Did the Indian Air Force strike the various structures at the madrasa in Balakot with lethality sufficient to have caused “heavy casualties”, as foreign secretary Vijay Gokhale told reporters on February 26?

Sections of the Indian media and of course BJP politicians believe it did and have even put a figure on the number of dead terrorists that ranges from 250 to 400. Pakistan has denied any damage or casualties and said the Indian payload landed on a nearby forest. On their part, international analysts have raised doubts about the Indian version based on their reading of pre- and post-airstrike satellite imagery of the madrasa.

While the truth is known to both the Indian and Pakistani governments, neither side appears keen to allow independent verification of its claims. The Pakistani military has prevented reporters from visiting the madrasa while the Indian government has also been circumspect about sharing imagery of the sort the US, Israeli and western air forces routinely release into the public domain.

In this vacuum, different people are resorting to different ways to settle the matter for themselves – including chest-thumping. In this clamour, there is now a debate among ammunition and aviation experts, who are trying to piece together what they know about the bombs the IAF dropped to figure out what might have happened on the ground.

Since World War II, missiles and their warheads have been designed to do things other than just be dropped and blow up. In the Balakot case, virtually the entire Indian media has reported that the IAF dropped 2,000-pound (lb) bombs over the madrassa. This claim, which has never been properly sourced, seems extremely unlikely based on post-airstrike satellite imagery.

It also reinforces the need for authentic, verifiable information about what happened in Balakot. However, with the governments’ silence and campaigns for the national elections gaining momentum in India, it is important to understand what is possible and why, and to keep from getting carried away.

According to media reports, the bombs were delivered using a guidance kit called SPICE, which can convert unguided bombs into guided ones. It is manufactured by Rafael Advanced Defence Systems, an Israeli company, and is used by the Israeli and Indian air forces.

The SPICE 2000, which can carry 2,000 lbs of bombs, is one of India’s most powerful (non-nuclear) air-to-surface weapons, depending on its configuration. And thanks to its precision guidance and long range, such weapons are often used as ‘bunker busters’: devices that can penetrate heavily fortified structures to blow them up from the inside.

At the same time, a bomb weighing 2,000 lbs (907 kg) can effect different kinds of damage on the ground, depending on its own specifications as well as those of the targets.

This forms the crux of the current debate, which takes off from sections of the media sharing higher resolution satellite images than were previously available of the Balakot madrasa after the IAF strike. The images show a clump of small buildings surrounded by a forest. Small dark smudges are visible on the roof of the main structure.

The confusion and uncertainty assailing the wider debate are relevant here. An official Indian statement claimed – before the images were released – that these buildings were a Jaish-e-Mohammad (JeM) training camp. Journalists who spoke to people living nearby say it is a madrasa and a school linked to the Jaish. Al Jazeera reported that the madrasa was run by the JeM and, according to Reuters, a signboard attesting to this was subsequently removed.

Whatever the purpose of the structure, it looks like a regular brick-and-mortar building. And many have claimed that the dark smudges are evidence of a SPICE bomb (or perhaps four SPICE bombs, since there are four smudges or holes) penetrating the roof’s outer shell to burrow in and kill everyone inside using explosives.

In the face of initial satellite images showing limited damage to the buildings, senior government officials had told reporters that the Pakistani army had been able to go back and put the roofs back on in two days, thus fooling the world that India hit nothing. But with the latest satellite imagery with its smudges on the roof, the briefing given to defence reports has changed. Now, the claim is not that the roofs were replaced but that the smudges/holes still visible on it are actually evidence of India having successfully struck its target.

The latest account of Indian “sources”, however, has been challenged by Western analysts.

George William Herbert, an expert on missile systems, tweeted on March 6 that a 2,000-lb non-penetrator warhead comprises 945 lbs of explosive filling and 1,055 lbs of metal casing. Assuming the filling is made either of tritonal (TNT + aluminium powder) or Composition B (TNT + RDX), the Gurney equation for a cylindrical casing indicates the explosion will set the metal – assumed to weigh 478.5 kg – off at 1.83-2.13 km/s. So if it went off inside a madrasa, the shrapnel would have obliterated the building.

