Arms Control Wonk

All Good Things…

By geoff | 11 August 2010 | 16 Comments

…must eventually come to an end.  I have enjoyed writing for ArmsContolWonk.com very much and it has been one of the more satisfying professional experiences I have had.  Unfortunately, at least for me, this post will be my last.  I am moving on in my career and no longer can afford the time etc. involved in writing these posts.  Jeffrey took a risk on letting me write a “techno-wonk” component for his outstanding blog and I will always be grateful to him for doing so.  While I’m at it, I want to say that Jeffrey was a pioneer in exploiting the potential of the web for informing the public with reasoned debate and I hope he gets the recognition that he deserves.  ACW has been a leader this in effort and I am proud to have been a part of it even briefly.

The spiffy new blog format Jeffrey has initiated for the blog easily allows me to see that this is the 142nd posting I have done for ACW.  A high fraction of these have been about missiles and missile proliferation.  In them, I have focused on what I call the “How” of proliferation rather than the “why” that has been discussed so much previously.  While I do think missiles and space are cool (who doesn’t?), I have always considered them to simply be a more visible way of understanding how developing countries acquire the advanced manufacturing capabilities needed for almost any WMD technology.  There are two reasons for this.  First, you can almost always distinguish success from failure with rockets, and especially satellite launches.  Second, missiles and space launch vehicles represent a national achievement that most countries are more willing to release information about than other technologies.  Consider Iran’s release of videos of the production of Shahab engines.  (It is true that Iran has released images of the components for the centrifuges but I think, on the whole, there has been more released for their satellite program.)  There are, of course, a number of reasons for this; reasons that I would have liked to talk about in future blogs.  Perhaps others on ACW will find this topic interesting and post on it.  I hope somebody continues to analyze.

Another future topic I would have liked to write about is motivated by the many recent examples of leaking.  As one friend of mine recently wrote: “Information wants to be free.”  I have a serious problem with that if the implication is that we can stop trying to contain proliferation sensitive information.  (If it is a statement about entropy and proliferation, it is simply a statement of fact.  You simply cannot have information shared among 850,000 holders of Top Secret clearances and expect something not to get out.)  One issue I wanted to work through on these pages was the relation between proliferation secrets and how successful proliferators are.   I have discussed many times just how ineffective “reverse engineering” the production line for making WMD is.  Proliferators need more than blueprints for effective proliferation, they need someone to teach them the shop-floor skills to take a “product’s” blueprint and create the process for its construction.  This is why the secret formula for VX really did not help Iraq when it tried to produce militarily useful quantities of the stuff.

But why make it easier for them?  It was just this balancing of assistance and difficulty that I wanted to explore.  I am sure others will take up this challenge on these pages.  The answer is just too important.

If what I consider my most important work here was understanding the “how” of proliferation, I had the most fun writing some of the more off-beat posts.  My favorite post—the one I had the most fun writing—was “Shocking Good Fun,” where I analyzed the atmospheric phenomena associated with a rocket going supersonic.  I also really enjoyed thinking about antipodal seismic signals of nuclear blasts.  That is another regret: that I have to leave before completely mining the wealth of ideas in “Caging the Dragon,” the excellent DOE report on containing nuclear explosions underground.

Of course, discussing past posts would be meaningless without talking about the contributions from you, wonk-readers.  I consider myself a generalist with a special interest in rockets (see above for why I think that is).  There are many real experts in the technologies I have written about and they have been kind enough to contribute their ideas and knowledge to these pages.  I have learned a lot from these experts and I am sure the general reader has a pretty good idea who they are.  (Several use pseudonyms.)

