Geoff FordenSafir Fin, Trajectory & Staging Details

click on the image for a larger version

Iran’s satellite launch pad position and orientation as determined in Jane’s Intelligence Review (G. Forden, “Smoke and Mirrors: Analyzing the Iranian Missile Test, April 2008, pp. 47-51). The actual image overlay has been replaced with a “cartoon” to avoid copy right issues. A not to scale missile cross-section outline has been inserted to indicate the orientation of the missiles. (The missile was not shown in the satellite image.)

I’ve been working on understanding some details of the Safir’s guidance and control systems, which as I indicated in a discussion of missile memos has a component of radio control. As preparation for that, I wanted to discuss some aspects of the Safir’s guidance that are rather standard: its first stage pitch program appears to be in the same plane as it would be if it was a more normal, internally controlled pitch program. In the course of re-examining some new videos and still images for this work, I also paid more attention to Iran’s simulations. I’m not sure that they have been talked about here, but those clearly show retrorockets on the first stage that are used to keep the first stage from ramming the second during staging. Similar boxes on the second stage might very well contain rockets for producing ullage pressure; forcing the oxidizer and fuel into the turbopumps before ignition. Unfortunately, the simulations do not show those firing (they are out of the field of view).

Fin and Trajectory Orientation at Launch

Most missiles have what some rocket engineers call a “face,” the side of missile facing toward the target. This is usually the direction of the number one fin and the Safir appears to be no different. During launch, the jet vanes (or in general, the thrust vector control system) not only counteract minor perturbation of the trajectory—such as if the wind tries to blow the missile off course—but also control the pitch-over of the missile toward its target. The program that controls this program for, for instance, a SCUD missile, follows a fixed program. At 10 seconds for instance, it shifts the average orientation of the jet vanes on fins II and IV five degrees, to just pick a number of degrees. (Again, the jet vanes continue to have short term variations around this value as needed to correct of minor variations.) As a consequence of this program, the missile pitches over in the direction of its target: the direction Fin I is pointing.

Fortunately, the Safir’s fins are also numbered in this way. Interestingly, it appears as if there is a significant amount of rotation of the Safir between when some of the pictures of it taken during the day and when it is launched during the night at approximately 18:30 (2/2/09) UTC. (That is the time when the Omid’s orbit crosses over the Safir’s launch site if its first orbit is “tracked” back to that crossing.) The fin orientation is shown in the image to the left just at ignition; notice the nice red glow as the igniter is fired. The gantry has been tipped over and is on the left of the Safir in this image while the transporter-erector is on the right.

The image at the top of this article shows the orientation of the fins relative to the track of the Omid as it is “tracked back” to cross over the launch site. The relative orientation of Fin I and the satellite’s track is close enough (considering all the possibilities for error) to confirm that the pitch program is, indeed, in the “standard” plane.

Staging Details

This Iranian animation of the Safir’s staging event clearly indicates that the first stage has retrorockets for moving the first stage out of the way and that the second stage fires after a brief independent coast. The animation also shows the first stage burning out at separation but before second stage ignition. I’m not sure about the timing of those later two events. The boxes that are the first stage retrorockets are very similar to boxes near the top of the second stage. It seems likely that they are also small rocket units that could be used to provide “ullage” pressure to force the propellants in the second stage into their turbopumps. If so, it seems likely that the reason the second stage’s turbopump is so closely aligned with the symmetry axis of the stage is to prevent its “thrust” from perturbing the symmetry axis of the engine cluster. If they did not do so, there would be at least a small off-axis force that would have to be compensated for by a constant average tilt for the two gimbaled engines.

It is interesting to note that these rockets do not appear on images of the Safir’s stages in its horizontal assembly building. There are, however, clear bolt holes to fasten them on to the missile’s airframe. If these are, in fact, rocket units, it might make sense to assemble them on the launch pad and might explain why there are walkways at about the right height.

Comments

  1. Jochen Schischka (History)

    Nice work, Geoff!

