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#1 Potato

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Posted 29 June 2017 - 06:44 AM

Hello,

 

I'm from Germany and I made a lot of smaller rockets the last 10 years.

But now I wanna built a bigger one ;)

 

The plan is a rocket engine with a ID of 45mm(1,77inch) and a length of 400mm(15,748").

I wanna use Rocket Candy with Sorbitol instead of regular sugar due to the higher viscosity of the Kno3/Sorbitol mixture.

Which ratio do you reccomend? 65:35 or better 60:40?

And how much fuel do I need? Do you think that one kg(2,2lbs) is enough?

I will cast it straight into the tube in one solid block. No separate grains.

And the most important question: Which nozzle size do you think will make the job well? I would say a diameter of 15mm(0,590inch), because that is 1/3 of the internal diameter.

The nozzle itself is casted out of plaster(gypsum) and is screwed in the tube.

Thank you for your support!

 

Kind regards

Dokami


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#2 dangerousamateur

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Posted 29 June 2017 - 02:15 PM

In my opinion you should start smaller and learn.

Because the questions you ask should not be asked when somebody moves to such monsters. Just my opinion ;)


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#3 nils

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Posted 22 February 2018 - 01:43 AM

I have recently made similar size rockets with good results. Outer diameter of my PE plastic tubes were 50mm and thickness of the wall 1,8mm.

Nozzle results were:

15 mm - no thrust

14 mm - little thrust

13 mm - more thrust

12 mm - ideal thrust, 2,5 kg on kitchen scale.

11 mm - over 5 kg of thrust or explosion

10 mm - mostly explosions, rarely ultimate acceleration.

 

12 mm nozzle version testing video:

 

If you use plastic tube - then I recommend hot bending and no screws:


Edited by nils, 22 February 2018 - 01:45 AM.


#4 stix

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Posted 24 February 2018 - 05:55 PM

I'm assuming we are talking about internal/core burner motors.

 

Nils, what you have neglected to tell us is the length of your fuel grain (I'd guess around 100mm). This is important and will help determine what throat diameter of the nozzle to use.

 

In your case 12mm works fine, but if you doubled the length of the fuel it would likely cato. Potato's planned motor is 400 in length which is 4 x the length of yours, and in my opinion too long compared to the width.

 

What's important to work out is the Kn Ratios. This is the amount of fuel burning (surface area) in relation to the nozzle throat area. Therefore the longer the fuel, the higher the ratio until it over-pressurizes - boom!! There comes a point where you wouldn't be able to make the nozzle throat wide enough.

 

The original post was from June last year. I hope Potato is ok because what he was planning was never going to work for many reasons. One thing he did get right was the amount of fuel required would be around 1 kilo (@ a density of 1.7 grams per cubic centimeter would equal 961.33 grams of fuel), providing that it was a core burner with the core running all the way through and the same diam. as the nozzle throat. But it would have cato'd anyway.

 

Cheers.


Edited by stix, 25 February 2018 - 07:44 AM.

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#5 JMan

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Posted 03 March 2018 - 11:26 PM

https://www.youtube....h?v=R93_4BWrC0A

I have used laser cut wooden nozzles but I would recommend cutting 4 or 8 notches (about 1/8 the diameter of the circle) around the circumference of the nozzle and press it into the fuel. This lets the fuel grip the nozzle and instead of blowing plastic shrapnel (espicaly if youre using pvc) your nozzle will act as a burst diaphragm to help build pressure for a (now) nozzleless motor. This is almost a hybrid by starting with a nozzle to get that high pressure for take off and lower pressure for sustaining flight.

As for the muxture I would recommend dry cooking a fuel consisting of sorbitol sucrose and kno3 in a 3:2:13 mix. Ive noticed this has many many benifits. The sorbitol will bind everything at a low temp and will flow to fill all gaps and air bubbles when directly filling a casing. Once dry the surcose causes the fuel to be very strong and solid but the sorbitol will keep it from cracking. I would not recommend only surcose because it does not bind to caseings well and is prone to cracking and air bubbles. I would also not recommend only sorbitol for such long burning motors. The fuel will get hot and will DRIP out of the motor unless it has a repetitively small nozzle.

