Shelling of the Markale Market in Sarajevo 5th February 1994 Review of Methods Used to Predict the Impact Velocity of the Mortar Bomb

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1 Dr DF Allsop Defence Academy of the UK Cranfield University Dept of Engineering and Applied Science Shrivenham Swindon SN6 8LA United Kingdom Shelling of the Markale Market in Sarajevo 5th February 1994 Review of Methods Used to Predict the Impact Velocity of the Mortar Bomb 1. Introduction 1.1 I am Dr Derek Allsop, a Senior University Lecturer with Cranfield University at the Defence Academy of the United Kingdom. I am a Mechanical Engineer and I teach and research into the design and testing of conventional barrelled weapon systems, their ammunition and their ballistics. A more detailed cv is attached at Appendix A of this report. 1.2 On the 5th February 1994 a 120mm mortar bomb was fired into Markale Market in Sarajevo that caused huge loss of life and very many injuries to the local population. At the time of the incident, and subsequent to it, a number of attempts have been made to predict where the mortar bomb was fired from. These were based in part on the fragmentation pattern on the ground caused by the bomb and the stabilizing tail boom buried into the Asphalt surface of the market square. 2. Weapon Description 2.1. A mortar is an indirect fire weapon consisting of a smooth bore tube from which a projectile is launched. The projectile is fin stabilised by means of a stabilising tail boom fitted to the rear of the projectile and is fitted with fins designed to ensure that the centre of pressure is behind the centre of gravity of the projectile whilst in flight. The angle of launch of the projectile varies from between 45 o and 85 o, the 45 o angle giving maximum range and the 85 o angle giving minimum range. 2.2 The recoil forces are very high because of the large calibre of the weapon and the heavy weight of the projectile. The recoil forces are transmitted directly to the ground through a base plate which spreads the recoil force over a large area. The barrel is supported on a bipod that allows the elevation and azimuth of the tube to be varied to aim the weapon in the required direction. A sighting system is fitted to the bipod or barrel for accurate alignment of the barrel with the intended target. 2.3 The projectile can be launched at different velocities using different augmenting charges fitted to the tail boom. 1

2 3. Range Predictions 3.1 To predict the range from which the mortar bomb was fired it is necessary to know the angle at which it was launched and the velocity at which it was launched. If the impact velocity and the impact angle are known then range tables can be used to give the launch angle and velocity. 3.2 It can be seen that key to the predictions of where the mortar bomb was fired from is a knowledge of the impact velocity of the mortar bomb. The evidence from which the impact velocity has been derived was from the stabilizing tail boom which was found embedded in the ground at the place of impact of the mortar bomb. The impact velocity predictions relied on: a. The accuracy of the measurement of the depth of penetration into the ground of the stabilizing tail boom b. The reliability of the method for prediction of the impact velocity of the stabilizing tail boom based on its depth of penetration into the ground. 4. Instructions 4.1 I have been instructed to review the accuracy with which the depth of penetration into the ground was measured and the reliability of the methods for predicting the impact velocity of the mortar bomb. 5. Impact Velocity of the Tail Boom 5.1 Consider what happens to the stabilising tail boom when the mortar boom hits the ground. Figure 1 shows a schematic arrangement of the 120mm mortar bomb. It has a cast steel body from which the high velocity fragments are formed. It is filled with TNT explosive. There is an impact fuze at the nose of the bomb and the stabilizing tail boom at the rear. 5.2 When the mortar bomb strikes the ground the fuze is initiated and the explosive is detonated. The casing of the mortar bomb is broken up by the shock wave and expanding gas, the shock wave travelling rearwards from the point of initiation by the fuze and along the longitudinal axis of the bomb towards the tail stabilising boom. High velocity fragments are created from the shell body. The fuze is driven forwards and the stabilising boom is driven rearwards. 2

3 Fragments ejected Longitudinal Fuze ejected Tail boom Ejected Figure 1. Schematic arrangement of the mortar bomb as it fragments. 5.3 In this instance the stabilising tail boom continued to travel forward because its rearward velocity caused by the explosion was less than the forward velocity of the mortar bomb. Thus the velocity at which the stabilising tail boom penetrated into the earth consisted of the impact velocity of the mortar bomb less the velocity at which the stabilising tail boom was ejected from the rear of the mortar bomb by the detonation of the explosive. 5.4 The stabilising tail boom was screwed into the rear of the bomb casing. The rear of the bomb casing fragmented and the high force exerted by the shock wave caused the front end of the stabilising tail boom to fail in shear as demonstrated by the angular failed surfaces shown in the photograph, Figure 2 3

