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PunkRocketScience
22nd September 2010, 07:10 PM
Hello all-

What do you use for your very large diameter airframes? I am working up a project with a 24-25" diameter and am looking for options to keep it as light as possible.

The two options I have seem used are either fiberglassed sonotube (fairly inexpensive but heavy) or composite bulkheads and stringers with foam inserts (fairly light, but expensive).

Does anyone have another option?

troj
22nd September 2010, 07:52 PM
We use concrete forms; Sonotube is but one brand.

It's a tradeoff of weight versus cost. Light ply over a frame would be a really slick way to go, but would get very expensive very quickly. The fiberglass over foam would be less expensive.

Cost is relative, as well, as is weight. For our big project, we're typically flying on a P or a Q, so weight isn't an issue.

Our cheapest big project, the materials for the airframe cost us maybe $500, and that was the Pershing II, which flew on a Q motor.

-Kevin

KenSparks
22nd September 2010, 09:22 PM
Thick sonotube-type tubes don't always need to be glassed, so you can save some weight that way (the 16" Scud we are flying at Plaster Blaster isn't glassed, and is flying on almost 16,000 Ns). Another option would be to glass the tubes, then strip out the cardboard (toss it in your swimming pool to make this easier) leaving you with a fiberglass shell.

If you want to discuss a bulkhead/stringer/foam insert structure, take a look at our web site on the Fatboy and/or e-mail me - I can spend hours discussing this option.

PunkRocketScience
22nd September 2010, 10:31 PM
Hey Ken!

I was looking at the Fatboy pics this morning! After reading Troj's post above, I was thinking about the bulkhead/stringer structure with door skins curved over it instead of foam filling in the gaps...

I was thinking about the fiberglass only option as well... But at $24 per foot, I hate to just throw away 16 feet of sonotube! Trying to figure out what else could be used as a form that would be cheaper and more disposable... Maybe cover the sonotube with mylar and return it "unused" when we are done forming the glass?

DAllen
22nd September 2010, 11:05 PM
Hey Ken!

I was looking at the Fatboy pics this morning! After reading Troj's post above, I was thinking about the bulkhead/stringer structure with door skins curved over it instead of foam filling in the gaps...

I was thinking about the fiberglass only option as well... But at $24 per foot, I hate to just throw away 16 feet of sonotube! Trying to figure out what else could be used as a form that would be cheaper and more disposable... Maybe cover the sonotube with mylar and return it "unused" when we are done forming the glass?

Contact local excavators in the area. They may have 24" sewer pipe they'd be willing to loan out for a fee. But be warned, 16' of 24" HDPE would be extremely heavy. I'd opt for seeing if they had a shorter piece and build it in sections.

Too bad you don't live around here, I have lots of connections with contractors who have piles of this stuff and they'd loan it to me in a heart beat especially if they knew it was for a gigantic rocket.

-Dave

KenSparks
23rd September 2010, 12:47 AM
Sonotube at $24 / foot is cheap compared to the cost of the materials in a composite bulkhead/stringer structure!

Mylar to wrap the sonotube with might be okay if it is heavy enough. You might also use vacuum-bagging material, wrap a layer around the tube, then lay up the glass, then a top layer of the vacuum-bagging stuff. Pull a vacuum on it, build a curing oven around it, and in a few hours you'll have a rocket body - but it still won't be cheap.

KenSparks
23rd September 2010, 12:55 AM
Sonotube at $24 / foot is cheap compared to the cost of the materials in a composite bulkhead/stringer structure! Heck, it's cheap compared to the cost of the glass and expoxy you'll need!

Mylar to wrap the sonotube with might be okay if it is heavy enough. You might also use vacuum-bagging material, wrap a layer around the tube, then lay up the glass, then a top layer of the vacuum-bagging stuff. Pull a vacuum on it, build a curing oven around it, and in a few hours you'll have a rocket body - but it still won't be cheap.

PunkRocketScience
23rd September 2010, 01:05 AM
Sonotube at $24 / foot is cheap compared to the cost of the materials in a composite bulkhead/stringer structure! Heck, it's cheap compared to the cost of the glass and expoxy you'll need!

Mylar to wrap the sonotube with might be okay if it is heavy enough. You might also use vacuum-bagging material, wrap a layer around the tube, then lay up the glass, then a top layer of the vacuum-bagging stuff. Pull a vacuum on it, build a curing oven around it, and in a few hours you'll have a rocket body - but it still won't be cheap.

Well... Yes, I know that it's fairly cheap compared to compositing materials... It's just kinda pricey as a throw away mandrel though! I don't mind paying for quality, I just want to avoid waste if at all possible...

Reinhard
25th September 2010, 01:35 PM
Take a look at John Cokers Wac Corporal:
http://www.jcrocket.com/waccorporal.shtml

Using sandwich techniques are not that hard, but the result will likely impress you. At least this was my experience, and I'm not very skilled in this area.

Reinhard

PunkRocketScience
25th September 2010, 04:35 PM
Take a look at John Cokers Wac Corporal:
http://www.jcrocket.com/waccorporal.shtml

Using sandwich techniques are not that hard, but the result will likely impress you. At least this was my experience, and I'm not very skilled in this area.

Reinhard

Thanks for the link Reinhard.

That is pretty much what I'm looking for, but finding a cheap 24" mandrel is coming down to being the issue.... Sonotube (I'm using that generically like Kleenex) runs about $24 a foot at the size I am looking at... Kinda pricey for a one shot, as I'm not likely to use it very often....

Scott Evil
25th September 2010, 10:57 PM
I tried the "Glass it Over and Soak it Out" thing once to try and make a light but strong 8.5" airframe.

Once I got the cardboard tubing removed, the remaining glass tubing was actually heavier than the original mandrel was and in my opinion not quite airworthy either. I used two layers of heavy "burlap" weight glass and two layers of light "finish" weight glass.

Lottsa bad technique on my end, I'm sure you guys could do a better job but once again, consider weight/materials vs. strength. Maybe just hardshell the sonotubing and run with it.

:2:

jderimig
25th September 2010, 11:30 PM
I haven't calculated the weight but consider using plywood rings and use countertop laminate for the skin. The latter you may be able to find cheap because there are always god-awful colors or patterns that need to get cleared out occasionally.

dave carver
26th September 2010, 04:49 AM
I saw in a local contractor's yard fiberglass duct that was large and kinda flexable, it looked to be 24" in diameter. With the right inner structure I thought it would make nice large rockets :cyclops:

Pantherjon
26th September 2010, 05:04 AM
Wild idea: Stringers and foam blocks? Make an 'X' frame, then the foam blocks and then turn round with a hot wire?:confused2:

As a mandrel for fiberglassing..

Scott Evil
26th September 2010, 10:49 AM
Maybe do it with dowels,plywood and aluminum instead of fiberglass. Don't forget a crapload of rivets :)