Take any 3fnc rocket that is marginally stable and has both nose weight and fins... remove either completely (or mostly) and it's suddenly not stable.
Another Crack at "Finless" Bullet Bill
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So you are saying -- if its not broke don't fix it...
If someone wants to use this as a test bed for evaluating GDS I am happy to print a set of parts and send them to you.
I cannot afford that many E12 engines
I cannot afford that many E12 engines
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Just needs more power ...
Well, more that I feel it's currently a few things going on at once vs. just gds, etc. so taping off the lower intakes or removing all the weight isn't really going to prove that either is the single stabilizing factor unless either was applied in a neglible amountSo you are saying -- if its not broke don't fix it...
You can have a finless rocket with just gds and you can also have a finless rocket with just a lot of nose weight, but either wouldn't be as efficient as one with both, which I feel you have here.
However, make no mistake... fins are the most efficient and effective at stabilizing for our rockets, so trying to stabilize otherwise is very inefficient.
BDB
Absent Minded Professor
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I’d be happy to take a crack at it. You could just send me the stl files.If someone wants to use this as a test bed for evaluating GDS I am happy to print a set of parts and send them to you.
I cannot afford that many E12 engines
I'd be happy to try it out as well. Closest container I have is for oatmeal; I'd have to get the measurements.
I have plenty of E12s to sacrifice; at some point the rocket would be the one being sacrificed to a CATO, though!
I have plenty of E12s to sacrifice; at some point the rocket would be the one being sacrificed to a CATO, though!
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As always, I am late to the party. One question, does GDS continue to work once the motor is no longer under thrust?
Would be a short flight (kind of like a saucer), but how does it fly with a zero delay motor?
Would be a short flight (kind of like a saucer), but how does it fly with a zero delay motor?
No, apparently GDS only works as the motor is burning and thus pulling air in for combustion. However, I have always done a combination of nose weight and GDS so rockets don't just suddenly go crazy after the engine thrust stops.
I posted a different rocket build here that shows the rocket backslide after reaching apogee (I think it is easier to see in the 2nd view I shared).
https://www.rocketryforum.com/threa...eamlined-finless-rockets.169672/#post-2211145
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INTERESTING! Makes sense, if the rocket is unstable going up (once thrust ceases) it will continue unstable (rather than ballistic) coming down.
hmmm, so if you launched with a zero delay motor, the motor ejects at burnout. Mass-wise, it may not become stable although it will be MORE stable than it WOULD have been with the motor in place during coast phase (Deducting tail weight is kind of like adding nose weight RE stability.). Not sure the loss of mass is a plus or a minus, probably a wash since it is unstable anyway.
so worst possible case is immediately goes unstable, but with no thrust, and inability to go ballistic, you basically have a hollow tube that will either tumble or backslide, so should be a safe recovery. Might make a nice small field rocket.
Rocket engines to not pull in air for combustion, as you very well know. The exhaust plume pulls in air by the venturi effect or something akin to it.
lakeroadster
When in doubt... build hell-for-stout!
Yea, in this case, GDS sucks, literally and figuratively.
Pem Tech
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Did you mean "cowbell"?
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I can't get enough ...
lakeroadster
When in doubt... build hell-for-stout!
I finally got out the spray paint... What do you all think?
If power and nose weight solves 90% of my problems, then GDS and base drag could take out at least 5% of the remainder. AND THAT AIN'T BAD!
I had to start with monochrome... Baby steps...I think you painted something. I'm not sure I've ever seen that before.
Had another very good flight today of another build with the eyes and 3D printed arms. I spaced this model in among the Viking launches from my younger kid's Scout Den. It was a very popular model with the kids! This is almost ready to go as a kit.
Launched again at METRA last weekend - stable again but did arch over a bit since we started it on an angle (I think the E12 might be a little underpowered since the model has so much nose weight). Still deployed recovery fine and overall good flight.
I have been wondering how much of the stability here is due to GDS (and how much is due to all that nose weight or other factors like base drag). So as one experiment I decided to tape the intake slot closed around the model with a piece of painters tape. Long story short, the model was no longer stable with the intake sealed off.
