More Half-Baked Designs Thread

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rear eject...​
Moves the CG forward, doesn't it? Which would seem to be the opposite of what's needed. Unless...

Do you mean a rear eject spool that's the full length of the body tube, with enough weight at the front that its ejection moves the CG aft despite the motor weight also being ejected?
 
Moves the CG forward, doesn't it? Which would seem to be the opposite of what's needed. Unless...

Do you mean a rear eject spool that's the full length of the body tube, with enough weight at the front that its ejection moves the CG aft despite the motor weight also being ejected?
A rear eject spool keeps the nose cone in place after ejection. Working out the details, is part of the design.​
 
Moves the CG forward, doesn't it? Which would seem to be the opposite of what's needed. Unless...

Do you mean a rear eject spool that's the full length of the body tube, with enough weight at the front that its ejection moves the CG aft despite the motor weight also being ejected?
See @hcmbanjo 's build of the Estes Skydart II to see exactly this technique in action. This post in particular shows a picture of the full-length pop-pod with weights up front: https://modelrocketbuilding.blogspot.com/2017/03/estes-skydart-ii-3229-build-step-22-pop.html
 
Moves the CG forward, doesn't it? Which would seem to be the opposite of what's needed. Unless...

Do you mean a rear eject spool that's the full length of the body tube, with enough weight at the front that its ejection moves the CG aft despite the motor weight also being ejected?
If you use an extended motor mount tube that goes all the way up to and ideally even into the nose itself, you can put a heft ballast weight on the forward end of the pop pod. Nose cone is glued or taped in permanently. That gives you very forward center of gravity on boost that is ejected as part of pop pod. Works extraordinarily well.
 
If you use an extended motor mount tube that goes all the way up to and ideally even into the nose itself, you can put a heft ballast weight on the forward end of the pop pod. Nose cone is glued or taped in permanently. That gives you very forward center of gravity on boost that is ejected as part of pop pod. Works extraordinarily well.

Sounds very similar to the Apogee Darkbird (and probably others as well). It was a bit tricky to get trimmed out, but we've had some really great flights with it. The rear eject pops the motor on a streamer and simultaneously releases the elevons to kick up slightly.

https://www.apogeerockets.com/Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/SR-72-Darkbird
 
Sounds very similar to the Apogee Darkbird (and probably others as well). It was a bit tricky to get trimmed out, but we've had some really great flights with it. The rear eject pops the motor on a streamer and simultaneously releases the elevons to kick up slightly.

https://www.apogeerockets.com/Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/SR-72-Darkbird
The DarkBird was where I stole the idea.

I modified it with the SR-73 Raven, cut the base off the nose cone, and ran the mount all the way up onto the cone.

Also figured out how to do a pop pod with a minimum diameter rocket, which was fun.

https://www.rocketryforum.com/threa...w-sr-73-raven-glider-and-new-gyskelion.38883/
 
I've had a number of rocket designs named Hotblack after the Hotblack Desiato sunship in Hitchhiker's Guide. But none of them have quite made it to fruition. This is my latest attempt. We don't have much description of the ship in the text. It's of a "simple, classic design ... very clean, very sleek." It's black, and it looks fast. With that design brief...

1719279531690.png

1719279565999.png

1719279832911.png

I'm looking at a 24mm cluster, light enough to fly on D12s, but strong enough to fly on CTI 2 or 3 grain motors. I'm thinking I'll cant the motors and fins a bit for a nice spin on ascent. The tailcone will be 3D printed, so I have a lot of design freedom there. I think the main fins are pretty good, but the T-wings need a little work. And of course I'll need to check out stability before going much further down the road. The nose cone will be something pointy, but isn't all that important in the grander scheme of things. My initial thought was to use BT-80 tube, but after looking this over I might want to switch up to 3" to give a more pronounced tailcone. I'm more or less limited to 1000' of altitude, so it won't hurt to be a little lower and slower.

The hardest part may be getting a good enough surface finish to justify the name.
 
