Why are the C11 motors 24mm when they are the same total impulse as 'standard' C motos?

One would think they would only be larger to contain more propellant, but I assume they contain no more propellant than standard C motos since they are the same total impulse.

Possibly due to nozzle geometries that could not be properly implemented in the 18mm case size? Not sure....

Can someone help explain the reasoning behind the casing size used?


Thanks,

Earl

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Earl L. Cagle, Jr.
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A C11 has a higher thrust spike than a C6 to get heavier models off the pad. Think of it as a short-burning D12 for small fields...


Bill

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Yep, I know that and what the motor was designed to do. I'm just curious as to WHY Estes put it in a 24mm case. All things equal, if it is the same total impulse as a regular C, it seems they could have packaged it in an 18mm case.

They obviously had their engineering reasons.....I'd just like to know what the rational is/was.

Earl

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Earl L. Cagle, Jr.
NAR# 29523
TRA# 962
SAM# 73
Owner/Producer
Point 39 Productions

Rocket-Brained Since 1970

We used to have a deeper cored motor with a bigger spike in the C5. Since it was an 18mm motor, it had a good spike and then a long, weak tail off. The C11 has a shorter grain, so you get more of the trust in a nice hard spike without much of a weak tail off. This is without the need of a deeper drilled core.

To get that same performance in an 18mm motor, you'd need a deeper core on an 18mm casing, and that would move out of the pintle press method and into a more dangerous drilling method.

The big question is why didn't Estes make the C11 a full 10 n/s motor? It's 8.8 n/s which is right where the C6 is.

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There was not enough space in the 18 mm casing of the C5 for any more than a 3 second delay.





Good question.


Bill

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It is well past time to Drill, Baby, Drill!

If your June, July, August and September was like this, you might just hate summer too...

Please unload your question before you ask it unless you have a concealed harry permit.

: countdown begin cr dup . 1- ?dup 0= until cr ." Launch!" cr ;

Give a man a rocket and he will fly for a day; teach him to build and he will spend the rest of his days sanding...

Yeah, I used to fly a lot of the C5 motors when they first came out.

On the C11 casing size question then, it was because of nozzle design considerations then. Ok, thanks. I just wondered why they used the 24mm case size and knew there had to be a good reason......just did not know what the reason was.

As for a ful 10ns in that motor, yes that would have been a nice benefit if they had done that.....and a curious question as to why they didn't.


Earl

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Earl L. Cagle, Jr.
NAR# 29523
TRA# 962
SAM# 73
Owner/Producer
Point 39 Productions

Rocket-Brained Since 1970

I'm not so sure it's because of the nozzle geometry... you can put a FREAKIN' HUGE nozzle on even an 18 mm motor... if you've ever seen the 'crater' in the back of a B14 you'll know what I mean... HUGE honkin' nozzle!

One thing that a larger diameter DOES buy you is a WIDER FLAME FRONT as the propellant burns outward and forward from the initial "dimple" where the ignitor goes... and a larger flame front equals more pressure, hence more thrust. (bigger post-ignition thrust spike). The only way to get a bigger flame front and change the burn characteristics for a bigger thrust spike from the C6 would be to either 1) go to a bigger nozzle and a deeper/wider propellant "dimple" (but then you have to pack the same amount of BP into the motor, meaning the propellant slug will be slightly longer or 2) drill the dimple to get more surface area for burning, like the B14... and we know THAT'S not going to happen! Switching to 24 mm is a 'cheat' to give the broader propellant slug more surface area with a similar size "dimple" and nozzle size.

later! OL JR

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Simply an additional option for 24mm rockets that usually need the higher liftoff thrust yet don't need to go as high.

Yes, you could use an adapter and a C6 motor, but that's like saying you shouldn't need an A8 motor for your Alpha, because you can use an adapter with an A3 motor.

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Roy
nar12605

And they won't do a B14 because they have to drill a deep core instead of using a pintle. I never said a word about nozzle geometry. The core depth is the issue in making a B14, not the core diameter. In fact, the wider the core, the sturdier the pintle, making the process more reliable and safer.

The C11 has a shorter grain so a shorter core will still penetrate close to the far end of the grain. The deeper the core in relation to the grain length, the more spike and less tail off (sustaining) thrust you get.

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If all performance and delay numbers were equal, an 18mm motor would be cheaper to manufacture. It would have made more sense for Estes to do an 18mm motor than a 24mm unless there were manufacturing issues that prevented it. Tunick was about profit, not variety.

The C5 didn't have the same thrust curve as the C11. There's no way they could get that same thrust curve out of an 18mm motor without going to a deeper core and sacrificing the 5 second delay. Even with that, I don't think they could do much better than the C5 without drilling a larger core. I imagine they would have been cato prone too.

In the end, the decision may very well have started out as just a 24mm "lite" motor, but I have to think they wanted a better load lifter than the C5 and knew they couldn't get something reliable and safe to manufacture in an 18mm casing.

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