Another rare warm February Day, so I was able to paint. This is done with 2 light coats of primer followed by 2 light coats then a medium coat of Chrysler Hemi Engine Orange. The paint is heat rated to 550 degrees. I will sand with 5,000 grit then wax.
Pretty, Be sure to take pictures 'cause that paint is going to cook off on the NC and leading edges of the fins if you put serious velocity to her. Kurt
This has to be one of the very best posts I've had the pleasure of reading here!!
Yeah. She's going to hit Mach 1.7.
But I thought your stability was over 2 cal for the whole flight? I read somewhere that small fins are fine as long as you use enough nose weight to keep it over 2 cal for mach flights.I am concerned about coning since my fins are a little small, but the flight should still be pretty stable. This is where the H255 pair may be a perform better than the H399/I-204 because the higher seconds stage impulse will punch it through transonic space faster.
But I thought your stability was over 2 cal for the whole flight? I read somewhere that small fins are fine as long as you use enough nose weight to keep it over 2 cal for mach flights.
In theory yes, however one thing that sims don't account for is coning. This can be a pretty significant issue when you have small fins and excessive nose weight.
In theory yes, however one thing that sims don't account for is coning. This can be a pretty significant issue when you have small fins and excessive nose weight.
To avoid coning:
Concentrate weight near the CG of the rocket instead of the ends.
Do not use 3 fins, instead use at least 4 or 5 of the same area so that you have at least one or two fins more perpendicular to wobble motion of the rocket.
Spin the rocket like hell.
I do have 3 fins, but even so, they are about 90% as high as the airframe is wide. Normally you want about 110% or so. These are pushing the envelope, but not near as bad as fins that are 75% or less. The subsequent rockets will have larger fins in relation to the size of the rocket...but not much larger.
Having the fins be at least, or just over, a body diameter is one thing. From what I can tell, your three fins have relatively short root chords, and no tip chord. Personally, I'd either increase span or switch to 4 fins. I'd take the slight performance hit over losing the rocket.
Good paper here, if you are math and physics challenged or impatient you can skip to the recommendations on page 10.
https://rsandt.com/media/Sounding Rocket Fin Design to Mitigate Roll Lock-In.pdf
How does "speed" give more stability? Isn't stability strictly determined by fin area/location and mass distribution?
Anything interesting in the raven file from the 2 stage flight this weekend? Did it fire the igniter output at the expected time?
N
Yes I have only 3 fins, but at this point, I have a built, sanded, and finished rocket. Recommendations such as more fins, larger fins, larger tip chord, etc. are irrelevant at this point. This is what I have and I am beyond reasonably sure it will fly successfully.
The stability of the sustainer is more than 2.5. By the time it reaches mach 1, the stability will be closer to 2.8. I have two programs that are both widely respected that say this will fly.
The Cp through the transonic region moves forward considerable. Depending on the mass distribution of your motor, the rocket can end up unstable above Mach 0.9. A min-diameter rocket (fly the motor) doesn't shift the Cg much; a big dumb rocket (motor is mostly behind the Cp) shifts the Cg forward as it burns. Therefore, the transonic stability problem is mostly seen in min-diameter rockets.
Another factor is the shape of the nose cone. The fins can end up within a cone of turbulent air. A long ogive shape gives minimal airflow separation. A conical nose could have airflow separation along the whole rocket, masking the effect of the fins. Small fins may not be in the steady air stream. A coning action in that case will occur as the fins wander out of the turbulent airflow.
Larger rockets tend not to survive problems with dynamic stability. Their higher longitudinal moments of inertia require more corrective force from the fins than a smaller, less dense, rocket. Testing with a subscale rocket won't necessarily predict dynamic stability problems.
I hear ya. My hybrid rocket attempt is also 3 fins and small profile (span ~100% or less of airframe diameter). However with my design the good thing is my mass in more concentrated towards the center of the rocket, but my CG moves backwards as the tank drains during the burn. I have high static margin (~4-5 cals) though.
Will test fly at NYPOWER on a J. That's what test flights are for.
Any progress?
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