Of course, any time you discuss several specific countries on the internet, you get some very strong reactions.  Reactions that have a tendency to get very unpleasant, very quickly.  As I carried out my duties to moderate these discussions—after all, the vast majority of our readers really do not want to read comments filled with unthinking hate—I would often see a comment that was sure to draw and escalation from the “other side.”  Sometimes, such comments would have a valid point to make or contain at least some interesting points of view and I would wonder why the writer needed to include the barbs in them that were sure to inflame people with different loyalties. In all cases, it might have had an immediate emotional release for the writer, but it always reduced the intellectual impact of the comment.

I came to expect that type of comment from some of the nationalities that we discuss in these pages, but I was very surprised to get it in full force on the very few occasions I might have hinted that I am not a fan of NASA’s manned space flight program.  (Not to mention the one time I talked about something all too many people want to think of as a UFO!)

Finally, I’d like to hear from you, wonk-readers, about which are your favorite posts that I have contributed and why.  And, perhaps more importantly, which were your least favorite posts that I have written.  Here too I’d like to know why.  I should say that I know I have made mistakes in these posts; mistakes I have tried to acknowledge.  But the point of my posts was never to lay out my “wisdom” before you, but to explore new ideas and do so in a time frame much faster than permitted by refereed journals.  That has inevitably led me to make mistakes (one howler—it was so bad, I deleted it before many readers had a chance to see it—involved me forgetting that the vacuum between the rotor and a centrifuge’s outer casing acted as a thermal barrier and greatly reduced the power requirement for a clandestine enrichment facility) so please be kind, gentle reader!

DF-21 Delta: Some Early Thoughts

By geoff | 6 August 2010 | 33 Comments

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.

Hangzhou Light Show

By geoff | 12 July 2010 | 36 Comments

Something interesting apparently happened last Wednesday (7/7/2010) in the skies over Hangzhou, China. As regular Wonk-readers will quickly agree, trying to extract reliable information from regular media reports is difficult at best. Unfortunately, almost all the news reports of this incident that I have seen of it have something like “…UFO over Hangzhou…” in the title. That makes it even harder to believe the reports. It doesn’t help that some of the pictures seem inconsistent with each other. (Editors seem to believe that UFOs in the title means you can photoshop the photos as you like.)

Some of the most interesting photos remind me of the images of the reentry of the Japanese Hayabusa spacecraft. Here, the break-up of the object appears behind buildings reportedly in Hangzhou has a tremendously large apparent angle, making it very, very close to the city of Hangzhou. Too close, in my view, to be credible. ( See here for a discussion of “apparent angle.”) I believe these are photoshopped and I am ignoring them. (I freely admit it could be a mistake to dismiss these images but they just don’t seem credible even if they are extremely well done if photoshopped.)

Instead, let us consider some of the most obvious causes of what appears to have been seen. First, NASA does not list any satellite decays—a satellite reentering the Earth’s atmosphere after being in an established orbit—for that day, nor were there any predictions for satellites even near that date. So it is highly unlikely that the “UFO” was a reentering satellite. Furthermore, there were no satellites put into orbit that day so we can eliminate a lower stage reentering.

A DF-21?

The most credible image shows an arc streaking across the sky soon after (near?) local sunset. Assuming this is actually the “UFO” and that it really was taken at Hangzhou, then we can say some interesting things about its trajectory. First, it appears in the Northwest and the end of the trajectory (in this image) is roughly due North. Let’s make that a little more quantitative. At sunset (which happened at Hangzhou that day at 11:00 GMT), the Sun had an azimuth of approximately 300 degrees and remained in that approximate azimuth for a considerable time after setting; though, of course, its elevation continued to go negative. Therefore, the image—assuming it was not zoomed and that it was a typical cell phone camera—shows the end of the trajectory having an azimuth of at least 350 degrees. (That’s 10 degrees West of North.) The peak of the trajectory appears at an azimuth of about 335 degrees. I would say there is about a +/- 5 degree error in my heading estimates, depending mainly on where the Sun really is in the image. Of course, that does not have to be the actual point where the trajectory had its highest altitude. It could simply be the highest point on the apparent trajectory: the curve of the Earth’s surface and the angle the trajectory makes relative to the observer could make a different point appear to have the highest elevation in the image.

click on the image for a larger version

The red line represents the reconstructed line of sight for the “end” of the trajectory while the yellow line represents the reconstructed line of sight for the “peak” of the trajectory. The white line is the ground track for a hypothetical DF-21 trajectory.

