Basswood or Plywood?

I am building a rocket that is 2.6" O.D. 33" long and will have a three 24 mm engine cluster. 4 fins, which would be bettter to use for fin stock, 1/8" Basswood or 1/16" birch plywood?

 

You could use either the basswood, the plywood, or even balsa (explained below). If you want to use plywood, I would suggest going with something thicker than 1/16". What are the fin dimensions? 1/16" plywood can get a bit flexy if the fin shape is too big. I have used 1/16" ply for fins on rockets with diameters ranging from 0.92" to 1.64", but not on anything larger. My Hercules is 2.34" in diameter and 33.75" long, and has a 3x24mm mount. I used 1/8" plywood for the fins. My Maxi Alpha (2.6" dia, 33.5" long) on the other hand, has 1/4" thick hard C-grain balsa for fins, which I papered on both sides. This had a tremendously positive effect on the overall weight of the model; it is the lightest rocket that I have in that size range. But it has a single 24mm mount and I fly it on black powder D's and E's. Do you plan on ever using composite motors in the cluster? This is a factor to consider as well.

MarkII

 

If you use standard size tubes and don't build the rocket like a tank, I think balsa could take it.

 

I've never used basswood and I've never seen 1/16" plywood, so I probably am not the best person to answer this, but....

I would use 1/8" (3mm) plywood. The reason is because

1. I assume this is not a high performance, high altitude rocket and weight isn't super critical.
2. It is using a cluster and will have a "heavy" bottom end.
3. It will need larger fins then average to lower the CP because of the "heavy" bottom end.
4. The larger fins will be more susceptible to breaking when landing.
5. I have a three engine cluster rocket and it has broken its fins on landing more then any other rocket I own.

I wouldn't worry about the extra weight of the 1/8" (3mm) plywood and I would highly recommend the extra strength. Use through the tube mounting of the fins and maybe a tip to tip coating of 3 or 4 oz. glass too. Balsa fins would hold up to the actual flights, it's the impact on landing that will bust up the fins.

 

I'd go with Birch Plywood or .063" white Styrene. I've done this with several BT-80 (2.6" dia) Upscales line the 3X Nova Payloader Night launch model below. The fins are one pice cut .063" Polystyrene sheet butt attached to the stock .021" wall BT-80 with 30minute epoxy fillets. the Model is a 4 D-12 cluster that recovers on a 36" Nylon Hemi Cute. the models mass without motors is 774.6g or 1.7lbs.
The Nova Payloader-II flys just about ever year at our night launches and has 19 logged flights since 1997 without a single fin related issue.

Hope this helpsl

 

TTW mounting of four fins on a 2.6" diameter tube that contains a 3x24mm mount could be a bit complicated, but if your 2.6" tube is standard Estes BT-80, you'll probably need to do it. My Hercules airframe is Semroc Series 225 tubing, which is heavy-walled. Three 24mm motor tubes fit into it like three 18mm motor tubes fit into the Astron Ranger; IOW, without any extra room. The three delta-shaped 1/8" plywood fins are surface mounted with Aeropoxy 6209 as the adhesive and (mixed with microballoons) as the fillet material. They are not glassed. (I have never done that.) They have a very strong and durable bond. Plywood (1/8") for the fins would probably work well for your project, especially if you use heavy-walled 2.6" tubing. If your fins aren't swept, they may not need any external reinforcement. Fiberglassing them (why tip to tip? - it's not like the plywood will break if it isn't reinforced; this is an MPR, after all) will add an appreciable amount of weight, and the strength that it would add may not even be necessary. (Depends on your design and the shape of your fins.) Don't add unnecessary weight to the tail end of the rocket. It doesn't need to be a tank.

MarkII

 

styrene is tough stuff and easy to cut (score with knife and snap ...then sand)comes in a bunch of thicknesses ,easy to sand,fill and paint..

That said ,I love basswood for LP stuff and even mid power ,easy to cut ,fill with SIG filler ,sands easy,strong, tight grain and strong for it`s weight.Just keep in mind where the grain of the wood going and you`re good to go.Bass wood comes in the usual thicknesses (1/16 ,3/32 ,1/8...most common)more then enough for what you need.
Plywood is nice stuff too ,comes in a bunch of sizes and thicknesses and strong as Russian female weight lifter.

Try the basswood ,you will like it.Just remember ,if it comes in hot.....it`s gonna bust no matter what you have strapped to the body tube.

Also, it makes a difference on how you treat the wood (CA ,epoxy, paper & glue etc.)

BB

 

Funny I've never seen a need for TTW fins on any Model Rocket under 3.3lbs.

If your model will be flying on BP motors and clusters Thur the wall fins is NEVER necessary, regardless of fin design or Tube thickness. I use Standard BT-80 (.021"wall) "estes style" tubing on all my Upscales with Butt mounted fin joints which are far more structurally sound then many on the board understand.

Frankly TTW fins are simply more work then required on just about any BP powered LPR and MPR models. Only real reason for using TTW fins is when you get into much higher thrust HPR motors and far heavier models.

Mark is exactly correct about not adding unnecessary mass to your lpr/mpr models, building as light as possible always improves performance without sacrificing good strength and longevity.

 

The 1/8" lite ply from Balsa Machining is great stuff and inexpensive, too. Strong, but very lightweight. After sanding each side of a fin with fine sandpaper and removing the resulting dust, I apply adhesive-backed label paper to both sides to create a very rigid composite structure and to ease finishing. I run water thin CA along every edge but the root and remove about 1/8 of the label paper on each side of the fins at the root to enhance fillet adhesion to the ply (otherwise, the fillet will be adhering to just the label paper).

