Design thoughts

[timothyterpsalot]

This is a rocket that I have been designing. I am curious to know your thoughts. I want to use it for my Level 2 cert. I eventually plan on flying it with an Aerotech K-250 to just over 15,000 ft. It is made of fiberglass. Fins will be aerofoiled. GPS in the nosecone.
Are there any challenges you would expect me to face with this? Any concerns? I'm just gathering thoughts, many heads are better than one. Thanks!

 

[rfjustin]

Fins are a little small, no? Span length from the airframe?

 

[powderburner]

rfjustin,

On what do you base your comment? timothyterps has obviously run a simulation and has a result that is supposedly "overstable" so theoretically the fins could be made even smaller.

timothy is asking for technical assistance and needs answers based on some level of analysis. If there is something wrong with his simulation then we need to find that glitch. If timothy set up his inputs correctly he may well have proper fin size already.

to timothy-

Looks good to me but I am not a rocsim user. Just a wild guess here but with a rocket that size, you are using someone's rail? Is it long enough to let your rocket accel to a safe flying velocity where the rocket/fin aerodynamics are all working OK?

 

[rocketsmith]

I might be mistaken but I think the fin tip chord should be parallel with the airframe. I believe this will help the fin tip vector move back to the trailing edge and rejoin the collapsing flow reducing some portion of this drag. If the fin leading edge is further out in the airstream it not only adds frontal area (drag) but will also create a lower pressure area at the fin tip which will create a larger vector and therefore increase drag. Also a lightly glassed paper airframe would probably be sufficient for a K250; fiberglass is heavy and really only necessary for high "G" motors that will go mach plus. You might be able to cut 30 to 40% of the weight; saved ounces are free altitude.

 

[timothyterpsalot]

The fins were a little troublesome. I found that if I changed the shape but not the size he rocket would lose stability. I am not sure if it was a glitch. I have had bad experiences with phenolic tubing and am trying to avoid it. I should consider the dynawind tubing though.
The rocket should be stable at the end of he launch rod. I know the k250 doesn't put out a ton of thrust but I'm trying to keep it light enough that this won't be a problem.
This rocket will hit mach 1 according to rocksim. I forgot to add that earlier.

 

[cjl]

The fins look a bit small to me too. Does it really predict stability with both the Rocksim and Barrowman methods?

I would be inclined to up the fin size a bit - if a rocket heading to 15k wobbles and arcs over, you have a LONG walk to get it back.

 

[powderburner]

I might be mistaken but I think the fin tip chord should be parallel with the airframe. I believe this will help the fin tip vector move back to the trailing edge and rejoin the collapsing flow reducing some portion of this drag.

OK, I'm stopping right there. I see no need to make this any worse.

Can you tell me what a fin tip vector is?

Can you tell me why it would move anywhere, let alone "back to the trailing edge"?

Can you tell me exactly what flow is collapsing? (presumably, in the vicinity of the tail fins?)

Where were you taught these aerodynamic principles?

I know I am out of school for 34 years, but I work in the aerospace industry, in the preliminary design area of a major company, and I have never heard of these concepts. Not to sound like I must personally approve any possible revision to fundamental aerodynamics, but I think I might have noticed changes like this sometime during those years at work.

And to think, all those MiGs with the clipped tails are now going to go out of control and crash?

 

[roadkill]

Hmmm...

2.9 caliber overstable = weathercocking...

...long hike...

Have you considered ARR's Blue Tube 2.0 for
body tube, that stuff is amazingly strong, I just
tested a 4 inch coupler and it easily took the brunt
of 300lbs axial load...

Lighter than Dyna Wind...

Those fins look kinda small to me too...

 

[daveyfire]

On what do you base your comment? timothyterps has obviously run a simulation and has a result that is supposedly "overstable" so theoretically the fins could be made even smaller.

