Construction Musings

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
Da Bomb
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Construction Musings

Post by Da Bomb »

What is every bodies favorite bottle shape, size, or your "go to" bottle?

I have been wanting to build an air frame with the 1.5 L Smart Water bottles. What are your thoughts and experience on this size and shape?

I have looked at two different construction techniques, and I would like to know what some of your thoughts are regarding these two techniques.

I would like if you could share some of the Pros and Cons of these two techniques along with flight characteristics and reliability.


1) Splicing bottle sections together to construct one integral "tank" assembly that tapers down to a section that a Gardena nozzle will be attached to that forms an air frame with six to seven liters of total volume.

or

2) Splicing two bottles together end to end to create individual "tanks" for assembly with couplers or Tornado Tube to construct an air frame with six to seven liters capacity. At the point of coupling with the Tornado Tubes, a fairing will be used to assist in maintaining shape, form, and aerodynamics.

I am looking at putting in an avionics bay for parachute deployment and the addition of a small altimeter.

Thank you in advance for your advise and experience.

Todd
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bugwubber
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Re: Construction Musings

Post by bugwubber »

You have to start with bottles that originally held carbonated beverages. The only exception is brands like aquafina Where they are obviously using bottles designed for carbonated beverages. Water bottles may be made out of PET but they go through a manufacturing process that focuses on looks and taste, not pressure containment.

That said, the curvy coke and Dr pepper bottles make interesting singles. Plain sided ones for multi

Mountain Dew bottles
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Straight and Dr pepper
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Straight sided (Pepsi I think) with a topochico nose
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And. Finally Dr pepper with mtn dew mid and coke 12oz top
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Re: Construction Musings

Post by bugwubber »

I guess my point is, I like variety and exploring different styles.
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Re: Construction Musings

Post by Blenderite »

Bug is definitely correct about the Smart Water bottles. I had actually tried them once and the bottle failed at around 45psi. I do not recommend these bottles at all.
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Re: Construction Musings

Post by Da Bomb »

Thank you for your responses. What is a could source for 1.5 L bottles? Tomorrow I will MIC the thicknesses of different bottles and post the results.

I the below picture is my two sons. This was our entry into their Cub Scouts water rocket activity for that year.

The patriotic themed rocket was one liter Smart Water bottles spliced together. The top blue section is just a fairing that contains a white parachute and the ejection mechanism. The mechanism consist of a large medical syringe that plunger fits loose but uses a large diameter compression spring with a low spring constant. The top of the top most tank is allowed to expand and hold the fairing in place when the rocket is pressurised. The parachute is stuffed under the fairing and the fairing pressed onto the top chamber and held into position. Launch occurs and the bottle relaxes back to some of it's original size and the spring then pushes the fairing up and exposes the folded parachute to the slip stream. The parachute deploys and allows the rocket to safely and gently fall to the ground. The white "aero spike" is from a farm supply store and is normally used to feed livestock from a bottle. This piece of rubber helps with impact if the parachute does not deploy properly. This worked well but was limited to only a couple hundred feet in altitude. We only used about 500 750 cc's of water and the system was pressurised from the on-site Craftsman air compressor to 120 psig.


The second rocket themed "Little Boy" was modeled from the first atomic weapon that was used in WW II. My son at that time fascinated with atomic weapons and nuclear science. The air frame is constructed from straight side 2 liter bottles. The top of the air frame is just an open "pay load" compartment that was constructed from just a mid section of a 2 liter bottle. The open portion contained a camouflage nylon rip stop parachute and small surprise from the other scouts. We bought and filled water balloons with water. We used about 100 cc's of water in one balloon per flight. The balloon acted as some ballast and assisted with keeping the parachute from deploying to early in the flight. The water balloon and parachute were designed to deploy at apogee. The water balloon would fall from the rocket and pull the parachute out from it's folded form. The balloon would hit the ground and burst into a splash of water and the rocket "bomb" would fall gently afterwards.

It became a competition among some of the scouts to see if they could catch the water balloon before it hit the ground. I do not think that anyone caught it but they did have fun trying. This rocket only achieved about 150 to 175 ABGL altitude. Aerodynamics played a huge role in it's efficiency. The box fins added a lot of drag and admittedly were not constructed of the right material, and flawed from assembly.

The best part is that we had fun building and flying the two rockets. I think even the crowd enjoyed them as well.

For the past couple of years we have just stuck with a single two liter bottle style rocket. My job kept me from having a lot of time to spend in the shop and with the rush of getting something out the door. Last year we tried to use an eight foot FTC rocket as an exhibition rocket with on board video. Boy that was certainly a CATO.

