Student Rocket Club Balsa Fin Improvement Help

[VinceC]

Hello Everyone,

A rookie here. Third year co-advising a middle/high school club. Started from scratch and always a lot of pressure not to let the students down. Lucky to have 10/12/14 students interested in meeting weekly and learning "on the fly."

With a lot of hard work and luck the Club was a national alternate in TARC last year. Their expectations are high this year.

Not a high tech, or "fancy" Club the Group sticks to off the shelf parts; cardboard body tubes, balsa fins, plastic nose cones, etc., and basic true and tried construction techniques. Although they have picked up a few very valuable pointers from this Forum. Thank you to all.

Last year one the Club's biggest challenges was the balsa fins being damaged upon landing. Hard to find open space around here and the ground is hard and there are piles of construction materials in the launch site that can pose problems during test flights. So the Club is looking for ways to reinforce their balsa fins or for alternative materials that will function/perform close to what they are used to.

There flight times and decent rates seem all reasonable so it is does not appear to be a recovery system issue. Maybe just bad luck, bad timing, or just the way it is - lol!

A few of the Club members mentioned they have found "dribs and drabs" of information on here about using paper/glue and fiberglass to reinforce the balsa wood. I thought I would "go fishing" and ask the seasoned veterans and experts here for their views on how to modify/improve the balsa wood fins and minimize their fears after each landing.

Thank you in advance for any information you would like to share.

Vince
Onteora Rocket Club.

Not updated for the start of their new season the Club does have a Facebook Page - Onteora Rocket Club.

 

[heada]

What actual damage seen? That will direct the discussion of reinforcement.

Damage to the edge of fins can be avoided by soaking the edges in thin CA and allowed to harden.

Fins the break along the grain can be papered, either with label paper, light skim of white/yellow glue and printer paper or even a light skim of epoxy and printer paper. Typically the fin composite sandwich is then pressed between large flat objects until dried/cured.

Fins that break off from the body tube can have the fin to tube joint reinforced by adding a paper layer across the joint from just the joint area up to fin tip to fin tip. It could be paper, fiberglass veil or any composite type material.

All of these add varying amounts of strength and durability but also mass. And they add mass in the area where adding mass is to be avoided so choose wisely and test alot.

 

[VinceC]

What actual damage seen? That will direct the discussion of reinforcement.

Damage to the edge of fins can be avoided by soaking the edges in thin CA and allowed to harden.

Fins the break along the grain can be papered, either with label paper, light skim of white/yellow glue and printer paper or even a light skim of epoxy and printer paper. Typically the fin composite sandwich is then pressed between large flat objects until dried/cured.

Fins that break off from the body tube can have the fin to tube joint reinforced by adding a paper layer across the joint from just the joint area up to fin tip to fin tip. It could be paper, fiberglass veil or any composite type material.

All of these add varying amounts of strength and durability but also mass. And they add mass in the area where adding mass is to be avoided so choose wisely and te

What actual damage seen? That will direct the discussion of reinforcement.

Damage to the edge of fins can be avoided by soaking the edges in thin CA and allowed to harden.

Fins the break along the grain can be papered, either with label paper, light skim of white/yellow glue and printer paper or even a light skim of epoxy and printer paper. Typically the fin composite sandwich is then pressed between large flat objects until dried/cured.

Fins that break off from the body tube can have the fin to tube joint reinforced by adding a paper layer across the joint from just the joint area up to fin tip to fin tip. It could be paper, fiberglass veil or any composite type material.

All of these add varying amounts of strength and durability but also mass. And they add mass in the area where adding mass is to be avoided so choose wisely and test alot.

Hello Aaron,

Thank you for the reply and wealth of information. Responses like yours makes this site great and I know they inspire me to share whenever I have something to offer.

Most of the damage has been along the edges with a few broken along along the grain and one memorable one that seemed to just get "crushed"! That one had a "hard landing" into a pile of culvert pipes no less.

Soaking the fin edges in CA seems very interesting as does adding a layer of paper/glue. Either option may be a good proactive measure included in the design and will provide improved results in the field.