While the rocket may be statically stable, the fins may not provide enough damping to recover from a significant disturbance, e.g. a strong wind gust as the rocket comes off the rail. I've seen this happen many times with small-finned rockets on large motors (most recently, https://www.tdkpropulsion.com/2009/06/16/rocstock-29/?nggpage=3 images 4-18). This effect can be exacerbated by long airframes with many joints in them, providing points for the vehicle to bend slightly and magnify any nutation.

Rocksim will do some limited dynamic stability calculations, however to really get a handle on the situation, the rocket needs to be spin-balanced and the fins attached completely straight. It's often easier to simply make the fins bigger and avoid riding the edge of dynamic stability in the first place.

Since the vehicle will be going supersonic, I'd also recommend plotting CP as a function of Mach number to make sure that you'll be stable, using an analysis tool (like RASAero or DATCOM) that has better modeling at high Mach.

 

[The EGE]

About the nose cone: is the conical nose your only option? Conical noses are decent at subsonic speeds, and very good at supersonic speeds, but your rocket will obviously be spending a while in the transonoc region, and for that you want a more rounded tip, like a parabolic cone, or else a special-shape nose like a Von Karman ogive or Haack.

 

[timothyterpsalot]

Thanks for the small fin not being able to recover tip. Hadn't thought of that.
I ended up with a conical cone after testing other designs in rocksim. It performed the best. Does rocksim not model transonic flight well?

 

[powderburner]

While the rocket may be statically stable, the fins may not provide enough damping to recover from a significant disturbance

Very true. Note that I used the qualifier "supposedly." I still have trouble thinking of a stability margin on only 3 or 4 as over-stable. Stuff happens. Stability is good. Better to have too much and burn some as you learn how the rocket really flies.

With the thrust-to-weight ratio that timothy's rocket has, it will very (VERY) quickly be up to pretty significant velocities. Side gusts will have relatively little effect on trajectory. The rocket will be most vulnerable at lower speeds, just off the launcher. This can pretty much be avoided by.....not launching on a windy day.

Rocksim will do some limited dynamic stability calculations...

I understand that also to be true, although I am not a rocsim user (or fan). Still, some level of analysis might be better than nothing?

Since the vehicle will be going supersonic, I'd also recommend plotting CP as a function of Mach number to make sure that you'll be stable, using an analysis tool (like RASAero or DATCOM) that has better modeling at high Mach.

An excellent idea. The transonic speed regime should also be analyzed. Are you listening, rfjustin?

...(like RASAero or DATCOM)...

Oh, man, haven't we found something better than DATCOM by now? That tool is AWFUL! I think chicken entrails will work better than that old pile-o-crud.

 

[madmax]

Powderburner, no need to jump down rfjustin's throat. The poster asked a question and a response was given. I was going to reply but decided against it.

 

[cjl]

While the rocket may be statically stable, the fins may not provide enough damping to recover from a significant disturbance, e.g. a strong wind gust as the rocket comes off the rail. I've seen this happen many times with small-finned rockets on large motors (most recently, https://www.tdkpropulsion.com/2009/06/16/rocstock-29/?nggpage=3 images 4-18). This effect can be exacerbated by long airframes with many joints in them, providing points for the vehicle to bend slightly and magnify any nutation.

Rocksim will do some limited dynamic stability calculations, however to really get a handle on the situation, the rocket needs to be spin-balanced and the fins attached completely straight. It's often easier to simply make the fins bigger and avoid riding the edge of dynamic stability in the first place.

Since the vehicle will be going supersonic, I'd also recommend plotting CP as a function of Mach number to make sure that you'll be stable, using an analysis tool (like RASAero or DATCOM) that has better modeling at high Mach.

Part of that isn't just the amount of damping that is provided, but also the relationship between the restoring moment and the rocket's moment of inertia. If the rocket is quite dense (as many MD rockets are), and has a small restoring moment due to small fins and a small margin between CP and CG, disturbances can get to a much larger amplitude before there is a sufficient restoring moment to bring the rocket back in line.

