Going Bigger and Bigger - What do I need to do??

[valkrider15]

I keep moving up the scale on my rocket builds. I have built a couple fully scratch models in the smaller sizes - 29mm motormount in a 38mm and 54mm tube, and have done some mods to larger kits (glassed tubes and fins on a 4" fanton , added zipperless and dual deploy) and extended my PML Callisto and added an eBay.

I now am looking at building a scratch rocket based around a 5.5" airframe and a 54mm motor mount. I's like to be able to fly up to full Ks in it and was wondering what build techniques I need to employee to make sure it has the strength to handle it.

I will certainly be glassing the airframe and fins with 2 layers of 6oz and 2oz over that. I've noticed many of the big rocket fin cans have allthread running through the centering rings. Is this something that I should do? I assume this helps to distribute the load across all the rings. Do you just sandwich the centering rings between nuts / washers on the allthread? Should the allthread be directly connected to the u-bolt for the recovery system? My centering rings are planned to be 3/8" baltic birch -I've seen this size and they seem very stout.

Are there any reinforcements required on mounting the fin can into the airframe? Does it make sense to add screws through the airframe into the centering rings for added strength or will nice and thick fillets with good epoxy be enough? Maybe add a layer of fiberglass around the rings as well?

What do you suggest on fin material? Plywood with glass / carbon or G10? If I go G10 will the .09" thick stuff be enough or should I go to .125"? If I go plywood / glass, what thickness should I use?

What are the other things I am missing? Just in the early planning stages so am open to any and all comments. Here is a picture of the parts so far. Thanks!

-Greg

 

[EMRR]

Originally posted by valkrider15
What are the other things I am missing? Just in the early planning stages so am open to any and all comments. Here is a picture of the parts so far. Thanks!

-Greg

Cash!

 

[valkrider15]

Well I got my tax refund and have put some aside for this project - so at least for the short term I will be covered. Even put a little extra aside for a second Altimeter. So now that the bankroll is covered back to design / construction ideas and tips.

 

[edwardw]

If you are going 5.5 - I would say put a 98 or 75mm mount in the rocket. You can always adapt down. As for the fins, I'd personally go with 1/8" G10. I would drill holes where the root attaches to the MMT and then when it sticks through the body tube so that you can make some epoxy rivets. If you epoxy both sides of your centering rings with some slower/thinner epoxy you shouldn't need any screws at all. I've never used all thread - but a lot of people do.

Edward

 

[valkrider15]

I debated about going to a larger size motor mount, but have just recently got my L1 and will hopefully get my L2 at NSL so I wanted to spend some time in the 54mm range. I also have a brand new 54mm case set that needs to get used.

I will be building the rocket in pieces and if I decide that I need more motor I can always just build a new fincan / booster for it. I am lookin at this as a learning experience so I don't mind if I outgrow it (although I think that will take a while for sure).

 

[edwardw]

I don't think you can ever have too much motor. Plus - you may someday have the urge to do a 75mm motor

Edward

 

[Johnnie]

...and when you want to do 75mm you use a 98mm mount and adapt down...it's a vicious circle. Wait till you want to put a 98mm motor in and need a 152mm mount to adapt down from

 

[Chuck Rudy]

Originally posted by valkrider15
I debated about going to a larger size motor mount, but have just recently got my L1 and will hopefully get my L2 at NSL so I wanted to spend some time in the 54mm range. I also have a brand new 54mm case set that needs to get used.

I will be building the rocket in pieces and if I decide that I need more motor I can always just build a new fincan / booster for it. I am lookin at this as a learning experience so I don't mind if I outgrow it (although I think that will take a while for sure).

I'm with Edward on this one.....I'm now building all my stuff as open as possible, then quick change clusters or big motors are now problem. If you've already acquired parts that's one thing but if you've not ordered a thing make a 5.5 incher with a 4" mount, then MMA down. Or use the plywood and all thread quick change method. And once you move to this method the quick change works in every rocket you build.

