Motor/Fin Relationship.

How far ahead of the motor/s can the fins of a rocket be and still be stable?
Example: Could a five foot tall rocket with the fins 6” from the rear be stable?
Assuming proper CP/CG is maintained.

You can take it for granted that the fins would be many and large; so as to move the CP rearward.
Or how about tube fins?

I am sure ive seem them like that, yet i never built one.

You could use software to model this and fly it virtually.

Try OpenRocket https://openrocket.sourceforge.net/

Your example would likely be stable, but you hit on the key of proper CP/CG relationship. As said above modeling it in Openrocket or RockSim will tell you for sure. You can crunch the Barrowman equations by hand as well. https://www.apogeerockets.com/downloads/PDFs/barrowman_report.pdf CG is also not hard to calc if you have the components on hand. Sum of the moments (the distance of the CG of the part where it is assembled in the rocket from a reference location (usually off of the rocket)* the mass) divided by the sum of the masses

How about a “Mid-Fin” arrangement?

Five foot long rocket with a set of big tube fins halfway up?

that's likely getting past workable with out lots of nose weight. The fins restoring force is based on the moment arm from the CG to the fins center of pressure.

So the only way a “Mid-Fin” might work would be if the motor/s were forward of the fins.

Think Flis’ canted motor mount at forward end of rocket with a set of big tube fins midway down and then a long trailing tube to act as a balance weight for the forward section.

Location and size of fins determine the CP. With the fins that far forward, the CP is naturally farther forward than on a standard rocket. So as everyone has eluded, the CG would need to be extremely forward. Canted motors in the NC? If you can engineer it, and get your CG a caliber forward of your CP, it will be stable.

But keep this in mind: it is very tricky to get all the motors to light at the same time, and if one fails, watch out!

mjennings said:

Your example would likely be stable, but you hit on the key of proper CP/CG relationship. As said above modeling it in Openrocket or RockSim will tell you for sure. You can crunch the Barrowman equations by hand as well. https://www.apogeerockets.com/downloads/PDFs/barrowman_report.pdf CG is also not hard to calc if you have the components on hand. Sum of the moments (the distance of the CG of the part where it is assembled in the rocket from a reference location (usually off of the rocket)* the mass) divided by the sum of the masses

Click to expand...

Ditto what mjennings has posted.
Sounds like you need a little further info. Think of the fins on any Free flight rocket the same as the Feathers on an Arrow or the flats on the rear of a weather vane. our fins do exactly at same thing by "damping out" by swinging the model back and forth around the CG until the flight path is swang back to the desired direction caused by pushing disturbance to the flightpath by Side wind gusts or other "Upsetting" pressures. The Closer the fins are to the CG the larger they need to be to correct upsetting disturbances. Ideally the further aft the fins can be placed the smaller they can be to do the job needed.

To sum up placing fins far away for the motor mount is generally not the best idea UNLESS they are still within the proper minimum 1caliber (one main body diameter) CP CG relationship.

Also note: Any fins forward of the CG have to be off-set by MUCH larger rear fins to cancel out the negative effect of such forward fins. Or make the forward fins free swings (Weathervaneing) so they have NO steering effect on the rocket.

Here is a very short version of what is happening. A stiff body will want to rotate about the Center of Gravity (CG). If you take a dowel and throw it up, it spins in the middle. Add weight to one end and find the CG then throw it and it spins about that point.

The other force is Center of Pressure (CP). Think of this like a wind vain, there is a point which if suspended the wind would no longer turn the rocket.

Now, the CG needs to be ahead of the CP so that the air movement (wind) of the rocket is strong enough to overcome the tumble about the CG. This has been determined to be about 1.5 times the diameter of the rocket. But also, if your rocket is longer and skinny, then there is additional complication as the body starts acting like a wing. For that look up superoc.