Magical formula of the nozzle

[Oinikis]

Hello. I've been out of pyro for half of year, and felling that i'll come back. now the thing is, these 6 months, i made gliders. alot of them. i made simpliefied versions of formulas for calculating lift, CG, and so on. and if i do calculations, i can make a dowel fly. now the point. since then i learned alot, and learned that calculating is the way to go, and here, where the questions beigns. i had an idea of rcandy rocket. the basic idea is, you have a tube with nozzle, and you cast your fuel independently in another, slightly smaller tube, then put the fuel-tube-thing into the tube with nozzzle, and close it off. now why i chose this. using reusables metal rocket motors, you can make a reusable rocket, which can be easily reloaded in a field, just clean, put another fuel section, close and ready to go. but i chose it for one even more important reason. basic coreburner rockets, expecialy rammed pyro rockets have very progresive thrust, meaning that it's easy to get it underpowered at start, or explode later. now using method i described, i can make the core in the fuel way wider than the nozzle, making thrust much more powerfull at the start, but much less progressive. i know this is widely used.now where the calculating goes in. basicly every unit of burning are, produces a certain unit of preasure. and a nozzle can expell only limited amount of gas, at a certain preasure. the same thing is in ohms principle, when calculating resistance. so now a certain ratio of burning surface/nozle are produces the same amount of preasure with same kind of fuel, regardless of rockets size. so now the ratio is directly proportional to preasure. and every casing can hold a certain amount of preasure. now if the ratio is too big, too much preasure will build up, and will require super strong casing, which might be too heavy, or too coplicated. and if the ratio is too small, not enough preasure will be built, and the rocket will have insuficent thrust, and also, the rocket will be very inefficent. now before starting experiments, i want to have basic idea of how big the ratio could be. that would save some time and resources.

http://i.imgur.com/YlsYVjP.png

so, there is very basic scetch of both types. the surface of burning surface is basicly radius time heigh times pi. as you can see, basic rockets ratio increases 4-5 time, meaning either it's weak, inefficent at start, meaning slow takeoff, which i a bad thing on winglet stabilized rockets, or it has very high ratio at the end of the thrust meaning the casing has to be really strong. ocasionaly, both things apply. however on wider core rocket the ratio increases only like 2 times, meaning that final preasure is not so big, to require very strong casing, but it shows high performance at the start, and reaches high speed very quickly, which is a good thing for winglet stabilized rocket. reason i chose rcandy for this is that it is castable, so it woul be esier to make, and it's low burnrate, so i can have reasonable amount of thrust time, even with less burning lenght. now what i want to find out is what ratios are comonly used, and with how strong casing. so if you have a rocket design using rcandy or rammed sugar fuel, please show me the dimensions of it, so i can see the ratios, and it will really help me designing prototypes. and information we (hopefully) get, will really help designing rockets, not only for me, but for everyone. thank you.

[gregkdc1]

Look here a lot of the work has already been done.

 

http://www.nakka-rocketry.net/design1.html

 

I would suggest to use bates grain geometry you will get an even more neutral burn. Also if you are using smaller aluminum tubes it doesn’t take that much to protect the case for the burning propellant. I will typically roll my bates grains up in 8-10 inches of card stock with a layer of aluminum tape on the outside of the stack when making motors out of 1" aluminum tube. This is more than enough for the casing liner.

 

I like to use a kn value of 150-200 with re-crystallized 60/40 dextrose propellant with 1% RIO added.

[Oinikis]

that's one thing i was looking for. i searched, but was unable to find these site run by singular man, which create wonderfull rockets. i want motors to be prepared as quickly as possible also.

EDIT: just checked bates grain, and it seems really cool, but i think it's for bigger rockets, when comparably fuel burns very slow. i'm happy enought with wide core.
EDIT: it gave me alot of info. thanks. now i have a basic idea of what the Kn ratio should be. now i need to start drawing. thanks.

Edited November 15, 2013 by Oinikis

[mabuse00]

I dont know if that can be casted so easily, but maybe a star- or cross-shaped core would solve the progressive trust issue more elegant.

 

Another thing to consider is that the fuel erodes, and it does so more near the nozzle than higher in the core. When I tried sorbitol fueled rockets they often burned through at the lower third of the core. It seems that near the end of the burn, the lower part of the grain is often already burned through, so the burning area does not raise anymore.

 

 

[Oinikis]

thanks, i'll keep that in mind. i have already designed a rocket 18 mm ID 4 mm nozzle core is 9mm fuel grain is 8 cm tall. the ratio starts about 150, and is 300 and the end. man that nozzle seems too small for me, but thats what calculations told me. i've read that nakka did soritol rocket with max ratio of about 600, and he said do not try sugar, becouse it'll blow up. he used PVC water pipes. so with thick paper tube 300 should be fine i guess. first i'll try to get this thing flying, then i'll experiment more. cross shaped core really made sense. also i calculated that fuel volume is 15 cubic cm, so about 27 grams of fuel. i want to master nearly neutral rockets, so i can combine rocketry with my glider making experience, and hopefully create something.

[mikeee]

If you use pvc pipe for motor tubes keep a safe distance when launching the rockets.

PVC pipe is nasty when it cato's, shards of sharp plastic fly in every direction.

If you know the power of your fuel, it will work, but it is still a matter or time before one cato's.

Cardboard tubes are a much safer alternative and no worry about cato's and flying debris.

[Oinikis]

"so with thick paper tube 300 should be fine i guess" maybe i didn't said it clearly enough, but i'll use paper tubes. i've got giant roll of heavy craft paper, and wood glue, so i can easily make pretty darn strong tube. i know that PVC shatters into shards, so i'm avoiding it.

[Oinikis]

well,dame one and it worked very nice. morning i made 300 grams of rcandy, and when i was measuring out some for melting and casting the fuel grain, i measured like a gram or few short, so fuel grain was like 1 cm shorter than intended. i wanted to do a static test, but my hobobro (my bro, who served in army, which now lives like a hobo, insults everyone, and drinks beer, and live at wrong timezone) came to action a forced me to do a flying test, so i did. he made nice legs for it to stand, and i put it upright, and with any fins, and with CG being in the middle of the rocket,it still flew nearly srtaight up. the burntime was like 0,5 seconds, but it took of like a bullet. the rocket has some evil writings, various accecories, and weird stuff on it, so it'll be surprise for someone. i wanted tocheck how it withstoof, but meh. thanks all for ye help. i've been lurking more and i saw that grain nac be shaped like a gear, to give nearly neutral burn (slightly progresive). i saw it was used to reach space. Casting of the fuel grain was very easy. i've made a special mold, with various core driving devices to make it straight. i really enjoyed the process, and it went very well. it used about 30 grams.anyone has more ides on what to try? maybe on weirdly shaped core. i also want longer burn time,but to be nearly neutral, becouse i would really like to combine one with a plane.