Thank you again and I can't wait until the Club stumbles along and finds our conversation.

Vince

 

[boatgeek]

Having done this for a while, I'd just suggest that you not use balsa. Plywood is a lot more durable, especially if you have a lot of stuff on your flying field. Yes, it costs some weight, but you save having to do rebuilds. I'm also a fan of through-the-wall fins, but I recognize that opinions vary on that.

 

[VinceC]

Having done this for a while, I'd just suggest that you not use balsa. Plywood is a lot more durable, especially if you have a lot of stuff on your flying field. Yes, it costs some weight, but you save having to do rebuilds. I'm also a fan of through-the-wall fins, but I recognize that opinions vary on that.

Hello, BG

Thanks for the idea to move away from balsa. When given a "nudge" the group does look at moving outside their comfort zone. Maybe one or two of the Members will purse the idea of using plywood. There are a couple of "builders" in the Club.

For example I was proud how they incorporated a number of methods for building motor mounts and attaching parachute shrouds. They built a motor mount that had redundant conduits to pass the kevlar string down through so they could fasten the kelvar to the outside retainer ring. But that is an entirely different conversation.

Thank you for the response and the great suggestion. It may not be "outside the box" for many but it will be for this Group.

Vince

 

[cls]

hmmm, landin on hard stuff. what shape are the fins? fins that go below the rear of the airframe will get dinged up. better to use a trapezoid shape.

and yeah, for TARC, plywood. 1/8" if you can get it.

 

[lakeroadster]

I'm a big fan of papered basswood, what I call "structural papering"... it's a DIY, which you might like to consider, more on that here:

Structurally Papered Basswood - SPB

 

[bjphoenix]

I paper all of my balsa fins these days for strength. If you can tolerate the weight then plywood or basswood would be stronger. I also mostly use fins that don't extend beyond the back of the rocket body so they don't hit the ground first.

 

[VinceC]

hmmm, landin on hard stuff. what shape are the fins? fins that go below the rear of the airframe will get dinged up. better to use a trapezoid shape.

and yeah, for TARC, plywood. 1/8" if you can get it.

Hello CLS,

Thank you for the reply. The Club appreciates the suggestion about location and shape of the fin. Maybe "obvious" but these valuable points may widen their "tunnel vision." And they will look for basswood/plywood and consider that a viable option.

Thank you again.

Vince

 

[VinceC]

I'm a big fan of papered basswood, what I call "structural papering"... it's a DIY, which you might like to consider, more on that here:

Structurally Papered Basswood - SPB

Good Morning,

Thank you for the reply and suggestions. The Club is bouncing the idea of "papering" around. They are trying to "wrap their head around" how to layers of paper is going to help - but that is part of the challenge of learning - lol. So they are going to try. And they are going to look into using basswood in place of balsa wood.

Thanks for the link and the photos. That really helped the Club see all of this first hand. Great looking work.

Vince

 

[VinceC]

I paper all of my balsa fins these days for strength. If you can tolerate the weight then plywood or basswood would be stronger. I also mostly use fins that don't extend beyond the back of the rocket body so they don't hit the ground first.

Good Morning,

Thank you for taking the time to reply. It looks like your practices summed up all of the above. The Club is definitely going to look at changing the shape of their fins, and they are going to investigate either papering balsa wood or using basswood.

So it looks like a great three pronged approach! Thanks again.

Vince

 

[Andrew Brown]

Vince,

I coach a TARC team up in Michigan. Anything we can do to help, please just ask. As a few have responded, plywood fins should fix your problem. An elliptical fin design also helps (surprising, this is actually the most aerodynamic at sub-Mach, but kids always seem to want the swept and pointy fins). Through-the-wall attachment as well. We've had simple cardboard tube/plywood fins last well over 30 flights, even with some hard landings. While papering balsa or basswood does help, keep in mind TARC has an egg payload, has to come down at 23-24 fps to meet time, and needs to survive enough flights to collect characterization data and have qualification launches. It takes a lot of beating.

