How about when half of it is liquid propellants? How about when it includes all of the deadweight—fuel tanks, turbopumps, and engines—associated with liquid propellant engines? How about when it makes no attempt to solve one of the major problems of solid-propellant technology: thrust vector control? Hi wonk readers! Jeffrey has kindly taken off my training wheels and let me be a guest blogger on his outstanding armscontrolwonk in my own right. I had thought I’d start my tenure here with a series of posts about Iran’s liquid propellant missile development program but I’m going to postpone that start until tomorrow. In stead, I thought I would weigh in on Iran’s solid-propellant missile. Ooops! I mean to say on their new liquid-propellant missile. Wait, that’s not exactly correct either. You know the one I mean; it’s the liquid-propellant missile with a solid-propellant motor in it. They call it the Sejil.
I’ve just come back from Croatia where the hotel’s internet was so flaky that I couldn’t do anything, so this post has been delayed by several days. I heard the news that Iran had launched a “two-stage, solid-propellant missile” with a range of 2,000 km literally as I was boarding the airplane on my way to London. I spent the entire flight wondering how I could be so wrong about the status of Iran’s solid-propellant development program. You see, there was no evidence that Iran had even experimented with thrust vector control (TVC) techniques suitable for solid propellant missiles. Graphite jet vanes, like Iran uses on its Shahab and its derivative missiles, corrode and fail very quickly in the high temperatures and corrosive environments associated with solid-propellant grains that use aluminum powder to boost their specific impulse. Fluid injection, like India used for the first stage of its SLV-3 (and the Agni I), requires considerable development both on static test stands with multiple degrees of freedom and flight tests on smaller missiles. The use of flexible nozzles also requires considerable R&D. None of which has shown up on Iranian short range solid-propellant missiles. Instead, they had seemed to be developing aerodynamic controls for these missiles instead of TVC. In particular, they have experimented with canards on some of their short range solid-fueled rockets; avoiding the entire TVC issue all together.
In contrast, and this was the really surprising thing, I think they used fairly large gimbaled engines to accomplish TVC for the Sejil. We know they are fairly large because the fuel and oxidizer tanks take up half of the first stage volume. The clincher that this was a largely liquid propellant missile, by the way, is the piping coming from the middle of what many supposed to be a solid propellant combustion chamber. Judging from the weld lines seen in other images of the Sejil, the missile uses over five tons of kerosene/nitric acid to power the four gimbaled engines in the first stage. (There is no indication that I can see that the second stage was anything other than an airframe with inert weight inside. Has anybody seen reports that it actually was live? This would explain the Fox report that it suffered a “failure” and only flew 180 miles.)
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Using liquid propellant engines to provide TVC for the Sejil does nothing for Iran’s development of solid-propellant missiles and is far from a major advance in their technology. It could be argued, however, that the Sejil does advance the liquid propellant technologies Iran has been trying to perfect with the August launch of the Safir with its cluster of twin gimbaled engines. Of course it’s possible, since I’m still working out the numbers, that adding a solid propellant boosting motor has substantially increased the payload capacity of this missile. In any case, Iran has continued its unique missile development path. Some of their innovations have been brilliant but I cannot help thinking that this one is pretty much a dead end.
There are still problems with my interpretation, namely, what I refer to as cowlings seem to hang down farther than I would expect steering engines to hang. That argues in favor of jet vanes. On the other hand, what is that pipe coming out of the middle of what is supposed to be a solid-propellant casing? Because of this, and other details, I believe that this is the correct interpretation, but I await your comments!
Tomorrow, I’m going to start a five part series of posts about Iran’s liquid propellant missile development program. The first installment will be a speculative piece asking about what evidence exists that Iran is trying to develop higher energy propellants than SCUD-type mixtures of kerosene and nitric acid. Until then, I look forward to your comments.
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12-11-2008, 11:01
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