Syria's Barrel Bomb Technology Relative To Aleppo Syria Attacks - The Good, The Bad And The Ugly

Richard M. Lloyd, Warhead Technology Consultant at Tesla Laboratory Inc. takes a look at the DIY barrel bombs used in Syria.

Introduction
The Syrian government over the last year has supplemented it's traditional conventional air campaign with cheap weapons that have been referred to as Barrel Bombs (BB). These Do It Yourself (DIY) weapons are derived from low cost cylinders that are filled with explosive, fuel and irregular shaped steel fragments. These DIY bombs are manually deployed by soldiers from Russian HIP helicopters. The idea is these helicopters can roam around Syrian neighbourhoods and drop these weapons with more accuracy than firing aircraft rockets.  The purpose of this paper is provide some fundamental frame work of how these bombs work, while exploring the myths and false information that has been associated with these weapons.

Initial Barrel Bomb Technology

The main objective of the Syrian barrel bomb program is to provide cheap and lethal damage on urban areas in Syria. When these bombs were initially developed they were deployed from HIP helicopters from low altitudes. These low altitude deployments would ensure the barrel bombs could maintain pin point accuracy and damage specific targets that the Syrian government wanted to kill. However, over time the Syrian rebels acquired Man Portable Air Defense System (MANPADS) that deterred these low launched bombings and drove the Syrian government to drop these bombs from altitudes near 7000ft. A description of a Syrian helicopter that has been shot down from a rebel who fired a MANPADS missile is shown in figure 1.

Figure 1 Syrian Helicopter Shot Down With Rebel Fired MANPADS

These rebel MANPADS attacks have forced the Syrian barrel bomb helicopters to release their bombs from high altitudes.  This increased altitude reduces the accuracy of these bombs and now their impact points are random.  Currently, barrel bomb are not attacking point targets, but rather they are attacking large urban areas killing humans and damaging infrastructure such as building.

Over the last year there have been many false claims made by Syrian civilians as well as the press that barrel bombs have been deployed, when actually they were Russian conventional bombs. These false claims have given barrel bombs false praise by claiming their damage potential is greater than what it really is.  Figure 2 shows photos of a HIP-8 helicopter dropping a barrel bomb relative to a Russian Hind Helicopter dropping a conventional Russian bomb. 

Figure 2 Comparison Of HIP Mi-8 Barrel Bomb And Hind Conventional Bomb

As stated, there have been many false claims in Syria that damage to Syrian cities has been caused by barrel bombs when the damage was actually caused by Russian conventional bombs that have significantly higher reliability than these DIY barrel bombs.

Barrel bombs designs that are observed though social media demonstrate that the Syrian government designed these barrel bombs with a fuse wick that requires a heat source (matches, cigarettes, cigars) to ignite.  The Syrian government must have made the simple calculation that if the helicopter is at a known altitude (H) and the fuse wick burns at a known rate (\(\begin{equation} \dot{b} \end{equation}\)) then the time that it takes for the bomb to detonate is \(\begin{equation} t=H/\dot{b} \end{equation}\). However, this simple equation that computes the time before detonation is false.  Whoever designed these barrel bombs does not understand that there is a 10% burn rate error in the fuse wick.  They also don't understand that if the helicopter is not at the designed altitude it will affect the time of fall.  In conjunction with these errors, they did not take into account the barrel size, mass, barrel bomb L/D ratio, tumble rate, drag effects, wind speed as well as the time it takes the soldier to deploy the bombs once it has been lit inside the helicopter. All of these effects must be accounted for if a fuse wick is going to be used.  Lets perform an example calculation that only considers the 10% error source of the fuse wick burn rate and assume all of the other errors are zero or negligible. The Syrian government would first be required to compute the terminal velocity of the barrel bomb by using the equation \(\begin{equation} V=\sqrt{2W/\rho AC_{d}} \end{equation}\).  The terminal velocity accounts for the weight of the barrel bomb as well as the air density, average cross sectional area and drag of the barrel falling to the ground.  A 500lb barrel bomb has a terminal velocity near 250f/s. Based on this calculation the time it takes the barrel to impact the ground is just 7000ft/250ft/s=28 seconds.  So, the selected fuse wick that has a known burn rate is cut to a length which would burn for 28 seconds and detonate just when it impacts the ground. However, since there is a 10% burn rate error, there exists a probability that it could detonate early.  If the burn rate is 10% too fast then the entire fuse wick would have completed its burn at 25.2 seconds.  This means the warhead would detonate 700ft before hitting the ground.  Examples of these early bursts have been demonstrated through many videos as shown in figure 3.

