SpudFiles

What did you make your nozzle out of. If you didn't reinforce it with a bunch of really thick washers at least, it will be melted away in an instant. You need to at least cast an epoxy nozzle, if not a ceramic one, in that threaded end cap. Also, you will get a much higher performance, if you have a divergent section of your nozzle, as right now you are not capturing any any of the energy from the expanding of the gas.

as right now you are not capturing any any of the energy from the expanding of the gas.

I know and i will put steel washers.

When you launch rockets alone, you launch rockets with BIN LADEN

(Sorry, I had to! )

What? (message too short)

When you drive alone, you drive with Hitler.

You know, that terrorism comment isn't that far off. From what I've read, Hamas millitants use KNO3+Sugar in their Quassam rockets.

I could be wrong though.

Hamas millitants use KNO3+Sugar in their Quassam rockets.

Well , if that is right, they would suck!

millitants shooting candy's rockets!

well the you obviously haven't been reading anything posted here if you think that they would suck... There is a group of people working on getting a KNO3+Sugar rocket into space. That is something that The US wasn't even able to do until half way through the century, using up a good portion of our national budget...

Go to a HPR launch and then tell me that a J or higher engine for only $20 or so ($50 if you are going to do a really good job on it) sucks...

Now i have All to make a pvc rocket moter except the KNO3/saltpeter/potassium nitrate
(same thing)

I know then fertilisers brands put kno3 in their products and it will be for me the cheapest way to get some.
who know a brand name who sell fertilisers with KNO3 in it?

Hmmm. Have ever heard of a program called Rocksim , it can build rockets quite well its accurate but it is costly for the full version ($100), have ever thought of the CP - CG realations . But there is a 30 free trial demo.

This how the whole rocket stabillity thing works

now if got a simulator and calculated where the Center of pressure is and you mark where that is right and you load the motor in and then balance the rocket on you finger and mark where it balances , you then measure the distance between the two points then minus that from the diameter of the rocket then you will get you stabillity margin.

A good margin is between 1.0 and 1.5. Trust me on this I AM A ROCKET BUILDER ( not quite a scientist ).

in the simulator when you put all the measuerments in , you weight you KNO3 motor and then sim that mass by adding a mass object and placing that mass object at the bottom of the rocket ( hopefully thats where the motor is going to be) . If it isnt stable then adjust the fin design until it is stable. Then print out the fin template and cut it out on a bala sheet, and then glue it to the body tube ( wood glue would do best) , make sure that thing is alined properly mate.

that was barely intelligible...

and as for rocksim:

Lentamentalisk wrote:then you really need to start using RockSim, the rocket modeling and design program.

I already mentioned it.

As for the rest of your witchcraft mumbo jumbo on rocket stability, in theory that will work, but in reality it is not nearly that simple. What matters is the dynamic stability, not where the CP and CG are (though those are a good indicator.) You do need to account for the fact that KNO3/sugar rockets are core burners, so the CG is going to change in very odd ways.

qldrocketscientist

Couple things about your approach,
1. The rocket should be balanced without fuel (but with the motor tube if appropriate). The rocket is going to be spending 99.999% of its flight time with the engine empty of fuel. (This assumes the rocket's fuel mass isn't the majority of the weight of the launch ready rocket.)

2. A launch rod is generally used to help with stability during the boost phase when the fuel weight might mess up the aerodynamic stability. But, if you rocket's balance is so sensitive that the presense or absence of fuel makes a difference than the COM is probably too far back (or the COP is to far forward). Water rockets typically contain much more water (by weight) than the rest of the weight of the rocket. At launch, the COM is way too far back for stability. Water rockets still work since the water is ejected very early in the flight.)

3. The COP can sometimes be estimated pretty well by just cutting out a piece of cardboard shaped as the side-view projection of the rocket. The COM of the cutout is very close to the COP of the rocket. This works pretty well with simple cylindrical shapes and simple fin designs. ROcksim would be great to have but it really isn't needed.

4. Tie a piece of string around the rocket so the string is around the COM point. Spin the rocket on the string over your head. If the rocket is stable you've got things right. If the rocket tumbles you COM is too far back or COP is too far forward. Again, this assumes that the majority of the rockets weight isn't fuel.

How quick is the burn on the Kno3 motor Quote ( the rocket is spending 9999% percent of its flight with the engine without fuel) .

When im usally designing rockets , counting of the size of the motor, i alway pick the biggest motor and then make the rocket with the motor loaded in the rocket, also the SWING test you were talking about , every rocket hobbyist always loads his motor, parachute and wadding when doing the swing test to make sure that it is going to be a stable flight .

your cut out cardboard method isnt really accurate as you say a barrowman calculation is better , here click on this link .

http://my.execpc.com/~culp/rockets/Barrowman.html
the second link is a calculator
http://www.webcalc.net/calc/0225.php

also NASA rockets use these calculation and have for a very long time.

The swing test is also not quite accurate as well .
link .
http://www.apogeerockets.com/Education/ ... tter53.pdf

Lentamentalisk i do have to agree on the motor that it will change the stabillity of the rocket in flight .

do i have a case .

h

the trick is that the rocket must be stable at all points of the flight. For the most part, homemade rockets are just an engine casing with fins and a nose cone tacked on, so there is not really much to worry about when it comes to stability. Model rockets, on the other hand, where you have a tiny engine, and a huge rocket surrounding it, have much more of the weight centered at the back of the rocket.
That was all old rocket theory though.

Enter: Dynamic Stability and the 21st century home computing devices.
Dynamic Stability is a concept far greater than you can imagine. It involves all sorts of shit that even I don't understand. However, it provides a means of stabilizing rockets that look like they should be doing loops while on the launch rod, and lawn-darting through your dad's new car.
As far as I can tell (correct me if I am wrong,) there is no real way to predict it, you simply have to keep plugging in numbers to RockSim, until it works out.

I have a very old copy of rocksim, that I could, in theory get to you guys if you really don't want to pay the $100 for the much improved, shinny new colorful versions that they have now. (mine is at least 6 years old, so just think about how computers have changed since then.)