Moonliner (3D Printed Parts + BT-80)

[BigMacDaddy]

I posted a teaser in the "What I did rocket-wise" thread but figured I should create a separate thread. I wanted to try my hand at creating a Moonliner [Inspired] TWA / Douglas Model Rocket. I am hoping that there is enough drag from the rear struts that this can launch without clear fin inserts.

Here are some pictures of the inspiration.

I decided to focus on the original 1955 version of this rocket as there have been a few different versions throughout the years...


I have been thinking about this for quite a while but started working on this more seriously a few months ago. I looked at a number of existing 3D designs but did not like the struts or other elements so decided to redesign this from scratch (I guess I always wind up doing this when making model rockets but rarely go all the way back to engineering drawings). Struggled for a while with this but I was finally able to get the necessary functions / features to work that allowed my to create the struts based on these engineering drawings.



This past weekend I printed and test assembled my first full prototype (and then immediately got to work on revisions).


Checked my version against the originals in photos (I knew I had made my body tube around 5cm longer than the real thing suggested but still wanted to check look and feel).



Next revision is coming now. First, I modified the nosecone and boat tail to combine a cone and tangent ogive transition portion since that seemed to more closely resemble prototype. I also simplified the boat tail and struts to reduce weight. I had a thought to make the struts foldable but that complexity just adds unnecessary weight given the goal of launching this model on smaller engines (E or maybe D). I am thinking that I (and others) can paint the slots onto the boat tail and body tube since it was going to add a bunch of weight if I build them in w/ 3D printed parts. Finally, tweaked a number of design details (e.g., canopy position as well as windows/porthole locations) and engineered parts to make assembly easier.


I will share swing test results as well as launch when it happens. Very open to feedback on design elements.

 

[Cape Byron]

Very open to feedback on design elements.

Black pinstripe tape will work for ‘faux slots’.

Beautiful work, as always.

 

[Scott_650]

Even if the landing leg strut “fairings” need enlarged to keep a good margin of stability this looks like a great rocket! What’s the current overall length of the prototype?

 

[BigMacDaddy]

Even if the landing leg strut “fairings” need enlarged to keep a good margin of stability this looks like a great rocket! What’s the current overall length of the prototype?

Thank you! The prototype shown is around 25" tall.

 

[BigMacDaddy]

If it turns out to be unstable I think it would be pretty easy to put some thin clear inserts inside the strut area to turn them into fins. I can readily make an adapter that snaps between the two strut supports to hold the PC sheet and can slot the underside of the struts.

The struts are around 20mm wide at the base and the shape is very draggy so not sure that will be needed.

Swing test as soon as my next version is done printing and I can assemble it.

 

[neil_w]

Cool.

I think that on this particular model, clear inserts would be extremely innocuous, if needed for stability. Since they're completely bordered.

Also: I think the transition at the base of the nose should be a lot more gradual than the way you have it in your second prototype there.

 

[BigMacDaddy]

Cool.

I think that on this particular model, clear inserts would be extremely innocuous, if needed for stability. Since they're completely bordered.

Also: I think the transition at the base of the nose should be a lot more gradual than the way you have it in your second prototype there.

Thank you for feedback... Not sure why the transition looks so much more gradual on the BT than on the NC. Only thing I can think of is I stretched the BT some to allow for the motor mount. Will play around with it.

 

[KenECoyote]

Cool.

I think that on this particular model, clear inserts would be extremely innocuous, if needed for stability. Since they're completely bordered.

Agreed too!

 

[BigMacDaddy]

Fresh off the swing test I am really pleased to report that this looks stable. CG was just in front of the struts for this test. No motor, glue, recovery system, etc... and 191g so far (just an order of magnitude sanity check). I think there should be plenty of room to get the final version of this flying on an E12-4/6 (and probably on a D12-3/5).

Ah, I am making the nose tip swappable so its possible to use the long narrow one for display and a more durable stubby one (like I have on the rocket here) for launching. I was pretty skeptical that the skinny sharp nose would survive sticking down into the mud (or even worse hitting frozen ground at a normal decent rate). The stubby one makes the nose look a bit like an X-15.

I am also a little worried about the struts on landing but I designed this for durability as much as I tried to keep weight down. For example, the struts have printed tabs that insert into slots in the shoulder of the boat tail and the wooden skewers insert a good distance into both the strut and the boat tail. Also I printed a couple of solid layers in the middle of each strut that gets a coating of ABS glue sandwiched between the two struts. My earlier version had no bottom layer when I printed the two halves but I thought the two halves did not glue together strongly enough. That middle layer here should also give some good structural support to the struts (particularly when combined with the 15% infill throughout).

