Experiments with/for kids

[Grumble]

I did an experiment to demonstrate the principles of structural integrity with my son, that I then showed a video of to all his Y4 and Y5 schoolmates as part of a science fair - they all loved it.

Take 2 bottles of Coke, one big (2L) and one small (500mL). Drop them from a height of about 3m. Take bets on what will happen. Discuss to the extent of their understanding.

[lpm]

I think both will remain intact.

[Grumble]

I think both will remain intact.

One bursts, which one and why?

[dyqik]

I think both will remain intact.

One bursts, which one and why?

The small one, because it landed on a sharp rock, and then the big one landed on it.

[monkey]

Which one's got the mentos in?

[Grumble]

Which one's got the mentos in?

Ha, some of the kids assumed that was the one I’d be doing.

[Boustrophedon]

When the Nephews and Niece next turn up, I intend to build water rockets. I used to do this as a year 8 practical lesson in school and I still have my massively over-engineered launcher.

Thing is, since I last used it fizzy pop bottles have been redesigned; the nice flange just below the screw threads has all but disappeared and the neck is much shorter, I had to re-make parts of the launcher, this involved a lathe and silver soldering.

It now works again, I tested it on the village hall playing field.

One problem I never really satisfactorily solved is attaching the fins. The fins are polypropylene corflute, like corrugated cardboard but plastic. I have not found a glue that will stick to both the polypropylene and the PET of the bottle. Cyano sticks very well to the polypropylene but not to the PET bottle.

Epoxy is the other way around.

Gorilla glue is fecking weird and just gets everywhere.

Hot melt glue sticks really well but melts the bottle.

Any ideas?

[monkey]

We've got a tube of E6000 at home, that stuff seems to stick just about anything to anything. Might be worth a go. It's thick and makes strings, so can be a bit messy.

[dyqik]

You could make loop type fins out of sections of another bottle, and then use a glue that sticks PET to PET.

Fins would look like (O), IYSWIM

Similar to the closed wing design - https://en.m.wikipedia.org/wiki/Closed_wing

[dyqik]

Or you could use the curvature of the sections to make a sort of spiral fin set up. A slight tilt to the fins would help the rocket to spin for even better stability.

[dyqik]

PETG used in water bottles melts at 240C (or at least, that's the temperature that I 3d print it at).

Careful use of a temperature controlled soldering iron could let you weld the fins to the bottle without ruining the structural integrity. Or one of those hand held 3d printing pens could be used to weld the PETG together with additional material.

[Millennie Al]

One problem I never really satisfactorily solved is attaching the fins. The fins are polypropylene corflute, like corrugated cardboard but plastic. I have not found a glue that will stick to both the polypropylene and the PET of the bottle. Cyano sticks very well to the polypropylene but not to the PET bottle.

Epoxy is the other way around.

Have you tried the obvious idea of applying a thin layer of epoxy to the PET, letting it dry, and then using cyano to glue the other stuff to the epoxy?

[bjn]

What if you somehow made the fins screw into the threads on the bottle? That would mean a very short pipe with the fins attached, with male and female threads at either end. So you screw the fins+pipe anssembly onto the bottle and connect your pump setup to the bottom of the pipe.

No idea how to easily fabricate that. 3D print it?

[Grumble]

You could make loop type fins out of sections of another bottle, and then use a glue that sticks PET to PET.

Fins would look like (O), IYSWIM

Similar to the closed wing design - https://en.m.wikipedia.org/wiki/Closed_wing

Or you could face the fins outward like )0(

[Grumble]

Or even use complete bottle sections as fins, oOo (but centred)

[Gfamily]

Or make two loops of the corflute to go round thr bottle snugly as collars, and attach each fin top and bottom to the loops.
That way you're only glueing the corflute to itself

[Martin Y]

Have you tried using hot glue while the bottle is full of cold water?

[Boustrophedon]

You could make loop type fins out of sections of another bottle, and then use a glue that sticks PET to PET.

Fins would look like (O), IYSWIM

Similar to the closed wing design - https://en.m.wikipedia.org/wiki/Closed_wing

I did use that design on my water-bottle rocket mortar. It was very dangerous on account of the bottle cap and schrader valve assembly being designed to knock off at the bottom of the tube. The fin worked well but wouldn't clear my launch mechanism. It's a thought though.

[Boustrophedon]

Have you tried using hot glue while the bottle is full of cold water?

Now that's a thought.

What if you somehow made the fins screw into the threads on the bottle? That would mean a very short pipe with the fins attached, with male and female threads at either end. So you screw the fins+pipe assembly onto the bottle and connect your pump setup to the bottom of the pipe.

No idea how to easily fabricate that. 3D print it?

Unfortunately I don't have access to 3D printers.

PETG used in water bottles melts at 240C (or at least, that's the temperature that I 3d print it at).

Careful use of a temperature controlled soldering iron could let you weld the fins to the bottle without ruining the structural integrity. Or one of those hand held 3d printing pens could be used to weld the PETG together with additional material.

Unfortunately the temperature PET softens at is less that 100 C, a PET bottle shrinks and crumples up in boiling water.

We've got a tube of E6000 at home, that stuff seems to stick just about anything to anything. Might be worth a go. It's thick and makes strings, so can be a bit messy.

Thanks I'll try that.

[Boustrophedon]

Thanks for all the suggestions folks.

[Boustrophedon]

The hot melt glue with water in the bottle looks promising.