TRF Summer Build Off: The Ellipse

Help Support The Rocketry Forum:

jqavins

Gone beardless since then
TRF Supporter
Am I the only one who's noticed that when two engine are in a cluster, the circle that encloses them has a lot of wasted space? I doubt it. Am I the only one who's thought that a body tube with an elliptical cross-section would contain a two engine cluster so much more efficiently? Or am I just the only one stupid enough to take the idea seriously?

I've worked out how I'll do it. I've figured out how to determine a safe value for the CP so the final swing test will go smoothly. The 3D printed nose cone and laser cut balsa parts are designed. An awesome paint scheme is... not coming to mind; I always have trouble with that part. The cluster will be two 24 x 95 mm, which provides me a pretty good range of total impulse options, from a pair of Estes C11's (21.8 Ns) to a pair of Aerotech F32's (153.8 Ns.)

Anyway, I'm typing on my tablet because my real computer is down at the moment, and may not be back up for a couple of days. I have the cone modeled in OpenSCAD, so I started to do the rest just to have some screenshots to post; I may have some pictures in a few days.

Last edited:

neil_w

TRF Supporter
That sounds great!

dhbarr

Amateur Professional
Yes! It seems to me that you could do a two-fin elliptical and it should be stable as a rock. Thank you for exploring this, so I can mark it off my "I wonder" list

jqavins

Gone beardless since then
TRF Supporter
Y'mean relying on the body width on its major axis and adding fins only to minor axis? I hadn't thought of that. No, I'm using four fins set at 45 degrees to the major and minor axes of the ellipse. Consider it step one, focusing only on the construction considerations. Other steps, like optimizing the fin configuration, can come later, whether I do it or someone else.

KenECoyote

Well-Known Member
TRF Supporter
Cool! The description has me picturing something like a space shuttle concept.

jqavins

Gone beardless since then
TRF Supporter
This first one will look a lot more like a "normal" rocket than that. Just imagine a 4FNC stretched sideways. But later models using the techniques I'm planning could do all sorts of things.

Oddrocs Rule!
TRF Supporter
Square tubes and now elliptical tubes! What is the world coming too?

rstaff3

Oddroc-eteer
The closest I've come is a truncated elliptical that originally housed a bottle of scotch...hic.

jqavins

Gone beardless since then
TRF Supporter
My computer's back up. Still a bit ill, but working well enough, sort of, pretty much.

Anyway, here are some pictures generated on OpenSCAD of the design as it stands.

The yellow nose cone will be 3D printed. The green fins will be laser cut from Balsa, unless I change my mind and use birch ply just so I don't have to worry about grain direction. The red is paper wrapped around a balsa interior; that will be made from laser cut ribs (some of which are also the centering rings) and and 1/16" square stringers.

I created two RockSim files, one with the body tube and nose as seen from the skinny side and another as seen from the wide side, and using the same fins. The CP is higher on the wide side version, just as the mind sim says it will be, so if I use that as the CP and make sure my CG is at least one wide view diameter forward thereof I should be stable. Also, I used regular elliptical fins in the sims, since swept ellipsese are more trouble to enter than they're worth, so there's a little extra margin there too. Of course I'll have to swing test it in both views before flying.

Laser cut and 3D printed parts are quoted and on order (though I haven't received confirmation from the vendor yet.) Assembly shouldn't take too long once the parts arrive.

neil_w

TRF Supporter
That looks cool. Are those fins perpendicular to the surface of the body, uh, "tube"? I'm staring at your pictures and going half blind and/or crazy trying to figure it out.

The swept-back fins should actually buy you quite a bit of margin, based on my past experimentation in OR.

jqavins

Gone beardless since then
TRF Supporter
Yes, they're perpendicular. Because of the nature of ellipses, there are two obvious ways of positioning the fins to be at a particular angle (45 deg.) and also perpendicular to the tube, and both of them are wrong. What I did was simply draw the ellipse in Visio, draw a square and rotate it to 45 deg., then position the square by eye to it's tangent to the ellipse. Those squares are some of the notches in the ribs that the stringers will lie in, so the fins will be attached directly over stringers; I might even scrape the paper away there and glue the fins directly to the stringers, then cover the resulting mess with fillets.

Oddrocs Rule!
TRF Supporter
Skin over frame, the way rockets should be built!

jqavins

Gone beardless since then
TRF Supporter
The laser cut and 3D printed parts arrived today.

The feqature inside the nose cone is a half loop that I plan to secure the shock chord to. More specifically, I should be able to loop a piece of spectra fishing line around it and secure the shock chord to that. I designed the hoop inside to that the base of the cone sits flat on the printer bed. The hole looks smaller now that the part is in hand; I may have to open it up file, but I'll try to get a line around it without doing that.

The holes in the aft pieces, obviously, are for the motor mounts. The holes in the forward pieces are for a parachute compartment (another bit of BT50.) That is there because I was concerned tat, if I brought the mid-section ribs all the way forward, the chute might snag on them. A tube seems more certain to give smooth deployment, at least in my head.

Assembly begins tonight. If I'm efficient (i.e. don't screw anything up) I may be able to show it off (unpainted, of course) at the National Maker Faire this weekend. (I only have two evenings because I'll be driving up to DC on Friday.)

rstaff3

Oddroc-eteer
Awesome looking parts!

