Crossing threads

[DrewW]

Alright, after binge-reading the scratch build thread from @neil_w which is massive and broad ranging; and after kicking off a conversation several weeks ago on how low did you go and successfully complete a Level 2 HPR cert, I decided to jump back into designing something to accomplish the following objectives:
1) Scratch build a level 2 cert rocket that is interesting different to look at (this implies no oddrocs)
2) With a K motor, fly a nominal apogee under 1000 ft
3) Dual Deploy
4) Keep under 25 lbs (without the motor & case)
5) Has a sweet name

Here is what I have so far...not quite there but close to my objectives

View attachment 448160

Introducing the Sweet'N Low
--------------------------------
Currently sim'd at 1100ft on a K665 (518ft on a J575)
Weight is at 24 lbs
Dual deploy
Sweet name

Thoughts?

 

[dhbarr]

Attachment didn't seem to take.

 

[DrewW]

Attachment didn't seem to take.

Thanks for the heads up. Re-introducing the Sweet'n Low

 

[neil_w]

Interesting! I like the name and the theme. Nice and draggy.

The narrow neck in the middle makes me nervous, even though I know fiberglass is strong it just *looks* like a weak point. Actually looks like the start of a plasma core, but that doesn't really blend with the "Sweet'n Low" theme. The fin can looks a bit busy to me. May play around with simplifying and/or reducing the number of fins?

 

[dhbarr]

If you're building up the aft fins from layers, leaving a u-shaped channel in the leading edge provides a startling amount of drag.

 

[DrewW]

Interesting! I like the name and the theme. Nice and draggy.

The narrow neck in the middle makes me nervous, even though I know fiberglass is strong it just *looks* like a weak point. Actually looks like the start of a plasma core, but that doesn't really blend with the "Sweet'n Low" theme. The fin can looks a bit busy to me. May play around with simplifying and/or reducing the number of fins?

@neil_w I agree on all points. I though the internally routed tube fins would be just about enough in the mindsim but not even close. I had to stretch it out to get positive stability, strakes are mainly for drag, fins are just barely what they need to be for 1.0 static margin with a K motor, and I still had to put 1.5kg of ballast in the nose. Going to have to play around with some other ideas. Just to ask, how did you model your helical fins in OR?

 

[DrewW]

If you're building up the aft fins from layers, leaving a u-shaped channel in the leading edge provides a startling amount of drag.

I’ll investigate this tonight.

 

[neil_w]

Just to ask, how did you model your helical fins in OR?

With great difficulty. Furthermore, they're purely for previewing appearance, and won't simulate in any sane fashion. Here's the file, enjoy the horror.

 

[dhbarr]

Figure 33 from Sighard Hoerner's Fluid Dynamic Drag, Chapter 3. Left column: Bodies of rotation; right column: Cross sections of 2D-bodies.

At the bottom right we guess that a square wave leading edge will be > 2.3 Cd -- it is the draggiest fin by thickness I can think of how to make.

Extra mega bonus points if the surfaces are intentionally very rough, say with truck bed liner or similar under the topcoat.

 

[jqavins]

Dual deploy from under 1000 feet seems like an odd choice. I guess it's in order to stretch your skills? May I respectfully suggest for your consideration, since DD is not a requirement for a cert flight, that you keep the cert flight simple and stretch into DD on the next one.

I though the internally routed tube fins would be just about enough in the mindsim but not even close.

Does that mean the little black nubs on the lower transition are the tops of tubes?

From what I've read, and I do not speak from experience, those will probably not function as fins at all. Once the length to diameter ratio gets large, air doesn't get through the tube, so it's effectively not a tube any more. And yours are really tall and skinny.

And they may also have pretty nearly dead air on top, being down in that valley.

Gang, who's the aerodynamicist around here? It sure ain't me.

(With the bright red CP dot sitting directly on one of the tube ends, for a moment I thought you were planning to mount lights there. Which would be cool.)

 

[DrewW]

You bring up a very valid point.

The L/D ratio of 13 on these tubes is probably not great in terms of aerodynamics in the traditional sense and I think I'll build a scale model of the final config to measure, or at least assess, stability and drag. I was hoping that the laminar shear on the internal walls of the tubes would work to add further drag while lightening up the lower half of the rocket (by reducing density). Additionally, what you're looking at for CG and CP are the relationships where I've removed the actual tube fin elements from the calculation; OR as far as I'm aware can't manage tubefins mounted internal to the body tube, so I have an identical set outboard of the tube in the "real" file and I have this representation as the "render only" file.

So I'm not an aerodynamicist but I occasionally work with them, the center line of the tube fins are outboard of the foreward body tube and the transition to a smaller tube in front should create a low pressure zone that sucks air in. While I can't model a angled cut on a tube fin in OR assume that they will have a leading edge angled cut that matches the lower transition. All of this should allow for adequate, or at least marginally acceptable air at the entrance, enough so to increase stability. Secondarily, if the air chokes in the tubes, I should be left with a significant base drag component which will help with stability and altitude minimization. Either way I think I win in some fashion in the balance.

 

[jqavins]

[T]he center line of the tube fins are outboard of the foreward body tube...

I had not noticed that. I had not noticed that the lowest section is greater in diameter than the highest. So yes, that surely makes things better than I thought, unless the tubes are fully choked, in which case it doesn't matter.

(I still think you should put lights on them. )