4 Asymmetric Damage Figure 2. Photograph of the 120mm mortar bomb stabilizing tail boom showing the asymmetric failure of the front end caused by the detonating explosive. (From REFERENCE [1]) 5.5 It can be seen that the failed surfaces form a point but it is offset from the centre line of the longitudinal axis of the stabilising tail boom. This can only be because the shear forces were asymmetric which would have caused the stabilising tail boom to rotate about its centre of gravity by an unknown amount. 5.6 It is also inconceivable that the stabilizing tail boom could pass through the remnants of the expanding gases from the explosion of 2.5kg of TNT without any form of disturbance. The stabilising tail boom would therefore have entered the ground having rotated through an angle of unknown magnitude. 5.7 Thus the final angle of the stabilising tail boom that has been so carefully measured by a number of investigators will be different to the impact angle by an unknown amount and cannot represent the angle of impact of the mortar bomb. 6. Rearward Ejection Velocity Component of the Stabilising Tail Boom 6.1 Attempts were made to calculate the rearward velocity of the tail boom from equations that were originally developed by Gurney and Sarmousakis in 1943 at the Ballistics Research Laboratory in the USA and subsequently modified by Mott, Linfoot and Taylor of the UK. These equations used a mix of theoretical considerations and practical firing trials to develop relatively quick methods of predicting fragmentation performance of simple munitions. 4

5 6.2 By conservation of energy they equate chemical energy released by the explosives to the kinetic energy of the fragments and the detonation products and ignore the energy used to fragment the shell body. 6.3 The original equations assumed that only one dimensional gas flow occurs which severely limits the geometry of shells for which it can be used. They were developed for simple very large bombs dropped by aircraft with very simple geometries and where the mass of the bomb casing was low compared to the mass of the explosive and there the energy used to fragment the shell was small compared to the total energy available. 6.4 The techniques and assumptions they used were taken forward and used in simple computer programs that could break down the shell into several segments and apply the relevant equation and coefficients. These equations are often used out of context without reference to their original boundary conditions and simplifying assumptions. None are capable of coping with the complexity of the interface between the shell body and stabilizing tail boom of a 120mm mortar bomb. 6.5 The Gurney approach was only ever meant to be a starting point for predicting fragmentation velocity in the development of shell bodies and that final values could only be found with any accuracy by validation by actual trials. 6.6 Modern hydrocodes are used to mathematically model fragmentation processes but these require a large amount of physical property data for the explosives and casing material which are often not available. They are not capable of predicting the point of material failure when the gases vent past the casing material and so cannot model accurately the final fragment velocities and still require validation from actual trial results. [REFERENCE 2] 6.7 Thus the only way of obtaining accurate values of the rearward ejection velocity of the stabilizing tail boom is to carry out trials and actually measure the velocities. This would require a minimum of seven but preferably ten firings to obtain a significantly statistical spread of results to obtain the standard deviation for subsequent probability analysis. 7. Measurement of Penetration of the Stabilizing Tail Boom into the Ground 7.1 The stabilizing tail boom was found buried in the ground at the place where the mortar bomb landed. It was below the surface of the ground and was removed by the French UN team investigating the incident on the same day that it occurred. 7.2 A video [REFERENCE 3] of the removal of the stabilizing tail boom clearly shows the actual finding of the stabilizing tail boom but not its removal from the ground. It does however show fine grained material being brushed away from the top of the stabilizing tail boom which was below the surface of the ground. 5

