New threads and interesting conversations directly in your inbox. Sign up now and get a daily summary of the latest forum activities!
Discussion in 'Mid Power Rocketry (MPR)' started by James Duffy, Apr 21, 2017.
Awesome Thread. Keep going.....
Okay, we'll forge ahead with the Bumper WAC build. I'll try to update this as frequently as possible, but please don't expect updates as frequently as during the fin can assembly.
Our first task is to select as accurate a scale as possible. The 1/19 scale figure listed on the box for both the mid-'70s and 2001 issues of the kit is simply wrong. The 1/16 scale figure listed on the assembly instructions for the '70s version is closer, but still off. The most critical dimension is the one we can't fudge much: the major diameter of the body tube. A real V-2 has a major diameter of 1651mm, and if we put a micrometer to the BT-101 supplied with the kit it measures out at about 99mm. That's a scale of 1/16.67.
Now, let's look at a couple of the dimensions on our just-completed fin can. The fin prototypical fin length of a V-2 fin is usually listed as 4067mm and our model's fin length is 240mm, which scales out to 1/16.95. (I knew from the earlier fin can that I did that the fin length would come in a bit short, which is why I suggested the longer, sharper fin tips when we cut out the fin skins.) Fin span on a real V-2 is 3564mm, and our fin can measures out at 213mm, or 1/16.7 scale. In reviewing all of that, we're just going to have to deal with the fact that our fins are going to measure out a few millimeters off, so our final, official, "build to the data" scale will be 1/16.7.
I plugged that into my trusty Excel spreadsheet to determine what the final dimensions of each critical element of the model should be, and will be consulting this extensively during the build process. That spreadsheet is attached below if you'd like to review it. Note that the dimensions listed are what they should be, not the dimensions of the parts supplied with the kit.
View attachment Bumper WAC Scale Worksheet.xlsx
We're going to start from the top of the Bumper WAC and work our way down, which means that our first step is to construct a WAC sustainer. We'll be building two, if fact, because you know we're going to lose one at some point.
Our WAC will be constructed from BT-20 tubing, which sizes out almost perfectly at this scale. The plastic Estes nose cone that we'll be using scales out a bit short, so we'll stretch the length of the body tube so that the overall length of the completed sustainer is correct. (If I were building this for FAI scale we would whip up a custom cone and have Shapeways print it.) The fins are laser cut from 1/16" plywood and attached through the wall, attaching to a BT-5 motor mount within.
A pdf of the Illustrator file for laser cutting the fins is attached below.
View attachment Bumper WAC sustainer fins 16-7 copy.pdf
The WAC fins are installed using a bit of epoxy, and alignment is ensured with the use of a guide laser-cut from scrap cardboard.
A pdf of the Illustrator file for laser cutting the fin alignment guide is attached below.
View attachment WAC Alignment guide copy.pdf
The two WAC sustainers will be set to the side for finishing at some undefined point in the future, and we'll next turn our attention to the V-2 nose cone. A bit of BT-20 scrap is placed on the tip of the cone to define a cut line for the hole into which we will insert the WAC sustainer.
Using a razor saw, slice the tip off of the V-2 nose cone along the cut line.
A sanding block is used to gradually sand down the cut surface of the cone. Take care to keep the sanding block perpendicular to the axis of the cone. This step will take a while, about thirty minutes for my model.
You'll be done when a BT-20 tube can slide into the hole cleanly and freely. The interior surface on my finished hole is ~19.5mm in diameter.
Staging of the sustainer on this model will be handled by a Perfectflite Minitimer 4, housed on an electronics sled/staging piston assembly installed in the V-2 nose cone. It will take a few steps to reveal all of this hoo-hah, but the first thing we need to do is cut a hole in the back of the nose cone to facilitate the installation of the electronics sled.
After rooting around in the kitchen a bit I found a Pilsner glass that was just the right size for marking the size hole needed (My wife became used to such aberrant behavior in the kitchen many, many years ago.) The glass was roughly centered on the back of the nose cone and a pencil line was drawn around it do define the cut line, which is about 3.5"/89mm in diameter. This is about the largest hole size that we would want to consider, as we will need a bit of lip on the base for attaching the mount plate for the electronics/piston sled.
The Pilsner glass is removed and, um, "tactically repurposed," after which the cut line is attacked with an Xacto knife. This step takes a while, so be patient. Eventually you'll have a nice big gaping hole in aft end of your nose cone. Note that I sliced off the nub at the center of the nose cone base first to facilitate easier handling.
Interesting. The two times I've done something like this I "hollowed out" the NC such that a stepless mooth fit was obtained and more surface area between NC & BT was achieved.
Seems like this way would be faster & less error prone, are there other advantages?
It seems to me that your way might be quicker and more accurate. Slip a section of BT-20 into the tube, illuminate the opposite side with a bright light, and you should be able to see the cut line pretty cleanly. I may have to try that on the next one of these I do. Thanks, good idea!
Having said that, we don't need to worry about anchoring the WAC against the tip of the V-2 nose cone with a friction fit, as we're going to build a replaceable staging piston assembly into the cone into which the WAC will insert. Just need to knock a few more preliminary steps out of the way first...
Our next step is to mark and cut the WAC fin slots into the forward end of the V-2 nose cone. We're going to mark the lines with a special laser-cut jig designed specifically for this purpose. Cut out the jig, slap it together with some CA, plop it over the end of the nose cone, mark the slot lines, and you're done!
A pdf of the Illustrator file for the alignment jig is attached below.
View attachment Bumper WAC interstage marking guide copy.pdf
I actually did something like this although I was not so methodical.
