My two cents... long post ahead!
I've been there, done that. This hobby has been my obsession since I was seven. I've built and flown A LOT of rockets in the past nine years. I went to my first LDRS when I was 12 years old. And there, with my dad, we successfully flew two J motors, several I motors, and lots of smaller rockets.
I have built a Quik Tube rocket, and it's a lot more involved than it seems on the surface. There's a lot more structure inside the airframe than there is on a phenolic or 'glass rocket because of the weak nature of the tubing. And the rockets get real heavy, real quick. On the Nike-Smoke I'm working on (which I am, I swear!
), the tubes only weigh about 2 lbs each. But this rocket is going to have to be capable of withstanding the force of at least 3, perhaps 5, 75mm M motors. So I'm gonna have to add a lot of structure behind it.
When I built "No Sniveling", I figured it might come in around 15-20 lbs. The original intent of that rocket was to fly the Kosdon 2550 case. But as things progressed, I couldn't get it to come in around that weight with the structural requirements I had for the rocket. It ended up at 25 lbs DRY. This is a 7.5" Quik Tube rocket. Building with 8", it's highly unlikely the rocket will be light enough for even very thrusty K motors while being strong enough for an M2500GG.
When designing rockets, three years (or more) will definitely make a difference. In the four years since I was 12, M motors became commonplace, composite technology advanced, EX became mainstream, and I gained a huge amount of understanding about the hobby. Granted, I probably could have flown an M at LDRS XVII, but something probably would have gone wrong in the flight. Under the huge forces of a K or L or M motor, things go wrong that you don't expect them to. Example: these motors (especially 98mms) exhibit quite a bit of vibration in flight. If you aren't prepared for it, this may set up a flutter wave in your fins much faster than expected, and tear them off.
Now Neil, you seem a bit ticked off at the moment, and what I'm about to say may tick you off a little more, but it's stuff that I've learned going through exactly the same thing you are. It sounds like all of these projects you're mentioning (PVC rocket, sonotube rocket, etc.) are being run on a shoestring budget. I think you'd be much happier with the outcome of your projects, and a lot less stressed on launch day, if you focused your time and energy on one project until it is completed, then move onto the next. Little things crop up in the midst of these projects -- you need assorted hardware, more epoxy, more fiberglass, etc. These all add up to a good chunk of $$ you spent on the project. If you short-change each project and don't complete it with the best materials, et al., you are risking an in-flight failure. After building and flying one Quik Tube rocket, I don't want to build or fly any more. I'd much rather go about it the usual way, with fiberglass or phenolic airframes and such. These things take a lot of time to build, and even more to fly. Also, prepping a K motor rocket is MUCH more time consuming than prepping an H or I motor rocket. I can get off 4 or 5 Hs or Is before lunch now, but I can barely fly one K motor, even with pre-prep the night before. An M motor takes me all day. Basically, you'll be lucky to get one big project off the ground at an LDRS, much less three.
Also, from what I've read, you don't have the construction or reinforcement skills to properly construct a project capable of withstanding a K motor, much less an M. These things require lots of epoxy and materials which will properly accept the epoxy. PVC does not count. In that particular rocket, upon ignition, 10:1, your motor mount will fly up through the rocket and off into the sunset because epoxy does not stick to PVC. If you're flying a 30 lb rocket with a 7:1 T/W ratio off the pad (not unreasonable), that's 210 lbs of force in shear against the airframe walls.
For what it's worth, my advice is the same as everyone else's. Work your way up slowly. Fly a couple Is, then a couple Js. Then a K or a small L. Enjoy the process -- you can fly 6 K550Ws for the price of an M1939W.
These larger motors pack a surprising amount of thrust -- a K motor will take a fiberglass 4" rocket 8000 feet, an L motor over 12,000 feet, and a mid-size M motor will take a 40 lb, 6" rocket over 11,000 feet. There's a lot more to flying these things than simply sticking together a bunch of pieces and flying the rocket.
But if you do choose to press ahead with this project, here are a couple things to keep in mind:
- Electronics -- what sort of electronics will you use? On an L motor or above flight, I'd use at least two units to back each other up. How will you contain them in the rocket? How will they be switched on? What will the gas seals be made out of? What will you use to fire the deployment charges?
- Recovery -- what are the hard points for the recovery system? Closed eyebolts or U-bolts are a must. Dual deployment or single deployment? Will you use a deployment bag with a pilot chute? Make sure you know how to pack a d-bag for successful deployment. What other flame protection will you use? Dog barf wadding does not count
- Airframe integrity -- is the tubing strong enough to handle the dynamic loads of the flight? What about the recovery forces? Will the design zipper on deployment? What sort of reinforcement, if any, are you using? How is it being applied? Pantyhose is not a wise choice for high-stress airframes.
- Fin integrity -- is the fin material appropriate for the power class and velocity anticipated? Remember, if you plan to fly M motors, the rocket is an M motor bird, and should be constructed as such. Will the fins flutter? If using plywood, are the leading edges properly treated to avoid delamination in flight? How are the fins attached?
- Motor mount integrity -- how many and what thickness of centering rings are you using? How are they attached to the body tube? Is there a method to positively retain the motor casing? Is there a method to positively connect the recovery system to the aft centering ring?
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That's all I can think of off the top of my head, and that's not all of it. I'm sure I left out some points. But you have to remember, the peak thrust on an M1315W is nearly 600 lbs. An M2500GG will have even more thrust than that. You're stacking a gorilla on your rocket in more ways than one Will it be built to handle it? And if it's not, will you be emotionally prepared to lose everything that you have put into it?
From my point of view, as a 16 year old, and having experienced the exact same thing. YMMV.