Study on structure of snap ring in solid rocket motor①

SUN Li－gang，ZHANG Duo，GUO Yun－qiang

(1.Coll.of Astronautics，Northwestern Polytechnical Univ.，Xi′an 710072，China;2.The 41st Institute of the Fourth Academy of CASC，Xi′an 710025，China)

0 Introduction

In the type of joint structure of solid rocket motor，there are two major types of joint at present;they are flange bolting and big threading.

The flange bolting is very simple and credibility，but it has the disadvantage of large negative mass and large structure dimension，so it can not be used in the specially design of some qualification for its space and structure mass limitation［1］.

For the big threading，the high machining accuracy is required，and in order to avoid disassembling difficulty which produced by the screw thread damage，the screw thread must be protected during the process of trial assembly and hydraulic test，because it has been locking together many times during the dismantling process in the past research and this phenomena leads to an entire product scrap.Moreover，the biggest problem is that thread connecting structure can′t meet the requirements of the fixed position in circumference direction［2－4］.

Compared with flange bolting and big threading，the snap ring has many advantages at some special occasion.Firstly，it has good layout because the small size of the joint structure occupy smaller space and has small mass.Secondly，snap ring is very safe and reliable and easy to dismount.Thirdly，the snap ring can fix the connections at a circumference position precisely.It is obvious that the snap ring is a joint structure，which has wonderful development potential，especially in the missile motor field.With the development of technology，solid rocket motor requires a more compact and more light structure.

Sometimes，flange joint and big threading can not meet the demand of solid rocket motor design.As a new joint structure，the snap ring has many advantages;it can reduce the structure space largely，decrease the negative mass.It has been believed that application of the snap ring structure in the SRM design must lead to a new technique revolution.

The snap ring joint structure has been studied since the early 1980s and it has been applied in the field of Solid Rocket Motor since 1980，such as TRIDENTⅡand MX and so on.

In 1990，we began to investigate the snap ring structure.It has been used successfully in the missile field，for example，the connection of module and disengaging device.In the field of solid rocket motor，although the Fourth Academy of China Aerospace Science and Technology Corporation(CASC)have done some research，it has not been used in the tactical rocket motor field so far.

In this paper，snap ring structure was used in engineering.The structure of snap ring has been designed for a tactical rocket motor.In addition，the FEM structure analysis was done to optimize its layout.The snap ring joint finally was verified by hydraulic pressure test.

1 FEM theory computations and test research

1.1 Snap ring structure

The present structure consists of several parts:connector 1 and connector 2，short snap ring，seal snap ring，compensating block，fixed bolt and cover plate.Every short snap ring and compensating block have the screw thread hole which make it convenient for the disassembly and fix position.The whole snap ring structure is illustrated in Fig.1.

The installation sequence of the snap ring structure is as follows:the sealing ring is put into the seal slot first，make the connector 1 and connector 2 assembly fit well，and then put the short snap ring into the connector 2 one by one from the inlet.After finishing the short snap ring assemble，adjust the position of short snap ring.Finally，assemble the seal snap ring.After installation，assemble the compensating block and cover board.

After hydraulic checkout or trial assembly，the disassembly method is the procedure just like this.Firstly，disassemble the cover board，take out the compensating block through the screw hole on the compensating block，and take down two seal snap ring.Then put the compensating block renewably，fix it on the connector by the bolt，and rotate the connector 2，then take out the short snap ring one by one.After disassemble the short snap ring，removing the compensating block and unhinging the connector 1 and connector 2 directly.Only to this step，the whole disassembly process is finished.

Most snap rings used in the past are just one piece of long snap ring，it needs high machining precision，and the technique is relatively complex.Furthermore，it is not easy to install and take down.All these are the serious limitation for the snap ring structure engineering application.For these reasons，at the present，the short snap ring can improve the processing.

