Measuring stress on drag device

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Bill Hanson

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Hi All,
I’m planning my L3. I have a 4” fiberglass MadCow DX3 XL, and planning on using an M1350 motor.
However, this combo busts the altitude waiver, bottom line is that I need a planned apogee at about 11,000 AGL. Thought first about adding weight, but need close to 4 KG extra which creates other problems.
Based on earlier discussion, I’m thinking of adding a doubled fiberglass centering ring that sticks out an extra 1/2” and bolting that to the thrust plate — that way I can take it off for a higher waiver. That solution sims out to right at 11k; exactly what I need.
My question is how best to measure or simulate the stress/flutter on the protruding ring. Max velocity per OpenRocket is Mach 1.1. My intuition is that since the centering rings handle the acceleration forces from the motor without problem, they should also easily handle the aerodynamic deceleration forces.
I think it’s fairly straightforward to measure the aerodynamic pressure in free air, but that’s not the situation at the tail end of the rocket. Is it a valid assumption to say that aero forces would be less at the tail than in free air? I’m pretty sure the tail end wouldn’t see supersonic airflow since it’s most likely in the subsonic area behind the shock wave, in which case the pressure in free air would always be greater.
Any ideas or suggestions greatly welcome!
Thanks,
Bill
 

Buckeye

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Worst case, assume Cp=1 (stagnation pressure) on the entire plate and see if it holds up.
 

AdAstraPerAspera

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I would advise against this. Build a larger diameter rocket for your L-3 if this is the field you are used to flying on. Unless you are used to flying high and recovering near minimum diameter rockets, a 4 in bird for an L-3 sounds like a bad idea. In fact, our local L3CC and TAP members specifically advise against it.

That said, if you really want to do this, get flight data on this drag thingy with smaller motors, and/or try it on a smaller rocket first. If it rips off, you will bust the waiver. If your estimated drag is low, you will bust the waiver. Drag on things like this will probably change drastically around Mach.

Talk to your TAP or L3CC. I really don't they will approve this.

The rocket you are building sounds like a great one, and could fly on some awesome L-2 motors. Get some experience with those, and take it to another feild, or build a bigger rocket.
 

Bill Hanson

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I would advise against this. Build a larger diameter rocket for your L-3 if this is the field you are used to flying on. Unless you are used to flying high and recovering near minimum diameter rockets, a 4 in bird for an L-3 sounds like a bad idea. In fact, our local L3CC and TAP members specifically advise against it.

....

Talk to your TAP or L3CC. I really don't they will approve this.

The rocket you are building sounds like a great one, and could fly on some awesome L-2 motors. Get some experience with those, and take it to another feild, or build a bigger rocket.
Thanks for your thought, even though I’ll admit I don’t like the answer.

But, just because I don’t like it doesn’t mean it’s wrong. I did want to avoid having to spring for a whole new kit and having a white elephant sitting in my garage. I get your point about the possible failure modes, which is why I wanted to be able to simulate or calculate the forces first rather than test by flying. Several thousands of military flight hours has instilled significant caution about such things. I suppose that popping out some drag tabs, sort of like I’ve seen with student active drag setups at IREC would be another way to go, but that sort of invalidates the KISS principle. Another possibility is just to paint the thing rough — 500 micron roughness sims out at about the same as the drag disk. Black chip guard should do that trick, but then I’m stuck with it.

The other possibility is that the club does have sporadic access to a launch area with a much higher waiver, but I would rather avoid having to drag a bunch of folks out there just on my behalf. Besides, half the fun is figuring out how to overcome the challenge and make it work within the constraints. I know the Apogee video on increasing drag also advises using a drag disk, but that was in the LPR/MPR range, and as you note, the aerodynamic forces in the trans/supersonic realm are quite different. That’s why I am looking for a way to calculate (at a minimum) the worst-case pressure on the drag ring. That way if the ring and bolts can handle it with a healthy safety margin, I can be confident it will stay attached and in one piece.

I’m having lunch with my L3CC after the holidays, so want to have some data or calculation pulled together so we can discuss this as an engineering question rather than an opinion question.

Again, thanks for your views on this. I may well end up going that way, but want to thoroughly evaluate the options first. The famous Winston Churchill aphorism about “You can always count on Americans to do the right thing ...” springs to mind here.

Bill
 

Bill Hanson

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Here are my "worst-case" calculations for aerodynamic stress on the drag ring -- please feel free to correct my work!

Given the drag disk extends 1/2" from the 4" airframe (114 mm vs. 98 mm), the extra area of the drag disk is pi*57^2 - pi*49^2 = 848 sq mm = 0.000848 sq m

Assume a flat plate (Cd = 1.21) of that area and a velocity approaching transonic = 300 m/sec, and an altitude of 5000' (approx max velocity point given altitude at the launch site. At 5000', rho = 1.0535 kg/m^3

Using the basic drag equation D = Cd*1/2*rho*V^2*A, and substituting the above values, gives a drag of about 49 newtons.

Since the Cd increases between 2 and 3x as velocity approaches Mach 1, that would (worst case) triple the drag = about 150 newtons, or about 34 lbs-force. Since the tensile strength of a grade 2 1/4" bolt is 2350 lbs, and using 3 bolts to attach it to the thrust plate, that's a well over 10x safety margin.