 

TTW has it's place in low power also. Looking at the Brighthawk photo, those are some huge fillets. TTW construction allows for minimal fillets with max strength. This would be ideal for scale rockets. Added weight of the extra wood is somewhat offset by not having to use large epoxy fillets.

Having said the above, I've only used TTW on two low power rockets. They are BT-80 upscales of the Red Max and the Citation Patriot, built as light as possible for low and slow demo flights. I used 1/4" balsa partly for semi-scale appearance and partly to add a little drag without adding much weight. The Red Max flies great on C11-3's but the delay is a hair short. I haven't run across any C11-5's to try on it. The Citation Patriot has a removable mount and can fly on a single D12-3, dual and triple 24mm mounts, and could handle composites if I decided to do so in the future.

I built a BT-80 based Goblin decades ago using surface mount 1/4" fins. I made the mistake of not putting a third centering ring on the stuffer tube near where the leading edge of the fins mounted. This allowed the body tube to easily flex to the point it chipped paint without the fins breaking. I cut holes in the aft centering ring and used canned foam. This worked great to stiffen the rocket for a while, but months later the stuff expanded more, causing swelling problems. It remained flyable, but I'll never use canned foam in a rocket again. It was eaten by a tree last spring at a school launch. The replacement will have similar TTW construction as the Red Max and Patriot. It will also assume duties as a low and slow demo model for schools, etc.

 

I recommended TTW fins if the builder was going to use heavier, denser fin material with standard weight BT-80. I mount the fins on the surface in nearly all of my own scratch-built MP and LP rockets; the one situation in which I am using TTW fins is with my Sandhawk, which has very thin fins that have a wide span. It is scale, so I don't want to add heavy fillets. But otherwise, surface mount has been they way I have gone with everything else. But getting back to the OP's rocket, if he went with something like 1/8" plywood and his airframe was standard-wall Estes BT-80, I would be concerned that the 0.020" wall tube would not be strong enough to hold the heavier fins without flexing. Surface mounting balsa fins on BT-80 is fine, as I did with my Maxi Alpha. I don't know about surface mounting basswood fins on that tubing; I have not built a rocket with that combination yet. If the OP uses BT-80 and goes with 1/8" plywood for the fins, he can built the fin can outside of the rocket, then cut slots in the BT-80 from the aft end and then slide the can in and bond it. The surface of the airframe won't have to support the fins at all, so he can use lightweight BT-80 without compromising the strength of the fin can.

My Hercules uses LT-225 for the airframe. This is heavier walled stuff (0.045") and it is quite strong enough to support the surface mounted 1/8" plywood fins. I never considered mounting the fins for it through the wall because it wasn't necessary and also because the 3 ST-9 motor tubes in the aft end leave very little room for fin tabs.

So I don't use that type of mount indiscriminately in my own builds, and I wasn't making a blanket recommendation for it here. I recommended it only for a very specific scenario.

MarkII

 

Here is a picture of the design of my rocket. I do not think it needs TTW fins EIther. I was just wondering if basswood, plywood or balsa would work better. I have never built a rocket this big before. I think I will go with basswood or balsa. I can always change it later. If they do not work. That is the joy of building your own. :cheers:

 

Looking at your design and your intention on the use of Basswood or Balsa, I think I'd choose to go with slightly thicker Balsa being careful to align the grain with the leading edge of the fins. Not sure what you plan on using for the fintip pods but keep in mind they are going to take all the abuse on landings. A decent size hemi chute should help lower your decent rate and reduce oscillation created with flat chutes but they are still going to want to flex a good bit on touchdowns. Depending on the diameter of your fintip pods you might want to consider use of thin walled polyethylene mailing or polycarbonate tube over cardboard or craft body tubing to help cushion some of the impact. Either will supply a good amount of flex in all but the coldest weather. They will require good soild epoxy rivet attachment method with fillets and you'll need to use a etching primer for plastics to keep the paint on should you go with either.

Another option might be to add a solid dowel to the lower 3rd of the pod in contact with some of the fin and the complete overhang. Or as Ted mentioned filling the pod tubes with expanding foam could help stiffen them without as much added mass.
Something to consider might be to shorten the overhang and/or add internal rings and dowel Landing legs, with or without spring loading shock-absorbers sort of like those used on the starship Vega or orginal design Mars Snooper or Snooper-II

 

Just saying, I have Cineroc film of a 3.0 lb rocket and you can see 2 of the four the fins rip off during flight. 1/8" Balsa with thru the paint fin attachment. The body tube shredded, this was in 1976, If LOC tube was available there would not have been a problem. The rocket was long, and with the two remaining fins, it glided backwards down to the ground...Rocket was Estes BT-101 18" with pnc-101 as a reducer to BT-70. (Road Dog)
Motors: 6 x D12-7, 9 x C6-7
The man in the photo is the famous Herb Desind...


The one below did have balsa fins too (1/4" thick), note the long root edge, makes up for balsa weakness: This is the Slobovian Avenger. Three Cinerocs.
Motors: 12 x D11-9, 8 x C6-0p, 20 D20-0p, 20 E5-0p. 1200 Nt-Sec
No shred, but ejection was way too soon, wish altimeters were available at LDRS 2...

 

...forget wood for fins, use 1/16" fiberglass. You can purchase fins already cut out or even have custom ones designed...

 

Cool Thread...

Previous post to yours was November 2009