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tom haggen
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[*] posted on 5-8-2004 at 21:45
Rocketry


Well I noticed there aren’t any rocketry sections, so I put this in the misc. section. However, rocketry is a science so what the hell? Rocketry has recently become a new hobby of mine. I have been launching A size rocket engines with some success down at the local field. My fuselage is about 1.5 inches in diameter and 15 inches in length. My fins are about 6-7" in length, and the nose cone was about 3 inches long.
Anyone who has some experience in model rocketry knows that the ESTES engines are sold in A-G, and even bigger sizes. First I was using A size engines in my rocket described above that also weighed about 3 ounces with the engine. Well for some stupid reason I decided to but an E size engine in this 3 ounce rocket, and it almost ended my rocketry career. By the way if you’re not familiar with the classification of these rocket engines each letter size means the thrust increases exponentially.
Anyway I launched this lightweight rocket with a dagger of a nose cone with an E size engine. To my horror the rocket flew sideways off of the launching rod and flew over these apartments in a helix formation into a residential neighborhood. I managed to pin point the landing site by a combination of guessing, smelling the spent fuel, and talking to a few eyewitnesses that were only like 5 years old. Thank god no one was hurt. Earlier before I launched the rocket I threw it at a wall and the nose cone was pointy enough to stick there. I guess I'm ranting to see if anyone can offer any advice on what specs I should use when designing a rocket that will be powered by an E size ESTES engine? Specs should include fin size, weight, length, and diameters.

Yours truly,
Tom Haggen




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[*] posted on 5-8-2004 at 22:20


I'm into rockets too(though havent been very active for some time).I cant give you much specifics about E size engines since Iike to build my own things.

Anyways what is the shape of the rocket and the wings? IMHO your problem is in the nozzle since(from what I'm getting) the rest of your rocket is still intact.Larger wings should remedy the situation though.And perhaps a smaller nose cone(less surface = less chance of problems :) )
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tom haggen
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[*] posted on 6-8-2004 at 07:10


Well I was reading threw an ESTES pamphlet last night after I wrote this post, noticed a few interesting things. I started looking at model rocket designs. Rockets 15-20 inches that only weigh about 2 ounces generally had a-c size rocket engines. The model rockets that were rated to use E size engines were almost always over 4 ounces and over 30 inches in length. I think if I just scale up my next rocket, including the wings of course, I won't have any problems. Hopefully I will be able to get some 900-1000 ft altitudes. :D

[Edited on 6-8-2004 by tom haggen]




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Democritus of Abdera
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[*] posted on 6-8-2004 at 14:14
Stability.


Mr. Haggen

If I may, I'd like to suggest that you read a little more about rocketry before the next launch.

Alot of what a rocketeer needs to know is counter-intuitive.

There are some serious misconceptions about the science of rocketry. (eg. the importance and purpose of fins in a rocket v.s. their use on a negative accelleration object/projectile like an arrow, dart or spear.

Here is the primary rule of stability for classically shaped rockets, the CG (center of gravity) needs to be between 1 and 2 body diameters, HIGHER (toward the nose cone) than the CP, Center of Pressure.

It may seem tedious to learn, but once you have a working understanding of the concepts of CP and CG Bob is your uncle!

You can then feel free to make up your own rocket bodies by the dozens for pennies out of kraft paper or whatever,
You can also fly very peculiar shapes that don't even resemble cylinders and know that they will fly before you even make them, rocket enthusiasts frequently fly old childrens toys just for kicks.

Once you understand a little of the math, you can fly a toilet seat, or even the whole damn toilet, if you feel like it!

Also, you really don't have to be concerned about the relative size of rocket to motor, you could literally fly a rocket the size of a tube of toothpaste with an engine the size of a water heater if you understand flight characteristics.

Look for a website by Richard Nakka out of Winnipeg, Manitoba.

There are also dozens of books at your local library, read 'em through and within a couple weeks you'll be a dyed in the wool rocket scientist, because; in the end it all comes down to Thrust and Aperture!~:D

Good Luck .




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tom haggen
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[*] posted on 6-8-2004 at 15:57


Maybe you could recommend some links. You seem to know a lot about the subject.



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[*] posted on 6-8-2004 at 16:28
...


http://arocket.mid-south.net/library/faq.html

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

http://www.rocketry.org/faq/

need supplies and live in the states?

http://www.skylighter.com/

(there are many others; however so shop around)

Because I dream Liquid Oxygen dreams; This next particular link makes my heart quiver....
http://www.flometrics.com/rocketpump.htm

[Edited on 8-8-2004 by Democritus of Abdera]




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[*] posted on 6-8-2004 at 20:54


When you put in a heavier motor than the rocket was designed for, you moved the center of gravity too far aft, thus losing your stability margin. The quick and dirty solution would be to add a corresponding weight to the front end, adjusting to make sure the rocket balances at more or less the same point. There may still be problems with the airframe not standing up to the higher acceleration. Also note that a "personel assisted ballistic recovery" is that much more likely to get said personel hurt or killed with a wieghted nose.

