Stability question

Estes Ascender RTF, from what ive been able to read/find there hasent been to many people who have used RMS motors in this rocket. Obviously, a.p is going to burn quicker than the b.p motors it seems designed around. Anyone out there have some info they care to share? I know the weight of a loaded CTI is going to be more than the F-15 b.p motor, is the diffrence in weight a concern? What about stability off the pad as a H-410 is not a b.p.F-15. Or am i just overthinking this ? Thanks in advance for any comment, much appreciated. Tom

Spent the day assembling and finishing the rocket.. Here it is.

Still looking for input if anyone can help it would be appreciated.

CPUTommy said:

Estes Ascender RTF, from what ive been able to read/find there hasent been to many people who have used RMS motors in this rocket. Obviously, a.p is going to burn quicker than the b.p motors it seems designed around. Anyone out there have some info they care to share? I know the weight of a loaded CTI is going to be more than the F-15 b.p motor, is the diffrence in weight a concern? What about stability off the pad as a H-410 is not a b.p.F-15. Or am i just overthinking this ? Thanks in advance for any comment, much appreciated. Tom

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As with any rocket the basic laws of stability remain in effect. The increased motor weight will definitely have an impact. Either calculate CG and CP and verify that CG is still at least one body tube diameter in front of CP with the heavier RMS motor. Or build (or see if you can download) a model of the Ascender for the free simulation software OpenRocket. This will calculate the CP and CG for you as well as the stability in calipers.

Also, you can check your thrust to weight ratio with your proposed motor, and make sure that you have at least a 5:1 margin.

Good luck.

You're not over-thinking it, in fact you're asking good basic questions that should be asked with any rocket you fly.
I did find a file for this one to plug into open rocket, which I can't recommend enough to answer these questions.
When you start getting into motors this size it becomes time to ask more questions actually.
Take all the time you want to think, question, double check, and verify.

All the CTI motors I've checked so far are showing stable numbers, from 1.3 to 1.5 calibers.
The 1 grain and 6 grain being best, as the 1 grain are lightest, and the 6 grain start putting weight ahead of the CG.

However, you really need to keep an eye on you maximum thrust. It's only a 2in 11oz rocket, very easy to over-motor it.
For example, the H410 you mentioned hits 111g and gets close to supersonic. Even the H160 is right at 50g and over 700mph.
Anything over 100n average impulse, regardless of total impulse, gets fast enough to risk airframe failure.
I honestly would only stick to motors up to about 60n of average impulse, and even then just the F's and G's if it were my own rocket.
Also have to watch your delay times pretty close. Due to the speed it'll be hitting, you're going to have a long coast phase.

-Hans

(Ok, I think I've corrected most of my grammar problems finally. Sorry about the continuous edits here.)

Hans is right on. Not only is stability a variable, but also speed. The faster a rocket goes, the more likely it is to have the fins rip off, or the cardboard airframe crumble, or drag separation pull the nose cone out earlier, which means a zipper (shock cord rips through the airframe).

Rockets that are designed for mach or near-mach speeds are usually made of much more durable materials like phenolic or fiberglass, and are epoxied together rather than glued.

You can use the web site ThrustCurve to see what kinds of speeds to expect for your rocket with different motors in it. Click on "Motor Guide".

When you say rip the fins off .. do you mean something like this ? All 4 at roughly the same spot, at least the fin can survived.. kinda...

View attachment 269724View attachment 269725View attachment 269726

As previously mentioned, stability is not the only issue. This is a very lightly built rocket designed to be launched on Estes 29 mm BP motors so strength is also a concern. The kit includes a motor block to be set at 4.5" to limit the length of the motor that can be put into the rocket. Estes also sells relabeled 29 mm SU AT motors that will fit in this rocket and will be safe to launch.

IMO this rocket is suitable for any model rocket motor you can fit into the stock motor tube with the motor block installed in the factory 4.5" position. If you go to thrustcurve.org and input the following rocket parameters: 2" OD, 11 oz. rocket weight, 29 mm motor mount, and 4.5" motor mount length and run all engine, you will get a list of engines that will fit and be safe to launch in the rocket. You will observe all the motors listed will keep the maximum velocity to 600 fps which is below a velocity that I would expect to see fin flutter for such a lightly constructed rocket. If you rerun the thrustcurve motor guide with a 15" motor mount (no motor block) you will see a lot more engine options. While they will fit if you leave out the motor block, IMO any larger, higher impulse motor that will push the maximum velocity over 750-800 fps should not be used to avoid to the possibility of a fin flutter shred or a column buckling failure(airframe foldover due to excessive dynamic pressure/acceleration). I personally would recommend similar priced and sized LOC kits for L1 flights simply because they are designed from the outset for high power flights with thicker airframes and stronger fins.

Bob

Note: This reply was written before the previous post which demonstrates the limitations of launching a nice model rocket kit on a Level 1 motor.

You could have fiberglassed the airframe with a couple of wraps of 3 oz FG and it would have been fine. At least the airframe would have been, as longer airframes will want to bend at the middle and the FG will stiffen it well to resist flexing. The problem is with the plastic fins. You would have been fine at Mach .6 or less. At Mach .6 to .83 it would have been dicey, especially at the higher end. Any faster than that and it would have been "all bets are off", at least for me. Once your rocket goes transonic, your fins need to be 1) stiff enough to resist flexing a fair amount and 2) be well-adhered to the airframe. If you had replaced the fins with 1/8" G10 you likely would have been fine.

I like the Ascender, but out-of-the-box it has it's limits.

Greg

Estes doesn't design kits for high speed. They design rockets to work with their motors, and especially the E2X kits are not designed with high velocities in mind.

I don't know why someone would want to glass an E2X rocket, since it is not a builder's kit to begin with.

If you want to fly high speed and big motors, I would recommend stepping up to either a PSII builder's kit, or a non-cardboard builder's kit.

I intentionally left out the motor block so that I could fly my RMS C.T.I. As for "column buckling failure(airframe foldover due to excessive dynamic pressure/acceleration)."

I know that personally.. I didn't think that the Estes tubes would do that as they seem much beefier than my last fold over.. Ill figure ill start "low" and see how it goes.. from low to heads up..