Design and Analysis of Fin Locking Mechanism for Missile Applications

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A Pyro based mechanism is used in flight vehicles for control of fins. Here, a Pyro pin puller based lock and release mechanism has been designed to lock (arrest the movement of) the control fins when the missile is under carriage condition and to unlock when the missile clears the nose tip of aircraft after launch. This is very critical mechanism as far as the launch of the missile is concerned. The dimensions of the individual components are derived from the classical design approach. The modeling of the components has been done by using solid works and the analysis is carried out using ANSYS.

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Currently in space and defence applications, pyro operated mechanism is widely used. These devises are used in separation mechanisms and flow control values. These are highly reliable systems and their usage improves the reliability of the overall system. In Defence applications one of the area where these pyros widely used in holding the control surfaces of air-launched weapons. One such system developed is Fin Locking Mechanism (FLM).

The above technique is also used for holding the fins in locked condition at the time of launch and unlocking them after attaining safe separation distance.

The configuring of design is carried out using CAD software (Solid Works). This is one of the widely used and user friendly software. Complete FLM system is modeled using this software and analysis is carried out using ANSYS software. The system reliability is verified by carrying out actual tests on ground. The details of design and its development and testing are discussed in the following chapters.

Purpose of the study

Controls surfaces are most commonly used in missile applications, these surfaces are most popularly known as FINS in the missile systems. These fins are realized by various methods in the past and lot of research is being carried out in this area since the beginning of missile systems. The control surfaces of missile system exiles aerodynamic loads, a FLM is used to arrest their movement. The only issue with the Fin locking mechanism lies in Pyro cartridge which is quite high in cost and standard Pyro cartridge are available in global market. The main reason for focusing on the Pyro cartridge is due to the sensitive nature of the circuit that affects total performance of the Fin locking system. Various studies are being carried out to improve the Pyro techniques as in the research carried out by Danali S.M.

In this context, the proposed work carried out in this project deals with a detailed study on the Fin locking mechanism and functioning of the system. At the same time this research allows to understand the design parameters of a mechanism that suits for the applications of missiles.

Aim and Objectives of the project

Aim: Design and analysis of Fin locking mechanism for missile application.

Objectives: To understand the various Pyro-techniques. Design aspects involved in Fin locking mechanism. Analyzing the proposed technique and obtaining the results from FEA. Summarizing the obtained results to draw conclusion from the propose technique.

Research Methodology

Initially the mechanism is configured for the available space in the weapon system. The sizing of the components and modeling of complete mechanism is carryout using CAD software and FEA is used for Analysis. Materials for various components in FLM were selected are listed in table 1.Qualified proto units are tested for evaluating the analytical values. The full qualification testing of the mechanism is carried out as per MIL-810F. After successful completion of ground tests, the FLM is tested in air-launch trials.

Scope and Organization of the Thesis

The outline of the thesis and organization of the thesis is given below: In the Chapter 1, the complete purpose of performing this research is presented along with defined aim and objectives. The statement of the problem is given with proper justification and importance of conducting this study. Later the approach of conducting this method was discussed in detail. In the Chapter 2, the literature review related with the Fin locking system and their background information has been collected for an easy understanding of the subject matters. A brief review on previous research carried out in the area of various Fin locking systems is elaborated. In the Chapter 3, a detailed design aspect, design specifications, mechanism configurations, assembly of system, working of Fin locking systems were is carried out. In the Chapter 4, focus was on the analysis involved in the Fin locking system and the experiment results are displayed in detail. In the Chapter 5, Assembly of FLM unit and working of Fin locking mechanism are explained in detail. In the Chapter 6, a detailed conclusive summary along with the list of outputs obtained out of this work.

Literature Survey

In the design of mechanisms for locking fins or control surfaces two fundamental designs are available, one is mechanisms operated by Pyro devices and another one is solenoid actuators. Selection of mechanism design is based on volume available in the product, power requirements, repeatability and reliability. All around the world designers prefer designing these mechanisms operated by Pyro devices. Though Pyro operated mechanisms are single shot devices they are reliable. In mission mode projects reliability is given priority than repeated usage of system.

Previous Research on FIN Locking mechanism

Similar work has been done by Seung-gyo Jang, Hyo-nam Lee and Jong-yun Oh (2014) they designed the mechanism for locking the missile in the canister. In this design each plunger operation needs an individual pyro.

Similar Research work has been done by Dana D. Hawthorne and HR Textron (2006) they designed locking devices which can be operated by solenoid release pins. In this design a locking device with solenoid device includes housing, plunger axially slidable member for biasing the plunger. Solenoid coil disposed in the housing around the plunger, for inducing a magnetic force move the plunger against the biasing member Such that the balls are movable radially inward into the recesses thereby releasing the locked member.

Similar Research work has been carried out by William W. Hsu, Glendale; Duncan D. Bragg, Simi Valley,(2001) they designed fin locking mechanism which can operated by a restraining device and spring. Upon a command Signal from a controller, preparatory to launch, a restraining device holding the Slide is removed and a Spring commences movement of the Slide towards a position which will take the links past their dead center position upon which additional Springs which loaded around each of the pins are activated to positively move the slide and extract the pins from the openings in the fins.