As for supersonic stability, I wouldn't be terribly concerned in most cases. Typically, a flat plate fin will become more stable as it goes supersonic, as the center of pressure will (roughly) move back from 1/4C to 1/2C on the fin.There are some odd cases of course, but with the appropriate stability from the beginning. In addition, most will be more stable later in flight, as the fuel burn will cause the CG to shift fowards.

 

[cjl]

Thanks for the small fin not being able to recover tip. Hadn't thought of that.
I ended up with a conical cone after testing other designs in rocksim. It performed the best. Does rocksim not model transonic flight well?

Not at all. Cones are pretty terrible for transonic and low supersonic flight actually.
This is what you should be aiming for if you want altitude:

 

[cjl]

I might be mistaken but I think the fin tip chord should be parallel with the airframe. I believe this will help the fin tip vector move back to the trailing edge and rejoin the collapsing flow reducing some portion of this drag. If the fin leading edge is further out in the airstream it not only adds frontal area (drag) but will also create a lower pressure area at the fin tip which will create a larger vector and therefore increase drag. Also a lightly glassed paper airframe would probably be sufficient for a K250; fiberglass is heavy and really only necessary for high "G" motors that will go mach plus. You might be able to cut 30 to 40% of the weight; saved ounces are free altitude.

The fin tip vector and collapsing flow?

What?

It's true that a taper like that may not be perfectly optimized, but I haven't got a clue what you're trying to say here. I've taken an aerodynamics class, and I'm in a more advanced aerodynamics class right now, and everything you said above is greek to me.

 

[bobkrech]

Beware of the comments Rocsim gives with the Static Stability Margin calculations. It will usually say short, stubby rockets are unstable when they are indeed stable, and will call long Toms like yours overstable when they are not.

The airframe can play a major role in the dynamic stability of the rocket and the CP will move forward and eliminate the "overstable" prediction in many cases. If the span is insufficient, the rocket may be susceptible to coning under certain conditions.

Look at the fin size on the AT Mirage and LOC Caliber ISP and Hyperloc 300 and similar rockets for a fin size comparison. Without doing any calculations I think your fins are a bit narrow and probably should extend outward 1 to 1.25 times the airframe diameter. For the least drag the trailing edge should be perpendicular to the airframe, and the in clipped delta design the outer edge should be parallel with the airframe and not rounded.

Bob

 

[sylvie369]

An excellent idea. The transonic speed regime should also be analyzed. Are you listening, rfjustin?

I think you might have lost track of whose design this was. Justin was just the first person to reply* - the rocket design is from Timothy.

* And his comment captures what I imagine almost all of us thought when we looked at the rocket - those fins look mighty small. Even if he doesn't have problems flying it, it might not have enough fin area to get to the pad in the first place. Were I RSOing it, I'd want a lot of evidence that he had good sims for it, and I'd probably ask for a second opinion.

I'm also not so sure about the thrust-to-weight ratio. A 5:1 ratio on a K250 motor would give a max takeoff weight of 11.24 pounds, and his sim says the rocket with motor already weighs 10.85 pounds (is the GPS in there too?). If the build is even a little heavy he'll lose the ratio, and I expect have some tipping off the rail. A non-vertical trajectory has to be just about the biggest thief of altitude that there is.

(Edit: Oh, okay, I just looked at the thrust curve for the K250, and it looks like he will probably be okay. There's a period of 80+ pound thrust at the beginning, which should move a 16+ pound rocket just fine. Never mind.)

 

[rocketsmith]

The fin tip vector and collapsing flow?

What?

It's true that a taper like that may not be perfectly optimized, but I haven't got a clue what you're trying to say here. I've taken an aerodynamics class, and I'm in a more advanced aerodynamics class right now, and everything you said above is greek to me.

The concept I am refering to is the drag vorticies created at the tip. I would elaborate further but I am at work and don't have time right now. I must admit, I haven't looked at any of my aerodynamics materials for quite some time and may be a little rusty but I thought this was basic aerodynamic theory, at least for transitional and mach plus speeds. In the 60's designers used to add small plates to fin tips to correct for this. They dropped it because the trade off of weight and bending moments created offset any reduced drag.

