as torque caused by engine axial elongation and rigid connection.At the same time the accuracy of the whole model can be verified in a simple way.

Thus the proposed decoupling method can obtain the measured value of five-component force by mathematical modeling.The decoupling can be rapidly achieved by computer processing,allowing later practical application.