REPORT : Mortar Attacks from Gaza : big problem for Israeli Iron Dome

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The Iron Dome anti-missile defense system is without a doubt the champion of Israel’s current conflict with Gaza.

Without it, the hundreds of missiles fired by Hamas into Israel day after day would have likely caused many deaths, and severe damage.

Israel’s air defenses that were busy in recent weeks repelling rocket attacks from Syria and the Gaza strip succeeded yesterday, for the first time, to defeat mortar attacks that targeted border villages.

Among the targets intercepted yesterday by the improved version of the Iron Dome were dozens of rockets and, for the first time, mortar bombs (likely 81mm) fired by the Islamic Jihad terror organization.

As a short-range weapon with typical high ballistic trajectory, the mortar’s accuracy and lethality make it a practical and dangerous weapon which is challenging to locate and defeat. That’s what makes the Iron Dome’s achievement remarkable.

Iron Dome relies on a radar that detects ballistic threats such as rockets, artillery projectiles, and mortar shells, as they appear over the radar horizon.

The radar tracks each threat, plotting the launch and projected impact point for each projectile, to enable the battle management system to determine which of them is a potential risk – those who would hit populated areas would be prioritized for intercept, over others that are projected to hit open spaces.

The firing event included several mortars firing a coordinated and condensed salvo of mortar bombs, aimed to achieve full surprise and maximum damage on the Israeli side.
A mortar team from the Palestinian Islamic Jihad prepares an 81mm mortar for firing. Note the fire observer in the background.

When the system engages rockets with a flight time of tens, even hundreds of seconds, it has enough time to process all that data and optimize the intercepts. But when facing mortars with a short flight time of 10-30 seconds, time is the most critical factor for intercept.

In the four years since operation ‘Pillar of Defense’, the last conflict that erupted between Israel and the Palestinians in Gaza, Iron Dome went through several spiral developmental cycles to expand the system’s capabilities against new threats, such as unmanned aerial vehicles and mortars.

Yesterday, the system demonstrated for the first time, in combat conditions, its capacity to defeat a barrage of mortar bombs.

Existing C-RAM assets cannot intercept mortar bombs, as they mostly provide an alert enabling the people at risk to take cover. Directed energy defensive approaches to C-RAM, based on high-energy lasers have yet to mature to achieve readiness levels for operational use, while other Short-Range Air Defenses (SHORAD) that are optimized against aircraft and air-launched precision-guided weapons lack the quick response to react and engage such ground-launched threats.

Among the improvements introduced with the Iron Dome were new surveillance modes for the radar, improving the detection and tracking speed, accelerated battle management processing addressing counter-mortar capabilities, and adaptation in the Tamir interceptor missiles to address these specific threats.

All these improvements enable the system to safely destroy most of the bombs before they hit their targets.

What performance characteristics make a rocket defense effective?

To successfully intercept an artillery rocket of the type Hamas has been firing, an Iron Dome interceptor must destroy the warhead on the front end of the rocket.

If the Iron Dome interceptor instead hits the back end of the target rocket, it will merely damage the expended rocket motor tube, basically an empty pipe, and have essentially no effect on the outcome of the engagement.

The pieces of the rocket will still fall in the defended area; the warhead will almost certainly go on to the ground and explode.

Destroying an artillery rocket warhead is a considerably more demanding mission than damaging other parts of the targeted rocket—or, in the analagous situation of aircraft defense, successfully damaging an airplane, causing the failure of its mission.

Analysis of photographs of contrails left by Iron Dome interceptor missiles can show whether or not an attempted rocket intercept could have been successful. Such analysis focuses on two connected facts:

To have a realistic chance of destroying an artillery rocket’s warhead, an Iron Dome interceptor must approach the rocket from the front—in fact, almost directly head-on. And for all practical purposes, an Iron Dome interceptor has no chance of destroying the warhead if the interceptor engages the rocket from the side or from the back.

