33 replies to this topic

[maxman]

Has anyone any info on rocket stick lengths? I was looking on google but keep finding contradicting info.
If I make a rocket more top heavy by adding a payload,do I need a longer stick? or more ballast?

There is so much about center of gravity,preasure and drag it seems confusing.
Should the stick be square section or round? Is it not to important? or is the stick just there to keep it upright till the tail end leaves the launch tube and the rocket gets up thrust?

I wouldnt fancy one nose diving where it shouldn't with a big payload on.

Hope some rocket guru can help!

[Stuart]

I once had a problem like that. The puropse of the stick is to balance the rocket in flight. If it tips to the side a little, then the weight and length of the stick acts as a lever and pulls the rocket straight again. If you ahve a payload, then you do need a longer or thicker stick. I dont think it is so much as length that makes the diffrence but in actual fact it is weight.

Most people say (I think) that your stick wants to be about 6 times the length of your rocket. Once again weight is the real factor here. If your stick was a couple of mil in dimeter, then 6 times the length should about right.

I am not sure when pressure or drag would come into it but you dont need to worry about it. It doesnt matter on the cross sectional shape of your stick

Stuart

[Phoenix]

I'm no rocket guru, I usually use shells (not that I'm a shell guru either), but I hope this helps. A common rule of thumb is that the finished rocket should balance at the nozzle. Hold your finger out, and, holding the bottom end of the stick, rest the rocket on it where the clay nozzle is. If the heading end tips down, and the stick goes up, the stick needs to either be longer or heavier. The rocket should balance, or the stick should _just_ tip downwards. This is because when the rocket is in flight, it is "balanced" on the jet of gas issuing from the nozzle. If the centre of gravity is too far above this point, it can tip over. I have bought rockets, which did not balance, and at best the wobbled in flight, and some even turned over and broke within a few metres of the ground.

However, there is also another factor to consider. This is that the stick acts like the tail of an aircraft. As well as levering the rocket straight, like Stuart said, if it moves to one side, the considerable airflow will be hitting it on one side, rather than moving straight past it. This will push it back behind the rocket, keeping it straight. Therefore so long as the rocket is not drastically unbalanced, you can get away a slightly top heavy rocket. However I personally prefer to make sure the rocket does balance, so that it does tip over on take off, before it has fully accelerated.

To balance the rocket you will need a certain amount of turning force at the pivot point (the nozzle). If you have a short stick, it will need to be heavier to achieve the necessary turning force. If it is longer, it will have more leverage, so you may be able to save some weight by using a thinner, lighter stick. The exra length will also increase the aerodynamic stabilisation.

For stick materials you could buy round or square dowel, but a cheaper option for small to mid sized rockets is probably the split bamboo canes sold at garden centres. These no doubt vary in size, but are generally suitably light and strong. For very small bottle rockets (like extinct whistle rockets) bamboo kebab skewers of 3mm diameter and 30cm long work nicely. Seek and ye shall find.

[Rip Rap]

"The Pyrotechnists Treasury" by Thomas Kentish is an fantastic resource & has an excellent chapter on rockets.
It tells how to calculate length of spindle, case, stick & case diameter etc. I will quote from his book:
"Taking the bore of the case as unity, the proportions will be as follow:-
1 = Internal diameter of case
1? = External Diameter
8 = Length of case
6 = Length of spindle
2/5 = Bottom diameter of spindle
64 = Length of stick
? by ? = Thickness & breadth of stick
These proportions are most readily calculated by taking the diameter in eighths of an inch (ie the stick is 8 times the length of the case).
I use the above when I make rockets & I'll tell you - Tom Kentish was spot on!

[alany]

This is a highly debated topic!

Older texts talk about balancing the rocket stick on your finger behind the nozzle, but they also say it is the unbalanced pressure inside the motor caused by the nozzle opening that makes them fly. In short, Kentish et al have the physics largely wrong.

Rocket sticks work by supplying drag, their geometry isn't all that important, as long as they supply enough of it to shift the center of pressure behind the center of mass. However most of the model rocketry sites just parrot the "1 calibre stability" as a law without any additional explination, which is quite dangerous.

A full treatment of the aerodynamics is lengthy, probably beyond the scope of this forum and I am unqualified to offer it, but I can post an email I wrote a while back which covers is more fully (actually I should probably write it up on my website).

For the stick to work the rocket has to be moving fast enough, there is a critical speed where a rocket becomes stable. That speed also depends on the angle of attack, if there is a crosswind 1 calibre stability may be insufficent, so critical speed is a function of angle of attack. However too much drag will cause excessive weather cocking. This is what most rocket builders have never been told.

A fireworks rocket isn't guided, once it starts to interact with the wind it will fly into it, there is nothing that can be doing about that. Gravity also effects its flight, and the kinematics can be quite complex to model. A stick stabilized rocket will also curve slightly towards the stick because the drag force is not applied colinear with the thrust and center of mass.

A stick about 4 or 5 motor lengths is usually sufficient, which is significantly shorter than those given by Kentish. It doesn't need to be thicker than 1/8 of the motor diameter and the cross section is not important. It should be as straight as possible, and any curve should be put under the rocket or it might start the rocket tumbling or veering badly.

