Stability of asymmetrical designs

rstaff3 said:

I predict a pucker factor of 3 on a scale of ten.

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You see guys, only a three out of ten pucker factor on the EKRANOPLAN. Now that is what I call CONFIDENCE!!

Daddyisabar said:

If it did end up doing the ground effect it would be totally kewl. You could just say "I meant to do that for extra scale flight points!" Flying something as a rocket that was initially designed to suck itself to the ground of water surface is. . . well. . . a bit on the odd side.

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To be fair, it wasn't designed to suck itself to the surface, but rather use ground effect to support it above the surface. It flies, just very low. Either way, more like a cruise missile than a rocket!

While I appreciate the merry banter and do sincerely look forward to the (eventual?) flight of the CSM, I'm still feeling a little vague about this.

Here's a good hypothetical: what if a took a standard 4-finned rocket and hacked off a fin. What would that rocket do in flight? If it would be something bad, how much of a new fin (compared to the others) would I have to put back on to avoid badness?

Sorry for the thread hijack. It's vague because nobody has a clean closed form solution. The CSM approached this case. Most of the fin area is side-to-side and up. The down component is merely the small tabs on the end of the main fins. The F104 Starfigher is even closer with two main fins in one plane and one fin perpendicular to those. The latter example says that it could be made to work. For some reason that sounds less likely than a rocket powered F104:confused2:

neil_w said:

While I appreciate the merry banter and do sincerely look forward to the (eventual?) flight of the CSM, I'm still feeling a little vague about this.

Here's a good hypothetical: what if a took a standard 4-finned rocket and hacked off a fin. What would that rocket do in flight? If it would be something bad, how much of a new fin (compared to the others) would I have to put back on to avoid badness?

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One way to compensate for the loss of the fin would be to simply angle the two opposing fins or rotate them a little further toward the removed fin. There are quite a few kits out there that use this for asymmetrical fin arrangements. OR does fine with changing the radial position but I can't find a way to angle them down in their original position.

It seems to me that the ring fin that you have in your concepts would give you a pretty good fudge factor though.

Do you have a design file for these? I am curious how you got OR to render the "booster" tubes.

Screaminhelo said:

OR does fine with changing the radial position but I can't find a way to angle them down in their original position.

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I can't speak for Open Rocket, but in Rocksim, you'd create a pod. The pod would be angled down, and you add a fin to that. That's at least how I did the model for the Orbital Interceptor, where the wing fins are on the centerline, but the tips droop to the same plane as the bottom of the tube.

rstaff3 said:

Sorry for the thread hijack. It's vague because nobody has a clean closed form solution. The CSM approached this case. Most of the fin area is side-to-side and up. The down component is merely the small tabs on the end of the main fins. The F104 Starfigher is even closer with two main fins in one plane and one fin perpendicular to those. The latter example says that it could be made to work. For some reason that sounds less likely than a rocket powered F104:confused2:

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For sure, the airplane-style designs generally have the most extreme asymmetry, with some having little or no down-facing fins. My mindsim says that if there's sufficient fin on at least one side, then in the face of a crosswind the rocket should weather-vane until it presents a symmetrical face to the wind, which would be OK. But my mindsim is still an alpha version, so results are not reliable.

The Orbital Transport is another good example with very little (but not nothing) on the bottom and a big draggy glider up on top along with the up-facing vertical stabilizers.

Screaminhelo said:

One way to compensate for the loss of the fin would be to simply angle the two opposing fins or rotate them a little further toward the removed fin. There are quite a few kits out there that use this for asymmetrical fin arrangements. OR does fine with changing the radial position but I can't find a way to angle them down in their original position.

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The Interceptor is the canonical version of this, although for that design (which is so freakin' brilliant, seriously) they drooped the wings and also put those small fins on bottom, and some how it all hangs together perfectly.

It seems to me that the ring fin that you have in your concepts would give you a pretty good fudge factor though.

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Possibly so, although that design is just an example; I'd really love to understand this better in the general case if possible.

Do you have a design file for these? I am curious how you got OR to render the "booster" tubes.

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They're just tube fins mounted to a phantom body tube. ORK files attached below.

EXPjawa said:

I can't speak for Open Rocket, but in Rocksim, you'd create a pod. The pod would be angled down, and you add a fin to that. That's at least how I did the model for the Orbital Interceptor, where the wing fins are on the centerline, but the tips droop to the same plane as the bottom of the tube.

