Weight distribution of sub-munitions fuze design

QIN Dong-ze(秦栋泽)， FAN Ning-jun(范宁军)

(1.School of Mechatronical Engineering，Beijing Institute of Technology，Beijing 100081，China;2.Department of Mechatronic Engineering，North University of China，Taiyuan 030051，China)

Reducing the rate of unexploded ordnance in clustermunitions is important.In November 2012，most countries had reached a consensus on the termsof“Protocol on ClusterMunitions(Draft)”. Self-destruction， self-neutralisation and self-deactivating devices can be installed or improved in submunitions.Self-failure can decrease the possibility of unexploded munitions，and self-neutralisation and self-deactivating can decrease the Hazardous of unexploded munitions.While there are some related presentations on cluster munitions unexploded ordnance，no literature about reliability systems analysis of typical sub-munitions fuze reliability design is published［1－2］.

1 M85 sub-munitions fuze structure and reliability models

1.1 M85 Action principle

M85 is a kind of sub-munitions developed by Israel Military Industries(IMI).It is a kind of dual-purpose improved conventional munitions which is widely equipped and has typical technical features.There are several types of military cluster munitions(artillery shells fired)produced and exported by Israel that are all loaded with M85 sub-munitions.Israel has also authorized some or all system components to be produced in other countries.M395，M396 and M397 that are used in 155 mm caliber，as well as all other caliber artillery that including 105 mm，122 mm，130 mm，152 mm，175 mm and 203 mm are all loaded with M85 sub-munitions［3－5］.

The action principle of M85 it that cargo projectile utilizes time fuze and sub-munitions is thrown from the end of cargo projectile.After being thrown from the projectiles，most of the submunitions disperse radially as the radiant type.The sub-munitions in the central axis have the same rotation directional correlative speed with the mother bomb.After the bullet is thrown，because ofthe high-speed rotation，centrifugal mechanism can overcome the resistance of spring to lift slider insurance with the action the centrifu-gal force.At the same time，under the aerodynamic forces，the ribbon clip on the top of the bullets falls off，and streamers are opened，so the sub-munitions can flight stably.Under the tension of the ribbon，firing pin lifts the insurance of slider，so sliders will move to right place to open the deceleration wing，which can reduce the rotational speed of the sub-munitions and ensure that the effect of a shaped charge’s attack.After the slider moves to the right place，self-destructive hammer ignites extension tube.If the sub-munitions is not ignited normally，the detonator will be ignited after 15 s，then sub-munitions destruct itself［5］.The M85 structure is shown in Fig.1.

Fig.1 M85 fuze components

1.2 Reliability mode

In order to analyze the problem，the M85 sub-munitions fuze is simplified into five reliability units(subsystems):stabilizing device，safety device，interrupter device，ignition device，and selfdestruct device.To simplify the research，we study the explosion sequence according to the circumstance when reliability equals 1.The following will calculate their ignition through reliability model.The comprehensive ignition reliability of sub-munitions fuze can be expressed as

where f is monotonic linear function;RWis reliability of stabilizing device;RSis reliability of safety device;RRis reliability of interrupter device;RTis reliability of ignition device;RSDis reliability self-destruct device.

Reliability of integrated ignition is composed of normal function of the target’s reliability and reliability of self-destructions.The unexploded ordnance rate is

The M85 model is shown in Fig.2.

Fig.2 M85 model

In order to show the hypothesis is feasible，the models are simulated by applying the Monte Carlo method，with the hypothesis structure reliability between 0.95－0.99.The simulation result of reliability(Fig.3)is lower than 99%(M85 reliability lower 99%)，showing that the hypothesis is feasible.

Fig.3 Monte Carlo simulation

2 Analysis of M85 failure reason

In 2006，there is a comprehensive test of the 9408 M85 sub-munitions in Norway.In the test，104 duds are produced，and 95 of them didn’t cancel the insurance and 9 canceled the insurance completely but they did not detonate.

2.1 External factors

According to the pictures，we can analyze that the environmental factors，the launch system factors，the mother shells factors，and the factors in the dispersal system all acted on the sub-munitions fuze.Consequently，the appearance of fuzes sub-munitions are damaged，and the internal body is deformed.Also，part of the fuze mechanism’s design isn't entirely reasonable，and with prolonged storage，so the fuzes failed.All of these factors resulted in the unexploded ordnance.