Herbert continued on Twitter, “The thousand pounds of explosive becomes hot gas at over a thousand degrees kelvin, and that’s about 1,000 cubic meters of air equivalent. [This] will approximately double the pressure inside a typical three-story building around 25 meters [wide]” – further contributing to explosive damage.

As a result, the most popular claims that the SPICE 2000 dealt damage on the inside but not on the outside don’t hold up. Forget about Pakistani forces replacing the roof in two days. If a SPICE 2000 with a 2,000-lb bomb had hit the madrasa, they would have had to refill the crater, re-lay the foundation and rebuild the whole structure in two days.

However, Herbert told The Wire he wanted to make it clear that he does not know what actually happened, that he wasn’t proposing any particular theory and was simply clarifying the technical aspects.

Now, this analysis did assume that the warhead on the SPICE 2000 was a non-penetrator Mk 84 (which uses tritonal, Composition H6 or minol for the explosive filling). If it had been a penetrative weapon, most of the weapon’s mass would’ve been contained in the casing so that the weapon can smash through a strong outer layer first.

For example, the BLU-109 is another 2,000-lb bomb that can be used with SPICE guidance kits. As a bunker buster, it can penetrate up to six feet of reinforced concrete with a casing that weighs 634 kg, to deliver a 240-kg payload of tritonal. A BLU-116 weighs the same 874 kg but carries only 109 kg of tritonal filling to be able to penetrate over 10 feet of reinforced concrete.

As Angad Singh, an aviations expert, commented on Twitter, “Depending on effects required at the target (for example, fragmentation) the explosive filling in the bomb could be even less. So there is no hard and fast rule that a 2000-lb class bomb will wipe out half a hillside.”

He also noted that if India’s defence procurement was anything to go by, the SPICE units were likelier to be all-up rounds, where the bomb is already configured and attached to the guidance kit at the time of purchase. However, he told The Wire, “We have no good information on the exact bomb mated to the Indian SPICE munitions,” although it was “not an Mk 84”.

As a result, he said on Twitter, India’s “Spice 2000 [could all be] earmarked for high-value targets” and “that all but guarantees they have low-mass warheads”.

On March 8, the Indian Express quoted an unnamed “top” military officer as saying, “Each warhead used by the IAF to target buildings on the campus of the JeM madrasa at Balakot … had a net explosive quantity (NEQ) of only 70-80 kg of TNT.” This is further indication that a low-mass warhead was used – and it also indicates the kind of warhead that might have been used.

This is because, if the filling was made of a high explosive like tritonal, the Gurney equation poses a problem. The shrapnel from a BLU-109 would still be released at 1.3 km/s and from a BLU-116 at 0.8 km/s. If, say, an NEQ of 80 kg of TNT was used in the BLU-116 configuration, it would still release shrapnel at nearly 1 km/s, and have a range of 14 metres. The madrasa is likely to have received significant damage any which way.

These numbers also hold for all conventional explosives of other kinds – not just bunker busters – as long as they use tritonal, which has a relatively lower Gurney constant of 2.3, similar to TNT, and which have a similar casing-to-filling mass ratio.

Second: considering neither the IAF nor the Government of India have released any official statements about which warhead was used, the radius of possibilities becomes longer.

A second military officer reportedly told the Indian Express:

It is a precision weapon meant to hit specific targets but without any collateral damage. … This time the target was Balakot. If the target was Muzaffarabad instead, which is heavily inhabited and where no collateral damage would be acceptable, we would need to take out the people staying in a particular room without causing any damage to the adjacent room. We have the capacity to do that with this weapon.

Why the IAF wanted to use expensive ordnance that minimised the damage to buildings that were located far away from any population is not clear.

