3d printed rocket.. help?!

Hi all
Im new to the Association and have been playing with rockets on and off over the years, just for fun mostly. I am in the midst of designing a rocket to be 3d printed in a flexible plastic. Looking at the attachment does this rocket look flyable?
I wasn't intending to have any extra tubing on the rocket and my purpose in creating it was to make as smaller rocket as i could. I did make a rocket once that was very similar in design and it flew brilliantly, with a short body and long fins, true the body was slightly longer, the fins were longer and it had three fins instead of four but it seems similar enough to me.
I tested the rockets parameters in the nasa rocket modeler simulater and it flew slightly further than a standard rocket design.
The guide tube is 4mm wide in the hole and i thought it might be a bit small if i made it smaller, i do have concerns of the change of flight path due air friction on the guide tube, more than the length of the tube , i usually use drinking straws.
The rocket will be loaded with a small bit of wadding to stop the rockets backfire from burning the inside of the cone or body and will also have a primitive parachute or ribbon to slow the decent.
my main concern is the rocket flipping due to incorrect center of gravity. as of the danger of the first flight i will be launching it at the beach, away from everyone.. also the solid engine heating and fusing to the plastic is a problem. there should be enough of a lip at the top to prevent the engine flying straight out the top of the rocket

Can anyone give me some feedback on this rocket design? and any changes i can make to have it perform better while still keeping the rocket similar to my design

oh i forgot to mention it takes a C sized engine.

With the motor near the front and the fins extending back beyond the motor, you've ensured that most of the weight will be forward while most of the fin area will be in the back of the rocket. That's good! The Center of Gravity (CG, the center of mass) of the rocket will be ahead of its Center of Pressure (CP, the center of the rocket's area). When the CG is ahead of the CP it generally means the rocket will be stable.

Your design reminds me of the Estes Scout - one of the first model rockets. The Scout used a neat method of recovery. It had a long engine hook (actually a piece of wire). When the ejection charge fired, it caused the motor to shift back to the far end of the hook so that the motor ended up sticking out the back of the rocket. This made the rocket unstable so it tumbled down. Your design may lend itself to the same type of recovery.

Oh, btw, that design looks really cool!

Please keep us updated. Your project is interesting.

-- Roger

thanks for feedback Jadebox.
I had a look at the scout and it is quite similar and i have made one in the past with long fiins and it did really well, although they can be hard to spot. Im having concerns about the fins breaking on landing if it comes down fin first.
also the rocket will cost about $10 to produce (does that sound expensive?).

if anyone knows of aerodynamic improvements i would be happy to hear, things like shape of nosecone, drag, fins, and weather launch tube will effect flight.

also is there any good aerodynamics simulators available for DL

I really like the design. The open airframe is unique.

Regarding price, $10 to produce would mean a $20 to $25 sale price which seems a bit steep for such a small rocket.

Google OpenRocket to find a simulator for download.

sounds cool, just DO NOT use a Quest C engine... it will melt the plasitc and get the motor stuck inside

have you tried open rocket..it's free..it's a design and simulator program:caffeine:

Leoski,

It is nice looking rocket and I'll be interested in your results. I am concerned about the plastic being described as "flexible". Typically, the materials used for model rockets are more rigid than flexible. Even the rigid materials will flutter with the right forces and frequencies applied but if your rocket is "soft" sitting on the pad, it may not be able to remain stable at speed. It could be your 3D printer's material is fine; the professionally produced SLA models I use at work are quite rigid but cost much more than $10 to produce.

Also, if you are worried about the launch lug (guide tube) throwing off the stability, balance it out with one between each pair of fins. It will add drag to the design but it will eliminate the influence on the path. If this is a Scout-sized rocket, then the extra drag won't hurt - a Scout will go out of sight on a 1/2A6-2 motor so a C will push this through the air and out of sight.

Try Open Rocket and let us know what happens. If your beach launch results in a splashdown landing, get the motor out as soon as you can so the swelling cardboard doesn't damage the plastic.

