Testing Motor Mount Strength

Hi,
I am new to mid-power rocketry and I have a fair few questions. I am entering a competition (G motor requirement) and am required to demonstrate that the motor mount can withstand and transmit into the body tube twice the maximum thrust of the motor and withstand a lateral force in any direction equal to a thrust misalignment of 5° at the maximum thrust value. How, exactly, does one demonstrate that?
Secondly, I have heard about "blue tubes" for building the fuselage out of. Are these appropriate for this thrust range?
Regarding the other strength testing criteria (fin deformation etc) obviously it would be good to know before building the rocket how thick the fins need to be, is there a way to know if the fins, fuselage etc are strong enough before actually building them and physically testing them? What type of adhesive is it best to use for the fins, motor mount etc?
I have gotten quite familiar with low powered rockets and the formality (testing motor mounts, fin strength etc) of all this is new to me!

Thank you for any advice,
Will

I don't know exactly how to strength test something like you describe, as most mid power rockets have through-the-wall fins, which transfer the load between the motor mount, fins, and centering rings. Do you have a bit more info about the contest and design you plan to build? That might help with strength calculations.

As for "Blue Tube" check here: https://www.alwaysreadyrocketry.com
It's a very strong variant of conventional cardboard tubes. Unless you are flying supersonic it's kind of overkill for G motors, and is very heavy compared to cardboard so if altitude is a concern be careful using it.

Thank you for the very prompt reply! The technical guidelines are here: https://ukseds.org/wp-content/uploads/2014/09/UKSEDS-NRC-2015-technical-guidelines1.pdf the part I am referring to is on page 5. This is still very early stages so we don't have a finished design yet. In the meantime I have thought of yet more questions: regarding the other strength testing criteria (fin deformation etc) obviously it would be good to know before building the rocket how thick the fins need to be, is there a way to know if the fins, fuselage etc are strong enough before actually building them and physically testing them? What type of adhesive is it best to use for the fins, motor mount etc? If blue tube is overkill what do you recommend? Thicker cardboard tubes?

You could make a dummy motor that serves as a platform to carry weights, stick it in the motor mount and stack on enough weight to equal twice the thrust. Stand the rocket up by its nose (use some sort of jig), and voila! To test 5 deg misalignment, rotate the whole thing 5 deg.

jmattingly13 said:

You could make a dummy motor that serves as a platform to carry weights, stick it in the motor mount and stack on enough weight to equal twice the thrust. Stand the rocket up by its nose (use some sort of jig), and voila! To test 5 deg misalignment, rotate the whole thing 5 deg.

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When you test the side loads for the 5 deg misalignment, you need to calculate how much force would be applied at want distance from the motor. Use thrust vectors to determine how much side force would occur at a 5 deg misalignment and then use that amount and the distance the weight will be from the nozzle to determine what weight you need to apply. Just as a Wild@$$ guess (WAG), if the max thrust is 80 Ns, you're probably looking at 50 grams or so when 30 cm from the nozzle. It shouldn't be much.

There are two ways to take the load from a motor: centering rings or thrust plate. Apogee components has a good explanation of the difference https://www.apogeerockets.com/Building_Supplies/Thrust_Plates/4in_54mm_Thrust_Plate.
Stacking weights on a spent motor is the way to go. However, for 5degree tilt it is the motor in the mount that would need to tilt, not the whole rocket. Instead of weights you could go with one of those pneumatic car jacks, but you've have to calibrate it. Harry Stine's book would be useful for most of your calculations, but mostly you want to simulate using openrocket or rocksim. If you need more sophisticated calculations then you could look of Model Rockets by Van Milligan or the book Topics in Advanced Model Rocketry (calculus needed). Good luck.