Oddrocs Rule!
TRF Supporter
Like the old chicken nugget commercial: PARTS IS PARTS!

But this will turn out to be a real golden nugget!

neil_w

TRF Supporter
Very nice, great use of laser cutting and 3D printing. Should be a fun(ky) rocket when you're finished.

What material/resolution did you use for the nose, how thick are the walls, and what does the finished part weigh?

The parachute tube makes good sense... Is anything other than the skin gonna hold the ejection gasses between the mm and that tube?

jqavins

Gone beardless since then
TRF Supporter
Very nice, great use of laser cutting and 3D printing. Should be a fun(ky) rocket when you're finished.

What material/resolution did you use for the nose, how thick are the walls, and what does the finished part weigh?
The walls are 2.5 mm. The resolution and material, well, um... This is my first use of a printed part. I sent the design file (STL file exported from OpenSCAD) to a service and he printed it for me. It weighs 28 grams.

The parachute tube makes good sense... Is anything other than the skin gonna hold the ejection gasses between the mm and that tube?
It will just be the skin. I do plan to strengthen the skin by a soaking with thin CA. I think it should do.

So, the first evening's progress is not what I hoped. I don't know what I was thinking in the hobby store, but I only bought enough of the square stock for two of the eight stringers; I'll make a quick trip out in the morning for six more, and see how fat I can get tomorrow evening.

rstaff3

Oddroc-eteer
This could be kittable, but might be pretty costly for that w/ the 3-D print?

neil_w

TRF Supporter
The walls are 2.5 mm. The resolution and material, well, um... This is my first use of a printed part. I sent the design file (STL file exported from OpenSCAD) to a service and he printed it for me. It weighs 28 grams.
It's probably PLA or ABS, resolution could be anything. It's good to know the difference if you're going to glue to it and just in general for future reference. For instance, if you wanted to smooth the surface out, if it's ABS you could do it with acetone, but for PLA that won't work at all. You should just shoot him an email and find out, just so you know.

neil_w

TRF Supporter
This could be kittable, but might be pretty costly for that w/ the 3-D print?
True that nose cone is not cheap, but it's also not prohibitively expensive (I can only guess at the cost though). The rest of it is cheap though. Could indeed make an interesting and unique kit. Let's see how it comes out.

Amsterdam

__________
That is just awesome. Paper is definitely the best/easiest/strong choice for a skin.. But it would look incredible with something transparent. maybe doped tissue, or some kind of Mylar?

i agree with the kittable comments. Looking real sharp. Excited to hear how this flies.

jqavins

Gone beardless since then
TRF Supporter
I've also wondered about making a kit of it, but I'm afraid that's for someone else; I lack that sort of entrepenorial drive. (Of course, if someone who has the drive wants to compensate me for the design, I wouldn't say no.)

I bought two sets of the ribs, fins, and nose cone. All together that was about $30 to$35 and I probably bought too much balsa, so I shouldn't think it prohibitive at all. Of course, any kit would have to include the stringers, the three pieces BT50, a paracute and shock chort, launch lug, engine hooks, etc.

Thinking in terms of a kit, this is a first prototype and I already have a couple of things I would change. I have eight 1/16" stringers, and if I do it again I would probably use four 1/8". And I would either make the hole in the bottom of the nose larger or use a different attachment method altogether. And that's before I'm half done building.

rstaff3

Oddroc-eteer
Maybe you can get a rocket vendor to produce one. If it was me I'd give the cutting/printing files to someone, all they's have to do is give credit on the packaging. Been done before.

jqavins

Gone beardless since then
TRF Supporter
Yeah, I may do that. But after it's built and flown.

Of course.

jqavins

Gone beardless since then
TRF Supporter
The frame is assembled and ready for the paper skin. So far, the build quality isn't what I'd like, but paper should cover a multitude of sins. And I have a design change in mind that should help make the next generation easier.

Once I've finished my lunch I have to do more job hunting, but I'll try to get the skin on tonight.

jqavins

Gone beardless since then
TRF Supporter
Well, I may have bitten off more than I can chew with this build. The first layer of skin is on, and it looks like crap. I've never built a rocket this way, and the frame and skin airplanes I built were a good forty years ago, and it shows.

There's still hope. What I think I'll do is fill the worst low spots before applying a second paper layer. That layer will be going over a full surface, not just ribs and stringers, so I hope it will take the right shape better. Of course I'll stagger the seams.

It may be that my design ability has simply surpassed my construction skill. We shall see.

neil_w

TRF Supporter
What kind of paper is that? Perhaps a light card stock (like 65 lb) would hold its shape a bit better and resist the crinkling that you're getting there.

I wonder if there's any reason someone couldn't actually roll proper spiral tubes in this shape, given a proper mandrel. Not at all suggesting you do this, just pondering theoretically. Does a rolled tube have to be round?

Oddrocs Rule!
TRF Supporter
Give it lots of DOPE!

jqavins

Gone beardless since then
TRF Supporter
It's printer paper, 20 lb, which is a lot heavier than the tissue that's used in those airplane kits. And yes, there will be dope.

I'm not too upset. As long as it flies successfully and doesn't look like total **** I'll be content for a first attempt. And I'm pretty sure I can pull off enough of a fix to not look like total ****.