Chuck

 

[valkrider15]

plywood and all thread quick change method?? Where can I get some information on this. Sounds interesting.

-Greg

 

[Chuck Rudy]

Originally posted by valkrider15
plywood and all thread quick change method?? Where can I get some information on this. Sounds interesting.

-Greg

Jerry O'Sullivan uses it for some huge motors, so it works very well. You can see some of his builds here

https://www.vahpr.com/project.htm

Here are two pics of the motor mounts ready to go into his 7.5" Nike Smoke, note the last plywood ring is oversized (to the 7.5" outside edge of the body tube) and carries most of the weight of the boost and spreads it out over the bottom of the body tube, requireing no glueing and making the quick change possible----it's definitely not a thrust ring, maybe it could be called the Boost Plate.

www.homepage.mac.com/wesrudy/NikeMMT.jpg
www.homepage.mac.com/wesrudy/NikeMMT2.jpg

Chuck

 

[BrnAgainRoc]

do what ed said. There is no such thing as to much power, but there is a thing called not enough. Id go with the 75mm mount that way you can adapt down or go higher when ever you feel to do so. I build all my rockets with larger mounts so i can fly a larger range of engines in it instead of being stuck on one size/ brand of engines.

 

[tinteffects]

going with the rest of the guys on this. Put the largest mm you can in it. Then adapt down. when I designed my L-3 rocket, I intended to use a 75mm 7600ns case. But I put a 6" mm and adapted down. Ihave been SO thankfull of the foresight.(My wife aint too happy about it tho...HMMM) also, to answer the other q's. glass the fins to the motor tube, slide in the assembly, then glass to the airframe and you'll be set to go. Happy flying!!!
You might want to adapt the N/C so that you can add or take out weight. I'll post how I did that as soon as I can get a pic. Hope this helps.

 

[valkrider15]

Based on the multitude of messages pushing for a larger motor mount I have decided to go to 75mm. That just seems SO DARN BIG, but I guess that is just something I'll have to get used to.

Having not come close to flying anything that large I am still concerned about building enough strength into the rocket to handle it. I'm confident in my build techniques so far and use quality epoxy (West Systems slow cure). I just want to make sure I have as much informaiton as possible before starting this build so that it will handle the stress of the big ones.

I plan on making the rounds at NSL in 2 weeks and asking lots of questions about building for L & M motors. So far it seems I should just keep upscaling everything I did with smaller rockets. Larger hardware (u-bolts, allthread), thicker fillets, more glass, line the tube with coulpers, more shock cored, etc. Am I missing anything here??

And thanks for all the comments / suggestions so far. This is a great place to learn for sure!

 

[nialloswald]

Based on the multitude of messages pushing for a larger motor mount I have decided to go to 75mm.

Why not go to 98mm? That way you have more options - like clustering 3 38mm MMT's. There's still enough space to fillet the fins etc - I helped build a 5.5" rocket with a 98mm MMT (for a HyperTek M motor) last summer and it was no problem getting it together.

The fin can/MMT assembly was built separately and then installed in the main body tube.

First test flight was on a Pro54 K530, reached about 2000ft.

As for reinforcements, I think you will gain more from reinforcing with composites (i.e. tip to tip glassing on the fin can) than making huge epoxy fillets. As well as adding strength to the fin-BT joint, it will add stiffness to the fins.

You could also use glass, carbon or kevlar cloth or tape on the fin-MMT joint. A small epoxy fillet to smooth the junction, then the fabric over that.

Another consideration is what sort of epoxy. Something like West System or AeroPoxy is stronger than hobby shop epoxy, and if you've got 5.5" tubes to glass the west systems dispenser pumps are worth every penny! (It is handy to have syringes for smaller mixes too). If applying a composite reinforcement over an epoxy fillet, or indeed a composite wrap over another composite wrap, the best strength is obtained when you get a chemical bond between the two layers. With something like West 105/206 this really means doing wraps on consecutive days.

