Rafael Derby design

I find that missile pretty interesting, in so much that it seems to differ in design choices from most of its contemporaries.

Most missiles of similar class seem to go for tail placed fins for manouvering and some sort of mid body wings or strakes for stabilization and lift.

Derby, however, seems to have reverted to Sparrow configuration in so much that it seems to use fixed rear body fins for stabilization while the front pair of steerable fins, wings actually, serve for lift and maneuvers.

But while Sparrow had its front wings positioned roughly mid-body, Derby has its front wings positioned visibly closer to the front tip of the missile. That would surely have to influence the moment arm, having in mind the center of mass. But while center of mass for sparrow missile (once the fuel was spent) was fairly close by just behind the mid wings. (which were, granted, pretty big) center of mass of fuel-spent derby missile seems to be almost as far from the steering fins as it is on conventional designs like amraam.

My questions are:
1) can anyone deduce some general ideas about advantages and drawbacks of the said aerodynamic configuration?
For example, is it more maneouverable? But causes more drag? Maybe its better at maintaining manouverability from the launch, while the missile is still heavy with rocket fuel?

2) what do those little fins, right behind the front fins, do? they also seem to be present on python 5.

3) do we know anything about derby's rocket motor? Has it been changed compared to python 5? if so, how? Is there any promotional cutout drawing of derby, showing the rough placement and dimensions of its rocket motor?

So, no. The Derby is canard controlled like Sidewinder or Python 4.


If you strip the front canards from a Python 4 leaving the second set, and add a slightly longer nose for a radar seeker, you can see where Derby comes from conceptually. The paddle vanes behind are used for yaw control on Python, so the same on Derby.


Its basically a modified, enlarged Python 4. Motor is supposed to be a slight modification of the Python 4 one.

At highly-supersonic missile speeds much of the overall lift is generated from the nose and forbody, which is balanced by the aft fins.

For Python, Debry etc. I assume that the canards would have to trim-out the change as the motor burns-out, but I don't know if they generate much lift overall.

totoro said:

My questions are:
1) can anyone deduce some general ideas about advantages and drawbacks of the said aerodynamic configuration?
For example, is it more maneouverable? But causes more drag? Maybe its better at maintaining manouverability from the launch, while the missile is still heavy with rocket fuel?

2) what do those little fins, right behind the front fins, do? they also seem to be present on python 5.

3) do we know anything about derby's rocket motor? Has it been changed compared to python 5? if so, how? Is there any promotional cutout drawing of derby, showing the rough placement and dimensions of its rocket motor?

Click to expand...

1.Hmm in my view it's to provide as much room for rocket motor as possible. By moving every actuators, batteries, cables etc to front.
So basically the main advantage of such configuration is packaging. Disadvantage however is that the canard control is suspectible to stall on high AoA unless a second pair of canard placed in front of it (Python4,5, Matra Magic, R-73..All use double canard configuration)

2.They're roll control fins

3.Hmm roughly half of the missile right behind warhead.

Here's an image from tactical missile design presentation by Fleeman. Seems pretty much you are looking for

Looking at these images and comparing them to python 5 it seems that derby has hardly any bigger rocket motor than python 5. Certainly if it is bigger at all it doesn't look more than 10-20% longer. Diameter seems the same, as missile diameters are the same. So what is left is fuel composition, perhaps it's optimized for different burn scheme, and the drag. Drag wise derby does seem cleaner than python 5, but it's hard to tell how much that'd influence range. Plus there's the drag from the trajectory, if derby goes up through thinner air and then coasts on a shallow dive.

The latter point not really applicable for spyder-sr i would imagine, which is why, i guess, spyder-sr is often quoted with just 20 km range. energy wise, derby just isn't in the same league as amraam or similar missiles. Rough image/drawing based comparison suggests derby has rocket motor of same length as early amraams, meaning rocket motor volume was some 25% bigger for amraam, due to fairly narrow body of derby missile. Then when the longer rocket motor came for later amraams that volume grew to a motor one third more volumenous.

Totoro, just in case you were unaware, the R-Darter/Derby was a joint South African/Israeli design and build. I'm not sure of how the design and work-share was split, however It has been hinted at the following - South Africa did the rocket motor, proximity fuse and some of the electronics (digital autopilot and IMU?), Israel did the radar seeker and servos. If true, then I would guess that the Israelis properly made a new or improved version of the R-Darter/Derby rocket motor for the Python AAM's (licensing issues, plus evolutionary development..?) .

Don't hold me to the above, just snippets I have heard and read over the years. Attached is a brochure from Denel, very scant in terms of info, however the cutaway drawing and other info may be of value to you. I guess not being able to develop as miniaturised a version of the radar seeker that the major powers could, it ended up taking up a lot more space than what AMRAAM's would for example. This is properly what lead to it's slightly strange design (and obviously shorter range).

Interestingly enough on all the South African brochures and show models, they always eliminate the two roll-control fins - BTW I was not going to post this in the South African missile thread as it is certainly not a prototype, however since you asked about it...

Thanks for the brochures and the info! I knew Denel had its hands in derby, but did not know they also made the rocket motor.

Also interesting to read in brochure that the rocket motor is boost sustain type. While late sparrow and early amraam motor had such profile, recent amraams reverted to a rocket motor with just boost grain, allegedly improving overall performance. That seems like another indication derby is comparable to early amraams in most regards (possibly excluding the seeker and electronics of course)