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Author: Subject: Rocketry Calculation
Deceitful_Frank
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[*] posted on 28-6-2006 at 02:38
Rocketry Calculation


Forgive my ignorance but I wanted to post this in SciMad but wasnt sure where to put it in order to please the mods but also to get the best response... I know that many HE hobbtist also dabble in rocketry and math :)


Hi chaps, I am wanting to improve my small rocket candy motors and have a quiery concerning Kn ratios and how to calculate the IDEAL throat diameter for a rocket of given design and dimensions.

I'm not asking to be spoonfed. I've researched rocketry and newtons laws to quite spme depth. I'm fairly profficient at math but differential calculus is where I must draw the line for the moment!

Basically I'm familiar with different grain designs and their effects, calculating Kn ratios etc but I need help determing the INITIAL Kn ratio required to get the craft off the ground as I beleive that at takeoff, any propellent burning and not having the force to counteract the downward pull of gravity and accelerate the rocket upward would be wasted.

IIRC This is how you design your nozzle... I'd be using an 80 gram candy (with or without Fe2O3) core burner bonded inside a strong 1" ID cardboard tube so it would act as its own casing on its progressive burn up into the sky. I think an initial upward acceleration of 2g would suffice to quickly gain stability on a suitable bamboo stick? What would happen when the propellant has burned to the casing and the cardboard receives the full working pressure... I'm not too concerned with at this time.

Thanks in advance
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Fulmen
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[*] posted on 28-6-2006 at 03:05


In case you haven't done this already, spend a few (!) hours at www.nakka-rocketry.net. You want to download the srm-spreadsheet (http://members.aol.com/nakkarocketry/soft/SRM.zip), it's exellent for calculations like these. He also has a spreadsheet for calculating ignition pressure (http://members.aol.com/nakkarocketry/soft/igniter.xls) that might come in handy.

As for using case-bonded propellant, it might pose a problem. The fuel is quite brittle and can easily crack if subjected to high stress.
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quicksilver
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[*] posted on 28-6-2006 at 06:40


Here are some spreadsheets on nozelbore, casing, etc, ect...I have used them and they are quite workable.
Don't worry about being "spoon-fed".... there is enough stuff to learn with a rocket hobby that all the help you can get is bearly enough :P

Attachment: rocket_stuff.rar (80kB)
This file has been downloaded 455 times





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enhzflep
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[*] posted on 28-6-2006 at 17:26


Not too sure where I got it from, but if you can find a spreadsheet named Grains 2000.xls (maybe grain without the s), this is a great place to start. Coupled with the program propep, this program can calculate the entire burn profile of a motor with 1 of about 15 grain geometries and with any fuel you can think of (or devise). This uses code originally developed by Nasa and can compute the burn of a motor to within a couple a percent.

Just the thing for all those rocket calcs. It can also ouput the motor data to a file that is compatible with other rocket software, so as you can calculate the actuall height and the deley period needed to obtain it.

The software can be a bit of a pain to set-up correctly and get functioning, but is well and truely worth it. Used it to calculate motors from 4mm end-burners with 1.2gms blackpowder all the way up to 26mm core burners with 60gms blackpowder. Had some D sized motors (24gms blackpowder in a 18mm tube - or more correctly, impulse up to 20NS, that would punch a _perfect_ hole through cement sheeting from a standing start 1.5 meters away :D)

With a little work, this software could easily deal with stinger missiles, and avoid the use of a stabilizer stick alltogether.

With lots of annoying bridging calculations, we used to use this software to make motors tha would start on the (relatively) expensive blackpowder and then progress onto double-base smokeless powder about half-way through the burn. We blew-up a lot of casings just trying to make it work, but the software made these babies work nearly every time. Only problem is that we could no longer see the rockets or the smoke they left during the day so had to fire at night. And the noise was something else - no more woosh or hissing of air, these things almost wistled as they were metal cased and operated up to 600 PSI.


[EDIT: Here's a page that contains links to Grains2000 + some Kn ratio calculators
http://www.lokiresearch.com/software.asp
Just remembered, this spreadsheet can output Wrasp motor data files.

Here's a link for Propep (needed by above spreadsheet)
http://www.rocketry.org/software/details.php?software_id=17

It's well the effort to integrate these programs. With Wrasp you can then take the optimized motor data file and use it to simulate a model rocket]


[Edited on 29-6-2006 by enhzflep]
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enhzflep
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biggrin.gif posted on 9-7-2006 at 19:25


Just thought I'd post the image for a laugh. It seems as though the figure of 1.5m was a little rosy however. A more accurate statement would be that the rocket hit the roof about 2m above launch position. Still, gives me a laugh every time I collect the mail.

Note that the 'nipple' on the left of the hole was left by the guidance stick...

HoleInRoof.jpg - 93kB
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