If you are putting it over cured (West 105/206 takes several days to fully cure) epoxy roughing it up slightly helps to create a mechanical bond between layers.

Oh, and one more thing I've been told many times - never sand into the glass cloth. The strength is in the fibres, and if you break them, you lose the strength. Build upwards with filler.

As I have been told many times before, epoxy alone adds weight more than strength.

I'm also somewhat dubious about how much strength is really gained from running half a ton of allthread down a booster - does anyone have good data/evidence to suggest how it helps?

Just my 2p. Hope it helps in some way.

FWIW the 'build it like a tank' approach doesn't appeal/make sense to me. We'll find out when I put a K in the 54mm booster I'm working on

 

[Smokin' Rockets]

Well to confuse you even more i'm going to disagree. At the stage your at, build a nice light 54mm rocket. No need to overbuild here. Shoot for under 10 # dry. It will perform great on J's and K's.

To build a 5.5" rocket for an L or M is where you need to reinforce quite a bit. That usually means weight. Not saying you still cant keep it light but odds are you will have an overweight pig that s a dog on J's and k's.

5.38 LOC tube is great stuff and really doesnt need glass. Use 1/4" ply fins or .093 G10 if they are small. Think of something like the LOC Magnum.

Fin flutter is the main cause of shreds on boost. Keep the fins stiff and they will stay on.

Althread is not needed and just adds weight.

Getting a rocket to perform well on J-K as well as L-M is very difficult without using expensive composites.

Maybe I'm just old school. Take your time, have fun, and take note of other setups. You will probably trash this rocket well before you ever get a chance to use that 98mm mount. I'm sure this wont be your onky rocket anyway.

GL....Bill

 

[Chuck Rudy]

Originally posted by Smokin' Rockets

Maybe I'm just old school. Take your time, have fun, and take note of other setups. You will probably trash this rocket well before you ever get a chance to use that 98mm mount. I'm sure this wont be your onky rocket anyway.

GL....Bill

As you can see there are two schools of thought. I also figured mine would have augered in long ago, but now two have a little 54 mm MMT and I'm sort of kicking myself for not having gone with the largest cavity it could take. Plus, I could cluster anytime. As it stands now it's not possible.

You have two schools of thought young man, pick your poison.

Chuck

 

[Ozymandias]

I respectfully disagree with the Tim Allen approach of "more power!" There's nothing wrong with building a 54mm mount and a 5.5" airframe. Case and point, my Nike Smoke is 5.5" with a 54mm motor mount. It will easily fly near out of sight on a K550. It's strong enough for the largest K's and even the 54mm Cesaroni L730. However, it's light enough to fly well on H motors. Hone on your building technique and keep your rockets light. If you get crafty, you don't even need a motor mount tube. Generally speaking, light rockets fly better! The trick is building them strong. I really like fillament wound fiberglass for my big rockets but it's pretty expensive in the larger sizes. Maybe you can wrap a fiberglass airframe on a mandral.

 

[Stymye]

good point ozzy

with virtualy every high performance hobby, light ,yet strong is the key...flying an anvil doesn't require much skill or technique, just more power( and money).

 

[nialloswald]

But if you make a light, strong rocket with a large MMT you have lots of choices. If you put a 75mm or 98mm 'host mount' in you can easily cluster within that - 3x38 or 3x29 if you like.

Also, a lightweight airframe with a 75mm MMT could be good for the HyperTek L motors, some of which have insanely long burns

 

[Ozymandias]

Putting in a 98mm mount is just way overboard. Again, if you want a nice rocket that flies well, just stick with a 54mm motor mount. Keep the rocket light and it will fly fantastically on full K's. If you try to build a rocket that will handle L and M motors too you'll have to build it so heavy that it will fly like a dog on the smaller motors. Furthermore, 75 and 98mm hardware and loads are very expensive. You can fly H-K for under $200 a launch. You'll have plenty of time to burn the big motors later on in bigger and better rockets.