Small fins on a supersonic rocket, do four fins help?

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Tominator 2

Tominator 2

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I have an aerodynamics question for you guys. Attached is an open rocket file for a 38 MD design. It was inspired by the loki dart that i posted pics of on TRF recently.
Anyways, like the loki dart and booster, the fins are pretty small. This file has them at 1.27"
Which is 0.30" shorter than the outside diameter of the rocket. Now people do the whole rule of thumb thing for rockets saying you have to have the fins be equal or greater than the diameter of the BT. Do you think things change with an extra fin?
Someone mentioned that the loki dart wasnt stable unless it had spin stablization from the launcher itself. But that might be a CG issue...
I would make sure the rocket would have plenty nose weight so the calibers of stability would be sufficient.
What do you guys think?

-Tom

View attachment 38mm rocket Open rocket 4 fins .ork
 

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Tominator 2 said:
I have an aerodynamics question for you guys. Attached is an open rocket file for a 38 MD design. It was inspired by the loki dart that i posted pics of on TRF recently.
Anyways, like the loki dart and booster, the fins are pretty small. This file has them at 1.27"
Which is 0.30" shorter than the outside diameter of the rocket. Now people do the whole rule of thumb thing for rockets saying you have to have the fins be equal or greater than the diameter of the BT. Do you think things change with an extra fin?
Someone mentioned that the loki dart wasnt stable unless it had spin stablization from the launcher itself. But that might be a CG issue...
I would make sure the rocket would have plenty nose weight so the calibers of stability would be sufficient.
What do you guys think?

-Tom
Click to expand...

Tominator 2,

Smaller fins will make things difficult in the stability. You must always remember when going past Mach 1 the Center of pressure moves forward by half a margin. If going to super sonic speeds your margin should be at least 1.5" or more. Hope this helps.
 
SinfulDarkLord: Your claim about the CP moving .5 margin forward is not really true.

If you go supersonic, the center of pressure for the fins moves from the subsonic 1/4 chord to 1/2 chord. However, at higher supersonic numbers the fins start mattering less and the nose shape starts mattering more, but that crossover seems to be around Mach 4.
 
CarVac said:
SinfulDarkLord: Your claim about the CP moving .5 margin forward is not really true.

If you go supersonic, the center of pressure for the fins moves from the subsonic 1/4 chord to 1/2 chord. However, at higher supersonic numbers the fins start mattering less and the nose shape starts mattering more, but that crossover seems to be around Mach 4.
Click to expand...

CarVac,

I am sure you know that it is true, if not well it is something I am willing to test to see for myself to see if I am wrong. The one thing I do know is that the rocket will still fly straight, but if you were to look closely the rocket will wobble left and right. I assume the super sonic forces keep the rocket in a straight upward position while it is going unstable through that phase. Well anyways I dont know these days. I got this info from the NASA engineers. Everyone has their different beliefs. Im being told now the sound barrier is 724MPH. I thought it was 768MPH. Everything is just whatever to me now lol.
 
GregGleason said:
The speed of sound varies with temperature.

https://en.wikipedia.org/wiki/Speed...on_and_implications_for_atmospheric_acoustics

Because temperature usually decreases with altitude, the speed of sound drops.


Greg
Click to expand...


Greg,

Thank you on that. It sums up a lot for me. Once again however all of it is just whatever to me since people have their different beliefs. The wikipedia link you post for example anyone can change it. Therefore millions of opinions and beliefs. I just like to follow on what seems more correct.
 
People use belief (rules of thumb) because the actual math is too hard for them to do in the time they have. Don't fall into that trap if you think it matters on your flight.

A good engineer can calculate an answer to 1% accuracy,
A very good engineer can calculate the answer to 10% accuracy in his head in under 30 seconds
A great engineer knows how accurate an answer needs to be in order to be safe.
 
So you guys are saying as long as the CP is good, then four small fins will be stable going Mach 2+?

Jordan T-Yeah, you are very right. I was looking at a professional sounding rocket for inspiration on this. Figuring that they made them small for a reason and they know it works.
 
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As long as the fins remain attached to the airframe..... :roll:

G.D.
 
Yeah make sure you glue down those fins pretty good. Mach 2+ is a different story.
 
Tominator 2 said:
I'm not really worried about the fins getting torn off. They will be CF, tip to tip. Smaller fins would have less of a chance of flutter too.
Click to expand...

Chance of flutter is a misnomer. Flutter is a vibration characteristic generated by resonance between the shockwaves from the air and the fins. Fin shape/taper is a far more important characteristic than size, especially when you're enlarging the cross section with T-T.
 
Tominator2,

You can run the numbers yourself using the Barrowman equations directly to estimate cp position. They are available free in several places on the internet. You can locate the cg easily enough by checking the longitudinal balance of the all-up rocket (loaded with motors, ejectn charges, batteries, electronics, as though ready for launch). You should allow around two calibers of stability between the cg and cp, and add nose ballast if required.

You should NOT add tons and tons of nose weight in an attempt to make it "more stable"

If you are going to try to achieve supersonic flight in any portion of the ascent profile, you should take great pains to be sure the fins are flat, straight, properly aligned with the airframe, and built with consistent leading edge shapes. Mis-aligned or inconsistent fin shapes will cause many drag and aerodynamic problems when the rocket approaches Mach 1, and any wobbles (or other deviations in flight attitude from the desired vertical flight path) will cause excess drag when you want the design to be cleanest and may keep you from going supersonic.
 