[Stuart]

Well, yesterday I found hundreds if not thousands of sticks for the job. Will take a picture on the weekend

Stuart

[Rip Rap]

Older texts talk about balancing the rocket stick on your finger behind the nozzle, but they also say it is the unbalanced pressure inside the motor caused by the nozzle opening that makes them fly. In short, Kentish et al have the physics largely wrong.

Hi Alan,
Older texts may not have the physics correct, when explaining WHY they fly, but I found Kentish was spot on in explaining HOW to get them to fly.
I have used his tables when making tooling & then construction of various sized rockets. Any failures I have had (very low percentage) have been attributed to poor storage &/or handling - ie damaged powder grain.

A stick about 4 or 5 motor lengths is usually sufficient, which is significantly shorter than those given by Kentish. It doesn't need to be thicker than 1/8 of the motor diameter and the cross section is not important.

Surely this depends on the density of the wood used. I always use common pine. Using Kentish's 64 = Length of stick & ? by ? = Thickness & breadth of stick, I have no problems.

All I know is that when I first started making rockets, I was largely using guess work, with predictable results. As soon as I started using Kentish's formula, everything started to work. No CATO's, good performance, straight flight, carry a decent payload etc.
I suppose I may be a bit biased towards him, but only because of the good results!

[maxman]

Are you saying 8x the motor length or 8x the whole device inc payload regardless of weight?
I have had problems in the past with "weather cocking" but I think that that is due to my green flower sticks not being straight when I get upto 24 to 30 inches long.

[Rhodri]

Just to add to the conversation.....

1. I find that kebab sticks (from Robert Dyas) work well for small (6 cm length, 1cm id) rockets with small payloads. These seem steady in flight and by a fluke seem to be of the correct length!

2. Used, washed and inverted "Yakult" bottles (those healthy yoghurt drinks) work REALLY well to mount on top after being filled with stars for the payload.

It's worth getting some of these little bottles and trying for yourself - you'd be surprised how well they fit, look and drinking the contents before loading is good for you!

What else could we ask for?

[alany]

Hi Alan,
Older texts may not have the physics correct, when explaining WHY they fly, but I found Kentish was spot on in explaining HOW to get them to fly.
I have used his tables when making tooling & then construction of various sized rockets. Any failures I have had (very low percentage) have been attributed to poor storage &/or handling - ie damaged powder grain.



Surely this depends on the density of the wood used. I always use common pine. Using Kentish's 64 = Length of stick & ? by ? = Thickness & breadth of stick, I have no problems.

All I know is that when I first started making rockets, I was largely using guess work, with predictable results. As soon as I started using Kentish's formula, everything started to work. No CATO's, good performance, straight flight, carry a decent payload etc.
I suppose I may be a bit biased towards him, but only because of the good results!

I am not bagging out Kentish for incorrect motor geometries, just questioning his stick lengths and his understanding of how rockets actually fly, his motors definately work in practice.

The standard naming convention (oz and pounds and stuff, which is stupid IMO) comes from before his time and relates to firearms projectiles, as he explains. The 3rds and 7/10s rules of thumb that I usually use come from the Chinese AFAIK? He seems to use 8*ID rather than 10*ID for motor length which is a little unconventional, however his 6*ID core (about 75% grain length ignoring the nozzle) is just slightly longer in the grain than the conventional 7*ID (around 70% grain length) when used with an 8*ID case.

My payload lifting motor, for comparison, is a little more than 5*ID in length with a 3*ID core (around 60% grain length). My propellant is more energetic than BP greenmix and actually uses spherical Aluminium as a phlegmatizer to allow the core-burning configuration.

His 60*ID stick formula is excessive IMO. While it makes an essentially unconditionally stable rocket that will tend to fly straight up it also reduces the performance by adding unnecessary weight and drag. That's my experence anyway. I use bamboo kebab sticks for most of my rockets, about 4-5 motor lengths not including the overlap with the motor.

Fins work better than sticks for rockets anyway. Fins give larger lifts at smaller angles of attack and offer less drag. Although making fins is a bugger, and attaching them is worse! I really like stingers myself, no stick or fins to worry about, but they suffer from different stability problems and can't really carry a large payload.

[Rip Rap]

maxman,
Sorry, I should have made it clear that 8 x the length of the rocket case caters for a rocket with a "standard" payload. "Standard" meaning a payload weighing up to three quarters of the total weight of the rocket, not including the stick.
For example - a 10mm id rocket = 80mm case, total weight (tube & powder grain)15g.
Stick length = 640mm with a total weight payload of up to 12g (approx.)
Of course, this is not correct to the exact mm.

Although making fins is a bugger, and attaching them is worse! I really like stingers myself, no stick or fins to worry about, but they suffer from different stability problems and can't really carry a large payload

- I totally agree Alan.

Rhodri - I like the Yakult idea!

[Jerronimo]

In my opinion it's much easier to stick with the balancing method, and use your common sence regarding stickdiameters, when you have a rocket that weighs for example 200 gram you are not going to use 5mm diameter sticks because they will be to long and prone to break.

I made some 30 rockets with hefty payloads, using the balancingmethod and have yet to launch one that's unstable in flight.

[Stuart]

I have pictures of the sticks I got hold of a few weeks ago. See the pictures section on my site to see them.


Stuart

[Pyromaster2003]

Hey Stuart, whats the diametre of them stick?

[Stuart]

Umm, duno, 2mm I think, I should be able to check tomorow

Stuart