The image above shows these reconstructed observation angles from Hangzhou; the red line represents the reconstructed line of sight for the “end” of the trajectory while the yellow line represents the reconstructed line of sight for the “peak” of the trajectory. I have added another line running roughly East to West from the Jiuquan Satellite Launch Center with a length of 1,800 km, roughly the range of a DF-21. Amazingly (perhaps an amazing coincidence) the halfway point of that trajectory corresponds roughly to the angle on which the apparent trajectory reaches its maximum. This peak appears to be about 25 degrees above the horizon in the image of the trajectory. Is that possible for a hypothetical trajectory so far away (~1,400 km) from the camera?

The apparent elevation of the peak of such a hypothetical DF-21 trajectory as seen from Hangzhou can be calculated from a simple geometric relationship as about 17 degrees. That is really amazingly close to the “observed” 25 degrees considering all the simplifications in the calculation I have introduced to make a quick calculation! So we certainly cannot rule out this possibility on geometric grounds alone.

Problems with a DF-21 Hypothesis

The major problem with a DF-21 hypothesis is that the peak of the trajectory occurs nearly 600 km above the Earth’s surface! So the arc we see in the image cannot be the trail produced by reentry heating. Perhaps we should rule a DF-21 launched from Jiuquan out on those grounds alone. But it is possible that continued discharge after burnout from the DF-21’s solid-propellant second-stage motor could be illuminated by the Sun if there was just the right geometry. Any other missile trajectory has to have the reentry much, much closer to Hangzhou. That creates lots of range safety issues and presumably are ruled out. (An errant missile trajectory would never have been allowed to get so close to such a large population center. All missiles, even SCUD combat missiles, have a self-destruct charge on board to prevent such large discrepancies from the planned flight path.)

So it seems to me that a DF-21 launch somewhere near Jiuquan and aimed at a point somewhere in the eastern Gobi desert is the most likely cause of this “UFO” even given the problem of illuminating the solid-motor discharge above the Earth’s atmosphere. Unfortunately, I doubt that we will learn anything new about the DF-21 from this image, given the uncertainty of where it took off, its heading, and its range.

A General Disclaimer

I have made a tremendous number of approximations and simplifications in this analysis in order to get a feeling for this event. Many of you will object to those and say more care should have been taken. I agree with you in principle but think such accuracy is unwarranted at this time considering the tremendous amount of uncertainty in judging which pictures are accurate and even what basic timing data to believe. Let me emphasize once again that this analysis assumes that the image at the top of this post is what caused the alarm in Hangzhou. I have no idea if that is the case. If some of the images I have discarded are really images of the phenomena, than this analysis’s assumptions are wrong at the start.

It is always hard to use media reports for quantitative analysis but particularly so for anything with UFO in the title. It seems that editors see that as a license for any kind of photoshopping they like. So, until better information is available, I think the most likely explanation is a DF-21.

Editor’s Note: Don’t even think about making a comment suggesting it was a real UFO, those comments will not get approved. This is a serious blog and is not interested in UFO theories. Get your own blog if you want to discuss that sort of thing.

Update (7/15/10)
The embedded video below shows both how difficult it is to get quantitative information off of the internet about “UFO” events and is a great example of a rocket plume in outer space. As an added bonus, there is a staging event at about 30 seconds. By the way, this is almost certainly a Progress-M launch from Biakonur.

Ill-Fated KSLV 1

By geoff | 10 June 2010 | 10 Comments

This morning’s news reports indicate that, once again, South Korea has failed to put a satellite into orbit. Some might wonder why we discuss this issue on ACW. However, it makes a very interesting case study for how countries acquire missile technology. And failures are much more interesting that successes! Stay tuned!