For plywood fins, I just have the kids cut them using a scroll saw, but a hand jigsaw works as well. If your school has a laser, that is the way to go. If you need plywood, we just buy it from Amazon, or I'd be happy to send you a sheet to try. We use the same material to cut our own centering rings.

For what it's worth, this is the stuff that we buy:
https://www.amazon.com/gp/product/B078JQ11QY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
It would be enough material for several years worth of TARC rockets.

 

[neil_w]

Good Morning,

Thank you for the reply and suggestions. The Club is bouncing the idea of "papering" around. They are trying to "wrap their head around" how to layers of paper is going to help - but that is part of the challenge of learning - lol. So they are going to try. And they are going to look into using basswood in place of balsa wood.

Thanks for the link and the photos. That really helped the Club see all of this first hand. Great looking work.

Vince

Papering with glue adds a tremendous amount of strength. If there's any doubt, just do tests. Paper a few scrap pieces of balsa or basswood and see, it only takes a few minutes. Doing your own tests is far far better than just listening to others' advice. Note that if you use something like 65lb cardstock (rather than 20lb printer paper) it helps even more.

The nice thing about basswood is that you can work it just like balsa, with knives and straightedges and hand-sanding. Plywood is great but moves you into slightly different techniques.

Papered basswood is a good first step up from balsa.

 

[K'Tesh]

I have a tutorial on papering fins... I find that papering really helps with both the finish and the strength...

https://www.rocketryforum.com/threads/papering-balsa-ply-fins-a-foolproof-method.65628/

 

[sriegel]

Another thing to consider is length of the shock cord. A longer cord between the chute and fins will help damp down swinging and mitigate the fins hitting the ground with a double whammy of descent and down swing.

You can also move the fins up the body a bit so the motor hits first. TARC rockets are usually overstable so you have wiggle room in placement.

 

[lakeroadster]

Good Morning,

Thank you for the reply and suggestions. The Club is bouncing the idea of "papering" around. They are trying to "wrap their head around" how to layers of paper is going to help - but that is part of the challenge of learning - lol. So they are going to try. And they are going to look into using basswood in place of balsa wood.

Thanks for the link and the photos. That really helped the Club see all of this first hand. Great looking work.

Vince

You might want to replicate something like this so they can get a hands on feel for what the paper is doing...

Using Wood Glue To Paper 1/16" Basswood - With A Strength Test

 

[Pem Tech]

If you don't want to mess with papering you can just use 1/16" basswood. Much easier to seal than balsa and much less prone to damage. That is all we use, outside of plywood.

 

[MidOH]

Longer shock cord.

Sand all trailing edges of the fins round. Glue on with the minimum amount of 30 min epoxy. Fillet with a minimum amount of 15 min epoxy. Seal the edges of the fins with super thin CA.

Prime and sand until glass smooth.

After that, my fins are pretty tough. I don't recommend papering. Too much weight, and weight breaks things as well. You can also use recycled materials. Plastic, fiberglass, carbon fiber. I'd say aluminum, but that's frowned upon, even though carbon fiber is just as likely to split someone's head open.

I've epoxy-d fiberglass fins onto paper tubes with good success.

 

[jmasterj]

You could also consider 1/16" FG for your fins if the weight won't be an issue. Slightly different skills required, but once you have those it's not more time-consuming than the good options offered above.

 

[bjphoenix]

You might want to replicate something like this so they can get a hands on feel for what the paper is doing...

Using Wood Glue To Paper 1/16" Basswood - With A Strength Test

I also did an informal test of papering:
https://www.rocketryforum.com/threads/experiment-with-fin-papering.173160/
I noticed that a bare balsa fin seemed very soft. When papered with printer paper each side it felt a lot harder and stiffer so I decided to do the test. I wanted to see if some papers would work better than others. I should have included some card stock.

 

[Jerry F.]

I see that some previous comments mention using a longer shock cord and I would like to add that if you are having problems with the rockets swinging or circling during decent and feel that this is adding to the impact at landing, try either cutting a spill hole (vent) in the top of the parachute, or using parachutes with a spill hole built in...this will significantly reduce the swing of the rocket below the parachute...