Photographs of Iron Dome contrails indicate that most of the system’s interceptors have either been chasing Hamas rockets from behind or engaging those rockets from the side. In both such cases, geometry and the speed of the interceptors and rockets make it extremely unlikely the interceptor will destroy the rocket’s warhead.

How an Iron Dome interceptor works. To understand why the Iron Dome interceptor must approach the artillery rocket from the front to be effective, it is necessary to understand the basics of how an Iron Dome interceptor is meant to function.

Figure 1 illustrates a theoretical front-on engagement by an Iron Dome interceptor against a Grad artillery rocket, a weapon initially produced by the Soviet Union in the 1960s, subsequently manufactured by many other countries, and now readily available to Hamas.

Figure 1. An Iron Dome interceptor engages a rocket in the proper orientation. The blue dashed line emanating from the forward section of the interceptor depicts the line-of-sight of its laser fuse.

The blue dashed line emanating from the forward section of the Iron Dome interceptor depicts the line-of-sight of its “laser fuse,” which creates a beam of light that reflects off the front-end of a targeted artillery rocket.

Via its control system, the interceptor can then determine when the target rocket is in the process of passing the interceptor.

The warhead in the Iron Dome interceptor is placed well behind the fuse assembly, a distance of roughly 3 feet from the laser-fuse aperture.

This arrangement gives the fuse enough time to determine where the front of the target-rocket is, to estimate how long it will take for the front of the artillery rocket to pass parallel to the artillery rocket’s warhead, and to detonate the Iron Dome warhead at the moment when it is in position to cause the rocket’s warhead also to explode.

The timing of this sequence of events is critical to performance.

The Iron Dome interceptor must account not only for the location of the target-rocket’s warhead, but also for the high crossing speed of the Iron Dome interceptor and the artillery rocket; for any off-parallel orientation of the Iron Dome interceptor relative to the artillery rocket; for the distance between the interceptor and rocket when the interceptor’s explosive warhead goes off; and for the speed of the shrapnel fragments shooting from the warhead.

Figure 2 shows how the fragments from the Iron Dome warhead would move, under the assumption that the crossing speed of the Iron Dome interceptor and artillery rocket—that is, their speed relative to one another—is about 1,200 meters per second.

Figure 2. Deciding when to explode: A conceptual diagram showing, via the blue arrow, the correct orientation if an Iron Dome interceptor warhead is to destroy a target rocket warhead.

The explosive in the Iron Dome warhead projects fragments at about 2,100 meters per second, perpendicular to the direction the interceptor is traveling. According to standard physics calculations (suggested by the red and yellow vector diagram at the lower right of the figure), the net direction of the cloud of fragments, as experienced by a theoretical observer sitting on the artillery rocket, is shown by the pale blue arrow passing through both the Iron Dome warhead and the artillery rocket’s warhead.

Figure 3 provides a slightly more vivid and detailed view of the outcome, if an Iron Dome interceptor works as intended. There is, however, only a limited range of possible outcomes that provide a high likelihood of success. Beyond that range, the possibility of success diminishes drastically.

Figure 3. A slightly more detailed view of the outcome, if an Iron Dome interceptor works as intended, spraying fragments at high speed into a rocket warhead, causing it to explode.

The many ways that Iron Dome can miss.

Because of the uncertainties in the exact crossing speed and geometry of two high-speed missiles, even a perfectly operating Iron Dome fuse may fail to place lethal fragments onto an artillery rocket’s warhead. In addition, unless the distance between the Iron Dome warhead and the warhead of an artillery rocket is small (roughly a meter or so), there will be a greatly diminished chance that a fragment from the Iron Dome warhead will hit, penetrate, and cause the detonation of the artillery-rocket warhead.

So a front-on engagement does not guarantee that an Iron Dome interceptor will destroy the warhead on the artillery rocket.

A front-on engagement geometry merely indicates that an Iron Dome interceptor has a greater-than-zero chance of destroying the target-artillery rocket warhead.

The consequences of a failure in fuse timing—in what was almost certainly an engagement between an Iron Dome interceptor and the artillery rocket—are shown in Figure 4 and Figure 4A.