 

[cjl]

The concept I am refering to is the drag vorticies created at the tip. I would elaborate further but I am at work and don't have time right now. I must admit, I haven't looked at any of my aerodynamics materials for quite some time and may be a little rusty but I thought this was basic aerodynamic theory, at least for transitional and mach plus speeds. In the 60's designers used to add small plates to fin tips to correct for this. They dropped it because the trade off of weight and bending moments created offset any reduced drag.

Typically, vortices are created at the tip of a subsonic airfoil. You can form them on a supersonic airfoil as well, but every case that I can think of with sizeable tip vortices has one thing in common - a reasonably large lift coefficient. If rocket fins are working as one would hope, they should be symmetric at a very small angle of attack. This would imply that they would have very little lift, and very little in the way of tip vortices.

 

[daveyfire]

Does rocksim not model transonic flight well?

Nope. (see attached.)

Part of that isn't just the amount of damping that is provided, but also the relationship between the restoring moment and the rocket's moment of inertia.

Bingo! Thanks for putting it better than I could :cyclops:

Oh, man, haven't we found something better than DATCOM by now? That tool is AWFUL! I think chicken entrails will work better than that old pile-o-crud.

What specifically do you not like about it? Yes, the input can be a bit laborious, and you may need to massage it a bit to make a solution converge, but I've found its results to be worth it. These issues are typical of any computationally-intense tool, and frankly I'd rather spend a few minutes massaging the numerical solver and get reasonable results than spend 30 seconds with a program that has lots of shiny sliders and the ability to model tube fins on sticks with pods that I don't trust at all.

(It's funny you brought Justin into this thread... he and I just had this conversation about a similar rocket a few months ago )

 

[rfjustin]

On what do you base your comment?

1. EMPIRICAL DATA
2. Simulations
3. Years of flying minimum diameter vehicles where fin size, specifically span off the airframe have the most profound or detrimental flight results.

Are you listening, rfjustin?

Yes.

It's funny you brought Justin into this thread... he and I just had this conversation about a similar rocket a few months ago

IMHO, put larger fins in the rocket...

op:



Justin

 

[bobkrech]

This is a rocket that I have been designing. I am curious to know your thoughts. I want to use it for my Level 2 cert. I eventually plan on flying it with an Aerotech K-250 to just over 15,000 ft. It is made of fiberglass. Fins will be airfoiled. GPS in the nosecone.
Are there any challenges you would expect me to face with this? Any concerns? I'm just gathering thoughts, many heads are better than one. Thanks!

Tim

You need to loaded the motor into the rocket before you rum RS and get the Static Stability Margin because CG is weight distribution dependent.

Did you load in the K250 motor and then run RS to get the Static Stability Margin, or is the value on the figure without the motor? If not, that's answers why the fins are so small.

Thanks for the small fin not being able to recover tip. Hadn't thought of that.
I ended up with a conical cone after testing other designs in rocksim. It performed the best. Does rocksim not model transonic flight well?

Search the forum for "nosecone shape". Cones aren't the best shape for any flight regime. Please read this thread. https://www.rocketryforum.com/showthread.php?t=6333&highlight=nosecone+shape

And his comment captures what I imagine almost all of us thought when we looked at the rocket - those fins look mighty small. Even if he doesn't have problems flying it, it might not have enough fin area to get to the pad in the first place. Were I RSOing it, I'd want a lot of evidence that he had good sims for it, and I'd probably ask for a second opinion.

My thoughts exactly.

Bob

 

[blackjack2564]

Your file looks very close to the BSD 54 Special [ also a 3in rocket] in size and fin shape.

Find the rocksim file on that rocket, increase you fin size accordingly and you should be good to go. I flew mine on a K-250 but alas the G-wiz did not function correctly and it became a fence post.

