Questions on build materials

[mangulator]

This subject may have already been discussed and if so I apologize.

I have never built or flew anything passed a "D" class motor and have only built and flew Estes Rockets.

I ran across a video on high powered rocketry and it peeked my interest. So I watched a few more of the "Class 1 through 3" launches and found them fascinating. I did a google search and went through a few websites and started to learn a little more on high powered rocketry. Im into photography and shooting video with a drone and I really liked the rockets that had a video camera on board.

I am a bit curious about a few subjects and am wondering if someone could help me with a few questions.

My first question is about some of the materials used in building high powered rockets. I have noticed most of the higher powered rockets have a lot of heavy metal parts used in their construction, for example the electronic bays and where shock cords are attached to the droge chutes and parachutes. I have not run across any builds or kits that use alternative materials and I'm curious as to why so much added weight is incorporated when I'm sure there are alternatives to save weight in these areas.

I have seen some great examples of rockets with the fins and tubes built of lightweight carbon fiber or fiberglass, but then I see that they have heavy duty steel 1/4-20 eye bolts and thread rod in their construction.

I'm sure that some of these parts could be made out of a lighter carbon fiber or resin material. For example, Im sure that a avionics bay designed with shock cord attachments molded in made out of carbon fiber or a strong resin would be lighter and just as strong. With 3D printing and CAD technology so readly avilable nowadays I'm sure it would be possible to design and build a complete avionics bay to custom fit any size rocket. Im sure its possible to design and use a 3D printer to make a mold in order to cast the part out of lightweight but strong material. I wonder if anyone has tried to create a bay designed around a set of particular components with redundancy in mind with ease of mounting.

Like I said Im really new to the high powered rockets and there might be rules as to what you can and cannot use in construction.

I have seen a lot of videos on construction methods when it comes to using epoxy or JB Weld, and the importance of putting it on liberally.
I can understand the importance of having a good bond.
However I have not seen anyone who has added screws to between the motor mounts or the centering rings and body by counter sinking, drilling and tapping out these parts. I would think doing so would add additional strength at very little cost especially if you still adhere to the liberal use of epoxy or JB Weld.

I also know that it is imperative to have a balanced rocket especially at these higher power levels and that is why it is so confusing to me as to why I'm seeing the use of heavy steel bolts used in places that to me would make it much harder to balance. I have read about rocket kits that may have a need to add nose weight, it seems that you would want a rocket that was strong and ridgid but light weight.

I was reading through some build instructions of a high powered kit and was wondering why the kit had all these heavy metal and plywood parts but had a fiberglass body and fins and a plastic nose cone. I thought to myself if I had this kit I would try and find a way to improve upon and or replace some of the parts and improve on some of the construction methods without altering it in a way that would make it dangerous to fly.

I'm thinking of getting into building a rocket that would allow me to attach a video camera and I'm learning as I go along. I'm going to do a lot of research before I get in to deep and really do not plan on getting into high powered rockets until I check out on some local clubs.

I'm just wondering if anyone had any thoughts on this.

 

[dhbarr]

Good stuff! Observing, reflecting, asking questions; wish I remembered to do that all the time. WRT safety note that standoff distances and range layout are big contributors to making it all work

There are a lot of ways to build a rocket "right", and even more ways to build a rocket "wrong". Most of what you see -is- overbuilt, probably because a) underbuilt ones do not survive and b) it's hard to resist adding a little more margin "just in case".

You absolutely can fly a small camera before needing to be high power certified -- your local club would love to help!

 

[prfesser]

Hello Mangulator,

One difference to consider: sport high-power is rarely concerned with weight unless it's for a special application (altitude attempts, rockets that break mach...). Sport modrocs are dealing with smaller motors, often with far lower total impulse, so it's much more important to consider weight and weight distribution. In most HP rockets, changing from (for example) a 1/4-20 eyebolt to a 3/8-16 eyebolt is considered trivial. In modrocs, frequently every gram counts.

Sport HP construction, except in extreme projects, uses much the same techniques as modrocs, though the materials and specifics may differ. Here are some commonly seen differences.

Modroc: balsa fins, surface-mounted; HP: plywood or composites (fiberglass, carbon fiber) with through-the-wall tabs.
Modroc: thin lightweight paper airframe; HP: heavier cardboard tube, or phenolic or other composites.
Modroc: balsa or thin plastic nose cones; HP: thicker-walled plastic or composites (sometimes nose cones are turned from basswood or other light wood)
Modroc: snap swivel on chute; HP: quick links or heavy duty swivels.
Modroc: white glue suffices for most construction; HP: carpenter's glue for wood and paper, or epoxy.
Modroc: rubber shock cords, plastic chutes; HP: nylon or kevlar cords, nylon chutes.

If you can build a model rocket that can withstand the largest motor you can stuff into it, you should be able to build (even design) a high-power rocket by some substitutions. My L1 rocket was a kit that used two D or E motors. Changed it out for a single 29 mm motor, epoxy construction and heavy fillets. I had no instructions and virtually no guidance other that what the kit provided, and apart from the motor mount change it was constructed pretty much the same as is any modroc. The rocket flew successfully at least half a dozen times before it pranged.

Best -- Terry

 

[Scott_650]

John Coker’s website is a solid reference for HPR rocketry - not the only way to do things, but very solid and accepted techniques and practices http://www.jcrocket.com/

 

[David Schwantz]

As far as metal parts for recovery attachment, there may be several 100# of force at deployment. A "snap" at apogee has broken many a shock cord. Using metal eye bolts, u bolts, all thread can eliminate a weak link. On a lot of the smaller stuff a wrap around the MMT and glue is great for attachment. But the use quick links allows for very easy replacement. A loop in the cord large enough to run the NC through can rid the need of an eye bolt in the NC. This can also be done at the Av bays. Threaded rod through the AV bay transfers the force from shock cord, through the bulkheads, and to the cord attached to the other side. You can run threaded rod down the center also to rid the need for two of them. Electronic sleds are printed all the time, and are very nice. But I would never trust a 3D printed part for recovery attachment.

 

[dr wogz]

• The forces applied are an exponential factor, not a linear factor. So, it' not just a mater of doubling things up.
• The strength of materials plays a role, and in this (most) case, metal parts offer the strength required, with a slight weight penalty. Cost can play heavily into this as well. And, parts availability. We can get titanium parts but teh cost & scarcity makes them out of reach for many.
• In some cases, being that little bit conservatives can play dividends on being confident.
• Are a few grams here & there significant to an overall rocket of 5 or 10 pounds?
• Some (sadly) will build their rocket to survive a crash rather than build it to fly as intended, hence the extras added; glue, parts, doubling up tubes, etc..
• Engineering is all about compromises.. You might be able to shed a bit here, but something else will likely increase (and this also applies to the all mighty dollar!)

And welcome to the forums! You'll find a lot here! (including varied opinions! )

Feel free to ask your questions & voice your concerns. We all will do our best to help & share some (persona) insight.