Figure 4. A view of damage apparently caused by the detonation of the warhead of this rocket when it hit ground.

 

The photo in Figure 4A shows the magnified front-end of the rocket; holes can be seen in the expended and empty rocket motor casing immediately behind the warhead.

In this case, it is nearly certain that the artillery rocket was engaged by an Iron Dome interceptor properly approaching the artillery rocket, front-on.

Unfortunately, it seems the timing commands from the fuse resulted in fragments from the exploding Iron Dome warhead hitting the artillery rocket after the warhead had passed.

The relatively low density of holes in the artillery rocket’s after-body suggests that the encounter also had a relatively high miss distance—possibly several meters.

And as can be seen in Figure 4, there is significant damage in the area where the rocket fell—damage almost certainly caused by the detonation of the rocket’s small warhead when it hit the ground.

This photograph illustrates that even when the Iron Dome interceptor is in a proper front-on trajectory, it can still fail to destroy the warhead of a target-artillery rocket.

Figures 5, 6, 7, and 8 are detailed diagrams that indicate how an Iron Dome interceptor would perform if it engaged an artillery rocket from directions other than head-on.

They show why the kill rate for an Iron Dome interceptor will be very low when the interceptor does not attack its target almost directly head on.

Figure 4A. Holes in an empty rocket motor casing suggest that an Iron Dome interceptor warhead exploded too late to detonate the target rocket warhead in the air.

As Figure 5 shows, even a moderately skewed approach to the targeted rocket will result in a drastically reduced chance that fragments from an Iron Dome warhead could be sprayed onto the rocket’s warhead.

Such small but crucial off-frontal errors could result from faults in the master guidance and control system of the Iron Dome interceptor.

Figures 6, 7, and 8 show interceptor engagements that approach the targeted artillery rocket from the side or from the back.

Figure 5. This vector diagram shows how a skewed frontal approach would tend to spread fragments from an Iron Dome interceptor warhead in directions unlikely to contact or explode a target rocket warhead. (Vector diagram speeds in feet per second.)

Figure 6. This vector diagram of an Iron Dome interceptor attacking a Grad rocket from the side shows how unlikely it would be for fragments from the interceptor warhead to hit the rocket warhead.

Figure 7. A vector diagram of a different sidelong approach, showing, again, that the spread of fragments from the Iron Dome interceptor would be unlikely to strike the warhead area of the rockets.

Careful inspection of the geometry of the fuse sensing beam and the spray pattern of the fragments from an Iron Dome warhead reveals two very serious problems with these kinds of engagements:

First, even if the fuse detects the artillery rocket in these angles of approach, it has no way of determining where the warhead is on the rocket.

Second, even if the fuse detonates the Iron Dome warhead, by chance, at a time when fragments might be sprayed in the direction of the rocket warhead, in almost all circumstances the result will be a very low density of fragments arriving at the artillery rocket warhead location.

Given the small number of fragments that can be dispersed by the Iron Dome warhead, this translates into a very high chance that no fragment will hit the warhead.

Making a successful interception even more problematic, the projected target area of the rocket warhead is very small, viewed from the front or back, rather than from the side.

Also, when an Iron Dome interceptor approaches from these side and rear angles, fragments from its warhead are very likely to hit the metal surfaces of a target rocket at low grazing angles, with fragments tending to bounce off the shell of the rocket body or warhead casing.

In sum, then, for engagement geometries that are not front-on, the probability that an Iron Dome interceptor will destroy the warhead of an engaged target-artillery rocket will be, for all practical purposes, nearly zero.

Understanding Iron Dome contrails.

If artillery rockets are fired at their maximum range, they can be expected to fall at angles of 60 to 65 degrees relative to horizontal in their descent to a target; they will fall at angles well above 65 degrees when fired at less than maximum range.

The very steep descent of artillery rockets is important to keep in mind when attempting to visualize what is happening when viewing the photographs that show only the smoke contrails of Iron Dome interceptors attempting to engage artillery rockets.

