Fiberglass or PML phenolic MMT's??

[threelapfancy]

Hey all. Adding a few more rockets to my collection this winter. Going to use Hawk Mountain tubing and NC's. We all know that fiberglass can be rather expensive. Can you guys shoot me opinions regarding using fiberglass tubing for motor mounts compared to phenolic when using fiberglass tube. I have several fiberglass rockets already but they all have fiberglass MMT's and centering rings. Any substantial differences between using phenolic MMT's and wood centering rings?

Doing this would sure keep costs down on the rockets.

BTW, these will be large rockets, flying M and N impulse motors.

Thanks!
Steve

 

[Pantherjon]

Structural differences? Not much really..Strength differences? Quite a bit..At least as far as the motor tube..The phenolic material is brittle and can easily crack..But, really, the diferences, I don't believe, are that significant in this application(just one HPR novice's opinion)..If what you are going after is to save a few bucks(and who isn't!?) then I think you will be fine going with the phenolic motor tube and plywood centering rings..I know of a couple people who have flown 'M's and 'N's with that combo with no problem!

 

[blackjack2564]

I would highly recommend sticking with a glass MM tube. Wood centering rings no problem, but as in a chain your rocket is only as strong as it's weakest link.

I have had many extreme flights go wrong for one reason or another, and a great many of them, the rocket survived intact.

The only 2 I would have changed anything were the 2 that were all glass rockets with phenolic MM tubes. One fell from 1000ft when the main tangled. everything was intact with the exception of the MM . the shear stress of 50lb rocket hitting hard pack ground at a slight angle transferred all the energy to the side of the mm and shattered it internally. No other damage other than paint, but yet the fincan was really irreparable without a total rebuild.

All for the sake of an extra 20 bucks, an entire 600 rocket was toast. Granted the upper part was re-used, but a new fincan was needed.

The other case was a 4 in glass rocket that flat spinned from 12,000 ft onto soft dirt and high grass, but by landing horizontal flexed the fincan. The glass absorbed the shock. The phenolic MM did not and was toast.

2 great rockets no longer flyable for 2 very different reasons, but both suffered from having the only none glass parts being MM tubes. [CR's were wood].

So yes, definitely stick with the much stronger glass. In the overall price of a project , it is not much of a money saver. If I HAD to save 20-30 bucks I would go with the glass and forget paint!

Especially on a M-N project. Both incidents cited were M's and other rockets of mine with glass mm HAVE survived similar mishaps.