UPDATE (6:13 PM 10 June 2010) The BBC is reporting that the KSLV-1 blew up something like 2 minutes after launch and that there is video of it. Gotta get me a copy of that!

Iranian Warhead Evolution

By geoff | 9 June 2010 | 29 Comments

The family of Shahab-3 warhead variants as compiled by Tal Inbar and Uzi Rubin. The NRV, the New Reentry Vehicle, is shown on the right. Note it’s a triconic design but with a larger base diameter than the Ghadr-1 warhead.

Wonk-friends Tal Inbar and Uzi Rubin are reporting a new warhead variant for the Shahab-3 family of missiles—and hypothesize that it will soon show up on the Sejiil solid-propellant missile. This new variant, also a so-called “triconic” design (why tri -conic? It has always struck me as “bi” conic.), got me wishing I had done a full aerodynamic analysis of “old” triconic warhead when it first appeared. Well, better late than never. Now I can compare all three designs: the conical warhead that appeared first on the Shahab-3, the “improved” warhead design that is often associated with the Ghadr-1 missile, and the New Reentry Vehicle (which Inbar and Rubin call the NRV, a naming convention I will adopt here).

The supersonic aerodynamics of these warheads are relatively easy to compare using the program HyperCFD, a program I tested and reported on in an earlier post. As expected, both triconic designs have larger drag coefficients than the simple conical design. This is because of the shock waves generated at the “breaks” in the aeroshell—where there is a discontinuity of the aeroshell’s form. What was surprising to me, however, was the relative positioning of the Ghadr-1 RV’s and NRV’s Cd (coefficient of drag). The NRV, with its more stubby appearance has a uniformly lower Cd. Perhaps I should have expected this: it is, after all, somehow “between” the two designs: the Ghadr-1 and the simple conical warhead. (Area, something vitally important to the actual drag of an object, has been removed from the calculation of the drag coefficient.)

The coefficient of drag, Cd, for the three warhead variants is shown here. Note that the first “triconic” design flown by Iran has the highest coefficient of drag, the new reentry vehicle (NRV) drag coefficient is slightly lower but still greater than the simple conic warhead.

As that first break approaches the rear of the warhead, any differences between the simple cone and the NRV should go away. That’s arguing purely on a continuity argument, which is perfectly valid but would have been more impressive if I had seen it before I ran the Cd calculations. On a perhaps more physical basis, it is possible that this decrease in Cd is related to the fact that the shock wave originating from the first break in the aeroshell is much closer to the shock wave originating from the second break. This proximity could reduce the amount of energy radiated in the shock waves, but I’m just guessing here. These differences in drag coefficient have only a very minor effect on the trajectory of the warhead as it reenters.

Let me emphasize that the “normal” triconic RV only matches the reentry velocity and acceleration profile so well because its base diameter—and hence its area—is considerably smaller than the other two base diameters. Refresh your memory of the design by looking at the graphic from Inbar and Rubin at that top of this post. When I take the same design and simply increase the flaring at the rear to fit the Shahab-3 rocket body (i.e. increase it to 1.25 m), then the triconic warhead slows down considerably. In fact, it nearly reaches terminal velocity and “gently floats” down to Earth. Well, if you consider 310 m/s floating.

Warhead Stability and Warhead Volume

As far as ascertaining the purpose of this evolution of warhead designs is concerned, the drag coefficient associated with each warhead variant is probably less important than the question of stability during reentry. Stability of any projectile using aerodynamic forces to achieve stability—either an guided rocket or a reentering warhead—is determined by the relative positions of the Center of Gravity (Cm) and the Center of Pressure (Cp). For any such object to remain stable, the center of gravity must remain in front of the center of pressure stability of a missile (either rocket or warhead) that relies on aerodynamics to keep on a straight path is determined by the relative positions of the center of pressure and thecenter of gravity. If the warhead is going to be stable in a “pointed end first” attitude as it reenters, the center of mass must be forward of the center of pressure. If it is not, the warhead will start to tumble.

