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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Need pointers on FTC nozzle fab

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rstaff3
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Need pointers on FTC nozzle fab

Post by rstaff3 » Fri Jun 06, 2014 2:37 pm

OK, I need your advice. I am trying to make nozzle/closures for FTC rockets using the basic method outlined on http://www.orgsites.com/va/bristolwater ... _pgg6.php3. Baically, you use a copper reducer as a mandrel and shrink bottle ends over that. I'm only on the step where you shrink a bottle to form an aligning spacer for the actual nozzles.

Attempt #1 resulted in a totally gnarly mess....too much heat. The shrinking of the 2nd bottle seemed to go well. Except, I can not get it to budge off the mandrel. BO: I've tried hot water, cold water, and even freezing the assembly over night.

My first question is how would you suggest getting off. Second, where did I go wrong. Third, would a coat of some type of grease help? Mann mold release? Vaseline?

TIA,

Dick



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Re: Need pointers on FTC nozzle fab

Post by rstaff3 » Fri Jun 06, 2014 3:33 pm

Update! I managed to tap the mandrel out by inserting a punch into the neck of bottle and tapping. I poked one hole in the plastic, but since this is just a spacer, it is still usable. I also burred-up the copper, which had now been ground and polished. I flipped the plastic part around and slipped it back on the mandrel and over a 3/4" PVC pipe.

So far so good, but questions #2 and #3 remain.

-D



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Re: Need pointers on FTC nozzle fab

Post by bugwubber » Fri Jun 06, 2014 5:44 pm

rstaff3 wrote:Update! I managed to tap the mandrel out by inserting a punch into the neck of bottle and tapping. I poked one hole in the plastic, but since this is just a spacer, it is still usable. I also burred-up the copper, which had now been ground and polished. I flipped the plastic part around and slipped it back on the mandrel and over a 3/4" PVC pipe.

So far so good, but questions #2 and #3 remain.

-D
I've just heat shrunk the bottle in place on the FTC cover. The polycarbonate doesn't shrink when it gets hot like the bottles.


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Re: Need pointers on FTC nozzle fab

Post by bugwubber » Fri Jun 06, 2014 5:45 pm

rstaff3 wrote:Update! I managed to tap the mandrel out by inserting a punch into the neck of bottle and tapping. I poked one hole in the plastic, but since this is just a spacer, it is still usable. I also burred-up the copper, which had now been ground and polished. I flipped the plastic part around and slipped it back on the mandrel and over a 3/4" PVC pipe.

So far so good, but questions #2 and #3 remain.

-D
I got all worried at first and filled the FTC full of water to prevent it from heating up, but then found out it didn't matter.


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Re: Need pointers on FTC nozzle fab

Post by rstaff3 » Fri Jun 06, 2014 7:31 pm

Thanks for the input. I may try that on a spare piece of FTC from a pyro project.

I put a thin layer of WD-40 on my mandrel and tried again...with success! The nozzle piece didn't come off easily but it leg go after a minute of coaxing.

I am a measure once, cut twice kind of guy. I jumped in without checking the fit of the brass reducer in a FTC. I found the resulting nozzle was quite loose. I have to think what I need to do.

I also need to go scrounging come our next recycle day!



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Re: Need pointers on FTC nozzle fab

Post by U.S. Water Rockets1 » Sat Jun 07, 2014 10:36 pm

rstaff3 wrote:Thanks for the input. I may try that on a spare piece of FTC from a pyro project.

I put a thin layer of WD-40 on my mandrel and tried again...with success! The nozzle piece didn't come off easily but it leg go after a minute of coaxing.

I am a measure once, cut twice kind of guy. I jumped in without checking the fit of the brass reducer in a FTC. I found the resulting nozzle was quite loose. I have to think what I need to do.

I also need to go scrounging come our next recycle day!
If you put a tornado tube on the threaded end of the bottle you can screw a scrap bottle into the other end and use it as a handle to yank any shrunken bottle off a mandrel. It works well for all shrunken bottle ends and not just for FTC rockets. A coat of carnuba wax will help get off any stuck bottles too. Just wax the mandrel before shrinking. It won't wash away.


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Re: Need pointers on FTC nozzle fab

Post by rstaff3 » Sun Jun 08, 2014 12:32 pm

Thanks, the tornado tube is a great idea for that end but, after trimming, there is still only a small section of the copper reducer to get hold of. The wax sounds best but my thought process was going through the options I have on hand. I may also try the Dow111. If it's good for the innards of a composite rocket motor I guess it should take the heat of a heat gun. Will have to be cleaned off well before gluing though...



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Re: Need pointers on FTC nozzle fab

Post by Water Rocket Expert » Sun Jun 08, 2014 9:46 pm

Just use a 16oz pop bottle and ftc as a mold. I wouldn't suggest the copper unless your really adament about precision manufacturing. Which none of us are that good at! I'll only speak for myself!


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Re: Need pointers on FTC nozzle fab

Post by rstaff3 » Mon Jun 09, 2014 2:54 pm

I doubt if I'll build lots and lots of these and think I can deal with the mandrel. My rocketry background won't let me accept thrust misalignment if it can be avoided (unless I put it there on purpose).



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Re: Need pointers on FTC nozzle fab

Post by U.S. Water Rockets1 » Sat Jun 14, 2014 10:05 pm

rstaff3 wrote:I doubt if I'll build lots and lots of these and think I can deal with the mandrel. My rocketry background won't let me accept thrust misalignment if it can be avoided (unless I put it there on purpose).
Have you done it in the past intentionally by design? For what reason? Was it a cluster of engines or something?


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Re: Need pointers on FTC nozzle fab

Post by rstaff3 » Tue Jun 17, 2014 3:14 pm

My background is low, mid and high power 'pyro rockets'. The common wisdom is the that stuff like thrust misalignment will rob you of performance and possibly make your rocket unstable. That being said, I've purposely canted the motors on several clusters :)



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Re: Need pointers on FTC nozzle fab

Post by U.S. Water Rockets1 » Sat Jun 21, 2014 10:23 pm

rstaff3 wrote:My background is low, mid and high power 'pyro rockets'. The common wisdom is the that stuff like thrust misalignment will rob you of performance and possibly make your rocket unstable. That being said, I've purposely canted the motors on several clusters :)
The purpose of angling to boosters is to increase the stability of the rocket, is it not?


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Re: Need pointers on FTC nozzle fab

Post by rstaff3 » Sat Jun 21, 2014 11:33 pm

You are correct. That is the rocket-sciencey purpose, but the angled plumes also look pretty cool :) If everything were perfect...motor thrust was exactly the same and the motors ignited at the exact instant, then canted boosters wouldn't help stability. However, motors do vary and commonly don't ignite at the same time. If you cant the motors into the rocket's CG, however, any mismatched thrust will not have as much impact. The FlisKits Deuces Wild was the first commercial kit with 2 canted motors. This has been known to fly stably even if only one motor ignites. I attached a photo showing a kitbash of the Deuces Wild (aka the Deuce XL5) with only one motor burning. It turned out the 2nd motor just went late. The flight was fully successful. The downside to this approach is that the effective thrust of the motors is reduced by the cosine of the cant angle.

http://www.apogeerockets.com/downloads/ ... ter226.pdf
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