Figure 3 Barrel Bombs Detonate Early From Negative Errors

These early bursts  have been widely seen in Syria, which are caused by an accumulation of the errors that potentially can exist.  It is unknown how accurate the helicopter pilot attempts to keep the helicopter at the designed altitude.  It is also unclear if the Syrian barrel bomb builders understand drag effects and fuse wick errors and probability timing . Another example of another error is a Syrian Solider lighting and pushing barrel bombs out of the helicopter. In this instance, time must be taken into account and there is obviously no control of how the bomb is thrown out where time must be accounted for.  A photo of a Syrian soldier pushing a barrel bomb out of a helicopter is shown in figure 4.

Figure 4 Description of Barrel Bombs on Syrian Helicopter Prior to Deployment

These early barrel bomb weights are around 100-300lbs and are ignited using fuse wicks. The soldier lights the fuse wick using a cigar because the wind would blow out a match or lighter.  A close up of the fuse wicks is also shown where they should all have the same length.  After the fuse wick is lit, the soldier quickly pushes the bomb from the helicopter. It is unclear how the Syrian designers of these bombs expected to time the fall exactly because there is clear evidence that many of these barrel bombs detonated early which is referred to as a negative time delay.

The above analysis has only considered what could happen if the total time is less than the required time to detonate the barrel bombs given a ground impact.  However, what would happen if the barrel bomb detonated late. Based on our example problem, the fuse wick would burn for a longer time than 28 seconds.  If this occurs, then the barrel bomb would actually hit the ground or a building while the fuse wick is still burning.  If this occurred, then based on video and photo evidence the barrel bomb would shatter upon ground impact or detonate if the fuse wick and bomb were not destroyed. There is clear evidence that barrel bombs can detonate given a positive delay time as shown in figure 5. This figure shows two potential out comes when the time delay is positive where the barrel bomb will either detonate or turn out to be a dud.

Figure 5 Comparison Of BB Detonating or Duding Based On Positive Time Delay

The next step is to try to understand why there is and explosion in one case and not in another. One explanation,assuming all is equal, is the effect of  the shock-wave through out the barrel bomb's front endplate which holds the fuse wick. The stress of a steel case impacting concrete, grass or sand has been investigated.  By using the shock Hugoniot equation \(\begin{equation}
P=\rho c\mu+\rho s\mu
\end{equation}\) , the interface pressure between the ground surface and steel barrel bomb is calculated.  The density of the material is \(\begin{equation}
\rho
\end{equation}\)while the bulk sound speed of the material is \(\begin{equation}
c
\end{equation}\) where \(\begin{equation}
\mu
\end{equation}\)is the particle velocity and s is the slope between shock velocity and material stress. These calculations show that the impact stress of a barrel bomb impacting concrete or rock is 3.5-5 times greater than if the barrel bomb impacted sand or dry soil.  This increase in stress will cause the endplate that holds the fuse wick in place to fracture from the barrel bomb which disables the bomb.