    Just some small additions:

    1.) Generally, i would advise against putting too much faith in computer animations (although in this special case, that particular animation seems to depict the staging process of the Safir quite logical) – please keep in mind that it’s no problem to find beautiful animations of e.g. interstellar starships traveling with several times the speed of light

    2.) Hmmm, the “heading of the Omid satellite” and the heading of fin I don’t seem to fully coincide (as i would expect) in the above picture…what is the source of this discrepancy, inaccuracy of the drawing – or inaccuracy of the launcher?

    3.) What does the “fin-I”-orientation (and we can find indications for the same procedure on the Shahab-1/-2/-3/Ghadr-1 and perhaps Sejil, too!) tell us about the guidance system of that missile? Similar procedure as on the Aggregat-4/V-2 (Horizont/Vertikant-guidance – hey, the Germans invented that type of guidance/aiming!) or the R-17/SS-1c/Scud-B (Горизонт/Вертикант-guidance) – totally different type of guidance system (e.g. solid-state strap-down, as some people assume)? I’m not so sure about this…

    4.) Some pictures (especially this one: http://www.b14643.de/Spacerockets_1/Diverse/Safir-IRILV/SAFIR_22.jpg) let me come to the conclusion that the retro- and ullage-rocket-packs are already mounted when the launcher is erected (all these final preparations like integration of the jet vanes, battery, payload etc. plus the fueling of the lower stage are perhaps made inside of or adjacent to that hangar, while the upper stage apparently is only fueled from the gantry in erect position -> see color-coded piping on the launching tower, possibly because of mass/strain concerns).
    Interestingly enough, in some pictures of the Sejil (like this one: http://www.b14643.de/Spacerockets_1/Diverse/Sejil/Sejil_12.jpg) rather similar bolts on the missile body can be perceived exactly where the “sidepipes” would be (which are missing in that particular photograph – also note the absence of any holes for e.g. liquid fuel lines).

  2. Geoff Forden (History)

    Jochen, As I tried to suggest, it is impossible to be more precise about the exact orientation of the fins with respect to the gantry then to see they are about 45 degrees away from it. Also, there is some uncertainty in the orientation of my overlay with GoogleEarth. All or which can explain the seeming discrepancy between Fin I and the satellite track.

  3. Jochen Schischka (History)

    O.k., Geoff, this basically confirms what i supposed (i know from first-hand experience how difficult this type of reconstructive work can be…).
    But on the other hand, i think we shouldn’t completely neglect launcher-precision, either. I mean, at 52° inclination, the Safir-2 entered pakistani “airspace” during second-stage-burn (albeit only grazingly); If the Iranians would have had the intention to avoid this, something in the range of ~55-66° would have been advisable, isn’t it?

  4. Jochen Schischka (History)

    Sometimes, it pays off to re-check facts: According to the NSSDC, Omid was injected into an inclination of 55.5° (see: http://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=2009-004A), not 52° as i had previously assumed; So the Iranians in fact avoided overflying any other country (as i would have expected), but got rather close to the edge of Pakistan with their trajectory.

    Nonetheless, maybe the Iranians initially aimed somewhat more straight to the south (like ~60° inclination – the center of the “geopolitical launch window”), but the missile went a little bit astray to the east (which could, at least partially, explain the small directional discrepancy in that picture). The effect on Omid’s orbital parameters wouldn’t have been dramatical in this case (but it would be highly interesting to compare initially planned and actually reached orbit to see if this speculation hits the mark).
    Interestingly, a course deviation in the range of up to five degrees (worst case) would have to be taken into account for a modified Scud-guidance-system on a missile with an overall burn-duration of ~8 minutes…

    BTW, did anybody else notice that, based on the NSSDC-data, Omid/the Safir-2-upper-stage almost perfectly overflew Male, the capital of the Maldives (but would have reached orbital velocity and sufficient height by that time, so that would not have bothered anybody)?

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