Yesterday, I discussed the possibility that Iran was learning techniques that might help them utilize higher energy fuels such as UDMH and nitric acid. However, the evidence for such use is rather minimal. The case we examined yesterday was not very convincing and, while there is still room for doubt (see Jochen Schischka’s comments), I think that that the balance of the evidence still leans toward Iran utilizing kerosene/nitric acid combinations. What is know is that Iran is trying to improve its engines and get a higher specific impulse—a measure of how energetic the combination of propellant and engine is—regardless of what fuel it is using. (The technical definition of specific impulse is the thrust delivered by the engine divided by the weight of propellant burned per second.) While Iran’s efforts to do this are most obvious on the Safir’s second stage, it is possible that Iran is also taking the lead away from North Korea in improving the specific impulse of the Shahab/Nodong/Safir 1st stage as well!

People often forget that the design of the engine has a direct effect on the specific impulse and usually compare the theoretical specific impulses of various fuel and oxidizer combinations. If you simply compare these theoretical values for UDMH/nitric acid and RP-1/nitric acid (RP-1 is a kerosene-like fuel similar to the kerosene-based fuel the SCUD uses) you get a ratio of 276/268, in vacuum, and thus an increase of only 3% by going to the more energetic fuel. But the design of the SCUD engine, and in particular, its thrust vector control mechanism robs the propellants of a considerable amount thrust! Consider the images below, which show the jet vanes for the Iraqi Al Samoud II, the SCUD-B, North Korea’s Tae’podong 1, and Iran’s Kavoshgar 1 (which I assume are identical to those on the Shahab and the first stage of the Safir).

Note the differences in specific impulses, Isp, between the Al Samoud II and the SCUD-B. It is true that these two missiles use different fuels (the Al Samoud uses TEGA 2, aka Tonka, while the SCUD-B uses TM-184; both are hydrocarbons) and will certainly have slightly different base specific impulses. However, it is widely understood that the jet vanes on the Al Samoud II, for instance, rob the engine of as much as 5% of its thrust. This would account for just over 10 s of Isp! It is not too hard to imagine that the SCUD’s larger Isp might, in part, be due to its jet vanes covering somewhat less of the its nozzle exit area. (This, by the way, is what the subtitle of this post is all about.) I’d be interested if any of you out there (John?) wouldn’t mind calculating this effect from first principles.

What I find most interesting about this comparison is that the Iranian jet vanes appear somewhat smaller to me and are, perhaps, angled outwards so that they interact with more of the peripheral part of the exhaust than North Korea’s Tae’podong 1. Is this an indication that Iran has diverged from the Nodong missile it is reported to have imported from North Korea to optimize its thrust? If so, it is an another indication that Iran is leading the way among the so-called proliferating nations: Iran, Syria, North Korea, … in developing better missiles.

This improvement is, of course, a minor one since it still uses jet vanes to control the direction of the first stage’s thrust. But as I mentioned in yesterday’s post, it is more effective to improve the specific impulse of a missile’s second stage. This is exactly what Iran has done by developing a cluster of two gimbaled engines for the second stage of the Safir missile.

Until I started writing these series of posts, I wondered why Iran was putting so much effort into gimbaled engines as opposed to developing a new, higher energy-density propellant combination. Now, I think, we can see the answer. By eliminating the jet vanes, Iran has produced almost the same improvement in delivered thrust as it would get if it had switched to a UDMH/IRFNA combination. Of course, it would be best if Iran did both but doing both at once would significantly increase the development risk and it makes sense for Iran to take these challenges one at a time.

Increasing the effective specific impulse of its engines by removing the jet vanes implies that Iran can cut down not only on the volume (and hence weight) of propellant but also the amount of tankage or dead-weight of the tanks. The next post in this series, on clustering engines, will examine the change in “dead-weight” associated with clustering engines. It will also probably be one of the most techno-wonkish of all this series of posts and I apologize for that in advance.