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OR doesn't support pods in any meaningful way. In many cases, although it is possible to create an OR file that *looks* about right, the actual sim is garbage. For example, in my posted designs above, the tail ring is not simulated at all, so it's effect on stability is completely unknown. Tube fins mounted away from the body require a phantom body tube, which screws everything up as well. So in these instances OR can still be a good for visualization but not analysis.

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neil_w said:

OR doesn't support pods in any meaningful way. In many cases, although it is possible to create an OR file that *looks* about right, the actual sim is garbage. For example, in my posted designs above, the tail ring is not simulated at all, so it's effect on stability is completely unknown. Tube fins mounted away from the body require a phantom body tube, which screws everything up as well. So in these instances OR can still be a good for visualization but not analysis.

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Well, then this alone helps me justify that I spent the money on Rocksim (even if it does have quirks that annoy me), because the number of designs I've done with pods (or used the pod tool) is not insignificant. I didn't know that OR didn't have that functionality, nor ring tails. Can it do tube fins on the body?

EXPjawa said:

Well, then this alone helps me justify that I spent the money on Rocksim (even if it does have quirks that annoy me), because the number of designs I've done with pods (or used the pod tool) is not insignificant. I didn't know that OR didn't have that functionality, nor ring tails. Can it do tube fins on the body?

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Yes, although I believe they're still labeled as "experimental".

I'd consider springing for Rocksim if it were cheaper, but at more than a Franklin it's just not worth it to me. I don't begrudge them their asking price, but I'd sooner spend that on a Chute Release or something. So instead of Rocksim I just leach off the hive mind here.

neil_w said:

Yes, although I believe they're still labeled as "experimental".....So instead of Rocksim I just leach off the hive mind here.

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Kind of going from MindSim to GroupSim. I'll stay away from the Star Trek puns that are screaming to be heard.

Thanks for posting the .ork files. I am always interested in seeing how folks massage the program to get the results that they do. I'll have to explore the designs at lunch but my version of MindSim says that they look like they would work pretty well. They may need a little tweak here and there but I think that you have more than enough fin area.

neil_w said:

Yes, although I believe they're still labeled as "experimental".

I'd consider springing for Rocksim if it were cheaper, but at more than a Franklin it's just not worth it to me. I don't begrudge them their asking price, but I'd sooner spend that on a Chute Release or something. So instead of Rocksim I just leach off the hive mind here.

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I've never been the hive type myself, which is probably why I didn't even know that OR existed when I bought Rocksim. Otherwise, I might've gone the other way, and would've been more limited in my designs, because I likely would've stayed within the limitations of the software. But I started with the Rocksim free trial, and when the trial period was over, I had enough time invested into a bunch of designs that I didn't really have a choice... :blush: The biggest issue I've got now is having the time to build all of this stuff...

EXPjawa said:

I've never been the hive type myself, which is probably why I didn't even know that OR existed when I bought Rocksim. Otherwise, I might've gone the other way, and would've been more limited in my designs, because I likely would've stayed within the limitations of the software. But I started with the Rocksim free trial, and when the trial period was over, I had enough time invested into a bunch of designs that I didn't really have a choice... :blush: The biggest issue I've got now is having the time to build all of this stuff...

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Back when I was starting with this stuff I tried to load the free trial on my Mac at home and had terrible difficulties, ultimately failing to get it to work at all. Maybe I should try it again.

For sure, two OR limitations are contraining my thinking: fins can only be mounted perpendicular to body tubes along the primary axis, and nose cones are likewise limited to body tubes on the primary axis. On the other hand, OR makes it possible to produce beautiful renderings of finished rockets, and makes paint-scheme and decal-design... well, not exactly "easy", but at least possible.

What about the swing test? Does that still work?

cls said:

What about the swing test? Does that still work?

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Not on paper designs.

For me, the pods feature on Rocksim has made it worth every penny of the upgrade cost. If OR gets these and can load the Rocksim files with pods, I'll have to reassess if Rocksim ever gets another upgrade. Rocksim also includes renders and can include decals, but the output isn't as nice as what I've seen from OR. I have never tried to add decals because I just don't care.

The swing test is extremely limiting, and always has been. And for those us design larger rockets, its not exactly practical.