When Iron Dome interceptors explode in the sky, but have contrails showing they have crossed the expected rocket trajectory in a side-on geometry or chased the artillery rocket from behind, it can be said, with a high degree of certainty, that no intercept could have occurred—assuming of course, an artillery rocket was even being engaged.

Figures 9, 10, and 11 are photographs taken during the artillery rocket attacks in November 2012.

Figure 8. An Iron Dome interceptor attacking a rocket from behind would have a low probability of spraying fragements into the rocket warhead. (Vector diagram speeds in feet per second.)Figure 9. A photo from November 2012 shows Iron Dome interceptor contrails that suggest ineffective sidelong or rear approaches to the target rocket.Figure 10. Another 2012 photo suggesting ineffective, non-frontal attacks by Iron Dome interceptors.They show contrails in the sky that indicate Iron Dome interceptors were attempting to engage target-artillery rockets from behind or from the side.

The geometries of the engagement are easily established; the artillery rockets are falling at high elevation angles relative to the ground, and the contrails show Iron Dome interceptors clearly approaching from above or sidelong to any reasonable estimate of a rocket’s descent path.

The photographs in Figures 12 and 13 show intercept attempts in July 2014 that are nearly side-on, and hence, have essentially a zero chance of destroying target rockets, if they are present.

Figure 11. More apparently ineffective Iron Dome attacks.Figure 12. Two intercept attempts in July 2014 that suggest Iron Dome interceptors attacked in a sidelong orientation unlikely to destroy the target rockets.

Observations colleagues and I made in November 2012 found no more than 20 percent of Iron Dome contrails indicating an engagement geometry that was front-on to the targeted rocket.

At that time we estimated the probability of destroying a SCUD warhead in a front-on engagement might be between 30 and 60 percent, meaning that if all other engagements affectively resulted in a zero probability of interception, then the overall intercept rate would be between 6 and 12 percent.

Given that less than 20 percent of the engagements we were able to get data on were actually front-on, our best estimate was that the intercept performance of Iron Dome was likely 5 percent or less.

Daytime visual photographs of Iron Dome debris clouds can show, in many cases, the evidence of a successful intercept, i.e., the destruction of the targeted artillery rocket warhead.

Since the Israeli government has been claiming a very high intercept rate—near 90 percent—it should be expected that visual evidence of hits would be common.

But we cacn found only one example of photographic evidence in which it is clear that such a head-on success occurred.

Figure 14 shows photographic evidence of the destruction of a rocket warhead by an Iron Dome interceptor. In this photograph, the Iron Dome missile is clearly on a trajectory that engages the falling artillery rocket head-on.

Figure 13. A contrail photo that suggests another sidelong approach by an Iron Dome interceptor. The large white arrows at the top and bottom of the photograph show the relative directions of the rising Iron Dome interceptor and the falling artillery rocket.

An inspection of the debris cloud shows that it is asymmetrical—indicating that two explosions have occurred nearly simultaneously.

This debris cloud formation is essentially the result of fragments from the Iron Dome warhead hitting the warhead of the artillery rocket and detonating it.

The explosive process that led to this observable debris cloud took less than one half of a millisecond, or essentially instantaneously from the perspective of an observer or with regard to the frame rate of a standard video camera, which would take a picture roughly every 30 to 40 milliseconds.

This photograph is the only successful engagement I have found during very extensive searches of voluminous photographic and video evidence of Iron Dome interceptor activity.

It could be argued that the details that can be seen in this photograph are sufficiently subtle that they might not be observable in all engagements.

This argument is probably correct.

All the same, it seems extremely unlikely that the Iron Dome system would be intercepting 90 percent of the artillery rockets it engaged, but result in only one photo among hundreds as evidence of a successful intercept.

It is absolutely clear:

There is no evidence in the public record to show that Iron Dome is performing at an intercept rate of nearly 90 percent.

Figure 14. What an Iron Dome hit looks like in the sky.Figure 15. Published warning times for artillery rockets of varying ranges attacking Israel from the Gaza Strip.

