However, there are plenty of cases where "canon" is broken. If the original design is long and skinny (e.g. Estes Trident), the goonified version will not be the "standard" length.
Question for Goony Builders
- Thread starter Bruiser
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However, there are plenty of cases where "canon" is broken. If the original design is long and skinny (e.g. Estes Trident), the goonified version will not be the "standard" length.
Certainly true, I just want to use the "correct" length as a starting point, and then adjust when necessary.
Man, this thread has gone astray! Wasn't it about whether or not Bruiser needs nose weight?
1) Goonies often do because they're short, but 2)...
1) Goonies often do because they're short, but 2)...
Boat tails are destabilizing, i.e. they push your Cp forward. If the Baby Bertha doesn't need nose weight and OR says your rocket does, that's probably why. Add the nose weight.
Bruiser
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I had no idea that a boat tail would push the cp forward. I was just thinking that they would help with stability because they make the rocket longer. Without getting to rocket science (is that a word?) why does it do that?
-Bob
-Bob
Nytrunner
Pop lugs, not drugs
In very unscientific terms, blunt backend create drag and vortices at the aft that "pull" on that area and contribute slightly to stability.
A tail cone smoothes the airflow out and reduces that base drag. You can see this effect illustrated in openrocket or rocksim
A tail cone smoothes the airflow out and reduces that base drag. You can see this effect illustrated in openrocket or rocksim
True, but not exactly on point. Rocksim actually ignores base drag unless you take steps to force it (adding a ghost part). OR might do better or might be the same, I don't know.
Transitions, however, move the Cp just because of how they interact with the air at a given angle of attack. "Transition" in this context means any change in the airframe diameter, including nose cones and tailcones.
If I remember right, the rules of thumb are that a feature with increasing diameter as you go down the airframe tends to attract the Cp toward itself, which means that nosecones are destabilizing while base flares are stabilizing; and reducing diameters tend to "repel" the Cp, so a tailcone pushes the Cp up. Transitions in between can wind up having either effect depending on where they are relative to where the Cp would otherwise be, so you've got to compute the effect using either A) the equations and a pencil, 2) a cardboard cutout, or III) software like OR or RS.
That is quite interesting. There is some OR experimentation in my (near) future.
I just came rushing back hoping I was in time to edit my last post before it was replied to. I need to clarify something that was not stated quite clearly. When I wrote "Rocksim actually ignores base drag unless..." I should have written that it ignores base drag with respect to stability. RS certainly computes base drag and its effect on performance.
There's a Peak of Flight article from some years back about forcing the base drag effect on stability by adding a ghost flare, i.e. transition with 0.01 mm forward diameter because you can't enter zero, aft diameter the same as the body tube's, and length (he dredges through his memory looking for the figure) equal to the body tube circumference, i.e. pi time the diameter. Or is it 2π times? Use a mass override to 0.01 grams because you can't enter zero. The article includes a derivation of the length to get the correct effect, which I don't remember at all.
Neil, as for your experimentation in particular, you might want to play around with designs that use nothing but a base flare for stabilization. No fins, no rings, just an upside-down funnel. Let your design imagination run off from there.
There's a Peak of Flight article from some years back about forcing the base drag effect on stability by adding a ghost flare, i.e. transition with 0.01 mm forward diameter because you can't enter zero, aft diameter the same as the body tube's, and length (he dredges through his memory looking for the figure) equal to the body tube circumference, i.e. pi time the diameter. Or is it 2π times? Use a mass override to 0.01 grams because you can't enter zero. The article includes a derivation of the length to get the correct effect, which I don't remember at all.
Neil, as for your experimentation in particular, you might want to play around with designs that use nothing but a base flare for stabilization. No fins, no rings, just an upside-down funnel. Let your design imagination run off from there.
pi * diameter is correct length. In OR you *can* enter zero mass. Note that you don't want to do your flight sims with the base drag corrector in place, because it'll add a lot of drag that doesn't actually exist.
Well, I'm initially most interested in how transitions and tail cones affect a normal finned rocket.
Nytrunner
Pop lugs, not drugs
Exactly. There is an affect on stability related to my post above.
OR and RS calculate based on the geometric components of the rocket, but the weirdness going on behind that geometry is unaccounted for, thus requiring the phantom cone.
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