I'm planning to use a doubled G10 fiberglass ring, so the other failure mode would be bending force on the fiberglass causing it to break. Given that the extra aerodynamic load is far less than the load on the fiberglass centering rings due to acceleration of the motor, I would assess that the chance of such a failure is negligible. I suppose that I could have the thing machined from aluminum, but I think that's overkill (but would sure look cooler).

The other potential issue is the additional stress on the thrust plate itself along with the aft CR. Since it's designed to compress against the outside airframe under acceleration (1765 newtons max for an m1350), the extra drag would actually reduce the load on the thrust plate/aft CR while the motor is cooking, and since the assembly is more than capable of holding that load, the extra drag is well within the structural capacity of the thrust plate and aft CR.

This covers the three failure mechanisms that I can think of -- drag ring separates from the thrust plate, drag ring itself breaks, and drag ring pulls out the thrust plate/aft CR. Are there other failure modes that I should consider?

Again, please critique my calculations and reasoning. What did I miss, and what other things should I be considering?

Thanks in advance,
Bill
 

AdAstraPerAspera

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A few things in no particular order:

The other failure is of course that it doesn't provide the drag needed.

98mm is the ID of the body tube. You need the OD.

Next, since it is on the aft end, you need to account for the boundary layer thickness. This is the slow moving (or rather, moving with the rocket) air right along the surface of the rocket.

The disruption caused by the plate also reduces the effectiveness of the fins. You gain base drag so this likely isn't a problem, but look closer into this if your stability margin is low.

You will also need to add in the pre-load on the bolt from torque.

Washers will help distribute the load from the bolts to the plate. Enough said.
 

Bill Hanson

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A few things in no particular order:

The other failure is of course that it doesn't provide the drag needed.

98mm is the ID of the body tube. You need the OD.

Next, since it is on the aft end, you need to account for the boundary layer thickness. This is the slow moving (or rather, moving with the rocket) air right along the surface of the rocket.

The disruption caused by the plate also reduces the effectiveness of the fins. You gain base drag so this likely isn't a problem, but look closer into this if your stability margin is low.

You will also need to add in the pre-load on the bolt from torque.

Washers will help distribute the load from the bolts to the plate. Enough said.
Thanks for your reply -- good points all.

Part of this exercise is because I'm certifying under NAR rules. The actual waiver max altitude is 12,664' (17,000 MSL - 4,336 launch altitude). Per the NAR checklist, the predicted altitude must be no higher than 90% of the waiver = 11,398'. So I have essentially two targets -- it can't simulate an apogee of more than 11,398', and it can't actually bust the waiver altitude of 12,664'. The "stock" DX3 XL with 200 gms of extra weight at the base of the nose cone, with an M1350 sims out at 12,338' apogee -- that fits (barely) within the waiver. Adding the drag disc is necessary to meet the 90% criteria, and that drops the simulated apogee to just about 10,000'.

Accordingly, if the drag disk does not produce the full anticipated drag due to boundary layer issues, then I'm still within the actual altitude waiver. I'm also going to use HyperCFD to try to get a better handle on the exact drag differential -- climbing the learning curve as we speak.

[Edit] Cd calculations (entire rocket)
Program -------- w/o drag ----- w/drag ----- Mach
HyperCFD ------ 0.50 ----------- 0.57 --------- 1.1
OpenRocket --- 0.79 ----------- 0.88 --------- 0.9
OpenRocket --- 0.67 ----------- 0.73 --------- 0.3

This tells me that the drag differential is relatively consistent across the velocity range, and the OR calculations are pretty much in the ballpark. [/Edit]

The stability margin of the rocket without the disk is 1.22 caliber, and that decreases to 1.09 caliber with the disk. Once I get the thing built and have a chance to look at the actual weight (which, based on my building history will be more than the stock weight) and balance, I will most likely add more weight to the nose to bring the stability margin closer to 1.5.

Good catch on the tube diameter -- should be 102 mm, slightly decreasing the drag area.
Great point about the torque load -- definitely don't want to over-torque the bolts! Washers for sure.

I've also attached the "with" and "without" .ork files for comparison purposes. Appreciate all the input!

Bill
 

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Bill Hanson

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I would advise against this. Build a larger diameter rocket for your L-3 if this is the field you are used to flying on. Unless you are used to flying high and recovering near minimum diameter rockets, a 4 in bird for an L-3 sounds like a bad idea. In fact, our local L3CC and TAP members specifically advise against it.

That said, if you really want to do this, get flight data on this drag thingy with smaller motors, and/or try it on a smaller rocket first. If it rips off, you will bust the waiver. If your estimated drag is low, you will bust the waiver. Drag on things like this will probably change drastically around Mach.

Talk to your TAP or L3CC. I really don't they will approve this.

The rocket you are building sounds like a great one, and could fly on some awesome L-2 motors. Get some experience with those, and take it to another feild, or build a bigger rocket.
Just to close the loop -- successful L3 cert on the 4" DX3 XL. Nothing fancy, but did go to a launch location in the area with a higher waiver. 12,500', Mach 1.1 (conservative calculation)

L3 certification!
 

BABAR

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Congratulations!
 

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