Open rocket simulation of Barromean Rings. And build.

[Tractionengines]

Attached is a SIM file for a design I am working on. It has 3 "ring fins" that are around the main body. I think I have seen an image or two of similar designs, from years ago. @neil_w Any thoughts on how well the OR 23.09 SIM of this could be. There's lots of warnings, but if it's in the ballpark for CP and Apogee. I would be more than happy.

 

[neil_w]

Attached is a SIM file for a design I am working on. It has 3 "ring fins" that are around the main body. I think I have seen an image or two of similar designs, from years ago. @neil_w Any thoughts on how well the OR 23.09 SIM of this could be. There's lots of warnings, but if it's in the ballpark for CP and Apogee. I would be more than happy.

Honestly, your guess is as good as mine. I believe some drag elements are being under-accounted, others over. Overall my gut tells me that OR is slightly underestimating drag, so I might be inclined to err slightly on the short side of delay, just to be safe (as opposed to your G80T-7 which is a bit long.

But really I'm just guessing, and if I were completely wrong I wouldn't be surprised. Let us know how it goes.

 

[BABAR]

@boatgeek did something like this

also @OzHybrid

aha, @neil_w post about FlisKits kit

https://www.fliskits.com/WPRESS/product/celtic-thunder/
(Others may have as well)

it’s a cool design.

https://www.rocketryforum.com/threads/project-venn.168935/
fwiw, it easily passes my MindSim evaluation.

 

[boatgeek]

I'm traveling for work right now, but when I get back I'll compare flight logs to my sim to see how they line up to one another. I think the approach I took in my build led to slightly-too-long predicted delays in OR, so I would err towards a shorter delay if you have it. My OR sim is in the last post of the thread helpfully posted by @BABAR. There are a lot of interactions and small cubbies that add drag that wouldn't be reflected if OR thinks there's only 3 large rings.

[edit] CP should be a non-issue. You have so much fin and drag at the back end you won't have stability issues.

 

[Tractionengines]

fwiw, it easily passes my MindSim evaluation.

CP should be a non-issue. You have so much fin and drag at the back end you won't have stability issues.

Yeah, fully agree with these statements.

I'm more evaluating real Apogee Vs SIM. I need to make sure not to break waiver, which is low at local field. I will have a Eggtimer Apogee in the nosecone, so even delays a little long are fine for motor backup.

 

[neil_w]

I'm more evaluating real Apogee Vs SIM. I need to make sure not to break waiver, which is low at local field.

In that case I would just play it conservative with smaller motors until you are able to calibrate the sim. As I said, I *think* that it'll fly lower than predicted, but I wouldn't bet the waiver on it.

 

[BABAR]

In that case I would just play it conservative with smaller motors until you are able to calibrate the sim. As I said, I *think* that it'll fly lower than predicted, but I wouldn't bet the waiver on it.

@neil_w , do Sims “scale”? Especially for OddRocs?

Meaning if @Tractionengines built this half scale, would the CG and CP and net drag be applicable to the full scale rocket?

A potential problem I see is that the fin can here is really a hybrid between ring fins and tube fins. My MindSim (FWIW, LOL) says that “all other things scaling equally”, smaller diameter tube fins are more likely to act like closed cylinders creating lots of drag but not much “direct” CP effect. The inner fin “chambers” are particularly diminutive.

Then again, for this design the inner smallest tube fin “chambers” are barely off centerline anyway (very very short hemispan for these inner segment, as opposed to the “outer” ring segments), so the inner segments probably aren’t doing much for CP (aside from drag) anyway.

 

[neil_w]

@neil_w , do Sims “scale”? Especially for OddRocs?

CP certainly does. Drag *probably* does. Mass and CG almost certainly don't (although might be fairly close).

 

[BABAR]

I don’t play with real Sims (too lazy.)

I see in the notes you are looking at a G motor. Is this big enough and do you have equipment and experience for electronic deployment?

Using motor back up, you are highly likely to get an apogee deployment.

Then start with lowest impulse motor that gets it off the rod or rail with good authority.