Kenn

Leoski,

I like the concept. I've been telling my friends that I've just got to get one of the coolest things around, a 3D printer. You've just shown me the overlap with my rocketry hobby, so I can see one in my future. Any way to make it overlap with my beer making hobby, so that there's just no way that I can live without a 3D printer?

As for your rocket, you asked for some ideas so I'll offer suggestions.

I notice that the launch lug is held on struts between the fins. Maybe you could move it to be flush with a fin for greater contact and strength. Maybe to be really cool (at least as I see it in my head), you could flare the base of a fin to be wide, and leave a channel in the fin large enough for the lauch rod.

I'm not seeing a recovery technique. Maybe the scout method that was mentioned, or maybe extend the upper tube by another 1" to 1.5" to accommodate a streamer or thin mylar parachute?

I'm not familiar with the properties of the plastic you are using, so I'm kinda guessing, but I think I see a weakness in where the fins mount to the strips that serve as the body tube. Think of the long fins as levers attached to a thin strip of plastic. Pushing on the tip of the fin inward toward the engine will exert a pretty high torque on the strip of plastic, possibly snapping the strip of plastic on a hard landing. This could be strengthened in an attractive way without needing to convert the strips into a fully cylindrical body tube. For instance, the base of the fin, where it connects to the body strip, could be extended like a stringer up and down the body tube. If you look at a picture of the fins on a Taser or on a QCC Explorer, you could see my idea. If you have a chance to experiment with several versions of your design, consider side torque as well by pushing the fins left and right, and confirm that the body strip doesn't twist too much either.

Other than those possible ideas based on what I don't know about the material you are using, I really like the idea.

PS. I reread your original comment, and I thought about motor retention. Maybe you could consider some kind of twisting lock to hold in the engine. Like on may Estes rockets, but with a stronger mechanism. And, just to be really exotic, maybe make an inner sleeve that slides over the motor and into the rocket. It that fuses to the motor because of heat, it might still come back out to be replaced with another sleeve that you made.

Leoski-

Very cool idea. I have worked with 3d printers - they open some interesting opportunities that you have started to exploit. Here are a couple additional suggestions.....

1) The layered plastic I've used is significantly stronger in the direction 90 degrees to the layers, and thin pieces tend to break cleanly at the layers when flexed - sort of similar to wood grain in balsa. Since it looks like you are making the entire body in one shot, if there is enough room in your printer to turn the model to have a horizontal axis while it is being made, then your fins will be much stronger and less likely to snap upon landing. You could (probably should) add in some more fillet material between the "tube" and the "fins" - we normally build those areas up for strength with glue - but you could just add more plastic there.

2) Other ideas to strengthen your design: make the rectangular "window" openings in your "tube" have generous fillets at the corners for added strength. Also - instead of flat fins, you could add some external ribs to the sides of your fins - that would look interesting, but would also give them some more flexural strength. You could also "carve" the name of your rocket as an opening through one or more of the fins - that would look cool too and would be really difficult to do in "normal" fin construction.

2) You can do some things with a 3d printer that are nearly impossible with other materials - I'd move the launch lug to be in the center of one of your fins for example. Another idea - add some "lattice" gantry work in the windows - maybe a chain-link fence like design. What about making the fins each have a curve in them when viewed from above/below. They would still cause the rocket to fly straight up but would be very unique - you could also add "cant" to the fins like the "texas twister" style rockets so that your rocket would spin during launch, but I think just slightly arced fins would be interesting.

3) you could also use this modelling technique to add in a "whistle" to each fin - look at the shape of a standard sports whistle - air blows rapidly across an opening and gets diverted into a second airstream to make an audible whistle - your air stream would be provided by the air that you were passing

4) If you wanted to sell rockets, I think you would want to incorporate features into them that were unique to your method of manufacturing - to have the most interest / audience. (Reminds me of the square tube rockets made by Newwayspacemodels a few years ago) As I am sure many on this forum can tell you, your chances of making much (any?) money on rocket business is pretty small - but if you can do things that are fun and end up ahead of break-even theres a lot to be said for it.

