Alcubierre Build Thread

[neil_w]

OK, let's get it started. I shall be building this guy:


Here is the obligatory Wikipedia link: https://en.wikipedia.org/wiki/Alcubierre_drive. Credit to @dhbarr for originally suggesting the name.

This build is considerably simpler than my last one, and I expect this build thread to be correspondingly shorter, although at my leisurely build pace it'll still take plenty of calendar time.

The structural design is finalized the but paint scheme is not. I'm still open to suggestions.

 

[Gary Byrum]

Subscribed!!

 

[dhbarr]

Woohoo!

 

[Mustang67]

Like it, very sharp.

 

[kuririn]

It's so cool, I gotta wear shades.

 

[snrkl]

I’m in...

 

[neil_w]

I supposed I might as well post this silly thing here, created several months ago when I was thinking up new ways to abuse OpenRocket.

 

[Bruiser]

Ok, so this is a cool looking rocket. What diameter body tube? Video was cool too. I'll be watching this one

-Bob

 

[neil_w]

It's a BT55, which I seem to have settled on for most of my scratch builds. It's a really nice usable size, easy to work with, models work well on 24mm motors, and they don't require too much storage space.

 

[neil_w]

Build Philosophy
I have learned over the course of my BAR career that I really do not enjoy making repairs to rockets. Therefore I will go out of my way to anticipate weaknesses and try to mitigate them, even if it sometimes leads to overthinking or overbuilding.

In the case of this rocket, it's pretty straightforward except for two issues: paint/construction sequence, and surviving landing.

To be fair, paint/construction sequence confounds me on many builds. I haven't worked out all the details for this yet, but I also don't need to at this early stage.

With regard to strength, the two areas that I worry about are the pods and the ring. In the original design, the ring was not attached to the dorsal and ventral fins; I *really* like the look of it that way. But in that case, if I think of the possibility of the top or bottom of the ring contacting the ground first, I simply can't imagine it surviving. Hence the fins are now extended to join the ring. As for the pods.... well, some of the details of how I'll address that will come later. First, though, let's fabricate the ring.

Fabricating the Ring(s)
As part of my strength strategy, I elected to construct two half-rings instead of a single solid ring (again, we'll come back to how this all fits in with the rest of the build). I did make a complete test ring a while ago, and it turns out that it's really a pain. The half-rings turned out to be much easier.

I used the same jig and similar technique as I did on the Starship Avalon: two laminated pieces of 1/16" balsa, clamped to a 4" PVC pipe coupler, yielding a ring with inside diameter of 5":
1) Cut two pieces of balsa to size, actually slightly oversized to be trimmed later
2) Apply Windex w/ammonia to each piece, work the piece until it'll conform to the PVC form. Unlike with the Avalon, here I'm going directly against the grain, so it was a little dicey and I actually broke a couple of pieces by moving too quickly.
3) Clamp one piece to the form.
4) Apply TBII to the surface
5) Wrap the second piece around, clamp everything together.
6) Let dry for a few days

Here's how it all looks clamped onto the form:

When dry, the pieces looked good:

However, when I checked the shape against the form, it turned out that they had released just a tiny bit of their curve:


It's pretty minor, but I will deal with it when I glue the rings to the wings. I also will be papering these pieces at some point. In the meantime there are other things to do.

To say that I'm enjoying creating bent pieces of wood is an understatement. I'm sure I will be going out of my way to find excuses to use this technique in future builds.

 

[neil_w]

Pods
It appears that construction is going to go very quickly on this rocket, although painting may take a while. It also seems like it's not actually that interesting of a build. I mean, it should be a really nice rocket when finished, but there's not all that much in the way of unique or exotic technique here. I suppose it's a good change of pace from the IRIS-T.

Anyway.

I decided to tackle the pods, which have more pieces than they ought to. They're BT-20 with a BT-5 stuffer, both to support the tail shroud and the TTW mount to the wing pylons.

Here's the first subassemblies:


In addition to applying CWF to the spirals and the nose cone (no CA for these guys) I spread a liberal amount all over the tubes. I'm not sure what I was thinking; it was a pain to sand off.

