VERY large, VERY strong fins?

[benjarvis]

Hi folks...


Does anyone have any ideas on materials or techniques for building some very large fins for a very powerful rocket?

To give some figures, I need a way of making some fins of about 30"+ span, can be up to 1.5" thick, have a root of perhaps 36", and need to withstand flight at 1500mph (mach2+).

I've built bigger fins, but not for anything supersonic...

Anyone have any aerospace-esque ideas?



Ben

 

[LFLekx]

Originally posted by benjarvis
To give some figures, I need a way of making some fins of about 30"+ span, can be up to 1.5" thick, have a root of perhaps 36", and need to withstand flight at 1500mph (mach2+).

1.5 inches thick, AND supersonic???

I'd imagine that you'd want to go with an aircraft-type framework of struts and ribs, with a thin skin over everything.

Plywood ribs, reinforced with CF and fiberglass, with heavily-reinforced struts. Cover with a thin sheet of balsa, and wrap in more CF and fiberglass.

I'd reccommend against trying to use foam-core techniques... at those speeds, friction-heating becomes a problem, in that it could melt your cores and cause the fins to buckle and fail.

 

[bobkrech]

Ben

Check out https://www.geocities.com/rdh82000/NmotorRocket/Construction.htm

A central FG support with nomex honeycomb should be lightweight and stiff enough to withstand the buffeting you will get at M>2. Just do the fin flutter calculation to make sure.

Robert has a very nice way to attach the fins to the airframe as well.

Bob Krech

 

[benjarvis]

Originally posted by bobkrech
Ben

Check out https://www.geocities.com/rdh82000/NmotorRocket/Construction.htm

Bob Krech

Cool.... that's a pretty good option...

I did consider some form of honeycomb composite as the basis for it, or honeycomb aluminium...

May well use a scaled up version of this sort of technique.

As for attachment, that's not so much of a problem, have a lot of room internally to play with and need the fins to be removeable so that can be a nicely engineered solution with location boxes for the root edge.

This is all just for a pie in the sky project I've been asked to do some feasability studies for... just wanted to see if it was vaguely feasable... I've done huge, and I've done high performance, but I've never tried to do both in the same rocket


Ben

 

[benjarvis]

Originally posted by LFLekx
1.5 inches thick, AND supersonic???

The last set of fins I made were 6" thick at the root edge, so 1.5" is quite thin really

Though they weren't going more than 300mph.


Good thought re: the heating on foam... this thing would theoretically be supersonic for 30 seconds+, so heating will definitely be an issue.


Thanks for the input so far folks



Ben

 

[Hospital_Rocket]

You might find some hints here:

https://www.nikeproject.com/

 

[llickteig1]

Guys around here use honeycomb core lay-ups for super strong, lightweight, large fins. Don't know where you are geographically, but here we have access to aircraft surplus, particularly floor panels materials for airliners which are very, very strong. Fiberglass and Carbon fiber layers laid-up over the cores should give you the strength you need.

HTH, --Lance.

 

[flyr98]

Agreed Lance,
Foam is fine as long as you lamanate with Kevlar first. This should keep the heat off the foam for the better part of that 30 second Mach portion. Balsa wood is a possability too. BTW, what kind of motors are we talking when you say "....mach for 30 sec."?? That's wild!

 

[benjarvis]

Originally posted by flyr98
Agreed Lance,
Foam is fine as long as you lamanate with Kevlar first. This should keep the heat off the foam for the better part of that 30 second Mach portion. Balsa wood is a possability too. BTW, what kind of motors are we talking when you say "....mach for 30 sec."?? That's wild!

I'm in the UK, but I have a source of Nomex and Alu honeycomb so that's not a problem. I think the honeycob/composite laminate route is very possibly a good way to go.

Like I said, it's just a preliminary evaluation I'm doing for someone for a commercial project... but if it happens it'll be flying at Black Rock, and it's a very large clustered/staged rocket. Can't say any more unfortunately :-(


Cheers again for the suggestions folks!


Ben

 

[Juerg]

With a 30" span and ONLY 1.5" thickness, your main problem will be fin stiffness.
Honeycomb alone will probably not do the job.
Do you have any load figures?
I'd probabyl try a combo of honeycomb and carbon fibre spars, to get enough stiffness.

Good luck

Juerg

 

[benjarvis]

Originally posted by Juerg
With a 30" span and ONLY 1.5" thickness, your main problem will be fin stiffness.
Honeycomb alone will probably not do the job.
Do you have any load figures?
I'd probabyl try a combo of honeycomb and carbon fibre spars, to get enough stiffness.

Good luck

Juerg

I'd have thought 1.5" was about right... but it can be thicker if necessary.

Scaling down, on a 3" span fin, that equates to 0.15" thick, which is well over 1/8th of an inch. I know I've flown rockets with 3" span 1/8th inch thick fins (solid G10) to mach 1.5+...

