Square Mid Power Rocket

Discussions on mid/high powered model rockets using F powered motors and above.

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Aquaman33
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Square Mid Power Rocket

Postby Aquaman33 » Mon May 27, 2019 12:15 am

I am thinking about adding a square rocket to my never ending list of projects.

Hoping that the wonderful wisdom of this forum can chip in with some advice.

Here's what I have learnt from doing several minutes :D of research:
- No reason why a square rocket won't fly.
- Usually a bit slower than their round cousins.
- Tend to weather cock more than their round cousins (due to flat sides).
- Most square kits are low power.
- Square rockets are a bit tricky to sim, but not impossible.

I am thinking it would be fun to make something with a 29mm motor tube. I have a handful of 29mm F and G motors, but no rockets to fly them in. You don't see many mid power square rockets, so I thought it would be something different.

For the body tube I am considering two options:
- Balsa/Fiber glass - I have a load of 2.5mm thick balsa that would work well, but I think it will need some fiber glass to strengthen it.
- Fiber glass only - Maybe just use a piece of steel box section or a piece of 2x2 timber as a mandrel.

I want to avoid plywood to keep it light.

Has anyone ever made a square fiber glass body tube on a mandrel?
I am wondering if it might get stuck because you can't twist it off the mandrel, as you would with a round tube.

Has anyone got any thoughts on fin configuration?

I notice that many of the square kits have their fins offset in pairs near to two of the corners. Like this:

Image

Or offset close to the edges of the tube like this:

Image

Is there a reason for offsetting the fins or is this just for aesthetics?

Others have their fins in the middle of the sides like this:

Image

Another configuration that might work is to attach them flat like this:

Image

Image

Any reason why that wouldn't work?

I'll probably just 3d print a tall pyramid nose cone like this:

Image

Image

Those are my thoughts so far. Keen to hear thoughts on fins and making square fibreglass tubes.

Cheers,

Derek

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SpaceManMat
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Re: Square Mid Power Rocket

Postby SpaceManMat » Mon May 27, 2019 2:04 pm

Great to see you trying something different. I’ve built a tardis and also an X Wing, so have a little experience with odd shapes before.

In my opinion stability is always a bit of an unknown and you really need to fly it to prove beyond doubt that it will work. If designing from scratch then I would aim for a stability of 1.5 - 2 and I think you would be safe. However I would treat the first flight as experimental so would put it out further than normal for that size motor, also would want to make sure it left the rail at a decent speed.

With the fin placement, I think it does not matter too much, however I think that the corners do see some funny air flow coming around the sides (think I’ve tried both corners and sides). What I would make sure of is that the fins have a decent span so you have lots of fin out in the clean air, definalety avoid greatly swept wings at are less than body width.

As for the partially offset fins, I think this may be a construction technique where the fin is run down the side of the motor tube. I guess the other reasons could just be because you can do it on a square rocket without it looking like you stuffed up (as would be the case for a round rocket).
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

Aquaman33
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Re: Square Mid Power Rocket

Postby Aquaman33 » Tue May 28, 2019 9:54 am

SpaceManMat wrote:Great to see you trying something different. I’ve built a tardis and also an X Wing, so have a little experience with odd shapes before.

In my opinion stability is always a bit of an unknown and you really need to fly it to prove beyond doubt that it will work. If designing from scratch then I would aim for a stability of 1.5 - 2 and I think you would be safe. However I would treat the first flight as experimental so would put it out further than normal for that size motor, also would want to make sure it left the rail at a decent speed.

With the fin placement, I think it does not matter too much, however I think that the corners do see some funny air flow coming around the sides (think I’ve tried both corners and sides). What I would make sure of is that the fins have a decent span so you have lots of fin out in the clean air, definalety avoid greatly swept wings at are less than body width.

As for the partially offset fins, I think this may be a construction technique where the fin is run down the side of the motor tube. I guess the other reasons could just be because you can do it on a square rocket without it looking like you stuffed up (as would be the case for a round rocket).


Thanks Mat.

I looked at your thread for the Tardis. Very cool.

I think I'll just put the fins in the middle of the sides. I like that look.

I am thinking square fins and I'll make them quite big. Also the rocket will be fairly long so should have a stability of around 2.

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SpaceManMat
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Re: Square Mid Power Rocket

Postby SpaceManMat » Tue May 28, 2019 8:26 pm

Aquaman33 wrote:
SpaceManMat wrote:Great to see you trying something different. I’ve built a tardis and also an X Wing, so have a little experience with odd shapes before.

In my opinion stability is always a bit of an unknown and you really need to fly it to prove beyond doubt that it will work. If designing from scratch then I would aim for a stability of 1.5 - 2 and I think you would be safe. However I would treat the first flight as experimental so would put it out further than normal for that size motor, also would want to make sure it left the rail at a decent speed.

With the fin placement, I think it does not matter too much, however I think that the corners do see some funny air flow coming around the sides (think I’ve tried both corners and sides). What I would make sure of is that the fins have a decent span so you have lots of fin out in the clean air, definalety avoid greatly swept wings at are less than body width.

As for the partially offset fins, I think this may be a construction technique where the fin is run down the side of the motor tube. I guess the other reasons could just be because you can do it on a square rocket without it looking like you stuffed up (as would be the case for a round rocket).


Thanks Mat.

I looked at your thread for the Tardis. Very cool.

I think I'll just put the fins in the middle of the sides. I like that look.

I am thinking square fins and I'll make them quite big. Also the rocket will be fairly long so should have a stability of around 2.


Sounds like a plan. Make sure you bevel the TE and LE, leaving them square creates aero issues and you don’t need more of those. Rear weight was always my biggest issue, but since you’re going with a more traditional length you should be fine.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

Kryten
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Re: Square Mid Power Rocket

Postby Kryten » Fri Jun 07, 2019 8:20 am

Flat attachment would work, and be very strong.
But you're essentially doubling the weight of the fins with no aerodynamic benefit. This isn't a big issue if the fins are lightweight.

A large flat area on a body tube has the least stiffness, which might explain why the fins are offset.
TTW attachment would overcome this
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