Neil, I will say that I'm impressed with some users' abilities to make pretty kick-ass renderings with OR, and kind of wish that I could do the same with Rocksim. But at the end of the day, I'm an engineer, not a graphic designer. I'd still choose the functional tools over the aesthetic ones. But I have found that Rocksim has lighting and decal tools as well that help in that dept, I just haven't spent a lot of time playing with them.

neil_w said:

Not on paper designs.

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I don't fully trust swing tests on really odd rockets. My assumption is rockets that generate lift or whose fins can get occluded by turbulence can do well on a swing and can do not so well in flight. These can also be sensitive to what motor you use. I still do a swing test on the odder rocs.

Maybe do this: Go out and get yourself the OOP SUNWARD MIRAGE kit, build it 24mm. It has a large delta wing mounted on one side of the tube and nothing else but a launch lug. Nice and long this kit is, put a bit of nose weight in and it flies "OK." It likes to roll over on its back and glide upside down during the coast, but it that is fine. Off the rod it goes and straight up during the powered phase and with an airplane rocket that is all you need to do. Just avoid any airplane action immediately after leaving rod or rail. Get some flying experience with a kit that has some "idiot factor" built in, but yet it "looks" very asymmetrical. (Actually these kits don't have a whole lot of idiot factor built in, but there is enough to keep most folks out of too much trouble.)

The kit instructions tell to you "add some nose weight" and swing test it yourself! They are also written in French so there is no excuse.

Do you fly it in wind? (With my best Napoleon Dynamite accent) "Gosh dern it! I spent so much time on this and By Golly I am going to fly it today!"

Do you fly it under powered? "Gosh dern it! The kit says you can build it 18mm. I need efficiency so I am gonna skimp on that no good stinking nose weight! Yeah, I heard some of these have crashed before."

Do you fly it off a short rod? "Gosh dern it! All I have is my old Estes launch pad!" Hey Dude: way too many quotation marks!

You fly it on a D12 3 and then a D12 5 for a longer glide, having passed the idiot test you are now an airplane rocket flying connoisseur. Yeah, I am actually going for more of a glide flight because it is cool, maybe a D12 7 would be awesome? Right?" Gosh dern it! Pad Fuhrer thinks that is a bad idea. But the D12 7 is the only motor I have left in the box! Guess I will just have to stomp away and go over and cry with the kid who thought he could fly his Patriot again after losing a fin on the first flight."

Then you read the rocket science stuff in the Apogee newsletter that says two finned rockets are inherently unstable. Now you are even more sad and confused. But I know there are guys out there flying the F-104 as a rocket! Maybe if I just put some pods under the Mirage I can make a B 58!

So Dude, answer my question! Can a rocket fly with big wings and a tail but nothing else? . . . Maybe, probably not, depends on a lot of factors, not a good idea, at best it will fly like the Mirage. Why don't you just put the landing gear down and add a drop tank? Hey Dude, now you are just talking crazy!

Say what?!? A commercially produced airplane rocket with one too few fins. This madness!

On my Mach 2 I offset the motor down right above the wing, since I figured the drag of the wing was larger than the drag of the tail...turned out I was close. Same with the bomarc I have the motor mounted high right under the wingline figuring the drag of the wing/tail was a bit higher than the ramjets. On airplanes I tend to like to put the thrust line right up the wing line or as close as I can and usually that is pretty close...You don't need symmetric tail surfaces for stability you just have to have the areas correct for stability margin. I think the major issue for assymetric designs is the assymetric drag which will cause them to arc unless you have the thrustline right.

rstaff3 said:

Say what?!? A commercially produced airplane rocket with one too few fins. This madness!

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It is OOP and it was from Canada! But look at the air intakes and with a big Delta wing on one side it could get by with out a true fourth fin. The Mirage was the most extreme, but the same basically held true for the others like the Liberator, Maverick, Phoenix and SU 47. Sometimes you can still get them on EBay. Great kits to start out with.


And then there was the famous Model Minutes Leduc from France. This was my post crash photo after the card stock motor tube blew out on the third flight. It used the angled tail fin method. Card stock can take a lot of damage!

Did a quick download and 5-minute playaround with Rocksim... the way it handles pods sure does open up a world of design possibilities vs. OR. Hard to say if it's worth the money for me, but it is tempting. Maybe I'll wait a while and see if there's *any* indication such capabilities will be added to OR any time soon.

I just spent some time playing with your designs and I think that they are both pretty stable. Since OR doesn't account for the aerodynamics of the ring fin, I think that an OR margin of .75 or greater would work fine. I did have to add a little nose weight to get it to work with 24mm motors but I think that they would both do well. The real fun has been trying some personal modifications though.