Another important parameter that must be considered that would determine if positive time delayed barrel bombs can detonate is the impact point on the bomb. If a barrel bomb impacts directly on the fuse endplate, then there is an increased probability that the bomb could fail.  Based on video analysis and ground inspection, these barrel bombs do not contain any fins to help control the fall of the bomb by providing stabilization.  These bombs randomly tumble and the probability that these bombs could impact on the fuse/endplate is just 1/6 or 16.6%. If we assume a uniform random distribution of barrel bombs that contain negative and positive time delays, then just based on this fact the probability of a positive bomb impact is 50%.  As of now, there is a 50% chance the bomb will not work and a 50% chance that the bomb will work.  Now lets account for the ground and the impact position of the barrel to the ground.  If the barrel bomb impacts concrete then the bomb will break apart and not work as intended. However, if the bomb impacts softer ground, it is probable, as observed, that these bombs will not break apart and wil function correctly. Our analysis assumes a 50% probability that the bomb will impact concrete/rock and a 50% probability that the bomb will impact sand or grass with light rubble.  This means the probability that the bomb will function is just the probability that the time delay is positive or negative and the probability that the ground is hard or soft.  This calculation is \(\begin{equation}
P=(1-0.5)(1-0.5)=0.25
\end{equation}\)which means there is a 25% chance these fuse wick barrel bombs will work. So, it requires the Syrian government to deploy four barrel bombs to get one bomb to detonate the way it's intended.  A description of two different barrel bombs is shown in figure 6.  The barrel bomb on the left is one that impacted on a soft surface,where the stress was not high enough to fracture off the fuse wick endplate, while the barrel bomb on the right is shown hitting concrete and fracturing off the fuse wick endplate.

 Figure 6 Comparison between barrel bombs impacts on soft and hard surfaces

Barrel Bomb Designs
The remains of these barrel bomb duds have been investigated to understand how the Syrian government has assembled them, and to determine how lethal these bombs can be.

It is evident from visual observations that there has been little to no quality control on these designs because of the many shapes and sizes that have been observed.  There appears to be clear design trends that clearly show that Syrian personnel have no weapon design and effects training.  Most Syrian barrel bombs contain many large steel components, such as cut rebar or large machine metal pieces such as large industrial bearings. These large metal chunks are inserted inside and mixed with the TNT explosive. The problem with this approach is when the explosive detonates, it's using a lot of its energy to accelerate these large chucks of metal which reduces its blast effects as clearly seen in figure 7. These added steel materials reduce the blast effects by making the explosive use its energy to accelerate it.

Figure 7 Large Metallic Metal Components Added In Barrel Bomb Explosive Mixture

However, if these large metallic pieces were to impact a human then they would cause catastrophic incapacitation. There are mathematical equations that predict the optimum fragment size that is required to kill a person by taking into account probability theory and applying weapon design equations. The probability of incapacitating a person is computed with this equation\(\begin{equation} P=1-e^-a({mv^{3/2-b)^{n}}} \end{equation}\)

Based on this equation, the optimum fragment mass can be computed to increase barrel bomb performance.  The barrel bomb case would break apart into thousands of small fragments from the natural fragmentation of the explosive shock wave interaction with the smooth case.

The fragment mass is m and the impact velocity is v where test derived constants are a, b and n. The velocity of the added metallic fragment is computed by using the Gurney equation which is\(\begin{equation} V=\sqrt{2E}\sqrt{C/M/(1+D/2L)(1+C/2M)} \end{equation}\)
The \(\begin{equation} \sqrt{2E} \end{equation}\)is the explosive Gurney constant which is around 2.4km/s for TNT. The explosive mass is C while the metallic mass plus the barrel bomb case is M. The parameters D and L are barrel bomb diameter and length respectively. The barrel bomb average case fragment can be computed by \(\begin{equation} \mu^{1/2}= Bt^{5/6}d_{i}^{1/3}(1+t/d_i) \end{equation}\) where B is a constant, t is the barrel bomb thickness and \(\begin{equation} d_i \end{equation}\) is the explosive diameter. The estimated number of fragments from the barrel alone is \(\begin{equation} N(m)=M/2\mu \end{equation}\)where the number of fragments in a particular weight class is calculated by \(\begin{equation} N(m)=M/2\mu e^{-(m/\mu)^{1/2}} \end{equation}\)