This year I have that fever again to build something for altitude, parachute deployment, and on board video.

I welcome all input from anyone. I have dusted off all the old URL's from the common websites, I am reaching deep into the bag of knowledge. Tell me some things that you all have tried that worked or was a not so good idea.

Thanks,

Todd

P.S. Sorry for the formatting of the paragraphs. The edit view looks okay, and the smiley thing is just weird.
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Cub Scout water rocket day circa 2011
Cub Scout water rocket day circa 2011
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Re: Construction Musings

Post by NautilusRockets »

I have pressure tested 87 individual 1L Glaceau Smart Water bottles from the US Midwest and the lowest failure point recorded was at 165PSI +- 3PSI. The only way I can fathom one failing at 45PSI is if it is significantly damaged. The plastic is 0.30mm thick along the bottle sides which is thicker than a significant number of different 2L soda bottles.

Of the 38 1.5L Glaceau Smart Water bottles tested, the lowest failure was at 132PSI +-3PSI. That failure was significantly outside the fail pressure of other 37 tested and I believe due to unnoticed bottle damage as the failure was not similar in appearance to the others.

That being said, do your own stringent pressure testing of any bottles that you intend to use as pressure vessels. Manufacturers can change suppliers of blanks, blow molds, PET formulations, etc. at anytime and bottles can also vary significantly between countries or regions.
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Re: Construction Musings

Post by Blenderite »

The bottles I tested were from several years ago, so perhaps they have changed them since I tested them.
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Re: Construction Musings

Post by bugwubber »

This is great to hear since those bottles have such. A great shape. I'll have to try some out
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Re: Construction Musings

Post by Da Bomb »

Measured three PET bottles today of different brands. As mentioned above differences in manufacturer of the preforms, molded shapes, mold variables, and process control was not included in today's test.

This was just one bottle from each of three vendors. All other conditions are unknowns. I am not sure of what specifications are required from each beverage distributor as far as how much statistical process control that they may require. I sure that this process is not a six sigma process.

Testing and results:

I picked what was available to me today. The method for checking the thickness of the PET material of each bottle involved removing the bottom from each of the bottles. An area was selected that was smooth and void of any style features that would result in a false or ambiguous measurement. The measurement was made in five locations on the circumference of the bottle. The measuring instrument was a Mitutoyo 0 - 1" micrometer with flat anvils.

1) First bottle test was a 2 liter Pepsi Diet Dr. Pepper. The average thickness was measured at 0.0130" and for you metric guys, I just used a conversion factor of mm = inches / 0.039370. The first reading came out to be 0.3302 mm.

2) The second bottle was a 2 liter CocaCola Classic. This bottle had the styling features that needed to be avoided to prevent false measurements from occurring. The average measured thickness of five readings was 0.0135" The equates to 0.3429 mm.

3) The third bottle was a 1.5 liter Smart Water beverage. This bottle had no styling and had smooth sides. The average measured bottle thickness over five readings was 0.0185" The metric size is 0.4699 mm.

Another test performed was that I had my son close his eyes and one at a time and had him bend the bottle wall and describe the differences in the amount of flex in the bottle thickness. This test might have been biased due to the bottles having a difference in circumference. He was able to differentiate between the different bottles and describe the flexibility of each bottle.

Conclusions:

The first two bottles are designed and manufactured for carbonated beverages. The third bottle manufactured for non-carbonated beverages. The third bottle has a thickness difference of 0.0055' ( 0.1397 mm). This equates to a 42.30% increase of PET material thickness in the Smart Water bottle over the first bottle designed for carbonated beverages. If desired we could compute the amount of hoop stress for a thin walled system and determine the ultimate yield strength (modulus of rupture) of the bottles in the experiment. Other variables would determine a variety in the outcome of the testing. Averaging the results would narrow the gap and produce more scientific results.

From my test results today, I could say that the Smart Water brand of beverage container could be used as a safe pressure vessel for water rocket construction. Additional real world hydro testing would be needed to validate the hypothesis that based on material thickness the Smart Water bottle could be used as a pressure vessel that could out perform it's 2 liter counter part.


The image below illustrates the method to test all three bottles today.
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PET wall thickness measurement w/ micrometer
PET wall thickness measurement w/ micrometer
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Re: Construction Musings

Post by bugwubber »

1 liter GLACEAU Smart Water bottle tested to 151 psi
RESULT: deformed, did not fail for 3 pressure cycles

Store brand light bodied 2 liter carb bev bottle
RESULT: bottle failed at 125 psi
Speculation- nozzle end failed, likely at seam.


Since it was the 4th, we used air only
50ft of hose.
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