Here is the video:
So intake slot makes it more stable. Why? I think the most likely explanation is that GDS is pulling in air from the sides and pushing it through the rear of the rocket (which basically becomes like a ring tail).
GDS could theoretically work even with sides closed since engine in recessed (in this case the argument is that the engine would pull air in the rear around the sides and then push it out the back in the center). However, this seems less efficient even if the intake is further back.
Also apparently base drag is not as impactful as GDS in stabilizing a short stubby rocket (when I simulated this in Open Rocket a base drag cone went a long way towards making this sim stable). I do not trust the OR simulation below since it violates too many assumptions. However, TIFWIW...
Lastly, I want to point out that the engine is recessed less than 1 caliber (even with the side slot closed) so Krushnic effect should not be a major factor (also the rocket took off at a very good speed similar to other launches).
My next question is -- will the rocket be more or less stable with a larger engine? I made the latest version of this rocket with 29mm or 24mm motor mount options. Changing the engine does not move CG much since the engine is recessed significantly (assuming the larger engine is not too long to recess fully inside the NC). I think the side intake slot is quite large (5/8" x 15.7" circumference = 9.8 square inches) and can accommodate a great deal of airflow (rear opening is 78.5 square inches). Given this it may be [/ hopefully is] even more stable with a larger engine but quite a bit of guesswork.
I have been wondering how much of the stability here is due to GDS (and how much is due to all that nose weight or other factors like base drag). So as one experiment I decided to tape the intake slot closed around the model with a piece of painters tape. Long story short, the model was no longer stable with the intake sealed off.
Here is the video:
So intake slot makes it more stable. Why? I think the most likely explanation is that GDS is pulling in air from the sides and pushing it through the rear of the rocket (which basically becomes like a ring tail).
GDS could theoretically work even with sides closed since engine in recessed (in this case the argument is that the engine would pull air in the rear around the sides and then push it out the back in the center). However, this seems less efficient even if the intake is further back.
Also apparently base drag is not as impactful as GDS in stabilizing a short stubby rocket (when I simulated this in Open Rocket a base drag cone went a long way towards making this sim stable). I do not trust the OR simulation below since it violates too many assumptions. However, TIFWIW...
Lastly, I want to point out that the engine is recessed less than 1 caliber (even with the side slot closed) so Krushnic effect should not be a major factor (also the rocket took off at a very good speed similar to other launches).
My next question is -- will the rocket be more or less stable with a larger engine? I made the latest version of this rocket with 29mm or 24mm motor mount options. Changing the engine does not move CG much since the engine is recessed significantly (assuming the larger engine is not too long to recess fully inside the NC). I think the side intake slot is quite large (5/8" x 15.7" circumference = 9.8 square inches) and can accommodate a great deal of airflow (rear opening is 78.5 square inches). Given this it may be [/ hopefully is] even more stable with a larger engine but quite a bit of guesswork.
Fantastic! Empirical evidence of GDS I would say.
lakeroadster
When in doubt... build hell-for-stout!
So I'm wondering if the arms sticking off the body is adding drag and helping with stability.. since they are behind the cg. Can you remove them and fly it again?
MidOH
Well-Known Member
Have you tried putting fins inside the tail ring for the motor thrust to blow across?
Thanks! There are actually 4 fins inside there now basically to bridge the gap between the two tubes.
I had more fins previously but was not sure if they mattered or helped.
I also considered doing angled fins to try to spin the model but was not sure that they mattered or helped and I got the model pretty stable w/o them so stopped experimenting too much.
MidOH
Well-Known Member
Spin wouldn't help. A real bullet spins at 30,000rpm to get some stability.
It might help some if the motor generated asymmetric force or if the model was built wonky and wanted to arch in a certain direction... Spinning the model might turn those tendencies into spiraling instead (I think).
Ultimately, I was not sure that I could spin the model with internal fins that had air pulled over them via induction/GDS (seemed like I should be able to but the model may not get to a reasonable speed to accomplish this).
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