To glass, or not to glass

[kelltym88]

I find myself in a quandry of sorts. I am working on a Madcow Solar Express. I am converting it to a 38mm MMT. Why? I like having more options. While I do not plan on flying it on a J420, or I600, any of the 38/240 or 360 loads would be a nice option. Madcow kits are of excellent quality. I have already epoxied the MMT in place and foamed the fin can.


So now I ask, does/should the airframe and /or the fins be glassed? The fins are TTW. Just looking for some opinions. Thanx.

 

[troj]

I don't know the Mad Cow kit you're referencing, as I've never seen a Mad Cow kit in person.

That said, the typical rocket in this range will work just fine without fiberglass. At the same time, fiberglassing it won't hurt, but it will add weight and may impair your ability to fly it on smaller motors.

How thick are the fins, and what are they made of?

A small (1" - 1.5" wide) strip of fiberglass in the fin roots will provide a significant increase in fin adhesion, provided the fins aren't a material that will flutter badly under higher thrust motors.

Of course, with all of this, I'll state the obvious -- if making the rocket capable of larger motors, make sure you determine what nose weight, if any, is necessary to compensate for the added motor weight.

-Kevin

 

[kelltym88]

https://www.madcowrocketry.com/catalog/solarexpress250.gif

The fins are 1/8" plywood. Weight is not an issue, as I usually tend to overbuild anyway. Yes, I plan to add nose weight, that's a given. I was thinking about adding a strip to the fins.

 

[troj]

The fins are 1/8" plywood. Weight is not an issue, as I usually tend to overbuild anyway. Yes, I plan to add nose weight, that's a given. I was thinking about adding a strip to the fins.

Neat design!

Looking at the fin shape, I'd probably fiberglass them -- those trailing tips like to break. I'd do that, even if I was planning to fly it on H motors.

Not tip-to-tip, but rather put a 3oz or 4oz layer of fiberglass on either side of the exposed portion of the fin. That, in conjunction with some joint reinforcement and you should be fine.

Now, all that said... I'll admit that if I were putting a 38mm motor mount in it, I'd probably fiberglass the tube, as well as laminate the fins. Why? Because I've whacked the end of one too many tubes moving them around, so I like to protect them from my abuse a bit.

I also don't mind fiberglassing; others find it a pain.

Is it necessary? No. Would I do it? Yes. 4" BSD THORs fly beautifully on K motors with no fiberglass.

-Kevin

 

[El Cheapo]

I'm doing the same thing to a Madcow Striker although I won't be starting it until maybe mid summer if then.

In correspondence with Mike from Madcow, he stated the Striker should be able to handle up to .85 mach before the airframe needs to be glassed. He also stated best bet is to sim the flight with various motors and see how fast it goes.

 

[Handeman]

Neat design!

Looking at the fin shape, I'd probably fiberglass them -- those trailing tips like to break. I'd do that, even if I was planning to fly it on H motors.

Not tip-to-tip, but rather put a 3oz or 4oz layer of fiberglass on either side of the exposed portion of the fin. That, in conjunction with some joint reinforcement and you should be fine.

Now, all that said... I'll admit that if I were putting a 38mm motor mount in it, I'd probably fiberglass the tube, as well as laminate the fins. Why? Because I've whacked the end of one too many tubes moving them around, so I like to protect them from my abuse a bit.

I also don't mind fiberglassing; others find it a pain.

Is it necessary? No. Would I do it? Yes. 4" BSD THORs fly beautifully on K motors with no fiberglass.

-Kevin

I completely agree with Kevin. There is no need to glass the rocket for any of the 38mm motors. There are all kinds of reasons to glass it that don't apply to motors/thurst conditions. The pointy fins will surely break on landing sometime. They may still break after glassing, but much smaller chance. Glassing the tube will make it much more durable when the load in the trunk shifts or the 12lb rocket tips over and lands on it. You don't need to glass it for flights, it's only up against the air while flying. It's the ground environment where there are lots of hard and unyielding things it has to contend with and much more likely to get bent, dented and broken.

