Mach Buster Design, what did i do wrong?

[firemanup]

Ok any opinions on this design from those out there...

The target motor is a CTI K445 or an AT K550, the goal is to break mach with an all paper/wood rocket with no fiberglass, the only fiberglass will be milled in the epoxy. Other than that traditional paper and wood parts and a plastic cone.

My biggest question is the fins i've chosen, they're a PML based fin that I modified slightly to get a little less area.

At this time i'm going with 3/16" birch ply I want them fairly thick to avoid any flutter, but again i'm just not sure on the fins yet if they're what i want to go with.

It's showing some extreme altitudes so I'll have to pick the calmest day I can find next season to send her up.

Rocksim file is attached..

Ideas/comments appreciated.

 

[Jerry Irvine]

Originally posted by firemanup
Ok any opinions on this design from those out there...

The target motor is a CTI K445 or an AT K550, the goal is to break mach with an all paper/wood rocket with no fiberglass, the only fiberglass will be milled in the epoxy. Other than that traditional paper and wood parts and a plastic cone.

My biggest question is the fins i've chosen, they're a PML based fin that I modified slightly to get a little less area.

At this time i'm going with 3/16" birch ply I want them fairly thick to avoid any flutter, but again i'm just not sure on the fins yet if they're what i want to go with.

It's showing some extreme altitudes so I'll have to pick the calmest day I can find next season to send her up.

Rocksim file is attached..

Ideas/comments appreciated.

Supersonic.

LOW drag
LOW weight
HIGH final thrust

IIRC the motors you list are neutral to regressive thrust curve shapes. Pick the most progressive one or go with higher thrust.

3/16" fins? They won't break! Go with 1/8" fiberglass if possible. If you are stuck on wood, fine.

Use this fin shape to dramatically lower frontal area and drag.

https://v-serv.com/usr/images/May-O3000.640.jpg

Fly this thing with the minimal possible weight!

I got a 38mm J880 to go something like M2.4 and that was a motor with fins on it.

Merry Jerry

 

[firemanup]

Jerry,

I just tried a fin set like the one you suggested, I actually thought they'd increase my stability from what I had but it became less stable with those so they're going to be out on this design.

I have modified them a hair though, decreased the distance from the body tube to the outer tip of the fin by a 1/4", again trying to have as minimal fin as i can have out there worrying about fin flutter more than anything..

I'm gonna stick with wood, or the birch ply, want to stay away from fiberglass and go with the goal of breaking mach with no fiberglass reinforcement or use...

Just a new goal for me... Here's the new file with the new fins, about the same just a slight change/tweak

 

[teflonrocketry1]

Jason,

To break Mach 1 on this design you will at least need a high thrust J motor (J415 or better). When you finally decide on the fin material post or PM me the details; I will run them through the FinSim program that predicts fin flutter. The range for landing on some of these flights (J motor) in a 10 mph wind is well over 3 miles, so I suggest dual deployment.

As per FinSim 2.3.0 using the file you posted: Fin Flutter at 2258 mph and divergence at 2607 mph. Stress analysis using through the wall fins and 1/16 epoxy fillets (West 105/205) at a 10 degree angle of attack puts the maximum velocity at 1014 mph. A 5 degree angle of attack allows for 1439 mph under the same conditions.

Bruce S. Levison, NAR #69055

 

[firemanup]

Bruce,

Extremely helpful, thanks..

Yes obviously dual deploy and it's designed with appropriate mass weights to simulate all the equipment for this.

The second attachment on this thread is what i'm leaning toward now as far as fin design.

With a max speed on a K550 being 882 mph and a K445 being 862 mph so it looks like i'm ok.

It is through the wall finned, i'll be using aeropoxy more than likely but i can't make fin fillets any less than about a 1/4" so I don't know how much difference that makes in your simulation that you ran.

Fin fillets will be from the fin to the motor tube, fin to the inside of the body tube and fin to the outside of the body tube.

 

[lalligood]

Would it hurt to suggest building a smaller version (either 29mm or 38mm) first & then upscaling for your 54mm effort? It would be a cheaper way to learn what should be some very valuable lesson(s).

Best of luck regardless of which path you choose! Please post your progress

 

[Stymye]

could the fins be airfoiled rather than square?

 

[firemanup]

Stymye,

I didn't know how to sim that in, but yes they will be..

Leading edge will be rounded and trailing edge will be knifed..

 

[firemanup]

Lalli,

Now what fun would that be...

Would make some sense but I don't think i will.. It will get shakedown flights on I's and J's first, but other than that it's gonna be a go as is...