In general, I often like the idea of T-wings imitating the fins they're attached to. Try copying the fins and scaling them down so the root matches the tip pf the main fin where they are attached. Two on each side, obviously.
 
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Nice, I like it, T fins might need to swoop back to more of a point a little more crescent shaped. I will admit that I do really like the double T tail configuration.
 
Nice, I like it, T fins might need to swoop back to more of a point a little more crescent shaped. I will admit that I do really like the double T tail configuration.
I didn't really know if it would work until one of my TARC students built a somewhat similar design for a competition we run. The rules for that require a prime number of fins, so most people go for 3, with the occasional 5 or 11. This year was the first year with 2. Since it flew well, I decided to run with the concept.

I'll try the @jqavins idea of mirroring the main fins first, but I'm also thinking that the T fins could be shaped like a guitar pick, perhaps built from some of the curves in the main fin. I don't really have a sense of how big the main or T fins need to be, which is a minor issue in working the aesthetics.
 
The T-wings don’t have to be flat on top. Angling dorsal, ventral, or both inward or outward (or dorsal in, ventral out, etc) might give you some variety,

The dorsal and ventral components also don’t HAVE to be symmetrical in size, shape, or sweep.

For durability, assuming you have the stability for it, having the outboard trailing edges forward of the motor nozzle may reduce fin breakage on recovery.

I’ve always wondered why the SR-71 dorsals weren’t perfectly vertical, but it adds to the cool factor.
 
Those fins look very small to me, purely with regard to appearance, no idea about stability. I'd like to see a render of the whole rocket to get an idea how it all comes together. You can model it easily enough in OR if you just approximate the tail "thing" as a tube for now.
 
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I agree that we need to see the whole thing. The T-fins may look small to me, but I can't tell without more scale; if the rocket is short then those are probably aesthetically fine, and if it's long then they are probably fine for stability. :headspinning: :dontknow:

I meant to say before that I Love That Tail! But I wouldn't model it as a tube. Boat tails hurt stability, so I'd model is as a regular boat tail with the aft diameter equal to... something. Maybe the aft end width? Probably overly conservative. Maybe to match the aft end area? Maybe average the aft end width and length? I have no idea. But some kind of boat tail.
 
I agree that we need to see the whole thing. The T-fins may look small to me, but I can't tell without more scale; if the rocket is short then those are probably aesthetically fine, and if it's long then they are probably fine for stability. :headspinning: :dontknow:

I meant to say before that I Love That Tail! But I wouldn't model it as a tube. Boat tails hurt stability, so I'd model is as a regular boat tail with the aft diameter equal to... something. Maybe the aft end width? Probably overly conservative. Maybe to match the aft end area? Maybe average the aft end width and length? I have no idea. But some kind of boat tail.
Next stop is probably an OR model to get fin and body proportions down for stability. The 3D printed tailcone will be moderately heavy, so I'll need to leave quite a bit of margin there. I was planning on modeling the tail as a boat tail. I've had pretty good luck with modeling by equivalent area, so figuring the area of the base and converting to a radius seems reasonably defensible from an engineering standpoint. At least I don't need the base drag hack [runs for cover].
 
I’ve always wondered why the SR-71 dorsals weren’t perfectly vertical, but it adds to the cool factor.
'Twas an early example of radar stealth, along with the inclusion of "chines" (which initially unintentionally but fortuitously also helped with wing lift characteristics). The edges of the wings also had internal wedge-shaped features for radar absorption.
 
