Why do missiles commonly have CP so far forward?

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neil_w

neil_w

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For reasons I have never completely figured out, missiles seem to have CP that is very far forward. They either have large fins moved somewhat forward of the aft end, or else they just have huge fins up front (e.g.: Nike Hercules). This means that models typically need a lot of nose weight.

I can cobble together in my head an assortment of reasons why this is the case but I'm not completely satisfied with any of them. Clearly it has to do with the fact that they use active guidance and need to be maneuverable, and also they likely have a lot of natural weight up front from the explosive payload. But that still doesn't completely explain why they are designed the way they are.

Can some of the experts out there enlighten me?
 
I'm no expert, but I have always heard the reasons you asserted in the second paragraph.


Sent from my iPhone using Rocketry Forum
 
Neil,
Here’s a good article that addresses exactly what you’re asking. See page marked 182 for Cp/Cg relationship:
https://www.jhuapl.edu/techdigest/views/pdfs/V04_N3_1983/V4_N3_1983_Cronvich.pdf
It depends on the missile’s purpose. If a missile needs to be most responsive to controls near the end of its flight, when Cg has typically shifted farthest forward due to use of propellant, having fins at the aft end can make a missile overly stable and thus less responsive when needed most.
 
Steve Shannon said:
Here’s a good article that addresses exactly what you’re asking. See page marked 182 for Cp/Cg relationship:
https://www.jhuapl.edu/techdigest/views/pdfs/V04_N3_1983/V4_N3_1983_Cronvich.pdf
It depends on the missile’s purpose. If a missile needs to be most responsive to controls near the end of its flight, when Cg has typically shifted farthest forward due to use of propellant, having fins at the aft end can make a missile overly stable and thus less responsive when needed most.
Click to expand...
Fantastic article, thanks. I knew you guys would come through! :)

Although most of the math is way over my head, the qualitative discussions are illuminating. The article gives a pretty good idea of all the conflicting demands on the design of a missile. The bit you summarized there on p. 182 is especially interesting.

Bat-mite said:
This is where scale doesn't translate well to hobby rocketry.
Click to expand...
Yup, vastly different objectives.
 
Like you surmised with the missiles, there are similar issues with aircraft, the Static Margin is distance between CG and the Aerodynamic Center/Neutral Point. A larger distance keeps the plane straight and level, like for a trainer where a gust that upsets the original attitude is met with a restoring torque to keep it stable. But with a smaller Static Margin distance the plane goes where you point it, with less restoring torque to fight against the pilot inputs, like for an aerobatic plane or fighter. The X-29 forward swept wing is unstable without active controls. Wright Brothers had to move themselves fore and aft in the sliding pilot seat to change c.g. location to maintain level flight, kind of like a hang-glider.
 
Oblique shockwaves, optimization to reduce induced drag by using a higher sweep angle, agility gains are welcome, and reduction of radar cross sections by reducing fin span are minor details. They are optimized further than hobby rockets around other constraints such as a launcher volume packing dimensions. Enemy system capabilities in tracking. The main point is the missile can deliver a heavy payload efficiently long ranges. They study range of missile for dollar spent. It has to meet certain targeting energy based equations if it's air to air. The military spends a lot of money developing motor and airframe configurations with new technology. NACA the old farts with paper lab manuals had the only supersonic data points for military grade zero lift missile airfoils. I applied those data points to a hobby rocket once for college team last year, it was a pita to manufacture and questionable risk for amount spent. It's interesting technology and it wasn't even advertised on other common college public databases like civil airfoils for subsonic research. A missile has a passive active semi active radar or laser guidance system to deal with any undesirable stability issues. Viktors missiles from Russian helicopters wobble in flight in a spiral. I don't think the military cares as much about stability margin until it's an ICBM or cruise missile i.e. something big. Or perhaps they sacrifice it for performance carefully? Depends on design goals they chose for a mission really.
 
Guidance, cooling, proximity/fusing/detection, war head, and then fuel were the order of placement for some of our air to air missiles. Our AIM-9 has less than 20lbs of explosive packed under a 8lb, fragmented, coiled steel rod, but weighs 180lbs. Certainly there had to be compromises in size and placement of those components depending on the range/speed and delivery/autonomy expectation of the weapon which would drive the design. Miniaturization of electronics and advancement of explosives have generally decreased the overall size of our carried munitions (comparing the AIM-7 vs the AMRAAM-120). Supersonic shockwave shaping requirements would drive design/placement also. Since model rockets don't carry those items packed in a not-to-scale tube thickness, the translation for CP seems a theoretical practice. BUT it is pretty amazing to see a Mach speed model rocket with charred paint along regions of the BT!
 