I've got some of the parts already needed, so I'll just have to order a few things to do it.

The biggest problem if it survives the flight which i think it will, is going to be finding it afterward... we'll see how that goes, and of course once I get started on it I will take pics

 

[lalligood]

Originally posted by firemanup
The biggest problem if it survives the flight which i think it will, is going to be finding it afterward... we'll see how that goes, and of course once I get started on it I will take pics

I know that saving weight is an important issue here but might I suggest using a beeper ? (It weighs just over 1oz!)

HTH,

 

[GL-P]

Stay away from swept fins!
(I can't read .rkt files cuz I don't have Rocksim tho)

Keep with shapes like 30-60-90

Very stiff and resistant of fluttering. Relatively good for stability too

Swept fins have "tails" that can flutter easily

I agree tho, scale the design down to 29mm and 38mm to test the building techniques. Don't want to lose an expensive rocket.

What kind of tube are you using? Maybe ypu might want to use a stuffer tube.

Good luck!

 

[firemanup]

Lalli,

Yes will probably at a minimum use a beeper, have plenty of those...

GL-P clipped Deltas..

 

[Jerry Irvine]

Originally posted by firemanup
Stymye,

I didn't know how to sim that in, but yes they will be..

Leading edge will be rounded and trailing edge will be knifed..

Sharp LEADING edges on supersonic.

Kill the mass of the dual deployment and use single motor eject.

J800
K1500+
L1500+

Shoot it over your head at a 30 degree angle from horizontal and lay in a foxhole about 0.5 miles away to hear the boom.

What safety code?

Jerry

 

[firemanup]

Gotcha on the leading edge Jerry....

Did some research and now i'm looking at unsymetrical edges also...

Bruce,

If i go to sharp leading and trailing edges or unsymetrical edges will that change the simulations you ran for fin flutter..?

Seems the unsymetrical fins add stability and correct the drag created by laungh lugs or rail buttons...

 

[teflonrocketry1]

The 1/4" fillets allow for higher speeds. As per FinSim 2.3.0 using the file you posted: Fin Flutter at 2258 mph and divergence at 2607 mph. Stress analysis using through the wall fins and 1/4" epoxy fillets (West 105/205) at a 10 degree angle of attack puts the maximum velocity at 1094 mph. A 5 degree angle of attack allows for 1581 mph under the same conditions.

Sharp wedge shaped fins have less drag at Mach speeds. FinSim doesn't yet take fin profile into account for the flutter analysis; only fin thickness, materials and fillet diameter/adhesive are used in the analysis.

Bruce S. Levison, NAR #69055

 

[firemanup]

Bruce sorry but you're a little over my head with the sims and i don't fully understand what you're saying.

Clarifying here....

This rockets design i'll get flutter at 2258mph.. and "divergence" at 2607, meaning they'll come apart at 2607mph..?

Stress analysis says these fins will handle a speed up to 1094 mph... at 10 degree angle of attack.

I'm not sure what that means, what is at a 10 degree angle..? The fins coming off the body tube are at more than a 10 degree angle aren't they?

If you can explain it to me it would be appreciated.. your posts are alot of help, thanks...

 

[cjl]

I think that it means that the air would be hitting the large flat side at a 10 degree angle. (i.e. if it was flying straight up, but was pointing 10 degrees off for some strange reason or if you angled the fins 10 degrees to make it spiral)

 

[teflonrocketry1]

Originally posted by firemanup
Bruce sorry but you're a little over my head with the sims and i don't fully understand what you're saying.

Clarifying here....

This rockets design i'll get flutter at 2258mph.. and "divergence" at 2607, meaning they'll come apart at 2607mph..?

Stress analysis says these fins will handle a speed up to 1094 mph... at 10 degree angle of attack.

I'm not sure what that means, what is at a 10 degree angle..? The fins coming off the body tube are at more than a 10 degree angle aren't they?

If you can explain it to me it would be appreciated.. your posts are alot of help, thanks...

These are different modes of failure for the fin; Divergence is like a twisting motion along centerline of the fin, Flutter is vibration or oscillation of the fin, Fin Stress is bending of the fin over the root edge and is related to fillet strength and mounting techniques (TTW). These are the maximum speeds the fins should be exposed to. The 10 degree angle of attack is the angle of the fin from the normal flight path. If a fin is mounted wrong or gets bent by more than this angle this is the maximal speed it should be exposed to. Since fin flutter is the lesser speed, and since a 5 degree deflection for and angle of attack is very unlikely; the fins should survive flight up to the flutter speed.

