Another Barrowman question

[graylensman]

If the fins are forward of the BT end by x, do you calculate that aft area as a "zero change" conical transition?

 

[powderburner]

In one of the versions I have seen, you account for the effects of the NC, and the fins, and any 'expansion' or 'reduction' transition.
But the BT gets left out? Has no effect? (Leaves *me* a bit puzzled)

 

[Stymye]

if you look straight down the nose of a rocket do you see the body tube? no

only at an angle of attack, but the barrowman equasions do not take angle of attack into consideration.

 

[rstaff3]

You don't account for body tubes in Barrowman. Extra tube behind the fins makes no difference to the CP referenced to the tip of the nose. It may of course make a difference to the CP/CG relationship and stability.

 

[graylensman]

powderburner - you bring up an interesting point of logic regarding body tube effects v. transition effects. Is it assumed that pressure is constant on a non-transition body tube based on nose/diameter calculations? And that a conical transition changes pressure, and thus is calculated?

And does that add additional complexity that the general rocketeer (read:me) would shy away from?

 

[rstaff3]

The reasoning is what stymye said. Barrowman is an approximation which makes estimating stability possible without a computer, lots of math, a windtunnel, etc. Dynamic stability is much more complex.

 

[powderburner]

The Barrowman approach has always confused me because it claims to worry only about small angles of attack and then assesses the fin effects as though they are 90 degrees to the airflow (flying sideways is NOT a small angle of attack?)

I have also always wondered at the way fin effectiveness is analyzed. Barrowman gives absolutely no credit for fins with a good airfoil section, and treats all fins as though they are flat plates with sharp edges. Theoretically, well-airfoiled fins would be *much* more effective than flat plates.

In the case of a truly fin-less rocket (that is, one without a flared aft end) the Barrowman approach gives the result that the rocket is unstable . . period. But we know that if enough ballast is added to move the c.g. forward into the front half of the rocket, the body tube itself will stabilize the rocket (somewhat), but under the Barrowman rules the BT gets no credit at all for contributing to stability.

I know that Barrowman was trying to simplify the stability analysis problem down to a workable level. He did make a valuable contribution to our hobby by developing a tool that allows us to make a rough estimate of the stability of our rockets before we risk life and limb. But the Barrowman approach only gives an estimate, and I doubt it was ever intended to address every peculiar design that we could think up.

 

[rstaff3]

Originally posted by powderburner
But the Barrowman approach only gives an estimate, and I doubt it was ever intended to address every peculiar design that we could think up.

Yes, it is a very useful tool and is fine for the 'average' rocket.

 

[Stymye]

I have also always wondered at the way fin effectiveness is analyzed. Barrowman gives absolutely no credit for fins with a good airfoil section, and treats all fins as though they are flat plates with sharp edges. Theoretically, well-airfoiled fins would be *much* more effective than flat plates.

thats right ,a big difference.. a fin that has a blunt leading and trailing edge will create base drag behind the fin.. much like a straight body tube will...well over 200% increase over a fin with a rounded leading edge tapering to a knife edge.

the same principal for the body tube , a tapered end "Boat tail"
same principal. if you have a widening transitition somewhere along the body tube, or a blunt cone it will become a much bigger component " as pressure drag" than the base drag. several times as much in many cases .... so calculating the effects of a transition far out weigh the effects of airfoiling in the static calculations(barrowman)

thats my theory atleast.

 

[teflonrocketry1]

Originally posted by graylensman
If the fins are forward of the BT end by x, do you calculate that aft area as a "zero change" conical transition?

If you are strictly following the Barrowman equations the answer is no; the body tube (and for that matter any part of it) is not taken into account by this method. Barrowman used a few simplifications to make a tenable thesis out of his project. One of the main assumptions is that the pressure along the body tube remains more or less constant at small angles of attack; so the body tubes contribution to the CP can be left out of the static stability estimation.

These equations will not work for all cases, such as with a long skinny rocket where body lift (boyancy) is a major factor or with short stubby rockets (CD spools) where Base drag (caused by the base vortex) is a major issue. Both of these factors are caused mainly by the body tube itself which has been ignored in the Barrowman equations. You might want to read the excellent report on these issues written by Robert Galejs at: https://www.cmass.org/member/Robert.Galejs/sentinel39-galejs.pdf

Bruce S. Levison, NAR #69055

 

[cls]

Bruce, thanks for the reference to the paper. it seems to explain something that has bothered me for a while: why do stubby rockets fly OK? I'm thinking Big Daddy (OK by Barrowman), Thumper Jr, Pterodactyl, Thumper, etc.

my daughter wants to build a 4" upscale Big Daddy but we haven't pursued it because barrowman (rocksim) says it needs a lot of noseweight.

 

[graylensman]

Thanks everyone for the input. It just shows how much is open for debate and discussion in our hobby.

My interest is being driven by a growing obsession with optimizing an egglofter design. Not so much for the competition, but for the desire to make the best rocket I can.

You know what's scary? I'm actually getting interested in working this math!