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Those fins look very small to me, purely with regard to appearance, no idea about stability. I'd like to see a render of the whole rocket to get an idea how it all comes together. You can model it easily enough in OR if you just approximate the tail "thing" as a tube for now.
The fins were /definitely/ too small. They're still too small, but at least they're closer.
I agree that we need to see the whole thing. The T-fins may look small to me, but I can't tell without more scale; if the rocket is short then those are probably aesthetically fine, and if it's long then they are probably fine for stability. :headspinning: :dontknow:
I was going to say that I was going to try to keep the main body tube at 24" long for storage, but then I looked at my storage shelves again and it'll be fine a little bigger. It'll just go in a different place.
I meant to say before that I Love That Tail! But I wouldn't model it as a tube. Boat tails hurt stability, so I'd model is as a regular boat tail with the aft diameter equal to... something. Maybe the aft end width? Probably overly conservative. Maybe to match the aft end area? Maybe average the aft end width and length? I have no idea. But some kind of boat tail.
I was definitely planning on modeling it as a boat tail rather than a tube once I got to OR. I've had fairly good luck with using equivalent frontal area for my various non-round rockets, so I made the end of the boat tail a diameter that gave an equivalent total aft end area. The fins are just standins for now--it's a lot easier to tweak trapezoidal fins about than to mess with freeform. Once I have the right approximate area and center, I'll start fooling around with making them look good. I'm also not sure why the pod fins are throwing a "thick fin" error--they're a pretty normal thickness.
1720487202582.png

One thing I notice right away is that I'm not going to be able to fly these on 24mm F motors--it's too much thrust for LPR tubes (23G!) and way too high for my home flying field. That's OK, we back off down to D12s and end up with an apogee around 500 feet. I still don't have enough stability, even with a hefty chonk of nose weight, so the fins need to grow some more.

If I make the body tube 24" long as originally intended, it looks more like this, though now the fins /really/ need to grow.
1720487518547.png
The perspective isn't exactly the same, but I tried to make the body tubes the same size by scaling the pictures.

After looking at both, I'm leaning back toward the smaller version, particularly since the original Douglas Adams description was that the sun ship wasn't very large.
 
Is your phantom body tube perchance not exactly 0 diameter?
They had a small positive value. After fixing making the pod fins 2mm thick, I checked and get the warning when the pod diameter is ~0.75cm or less. I've re-set them to the pod fin thickness so they don't mess with drag much.

Thanks!
 
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The perspective isn't exactly the same, but I tried to make the body tubes the same size by scaling the pictures.

After looking at both, I'm leaning back toward the smaller version, particularly since the original Douglas Adams description was that the sun ship wasn't very large.
I like this one better too. My mind sim says the main fins are a good size, but the winglets need to grow. My knd sim has no points of comparison for winglets, but my gut feel says a little bit, if that's really where it's needed, just might go a long way. Try growing them by 10% in length and width, so 21% in area, and see how far that gets you. I'll curious to see if my gut ks all wet.
 
As I'm fooling about with this design, I'm discovering that I have half of the information I need. Is it possible for OR to display the CP separately in the Y and Z directions? (assuming X is along centerline) I'm pushing and pulling on the two different fin sets but I'm not sure where my limiting stability is.

If not, I can temporarily delete the pod fins and make the main fins a 4-fin set to get stability set in that direction, then revert back to 2 fins and re-do the pod fins.
 
No. Well, yes and no. But no.

The CP and CG will always be computed and shown along the center line. But, if I understand what you really need, there is a different way. (Disclaimer: I'm 90% sire OR does this, but I'm an RS user.)

The CP is computed relative to a sweep of wind angles around the rocket. Imagine you're using the cardboard cutout method. Now imagine rotating the rocket around its centerline so the silhouette is changing. Compute the CP as the rocket rotates, every - I don't know, let's say - two degrees. The CP shown on the rocket overview screen is the worst case of all those 180 cutouts. Of course, the computation has all the superior accuracy you'd expect, when it's computed in all 180 positions. This much I'm sure OR does. Usually, knowing that you're being told the worst case is all you need.

But, you can also get a polar coordinate plot of the static margin vs. the hypothetical angle of projection of the silhouette. With that, you can tweek asymmetric features to keep the margin at its highest from getting too great, i.e. too far above the worst case, or to improve matters in the worst direction without going overboard and ruining your design concept elsewhere.
 
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