These equations have been exercised pertaining to the Syrian barrel bombs.  The analysis clearly shows that these weapons are under-designed and their lethality in urban areas can significantly increase by changing their approach and understanding of weapon effects.  Even though these embedded large chunks of metal are highly lethal against humans, the probability of being hit by one is extremely low. Given a 40ft stand-off distance of a human from a barrel bomb explosion, there would be a 96% probability of death given a hit from one of these fragments,  However, the probability of being hit from these chunks of metal is only 3%, which gives a total probability of incapacitation of 2.8%. An example calculation is shown in figure 8.

 Figure 8 Barrel Bomb Probability of Incapacitation and Probability Equations

The blast effects have been investigated, and where the pressure-impulse to incapacitate a human has been calculated.  The blast damage is directly related to the amount of explosive as well as the metal casing surrounding it. The lethal blast radius has been calculated, but it is not reported in this version of the report.

Changing Barrel Bomb Technology
There are several significant changes that have been observed in Syrian barrel bomb technology.  It is evident from these changes that the military determined they were not getting explosive detonations on the ground and their bombs were not working.  The Syrian government has switched from using fuse wick fuses to impact fuses. As stated earlier, these fuse wicks have minimal chance of working because of their time delay errors.  However, the addition of impact fuses accounts for all of the errors that are associated with fuse wicks. These new impact fuses are clearly DIY and in conjunction with these new fuse concepts, the sizes of the barrel bombs have significantly increased.  Today's Syrian barrel bomb weights can exceed over 2000lbs of TNT, making them highly lethal given a high order explosive detonation in urban areas. A description of a recovered barrel bomb with these new design features is shown in figure 9.

Figure 9 Newer barrel bombs designed with DIY impact fuzes

These impact fuses were created using detonation cord to ignite the bulk TNT explosive charge. It is unclear what the exact reliability of these fuses are and how well they actually work. However, it is expected that the Syrian builders have performed some testing of this device to have enough confidence that it will work given that the barrel impacts the ground on the fuse. In order for these new bombs to work they must impact on the fuse which means they must impact on the front of the barrel.  The only way to accomplish this is to add fins to stabilize the bomb so it won't tumble as it falls to the ground.

This recovered barrel bomb did not operate and detonate correctly because the fins appear to be too small.  The explosive weight of this bomb is near 2000lbs with small fins attached to the middle of the bomb. Figure 10 is a picture of the small fins that were welded to the center of the bomb.  A picture of a typical Russian bomb is shown, which clearly shows these bombs are designed with fins in the back of the bomb and not in the middle.  It has been concluded with high confidence that this massive bomb still tumbles and the probability of falling on the impact fuse is random.  It appears the fins are designed too small and the barrel bomb is randomly tumbling to the ground.  If we assume the probability of the fuse working correctly is 75% and the cylindrical barrel bomb has 6 sides, then given random tumbling, the probability of success is 1/6 or 16.6% given 100% reliability of the fuse. However, if the DIY fuse has a reliability near 75%, then the probability of success is 12%.

Figure 10 Syrian barrel bomb with small fins attached to the middle

It appears the Syrian builders have discovered this problem with their fins and their inability to generate a stable and aligned bomb.  There is clear evidence that the Syrians have moved these fins from the middle of the bomb to the aft end, which is the design norm of all the gravity bombs in the world. Figure 11 shows a large barrel bomb just after it has been deployed.

Figure 11 Large Barrel Bomb Deployment  Showing Initial tumbling Of Weapon

These new design features have increased the probability that the barrel bomb will fall aligned with stable flight and impact the fuse. We have already estimated that the fuse has a 75% chance of working given an impact. Based on these fins relative to known bombs around the world, it appears that the stability of the bomb will increase, but it is still unclear whether the fins are large enough and far enough to the back of the bomb to provide good stability.  A conservative estimate of the fin performance based on Russian and US fins relative to the Syrian barrel bombs is 50%. This estimate is based on analyzing Russian bombs of similar weight, and measuring their fin distance and diameter relative to the recovered barrel bombs. Given these two performance probabilities, the probability that these new Syrian barrel bombs will work correctly is 37.5%.