They may well actually be more like 22" to 25" span now... I was thinking something like 1/8th G10 core (or 1/8th carbon if I get scared) with a layer of 1/2" thick nomex or ali honeycomb on either side, then vacuum bag the whole thing in carbon cloth.

I'd have thought that should survive a long period above mach... so long as the heating isn't TOO great.

Cost is not an issue, neither is weight (to some extent anyway)... the other option is just get em machined out of 10mm ali plate... but that WILL be heavy


Ben

 

[andysrockets]

This sounds like an interesting project, and one that really needs an aerodynamicist and mechanical engineer. Sadly I'm a leccy engineer though.

I would have suggested either Alu or CF Skinned Hexcel type honeycomb, as these are very light and pretty stiff too. Not something you can easily gash together by hand though - really needs to be properly manufactured.

Failing that, I'd question if ribs and spars (maybe I-beam, which would allow rivetting) skinned with alu sheet would work? Certainly by turning the plane of the structural components by 90 degress relative to the normal plane of the fins would increase lateral stiffness.

Still to overcome though would be torsional (ie twisting) stiffness. ie twisting around an axis running through the plane of the fin. Worst case is a fin that is apparently very stiff, but would actually flutter like jelly.

 

[LFLekx]

Originally posted by benjarvis
The last set of fins I made were 6" thick at the root edge, so 1.5" is quite thin really

Hoo, Boy - do you build MONSTERS!

 

[benjarvis]

Originally posted by LFLekx
Hoo, Boy - do you build MONSTERS!

Yeah, I guess 38ft tall is pretty big

This new project will be cooler though, just hope it happens!



Ben

 

[Karl]

I had a quick scout around that 'N Rocket' website and looked at the flight pics. It appears that his rail buttons had metled, even for that very short time on the rail. If your going in the direction of using a rail, maybe use aliminium buttons? OT I know

Keep us updated on this one Ben! Sounds like it's too cool to miss out on!

Good luck with the project and I hope it comes togethor in the end!

Karl

 

[rocwizard]

Originally posted by Karl
I had a quick scout around that 'N Rocket' website and looked at the flight pics. It appears that his rail buttons had metled, even for that very short time on the rail. If your going in the direction of using a rail, maybe use aliminium buttons? OT I know

Karl

Robert's buttons melted because the rocket went Mach 2.6 on the N4000, not because of the rail friction. On a rocket of the magnitude Ben speaks of, I highly doubt it would use a mere HPR design rail/buttons setup.

Anyhow, awesome project form th sound of it Ben. I really hope to see it come to fruition!

 

[speedyweasel]

Originally posted by rocwizard
Robert's buttons melted because the rocket went Mach 2.6 on the N4000, not because of the rail friction. On a rocket of the magnitude Ben speaks of, I highly doubt it would use a mere HPR design rail/buttons setup.

Something to consider would be retractable rail buttons (sounds strange, but pretty easy to implement). A gentleman on the THPRA list (and other lists, but that's where I met him) sent me some good photos of his retractable button design on a minimal diameter (can't be minimum or the buttons wouldn't retract, right?) 75mm CF rocket. Brilliant design, fairly simple, extremely clean slip stream, lightweight... good stuff. I'll ask his permission to post the photos.

 

[benjarvis]

Originally posted by SpeedyWeasel
Something to consider would be retractable rail buttons (sounds strange, but pretty easy to implement). A gentleman on the THPRA list (and other lists, but that's where I met him) sent me some good photos of his retractable button design on a minimal diameter (can't be minimum or the buttons wouldn't retract, right?) 75mm CF rocket. Brilliant design, fairly simple, extremely clean slip stream, lightweight... good stuff. I'll ask his permission to post the photos.

Yep, I've seen the RRS guys use spring loaded retractable rail lugs before.... lovely idea on big high performance rockets.

This project is gonna be draggy as hell anyway, so I don't think rail lugs are a big problem.

And no, they won't be plastic

The Lugs on Beautiful Blue were custom made ball-raced quadrouple aluminium roller sets on a 10mm thick aluminium plate, bolted on to an internal steel frame. Each lug weighed over 2lb ;-)

This won't need anything QUITE that big


Will have to see how things progress... should get a go/no go by the new year.


Ben

 

[Juerg]

Originally posted by benjarvis
I was thinking something like 1/8th G10 core (or 1/8th carbon if I get scared) with a layer of 1/2" thick nomex or ali honeycomb on either side, then vacuum bag the whole thing in carbon cloth.

You can leave the core sheet away, this doesn't do a lot more than adding weight! (being the "neutral fibre" as far as bending the fin is concerned)
What you need is pretty strong fin-skins made in a mold / autoclave and then sandwiched together with the honeycomb.

Juerg