Here are 14 facts you may not know about the Iron Dome system:

1. Iron Dome is the world’s only dual mission system that provides an effective defense solution for countering rockets, artillery and mortars as well as aircraft, helicopters, UAVs and PGMs. It can detect and intercept rockets and artillery shells headed for population centers within a 43.4-mile (70-kilometer) range.

2. A toy car sold by Toys R Us inspired developers in building the Iron Dome. One of the leading developers recently told Hayadan , the Technion-Israel Institute of Technology’s magazine, that due to schedule and budget constraints, some of the missile components were taken from a toy car he had bought for his son at a local Toys R Us store.

3. The Iron Dome system was designed to be operated easily by an average woman soldier 160 centimeters in height and 48 kilograms in weight, according to Hayadan.

4. Iron Dome can handle multiple threats simultaneously and efficiently. The system only intercepts an incoming rocket if it is deemed a critical threat. A unique interceptor with a special warhead detonates any target in the air within seconds.

5. The cost of launching a missile from the Iron Dome at a threatening rocket has been reported to cost anywhere from $20,000 to $100,000. The rockets fired by terror groups at Israel are estimated to cost between a few hundred to a few thousand dollars.

An Iron Dome defense system missile intercepts a rocket fired at Jerusalem from Gaza last Thursday. Photo by Flash90.

An Iron Dome defense system missile intercepts a rocket fired at Jerusalem from Gaza last Thursday. Photo by Flash90.

6. It took less than four years to develop the Iron Dome system from an idea to the drawing board to combat readiness. In 2007, a year after the Second Lebanon War, then Defense Minister Amir Peretz chose the Iron Dome to be developed as Israel’s defensive answer. In March 2011, the Iron Dome was declared operational. In April that year, the advanced missile interception system successfully shot down its first Grad rockets fired by Hamas from the Gaza Strip at Israel.

7. When the Iron Dome system was chosen to be developed into Israel’s defensive solution against short-range rockets, many critics predicted it would never work. One of the project leaders said: “We knew that eventually our critics would get our response, which came in April when the first operational deployment destroyed eight out of eight rockets aimed at Ashkelon and Beersheba.” Indeed, the strange-looking battery contraption was hailed as the hero of Operation Pillar of Defense. Today, operators of the system report a best-in-the-world 90 percent success rate.

8. Israeli contractor Rafael Advanced Defense Systems and Israeli company mPrest Systems designed and programmed the core of the Iron Dome management system.

9. Iron Dome operates in all weather conditions, including low clouds, rain, dust storms or fog.

10. Aesthetics were important to the designers and developers of the system. One developer told Hayadan: “I wanted the battery system to look super-modern and threatening, because it was obvious that within an hour of its use it would be featured on the likes of CNN and Al-Jazeera.”

11. During Iron Dome’s deployment, the IDF realized that it is also effective against aircraft up to an altitude of 32,800 feet (10,000 meters), according to a report by the Hebrew-language Flightglobal magazine.

12. Iron Dome is jointly funded by Israel and the US. Israel provided initial funding and development, which allowed for the deployment of the first two Iron Dome systems. In 2010, the US government contributed $205 million toward its development. In 2011, Haaretz published a report stating that Israel would invest $1 billion in Iron Dome batteries. In 2012, the American government approved another $70 million package for further R&D. In 2014, the US Senate Appropriations defense subcommittee agreed to provide $351 million for Israel to secure the Iron Dome system.

13. Iron Dome is the first of a planned three-part defense system – Iron Dome, Magic Wand, Arrow — that could be operational by the end of the year, according to Rafael. Magic Wand is designed to intercept projectiles with ranges between 70 kilometers (45 miles) and 300 kilometers (180 miles), like the large arsenal of Hezbollah rockets in Lebanon. The Arrow system is for longer-range threats from Iran. The three components will complete what Israel calls its “multilayer missile defense.”

14. The developers of Iron Dome — from Rafael and the Ministry of Defense Administration for the Development of Weapons and Technological Infrastructure – won the prestigious 2012 Israel Defense Prize for their technological breakthroughs in developing the groundbreaking system.

 

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