 

[techrat]

HA! I was just thinking about this as a scratch build, figuring, like you, that the extended struts would act as a fins, and you could get away with it as-is if built correctly, but it looks like you beat me to it.

 

[BigMacDaddy]

Cool.

I think that on this particular model, clear inserts would be extremely innocuous, if needed for stability. Since they're completely bordered.

Also: I think the transition at the base of the nose should be a lot more gradual than the way you have it in your second prototype there.

Hope the lines look better on the new nosecone (from the swing test).

 

[BigMacDaddy]

HA! I was just thinking about this as a scratch build, figuring, like you, that the extended struts would act as a fins, and you could get away with it as-is if built correctly, but it looks like you beat me to it.

Cool thanks -- great minds or something like that...

I was not planning to be at Metra -- and maybe I can find someone who is going out there to bring some \

 

[BigMacDaddy]

These are not the most efficient fins ever -- 6 hours to print just the struts... Definitely faster to cut plywood fins

 

[Cape Byron]

These are not the most efficient fins ever -- 6 hours to print just the struts... Definitely faster to cut plywood fins

But they look great, and that’s what matters.

 

[jqavins]

I think that on this particular model, clear inserts would be extremely innocuous, if needed for stability. Since they're completely bordered.

And the bordering members are wide, i.e. big in the thickness direction of the clear panel, which would help a lot to hide the panels, if panels should be needed. The only reason I didn't post this thought sooner is that I wanted to read down to see if someone beat me to it. And now that I think about it, panels would also strengthen the whole strut-leg assembly.

Also: I think the transition at the base of the nose should be a lot more gradual than the way you have it in your second prototype there.

Hope the lines look better on the new nosecone (from the swing test).

I'd say the real thing looks ogive a good half way up.

Black pinstripe tape will work for ‘faux slots’.

Or you could make the slots real even though the folding mechanism isn't. Just leave open spaces in the tail cone, or internally boxed in spaces, even though they will remain empty. Open if not boxed in would also reduce mass a little.

Ah, I am making the nose tip swappable so its possible to use the long narrow one for display and a more durable stubby one (like I have on the rocket here) for launching. I was pretty skeptical that the skinny sharp nose would survive sticking down into the mud (or even worse hitting frozen ground at a normal decent rate). The stubby one makes the nose look a bit like an X-15.

I am also a little worried about the struts on landing but I designed this for durability as much as I tried to keep weight down. For example, the struts have printed tabs that insert into slots in the shoulder of the boat tail and the wooden skewers insert a good distance into both the strut and the boat tail. Also I printed a couple of solid layers in the middle of each strut that gets a coating of ABS glue sandwiched between the two struts. My earlier version had no bottom layer when I printed the two halves but I thought the two halves did not glue together strongly enough. That middle layer here should also give some good structural support to the struts (particularly when combined with the 15% infill throughout).

If you're sufficiently confident about the strength of the strut-leg assemblies, you could use a long shock cord connecting the nose cone to the body and attach the parachute close to the nose. Then the struts touch down first and the nose cone with its delicate tip hits a bunch slower. Anchor the shock cord to the body with a Y harness, and the body might stick the landing.

great minds or something like that...

Yup, that's me; a great mind or something like that.

 

[BigMacDaddy]

Ok... Some adjustments I made in the last week or so...

I adjusted the scale of the struts down by 10% and increased the angle they mount to the body from 20 degrees to 25 degrees. The struts looked too big and angle did not look right to my eye. Checked the original plans (text is too difficult to read so used a trusty compass image to measure drawings.) I also made some final tweaks to a number of other elements and printed a new set of parts so I can confirm everything is good for production.

 

[BigMacDaddy]

I also launched this rocket this past week. Out of the 5 that I launched this one was the only one not stable... Ah well, can't win them all and I guess you can only trust swing tests so far. I think it just needs a bit more nose weight. This was with 1oz in nose and a D12-3 engine. I will try with 2oz in nose and not sure if I will use a D or E engine.

Anyway, for your entertainment.



Looked nice on the pad...

 

[jqavins]

Oh no, not another learning experience!

 

[Richard N]

Are you going to test clear fin inserts? They would make the legs much more structurally sound and smooth the turbulent flow around the legs and enhance stability.