After gluing in the stuffer tubes, I realize I had made a dumb mistake by not letting one of the centering rings extend out beyond the back of the pod to support the tail shroud. So I did the same thing I did on the IRIS-T: glued a tabbed ring of cardstock into the tail end of the BT, as shown below:

Glue is applied to the exposed tabs and to the ring around the end of the BT-5, and the shrouds are glued on. In addition to helping center the shroud properly against the BT, this make for a *really* strong shroud support. It feels rock solid.

These shrouds gave me a lot of trouble for some reason, BTW. I seem to struggle with getting the small ones just right. The seams will be noticeable on these, despite application of CWF. I don't really care, they won't be too visible anyway.

I want to paint the insides of the tail end of the pods, probably flat black. there's no way I could get the paint all the way in there, and I didn't want to be able to look in and see unfinished stuff in there, so I cut a couple of balsa bulkheads and installed them 1" from the end. That should be easy enough to paint. I kind of feel like I should do something more interesting in there, but I haven't thought of anything yet.


The two little fins on the pods were cut from 1/8" balsa and papered.


Finally they were glued on to the pods. On the other side, I cut the slots for the TTW mount. I think I made the tabs too small to actually accomplish anything other than perhaps helping with alignment, but too late to change (I've already cut the wings and I'm not going to redo them just for this.)

Here are the completed pods:


At this point I realized that I completely spaced out when marking for the fins and the slots, and forgot to account for the location of the seams on the tail shrouds. So the positions are completely different on the two pods. A minor flub, but nothing I can do about it anyway.

I decided to apply the filler primer to the whole subassemblies, since there's no difficult nooks and crannies to sand around. I think I got all the joints pretty good and smooth, these should look good after finishing.

 

[Gary Byrum]

Looks like you made the shrouds with the overlap method. I still do that too. But if you CA the entire shroud and use an extra coat on the seams, those can be sanded flat to get rid of the step down appearance. I usually get them down as far as possible and if necessary, use a little Bondo spot putty over it. That fills any pits or leftover grooves and can sanded to form. Works well if you do it right.

 

[neil_w]

Looks like you made the shrouds with the overlap method. I still do that too.

I haven’t had good results making them the other way. I should probably practice.

But if you CA the entire shroud and use an extra coat on the seams, those can be sanded flat to get rid of the step down appearance. I usually get them down as far as possible and if necessary, use a little Bondo spot putty over it.

I totally forgot about this technique. Will remember it when time comes for the main tail shroud (which will be annoying since it will have the engine hook going through it, almost exactly the one on the Avalon).

Crap, I need to check to see if I have any engine hooks in stock.

 

[Gary Byrum]

I really don't know if it's a given technique, I just stumbled on the idea because I'm no good putting shrouds together the "other way" either. It's not too late to do those pod shrouds even if you already have them glued on. I usually do it on purpose so I can get rid of the seam between the shroud and tube all at the same time.

 

[K'Tesh]

Neil... Lord of the Rings!!!

I'm in!

 

[neil_w]

My build sequence is kind of all over the place on this one.

There's only a bit to say about the motor mount.

1) It's a bit longer than typical so that the front ring won't interfere with the TTW wings (more on this later).
2) The centering rings were a bit loose on the tube, so I glued a ring of copy paper around where the rings go, and that tightened them up.
3) The blue tape is temporary, to be replaced by electrical tape.

After my struggle with the tail shrouds on the pods, I was determined to do better on the main tail shroud. After initial assembly it looked really good:


Next I applied CA liberally to the seam area, and then sanded it down the ledge (Gary's suggestion from earlier), and it worked pretty well. I didn't sand it *flat*, but definitely knocked down quite a bit, leaving me needing only a small amount of CWF to smooth it out the rest of the way. Success.

I also made the cutout for the motor hook. This is now the third rocket in a row where I've had a tail like this (the Avalon and the IRIS-T being the others), and this is the first one I really nailed. This time I cut out the slot with a hobby knife, cut from the *inside* of the ring. I got a very clean and accurate cut this way. I might need to deepen it a bit, will have to see if it provides enough room for the hook to move. Here's what it looked like after the treatment:


And here's a dry fit on the motor mount, with a scrap piece of BT55 left over from the IRIS-T build. Looks very good so far.