You will have to come in under 6lbs to reach that altitude on that motor without drastic design. I finally did it with a 59in version light weight glass tubes. ready for fight sans motor..... 5.3lbs on the dot. Flew the new version K-250.[use your own closures]

Altitude reached was 15,308 ft. Mach .8 I would use cardboard with full length couples if I did it again. My old 54 special only weighed 3.8 lbs built that way. mach=no issue on a K-250

Here is nc shape vs speed functionality graph of which all are speaking.....judge for yourself

 

[timothyterpsalot]

I thought that cones were good for supersonic flight? Or is it not until higher machs that the design becomes more efficient?
The rocket is stable by a margin of 3 using rocsim method of caculation but only like a margin of .2 with barrowman method. What should I think of this data? I heard the for the most part rocsim method is moe reliable because it accounts for the dynamic stability margin. Is that correct? Which should I trust?

 

[sylvie369]

I thought that cones were good for supersonic flight? Or is it not until higher machs that the design becomes more efficient?
The rocket is stable by a margin of 3 using rocsim method of caculation but only like a margin of .2 with barrowman method. What should I think of this data? I heard the for the most part rocsim method is moe reliable because it accounts for the dynamic stability margin. Is that correct? Which should I trust?

.2?

Not 2, but .2? You're getting a .2 margin with the Barrowman equation? I wouldn't fly that, regardless of what the Rocksim method tells you.

I definitely think you need to try larger fins. I believe that this rocket as designed is dangerous.

 

[timothyterpsalot]

Sorry, I wasn't clear. The .2 is from a "revised" design that isn't finished. I was wanting the question to be focused on the numbers, not the design. The initial design almost had a margin of 1. I am more curious about the numbers. When Barrowman says it is stable, rocsim says it is WAY over-stable. What's up with that?

 

[bobkrech]

I thought that cones were good for supersonic flight? Or is it not until higher machs that the design becomes more efficient?

Cones are easy to make, not more efficient.

Sorry, I wasn't clear. The .2 is from a "revised" design that isn't finished. I was wanting the question to be focused on the numbers, not the design. The initial design almost had a margin of 1. I am more curious about the numbers. When Barrowman says it is stable, rocsim says it is WAY over-stable. What's up with that?

Your rocket has a length to diameter ratio of 27. You want to have a stability margin well in excess of 1 for a long rocket because the CP will move forward at increasing angle of attack, greatly reducing the stability margin.

Please remember you have determine the CG for the rocket in the launch state with the motor loaded. The unloaded numbers are meaingless.

Please read the first two references to see what the problem really is.

https://projetosulfos.if.sc.usp.br/artigos/sentinel39-galejs.pdf

https://www.tripoli.org/tra_ca/tech_reports.html

https://www.jmrconline.org/Stability.pdf

https://rocketdungeon.blogspot.com/2005/11/i-may-be-unstable-but-i-dont-like-my.html

Bob

 

[Adrian A]

I thought that cones were good for supersonic flight? Or is it not until higher machs that the design becomes more efficient?
The rocket is stable by a margin of 3 using rocsim method of caculation but only like a margin of .2 with barrowman method. What should I think of this data? I heard the for the most part rocsim method is moe reliable because it accounts for the dynamic stability margin. Is that correct? Which should I trust?

Believe Barrowman when it says margin of 0.2. Run far away from a rocket with 0.2 Barrowman stability, even if Rocksim says it's over 1. I had an F10 altitude rocket with about 1 caliber of Rocksim margin that was unstable until it burned off enough fuel, at which point it accelerated toward the flight line and buried itself in the ground several feet from panicked spectators. After that I learned that Barrowman was predicting about 0.1 calibers of stability margin for that rocket, and I've never used Rocksim's stability calculations again.

Also, I'm not a fan of using calibers as a relevant figure of merit for stability, especially for a long rocket. I recommend you consider the margin not in terms of diameters of stability margin, but as a fraction of your rocket length. On a 7' rocket, do you really want just 3 inches of lever arm for the center of pressure to push your rocket straight? I would want 7"-10" of stability margin if it were mine.