The so-called DF-21D is much in the news recently, mostly because it hasn’t shown up yet. It is reputed to be the anti-ship version of China’s short-range workhorse, the DF-21. (China uses some version or other of the DF-21 for short-range ballistic missiles, anti-satellite weapons, and ballistic missile defense.) I thought I’d start the analytical discussion of this virtual missile by making some simple calculations about what sort of transverse accelerations its terminal phase guidance and control systems are going to need.
The first point to make is that (unless it is using a nuclear warhead) it is going to need terminal guidance to fine tune the warhead’s trajectory as it reenters the Eearth’s atmosphere. This is true regardless of how well China needs the position of the target carrier—the only target worthwhile shooting at. Consider the scenario China’s military must assume: as soon as a DF-21D is launched (and hence detected by US early warning satellites) every carrier anywhere near the missile takes off at maximum speed in some random direction. If the DF-21D is launched at maximum range (again something China’s military planners would need to assume), each ship could be some 13 km away from where it was a the time of launch. The DF-21D would have to correct for that change sometime during its flight. The most logical place to correct for those changes are sometime after the end of the boost phase since the target carriers—the only targets worth shooting at—can zig and zag at anytime.
Thrusters vs. Fins
The answer is, of course, both if you got ‘em. But each mechanism for changing the warhead’s trajectory will require its own target tracking system. Ideally, you want to make changes in trajectory as early as possible since the longer you have to accelerate to the new trajectory, the lower the magnitude of the required trajectory (and, among other things, the more control you have over the final result). If the DF-21D warhead uses infrared sensors—putting aside the question of whether or not China has the required technology for a moment—then it will have to use them during the coast phase of its trajectory. Otherwise, the heat of reentry will blind the sensor if it tries to use them after it reenters the atmosphere, say something like 50 km altitude to pick a round number.
At these altitudes, the warhead cannot use aerodynamic surfaces to change its direction. So it will need thrusters—little rocket engines—to change its direction. Of course, China does has plenty of experience with fine tuning trajectories with small thrusters from its satellite insertion operations. The most likely method China might use for such a platform is a “bus” that holds the warhead while little thrusters change its position. What sort of thrust would they need? Assuming the warhead makes its corrections as the warhead passes below 100 km altitude in order to minimize the time the target has for changing its direction (again, I’m pulling these numbers out of thin air) it would have enough umph to change the velocity of the warhead/bus combination by 0.6 km/s. (This is calculated by assuming the thrusters need to change the direction of the warhead by 13 km in the 22 seconds the warhead has between when it passes 50 km—the minimum altitude I assume it can still use IR sensors). That, in turn, requires a little more than three G’s (three times the acceleration of gravity). That is probably about the requirements needed for China’s ASAT weapon tested in January 2007. So that seems possible.
If the warhead shuts down its IR sensor as it passes 50 km altitude, it is about 22 seconds before impact. It is too much to hope that the carrier can change its direction or even its speed in those few remaining seconds so the we can expect; the George H. W. Bush displaces 100,000 tons! That means the warhead can “safely” extrapolate the position the carrier will be 22 seconds after its tracker shuts down. During those 22 seconds, the Bush could travel 370 meters, which is about the length of the Bush (333 meters) but five times the beam of the Bush (77 meters). How likely a hit will be will depend on two things: how accurately the tracking system can determine the position and velocity and how finely it can tune its acceleration to match the desired trajectory.
If, for some reason, China relies solely on aerodynamic surfaces for maneuvering then it will have to wait until it gets even closer to the Earth’s surface for really effective control. Let’s assume it needs to wait until its 30 km above the Earth’s surface before the warhead’s fins “bite.” Of course, it could have stored the needed maneuvers from an IR sensor that shut down several seconds before it started maneuvering. On the other hand, it could use a radar to track the target since 50 km is well within the range of most radars mounted on fighter jets today.
At 30 km, the warhead is 13 seconds before impact. If it has to do all its maneuvering to cover the 13 km assumed miss distance, than it will need to change its velocity by nearly 1 km/s. That, in turn, will need an acceleration of 7 G’s. That is certainly possible achieve using only aerodynamic surfaces (SCUD warheads probably had nearly 10 Gs of transverse acceleration as they corkscrewed during their reentry during the first Gulf War). However, it needs to be very finely tuned and that seems the hardest point. No matter what, it would require considerable testing to develop.
Is a DF-21 Anti-Ship Missile Possible?
These rather simple calculations have shown that both types of guidance and control for an anti-ship ballistic missile are possible. But both would be pushing China’s technology considerably. For instance, China can most likely build mid-infrared detectors for military space applications. These might be used for their missile defense interceptor, even though they are barely applicable for anti-satellite weapons. Could they be used for an anti-ship application? Possibly. They could certainly see through most clouds so cloud cover is not an issue. But it would take more thought than I have given it to know that it could discriminate between a ship and the ocean. Radars, which with their limited range would require aerodynamic maneuvering, seem even more problematic because of the need to control large accelerations.
So, while I cannot rule out the DF-21D on first principles, it would need a sustained test and evaluation program no matter what technology it used. I, for one, am unaware of China undertaking such an extensive test program.