Bruce S. Levison, NAR #69055

 

[11bravo]

Knowing nothing about it really, here is what I came up with as a mach buster.
I may have to come up with some sort of (I really hate to use the common term) uber strong chute.

BTW- what is everybody using as the speed of sound?
There seem to be as many values as there are people to give them to you.

Greg

 

[powderburner]

Originally posted by Jerry Irvine
Sharp LEADING edges on supersonic.

You want sharp leading and trailing edges, and tip too.

Further, if you taper the chord of the fin from root to tip you need to taper the thickness in the same proportion. For example, if the root chord is 15 inches and the tip chord is 3 inches, you need to taper the thickness of the fin so the tip is also 1/5 as thick as the root. Obviously this is going to make for fins that are fairly delicate, but you said you wanted to go supersonic and that you wanted to reduce the drag on the fins . . .

 

[KarlD]

Originally posted by Jerry Irvine
Sharp LEADING edges on supersonic.

Kill the mass of the dual deployment and use single motor eject.

J800
K1500+
L1500+

Shoot it over your head at a 30 degree angle from horizontal and lay in a foxhole about 0.5 miles away to hear the boom.

What safety code?

Jerry

Not too go too far off topic but Jerry,

Do you have a USR kit that with some fiberglassing would do sonic?

Thinking of a nice Fall Boom flight...

 

[firemanup]

Thanks Bruce.. got it now..

 

[Damian]

Fin flutter simulation.

This sounds very interesting......

How do I found out more?


Damian

 

[Damian]

Bad form to reply to ones own post. I know.

Found it and downloaded it.

Very interesting.

D

 

[teflonrocketry1]

Originally posted by 11Bravo
Knowing nothing about it really, here is what I came up with as a mach buster.
I may have to come up with some sort of (I really hate to use the common term) uber strong chute.

BTW- what is everybody using as the speed of sound?
There seem to be as many values as there are people to give them to you.

Greg

Greg,

Cool design; 18 inch long fins on a 2.6 inch diameter airframe! Has the design been built and flown? What about using a drouge 'chute to slow it down?

The speed of sound depends on temperature, air pressure, and the altitude above sea level amoung other things. I like to use 750 mph as a generic value for Mach 1.

Here is the link to AeroFInSim software for those who need it:
https://www.aerorocket.com/finsim.html

Bruce S. Levison, NAR #69055

 

[11bravo]

Bruce-
Thanks!
No it hasn't been built, and as I am not certed even level 1, it will be a while.
However, as my real interest in rocketry leans towards the "Further, faster, higher" made famous by one of the astronauts in "The Right Stuff" (Gordon Cooper or Scott Carpenter???), it may just be something to keep in the back of my mind.
The fin length came from two places: to keep the span down, lengthening them gives more surface (stabilizing) area; and the old fashioned, 'yeah, that looks about right' in the fin editor.
The comment about needing a strong 'chute was at a time that I was just letting the rocket fall from apogee to about 500' and using an altimeter to pop the chute then. I was getting deployment velocities in the 500 fps range.
Now the plan is to have it pop at apogee and fall like a retarded rock (not mentally challenged, but have the fall slowed) with and 18" 'chute with a 9" spill hole. Problem with that might be 10-15" of dirt stuffed up the back of the motor.
RockSim has gives descent rates in the mid 70's fps for that setup.
Not something I'd want to catch, but to keep the weight low and unless it encounters a rock, should be OK.
Oh, by the way, I've renamed the rocket to

Got Shred

Should be a question mark after that, but cannot have one in a file name, oh well.

That name should divulge some of what I currently expect from this rocket.

Thanks for the link, I'll check that out too.

Greg

 

[bsexton]

Jason - glad to know you're still pursuing the hobby. Sounds like a great project. Good luck and Happy New Year!

 

[Neil]

Hm... You should look into the Loki J1001... I havnt seen a thrust curve yet (there isnt one up on the website yet), but it sounds like itd pack a heckuva punch striaght off the pad... 38MM, seven hundred and sumpthin N-s, oughta be enough for Mach with a smallish rocket.

You did use my fauvorite paint scheme too...

If it were me, i'd go with a 38MM phenolic tube, a PML 38MM Urethane nosecone, .062" fiberglass Delta fins with a long sweep and small span, a J1001 from Loki, and a big fat beeper

Your design will work also, with a big enough motor. I'm definetly a fan of minimum diameter for machbusters... Though, if you put a big enough motor in anything... (goes and checks all his designs... Cackles an evil cackle... )

Good luck!


(we expect build pics now, you realize that, right? )

 

[quickburst]

The speed of sound in air at sea level is 760 mph.

QuickBurst