Nuclear Friday: Not Quite Ballistic

Two weeks ago The Board promised an investigation of the warhead maneuver capabilities of the RS-24 Yars (NATO designation SS-29) missile. Russia has promised that this missile will be able to defeat any defensive capabilities that are likely to become available within the next twenty years.

As I explained, no currently deployed defensive system is claimed to be able to counter any but the most crudely designed intercontinental-range threat. But once deployed, systems like Aegis and Aegis Ashore, along with second layer systems like THAAD and PAC-3, can be gradually upgraded. This may be the beginning of an arms race, a decades long competition between defensive systems and upgraded offensive weapons.

To understand this potential conflict, we need to take a look at the capabilities of the Yars ICBM and attempt to understand how its penetration capabilities work. My analysis will be a bit speculative due to secrecy around the Yars project, but I think that with a bit of investigation into the history of ICBM development I can make some good guesses about how the RS-24 might work.

Yars is Not quite ballistic.

nbyars
Trump wished we had something like this for his parade.

So what is a ballistic missile anyway?

“Ballistic” is derived from a Greek word that means “thrown-like”. It’s related to the English words “ball” and “bowling”. Ballistics is the study of the paths of objects affected solely by gravity and aerodynamic drag. A thrown ball or a cannonball are simple examples of ballistic objects. They are given an initial velocity and then take a path largely influenced by gravity, though aerodynamic forces become much more important at high speeds.

Ballistic trajectories are parabolic at low speeds and short distances. As ranges of artillery became longer people figured out the need to treat the path as elliptical rather than parabolic and also to take account of the rotation of the earth. All of these considerations are part of ballistics.

Things are a little bit more complicated in the case of rockets. A shell is not accelerated once it leaves the barrel, but rocket motors provide thrust over a longer period of time. A ballistic missile is simply one that makes no adjustments to its own course after its rocket engines have been completely exhausted. The German V-2 was the world’s first strategic ballistic missile.

Aiming such weapons is a technical challenge. Rocket motors have to be designed very consistently and the missile does need a complex guidance system to orient itself with the correct angle and speed for the warhead to reach the target without guidance. The V-2 performed these tasks passably well given the limitations of its technology, but it would be a while before an effective intercontinental range missile was possible. The Soviet R-7 was the world’s first ICBM, making its debut by launching Sputnik in 1957.

This began the classic age of large single-warhead intercontinental ballistic missiles. It did not last very long as by the early seventies long range missiles made their first steps to becoming not quite so ballistic.

Let’s get on the MIRV Bus:

Early thermonuclear devices were bulky and heavy, these are the classic H-bombs of the 50s and 60s. But advances in US bomb design would replace these monsters measured in megatons with compact devices measured in a few hundred kilotons. Multiple smaller devices can more reliably destroy targets in a given area than one much larger bomb can. Remember, the point of nuclear warfare is to obliterate the ability to make war. Machines and infrastructure are the most important targets.

I’m glossing over a lot of strategy and arms control debates here, but what happened is that very quickly most long range weapons of every major nuclear armed power were either converted to or replaced with MIRVs, Multiple Independently-targeted Reentry Vehicles by the mid seventies.

MIRV’s were the first step away from the classic ballistic missile concept. Warheads are packed into a final stage called a “bus” which has targeting information and a small amount of thrust capacity to orient itself in different directions and to give a little bit of push to each of the warheads in the bus.

nbmx
Warhead configuration in MX missile

What I find interesting is that the Yars has almost as much lift capacity as the now decommissioned American MX, yet it carries fewer warheads. I believe this is a hint to how Yars works and I will get to that in a moment. But remember, Yars has fewer and smaller warheads than one might expect.

nbmxkwaj
MIRV test at Kwajalein, the most targeted place on Earth.

Slipping Past Defenses:

In the sixties and seventies it seemed that Anti-Ballistic Missile systems might become a major part of every armed power’s strategic capabilities. There are three ways to sneak past a seventies style ABM system. Speed, spoofing, and maneuver. The US focused mostly on making faster warheads that would be harder to track and intercept. The sleek shape you saw in the picture of the MX warheads above was developed in the late sixties and became the standard for most US missiles by the mid seventies. These warheads had almost no heat shielding, but the warhead design was robust enough to survive the heat of reentry. US warheads before this time and almost all Soviet missiles until the end of the Cold War were covered in a blunt heat shield to protect the warhead. This made for easier interception by an ABM system. The Soviet Union focused on putting decoys and other spoofing devices on their missiles to confuse US ABM systems.

Because of the ABM limitations in the SALT I Treaty, defense penetration systems turned out not to be as strategically relevant as predicted, and when the US dismantled its only nuclear armed ABM system soon after it was completed, the Soviets stopped deploying decoys. However, the Soviet Union retained their Galosh ABM and the UK responded with the Chevaline decoy and spoof system for their submarine launched missiles. This was the most extensively deployed spoof system of the Cold War and was in service until 1996 when the last sub carrying them was replaced with a vessel carrying Trident D5.

While the sleek warheads of the US seemed fast enough to defeat Galosh, the US continued research into maneuverable reentry warheads. In 1978 the NATO board and German government asked for a counter to the Soviet SS-20 IRBM. Pershing II was the answer. It had a much lower yield than Pershing I, but it had the range to reach command and control targets near Moscow, and it could maneuver its warhead. It was the first maneuverable warhead with terminal guidance of its type. Read the link here for a cool picture and description. The Pershing II could turn sideways and slow down to move in a flatter arc so that longer range Galosh interceptors would miss it, and then tip into a steep descent while taking four radar “pictures” to make small adjustments to its course with low power thrusters connected to terrain contour matching system.

I’m pretty sure the capabilities of the Pershing II hold a clue to how Yars works. The Soviets hated and admired that missile. Yars works differently and it has a much longer range, but I’m pretty sure that Pershing II is a clue to the secrets of Yars.

So how does The Board think Yars work?

The secret is in the stats. You can’t see it in the videos I posted two weeks ago, and it’s not in any pictures, but the good folks at CSIS have the stats. The bus, or post-boost vehicle as they call it, is 2.7 meters long. That’s nine feet y’all. Doesn’t sound like much but that’s huge. I showed you the MX warheads, and the MX had just a little more thrust than the Yars, but take a look at this picture of the MX bus:

nbbusmx

That ain’t no 2.7 meters, 1.5 at the most. The reason Yars has just three small warheads is because its bus is so huge. Keep in mind that the bus isn’t released until the missile is outside the atmosphere. A giant bus with three tiny warheads could have enough power to change course several times and spread warheads over a larger area than earlier MIRV missiles. Russia Today boasts of the maneuverability of the warheads. I think it’s the bus that’s doing the magic here. The warheads themselves probably can make only one Pershing II style maneuver and are guided by GLONASS receivers, which is kind of an alternative Russian GPS. Keep in mind that because Yars warheads move faster and are descending from higher than Pershing II ever did, they may be able of making different kinds of maneuvers in different layers of the atmosphere even if they can make only one move.

Overall, I think the maneuverability of that huge bus on Yars is its primary strength

This is a damn good missile. Well played!

 

 

 

 

 

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