Kerry

KerryQuinn said:

thin pieces tend to break cleanly at the layers when flexed - sort of similar to wood grain in balsa.

Click to expand...

Kerry, for my edification, is it possible with a 3D printer to print diagonally in the vertical direction? That is, when the fins are printed, have the printing build up and inward toward the center of the tube, specifically to match the effect of balsa grain, and how we orient it for greater strength?

BTW, if that's an XWing Goony as your avatar, I love it. My X-wings are nearing the top of my queue (sometime this year, I think), and I'll have to goony them too.

So, I'm assuming from your calling it 'flexible plastic' you are buying this through shape-ways or the like?

I have serious doubts about the fins staying on the rocket. there is very little to prevent the plastic from flexing in the very fast air-stream given how little material is in the design.

wow some great ideas guys.
yes i will be getting it printed at shapways, using the strong and flexible material, which is quite strong and flexes without breaking quite well. Iam not too concerned with the fins breaking during launch because of the print process doesn't leave a weak point at the fin join. As suggested i moved the launch tube into the wing and am experimenting with different landing ideas.
the idea behind this rocket was to have is at compact, minimal, streamlined and easy to launch as possible (no gluing or build). I was not really thinking of selling them, though if i can keep the price low and get interest that could change, i may be able to get the price to $5 a print if i print a few (over 10 per print).
One landing idea is to have propeller like blades spring out on backfire as i would rather in lands nose first ( i dont like the idea of the fins hitting the ground first)

It's called flutter. Fins with a lot of flex will wobble a lot (like a flag in the wind) and often rip off the rocket mid flight. Your fins are very large and have very little attachment strength, so are particularly susceptible.

It's usually only something we think about in big fast rockets, but might be a problem here because of how sparse you are trying to go with the design.

I could be wrong here, but my gut is telling me this is a very questionable design, structurally speaking.

Thanks Landru.
I see wheat you mean, i had thought the long straight section of the fins would wobble, the plastic "should" hold-up as far as the join at the wing to the body goes but the long fins fluttering cause undesirable effects on the flight (or could just snap)... is there a ratio between the length and flexibility of the fins and the amount of flutter? due to the creation process the wings are perfectly aligned and bladed, to cut through the air with minimum resistance so im hoping the flutter will be reduced.. could you shed some light on the whole flutter thing.. is flutter caused by the rocket racing through pockets of different air density? or by imperfections in the rocket its self?, vibrations maybe?.. i can imagine dragging a flag through the air and seeing the flutter.. hmmmm.. would the windows in the rocket, where u can see the engine create a air vortex ( like in the back of a ute or pickup in america.. im in Australia) and cause the amount of air going over the fins to be reduced? (or am i dreaming?)

on another note, can anyone tell me what nose cone shape will cut through the air with least resistance?

I dont get on option of the orientation the rockets gets printed in, though im not sure it will make any difference, besides for a slight directional grain on the exterior the print process shouldn't be weaker like balsa with pockets of less density.

KerryQuinn, thanks for some great ideas... I may have to strengthen the wings slightly and the whistle idea is cool

any other ideas how i could reduce the overall materials while keeping the rocket strong and to its original intention?

leoski said:

Any other ideas how i could reduce the overall materials while keeping the rocket strong and to its original intention?

Click to expand...

I'm not sure it really needs to be stronger. You might make a prototype. If it seems weak in a specific place, you can look at make that spot stronger.

Having said that, I wonder if you can take advantage of the process you're using to create kight, but rigid structures. Someone mentioned adding ribs on the outside of fins. Another thing would be to make the fins more complicated than a flat sheet. You could create an airfoil shaped hollow fin with some internal structure. I don't know how precise the machine you use is, but if you might be able to create a built-up fin that was actually lighter, but still stronger than a flat fin.

Did that make any sense?