The present invention is directed to a locking mechanism operated by a Pyro (EED) for use on a missile having a moveable control fins extending from an outer surface thereof, the locking mechanism includes a locking pin for each of the moveable control fins which is adapted to extend through the outer surface of the missile into an opening in the control fin to retain the control fin in a fixed position prior to launch. When control surface are to be unlocked command is used to fire the Pyro and high pressure gases excess pressure on plunger and plunger retracted down by shearing the shear pin.

Basic function of mechanism

In operation of a fin locking mechanism constructed in accordance with the principles of the present invention, the missile would be assembled with the fins in their locked position. That is, upon assembly of the missile the fins would be attached to the members and would then be positioned such that the opening would be immediately adjacent the opening in the surface through which the pin would extend. The restraining mechanism would then be engaged to assure that the plunger remained in its pin extend position. After assembly of the fins in their locked position, missile will be loaded upon the aircraft and the aircraft would take flight toward the predetermined area so that it may accomplish its mission. Upon detection of the desired target whether it be a surface target or an air target, depending upon the particular mission of the missile. The desired time of launch an appropriate control signal from the controller would be applied to the Pyro cartridge releasing the restraining device from engagement with the slide member. When the unlock command is received and the Pyro cartridge is activated to release high pressure gas towards the plunger. As soon as the plunger commences to move towards the downwards shear pin immediately shear it off. The high pressure gas generate the greatest amount of force are activated and move the plunger forcibly downward. Since plunger are coupled inside the cylinder then simultaneously moved thereby positively extracting the pins and unlocked the fins. In this condition the control signals received from the guidance system in the missile can appropriately start functioning cause the fins to move appropriately to control the flight path of the missile.


So far in this chapter Fin locking mechanism and designing techniques have been reviewed to establish a clear understanding of a fin locking system basics, functioning and latest developments. In the Chapter 3, detailed design considerations will carry out to extend fin locking mechanism.

Design aspects of system

Typically, the fins of a missile must be locked during transportation on an aircraft or other launch vehicle. However, at launch time, they must be quickly unlocked to be ready for missile flight. For example, a missile carried on an aircraft has its fins locked prior to launch. When the pilot is ready to fire the missile, a signal is sent from the cockpit which starts the missile’s battery. When the cockpit has confirmation that the missile is powered, it signals the missile’s fins to unlock and wiggle to verify function. When fin release and function is confirmed, the cockpit signals the missile launcher to unlock and launch the missile. In a combat situation, where time is critical, it is essential that the fins unlock as quickly as possible.

Prior approaches include a complicated system employing a gas generator. To affect fin release, this prior system collects gas and distributes it through a manifold to each fin piston, which compresses to release the fin. This system has many parts and is slow, taking on the order of 70 milliseconds to unlock the fins. Another prior approach utilizes a shear pin which fractures to unlock the fins. Thus, a need remains in the art for a less complex, more reliable system which can lock the fins of a small diameter missile yet quickly unlock them for launch.

Basis of Design

Basically design is made depend on the volume of mating component. Initially the basic dimensions of the individual components are derived from the classical design approach the modeling of the component as per the derived basic dimensions. Some of the basic parameters are to be considered for design the mechanism. Sizing of individual components are taken in to consideration by checking with mating part. Total four control surfaces are assembled in missile so design should do with less complexity. Design is made to hold two control surfaces for each FLM unit. Generally this mechanism comes under Pin-Puller type. Design parameters are to be considered carefully for designing pin puller type mechanisms. These FLM units consist of two cylinders, two plunger pins and two shear pins. Plunger pin and shear pin are assembled in cylinder. Each cylinder assembly holds one control surfaces. Material selection is also an important factor for design of FLM unit. Standard Pyro cartridges are assembled to FLM unit for functioning of mechanism. As Pyro cartridge release high pressure gas the cylinder should with stand pressure load. Design of cylinder is important consideration cylinder thickness and cylinder dimensions are more critical for mechanism. Design is made depend on the hole size of fin trailing edge the plunger pin is made by hole diameter of trailing edge of fin. Cylinder is designed based on the plunger and its stroke length and a hole is provided on the plunger in which shear pin is assembled.


In this research, a detailed study on Fin locking mechanism has been carried out to prove that this is one of the reliable mechanisms for missile applications. The calculated pressure load of 32.5 Mpa is applied on the mechanism with closed condition and checking for maximum stress inside the FLM unit. The unit will be pressurized using Hydro-static pressure facility in steps of 10bar up to 1.5 times of failure load on the shear pin and will be held for 60 seconds. In both the cases FLM unit have performed its intended function successfully. Pyro cartridge in FLM system plays a vital role and it provides high pressure gas to a greater extent towards the cylinders. For an applied input voltage, the Pyro Cartridge tends to be active and releases high pressure gas. On the other hand, various suggestions were made to improve the material of the components to increase the strength and to avoid failure of mechanism. Repeated ground tests are conducted on FLM System to check the performance of device. For a mechanism to be successful all the components which are involved are to be worked according to their intended function. Apart from the above discussions, Fin locking mechanism is used in most of the conventional missiles, rockets and bombs.

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