Semroc X-21

[PaulK]

While everyone else is building their lander, I decided to start my semroc X-21 today (after reading the instructions through a few times). I have some questions for folks that have built a clone or original. First, for the engine bulkhead, the instructions say to push the spacer tube all the way in. Gee, this doesn't seem right, I'm used to leaving 1/4" out the back. I realize the engine is supposed to kick, so I suppose I don't need the engine to stick out the back, but still... Was the original spacer tube shorter than an engine? Second question is how to sand the root stress supports. The instructions say to "taper sand the supports as indicated on the balsa". No indication. I think this means sand the taper so the thick edge is on the BT, then tapered away from the BT down to the wing. The original instruction pic seems to indicate this. Lastly, the release clip thingy is unclear. The original instructions don't say much about it, and the semroc instructions say to substitute a straight pin. Huh? I read the post here (and EMRR) about using a piece of soda can, though it sounds like something less sharp would be better. I'm guessing the original was simply a thin piece of metal.

Other tips are of course welcome. And I'll post my tips as I go along. No pics yet, as I've just started, stay tuned, and thanks for your help!

-Paul

[sandman]

Paul, the X-21 is my next project.

I haven't read the plans too closely yet but so far the Centuri plans are a bit vague in spots.

I guess I'm spoiled with all of the step by steps with pictures that everyone else uses.

I haven't even opened mine yet to see what SEMROC has to say.

[stefanj]

Many old Estes plans showed motors mounted so they didn't extend from the rear of the body tube. You used a dowel to shove the extended casing out the rear.

I built my Cloud Buster and Mark this way.

It is possible that the X-21 shared this "feature."

[barone]

I haven't even opened mine yet to see what SEMROC has to say.
Gordon,

You have to download the plans so you are actually using the old Centuri plans. The Semroc plans have some construction notes for the downloaded plans and a nice exploded view. The balsa is all laser cut, as you'd expect in a Semroc kit. The first downloaded plans I got are kinda hard to follow though. Mainly because the print is small.....gonna do it again but increase the size so I don't need the magnifier along with the reading glasses. My first read thru did leave me kinda baffled about the release mechanism. After I finish the Mars Lander, I'm going to go through the plans again but slower......First reading had me wondering about the same things Paul has mentioned.

Don
NAR 53455

[ghrocketman]

I haven't been able to figure out the strategy that Semroc uses to determine when plans are included with their kits. It seems that all the CC: kits have no instructions, but all the Retro-Repro ones do. Maybe it has to do with whether or not they are an exact clone; for example the SST Shuttle and Space Plane are, but the Mars Lander is not.
No matter for me anyway, as I cannot build a Mars Lander (I have #7) or an X-21 (I have #15) until I order builder (higher production #) kits.

[sandman]

Don, I meant the downloaded plans. There is only the blowup pic (top and bottom) to go by. :cool:

The cc and Retro Repro thing I think has to to with copyrights.

[PaulK]

Many old Estes plans showed motors mounted so they didn't extend from the rear of the body tube. You used a dowel to shove the extended casing out the rear.

I built my Cloud Buster and Mark this way.

It is possible that the X-21 shared this "feature."

StephanJ,

The X-21 has a motor bulkhead, so there would be no way to push the engine out from the front. The idea is that the motor kicks itself out. The only problem would be if I for some reason need to get the motor out without launching. ("Oh dang, did I accidentally put a C6-5 in there?")

-Paul

[PaulK]

Heres my first tip. When gluing on the root stress supports, glue on both sides to one wing at the same time. I glued on only the bottom stress supports, left overnight to dry in the cradle, and this severely warped the wings. I suppose the shrinkage of the wood glue caused this. I'm trying to unwarp them now with moisture & weights. Stay tuned...

-Paul

[PaulK]

The X-21 is ready for priming. The warping issue was solved by moistening the wings and weighting down overnight. I also noticed that one of the 1/16" wings had some defective balsa, it was very thin in the middle (around 1/32) which it made it warp much more than the other. So I built it up with some scrap 1/16 and sanded it down. Wish I would have noticed that before I glued it together :rolleyes: .

-Paul

[bikegod]

I had the same thing happen to me, only while it was drying. It curled just a little on the edges.

I slid some glue in on my xacto and then tape the edges down with the elevon scraps. Seems to have worked fine, but I also did the "thin sanding" after they were glued down.

Question: Is the payload rear connector glued to either the body tube or clear tube? Mine slides around very easily though the nose cone is tight.

Not a lot of room in there for payload either.

[PaulK]

Question: Is the payload rear connector glued to either the body tube or clear tube? Mine slides around very easily though the nose cone is tight.

Not a lot of room in there for payload either.

I glued the coupler to the main tube, not the payload. I suspect gluing it to the payload would not look very good. I plan to build up & paint the shoulders for tight fits.

The shoulders sure do eat up most of the payload section. Well, seems mostly for looks anyhow.

-Paul

[bikegod]

I guess you could look at how many washers you have in there.

I am kinda lost on the elevon/wire details. Would you mind posting a pic/closeups when you can. I had the day off yesterday and did 90% of the kit (basically ready for paint and wiring). I added some more glue to the fillets and touched off some of the rough spots, though I am going to let the Kilz do most of the work.