-- Roger

the hollow fin idea would be great for a bigger rocket, but i have reached the limitations of the printing process already.. minimum wall thickness is .7mm, and all my walls are 1mm (didnt want to go to .7mm because of the weakness)
The fins are 1.5mm at the top and taper to 1mm towards the bottom currently (sort of airfloil).
in theory, the rough texture created by the print process should act like the dimples on a golf ball and make a small pocket of air surrounding the entire rocket and reduce friction (so after the test flight i may be able to reduce the materials even more)

leoski said:

One landing idea is to have propeller like blades spring out on backfire as i would rather in lands nose first

Click to expand...

A quick-and-dirty method to land nose first, without adding rear ejection, is to fasten a string to the bottom of the rocket on the outside, and tuck it into the rocket by the nose cone. This string is where the shock cord is attached, which is then attached to the nose cone and streamer or parachute.

Not real pretty, but is effective, and avoids the mass and engineering of the propeller blades.

I had thought of that david, but your right it might be a bit rough for such a precision build rocket, not to say i wont end up doing that, but we'll see

I put the rocket through openrocket using a C6-7 engine and the results as follows..

ps. having not used openrocket before and not knowing how a standard rocket would perform i cant say weather it performed well, can anyone tell me how it went?.. the last simulation is closest to my actual design

I'm not concerned about the fins themselves but where they meet the body tube. Because of the "windows" you have cut out there is precious little from keeping the fin oriented 90deg from the body tube. There will certainly be some forces acting in that direction, too. 1mm thickness might not be a problem, looked a little thinner from the image.

Flutter can be 'triggered' by a lot of things, from vortex shedding frequencies to just general flight correction (forces imparted on the fin as it keeps the rocket straight)

https://www.info-central.org/?article=138

Anyway, you really should just get one made and see how it goes. That's the best part of rapid prototyping: being able to iterate designs really quickly for a relatively small cost.

Leoski, the string down the side is what I'm considering for my cardmodel build of the 2001 Space Odyssey ship, the Discovery. (See the build in the cardmodel thread.) My build is putting it about 9 feet long, and being card model, it is fragile. I want it to come back down and land horizontally instead of vertically, so I think that I'll run a string to the base and to the front, and fasten the midpoint to a parachute.

After you do your first test, if those slats down the side are too succeptible to torque effects and break, like Landru has mentioned for flutter, and I've mentioned for impact, you might consider making the gaps be 8 diamonds, or some other shape, instead. That way, you could fabricate a ring all the way around the middle, right in the center of the fins. That counterforce would be more directly applied to the fin, instead of some distance away.

BTW, you asked about the best shape. Here's the wiki entry. It says "For aircraft and rockets, below Mach .8, the nose pressure drag is essentially zero for all shapes. The major significant factor is friction drag, which is largely dependent upon the wetted area, the surface smoothness of that area, and the presence of any discontinuities in the shape. For example, in strictly subsonic rockets a short, blunt, smooth elliptical shape is usually best." So, you've got plenty of options since you won't be approaching the sound barrier.

as far as i can tell from my short amount of research into flutter, if i reinforce the body it wont stop the flutter but it will reduce the chance of the fins doing damage to the body or tearing off, if i thicken the wings by double it will reduce the flutter by 8 times.. so if i notice flutter or a fin just tears off then i will address the issue, but if i reinforce the body now i may not be able to tell if there is flutter in the first place and my model will not fly at its optimal as it could, if that makes sence...if i have no problems with destroying fins or flutter i may make fins thinner and body... i wonder what the limit for minimal materials is, how low can i go.

leoski said:

i wonder what the limit for minimal materials is, how low can i go.

Click to expand...

A dozen rockets or so, a pencil and graph paper, and soon you'll no longer be wondering. You'll know!

latest openrocket test... this is pretty close to actual model.

says stability is 1.01 is that ok, i know if its under one its bad, so how does mine sound?

DavidQ (and others wondering)...

