Need some advice for pushing my fiberglass Painkiller 3 a bit harder

[HHaase]

Ok, this is a weird post, but I feel like a noob even asking this one. I'm looking to start flying some of my stuff higher/faster than I've done previously, deep into the K/L size range. The most I've done so far is a barely-K sized motor, and high-ish subsonic. So I could really use another set of eyes on my numbers to see what I may be missing.

My current highest performance rocket is my Painkiller 3. I just honestly don't know when I need to be concerned with the structural strength when it comes to motors and forces during flight. I can fit a 5 grain 54mm CTI motor, just barely, and both the K570 and K600 hit my target flight of 10,000ft and supersonic. I did full injected internal fillets with West Marine G-Flex. External fillets are G5000. I'll be using my Raven altimeter for dual deploy control. This rocket has flown before on a K360 without issue.

OpenRocket gives me very similar performance with both of these motors. About 45mph off the rod, peak velocity just under 850mph, 15-18g for acceleration, 1.18 calibers of stability at launch, and I won't bust the waiver. Logically, this all looks like an exciting but perfectly achievable flight. But my instinct is telling me I'm overlooking something big.

Am I missing some other parameter that I should be looking at?

 

[dhbarr]

That's all g10/g12 fiberglass parts, right? 54mm in a 3in? Structurally i would think you're fine.

But isn't 1.18 a little low for something that's going to spend that much time in the transsonic region?

 

[richP]

Tracker?

 

[HHaase]

I’m leaning that way too, the more I research. I know the Cg is going to move forward pretty fast with this rocket and motor combo. But it just may not be the right rocket to push this hard. If I’m not comfortable with the answers to questions like that I won’t proceed.

It is a fully g10/g12, 3” with 54mm mount. And yes, with tracker.

 

[OverTheTop]

Sounds like it should be no drama at all.

 

[richP]

I’m leaning that way too, the more I research. I know the Cg is going to move forward pretty fast with this rocket and motor combo. But it just may not be the right rocket to push this hard. If I’m not comfortable with the answers to questions like that I won’t proceed.

It is a fully g10/g12, 3” with 54mm mount. And yes, with tracker.

OK. I didn't see a tracker in your OR file and figured it would move the CG forward a bit if you installed one; in addition to helping recover the rocket.
Otherwise, I think you'll have little to worry about. You could probably push it even faster/higher, if desired. Just don't forget the pressure relief holes.

 

[HHaase]

OK. I didn't see a tracker in your OR file and figured it would move the CG forward a bit if you installed one; in addition to helping recover the rocket.
Otherwise, I think you'll have little to worry about. You could probably push it even faster/higher, if desired. Just don't forget the pressure relief holes.

I am pretty comfortable at this point that the structural elements of the rocket are up to the task. But I'm still unsure of the stability.

I may have factored the eggfinder into the weight of the nose cone already, but I can't remember. I was still fairly new to using sim software during this build. I'll have to just re-weigh the nose and verify that. Easy enough to do. If I missed that, it would be a big help since I mount my trackers up inside the nose cone. I'll also measure the actual no-motor Cg as well, to make sure the sim didn't mis-calculate something.

Another new element to me is that the motor is long enough that it extends forward past the Cg. I'm completely at a loss in understanding how that will alter stability during flight. If I could fit a 6 grain motor (I can't, too long), then I'd actually have an improved stability margin at launch. That and the propellant is going to be almost right at 25% of the total launch weight. So I do know the Cg is going to rapidly shift forward by the time it reaches burnout. I just don't know what will happen in the first two seconds. This is the area my experience fails me.

At least I don't have to worry about the recovery gear shifting aft under thrust. There's essentially no room for it to move with a 5 grain casing installed.

-Hans

 

[boatgeek]

I would run a stability vs. time graph once you have the sim updated with a good dry weight and CG. If you're barely kicking past Mach, you'll have burned off a lot of propellant and improved stability quite a bit before you hit transonic.

 

[HHaase]

Looks like I had already accounted for the weight of the tracker in the nose, and had already done an over-ride for the measured Cg of the rocket. So the numbers are legitimate in that regard. That got me into a more in-depth look and I did finally find a way to plot the Cg in relation to Mach speed.

With the K570 classic, the stability margin is 1.31 calibers at liftoff and it does climb rapidly. When the rocket hits supersonic the margin has reached 2.5 calibers, and the margin peaks at 2.8 calibers at burnout. Time above mach is only about 1.5 seconds.

The K780 blue streak has a very similar Cg/Stability curve up until supersonic. After that it peaks at closer to 3.2 calibers in margin at mach 1.22 instead of mach 1.1

These two propellants seem to be the most efficient from a weight to impulse standpoint, and give me the most stability while still hitting my altitude/velocity goals.

 

[DizWolf]

I can't remember the motor, but I know GaryT isn't one to leave much room in the booster. This is an old photo of his painkiller 3. Structurally, I can't imagine any motor that fits giving it an issue. Stability, my gut says should be fine, but those are kinda smallish fins...