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Sugar rocket pressure problem


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#41 NeighborJ

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Posted 23 March 2017 - 07:42 PM

The Saturn V rocket having variable thrust liquid fueled engines would have very different results, being able to compensate it's thrust to achieve an optimal power to weight ratio at any size scale. Still time would still need to be scaled down along with the rocket otherwise one could expect a scaled down rocket to achieve orbit. My head is spinning trying to understand what this would mean. Basically I would expect there to be a lot shorter burn time so I couldn't expect it to go nearly as high.
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#42 stix

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Posted 24 March 2017 - 03:20 AM

. . . Still time would still need to be scaled down along with the rocket otherwise one could expect a scaled down rocket to achieve orbit . . .

 

Yeah NJ that does seem to be the case.

 

The "scaling theory" was fleshed out a bit in this thread.

http://www.amateurpy...fuel-burn-rate/

 

Although, that thread was more about scaling UP from a smaller motor, but the issue is the same as you suggest, as in time would need to be scaled as well. The only way of doing that (when scaling up) would be to add a burn rate modifier.


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#43 NeighborJ

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Posted 24 March 2017 - 08:25 PM

I didn't read the whole thread but it all makes sense and I can see other issues. If the large rocket motors lift off with a minimal amount of g force to protect the passengers and payload then a scaled motor would lift even slower. Without any guidance system to stabilize it, then it would be almost impossible to achieve a stable flight. Essentially a smaller motor would require tuning to achieve an equal amount of g force as the big ones or have a scaled down amount of gravity and crosswind.
It is simply easier to design a motor for a specific application. I never had calculus so I wouldn't know where to start with the math but I do understand the concepts and theories which are all pointing to the impossibility of creating an exact scaled replica of a large rocket.
I enjoy these kinds of threads, they tend to open my mind to new ideas and brush the cobwebs out of my head.

#44 stix

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Posted 26 March 2017 - 05:17 AM

. . . It is simply easier to design a motor for a specific application . . .

 

Yes NJ, that is one way.

 

The other way/method is to design a motor, test it, evaluate the data, then extrapolate that data and apply it to your new design and then predict the outcome. "Outcome" being very subjective. When scaling up using small motors (approx. 20mm - 40mm ID) this method does seems to hold true for this size. As we go bigger then the fuel burn rate and pressure will come into play more.

 

What I'm talking about is "scaling up".

 

If you're into mucking about with various fuels, grain geometry, nozzle designs (lots of fun), then it makes sense from a cost and time point of view to start with smaller designs, then go bigger.

 

kramrocket has done some good tests and my effort will be to show the results in a way that can easily be understood.

 

Sorry that this thread has gone off too far a tangent. I'll try my best to get this together over the next week and post a new topic for those who may be interested.

 

Cheers.


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#45 JMan

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Posted 06 April 2017 - 08:52 PM

Sorry I was away for spring break and came back to a lot of work I don't really mind the tangent it carries a lot of stuff to be learned and interesting topics but I tried to make a new batch today and over cooked it bummer.

But talking about the scaling the burn rate is measured by a linier length (radius) so it would scale down porportionally assuming the pressure stays the same (it won't for the following reasons) the pressure/thrust is measured in surface area of burning propellant so it will be scaled by an exponent of 2. This includes pressure and thrust which is why your Saturn v calculation was so much higher it's not 1/2 rather 1/4 (just 1/2 squared). Weight is measured by volume scaling it down would be by an exponent of 3 so although burn rate decreases directly and pressure decreases by a factor of x to the -2 your new weight now weight x to the -3 your 100:1 rocket will now weigh 1000000:1 (100 squared it 1000000 right?). Then you all talk about keeping it stable with the low thrust to weight well that is no longer a thing, in fact its thrust is much higher (i think at least I'm sitting in bed not doing math right now) but smaller objects are much stronger porportionally.

It's kind of hard to explain and I doubt you want to read much more but vsauce 3 has a good video on it (could Godzilla exist) it describes how getting larger makes things not as strong and vise versa

#46 JMan

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Posted 07 April 2017 - 10:50 AM

Also a new question but has anyone tried dehydrating their rocket fuel rather thank cooking it? I'm thinking that the water would be driven out much more throughly than cooking it and it not only would help the burn but also decrease the weight slightly.

i was thinking of trying to cook most of it off to where it's an apple sauce consistency then pour that into my rocket, dehydrate it and pour some more (assuming it shrunk) and repeat untill its full of perfectly dry and air bubble-less fuel perfectly conformed to my rocket case

#47 NeighborJ

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Posted 07 April 2017 - 03:27 PM

JMan I tried that same experiment with less than satisfactory results. You may have better luck but mine took weeks to dry over a space heater and the KNO3 was recrystalizing on the surface. The resulting fuel was very slow.
Ideally it can be made without adding water and with much stirring if you have a skillet with exceptional temperature control but it is very easy to ignite the whole batch if the skillet heats unevenly or overshoots it's setpoint. I've read a thread here somewhere of someone granulating cooked fuel then pressing it into a motor and it was claimed to have given good, hot, consistent results.
I think at some point anyone who has made sugar rockets looks for an easier casting procedure but there are few safe options without changing the type of sugar used or highly precise equipment.

#48 JMan

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Posted 07 April 2017 - 08:49 PM

Yah that's how I started by melting the sugar and mixing the kno3 but the dissolving method seems to work best and actually easiest I think but right now I have a simple radiater (water of course no flame or sparks) that my finished castings sit atop to stay dry I live in Ohio and weather is quite unpredictable (is was 80 wendsday and Thursday night Friday morning it snowed?!?) so that radatier keeps any moisture away rather than a zip lock bag

I tried zip lock bags and even the trunk of my car and, contrary to popular belief my hummad hot trunk stores the casting better than a bag sitting in the house? I'm seeing if maybe this radatier drives out even more moisture than cooking and sealing. I'll keep you updated but I just can't seem to get my batch down after break the test burns boil before ignition(too much water) yet when I cook it slightly more they crumble like chips ahoy cookies (surprisingly similar actually) so I packed it while it was in the playdough-ish phase maybe a little dryer (peanut butter like idk how to describe them in technicals terms) and packed and cored and left to radiate

#49 dagabu

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Posted 16 April 2017 - 08:52 PM

Also a new question but has anyone tried dehydrating their rocket fuel rather thank cooking it? I'm thinking that the water would be driven out much more throughly than cooking it and it not only would help the burn but also decrease the weight slightly...

 

JMan,

 

That is a great question but think about other answers. I see the use of heat to dehydrate the fuel but I offer this instead. 

 

Vacuum. I have a couple vacuum pumps I could use to evacuate the mixed liquids to precipitate out the finished fuel.  Heat would certainly make the process much faster and much safer IF the crystals dont drop out independently. 

 

I theorize that the same result will happen as when drying it out over heat over time, I think its the stirring that keeps the KNO3 and Sucrose in a homogeneous mix.   


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#50 Redrocketman

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Posted 19 April 2017 - 01:28 PM

Couple of points. The core diameter MUST always be larger than the nozzle diameter, or erosive burn will occur, and the core itself will be acting as the nozzle, not the nozzle!! Sounds funny but it's true. Personally I strongly advise against threading PVC pipe when putting it under such extreme pressure and heat fluctuations- the 2 main enemies of PVC. It's only creating a weak spot in an already venerable motor design. I'm not saying it to take the piss, I soley make PVC motors, from a 3/4" G-160 to the rather scary K-550, soon to be tweaked to a K-1100!! But, all possible avenues to lower the risk of a failure are taken and still they sometimes happen, don't add another weak spot!!




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