Page 1 of 1

Negative reinforcement question

Posted: Wed Feb 06, 2019 1:08 am
by anachronist
Classes D and E require an unreinforced bottle. Reinforcement to allow for greater pressure isn't allowed.

MY problem is, I need a way to prevent the bottle from collapsing under negative pressure!

At maximum velocity after the reaction mass is consumed, there is such a strong region of low pressure behind my rocket's nozzle that it sucks the air from the bottle until the bottle caves in, resulting a 200 mph bent rocket, often ripping the nose cone off! It happens so fast, I am not sure if the bottle is simply buckling from accelerating the weight in the nose, but I'm pretty sure it's from the vacuum behind it.

So, is it permissible to tape some bamboo skewers to the outside to stiffen the wall of the bottle, or have some sort of expandable 3-D printed plastic contraption inside the bottle to prevent buckling? These things would have a weight penalty without increasing the pressure capacity of the bottle. Fins that extend up the wall of the bottle might accomplish this too.

This isn't "reinforcement" to make the bottle withstand more pressure, it's just reinforcement to make it more structural.

Old-fashioned 2-Liter PET bottles came with a reinforcing ridge around the circumference (example image), but these days they don't. I've experimented with a Coke bottle, which has an hourglass figure, but the part that's cylindrical still wants to collapse. I have a couple long slender 2 liter bottles with a ridge around the circumference but I brought them all the way back from Asia, so I'm saving those for a 'final' project, not experiments.

-Alex

Re: Negative reinforcement question

Posted: Wed Feb 06, 2019 1:09 am
by anachronist
Sorry, messed up the link, and can't edit my post. Here's the bottle: http://www.aircommandrockets.com/labs/p ... 50_001.htm
I wish those were still available.

Re: Negative reinforcement question

Posted: Wed Feb 06, 2019 1:11 am
by anachronist
Hmm, apparently Air Command doesn't permit linking out to their pages. Oh, well.

Re: Negative reinforcement question

Posted: Sun Feb 10, 2019 9:59 am
by WRA2
anachronist wrote:
Wed Feb 06, 2019 1:08 am
Classes D and E require an unreinforced bottle. Reinforcement to allow for greater pressure isn't allowed.

MY problem is, I need a way to prevent the bottle from collapsing under negative pressure!

At maximum velocity after the reaction mass is consumed, there is such a strong region of low pressure behind my rocket's nozzle that it sucks the air from the bottle until the bottle caves in, resulting a 200 mph bent rocket, often ripping the nose cone off! It happens so fast, I am not sure if the bottle is simply buckling from accelerating the weight in the nose, but I'm pretty sure it's from the vacuum behind it.

So, is it permissible to tape some bamboo skewers to the outside to stiffen the wall of the bottle, or have some sort of expandable 3-D printed plastic contraption inside the bottle to prevent buckling? These things would have a weight penalty without increasing the pressure capacity of the bottle. Fins that extend up the wall of the bottle might accomplish this too.

This isn't "reinforcement" to make the bottle withstand more pressure, it's just reinforcement to make it more structural.

Old-fashioned 2-Liter PET bottles came with a reinforcing ridge around the circumference (example image), but these days they don't. I've experimented with a Coke bottle, which has an hourglass figure, but the part that's cylindrical still wants to collapse. I have a couple long slender 2 liter bottles with a ridge around the circumference but I brought them all the way back from Asia, so I'm saving those for a 'final' project, not experiments.

-Alex
I think we will need to have a discussion involving lengthwise reinforcement and a vote for a rule modification to include something like that. I see this as more of an issue with class E where someone might build a long skinny spliced rocket and experience the issue you are describing due to the rocket bending during acceleration. In the past i have seen people use balsa wood and run it the length of the rocket "insane air" (a previous record holder) used this design to limit flexing on an unreinforced FTC rocket. The biggest challenge will be in preventing teams from creatively applying this type of reinforcement to enable to pressure vessel to hold more pressure. Maybe limit the material to balsa or or paper.

Re: Negative reinforcement question

Posted: Sun Feb 10, 2019 10:01 am
by WRA2
anachronist wrote:
Wed Feb 06, 2019 1:11 am
Hmm, apparently Air Command doesn't permit linking out to their pages. Oh, well.
Link works now. Must have been a technical issue with the site.

Re: Negative reinforcement question

Posted: Sat Feb 16, 2019 9:13 pm
by anachronist
WRA2 wrote:
Sun Feb 10, 2019 9:59 am
I think we will need to have a discussion involving lengthwise reinforcement and a vote for a rule modification to include something like that.... The biggest challenge will be in preventing teams from creatively applying this type of reinforcement to enable to pressure vessel to hold more pressure. Maybe limit the material to balsa or or paper.
Well, class D is already limited to 100 psi, so letting a bottle hold more pressure is irrelevant, no rule change needed. Class E could be clarified to say that any structural modifications cannot affect the pressure capacity.