That said, one option that fits the bill is a fuel-air explosive (FAE), which – according to the Defence Research and Development Organisation (DRDO) – is “highly effective against soft targets like light vehicles, drop tank, trenches, bunkers and antitank mines”. They use fuels to consume oxygen from the air and burn at over 1,500º C for a long time. They are effective against targets enclosed in inaccessible niches like caves and tunnels.

As a result, the dark smudges in the images could be burn marks from the use of an FAE flown with a SPICE 250 kit – which means the total weight of the weapon was only 113 kg (250 lbs). This mass is close to an FAE developed by the DRDO that can carry 38 kg of propylene oxide and deal damage in a circle of radius 8 metres. If an NEQ of 70 kg of TNT was used, then each FAE could have carried 18-19 kg of propylene oxide, adjusted for the amount of physical damage dealt at 7-9 metres.

This is sufficient to have killed people inside a madrasa-sized structure, and the multiple dark smudges on the roof of the main structure could simply be signs of fire damage. However, there is the overpressure to deal with.

Herbert explained to The Wire that FAEs have a reaction detonation pressure determined by the materials used and how they mix with the air. This is called the Chapman-Jouget detonation pressure (PCJ). And if the FAE is detonated inside a structure, the fuel “tends to fill” large parts of the structure and pressurise it from the inside.

For a typical FAE, the PCJ can be hundreds of pound-force per sq. inch (PSI). A hundred PSI is equal to 6.8-times the atmospheric pressure (atm). This kind of pressure, Herbert said, “tends to break every wall apart very effectively” but does not throw the walls “very hard or throw fragments very far.”

For its part, the DRDO has estimated that the blast pressure of a rocket-delivered FAE is 0.8 kg-force/cm2 at 16 metres. This is a little less than the atmospheric pressure that regular buildings can withstand. Extrapolating the findings of one DRDO study, 18.5 kg of propylene oxide has a blast peak overpressure of 2.1-3.4 atm at about 8 metres from the canister. Even if multiple units were not fired, structural damage seems likely.

The satellite images also show burn marks of varying sizes, as well as a few craters. Col. Vinayak Bhat (retd.) reasoned in The Print that the smaller burn marks, found on the landscape surrounding the building, could have been human-made whereas the larger ones could have been the result of FAEs. Assuming they are contemporary, this suggests FAEs with a larger impact range could also have been used.

One possible way out, as Col. Bhat suggested, is that there were two waves of IAF fighters. The first carried FAEs used on the madrasa and against fleeing people. Then, a second wave carried high-explosive weapons to bomb the surroundings.

But while this seems to be able to explain some of the features of the satellite images, the theory does not square with the detailed briefings that reputed defence reporters like Indian Express‘s Sushant Singh received from the government, which spoke of only one group of four Mirage-2000s firing their precision-guided munitions from the Indian side of the Line of Control.

Angad Singh also told The Wire that he does not think an FAE was used – “certainly not the DRDO one, which as far as I am aware, is not in wide service yet.” He added that “the attack direction and profile seems to suggest SPICE 2000, not 1000 or 250.”

An FAE mated to a SPICE 2000 seems excessive: while it could explain the burn marks on the ground, it doesn’t explain what appears to be an erect, intact structure. If the kit had been mated with a high-explosive, then the unnamed military officer’s comment implies that the casing on the weapon was really heavy.

This in turn could mean one of two things. First, that the madrasa had a roof full of holes/smudges to begin with, and that they are not signs of damage.

Second, the madrasa was – or presumed to be – very heavily fortified. If the madrasa wasn’t fortified, it wouldn’t be standing. But if it was, there is no way to confirm.

So there we have it: multiple intersecting theories, led by a SPICE kit and low-mass warheads that may or may not have been FAEs, Mk 84s or something else – something the Government of India is keeping mum about. At the centre of all this stands the Ship of Theseus: a madrasa that journalists are being kept away from, a building that may or may not be fortified, which even may or may not be the same building it was before.

The article was originally published in The Wire.