Fin flutter software

[MFr]

Hello, I'm designing my first supersonic rocket and I'd like to calculate the flutter of the fins. The formula in Technical Paper 4197 is only valid for subsonic.
I'd like to know if anyone knows how and what documentation to read.

I can't get aerofin sim, I've contacted the author but no answer. Could someone share the files with me?

 

[MarsLander]

Dennis J. Martin's (author of TN4197) methods were validated to about Mach 1.5.

 

[cbrarick]

he won't talk to me, either. Wish i had a current copy

 

[kjhambrick]

@MFr --

I didn't hear @MarsLander toot his own horn so I will ...

See TRF Flutter Tools for You by @MarsLander

I've tried his Excel programs and they seem to work well.

Maybe they'll help up to Mach 1.5 ?

HTH

-- kjh( toot toot )

 

[G_T]

Anything good for up around M3.5 - M5?

Gerald

 

[Handeman]

Anything good for up around M3.5 - M5?

Gerald

Good question. But, not to be snarky, if you're building something that fast, you should probably be telling us Mach 1 - 1.3 guy about it. I don't think there is more then a handful of flier on TRF that have ever gotten even close to Mach 3. Those speeds are really way beyond what most of use even dream of seeing. I've been told the "rule of thumb" is if your rocket is doing Mach 5 as it passes 30,000 ft. it will reach space. I can only dream of getting one of mine to Mach 2 and maybe over 20K.

 

[G_T]

Warning, very OT but since you asked...

Machingbird

Design is a 2 stage, but somewhat different. It isn't very big, but the flight profile is not quite what others have been trying. The overall design and flight profile has been through countless iterations. I started working on this years ago. It took me a long time to figure out a way to make it so it might work.

Full composite 98mm booster with 4 small fins, staging an 88mm sustainer with three small fins and a slight boattail. No asymmetric cores, to eliminate mass asymmetry to help minimize odds of coning. The sustainer is also a bit stubbier than the usual trend for a high altitude 2 stage attempt.

Booster burnout @ 6100ft, M1.9. Sustainer hot fired off the booster a few seconds later @ 15Kft, M1.4 (target for separation to keep it above the transonic range so it can stay as straight as possible).

This is a safer design since there is no igniter built into the sustainer; the booster lights it. If something goes wrong, the sustainer can't light because the booster won't be there. Before it is stacked, it can't light. This sort of design can only really work due to the booster not being much larger OD than the sustainer. Essentially the sustainer plugs into the booster. There is a little penalty for doing it this way but I think the tradeoffs are worth it. Heck, the interface is a 3D printed sleeve; I don't even need to machine a coupler.

Max Q @ about M2.7, near 27Kft. Sustainer burnout @ a bit over 52Kft, M4.81 in the sim, around 23 seconds into the flight. Sim is to 346Kft. N to M motors and the M is quite unusual for thrust profile and duration, a dual propellant double-taper. The slower burning of the sustainer propellants tests at under 1/8"/s and has some other things unusual about it, though it is still APCP. The N is a slightly modified Bates, not all that special really. Everything is case bonded. No casting tubes; no conventional liners.

For the sim, derated sustainer propellant a little, derated nozzle efficiency a fair bit, and I think I could possibly build it for the mass budget. Going by memory it loses about 10Kft per pound over design mass. It's a modest sized rocket with a modest total propellant mass.

I did a lot of work on the flight profile to try to make it as survivable as possible, trying to keep the speed low until it gets high. If it were to survive a first attempt, the motors can be stretched for higher potential altitude.

I no longer have the equipment to do machining like I used to, so it may not get built. Work doesn't help either. I do have much of what is needed for it though; Kate, NC, tubes, chemicals...

I'm planning on bringing at least something to fly at Black Rock this year though probably a much more modest single stage. And the EX hybrid THRP-1 to LDRS.

Gerald

 

[TonyL]

Hi Gerald,
To answer your question: As far as I have been able to find, ZONA is the only commercial code that models conditions in the mach 3 to 5 region. There is a lot more going on thermodynamically at mach 5. That said, finding someone to do the work looks like it would be an expensive proposition. Conventional NASTRAN/CFD codes have valid physics modeling up to about mach 3. Open foam is a candidate for access to CFD modeling, NASTRAN is free if you like to 'punch cards'.

For those who have been researching this, my Aeroelasticity instructor showed that the Theodorsen method, and ultimately the U-g, k, and p-k methods are the most valid at k<.1, where the structural response is much slower than the air velocity and linear modeling approaches are valid, while I find my models are in the k>2 region where the assumptions of these linear methods are far from the correct physics.
In terms of what to do, AeroFinSim is the only tool that seems realistic in its approach, and even that still needs correct frequency and modulus values to produce something other than garbage. My strategy is to use more conservative margins against the reported flutter boundary such as it is, and then flight test.

What one can do is look at fins that have survived the relevant velocities and dynamic pressures and 'scale down': If one keeps the same thickness and the same shape and just scale it uniformly to a smaller silhouette, it will be expected to have a higher flutter velocity than the larger fin.

br/

Tony

 

[G_T]

Not even necessary to scale down - at least one of the earlier sounding rockets had Ti fins on the 114mm sustainer and a similar velocity at altitude, and worked. The fins I have in mind are dynamically much stiffer and notably smaller. YMMV of course! Yes, I'm more concerned with heating than flutter. If I thought I could get away with it, the fins would be aluminum though not 6061. 6061 doesn't handle heat well at all. But heating is why I'd prefer the fins to be 6Al4V Ti. But I can't machine that, hence the problem. Actually unless the parts are quite small I can't machine anything any more. My old Bridgeport is gone, etc.

Gerald

 

[TonyL]

Sadly Ti6Al4V and stainless are the main alloys that take the heat well and not so much aluminum, but machining stainless should be a bit better than Ti. The other knob is the sweep angle, as leading edge heating goes with the sine of the angle from the body. If one makes a meaningful change in sweep angle then leading edge temperature should reduce.

br/

Tony