 

[BABAR]

Hello Everyone,

A rookie here. Third year co-advising a middle/high school club. Started from scratch and always a lot of pressure not to let the students down. Lucky to have 10/12/14 students interested in meeting weekly and learning "on the fly."

With a lot of hard work and luck the Club was a national alternate in TARC last year. Their expectations are high this year.

Not a high tech, or "fancy" Club the Group sticks to off the shelf parts; cardboard body tubes, balsa fins, plastic nose cones, etc., and basic true and tried construction techniques. Although they have picked up a few very valuable pointers from this Forum. Thank you to all.

Last year one the Club's biggest challenges was the balsa fins being damaged upon landing. Hard to find open space around here and the ground is hard and there are piles of construction materials in the launch site that can pose problems during test flights. So the Club is looking for ways to reinforce their balsa fins or for alternative materials that will function/perform close to what they are used to.

There flight times and decent rates seem all reasonable so it is does not appear to be a recovery system issue. Maybe just bad luck, bad timing, or just the way it is - lol!

A few of the Club members mentioned they have found "dribs and drabs" of information on here about using paper/glue and fiberglass to reinforce the balsa wood. I thought I would "go fishing" and ask the seasoned veterans and experts here for their views on how to modify/improve the balsa wood fins and minimize their fears after each landing.

Thank you in advance for any information you would like to share.

Vince
Onteora Rocket Club.

Not updated for the start of their new season the Club does have a Facebook Page - Onteora Rocket Club.

Out of box solution. use A forward swept rear fin edge. Rocket will then usually impact on the motor casing, which especially for single use motors are discarded anyway so who cares?

 

[VinceC]

Vince,

I coach a TARC team up in Michigan. Anything we can do to help, please just ask. As a few have responded, plywood fins should fix your problem. An elliptical fin design also helps (surprising, this is actually the most aerodynamic at sub-Mach, but kids always seem to want the swept and pointy fins). Through-the-wall attachment as well. We've had simple cardboard tube/plywood fins last well over 30 flights, even with some hard landings. While papering balsa or basswood does help, keep in mind TARC has an egg payload, has to come down at 23-24 fps to meet time, and needs to survive enough flights to collect characterization data and have qualification launches. It takes a lot of beating.

For plywood fins, I just have the kids cut them using a scroll saw, but a hand jigsaw works as well. If your school has a laser, that is the way to go. If you need plywood, we just buy it from Amazon, or I'd be happy to send you a sheet to try. We use the same material to cut our own centering rings.

For what it's worth, this is the stuff that we buy:
https://www.amazon.com/gp/product/B078JQ11QY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
It would be enough material for several years worth of TARC rockets.

Hello Andrew,

Thank you for the generous reply. You seem to resonate what others have offered and really provided some valuable TARC information to consider.

I am having the Club review this thread to see what they think. After all it is their rocket. If I had a vote I would vote for a fin shape with tabs that does not extend beyond the body tube and is constructed from plywood.

We will see what happens. The Club placed 113th last year and was an alternate but I think the scale slid more to beginners luck than design! They want to do better this year and I think will be happier if they are not repairing broken fins on a semi regular basis.

They can strive for better design, engineering, and testing and I will keep my fingers crossed and keep praying!

And the link was fantastic. Thank you for head start.

Vince
Onteora Rocket Club

 

[heada]

Shape, material and attachment method all play together to give a trade-off between performance and resilience. Switching from balsa or basswood to plywood, using through the wall (TTW) fins so that they're attached to the motor mount tube as well as the body tube and using fins that don't go past the aft end of the rocket is the best combination for resilience but the trade-off is weight.