They're actively guided, so they don't need the same stability margin as strictly ballistic objects (i.e. hobby rockets).
 
Bat-mite said:
This is where scale doesn't translate well to hobby rocketry.
Click to expand...

Well, for some scale subjects...absolutely.
However - there are some missiles rockets (thinking a lot of early sounding rockets here) that seem to be quite easy to translate - as they had a pretty good CG/CP relationship to begin with in the full size.
A lot of those early Estes scale kits did just that.
 
MaxQ said:
... some missiles rockets (thinking a lot of early sounding rockets here) that seem to be quite easy to translate - as they had a pretty good CG/CP relationship to begin with in the full size.
Click to expand...

Not sure if this is true, but did WAC mean Without-Attitude-Control, i.e. fin stabilization only? (one of the early WAC Corporal sounding rockets)
 
Andrew_ASC said:
Oblique shockwaves, optimization to reduce induced drag by using a higher sweep angle, agility gains are welcome, and reduction of radar cross sections by reducing fin span are minor details. They are optimized further than hobby rockets around other constraints such as a launcher volume packing dimensions. Enemy system capabilities in tracking. The main point is the missile can deliver a heavy payload efficiently long ranges. They study range of missile for dollar spent. It has to meet certain targeting energy based equations if it's air to air. The military spends a lot of money developing motor and airframe configurations with new technology. NACA the old farts with paper lab manuals had the only supersonic data points for military grade zero lift missile airfoils. I applied those data points to a hobby rocket once for college team last year, it was a pita to manufacture and questionable risk for amount spent. It's interesting technology and it wasn't even advertised on other common college public databases like civil airfoils for subsonic research. A missile has a passive active semi active radar or laser guidance system to deal with any undesirable stability issues. Viktors missiles from Russian helicopters wobble in flight in a spiral. I don't think the military cares as much about stability margin until it's an ICBM or cruise missile i.e. something big. Or perhaps they sacrifice it for performance carefully? Depends on design goals they chose for a mission really.
Click to expand...

Once again you are off on an irrelevant tangent Andrew_ASC. The simple answer to the OP's question has been given several times, military missles have CP/CG relationships that the hobbyist can find frustrating because the real missles have to be maneuverable and something that is borderline unstable can with ACTIVE stabilization be very maneuverable, most missles are actively stabilized.
 
GlenP said:
Not sure if this is true, but did WAC mean Without-Attitude-Control, i.e. fin stabilization only? (one of the early WAC Corporal sounding rockets)
Click to expand...

Yes....I've heard that...not sure if WAC = "without attitude control" was merely a popular "backronym" ...but it sounded good.

I think most of the popular Estes "scale" models we all built - didn't need much more than a washer on the NC, but then we digress.
There are some scale missiles like TLP Nike Ajax and Roachworks Nike Hercules that offer scale like appearance by amping up the nose weight, or reducing the forward fin size ...but now I digress further.
 
MaxQ said:
Well, for some scale subjects...absolutely.
However - there are some missiles rockets (thinking a lot of early sounding rockets here) that seem to be quite easy to translate - as they had a pretty good CG/CP relationship to begin with in the full size.
A lot of those early Estes scale kits did just that.
Click to expand...

Sounding rockets don't need to be maneuverable like most missiles do. Apples and oranges.
 
rstaff3 said:
Sounding rockets don't need to be maneuverable like most missiles do. Apples and oranges.
Click to expand...

Yes, sounding rockets and guided missiles are clearly not the same, but neither are all missiles, - so some "scale" subjects (ie. missiles) do "translate well to hobby rocketry".
 
Last edited:
MaxQ said:
Yes, sounding rockets and guided missiles are clearly not the same, but neither are all missiles, - so some "scale" subjects (ie. missiles) do "translate well to hobby rocketry".
Click to expand...

I guess I am mostly quibbling about semantics and agree many sounding rockets are naturally stable when built as a sport rocket. Not so much missiles with forward control surfaces...but not all have those. However, sounding rockets are not missiles. Missiles are shot at a target. But I guess you can say a particular area of the upper atmosphere is a target for the sounding rockets.

Sorry this mini rant was caused by lack of coffee. Somehow, I slept late :)
 
rstaff3 said:
I guess I am mostly quibbling about semantics and agree many sounding rockets are naturally stable when built as a sport rocket. Not so much missiles with forward control surfaces...but not all have those. However, sounding rockets are not missiles. Missiles are shot at a target. But I guess you can say a particular area of the upper atmosphere is a target for the sounding rockets.

Sorry this mini rant was caused by lack of coffee. Somehow, I slept late :)
Click to expand...

Roger Wilco. And you are quite correct on all points.
I may have misread John's point.
 

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