click on the image for a larger version

The trajectories for the bus/warhead/decoy complex (in red) and a cloud of chuff released just before burnou (in yellow)t.

(With apologies to Langston Hughes.) The Missile Defense Agency (MDA) might paraphrase Langston Hughes to say to potential adversaries “Chuff your rocket like you ought to do but [please] don’t let your rocket chuff [us].” It turns out that the MDA requires any potential incoming missile to behave extraordinarily well. Any deviation from absolutely perfect behavior could throw the missile defense into a dizzy. The latest example of this is the 31 January 2010 failure of the Ground-Based NMD interceptor to hit its target. One of two contributing reasons for the failure was a “chuffing” of the LV-2’s solid propellant stages, which apparently changed what the Sea-Base X-band radar sees enough to prevent it from adequately assisting the interceptor close with the target. (Or, perhaps, help it discriminate between the simulated warhead and the officially sanctioned decoys.)

That, of course, needs some explanation. Chuffing is pretty much what it sounds like; a type of puffing. Solid propellant motors (like liquid propellant engines) can suffer from instabilities in the combustion of their fuel and oxidizer. Chuffing involves a sudden extinguishing of the flame burning inside the rocket with a consequent loss of pressure, perhaps down to a vacuum if this happens at high altitudes. Gases released from the heated inner surfaces then spontaneously catch fire and the process of burning and snuffing out starts all over again. This can happen as much as one hundred times a second and it doesn’t take much imagination to believe that there is a lot of variation the amount of aluminum particles coming out the end during this process (aluminum is added to many types of solid-propellant rocket motors to increase the burn temperature and hence thrust).

Here is my guess on how chuffing completely changes what the SBX sees: The LV-2 target vehicles consists of the first two stages of a Trident C-4 missile a bus with the simulated warhead and the officially approved decoys. In my hypothesis, the second stage experiences chuffing near the end of its burn time, releasing excessive amounts of aluminum (in unburned chunks of propellant that broke off during the trauma of cyclic pressure drops). This cloud propellant chunks containing aluminum is, naturally, released with a lower velocity than the warhead and decoy bus because the bus/warhead/decoy complex continues to be accelerated. This difference in velocity, however, does not need to be very large if the chuffing lasted all the way up to burnout.

It turns out, however, that the cloud, traveling on its lower trajectory, actually appears to move ahead of the warhead because it is also on a lower trajectory. I have simulated this as shown in the diagram at the top of this post and, assuming that the SBX is some 1000 km West of Vandenberg, it could actually shield the warhead/decoy complex during the important period when the bus released the various bits and pieces.

click on the image for a larger version

You would think that an X-band radar could discriminate between chunks of aluminum and a warhead. That is certainly what Lincoln Labs thought in 1999 when they gave a briefing on just this issue. ( Here is their graph showing all potential sources of confusion, both “natural”, meaning stuff unavoidably produced during target vehicle flight, intentional countermeasures. Note II.b, the right diagram in the middle row, which shows fuel debris cloud as an important factor.) A little later in the briefing, they show a graph of measured “length” of the objects and their mean radar cross section (the right most diagram). The box, almost totally obscured by the “deployment hardware” square, shows that they expect the debris from chuffing to have very low length while the RV (and empty stages trailing along behind) have very large lengths. The battle management computers and filters on the individual sensors—including the SBX—would use these differences to discriminate between targets and decoys (and chunks of unburnt fuel). In this flight test, however, they weren’t able to.

Whether or not this is going to be a fundamental flaw in the whole missile defense development program remains to be seen. But remember that this is just a result of a fully functional target vehicle behaving in an “acceptable” manner. Based on MDA’s history, we might expect them to put increased attention to making sure the target booster does not chuff. And if they do attempt to solve the more general problem of discriminating between the nuclear tipped warhead and small bits of unburnt rocket fuel, the chances are they will come up with a new algorithm that is good only for chuffing from Trident C-4 stages.

We haven’t even started to think about what an aggressor might do to make life difficult for a missile defense system. Ted Postol has been working on similar issues and will be discussing his results in a forthcoming article. From what I’ve seen of his article, it’s going to prove very interesting. As for today, I think we have a pretty good idea of why chuffing contributed to the failure of the January 2010 NMD flight test.