Calculated stability of a rocket at take-off

[stix]

This is probably more related to amateur rocketry than to pyro rockets but nevertheless useful for both - I think.

 

I did read some 30 or so years ago (gulp!), and in the back of my mind this has always been an important view to consider. There is a general assumption that can be made regarding the stability of a rocket motor at take-off, as in the relationship to the force (thrust) and g-force over time.

 

It was this:

Over the first .5 of a second, if the g-force of the rocket hasn't reached 5 g's then it's considered unstable.

 

G-force is calculated by dividing the current force by the current weight, minus 1.

 

Has anyone read about this?. Any other constructive or alternate views are welcome.

 

Cheers.

 

 

[calebkessinger]

Man I wish I could understand anything you just said.

Too much for my feeble mind.

 

but I like ROckets!

[Sulphurstan]

Interesting. What is the "current force"? I m always concerned, once it is about rocket stability.. I would like to investigate this a little more...
I've got an excel chart, pretty much detailed, for height calculation, where g (9,8 M/S) is involved, I may send it to you, if you want, that may help..

Edited December 20, 2017 by Sulphurstan

[stix]

Interesting. What is the "current force"? I m always concerned, once it is about rocket stability.. I would like to investigate this a little more...

I've got an excel chart, pretty much detailed, for height calculation, where g (9,8 M/S) is involved, I may send it to you, if you want, that may help..

 

The "current force" is the force at the time the measurement is taken.

 

For example:

Lets say that at 0.5 seconds into the burn, you measure a force of 2 kilos of thrust. Your total weight (at launch) is 350 grams, ie. including your pyrotechnic header or camera & recovery system depending on what the rocket is used for, so:

 

2000 / 350 = 5.7

We then subtract 1, and get a final result of 4.7

 

The reason we subtract 1 is because that means that the rocket has now alleviated it's own weight, and the rest (4.7) is positive acceleration (assuming the rocket is going straight up). Obviously to get this sort of info you need to first test on a test stand.

 

Here's a worked example from an actual motor:

 

 

At the top r/h corner of the graph you'll see the cursor info panel. This shows the sample number/time/force/g's at the exact position of the cursor (the black vertical broken line).

 

In the blue "specifications" panel at left you can change the "Launch Weight". This will give immediate feedback showing how many g's and therefore it can easily be determined if there will be enough acceleration at take-off for stability. This also "assumes" either a long stick or launch rail (around 1mtr long).

 

Yes, there are a few assumptions, and this is only meant as a reasonable guide, but useful I think. The g's value can be converted to velocity, but without complicating anything it's easy to remember: 5+ g's at 0.5 seconds.

 

I can't find the old notes that I read this from and was wondering if anyone else had heard of this as a general rule.?

 

@Sulphurstan

 

I'm currently working on altitude calculations for the software using formula's from a model rocketry book, but happy for you to post the table as it's always good to check results from other sources.

 

Cheers.

Edited December 23, 2017 by stix