Thread by @FRHoffmann1

It seems that the Biden Administration has made the decision to provide Ukraine with MGM-140 ATACMS maneuvering missiles. In this thread, I want to explore the warhead effects of ATACMS, specifically discussing overpressure, fragmentation, and penetration effects. 🧵👇 1/25

My analysis focuses on the ATACMS M57 unitary warhead variant, which appears to be one of the main variants in play for Ukraine. However, many aspects discussed also apply to the M48 variant. Towards the end, I also reference the M39A1 variant. 2/25

I view this thread as largely complementary to the works of others who have done an exceptional job at explaining the overall capability profile of ATACMS. I highly recommend checking out the excellent threads by @ColbyBadhwar and @dillonrpayton on this topic. 3/25

The M57 variant which has an official range of up to 300km and carries the 227kg WDU-18/B unitary warhead. Unclassified data suggests that the M57 variant has an accuracy of 9m CEP, meaning 50% of ATACMS will fall within a 9m radius of their designated target. 4/25

Note that CEP (circular error probable) constitutes a probabilistic method of accuracy evaluation. Real-life results may differ. In addition, ATACMS’ 9m CEP is the unclassified value provided by the DoD. The classified CEP may be lower (resulting in greater accuracy). 5/25

The WDU-18/B warhead consists of 98 kg explosive material and 128.8 hull material, including the fuse. The warhead uses DEXTER high explosive, which has a TNT equivalent of 1.05 and a detonation velocity of approximately 6630 m/s. 6/25

The M57-variant achieves most of its destructive power through diffraction loading where the peak overpressure of a shockwave, generated by the detonation of high explosives, is applied to several sides of the target nearly simultaneously to destroy it. 7/25

The extent of damage caused by a shockwave depends on its peak overpressure. Most reinforced concrete structures crumble at 10 pounds per square inch (psi) overpressure, while many other types of buildings and structures can be destroyed at lower levels. 8/25

The human body can withstand relatively high blast overpressure. For instance, lung damage typically occurs only at around 15 psi. Individuals who die from blasts typically do so because they are trapped inside a collapsing structure, rather than due to the shockwave itself. 9/25

The graph below illustrates the lethal radius of the M57 variant for various types of targets, categorized by their hardness. For 10 psi targets, the M57 ATACMS variant has a lethal radius of 18.1 m, which increases exponentially as the target hardness decreases. 10/25

Considering ATACMS' unclassified accuracy of 9m CEP, it boasts a single-shot kill-probability (SSKP) of 94% for 10 psi targets (not factoring in systemic error). When it comes to softer targets (1-4 psi), ATACMS' SSKP based on unclassified data approaches 100%. 11/25

If we assume that the actual CEP is 7m or 8m instead of 10m, which is a realistic consideration given the general overperformance of US equipment in this war thus far, the SSKP remains consistently above 97%, even for targets at 10 psi. 12/25

In addition to overpressure effects, the detonation of a warhead also leads to fragmentation effects. These effects can significantly expand the lethal radius. This is because the shockwave dissipates more rapidly than the fragments are slowed down by wind resistance. 13/25

Unlike the M30A1/M30A2 GMLRS-variant, whose warhead is loaded with preformed tungsten spheroid fragments, ATACMS’ unitary warhead is not pre-fragmented. The resulting fragmentation effects will therefore be uneven and vary strongly from one detonation to another. 14/25

For the sake of argument, let’s assume the 128.8 kg hull material of the WDU-18/B warhead splinters evenly into 7,500 fragments with an average weight of 17 grams and a cross-sectional area of 3 cm. In this case, the initial fragment velocity amounts to 1640 m/s. 15/25

Assuming that those fragments have a high drag coefficient (due to their non-aerodynamic shape) and considering the normal density of air as 1.2 kg/m^3, the fragments will decelerate to less than 600 m/s at a distance of 80 m, and less than 500 m/s at a distance of 100 m. 16/25

At 600 m/s, fragments that weigh 17 grams transfer a kinetic energy of ~2.9 kJ upon impact; at 500 m/s they transfer ~1.9 kJ. This can cause light to moderate damage against lightly armored vehicles and aircraft, but not enough to cause substantial damage (on average). 17/25

More importantly, as the average number of fragments striking a target decreases proportionally to 1/R^2, the likelihood of being hit by a sufficient number of fragments is relatively small if the detonation produces only a relatively small number of fragments. 18/25

If we assume the target presents 15 square meters of surface area to the warhead (e.g., a 5mx3m truck) and that the detonation produces 7,500 fragments, the target will be hit, on average, by 1.39 fragments at 80 m distance and by 0.9 fragments at 100 m distance. 19/25

This implies that the ATACMS M57 unitary warhead variant, like Storm Shadow/SCALP-EG and Taurus, is primarily designed for point effect purposes and has limited capability for creating area effects. To achieve area effects, the M39A1 variant is required. 20/25

The M39A1 variant is equipped with M74 submunitions that scatter over a wider area. Upon detonation, they create a dense and overlapping fragment cloud, which can cause significant damage to a broader range of targets due to the size, weight and velocities of the fragments. 21/25

As others have pointed out, this would be extremely useful for Ukraine, in particular to threaten and destroy aircraft hangars and airfields. If Ukraine does not receive the M39A1 variant immediately, they will hopefully have access to them in the near future. 22/25

Lastly, a quick note on ATACMS' hard-target penetrability. Penetrability is largely a function of warhead configuration, mass, and velocity. Considering that ATACMS has a very high terminal velocity (~ Mach 3), it can achieve some degree of hard-target penetration. 23/25

ATACMS is outmatched by Storm Shadow/SCALP-EG in terms of hard-target penetration, however, which are optimized to engage hardened and buried targets. ATACMS is more useful to engage softer targets of time-sensitive nature and will likely be used in that fashion. 24/25

In conclusion, it is widely understood that ATACMS alone will not win the war. However, it plays a crucial role in complementing Ukraine's long-range strike arsenal and buys the Western alliance time to search for a more sustainable long-term long-range strike solution. 25/25

Here is @ColbyBadhwar's thread I referenced above:

https://twitter.com/ColbyBadhwar/status/1703757651623162271

And here is @dillonrpayton's thread:

https://twitter.com/dillonrpayton/status/1703756138880381033

Lastly, a big thank you to @ColbyBadhwar and @John_A_Ridge for patiently answering all my questions. If you're not already following them, I highly recommend doing so.