6 7.2 The stabilizing tail boom was replaced the following day by Dr Zecevic and the depth of penetration was measured to be between 220mm and 250mm. This is shown on the same video. [REFERENCE 3]. 7.3 Comparing the video of the Stabilizing tail boom when it was first found embedded in the ground with that of the video of it embedded in the ground the following day it can be seen that the depth is very similar. The stabilizing tail boom was damaged at some point and the damage can be clearly seen and the orientation of the tail boom appears to be very similar in both videos. 7.4 What is not shown on the video is the actual removal of the stabilizing tail fin and the reinsertion the following day. The ground into which the Stabilizing tail boom penetrated was sand and gravel which has no cohesive strength. Thus as it was removed there would have been sand and gravel falling from the sides of the hole and partially filling it. 7.5 Likewise when the stabilizing boom was reinserted into the hole more of the sand and gravel would have fallen into the hole adding to the material that was already there. It should not have been possible to reinsert it to the same depth from which it was removed. The reason for this is explained in section Calculation of the Velocity of Impact of the Stabilizing Tail Boom. 8.1 The accuracy of the measurement of the stabilizing tail boom penetration into the ground is important because investigators have used this to help predict the impact velocity of the mortar bomb with the ground. The prediction methods used were based on trials data obtained from the firing of different projectiles into different types of soils at different impact velocities. 8.2 Depth of penetration into the ground by any object is governed the density of the soil, its strength and the friction acting on the surfaces of the object in contact with the ground as it penetrates into it. Friction acting on the projectile has by far the greatest effect on the retarding force acting on the projectile and the greatest effect on friction is the yaw angle which the yaw angle enters the ground and any form of lubricant that is present. 8.3 Yaw angle is of crucial importance because this increases the surface loading and thus the friction forces. The amount of yaw is not known and so the effect on penetration depth is not known. 8.4 The moisture level of the ground is an important factor because water acts as a lubricant and reduces friction. Moisture levels were not measured and the moisture levels for the test results used for impact velocity predictions is not given. 8.5 Also, the stabilising tail boom would start penetration when the front part impacts the ground which is 40mm diameter. After 70mm the fins will have impacted the ground. There are twelve of them and they extend to 120mm diameter so that the sliding surface area in contact with the soil would have increased by a very large amount. The friction forces would also have increased considerably but the method used to predict impact 6

7 velocity from depth of penetration does not take this into account [REFERENCE 4]. However there is a more important consideration which has not been taken into account and is described below. 9. Effect of the Fuze on Penetration of the Depth of Penetration of the Stabilizing Tail Boom. 9.1 When the mortar bomb detonated the fuze would have been driven forward into the ground. Investigators [REFERENCE 5, 6, 7, 8, 9] have mentioned that this produces a hole (known as the fuze trough or fuze furrow) that is often sufficiently intact in shape to allow the angle of impact and the heading of the mortar bomb to be calculated from the ovoidal shape of the mouth of the hole. (Unfortunately this was not the case for this incident because the fuze trough was not clearly defined.) 9.2 The initial velocity of the fuze as it was driven into the ground would have consisted of the velocity imparted to it by the explosive gases plus the forward velocity of the mortar bomb at the time it impacted the ground. Thus the resultant velocity of penetration of the fuze is likely to have been considerably greater than the resultant impact velocity of the stabilizing tail boom. 8.5 When the stabilizing tail boom impacted the ground it would have entered a preformed hole which would have been a similar diameter to that of the maximum diameter of the fuze. Fragments from the fuze were found scatter on the surface [REFERENCE 10] but the main body of the fuze would have been buried deep in the ground under where the stabilising tail boom was found. Thus the depth it penetrated would have been a characteristic of the impact of the fuze and not that of the stabilizing tail boom. Figure 3 shows the arrangement of what would have occurred. Stabilizing Tail Boom f Fuze Trough Fuze Ground Figure 3. Relationship between the stabilizing tail boom and the fuze trough. 7

8 8.6 The diagram in Figure 3 assumes that the final position of the fuze was deeper than that of the stabilizing tail boom. This appears to have been the case. If the tip of the stabilizing tail boom had impacted the remains of the fuze there would have been deformation on the pointed tip of the damaged stabilizing tail boom (See Figure 2) but there appears to be no additional damage to suggest that this may have happened. 8.7 A gap between the fuze and the stabilizing tail boom would explain how it was possible to reinsert the stabilizing tail boom back into the hole to the same depth the day after it was removed despite the almost certain ingress of the fine grained material surrounding the point of impact and from the side of the hole through the sand and gravel. 9. Abrasion Marks on the Stabilizing Tail Boom 9.1 There is other evidence found on the stabilizing tail boom not pick up by the crime scene investigators as shown in Figure 4 which is another photograph of the stabilizing tail boom. The ground into which it penetrated was sand and gravel which is highly abrasive. Only the stabilising fins show signs of abrasion; there are no signs of abrasion on the small diameter front end where it would expected to be if it had been forced through a layer of sand and gravel. Abrasion Marks Figure 4. Photograph of the stabilizing tail boom showing the abrasion marks on the fins. None can be seen on the small 40mm diameter front part. (From Exhibit P1967). 10. Other Methods of Measuring Impact Velocity of the Mortar Bomb 8