Broadly speaking, hobby rocketry can be divided into two classes, consumer and "amateur", or experimental. Consumer rocketry tends to be governed by groups and companies who produce OTS equipment and safety guidlines. Traditional "model" rocketry is the model here, and is very forgiving due to the safety guidelines in place. Once you get beyond "D" class motors, or so, you move into "high power rocketry" (HPR). HPR is less forgiving and not suitable for urban settings, in general. Many consumer rocketeers have a lackadaisical approach to safety, and I fully expect a serious accident to occur at an HPR launch anytime.

Amateur rocketry is a different breed, mostly from a philosophical standpoint. Amateurs tend to be more interested in the engineering of rockets than in watching them fly. They usually focus on propulsion, and build thier own power plants. For many amatuer rocketeers, a launch is a rare event. Lacking the canned knowledge of commercial rocketry, safety in amatuer rocketry relies on the discretion and discipline of the rocketeer, and the field is brutally unforgiving. Impetuous persons need not apply.

Your current problems are probably best solved by consulting any one of the various consumer rocketry forums--rec.models.rockets is traditional, but there are many alternatives.

Democritus' links are good, but apply more to amateur activities. Required reading for is Nakka's site, his work and documentation is of top caliber.
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tom haggen
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[*] posted on 6-8-2004 at 23:15


Before I go reading into those links I would just like to ask a brief question. Should the center of gravity of my rocket be the halfway point between the end of my nose cone to the bottom of my fins?



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[*] posted on 8-8-2004 at 18:18
Mr Haggen..


Rocket kits are available for the amotivated consumer.

I provided the links believing you were a rocketry fledgling in need of direction, not a fetus in need of an umbilical cord.

my mistake......won't happen again.




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[*] posted on 17-9-2004 at 22:53


"before I go reading into those links I would just like to ask a brief question. Should the center of gravity of my rocket be the halfway point between the end of my nose cone to the bottom of my fins?"
It should be as far down as possalbe.High up it is when the rocket tips it will accelrate it more. The CoG will be in the middle if the rocket was flying flat, but for rockets the go straigth up low down is more stalbe.
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[*] posted on 22-9-2004 at 01:10


I'am in the stage of designing a liquard rocket, but i need to get some numbers and a fomula.
I have a 2l/min pump, and was planning on useing a 6:1 O2 ratio, but i need to work out the exhust velocity to test my design before building it.
Any help appracted, thanks

[Edited on 22-9-2004 by tokat]
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[*] posted on 22-9-2004 at 17:42


Tokat: I don't mean to insult you, but you really have a lot to learn before you build anything. In your previous post, for example, you claim that the CG should be as low as possible for vertically launched rockets. In truth, the CG should be as high as possible, regardless of orientation.

Regarding exhaust velocity, it's not an easy problem. You need a computer program (like proPEP) in order to do it well. These programs are freely available. Needless to say, characteristic exhaust velocity depends on both the exact propellant mixture burned and the chamber pressure. Actual velocity also factors in nozzle effeciency at the given atmospheric pressure.
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[*] posted on 22-9-2004 at 18:26


More accurately, the centre of pressure (CP) should be aft of the Cg. Cp is an aerodynamic function of body length, body diameter, fin area, and fin position.

This can be tested on the finished rocket by suspending it on a thread at the Cg, and holding it in a breeze. If the rocket swings to point into the wind, then the Cp is behind the Cg, and should result in stable flight. Just how stable is another question altogether.
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[*] posted on 22-9-2004 at 23:23


"Regarding exhaust velocity, it's not an easy problem. You need a computer program (like proPEP) in order to do it well. These programs are freely available. Needless to say, characteristic exhaust velocity depends on both the exact propellant mixture burned and the chamber pressure. Actual velocity also factors in nozzle effeciency at the given atmospheric pressure." i've got acoulpe of rocket sims, but looking for the isp i think, for 100-400 for chem rockets, i found some fomulas off nasa web site to work out how far a rocket will travel ,but with out know the speed like 7800ms for tnt i can't compelte these. From there the pressure in the chamber shouldn't(hopfully) be to hard. Its gas tubine/rotary engine, the fuel isn't going to be genarting the thrust directly, to hot fans to turn the comprssors, its preaty much a proller. The fiquares for rocketry about fuel rates/prssure and such will be compatlbe.

"More accurately, the centre of pressure (CP) should be aft of the Cg. Cp is an aerodynamic function of body length, body diameter, fin area, and fin position. "
In chem rockets burning it from a horoztal face then that will come into effect ^. As Cp should natrual be behind CoG.

"In truth, the CG should be as high as possible, regardless of orientation" i don't understand it, the idea that the thrust/throat(high prssure mixing with low is the point doing the pushing, if the CoG is at the top it will want to tip over like in fomula one cars they are as low as possalbe.
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tom haggen
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[*] posted on 3-4-2005 at 15:04


I'm thinking about making my own engines. Does anyone have any suggestions on how I can make my own clay nozzles to put in my engines? Also do you guys think that KMnO4 is a safe oxidizer to use?

[Edited on 3-4-2005 by tom haggen]




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[*] posted on 3-4-2005 at 16:36


Tom, I'd save the Permanganate for something else, as it's a little too unpredictable and chemically reactive. A rocket needs to be predictable, and I'm sure there has been more work done with more stable propellants. No doubt you could make something work but why reinvent the wheel? Check out Nakka's excellent web site and view some of his tests and videos, and links:
http://www.nakka-rocketry.net/




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