I have never done anything this complex before and I am really loving it. I actually have 3 kits in progress (well the other two are already ready for paint). I have some drawing board plans and parts so I definitely have something to do while the paint is drying.

Oh, and thanks for the earlier pics. Let's me know that I am on the right track.

[PaulK]

Final paint coat and pinstriping is now on. I decided on a fluorescent red to make it more visible (fluorescents sure don't show up good on pics). The top side is pinstriped similar to the 64 catalog, haven't figured out how to do the checkerboard yet. I decided not to glue either shoulder visible thru the payload. I just applied some white tape to make a snug fit and highlight the payload section. Just need to put on the elevon cords, figure out how to make a release clip, and balance her out.

-Paul

[Ltvscout]

Final paint coat and pinstriping is now on. I decided on a fluorescent red to make it more visible (fluorescents sure don't show up good on pics). The top side is pinstriped similar to the 64 catalog, haven't figured out how to do the checkerboard yet. I decided not to glue either shoulder visible thru the payload. I just applied some white tape to make a snug fit and highlight the payload section. Just need to put on the elevon cords, figure out how to make a release clip, and balance her out.

-Paul
Whoa! That's a beauty! I hope to be at one of our club launches that you're at so I can see it fly this Summer.

[bikegod]

Checkerboard could be made by cutting small squares of TrimKote and laying them on their contrasting color (black squares on white or white squares on black for instance) but that could prove tedious (though you could do it in front of the TV). I think TrimKote comes in some checkerboard patterns, though I don't know if they are the right size or not.

I haven't seen the original catalog shot unless it is the same pic on the Semroc website.

I am still lost on the Elevon/Wiring/bungee cord details. I assume that the elevons should be in the down position at launch but would need to be up somewhat for the flight down. The diagram is kind of minimal. Any help here would be greatly appreciated.

[PaulK]

Thanks Scott. :) Finished the elevon cords today. Here are some detailed pics, along with my version of the release clip. The instructions say to use a pin, I used a 5mm x 25mm piece of aluminum sheet cut from some scrap, filed & sanded to get smooth edges, and taped at the point where the thread runs thru to further protect the thread. It seems to fit nicely in the BT, we'll see how it works when I launch it.

Yes, the elevons are flat for launch, held by the release clip & thread. When the engine kicks, the clip is released, the elastic looped around the LL pulls them up. The wires are bent to control the maximum angle. The centuri catalogs are on the ninfinger site.

-Paul

[bikegod]

Very clear and concise, thanks a bunch for the pics!

I still am not understanding the how the wire is used to control the max elevon lift. Maybe I am just being dense.

Like the red. I am considering Neon orange or yellow. I would really be bummed to lose this one.


I have more sanding and another coat of primer possibly. Still have some grain showing up.

[PaulK]

You're welcome. I agree a fluorescent color is good on this small bird. Heres a pic of the elevon stop wire, hope it helps. I glued it on with aileens tacky glue, since it dries clear. Glide tested it today, it needs surprisingly little angle on the elevons for a flat glide.

-Paul

[Ltvscout]

You're welcome. I agree a fluorescent color is good on this small bird. Heres a pic of the elevon stop wire, hope it helps. I glued it on with aileens tacky glue, since it dries clear. Glide tested it today, it needs surprisingly little angle on the elevons for a flat glide.

-Paul
Ok, now I want you to put a grasshopper in the payload section and glide test again. :D That's what's wacky about this design. A payload section on a glider!

[bikegod]

I think the grasshopper would feel a little cramped in the payload section.

Maybe a small cricket from PetCo.

On a scale, what G-Force do you think they would experience?

Thanks for the pic! That really helps! I was totally clueless on the placement of the wire. I assume it is on both sides, right?

[PaulK]

Ok, now I want you to put a grasshopper in the payload section and glide test again. :D That's what's wacky about this design. A payload section on a glider!
I suppose the whole idea of this glider was to look cooler than the space plane. And we need a place for the pilot, right? ;) There is also the wee problem that the 2" payload section gets reduced to < 1/2" after the 2 shoulders & screw-eye. Hmmm, a grashopper decal might be in order...

I assume it is on both sides, right?
Yup, one wire per elevon.

-Paul

[stefanj]

You could increase the size of the payload section by cutting back the nose cone and bulkhead shoulders to a bare minimum.

You could also hollow out the nose cone base, allowing the weights to be placed well forward and perhaps friction-fitting them into place.

Even so, I suspect that payload capsule is pretty much cosmetic.

[bikegod]

Dang, I am so excited to finish this and see it fly. Going to a big field this weekend for a kite festival. We will be first on the field so I was thinking of getting a launch or two in before the crowds (10k expected this year). I, however, hate to fly em unfinished, and am now back at work and not getting home until after 9. Can't have too many late nights.

I think I can pull it off if I drop the other 3 that I am working on.

[lessgravity]

This forum was very helpful with this build. It helped clear up some of the gray areas I had reading the "small print" instructions. Anyway, here she is (http://rocketry.wordpress.com/2006/05/28/x-21-build/)

[PaulK]

Thanks, and nice looking X-21. How'd you do the checkerboard patterns?

I launched her for the first time today. It really was too windy, so used a 1/2A6-2. The wind really messed up the glide, well at least I didn't have to walk too far. Here is a launch pic. I'll try again next weekend at the WOOSH vintage rockets theme launch.

-Paul