9 10.1. It is important to consider how else the impact velocity of the mortar bomb could be derived. The only evidence that could help with this is the stabilizing tail boom embedded in the ground. It can be seen that the depth of penetration is governed by the penetration characteristics of the fuze so is of no help Prediction methods for obtaining the velocity at which the stabilizing tail boom is ejected from the rear of the mortar bomb are not sufficiently well developed to give an accurate answer Static fragmentation trials would give the ejection velocity by actually measuring it. Sufficient firings would be necessary to give a statistically valid result and to give the statistical spread and confidence levels in the measured results Whilst the actual impact velocity of the stabilizing tail boom would not be known the trial results would set the minimum velocity for the stabilizing tail boom to continue travelling forward and penetrating into the hole left by the fuze. This may or may not be useful depending on what the actual velocity is. The closer the velocity is to the maximum possible impact velocity of the mortar bomb the narrower will be the range of possibilities for the impact velocities. 11. Conclusions 11.1 The shelling of the Markale Market in Sarajevo 5th February 1994 was a unique one off event. There was very little evidence on which to base a forensic investigation into where the mortar bomb was fired from Key to establishing the range from which the mortar bomb was fired is the impact velocity of the mortar bomb The method of calculating the ejection velocity of the stabilising tail boom from the rear of the mortar bomb was over simplistic and not capable of coping with the complexity of the of the geometry of the construction of a mortar bomb to give a reliable answer Comparing videos of the Stabilizing tail boom embedded in the ground before it was first removed and after it was reinserted to be measured the following day it appears they are similar The method of predicting the impact velocity from the depth of penetration was flawed because no consideration had been given to the fuze trough With the current information available from the scene of the incident it would not be possible to predict with any accuracy the range from which the mortar bomb was fired within the distances of the maximum and minimum ranges of the 120mm mortar. Dr DF Allsop 20th January

10 REFERENCES [1] From Exhibit P1967. [2] The Design of High Explosive Fragmentation Munitions using a Modified Gurney Velocity Equation for a Sphere Transformed into a Shell. D.J.J. Jaya-Ratnam 1997 Phd Thesis Cranfield University (Supervisor Dr D.F. Allsop) [3] Exhibit P1450 [4] Dr Zecevic Expert Witness Report (English Translation) 2003 "Shelling Incidents" Expert Report "Markale Market" Incident. DEF35141.doc/ [5] Capt Verdy. UN Shelling Report 06 FEB 94 [6] Mr Richard Higgs. Information Report. Consolidation Report on Firing Incidents involving Mortars in the Sarajevo Area dated 1 and 12 June 1993, 22 JAN 1194, 4 and 5 FEB 1994, 18 JUNE 1995 and AUG [7] Karadzic trial, 13 December 2010 T [8] Karadzic trial, 13 December 2010 T [9] Prosecutor v Karadzic, Case No. IT-95-5/18-T, Transcript of 24 FEB 2011, p.5 [10] Witness Summary. Mr Richard James Higgs. 92ter#; p.15 Galic Trial, 4 March 2002 p

11 APPENDIX A CV for Dr Derek Allsop MSc, PhD, CEng FIMechE I am a Chartered Mechanical Engineer and for the past twenty four years have been employed at Cranfield University at what was the Royal Military College of Science and is now the Defence College of Management and Technology. For the first eight years I was Deputy Director of the Weapons Assessment and Technical Support Unit undertaking research and consultancy on weapons testing and design for the MOD. I then moved to the ballistics group as a Senior Lecture where I lectured on engineering design at undergraduate level and weapons and ballistics at post graduate level. I also undertook research into weapon systems, ammunition and ballistics and supervised PhD students researching in these areas. Twelve years ago Cranfield University introduced a Masters Degree in Forensic Engineering and Science and I developed the teaching on Forensic Ballistics. Two years ago a suite of MSc degrees in forensic subjects was introduced and I became the academic lead in the Forensic Ballistics Masters Degree Below is a list of research and consultancy work undertaken during this period: 1. Hit Chance investigations for briefly exposed targets using shoulder mounted 7.62mm and 5.56mm calibre weapons. 2. Five year MOD research programme into erosion and wear in small arms gun barrels. 3. Performance of the Winchester Olin 12.7mm Calibre Sabotted Light Armour Penetrator (SLAP) Round to established the characteristics of this very high performance round. 4. Trials and assessment of vehicle gun mounts for the supply of gun mounts for the All Arms Air Defence role of the 12.7mm machine gun. 5. Test and evaluation of ammunition for the 12.7mm round for use in the air defence and anti-light armour role. 6. Feasibility study into 45mm CTA cannon and ammunition for use in Light Weight AFV s. 7. Development of a mathematic model to assess hit probability for high rate of fire medium calibre cannon against high speed aircraft and surface craft. 8. Analysis of trials data for grenades submitted for consideration to replace the in service grenade. 9. Assessment of 40mm Grenade Launcher to establish suitability for use by the infantry. 10. Development of diffusion bonded Stellite lined barrels to reduce barrel wear under severe firing conditions. 11