The center of pressure is solely determined by the exterior shape of the warhead and can be easily calculated by the same program, HyperCFD, that I used to calculate the drag coefficient at supersonic velocities. Unfortunately, the center of mass is completely dependent on the arrangement of the mass inside the warhead and, as such, is not knowable to outside observers. However, we can still make some interesting calculations if we assume that the warhead is filled with a uniform material and fill it from the tip of the warhead back. In the calculations that follow, I use a density for this material of 1.8 g/cc, the theoretical density of RDX, a popular high explosive. I should note that this is considerably greater than the average density of the first plutonium weapon, the Fatman, which I calculate to be approximately 1.2 g/cc but much less than more advanced nuclear designs. What we will do is look at the relative positions of the Cp and Cm as we fill the warhead to various levels, always starting at the tip and filling backwards. (This is exactly what happens when you fill a SCUD warhead with conventional high explosive.) But first, let us fill each warhead right to its very end:

The positions of the Center of Pressure (Cp) and Center of Gravity (Cm) for the three Iranian warheads assuming a uniform fill from tip to back. Note that all three are unstable when completely filled because the Cp is in front of the Cm.

Since none of them can be totally filled, we can ask the question: which variant is capable of holding more mass as the variant just ceases being unstable? While the final answer is as we might have expected, I was surprised by the intermediate answer. So surprised that I almost did not post this analysis. (Perhaps some are wishing at this point into this every lengthening post that I did not!)

If we leave the rear portion of each warhead empty, we can calculate the position of the Cm for different total fills. This plot is shown below:

The position of the Center of Mass (shown along the Y-axis) of each warhead variant is shown when it is filled to different points along the warhead length. To compare the three variants (the NRV and the simple conic warheads are each 3.5 m long while the original “triconic” warhead is 2.75 m long), the level of fill is written normalized to each warhead’s overall length. (The triconic warhead appears to end earlier than one but that is an artifact of my plotting program, which stops at the last binned mass.)

Note that the vertical lines are the positions of neutral stability, where the positions of the Cp and Cm are equal. If we fill a warhead to less than its neutral stability point, than the warhead will be more stable on reentry. If we fill it past the point of neutral stability, it will be more unstable . Note that the simple conical warhead appears to be a better choice since its neutral stability point takes a fill that is closer to its rear. (This is the place I got stuck on and almost didn’t get past.) But this result is somewhat misleading. We need to look at the mass of the fill for the three warhead variants—and it turns out that the “central barrel” of the NRV makes all the difference.

The mass of each warhead variant as a function of the high explosive fill. The vertical lines are the lines of neutral stability for each warhead variant. Note that the NRV, while of necessity having a shorter fill length if it is going to be stable, is actually capable of carrying more mass than the simple conical warhead. (Also note that the weight of the nosecone skin is not included in this calculation.)

A Purely Conventional Warhead?

Truly understanding the reasons for the evolution of the warhead variants would require an understanding of what nuclear warhead designs Iran might use. However, the switch from the initial triconic warhead—which came out some what after the Shahab-3’s simple conical nosecone—to the NRV makes sense for a purely conventional warhead since it can carry considerably more high explosive even the simple cone. Furthermore, the differences in dimensions of the NRV and the simple cone seem rather minor and would clearly be very dependent of some unique feature of a hypothetical nuclear warhead design.

Now It Can Be Told: Inside BOB

By geoff | 3 June 2010 | 36 Comments

(posted at 2:01 GMT 4 June 2010)

One of the great things about writing for the Wonk is that people tell you things, including what’s inside what has been called here the “Big Odd Box” in Burma. Last January, I was invited to join a group of experts in Oslo, Norway, to review a ton of electronic document