Show me whatcha got!

neil_w said:

Did a quick download and 5-minute playaround with Rocksim... the way it handles pods sure does open up a world of design possibilities vs. OR. Hard to say if it's worth the money for me, but it is tempting. Maybe I'll wait a while and see if there's *any* indication such capabilities will be added to OR any time soon.

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...aaaaaand once again I cannot get the app to run on my Mac at home. That'll certainly put the kibosh on Rocksim if I can't get this resolved. Will see what Apogee tech support can do for me.

Nothing radically different, mostly playing with lines and angles at this point. I'll upload what I have tomorrow but it is time for a shower and bed right now...it's been a long day.

I believe Tim uses a Mac, so you better believe RockSim on Mac can be made to work.

I was going to suggest that you try the RockSim demo version to see if the advantages are worth the price, but you beat me to it. "Worth it" is an individual judgement call, but the advantages are undeniable.

Did you get a chance to check out the radial stability plot? RockSim will compute the CP relative to to different radial angles on attack and display the whole plot of static margin. I believe that the single margin number given is the minimum from any angle. You get it from the "Rocket" menu, "Stability Analysis."

The first rocket btook me about 5 minutes. Note the the stability is positive from some angles but not all.

The second took a bit longer since one has to use pods to put the anhedral on the wings. But it was still pretty quick. And now it's stable all the way around. (Confession: I had to move the fins and add nose weight. The nose weight I added, 30g, may have been enough by itself, or that might have been marginal.)

OK, enough schilling for RockSim. It certainly has its drawbacks. The decal ("texture" feature) and lighting for the renderings are a humungous PITA which is why I almost never use them. (But see my one and only post in the card stock subforum.) I don't know if OR is any better.

I have a few designs with partial tube fins, which are done by applying ring tails to pods an the surface so that a portion of the ring is buried within the body tube. RockSim renders that fine, but I don't know if the stability calculation (i.e. the CP calculation) correctly accounts for the fact that only a potion of each ring is actually in the air, so I don't trust the results.

In other words, it ain't perfect, but the pods do open up, not merely a few possibilities, but worlds of possibilities the OR doesn't support.

jqavins said:

I believe Tim uses a Mac, so you better believe RockSim on Mac can be made to work.

I was going to suggest that you try the RockSim demo version to see if the advantages are worth the price, but you beat me to it. "Worth it" is an individual judgement call, but the advantages are undeniable.

Did you get a chance to check out the radial stability plot? RockSim will compute the CP relative to to different radial angles on attack and display the whole plot of static margin. I believe that the single margin number given is the minimum from any angle. You get it from the "Rocket" menu, "Stability Analysis."

The first rocket btook me about 5 minutes. Note the the stability is positive from some angles but not all.
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The second took a bit longer since one has to use pods to put the anhedral on the wings. But it was still pretty quick. And now it's stable all the way around. (Confession: I had to move the fins and add nose weight. The nose weight I added, 30g, may have been enough by itself, or that might have been marginal.)
View attachment 296372View attachment 296374
OK, enough schilling for RockSim. It certainly has its drawbacks. The decal ("texture" feature) and lighting for the renderings are a humungous PITA which is why I almost never use them. (But see my one and only post in the card stock subforum.) I don't know if OR is any better.

I have a few designs with partial tube fins, which are done by applying ring tails to pods an the surface so that a portion of the ring is buried within the body tube. RockSim renders that fine, but I don't know if the stability calculation (i.e. the CP calculation) correctly accounts for the fact that only a potion of each ring is actually in the air, so I don't trust the results.

In other words, it ain't perfect, but the pods do open up, not merely a few possibilities, but worlds of possibilities the OR doesn't support.

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I wish that screen worked on Rocksim for me. Somewhere in the upgrades of the program, the computer, and all the other stuff, that screen stopped working. The plot appears to also appear on the base view of the design screen, but I don't fully trust the results.

I forgot to share this yesterday. I was just playing around with the second design, I haven't really changed it much. I am sorting through the MindSim optimization options right now, I'm seeing some sort of wingy thingy.

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More OR fun this evening!




The horizontal stabilizers are obviously in an impossible location, the plan would be for them to attach to the tube fin in the vertical stabilizer.

There are some things that I would want to do if I were to ever build this that are beyond my knowledge and abilities right now but here is the file for those that are interested.
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