The reinforcements I had in mind wouldn't affect the pressure capacity. I was thinking of taping bamboo skewers to the sides, or 3D printing some sort of expandable plastic framework that fits inside the bottle. Would that be allowed?

Long fins taped to the sidewall would also stiffen the bottle.

PET bottles have evolved over the years to save manufacturing costs, and nowadays a 2-liter bottle is so flimsy it can't take negative pressure without buckling. And negative pressure seems unavoidable given the aerodynamic shape of a 2-liter bottle.

Smaller bottles still work rather well. Even with the same wall thickness, the ratio of wall thickness to volume is higher with smaller bottles.

-Alex

Re: Negative reinforcement question

Posted: Sun Feb 17, 2019 9:32 pm
by WRA2
anachronist wrote:
Sat Feb 16, 2019 9:13 pm

The reinforcements I had in mind wouldn't affect the pressure capacity. I was thinking of taping bamboo skewers to the sides, or 3D printing some sort of expandable plastic framework that fits inside the bottle. Would that be allowed?

Long fins taped to the sidewall would also stiffen the bottle.
Hi Alex,

The internal framework idea would be acceptable without any changes to the rules as they stand now.

As far as taping things to the outside, I think we would need to come up with some limits as to what could be done. A team "could" use fiberglass reinforced packing tape to "attach" the bamboo skewers which would increase the bottles pressure capabilities. 15 years ago teams were reinforcing FTC tubing using the fiberglass packing tape and getting them to hold 300PSI. The number of these lengthwise "reinforcements" would need to be limited and the method of attachment defined. Another possibility are a few "hoops" around the bottle to limit collapsing (the number would need to be limited).

The "negative pressure" idea is really interesting and could lead to some really fun experiments. Are you sure that it is not an issue where the bottles are collapsing due to lengthwise bending while the rocket is accelerating? Or maybe a combination of both. The best way to test this might be to build an external frame to put the bottle in which does not touch the bottle except for at the ends and fly it with some cameras to observe the bottle collapsing. This would eliminate the lengthwise forces trying to "fold" the rocket in the middle. (such a framework as described would also be allowable under current rules) the drag and weight would reduce altitude performance but might be useful for a multi stage rocket where a 2nd stage filled with water would increase the flexibility on the 1st stage.

Re: Negative reinforcement question

Posted: Mon Feb 25, 2019 1:41 am
by anachronist
WRA2 wrote:
Sun Feb 17, 2019 9:32 pm
anachronist wrote:
Sat Feb 16, 2019 9:13 pm
The reinforcements I had in mind wouldn't affect the pressure capacity. I was thinking of taping bamboo skewers to the sides, or 3D printing some sort of expandable plastic framework that fits inside the bottle. Would that be allowed?
As far as taping things to the outside, I think we would need to come up with some limits as to what could be done. A team "could" use fiberglass reinforced packing tape to "attach" the bamboo skewers which would increase the bottles pressure capabilities.
As I said, for class D that's irrelevant. There is already a 100 psi limit in class D. A typical bottle can take at least 150 psi. So if reinforcement happens to increase the pressure capacity, that shouldn't matter for class D, because of the 100 psi limit. I honestly don't understand the concern here.

-Alex

Re: Negative reinforcement question

Posted: Tue Feb 26, 2019 7:37 pm
by WRA2
anachronist wrote:
Mon Feb 25, 2019 1:41 am
WRA2 wrote:
Sun Feb 17, 2019 9:32 pm
anachronist wrote:
Sat Feb 16, 2019 9:13 pm
The reinforcements I had in mind wouldn't affect the pressure capacity. I was thinking of taping bamboo skewers to the sides, or 3D printing some sort of expandable plastic framework that fits inside the bottle. Would that be allowed?
As far as taping things to the outside, I think we would need to come up with some limits as to what could be done. A team "could" use fiberglass reinforced packing tape to "attach" the bamboo skewers which would increase the bottles pressure capabilities.
As I said, for class D that's irrelevant. There is already a 100 psi limit in class D. A typical bottle can take at least 150 psi. So if reinforcement happens to increase the pressure capacity, that shouldn't matter for class D, because of the 100 psi limit. I honestly don't understand the concern here.

-Alex
Hi Alex,

I am referring to class E. Class D is limited to 100 PSI so any added reinforcement would simply increase the weight of the rocket. For class E there is no pressure limit Leaving it up to the competitor to find a bottle that can hold more pressure. You are also not limited by volume in Class E so it is also open to spliced bottles, FTC tubes, etc. with the only limit that you cannot apply material to the pressure vessel to "enhance" it's pressure holding capability. If you can find a bottle that can hold 200 PSI, go for it. That is where the concern is. Teams competing in that class could use the "negative pressure" reinforcement to hold more positive pressure.