Barrel Bomb Design Variations
There have been several recovered barrel bombs that are designed differently compared to most traditional barrel bombs. Some of these barrel bombs contain smaller warheads that are embedded within the explosive to ignite the much larger barrel bomb explosive.  This design technique uses the small warhead as a booster, which is really not required.  An example of this is shown in figure 12.

Figure 12 Syrian barrel bomb using small warhead as a booster

The explosive has appeared to be soaked in oil which would give the same features as ANFO. ANFO is an explosive mixture which contains some small percentage of oil.  This oil actually reduces the explosive detonation velocity which reduces the blast and fragment acceleration power of the explosive.  This bomb has an impact fuse, but the most disturbing observation is it has no fins. Without fins, this bombs reliability is very low and the odds of it working are small.  In fact, all objects that fly through the air eventually turn and fly with the side with its greatest presented area.  If this rule of thumb is applied, this bomb will never rotate and align its front surface to the ground.  The highest probability of the side with the fuse impacting is just 1/6 or 16.6% which assumes there is an equal chance of any side impacting. Now, if the reliability of the fuse is 75%, then the probability that the barrel bomb will function is 12.4%.                

The Syrian government over the last year has significantly increased it's barrel bomb sizes from hundreds of pounds of explosive to 2000 pounds of explosive. However, there is strong evidence that these weights are larger than this.  A video exists that shows the Syrian military throwing barrel bombs from a helicopter with a large fuel auxiliary tank. This video shows to the right a large yellow tank, which is the helicopter's internal fuel auxiliary tank. This tank can contain over 960 litres of fuel ,or, if it was converted to a high-explosive barrel bomb, it would contain around 3000-3500lbs of explosive as shown in figure 13.

Figure 13 Syrian Military Using Helicopter Fuel Auxiliary Fuel Tank As Barrel Bomb

With the need for larger barrel bombs and the Syrian military's need for larger explosions, there is reason to believe that the military could be using these auxiliary fuel tanks as Fuel Air Explosives (FAE).  There is no clear evidence of the military converting these fuel tanks to FAE'S but there appears to be very large unexplainable explosions in Syria that that are clearly made from unknown FAE weapons. If the military made these tanks into FA'S and they were able to master the weaponization of deploying fuel and igniting it, these tanks would generate explosions greater than 15 tonne TNT equivalent weights. One of these unexplainable explosions that has not been attributed to ground fuel explosions, fuel or high explosive depots or anything but a dropped bomb is shown in figure 14.  This unexplainable explosion distinctly shows that a FAE or fuel weapon was used in Syria that generated a fireball cloud with a diameter near 2000ft.  The TNT equivalent yield is estimated from the height of the dust cloud where many yield equations have been derived for nuclear and large explosive events in the 1960's. This explosion did not persist like known explosions that were started from fuel depots or fuel lines from conventional bombings. This explosion occurred and persisted for about 120 seconds and then disconnected from the ground.  Also, there have been several barrel bombs that have been observed from helicopters that appear to have platforms that are attached to wheel tracks.  These bombs would weigh over 3000lbs and would require a sled system to move it on and off the helicopter.

Figure 14 Potential Syrian derived weapon, which supports unexplainable explosions

This explosion would generate significant damage to all buildings within a 600ft diameter. The analysis assumed the fuel is ethylene oxide which has an energy content of 28000J/g while TNT explosive is around 4180J/g.  This large increase in energy is another reason why the Syrian military may be investigating weaponizing auxiliary fuel tanks into FAE weapons.  It is known that Syria possesses Russian ODAB 500 FAE weapons that only have a TNT equivalent yield of one tonne.