When launched in different platforms or charges，submunition initialfactors have obvious difference when dispersed because of the differencein launch environment，thereby causing some influence on the normal ignition of submunition fuze.It has been proved by experiments that artillery propulsion charge has exact impact on undetonated submunition rate，and the change in charge leads to the change in submunition initial velocity and rotational speed，therefore causing significant impact on submunition fuze centrifugal arming device.

In general conditions，when launching platform is different，dispersion environment also varies greatly，the same submunition has different comprehensive ignition rate when launched in different platforms.

Most dual purpose improved conventional submunition is encapsuled in carrier projectile，and leverage effect exists in separation process which may impair the fuze.

The ignition performance is pertinent to target media.If submunition doesn’t hit the target，it may drop on soft groundsand dunegrass landjunglewater pit，then unignited projectiles is likely to exist.

Submunition fuze belongs to success-failure system when its working property is taken into account，but storage life exists when long-run storage performance is considered.Submunition fuze is affected by various environment stimulating factors in storage process，which may change its performance parameters，causing the whole system’s reliability to decline.

2.2 Fuze design factors

For the deformation of sub-munitions，they are mainly caused by the launch system factors，dispersal system factors，mother bomb system factors and environmental factors.For the reasons of operational requirements，cost and management mechanism，it is unlikely to improve the above systems in cluster munitions.The main reasons for the undeformed sub-munitions fuze failing to work are as the following:

① Fuze itself fails，unable to complete the process of cancel the insurance and detonate the warhead.These failures are mainly caused by fuze design defects and manufacturing errors，misoperations in transportation，storage，and service processing.

② Due to volume restrictions.The functional modules of the fuze device adopt series connection design，so that one component's failure will prevent their components from operating normally.

③Self-destructive mechanism design is not reasonable，which leads to the low efficiency of self-destruction mechanism.Self-destructive agencies should not start after the security system canceled the insurance completely.The best choice is that it should be independent of the fuze safety system completely and start after the throwing.

④ Stable institutions，security system and the design of ignition mechanism need to be improved.If the stable institutions work in poor conditions，the security system is hard to cancel the insurance.At the same time，the problem of stable institutions caused the bad location of submunitions，so the ignition institutions are difficult to ignite plants，sand and other soft targets effectively.

⑤Same detonators are used in single detonator，self-destructive mechanism and the ignition institutions.If the detonators failed because of long time storage，the sub-munitions will not be able to detonate.

Tab.1 Analysis of the causes of M85 bomb unexploded

3 Analysis of each device parameter sensitivity

According to the picture information the stability device failed 54 rounds，and self-destruct mechanism failed 9 rounds.

Since some of the data is missing，the sensitivity of every mechanism in the fuze has to be analyzed.The reliability of stabilizing device and self-destructive device is known.The reliability of other two mechanisms is fixed.Only one reliability varies.Putting the values above in Eq.(3)，the outcomes are shown in Tabs.3 －5.

Tab.2 Number of failures in unexploded bomb fuze device

Tab.3 Analysis of the insurance device sensitivity

Tab.4 Analysis of the flameproof device sensitivity

Tab.5 Analyis of the ignition device sensitivity

Tabs.3 －5 show that the safety device sensitivity is slightly higher than the interrupter device and ignition device.Interrupter device and the ignition device have the same sensitivity.Judging from Tab.2，in the M85 sub-munitions fuze failure function，stabilization device has the maximum weight followed by safety device，interrupter device，ignition device，and self-destruct mechanism，which helps sub-munitions fuze design.Because considerable number of sub-munitions fuzes use the ribbon structure and the centrifugal insurance institutions，we can use the failure case of M85 sub-munitions fuze as a reference.Ribbons are the key components of sub-munitions fuze.

4 Conclusion

Theoretical study shows that there are various reasons which cause the cluster munitions’sub-munitions'misfire.To analyze the experimental data comprehensively，we define the factors that impact fuze.The sensitivity of all the mechanisms in fuze have been analyzed，and the sequence of device effect in M85 sub-munitions fuze design have been figured out，which provide a reference for the establishment of sub-munitions fuze failure function and othersub-munitions fuze’s redesign.

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