Capsule Fail

[Bone Daddy]

I've been playing with capsule shaped rockets and had an unexpected failure this past weekend at the monthly DARS launch.

The yellow capsule is BT-20 and flies stable on A,B and C motors.
I wanted to have parachute recovery so I built the white one with BT-50.

I loaded it up with a C6-0 expecting it to be stable and it pretty much went squirelly right off the rod.

2nd flight was on an A8-3 and it too was unstable.

I've built and flown many saucer and saucer like objects and would have bet heavily that this one would be stable.

The only thing I can think of is that the slope of the capsule is too steep.
I'm basing this on the fact that my Savory Asiago saucer was unstable which I attributed to its shape.

The shape of the capsule involved no rocket science, it was the biggest one I could fit onto one piece of bond paper.

Bones

 

[Oldschool77]

Looks about as steep as a ''Birdie''(Kinda looks like one). My scratch saucer mnt is at top. Perhaps your mtr mnt is not up far enough due to your recovery sys. Maybee extending the skirt a tad?

 

[Bone Daddy]

Both have full length body tubes.

It would be pretty easy to add some nose weight by adding some type of nosecone.

I'm just kinda baffled why one would work and one would not..........................

Bones

 

[kjohnson]

Yes, your cg is too far back on the white cone. There is an old Centuri tech report that tails about that but for something that shape, you'd want to recess the motor mount further up in the cone. See the aforementioned birdie and the Centuri The Point as examples.
kj

 

[Bone Daddy]

My curiosity revolves around the concept of base to height ratio and stability.

The yellow cone is 5" x 3.5" approx 1.4 : 1 and is stable.

The white cone is 5.25" x 5" approx 1.05 : 1 and is unstable.

From the above, I would seem that if the white capsule had a base to height ratio of at least 1.4 : 1 it would be stable. Even with the motor at the base of the capsule where I want it to be. This would mean it would need to have a 7" base.
This would actually give me more the look I was shooting for.

But before I try this, the plan is to fly the white capsule tomorrow with about 12 grams of nose weight added.
Report to follow.

Bones

 

[Bone Daddy]

OK, I decided to go ahead and try to cut out a 7" - 5" shroud.

I'm using VCT for the template.

I had to piece together the patter from 4 sheets of paper and transfer to 2 pieces of bond paper.
Not expecting this one to be that great, but it should give me an idea of what I'm up against.

I really need to find some legal size bond paper.

This rather arduous build is being accompanied by the incredibly beautiful and talented Miryo of BEG fame:

https://www.youtube.com/watch?v=yjr485xFf7A&feature=related

Bones

 

[Bone Daddy]

Turns out it wasn't as bad as I thought it would be to piece together the pattern.

It actually fits on a single sheet of bond paper.

With a little luck, Miyro I will be ready for a test flight tomorrow...........................

Bones

 

[Salvage-1]

Yup, looks like you need to move the engine forward. I have got paper cups, automotive funnels, etc flying and stable, but the motor needs to be as far forward as possible.

 

[Rex R]

drawing pads are bigger, I've got one w/ 12" x 18" paper .
rex

 

[Bone Daddy]

Thanks for the tip Rex.

Didn't get a chance to fly today.

I'll try again tomorrow.

Bones

 

[kjohnson]

My curiosity revolves around the concept of base to height ratio and stability.

The yellow cone is 5" x 3.5" approx 1.4 : 1 and is stable.

The white cone is 5.25" x 5" approx 1.05 : 1 and is unstable.

From the above, I would seem that if the white capsule had a base to height ratio of at least 1.4 : 1 it would be stable.

Well I dunno about that, but the CP of a cone should be at 2/3 of the height of the cone from the tip. (see page 9 of https://www.spacemodeling.org/JimZ/manuals/tir-33.pdf ) so the CP of your 5" high cone would be about 3.3: from the tip. Your CG should be somewhere ahead of that. If your ratio happens to work for one cone and not for another of a different size, I'd look back to your CG placement.

kj

 

[Bone Daddy]

Continued my look at capsule shaped rockets at the weekend DARS launch.

First up was a BT-50 capsule with a much wider base. Perfectly stable, but obviously high drag during flight.

Next up was my unstable capsule with added nose weight.
Beautifully stable. Pretty much straight up and straight down. Note that on the this flight the added nosecone was taped to the saucer.

She flew a second time with a parachute. Again stable.

I have not weighed the nosecone yet. I'll try to do it tomorrow and post.

Bones