13 replies to this topic

[chris.h]

Hi all

I am brand new to this forum as you can proberly tell, so bear with me. My interest at the moment is BP rockets, small and large. The amount of information available through these pages is excellent if you are prepared to look.

One piece of information I don't seem to be able to find, even though I have read virtually all threads, is the LENGTH rockets should be.

As an example, is there an optimum length of the tube for a 2 and 4 oz rocket? How long should the core be if a core burner? How long should the core be?

I have made a few with different tube lengths with various degrees of sucsess, but it would be good to standardise things.

Any help would be apreciated

[Pretty green flames]

Some general specs

Casing lenght: 10 Tubes ID's, for example 4oz. has and ID of 1/2" so the casing lenght is 5"
Core lenght: 7 ID's

That's basicly it.

[chris.h]

Some general specs

Casing lenght: 10 Tubes ID's, for example 4oz. has and ID of 1/2" so the casing lenght is 5"
Core lenght: 7 ID's

That's basicly it.

Cheers,

That is exactly what I needed. Great site by the way.

[GBthriller]

Some general specs

Casing lenght: 10 Tubes ID's, for example 4oz. has and ID of 1/2" so the casing lenght is 5"
Core lenght: 7 ID's

That's basicly it.

I'd like to add; "Amateur Rocketry" book says the core distance to the delay is equal to the distance to the inside of the tube wall. I suppose that means the 'burn-out' is all at once and doesnt end-burn for a period. Although, maybe you want it to end-burn for a while... As always, there is only ONE answer.

Edited by GBthriller, 02 December 2005 - 03:32 AM.

[Maineiac]

Is this saying that there is a solid charge of BP propellant above the spindle equal to the ID of the rocket? If so then that sounds about right in my experience. This is diminishing as soon as the rocket is ignited though and can be shortened a bit by adding the glitter delay discussed below.
There will be some "end burning" but it gives next to no thrust to the rocket especially one carrying a substantial payload. The actual core of the motor is gone in probably 1 to 1.5 seconds in a 1# rocket. After that it is merely delay til the passfire is hit. This is why you can switch compositions above the core to say, a glitter formula. My favorite for this is Winokur 39 as it locks in nicely. D1 will work decently as well. This adds a very nice tailed effect which varies from the charcoal tail given off during ascent. This delay comp is only added after the solid charge of rocket propellant is pressed above the top of the core. It pays to mark your drifts once you figure out where you want these changes to take place. Drill through the clay top plug and into the delay to set your timing.

[chris.h]

As I understand it, the cored propellant would give you the main lift throughout the rockets accent, and then the solid portion would end burn, allowing the rocket to "cruise" before the payload is ignited. Does that sound correct?

[Pretty green flames]

Yes that's right. Core burners develop a lot of lift so they continue to fly upward long after the cored part has been burnt. You will also have to decide on what type of payload to use. I use only 1 id of solid propelant above the spindle as my payload has a 3sec. delay of it's own. 1ID of solid propelant above the sindle and 1ID of clay, then the passfire hole is drilled and the rist is filled with granulated BP to ensure payload delay ignition.

[broadsword]

I just use a rocket stick that is 6 times longer that the motor. If i have a large header on the rocket then i will make it a little longer to compensate!

[Jerronimo]

A trick that has always worked for me is to balance the rocket just under the nozzle on your finger.

[Maineiac]

Stick length, balancing...the "black arts" of rockets. Here's what works for me on various motors:

2oz Stick-3/16" Round dowel, 18" long (bp)

4oz Stick-1/4" Square, 24" long (whistles)

8oz Stick-3/8" Square, 36" Long (bp)

1# Stick is 1/2" Square, 36"-40" Long (bp)

All sticks are clear pine which yields a nice balance of strength/weight and is either free or cheap. Square sticks have proven better for guidance. The round sticks on the 2oz seem to not matter. After firing many dozens of these I've "not yet" had one go errant. The fact is that they are so fast they tend to go straight up with not much arcing involved. They are also never tasked so much as to slow them down a whole lot.

I only bother to balance the 2oz and 4 oz motors. The larger motors there is no need and it would be detrimental to do so. Consider a 36" .5" square stick attached to a 1# motor. There is typically 4" of stick contacting the case thus leaving 32" in free air. Try to balance a rocket with a header that weighs only 300 grams. You would need to; A) add a ton of weight to the end of the stick B) use a much heavier (thicker) stick or C) a much longer stick. Why take away from the rockets capability? I used to try to balance all my rockets until I was told not to bother by someone who knows quite well. Too, eventually it was impossible and thus it was abandoned on the bigger sizes. The small rockets that may carry 15-50 grams? Sure, balance them but only within reason. If your motor is worthy then there is little need. You will just be wasting lift capacity on a stick with more potential energy on its way back down.

Edited by Maineiac, 05 December 2005 - 02:05 AM.

[Jerronimo]

That's an informative post Maineiac, will keep it in mind when I start production for newyears eve.

[fishy1]

what's the choke diametre again?

i think it was 1/2 the ID, but i can't remember.


thanks, i'm making up some new tooling at the weekend, and i want it to be decent.

[Maineiac]

Your recollection is correct. 1/2 the tube ID is the traditional nozzle size.

[fishy1]

Your recollection is correct. 1/2 the tube ID is the traditional nozzle size.

ok thanks, just wanted to make sure.