 

[BABAR]

CP certainly does. Drag *probably* does. Mass and CG almost certainly don't (although might be fairly close).

Seems that’s okay, as you can easily ground check both the true CG and true mass, and within limits make verifiable adjustments on both.

 

[neil_w]

I think I take back what I said about drag. First of all I'm not even sure what it *means* to say that drag "scales". Secondly it's probably not what you were asking, so best to leave it out of the discussion.

 

[BABAR]

I think I take back what I said about drag. First of all I'm not even sure what it *means* to say that drag "scales". Secondly it's probably not what you were asking, so best to leave it out of the discussion.

I kinda wondered. I’m a little curious about the CP scaling as well for odd rocs. I’m thinking that with OddRocs, you have a lot more nooks and crannies, and I’m thinking that narrower crannies and nooks may have effects Not just on drag, but will screw up laminar flow more and that will muck up CP.

Then again, I always have my doubts about the Sims once you get beyond 3FNC or 4FNC.

@neil_w , your fleet certainly qualifies as “avant-garde” (at least per @kuririn ’s definition). How close has Open Rocket altitudes and delays predictions matched your flights? I guess I should also ask @Ronz Rocketz , as he almost always flies altimeters, and flies Things like Asteroid Hunter and some other complex designs.

 

[neil_w]

@neil_w , your fleet certainly qualifies as “avant-garde” (at least per @kuririn ’s definition). How close has Open Rocket altitudes and delays predictions matched your flights? I guess I should also ask @Ronz Rocketz , as he almost always flies altimeters, and flies Things like Asteroid Hunter and some other complex designs.

Sadly I have flown very little with altimeters, so I haven't been able to do any serious sim-tuning or accuracy analysis. Generally my delays have been pretty accurate, but that does not mean that the altitude prediction is. So... no answer yet.

 

[Tractionengines]

Started the build...
- 7" dia x .045wall x 2.5" tall Rings Fiberglass.
- Estes 4" body tube.
- Estes 4" Nosecone.
- 4"x38mm CRs
- 38mm x 10" motor tube
- Recovery... T.B.D.

Drew up the ring layout. Then figured out slots needed for top/bottom. Cut the initial ring slots, looks like it will work.

 

[BABAR]

Looks like a “paint as much as possible before construction “ job.

 

[BABAR]

Standoff launch lug, or can you thread a rail through an outer ring space?

 

[Tractionengines]

Looks like a “paint as much as possible before construction “ job.

Yep, and don't worry about 100% coverage in areas that won't be seen...

Standoff launch lug, or can you thread a rail through an outer ring space?

The rings are 7" diameter. So there's plenty of room to pass a rail through one of the three "triangular" spaces. The rail buttons only need about .050" washer under them to account for the thickness of the ring.

 

[BABAR]

Yep, and don't worry about 100% coverage in areas that won't be seen...

great minds…..

 

[JoePfeiffer]

My guess is it'll have a lot more drag than the sim says (the tube fin code assumes no obstructions in the tube), and will be at least as stable as predicted.
The earlier suggestion that you send it up on the smallest motor you can that has adequate speed off the rail, see how high it goes, and adjust from there is a good one.

 

[Tractionengines]

The earlier suggestion that you send it up on the smallest motor you can that has adequate speed off the rail, see how high it goes, and adjust from there is a good one.

That will be the plan. It looks like I can even keep it a class 1 flyer, so I can fly anywhere.
Also, the Eggtimer Apogee for deployment lets me not worry about motor delay time.

 

[BABAR]

That will be the plan. It looks like I can even keep it a class 1 flyer, so I can fly anywhere.
Also, the Eggtimer Apogee for deployment lets me not worry about motor delay time.

My ignorance is showing. So with electronic deployment, do you just use the electronics to detect apogee, and then select a motor that has a delay that is at least a little OVER predicted time of apogee. Is that motor back up? And the motor ejection charge SHOULD fire every time, but assuming electronic succeeds prior to motor ejection, the motor charge is just firing out an already empty tube?

 

[dhbarr]

These fins are short enough that if you get into about m 0.8 they'll function like gridfins, similar to a 7-around-1 tuber.