1.2 FEM analysis model

According to the characteristics of the snap ring joint structure，the computation is developed by taking the configuration dimensions as the optimize variable and the stress level on the snap ring position as goal function.Through the calculation，the follow technique problems can be solved:

(1)The deformation of the snap ring under internal pressure.

(2)The stress concentration on the snap slot under internal pressure.

(3)The fit clearance on the snap ring.

In this section，we simplified the design of snap case and did not consider the effect of fix position bolt on the structure.The case geometry shape，constraint and the load are symmetry for the symmetry axis，so are the displacement，strain and stress.As a result，the two dimensional axisymmetric model was adopted to calculate the deformation of the snap ring case under internal pressure.The FEM calculation was done by ABAQUS.In order to obtain the reasonable stress results，the mesh of the model was refined and Axisymmetric Solid 4 Node Quadrangle 10 Ring Element was used.The FEM model is shown in Fig.2.Uniform pressure p was applied on the inner surfaces.

Step type:static，and the inertia force is ignored.

Boundary condition:in order to simulate the hydraulic pressure experimental more actually，the computation model has been simplify properly without considering the effect of cover board and joint bolt on the loaded system.

Load condition:20 MPa internal test pressure is applied to the internal wall of the case.The friction coefficient between the snap ring and structure is 0.1.

Material property:the model material is 30CrMnSiA，which is assumed as the linear elastic material.The material parameters are shown in Table 1.

Table 1 Case material parameters

1.3 Test verification

The test was developed to verify the FEM calculation of the snap ring.The test pieces are shown in Fig.3.

1.3.1 Strain measure point

The snap ring was tested by the hydraulic pressure test after it had been sealed，the strain sensors(strain gauges)were placed on the exterior surface parallel to and perpendicular to generating line of case，the axial and circumferential strain gauge were placed at every point.The measure position is shown in Fig.4.

1.3.2 Displacement measure point

The case displacement measuring point is based on the position of hydraulic pressure test;the circumferential displacement which is over the middle of the case and the axial displacement of the two hydraulic pressure boss end plane are need to measured.The total is three routes of displacements.The bilateral axial displacement meter on the end plane must be located on the same busbar.The specific measuring positions are shown in Fig.4.

2 Calculation and test results

2.1 FEM calculation

The snap ring′s structure was analyzed by structural optimization method and the following figures indicate the load condition.The calculation results of stress and strain are showed in Fig.5 and Fig.6.

The stress and strain distribution indicate that the snap ring stress distribution is smooth under internal pressure condition after optimization of the system.The design safety factor meets the requirement of practical application.

2.2 Hydraulic pressure checkout

The procedure of hydraulic pressure check is 0 MPa→5 MPa→10 MPa→14 MPa→16 MPa→18 MPa→20 MPa(dwell time=1 min).

Table 2 Results comparison between experiment and FEM calculation

Table 3 Original data of measuring point displacement

The experimental results indicate that the FEM model and boundary condition are correct and the error is in the allowable range.

3 Conclusion

From the results of experiment and calculation，we know that the snap ring has saved connection space and its connection and sealing are reliable.It can be said the it is a new－type potential connection structure especially in the small joint.Furthermore，some useful details have been obtained as follows.

The snap ring had a certain plastic deformation under the internal pressure，so when we carry out a design ，the selected materials should be heat－treated in order to enhance the snap ring′s strength and toughness so that the plastic deformation of snap ring can be reduced to the allowable situation.

The chamfer under the bottom of snap ring slot has great influence on the stress level.The research results show that the size of chamfer should be about 1 mm.

Joint buckle affects the whole structure deformation largely;increasing the buckle number can enhance the supporting capacity of the case by 40%.

Snap ring should be designed into multiple short snap ring structure，thus it is easy to assemble on the base of reliable connection.

In order to ensure the load uniformity on the multiple short snap rings，the thickness of the snap ring should be controlled strictly.

The snap ring mainly endure the shear force.The loading and layout of the snap ring are reasonable relatively and it can not be reversed when the depth－width ratio is equal to about 0.5.

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