Aleppo barrel Bombs Attacks
It was reported that Syrian government aircraft dumped barrels packaged with explosives on at least four opposition-held neighbourhoods of Aleppo on December 18, 2013. This would be the fourth day of increased air strikes on this contested northern city. It appears this air campaign killed more than 100 people in the first three days alone.  A map of the potential barrel bombings is shown in figure 15.  The damage radius of these bombs looks very large from observing videos of the explosions.  In fact, it is still unclear if all 11 of these explosions were solely from barrel bombs.  The Syrian people have a history of calling all bombs dropped from helicopters barrel bombs, which is false. There has been many reports that all of these attacks are barrel bombs but given the probability of success, this seems questionable and further analysis is required. However, lets assume all 11 explosions were from barrel bombs and lets also assume that all 11 barrel bombs possessed fuse wicks.  Since the probability of a fuse wick barrel bomb working is 25%, it would require around 88 barrel bomb drops to achieve 11 successful explosions based on the probability equation of \(\begin{equation} P=1-(1-P_{k/h})^n \end{equation}\)

The single shot probability of success is 25% and given a 90% probability requirement it would take 88 fuse wick barrel bombs which appears to be an unrealistic scenario where Russian bombs would be mixed in. We know that the Syrians have developed or increased their capability by adding impact fuses to account for the timing errors of the fuse wicks. Let's assume that the barrel bombs contain impact fuses but do not contain fins that are able to stabilize the bombs for a successful impact on the fuse. The single-shot probability of these barrel bombs is 12%. This means that the Syrian military would have to drop nearly 187 barrel bombs to obtain 11 explosions. This large number of barrel bombs also does not appear realistic, as the Syrian military uses no or very small fins in the middle of the bomb.    

Figure 15 Aleppo Bomb Impact Points From December 2013 Attacks  

The next calculation will consider the most advanced barrel bombs it is believed the Syrian military has. The single-shot bomb probability of success is 37.5%, which means it would require five bombs to achieve one successful explosion.  The total number of bombs that the Syrian military would have to drop to obtain 11 explosions is 55.  It is unknown how many helicopters were used in these bombings, but if the capacity of a HIP helicopter is considered, then each helicopter could only hold around 3-4 bombs of 1000lb size.  A single video was found of a very large barrel bomb that appeared on the internet December 21, 2013.  It is believed this barrel bomb could have been used in the Aleppo attacks. Figure 16 shows this barrel bomb and it specifically shows that this bomb has an explosive yield of 1500-2000lbs.

Figure 16 Large barrel bombs filmed and posted online on Dec 21, 2013

However, if the military has mastered the fuel air explosive dispersal technique, then this bomb would have a yield of 6 tonne TNT equivalent weight.  The damage on the ground strongly supports that very large bombs were used to generate the amount of damage that is seen in many of the videos.  Another key observation of the barrel bomb is the fins are on the back of the bomb, and not in the middle. It appears that this bomb concept has a good chance to be aligned and stable to allow for a lethal fuse impact. A closer look at the barrel bomb from the December 21 attack is shown in figure 17.

Figure 17 Close up of the barrel bomb from video

Summary

It is unknown how many bombs in Aleppo were DIY barrel bombs and how many were Russian conventional bombs. If we estimate that there were three helicopters used in this campaign and each helicopter contained three 2000lb barrel bombs, then based on each helicopter performing two separate missions, an estimate can be made as to whether it's feasible that all the explosions in Aleppo were from barrel bombs.  The calculations show that each day, there were 18 barrel bombs dropped with only 3.6 explosions per day. After three days of bombing, the total number of barrel bombs dropped is 54 which correlates with the 55 barrel bombs required that must be deployed to achieve 11 explosions. It appears possible that all the attacks in Aleppo could have been DIY barrel bombs because these attacks occurred over 3-4 days, as it is clearly possible that 55 barrel bombs could have been dropped with their new and enhanced designs over this period of time. The video analysis of the overall damage strongly supports that very large barrel bombs were used.