Many people have written in objecting to my theory that the Sejil uses liquid propellant steering engines for thrust vector control and, I have to admit, that they have had some very good arguments. Perhaps the most persuasive arguments have been:

  • The boxes hang far below the end of the solid propellant motor’s nozzle and would subject any such engines to temperatures and a corrosive environment that would destroy them.
  • Placing steering engines farther up the boxes, away from the exhaust from the solid propellant motor’s nozzle, would require that the SPM’s nozzle be considerably smaller than the missile’s diameter. This would make the compression ratio inside the motor considerably greater than the expansion ratio as it leaves the nozzle.

People who raise the first point do, of course, ignore the fact that the boxes, which in the jet vane theory contain the servo mechanisms, would also be subject to these same environmental challenges. Pictures that show Soviet designed ICBM’s that use jet vanes for their large solid propellant motors clearly show that these servo mechanisms are protected by the nozzle itself, with holes cut out for the jet vane supporting arm to pass through. Of course, jet-vane theorists could well say that is the reason the missile is said to have failed: the boxes burnt through and the missile flew off course.

People who hold the jet vane theory also say that the side pipes are unrelated to TVC. Some of them say that their purpose is a mystery that will remain for some time. Others have suggested that they might be retrorockets to move the first stage safely away from the second during staging. (This is important as one of the Falcon-1 flight tests showed when the jettisoned first stage bumped into the second and broke the engine.) These people have not, in my opinion, offered a good reason for putting the retrorockets near the empty stage’s center of mass.

Wonk-reader Babak has sent in a new (at least to me) picture he captured from a video of the launch that shows a close up of the boxes and, most importantly, the space directly beneath the solid propellant nozzle. The question I have is: where are the jet vanes? There is something there, but it is very thin and appears, at least to me, to be venting something. Surely the jet vanes for such a missile would be much thicker than what ever that thing is. (They would have had to be very successful in minimizing the size of their jet vanes, as I discussed in Moving ‘em Out to Move Them Up ,to make them invisible in this photo!)

If anything, I think this photo yields further support for the liquid-propellant steering engines and not just the negative evidence of not showing jet vanes. Where I originally thought the boxes were cowlings to protect them from the air stream, it now appears that they might encircle the engines to protect them from the harsh environment found under the solid propellant motor. The rectangular appearance of the boxes could then be explained by allowing the engine to swing back and forth perpendicular to the missile body to steer the missile. After all, with four such steering engines, you only need to provide each engine with a single degree of freedom to completely control pitch, roll, and yaw.

So as Wonk-reader Tal Inbar notes, we may never know for certain what Iran is using for TVC until we see them parade the missile down the streets of Tehran. Until then, I feel that the preponderance of evidence still favors liquid-propellant steering engines though I have to admit your arguments, wonk-readers, have certainly shaken my conviction.