 

[Gary Byrum]

Next I applied CA liberally to the seam area, and then sanded it down the ledge (Gary's suggestion from earlier), and it worked pretty well. I didn't sand it *flat*, but definitely knocked down quite a bit, leaving me needing only a small amount of CWF to smooth it out the rest of the way. Success.

Actually, you could sand it flat if you remember that you are only lowering the edge. There's plenty of remaining overlap to maintain adequate thickness.

 

[Scott_650]

Wow, I’m about a hundred pages into “Dark Run” by Mike Brooks, a pretty good (so far) space opera where the ships have Alcubierre drives for FTL travel - though the main characters’ ship is described as being cube-shaped with a drive ring. Yours will undoubtedly fly better in an atmosphere.

 

[neil_w]

Actually, you could sand it flat if you remember that you are only lowering the edge. There's plenty of remaining overlap to maintain adequate thickness.

Yeah, I just got to the point where I felt like I was pushing my luck, given that that area is a weak spot due to the motor hook cut. Still it came out very good, probably one of the best shrouds that I've made.

Wow, I’m about a hundred pages into “Dark Run” by Mike Brooks, a pretty good (so far) space opera where the ships have Alcubierre drives for FTL travel - though the main characters’ ship is described as being cube-shaped with a drive ring. Yours will undoubtedly fly better in an atmosphere.

This is always a bit of a conundrum when trying to design sci-fi rockets. They need fins to actually fly, but in space the fins would be useless. So rockets like this one end up looking like it's designed to fly in the atmosphere. There is of course also plenty of precedent in pop sci-fi for ships that can magically fly in the atmosphere despite having no aerodynamic qualities whatsoever. See: Millenium Falcon.

And so I eventually abandon all pretense of trying to actually come up with a coherent story about why this (or any) rocket looks the way it does, and just stick with "it looks cool".

 

[Charles_McG]

There is of course also plenty of precedent in pop sci-fi for ships that can magically fly in the atmosphere despite having no aerodynamic qualities whatsoever. See: Millenium Falcon.

Who needs lift when you have THRUST!

 

[Rockyt]

Who needs lift when you have THRUST!

Don't forget the Enterprise!

 

[K'Tesh]

Who needs lift when you have THRUST!

 

[Charles_McG]

Precisely.

 

[Bruiser]

Looking real good, coming along great! Remember what you told me about boat tails. CA, sandpaper and CWF are your friends and it worked for me.



-Bob

 

[neil_w]

Yep.

That one came out great.

 

[Gary Byrum]

Hey Bob, let's see a profile shot of that rocket. Looks like you did some layering on the fins.

 

[Bruiser]

Gary, the fins are layered. I don't want to take Neil's thread off track though. I have a build thread over here https://www.rocketryforum.com/threads/build-thread-mim-23-hawk.147973/

-Bob

 

[neil_w]

I inserted the motor mount, and then once again put in one of my tabbed rings to hold onto the tail shroud:

This method might be a bit more effort than it needs to be but it sure does work well. After installing the shroud, I finished with some more CA and CWF and the tail is looking good:

Prior to doing this, I slotted the tube for the wings. Each will have a small 1" tab just to assist with alignment (idea gotten from Bruiser's Hawk thread). As the wings are extremely long, it'll be helpful to just have something to lock them into position. More on the wing construction later.

Unfortunately, I cut the slots 1" too far forward (quel idiot!), so I then had to elongate them to 2":

You can see the front centering ring in the slot to the left; that's how I first realized the slot was in the wrong place. No big deal; the wings are 12" long so losing 1" of root bond won't matter, and it'll all be hidden underneath fillets. Annoying carelessness though.

The body tube assembly is now ready for filler/primer.

 

[Nytrunner]

Man, this is going so much faster than the iris. Isn't it a whole lot easier when you're the one that can decide what's the right shape instead of pouring over pictures?

 

[neil_w]

Man, this is going so much faster than the iris. Isn't it a whole lot easier when you're the one that can decide what's the right shape instead of pouring over pictures?

Too fast, to be honest, though it'll slow down when it gets to painting time. I'll be onto Plasma Dart II by winter time I'd guess.

The IRIS took extra long not just from trying to following pictures, but also because there were so many different details that required me to invent techniques, and then practice. This one is just build build build, using techniques from previous rockets.