Just did a fin flutter calculation using an aerospace engineer's article in apogee newsletter. With 0.04" thick CF fins, the fin flutter value is 3,697 FPS. Or 2,520 mph.
https://www.apogeerockets.com/downloads/Newsletter291.pdf
I used the shear modulus from infocentral.
Shear modulus=7.8x10^6 psi
Fin thickness= 0.04"
The biggest motor i would use is the 6XL J530 imax. Rasaero says about Mach 2.6 depending on weight without motor.
 
powderburner said:
Tominator2,

You can run the numbers yourself using the Barrowman equations directly to estimate cp position. They are available free in several places on the internet. You can locate the cg easily enough by checking the longitudinal balance of the all-up rocket (loaded with motors, ejectn charges, batteries, electronics, as though ready for launch). You should allow around two calibers of stability between the cg and cp, and add nose ballast if required.

You should NOT add tons and tons of nose weight in an attempt to make it "more stable"

If you are going to try to achieve supersonic flight in any portion of the ascent profile, you should take great pains to be sure the fins are flat, straight, properly aligned with the airframe, and built with consistent leading edge shapes. Mis-aligned or inconsistent fin shapes will cause many drag and aerodynamic problems when the rocket approaches Mach 1, and any wobbles (or other deviations in flight attitude from the desired vertical flight path) will cause excess drag when you want the design to be cleanest and may keep you from going supersonic.
Click to expand...

Yeah i want to aim for 2 calibers. You have a great point about misaligned fins adding unnecessary forces. I definately will make sure they are straight as possible.
Thanks for the advice.:)
-Tom
 
From my experiences, I would like to add that even if the rocket appears stable in RockSim or Open Rocket or one of the other programs, it can still be unstable with small fins. I have attempted to build a number of performance rockets with small fins where the fins were incapable of maintaining stability. The fins being so small kept them from being able to correct a change of direction due to wind or other outside influences. These rockets were good for paperweights and providing some un-intended excitement. Again, the software showed they were stable, with a couple of calibers of safety margin, yet in practice, were unstable. On a couple of them, the fins were narrow in chord width, and were long on span, being a full diameter of body tube per fin, but having a short root length, much like the wings on a cruise missile. The fins were just not keeping it stable. I guess my point would be to be very careful and test. What looks and calculates to be stable may not be. MY fins were also very thin, being made of laminated material so they were strong yet only about .045" thick. Bigger fins have worked fine, but I was trying to tweak to get better performance and higher altitudes. When I cut the fins down to the minimum size and still remain stable per the software, I found failure.
 
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BEAR said:
From my experiences, I would like to add that even if the rocket appears stable in RockSim or Open Rocket or one of the other programs, it can still be unstable with small fins. I have attempted to build a number of performance rockets with small fins where the fins were incapable of maintaining stability. The fins being so small kept them from being able to correct a change of direction due to wind or other outside influences. These rockets were good for paperweights and providing some un-intended excitement. Again, the software showed they were stable, with a couple of calibers of safety margin, yet in practice, were unstable. On a couple of them, the fins were narrow in chord width, and were long on span, being a full diameter of body tube per fin, but having a short root length, much like the wings on a cruise missile. The fins were just not keeping it stable. I guess my point would be to be very careful and test. What looks and calculates to be stable may not be. MY fins were also very thin, being made of laminated material so they were strong yet only about .045" thick. Bigger fins have worked fine, but I was trying to tweak to get better performance and higher altitudes. When I cut the fins down to the minimum size and still remain stable per the software, I found failure.
Click to expand...

Yeah, i want to do some barrowman CP calculations by hand and compare what rocksim gets. My goal would be at least two calibers of stability and a long launch tower on a calm morning should leave me with better chances of a stable flight.
 
Why not try building a LPR with a scale Cg and accurate scale fins?

Seems like a C motor going unstable is better than a J motor, especially when I am LCO. :duck:
 
A dart is not a good example to base a ground launched conventional rocket off of. The Loki Dart I have on my desk, a Marquardt copper chaff dart, has the CG at the middle and it does not have the weight of the time delay/ejection charge in it. The Loki dart also does not have to deal with low speed stability, it is not on its own until Mach 5 or 6

To ballast a regular rocket to the same CG would defeat the gains from the lower drag of small fins.

We get rockets at BALLS that are made to the minimum simmed stability, an example are the three stage rockets in the last few years. In the perfect computer world they are great, but in reality the small fins at low speed can not return a long heavy rocket to straight resulting is a scary tumbling rocket with motors yet to fire.

Now having the minimum size fins on stage 2 of 2 pushed by a booster with a “nice” stability margin is more likely to succeed.

Mark
 
MClark said:
A dart is not a good example to base a ground launched conventional rocket off of. The Loki Dart I have on my desk, a Marquardt copper chaff dart, has the CG at the middle and it does not have the weight of the time delay/ejection charge in it. The Loki dart also does not have to deal with low speed stability, it is not on its own until Mach 5 or 6

To ballast a regular rocket to the same CG would defeat the gains from the lower drag of small fins.

We get rockets at BALLS that are made to the minimum simmed stability, an example are the three stage rockets in the last few years. In the perfect computer world they are great, but in reality the small fins at low speed can not return a long heavy rocket to straight resulting is a scary tumbling rocket with motors yet to fire.

Now having the minimum size fins on stage 2 of 2 pushed by a booster with a “nice” stability margin is more likely to succeed.

Mark
Click to expand...
I understand where you are coming from. And a two stager working better with small fins makes sense. I ran some barrowman calculations and got a Cp of 27.7". Which is 0.1" off of what open rocket says. Not bad.
My hope would be to able to fly it with 2 calibers of stability and using a longer launch tower.

A good example of a couple small finned 38mm rockets are kurt von delius's altitude birds.
3.jpg

Here is another.
View attachment 101241
 
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