Besides, when you start pushing the 38/480 and 38/600 cases the glassing might give you a little more comfort and that L2 in the 38/720 won't be far behind.

 

[kelltym88]

Already L2. Besides preventing the rocket from getting damaged, glassing would be necessary if one were attempting to reach/break mach, correct? The pressure at that speed could crush a non-glassed tube, but since that is not my goal, and I plan on keeping the initial thrust of whatever motors I use to no more than 50lbs. at lift off, then I guess I'm OK.

Thanx all for your input, I will probably but a layer of glass over the fins, otherwise, I'll leave it like it is.

 

[Handeman]

I don't think that mach speed puts that much force on the tube directly. The issue with mach speed is the fin flexing/flutter. Those forces are transmitted to the mounting points of the fins, which of course is the body tube.

 

[MarkII]

I don't think that mach speed puts that much force on the tube directly. The issue with mach speed is the fin flexing/flutter. Those forces are transmitted to the mounting points of the fins, which of course is the body tube.

My :2: (if it is even worth that much)

What you said about the airframe makes sense. If the pressure as the rocket approaches Mach is primarily frontal, then the body tube is not the component that is most at risk of failure. It is quite difficult to collapse a cylinder longitudinally, even when that cylinder is "only" made out of (very thick walled and highly compressed spiral-wound) paper. From what I have read about the history of attempts to reach supersonic speeds in aviation, the failure always occurred when the wings were torn off (or fluttered so badly that the aircraft went unstable).

MarkII
(Full disclosure: I'm not HPR-experienced, by I'm learning...)

 

[kelltym88]

OK, wait a minute here. I was always under the impression that as one gets into bigger motors, glassing the tube was necessary to avoid it from collapsing. I understand glassing the fins, in fact I've had my share of shreds thank you.


So, could you build an "M" class rocket w/o glassing the tube? Could you put a 54mm MMT into an Estes Big Daddy w/o glassing the tube?

 

[Handeman]

OK, wait a minute here. I was always under the impression that as one gets into bigger motors, glassing the tube was necessary to avoid it from collapsing. I understand glassing the fins, in fact I've had my share of shreds thank you.


So, could you build an "M" class rocket w/o glassing the tube? Could you put a 54mm MMT into an Estes Big Daddy w/o glassing the tube?

Yes and Yes.

I don't know about the Big Daddy, but I know there have been successful L3 rockets built without fiberglassing the tubes. The Big Daddy might hold up fine with it's thinner LPR tubes if you stick with 54/426 motors.

The "but" in this is design and motor selection. When you put a 54mm MMT in a rocket, the tendency is to put the biggest motor that will fit into it. At least I tend to get to that point. If that motor is bigger then what the rocket was originally designed for, all bets are off.

 

[troj]

So, could you build an "M" class rocket w/o glassing the tube? Could you put a 54mm MMT into an Estes Big Daddy w/o glassing the tube?

Barry Lynch (owner of LOC/Precision) isn't fond of composites, and did his Level 3 without any.

At LDRS last year, we flew a Q motor with zero structural composites -- all we used was a thin film of fiberglass over the pink foam nose. We ended up horribly overbuilding via other methods.

If you're going to get up close to Mach, then you either need small, aerodynamic fins, or they need to be fiberglassed. Either way, they need to be stout.

Avoid long, unsupported lengths of tubes, and you'd be surprised at the velocity you can get without any composites. The 3" BSD THOR I have does this a creative way -- the entire rocket is lined with couplers, creating a double wall. Much stronger than standard tube, without much additional weight, and it's easier (and cheaper!) than fiberglassing!

There is a tremendous misconception in this hobby of the "need" for composites. People were flying Ms for quite a while before the magic of composites really hit the hobby, and they were doing it without any problems. Bruce Lee got his Level 3 on Super Mario, a 7.5" rocket that's about 10' tall, that has zero composites. He flew it several times afterward, as well. The death of it was when it landed in water, which destroyed the tubing.

Where composites are "needed," in my opinion, are on the high-performance flights. Most of the flights you see don't need them.

-Kevin