12 11. Analysis of the performance, including firing trials, of the 20mm Giat Cannon fitted to the Offensive Action Vehicle for Special Forces 12. Development of a Specification for Testing and Evaluation of the Combat Effectiveness of Fragmenting Ground Burst Grenades. 13. Development of test a bed SA80 with variable rates of fire and subsequent testing and analysis on the effect of rates of fire on dispersion. 14. Analysis of Potential Contenders for the MRAV Self Defence Weapon. 15. Testing and analysis of CRISAT Combat Body Armour to establish the protection levels against a wide range of calibres, bullet structure and fragments. 16. Ballistic tests of 5.56mm and 7.62mm ball rounds after penetrating CRISAT CBA. 17. Design and build of technology demonstrators of IW s and LSW s using electronics to control the functioning of small arms as part of the FIST programme. 18. Development of a mathematical model for predicting fragmentation dispersion for accurate assessment of fragmenting munitions performance. 19. Assessment of the performance of larger calibre of anti-materiel rifles. 20. Design and performance study into the optimum calibre and weapon format required for engaging current and future anti-ship missiles. 21. Ring aerofoil studies to investigate their suitability for use for grenades and baton rounds. 22 Design and build of a spinning barrel test rig to investigate projectile spin rate requirements for projectiles with complex geometries. 23. Analysis of current and future target arrays was undertaken and the performance of the current and future GPMG s and Automatic Grenade Launchers was undertaken to establish their effectiveness. 24. Acted as an expert witness in two terrorist related cases covering the ballistics of the 82mm mortar and the manufacture of assault rifle components. 25. Acted as expert witness in an anti-aircraft gun design arbitration case. 26. Undertaken research into the ballistics of 17 th century weapons to aid battlefield analysis. 12

13 27. Research into the performance and effectiveness of Non Lethal Weapons. 28. Prediction of the safety trace of shotguns for the proposed Olympic Games shooting competition venue. 29. Research into the aerodynamics of projectiles that develop lift. 30. Pre-production study for the MOD on the new 40mm Cased Telescoped Ammunition Cannon and its ammunition. 31. Safety trace studies for a new Bangalore Torpedo. 32. Conducted bounce and roll trials of simulated 30mm and 60mm calibre 15 th century cannon. 33. Shotgun steel shot ricochet safety studies. 34. Supervisor of a PhD programme of research to establish the optimum performance of baton rounds used in the less than lethal role. 35. Supervisor of a PhD programme of work to investigate gas flow studies for the optimum design of sound moderators for maximum performance with respect to mitigation of the signature of weapon firing. 36. Supervisor of a PhD programme of work to investigate the wear in gun barrels. 37. Supervisor of a PhD programme of work to investigate the mathematical modelling of fragmentation of high explosive warheads. 38. Supervisor of a PhD programme of work into the mechanical properties of soft tissue and the subsequent mathematical analysis of the performance of terrorist improvised explosive devices. 39. Supervisor of an MSc by research into the ballistic performance of 17 th Century Muskets. 40. Researched the deformation of fixed shot gun chokes by non-toxic shot material. 41. Researched the cause of damage to removable shotgun choke inserts. 42. Acted as advisor to Birmingham Proof House and the British Association of Shooting and Conservation on weapon failures 43. Carried out a research into weapon failures due to barrel obstruction. 44. Carried out research into 20/12 bore weapon failures. 13

14 Current research investigations include the proofing of sound moderators and the proofing of Wildcat cartridges for Birmingham Proof House, ground penetrating studies for a gun driven pile driver, disruption methods for buried IED s and the development of a 60mm mortar training round. Books written: D.F. Allsop and L Popelinsky et al, Brassey s Essential Guide to Military Small Arms Design Principles and Operating Methods. Land Warfare: Brassey s New Battlefield Weapons Systems and Technology Series into the 21 st Century. D.F. Allsop and M.A. Toomey, Small Arms General Design, Land Warfare: Brassey s New Battlefield Weapons Systems and Technology Series into the 21 st Century. D.F. Allsop. Cannons, Land Warfare: Brassey s New Battlefield Weapons Systems and Technology Series into the 21 st Century. 14