Think of a 3D printer like a 2D X-Y old pen-plotter that "draws" your object one XY layer at a time. It draws with a thin molten plastic bead that snakes back-and-fourth row by row by row, Layers are typically around 0.005"-0.015" thick or so depending on the plastic and the printer. Once that entire layer is "drawn" the table supporting the layer you just drew drops down by 0.015" and the process repeats - this time drawing the next layer up from the one you just completed. Since the entire layer has cooled and solidified by the time you come back with the next layer, there is usually less bonding, and therefore less strength between these layers than there is between the side-by-side molten beads drawn within the same layer. Its not possible to draw diagonally (not at least on any 3d printer i'm aware of).

Leoski -
Flutter comes from un-even shedding of air "vorticies" interacting with thin flexible parts of a rocket (or plane). These instabilities are always there, even on shorter and stiffer structures, but its only a problem when the frequency that the vorticies are generated is around the same frequency as the natural vibrational frequency of the structure itself. As your structure gets longer and skinnier and/or made from more flexible materials, the natural vibration frequency of the structure goes down and you can run into problems. That said, there are a few things you can do to help reduce flutter (these are true for all fins, not just 3d printed plastic ones).... make the fins stiffer (as you said), use stiff materials - I'm not sure which material you are using - but it is possible to 3d print using materials like ABS plastic, which is actually significantly stiffer than balsa so it would be LESS likely to flutter. You can also add external ribs as I mentioned earlier - but another thing you could do is sand the leading and trailing edges to be more airfoiled as this will make the vorticies themselves both smaller, and occur at a higher frequency.

I wouldn't be afraid to go min wall thickness (0.7mm) on a hollow airfoil shaped fin - I think that would be strong AND be very cool looking. on the main vertical section of the tube-with-cutouts, I'd probably go thicker - maybe 1.5-2.0mm, but most of all, I'd just pick one and get it printed - take a bunch of pictures ahead of time and then go and launch it (away from people, buildings and cars just in case) and then come back and write it up here on TRF - it is a cool project you are working on.

As for your OpenRocket result, we can't tell much from the tables you posted. If you post your .ORK file and/or screen shots showing plots of the altitude, pitch and yaw vertical velocity and stability vs time for launch with 10-15mph wind people here would be better able to comment on your flight stability and suggestions for change.

As for your nose cone, the shape wont matter much as long as it is pointy. Make sure you print it HOLLOW and then fill it with clay or something heavier.

Good luck
Kerry

DavidQ-
Yes, my avatar IS a goony X-wing - and the main body "tube" is a square tube from newway spacemodeling, so I made the fins and the "engines" square as well. There are better pictures of it and some other rockets I made from square tubes here:

https://www.newwayspacemodels.com/index.php?option=com_content&view=article&id=67&Itemid=72

It flies great on a D12-6. Glad you like it.

-Kerry

finally got around to getting it printed, will launch it any day now

That is so cool! It looks really good! I'm glad that you were able to finally complete your project. Have you launched it yet? If not, be sure when you do to wrap the parachute/streamer in a piece of wadding so it won't melt. Looking at some of your previous posts, I saw that OpenRocket predicted over 2000ft! :y:
Let us know how it goes!

This looks really neat! Post pictures of the flight and maybe some data if someone is tracking it.

If this flies well and you decide to mass produce them, you'll make a lot of money! I'd buy a couple for sure. You probably can't 3D print enough for mass production, but I'm sure a plastics company could create molds and produce them by the truck loads.

Good work!

Man, you could almost injection mold that thing! That could be a 1 dollar rocket if you wanted to invest the time and money.

It looks so wrong in so many ways like the super thin engine retention ring, and the potentially melted streamer, and the crazy fin ribs, and the brittle 3d print material, but I really hope it all works out!

I would start with an A engine or something, that way you have a higher chance of retrieving it. If you are trying to evaluate the design for future use it's hard to evaluate when you can't find it after flight. If the first one works then launch it to the moon.