Of course, as a business model you could wind up selling a lot of leg replacement kits : )

 

[BigMacDaddy]

Are you going to test clear fin inserts? They would make the legs much more structurally sound and smooth the turbulent flow around the legs and enhance stability.

Of course, as a business model you could wind up selling a lot of leg replacement kits : )

I actually put slots in the struts so that it would be easy to add inserts and I am planning to print a small connector that can be put across the two wooden dowels so you can mount an insert on that side of the triangle. However, my main approach will be to test with more nose weight.

 

[Cape Byron]

Nose weight good. Clear fin bad.

On this design.

Only my opinion.

I’d be happy to lose 50 metres in altitude to avoid plastic fins.

Opinion only. YMMV.

 

[techrat]

Agree with above. Not a fan of clear plastic fins, which is why I won't buy the Estes SpaceX model -- I'd rather scratchbuild my own Falcon9 with the landing legs deployed and use those as the fins. Same with the TWA Moonliner. It can fly with just the legs as fins, and we've seen wackier designs that do fly -- the old Estes Mars Lander was exactly that. A little engineering can make it work. I am convinced of that.

 

[jqavins]

I agree in heneral in that I dislike clear fins. Except that in this case, their being framed in would camouflage them such that it wouldn't be so bad. Just my two cents, of course.

 

[Richard N]

Clear plastic fins are a practical solution to the difference between our model rockets and full scale rockets. Full scale rockets do not use fins because they are stabilized by guidance systems steering gimbaled engine nozzles. Our model rockets do not have guidance systems or gimballing nozzles and instead use fins and proper balance. There is no other practical way the Estes Falcon 9 would be stabilized other than fins.

By launching a model Falcon 9 with its legs extended to take the place of fins, I wonder what part of the SpaceX rocket's flight this is attempting to emulate?

The only time a Falcon 9's legs are extended in flight is when the first stage alone is within the last few seconds before landing.

 

[jqavins]

Clear plastic fins are a practical solution to the difference between our model rockets and full scale rockets. Full scale rockets do not use fins because they are stabilized by guidance systems steering gimbaled engine nozzles. Our model rockets do not have guidance systems or gimballing nozzles and instead use fins and proper balance. There is no other practical way the Estes Falcon 9 would be stabilized other than fins.

By launching a model Falcon 9 with its legs extended to take the place of fins, I wonder what part of the SpaceX rocket's flight this is attempting to emulate?

The only time a Falcon 9's legs are extended in flight is when the first stage alone is within the last few seconds before landing.

Transparent fins are not invisible, so they are aesthetically bothersome. The extended landing legs are inaccurate for ascent, so are aesthetically bothersome. Since emulating any part of the flight is not necessarily to point of a build, one pick's one's poison.

 

[neil_w]

Clear fins are not so bad in flight, where they mostly disappear (if they're kept clean). More annoying on display.

The most visible part of the clear fins is generally the edges, which would be invisible on this model due to the framing. That is why I don't think they'd be so bad here. But I understand why some would want to avoid them.

 

[jqavins]

I do wish one could get polycarbonate to cut fins from that are antireflective coated like glasses. In the picture above, the edges are not what's visible, and a good antireflective coating would make then disappear all the better. Of course, that's a still photo; viewed live on the field I bet they are even less visible.

(A long time ago I heard about a multi-layer antireflective coating that my father saw at a trade show. By using multiple layers and taking advantage of the variation of the index of refraction for varying wavelength, the reflections can be cancelled much better over a range of wavelengths than they can be by a single quarter wave layer. The display was of a pane of glass coated over most of its area, with an uncoated square in the middle. The uncoated square seemed to be floating, as the rest of the glass was invisible for all practical purposes. Although the general approach was disclosed at the show (such a possibility was considered obvious anyway) the specific materials, relative thicknesses, and processes were classified.)

 

[tsmith1315]

I do wish one could get polycarbonate to cut fins from that are antireflective coated like glasses.

I’m an optician, so I’ve thought about that many times!

The machines are made to handle round, uncut lenses, 60-80mm or so in diameter. No large, flat plates. Typical rack full to be coated:

 

[Richard N]

Here's a rocket with extended legs and clear fins : )

 

[BigMacDaddy]

So since I needed to launch this again I decided to build another one reflecting the updated sized struts and new angles on the struts. I think it is closer to scale (as far as I can tell from pictures, engineering drawings, etc...). 2oz in nose and E12.4 I think but I better not forget to measure tubes before I glue it together like I usually do...