Subsonic should be grand, might weathercock more like a 4FNC than a 6-around-1 with taller tubefins.

 

[Tractionengines]

My ignorance is showing. So with electronic deployment, do you just use the electronics to detect apogee, and then select a motor that has a delay that is at least a little OVER predicted time of apogee. Is that motor back up? And the motor ejection charge SHOULD fire every time, but assuming electronic succeeds prior to motor ejection, the motor charge is just firing out an already empty tube?

Yep. Exactly. The Eggtimer Apogee senses air pressure, as soon as that stops dropping; you've reached apogee. So it fires a deployment charge. Then your motor delay is picked 2-5 seconds longer, and fires through an empty tube. (The Apogee actually fires at 1second after lowest pressure, so don't select motor delay too close to calculated.)

If the chute is hung up or failed to be pulled out, the motor charge as backup can "save you", sometimes resulting in a zipper, but that's better than coming in ballistic.

 

[BABAR]

These fins are short enough that if you get into about m 0.8 they'll function like gridfins, similar to a 7-around-1 tuber.

for the less well informed, like me.

Looks like transonic is the bugaboo

https://en.m.wikipedia.org/wiki/Grid_fin

 

[Tractionengines]

These fins are short enough that if you get into about m 0.8 they'll function like gridfins, similar to a 7-around-1 tuber.

Sbsonic should be grand, might weathercock more like a 4FNC than a 6-around-1 with taller tubefins.

WAY, WAY Down the road... a couple of BIG motor flights could be fun, but I need to tune the SIM first.

IF the SIM calcs are anywhere close... Right now a J270 pushes it to 3000ft and M.7 at 20Gs, and a J435 (biggest DMS 38mm) is also at 3000ft, but gets there a lot faster, pulling 38Gs off the pad and max speed is M.85...not sure the outside portion if the fin will hold up to that... both are good with a 4000ft waiver though.

 

[Tractionengines]

for the less well informed, like me.

Looks like transonic is the bugaboo

https://en.m.wikipedia.org/wiki/Grid_fin

Your Ref. is interesting as I also have a MOAB on the drawing board... which is fully stabilized by the grid fins...rear eject, so it parachutes down nose first... but that's LATER...

 

[dhbarr]

for the less well informed, like me.

Looks like transonic is the bugaboo

https://en.m.wikipedia.org/wiki/Grid_fin

The good news is, short tubefins / gridfins punch through just fine if your material doesn't flex.

 

[BABAR]

The good news is, short tubefins / gridfins punch through just fine if your material doesn't flex.

post # 14 tube fins 7” fiberglass.

Not something I am familiar with as a “stand - alone” fin material. Gotta be tougher than my tube and ring fins, usually single layer large body tubes or Quaker Oatmeal cartons (not a completely shameless plug, the name brand cartons are much heftier than the generics. All taste the same to me.)

As a sport rocket, not an altitude seeker or Mach breaker, thrustcurve.org should help pick the lowest motor that gets it off the rail adequately. Should aim to stay subsonic.

Combined with electronic apogee detection and deployment, easiest to go medium long on delay.

In case of electronic failure, some sort of antizipper device might be nice. Here’s a favorite of mine

https://www.rocketryforum.com/threads/blaines-kevlar-anti-zipper-using-foam-ear-plug.154996/

 

[Tractionengines]

post # 14 tube fins 7” fiberglass.

Not something I am familiar with as a “stand - alone” fin material. Gotta be tougher than my tube and ring fins, usually single layer large body tubes or Quaker Oatmeal cartons

They are "cutoffs" from work. We make hi-temp fiberglass tubes from ~2.5 to ~8.0" diameter in .5 inch steps, usually with .035-.050" wall thickness.

I kept these from going to the scrap bin a couple years ago, with this in mind.

 

[BABAR]

They are "cutoffs" from work. We make hi-temp fiberglass tubes from ~2.5 to ~8.0" diameter in .5 inch steps, usually with .035-.050" wall thickness.

I kept these from going to the scrap bin a couple years ago, with this in mind.

Well, it’s not a toilet ball float, but it’s creative!