A fin shape I recommend for very good performance as well as resilience is here:
https://locprecision.com/collections/rocket-components/products/fs-pfyhlocFin shapes to avoid are ones that extend past the aft of the tube like here:
https://locprecision.com/collections/rocket-components/products/fs-yshawk238
That first fin shape, made from quality 3/16th's plywood, using through the wall fin tabs with good glue fillets inside and out will be stronger than the body tube by far. Using 3/16th's quality plywood should not add much mass compared to a similar size/shape fin in balsa/basswood. Using fin tabs that go through the body tube to the motor mount changes the lever action when landing on a fin and nearly eliminates a fin being broken off (I smell a physics lesson in there somewhere).

 

[cls]

Using fin tabs that go through the body tube to the motor mount changes the lever action when landing on a fin and nearly eliminates a fin being broken off (I smell a physics lesson in there somewhere).

... But, when it does break, it is much more difficult to repair.

So, to the OP, whatever design you choose, think about being able to easily make several booster sections that are nearly identical in fin alignment and weight.

With success, you'll fly many times, and the rockets take a beating. It's nice to be able to swap in a new one without having to do too many calibration flights.

 

[VinceC]

... But, when it does break, it is much more difficult to repair.

So, to the OP, whatever design you choose, think about being able to easily make several booster sections that are nearly identical in fin alignment and weight.

With success, you'll fly many times, and the rockets take a beating. It's nice to be able to swap in a new one without having to do too many calibration flights.

Hello cls,

Thank you for taking the time to give us another "consideration" outside of the box to think about. I think at the end of last year a couple of Club members were thinking about duplicity and even triplicity - especially when one rocket was almost lost during a test flight and the competition completion date was fast approaching.

Thank again for your words of wisdom.

Vince

 

[techrat]

If the students are building off-the-shelf kits which include balsa fins, the easiest way to add strength is to add paper. A thin layer of glue on the fin, and some simple printer paper (cut to the right shape), on both sides, then lay a book on top while it dries so the fin doesn't warp, adds A LOT of strength for very little weight penalty. I've done this many times and it works fantastic. If your students aren't afraid to fabricate portions of their rockets, then Basswood or plywood fins are also an option that can provide considerable strength improvements, although the plywood option though very strong comes with a weight penalty. And obviously, as others have noted, through-the-wall tabs into the motormount can also improve durability. And though I prefer a fin-standing rocket, placement of the fins just slightly above the edge of the body tube so that they don't protrude below the body tube is also a way to minimize damage.

 

[neil_w]

Out of box solution. use A forward swept rear fin edge. Rocket will then usually impact on the motor casing, which especially for single use motors are discarded anyway so who cares?

That is not out of box at all, it is a standard design decision and highly recommended for surviving landings. Look at the world of HPR kits... I would say a pretty large majority have forward-swept trailing edges. As rockets get larger and heavier, the load sustained by fins on landing becomes proportionally (or maybe disproportionately) larger. Let the rocket land on its motor.

Aesthetic considerations may in some situations point in a different direction, but that's not usually the case for TARC rockets.

 

[BABAR]

That is not out of box at all, it is a standard design decision and highly recommended for surviving landings. Look at the world of HPR kits... I would say a pretty large majority have forward-swept trailing edges. As rockets get larger and heavier, the load sustained by fins on landing becomes proportionally (or maybe disproportionately) larger. Let the rocket land on its motor.

Aesthetic considerations may in some situations point in a different direction, but that's not usually the case for TARC rockets.

True, but seems rarely utilized in the low power world. Aside from aesthetics the only performance cost is a slightly less efficient fin, as with no other design changes, a fin of same shape and surface area inverted to sweep forward does move the CP slightly forward. Since most kits are slightly over stable, probably a non-issue (example @lakeroadster ‘s Ahpla), and for scratch builds likely can easily be managed by increased fin size or nose weight in OpenRocket or other sim programs.

 

[BABAR]

Another fin preserving option is to attach soda straws to the outside fin edge that extend an inch or so tailward of the fin. They can be attached with tape.

DEFINITELY a cost in drag, likely not “sim-able” (although @neil_w is pretty good!)

The straws hit terra all too firma first and bend, basically an easily replaced shock absorber.

Slightly different recover mode but same concept. Worked great

https://www.rocketryforum.com/threads/frangible-assisted-recovery.165724/