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Rich_Insane
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[*] posted on 20-10-2009 at 15:11
Rocket thrust calculation?


Hey everyone,

I was wondering how one could calculate the thrust and pressure a rocket nozzle exerts. I found some formulas, but they all are much too complex. i remember that long ago, I learned a formula that only needed nozzle area and some other easy stuff. Any ideas?

I only need the pressure in psi. Let's say I have a "warhead" (just a generic term for the payload weight) that weighs 500 grams. How much force from a KNO3/Sucrose rocket motor through a nozzle of say 1.5 cm diameter is needed?

Any ideas, or is this one of the unsolvable?
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DDTea
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[*] posted on 21-10-2009 at 22:10


This is solvable, definitely. A bit of integral calculus might be useful though... One website that I've found very useful for rocketry is Richard Nakka's rocketry site... http://www.nakka-rocketry.net/ I know he addresses the issue of nozzle design in great detail. Take a look if you haven't already.



"In the end the proud scientist or philosopher who cannot be bothered to make his thought accessible has no choice but to retire to the heights in which dwell the Great Misunderstood and the Great Ignored, there to rail in Olympic superiority at the folly of mankind." - Reginald Kapp.
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chemoleo
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22-10-2009 at 14:33
franklyn
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[*] posted on 22-10-2009 at 18:15



Thrust is just the chamber pressure per unit area multiplied by the area of the
exhaust nozzle throat , in other words the force generated. It must be greater
than the rocket's weight at ignition or else it won't begin to move until enough
propellant has burned out to become lighter then the thrust generated. Pressure
developed is a matter of how much gas is being produced per unit time while
it is at the same time venting out of confinement. If you have available a chart
showing the rate of burn of some known propellant composition relative to a
range of pressure ( burns faster at higher pressure ) all the work is already done
for you there. Select the pressure which will be determined by the burst strengh
of the rocket casing ( the same as the thrust calculation ) and read off the rate
of burn , the amount of material burned per unit time to produce the required
quantity of gas volume. So far this is a dimensionless abstract excersize and
the surface area of the combustion chamber of the solid booster must now be
calculated by determining from the density of the propellant the laminar volume
that will burn per unit time in relation to the size of the throat area of the exhaust
to correspond to the selected pressure and burn rate. The motor will be sized
according to the overall weight plus payload to be carried.
You can see from this there can be no simple calculation since so much is
interdependant. Change one value and all the rest are affected.
Since all this depends on any change in the rate of combustion , resorting
to calculus is unavoidable. The major reason technolgy did not advance
much prior to the development of calculus. See ' Making of Rockets (1696) '
Included among others here _ http://www.megaupload.com/?d=9AKP7V4G
Aware of the impediment this makes for amateurs ( not all are rocket scientists )
others who have gone before provide assistance in this area of endeavor ,
a good place to start is _ http://www.nakka-rocketry.net

* Note that liquid rocket motors being throttleable are less resistent to
optimization since one can always force through more fuel , up to a point.
- A related post
http://www.sciencemadness.org/talk/viewthread.php?tid=8746&a...

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Rich_Insane
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[*] posted on 26-10-2009 at 16:36


How would one calculate the pressure. Are there pressure tables available for different mixes? As in tables.
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schatz
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smile.gif posted on 15-12-2009 at 08:20
Rocket thrust calculation?


Dont know about calculating, but how about parctical measurments?
From way back I can tell you that we had a simple setup, using an 8mm film camera, bathroom scales /readout mirror. We eventually graduated to adding a huge surplus hydraulic pressure guage.
If I remember correct, the thing worked in a fashion; That is we could plot time/thrust/pressure graphs etc; Vastly improved blind trial and errors in nozzel and grain design.

But what did we know; Now, older wiser and uglier I wish that we did really put that sounding rocket in the strat
:)
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bbartlog
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[*] posted on 15-12-2009 at 08:40


Quote:
How would one calculate the pressure. Are there pressure tables available for different mixes?


See franklyn's post, above. The burn rate increases with pressure and heat. Some particular mix doesn't produce a particular pressure, rather it burns at some rate (which itself increases with pressure and heat) and you then need to size your exhaust nozzle to allow the escape of a sufficient amount of hot gas to keep the chamber pressure within the design limits. Or you can approach the problem differently, looking at the exposed surface area of your grain and changing the grain geometry to make the gas production fit your nozzle area and pressure constraints.
The theory and math are hard, and small mistakes can lead to exploding rockets, which is why most hobbyists start with a known-to-work set of parameters and then do tests by varying things slightly.
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[*] posted on 27-12-2009 at 04:39


Go to http://www.nakka-rocketry.net/softw.html and get SRM.XLS. I have used this alot and find it great for motors with over 100g of fuel. It bugs sometimes with small motors. Make sure you use MS Excel and not another program as this can bugs it also.
If you are making smaller motors like I do just try it and if you get a bug add a few grams of fuel.

[Edited on 27-12-2009 by Bowdlerize]
franklyn
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[*] posted on 29-12-2009 at 02:07


http://www.rocketmotorparts.com/links.html

http://www.rocketry.org/orgs/orgList.php

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franklyn
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[*] posted on 22-1-2010 at 14:13


Rocket Manual for Amateurs - By Capt. Bertrand R. Brinley
(Ballantine Books - 1960) 385 pages , cost then $0.75 cents :)
This is the only source of this book apart from "Torrent" distributuion
http://www.rocketryplanet.com/content/view/2766/95/
You can obtain all of it , but the relevant content to your query is

Capter 4 - 24 MB
www.rocketryplanet.com/images/pdf/RMA-Chapter-04.pdf
Appendix (E) - 45 MB
www.rocketryplanet.com/images/pdf/RMA-Appendix.pdf

it's just arithmetic with roots and powers , no calculus.

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