U.S. Hypersonic Strategies Part 3

You played the board game, which demonstrated an odd fact. By making random sideways movements, the red checker “evaded”  your black checker, which you attempted to guide with your purposeful hand. How could the brainless strategy of the red checker outwit your intelligent black checker?

There will be no math in this post, which continues to emphasize accessibility to the audience. It must remain rough, since the performance figures for Russian and Chinese hypersonic vehicles are not available to open source.  I’m hesitant to introduce the concepts of kinetic and potential energy. There are some interesting twists that go a little beyond freshman physics.

The game imitates the situation of the First Gulf War, when Patriot PAC-2 missiles failed to intercept — or failed to destroy, Iraqi Scud missiles that spontaneously disintegrated into pieces, tumbling through the atmosphere in complex paths. It was  noticed that  interception and destruction are practical distinctions, with much damage caused by still-incendiary boosters landing on soft targets, even if the warheads did not detonate.

The above is offered partly as a lesson in confusion. It includes issues that are barely related to intercepting hypersonic vehicles. Confusion typifies the subject, because missile defense is not one problem. It consists of at least four problem-regimes, some separated by mere seconds, as the aggressor missile transitions from one regime to the next. Each requires a different counter-weapon. These regimes were understood to be constants of the problem, because in the formative period of ABM theory,  only two kinds of missiles were subject to strategic consideration: ballistic, and cruise. The ability of subsonic cruise missiles to evade, even in the current day, could have been a show-stopper, but imagination has no limits. Hence, Star Wars.

The problem  was based on a missile that is thrown like a stone. After a few minutes of powered flight, the missile is going about 15000 miles/hour, arching over the atmosphere with no opportunity to change direction. Above the atmosphere, the warhead separates from the booster, and proceeds like a thrown stone to the target.  At the cost of some additional weight and complexity, the simple warhead can be replaced by a steerable “bus”, which makes modest adjustments in speed and direction to drop off its passengers, MIRVs, (multiple independently targetable reentry vehicles).

Each MIRV is a thin cone, designed to withstand the incredible heat of reentry, with small rockets, “thrusters”, to keep the pointy end forward.  In fact, a MIRV vehicle could be steered a little when it descends far enough into the atmosphere, but it wasn’t thought worthwhile, as it would add weight. As early as 1963, a  U.S.  experiment with a surplus Thor warhead showed that steering is quite feasible.

The ability of a reentry vehicle to convert forward speed into sideways movement is critical to the problem. When the MIRVs, or MIRVs attached to the bus, are above the atmosphere, they proceed like stones. When they enter the upper atmosphere, the path becomes a little complicated. This is why it’s preferable to intercept a warhead in the vacuum of space.

You may note with pride that the ancestral hypersonic vehicle, the source of current frustration, was an American innovation, the Pershing II. It was designed to fly low, in a “flat trajectory”, with a single programmed “jink” as it entered the atmosphere.  Modern hypersonic vehicles greatly extend this idea, replacing predictable flight with the unpredictable.

The  problem of ballistic flight resembles the bull and the matador. The bull has more energy, the matador has more agility. The matador dodges the bull, whereas the interceptor gets in front of the bull, but otherwise, the analogy is strong. But what if the bull has the agility of the matador, as well as its speed?

The buzzword “hypersonic” is a detour to understanding. Speed is an issue in some cases, such as anti-ship, but a hypersonic missile  is slower than an ICBM. To note that a hypersonic missile “flies through the atmosphere” doesn’t quite nail it. So let’s nail.

The antimissile, as it exists today, launches a payload, the Exoatmospheric Kill Vehicle.  The EKV does not “fly in air” With very small, precise rockets, it places itself with extreme precision in the path of the red checker. Like  the matador, the EKV has no forward energy of its own, but  agility of lateral movement. In the vacuum of space, the MIRV “red checker” proceeds unalterably to its doom.

A strategic hypersonic warhead/missile has energy of forward motion. Flying  in the atmosphere, it is designed to have “lift”.  By banking, it converts a significant part of forward energy into sideways motion.  An airplane does this more efficiently, but the hypersonic missile is going very fast, so it has much more to start with. This forward energy is like a “battery” that the missile can drain to make changes in direction. It obtains a large initial velocity from a  booster stage. The Russian Avangard has an integral scramjet that allows it to exceed the performance of the Tsiolkovsky rocket equation.

The rocket equation is cruel to the EKV.  Take a look at a picture of one of the EKV designs. There are other pictures around the web you may wish to examine. The weight of the EKV is variously stated to be 120 or 140 pounds. It appears to have two propellant tanks  and two oxidizer tanks. The amount is not critical to the argument, so assume  50 pounds total, of which 15 is propellant.  This is not a lot of energy; it’s much less than a car tank of gas. It can’t match the amount of forward energy the adversary can convert into sideways motion.

As originally conceived, it didn’t have to. The EKV has a very high forward velocity/energy, provided by the Ground Based Interceptor. But it cannot convert any of that forward energy into lateral movement. It is specialized to operate in space, and is protected by a shroud until it gets there. Lateral movement must come from the amount of fuel you could fit in a Jerry can. And unlike the Avangard ramjet, it cannot escape the rocket equation.

Takeaway: The bright child of U.S. antimissile efforts, and hypersonic adversary missiles, are so different, never the twain shall meet.

Next: Conceptual Thinking.



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