If your motor is long this mix will also allows the spindle to come out very very nice. You must insert the spindle while it is hot and spin it until it cools. It will become harder and harder to spin until the core solidifies and your spindle will spin with no resistance. I would recommend using a NON WATER ice pack to help cool it faster so you do t have to spin the spindle for so long.

Im not sure how long youve been using sugar rockets but Im going on about 5 years and Id love to see you make a wonderful reliable rocket. Just be careful and if ever in doubt, dont. You are taking the risk and it is rocket science if you mess up its a bomb squad/ fire department/ paramedics.

Post some pics/vids of your motors wed all love to see them!

Edited by JMan, 03 March 2018 - 11:27 PM.


#6 stix

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Posted 04 March 2018 - 02:46 AM

Have to say I did like Nils video with his method for heating and folding over the pvc tube.

 

@ JMan

 

Your sorbitol/sucrose/kno3 mix is similar to my current standard mix. Are you melting the sucrose (ie. 186°C) or melting just the Sorbitol and leaving the sucrose as a filler?

 

I wouldn't recommend "case-bonding" as it's too easy for the flame to propagate up the sides.


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#7 JMan

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Posted 04 March 2018 - 08:07 AM

Stix, I am cooking at 100-110 c and using the sucrose as a filler. Ive never had a problem with the flames traveling up the sides when using sorbitol, however, with sucrose only it has happened (although its been the least of my catos) the sorbitol should really bind with the pvc if youve heated it up enough
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#8 stix

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Posted 07 March 2018 - 03:05 AM

Ah yes. Similar to my composition KNSUSB18+C, which I think we discussed a while back.

 

Glad you've got some real results - I've got a few cast grains ready to go but have never tested in an actual motor. I'm interest to know how you made it "pourable" and what was the composition that worked for you?

 

Cheers.

 

Oh yeah, I guess this thread has been hijacked, Nils first, then me, then you. Might be better to start a new thread?


Edited by stix, 07 March 2018 - 03:10 AM.

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#9 JMan

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Posted 07 March 2018 - 10:37 PM

Yes it was your post which inspired me to try the sucrose sorb mix and the way I insert it into the motor isnt really pouring.

Ive tried that but it cools too quick so I roll them a little smaller than the tube and let them cool on a heat sink then drop them in and let the motor caseing sit on the hot plate (its really cus its such a small motor) and once its up to temp I tap it on the ground a dozen times and repeat.

The right mixture might not be the same for larger motors but I think its really the consistency. The added sucrose adds so much strength to sorb grains (or sorb adds so much malaubility and workable temps to sucrose grains)

Once I get it 100% repeatable with common materials Ill post a thread so hopefully this weekend (and hopefully it will help potato with his/her motor
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#10 stix

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Posted 12 March 2018 - 06:36 AM

I'm glad you have done some tests with that formula. Seems like it works well. I ended up slightly heating the fuel in the forming apparatus in the oven then pressing again. Sorbitol makes that part much easier as in a final pressing to consolidate the mix (no voids).

 

After a few months of testing, the "best" formula that I came up with was:

 

KNO3     62% (fine powder)

SURCOSE  20% (fine powder)

SORBITOL 18% (doesn't matter because it melts)

 

+ a tiny amount of carbon black.

 

The tests were based on "castability" that is, not necessarily pourable, but formable. The tests also rated "strength", "flexibility", "Set-Up time" etc....

I'm interested to know what final mix worked for you?

 

In the end, I got "testers block" similar to writers block. I couldn't test simply for the reason that I can't test at home, and going out somewhere to test, becomes a drag. All the testing became overwhelming and I went on to something else to give my brain a rest. Never tested in an actual motor, but I have made grains ready to go. I may do so - your post has inspired me to finish what I started. :)


Edited by stix, 12 March 2018 - 07:04 AM.

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#11 JMan

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Posted 12 March 2018 - 10:14 PM

I havent tested much different amounts as much Ive ive been trying to get an easy made motor. My mix is 14-21-65 I like sticking to the exact chemicle mx if 7-13 (35-65). But give me a weekend or two more and Ill have a new post explaining all about it. My biggest problem has always been voids and coreing the rocket this solves both :)

Edited by JMan, 12 March 2018 - 10:14 PM.

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#12 stix

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Posted 14 March 2018 - 12:04 AM

Sounds good.


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