I also decided that the Estes Maxi-Brute fins were too flimsy and needed reinforcement. I mixed some epoxy with micro-balloons and then buttered the mixture into the root of the fins before bonding them together. As I recall before bonding the two fin halves, I used a sanding block on each fin half so that the two pieces would fit together.
With the locations for the WAC fin slots in the V-2 nose cone marked, we can go to work cutting the slots. Be advised: this is a miserable job, but it is doable. Just be patient, and you'll get through it without wanting to shoot up and die in a gutter.
A pretty wide range of tools were used for this task. The initial cut lines were done on either side of the marked lines with a standard #11 Xacto blade, but most of the plastic excavation was done using a plastic modeling tool called a panel line scriber. You can get one from a hobby shop specializing in plastic models or from Squadron:
This tool is dragged along the cut line, and pulls out curls of plastic faster than using an Xacto alone. With the slots opened up they were widened and refined with a blade, a small square jeweler's file, and a standard emery board. The jeweler's file is great for getting the base of each fin slot uniform and square.
Note that the forward end of each fin slot is a bit wider than the base. This is prototypical, as shown in the drawings attached at the bottom.
Eventually the slots will be wide enough to accommodate the aft end of the WAC sustainer. A fairly loose fit is preferred as the sustainer will be retained in the V-2 nose cone by a piston assembly that we'll build in a bit.
As noted earlier, the conversion from a V-2 into a Bumper WAC will require some unique parts for the staging electronics that will be installed in the nose of the model. Also, we'll be cutting some new centering rings from the same plywood as these parts.
Here's a photo of the parts fresh from the laser cutter, along with a description of each component.
We'll begin with the avionics/piston mount. A pair of 4-40 blind nuts are pressed into the holes on the part, then anchored with a touch of medium CA. That assembly is then epoxied into the aft end of the V-2 nose cone.
The avionics/piston mount will be assembled in the next step. A pdf of the Illustrator file for the 1/8" thick plywood parts is attached below.
Next up is the avionics/piston mount sled. The upright is attached to the base with epoxy.
All of the other joints can be completed with medium CA.
A short section of BT-50 tubing is installed to create the base of the staging piston. In a later step we'll put together a replaceable extensible piston assembly out of BT-20 and BT-5 tubing and centering rings that will connect this electronics sled to the aft end of the WAC sustainer. In addition to providing a secure physical connection between the booster and sustainer, the extensible piston will ideally provide a bit of protection for the booster nose cone from the ignition of the sustainer.
With the Perfectflite Minitimer 4 in place the unit looks like this. A 9V battery will provide power for the electronics, and will attach to the base of the sled with tape and a zip tie.
Very nice, makes me wish for a laser cutter of my own or even just local access to one.
As noted before, access to a laser cutter has completely changed my approach to just about any construction task in this little hobby.
Something to consider: just about every trophy shop in the world has a laser cutter sitting idle for about 90% of the work day. Most of these shops are run by laid-back, semi-retired types who would be thrilled to put their cutter to work for a few hours every now and then. Stop by and chat with the owner, and maybe he'll show you how to use the thing in exchange for $20 or so every time you use it.
That's a recommendation to think about for sure, an acquaintance of mine owns a cabinet shop and has a large CNC router that I have considered trying to work a deal to use as I know it can sit idle for quite a while during the time it take to assemble the custom cabinets he makes.
The motor mount and centering rings have been assembled using epoxy, and the Bumper WAC and V-2 versions of the model have both been stacked for the first time.
Measuring the models against the available references revealed that the models were too long, so the body tubes were shortened a bit to make things accurate. (7mm was removed from the V-2 model, 14mm from the Bumper model.) I also elected to replace the body tube from one kit, as it had become ratty with time. Estes packaged these kits with a tail or nose cone reversed into the body tube, and it is not uncommon for the ends of the tube to be stretched out. The electronics sled and a spare WAC sustainer are displayed in front of the models.
If you're interested, a pdf of the 1/16.7 scale workshop drawing for the Bumper WAC model is attached below.
View attachment Bumper WAC workshop drawing 16.7.pdf
At no point do I intend for this to devolve into a full-blown scale obsessive project (you can see one of those here), but there is simply no way I am going to put this abomination on my model:
So, let's make some quick & easy hatches to put on the tail section. A simple drawing is whipped up in Illustrator for a selection of hatch shapes and sizes that roughly conform with the hatches on the evil and unclean sticker sheet shown above. We'll also add some rivets to make things a bit more sexy and scale-like. Each hatch type will be represented in two forms. The first is a "rivet map" that will be used to emboss that pattern onto sheet styrene, and the second is a template for cutting that hatch from the sheet. (A pdf of the Illustrator file for the rivet map and hatch templates is attached at the bottom of this post.)
This is what these templates look like after cutting them from 3mm acrylic sheet on the laser cutter. Look closely at the big acrylic sheet on the left and you can just make out the rivet pattern holes.
A sheet of .010" thick styrene sheet is taped onto the large acrylic piece and placed onto a light table so that the rivet locations can be clearly seen. If you don't have a light table sunlight through a window will do just as well. A small round-end scrapbooking burnisher (borrowed from my wife) is used to emboss each of the rivet locations.
I failed to get any photos of this process, but the technique is pretty well documented over on my Mercury Little Joe thread.
Each of the hatches is cut out and trimmed using one of the individual acrylic hatch templates. A quick touch-up on the rounded corners of each and the hatch os ready for installation.
The hatches were glued to the tailcone using an assortment of references, the best of which, ironically, was the finishing guide from the '70s version of the V-2 kit. (I'm sure that someone will know where that can be found on the web and post a link here.) Tamiya thin plastic cement was used for this step. A bit of cleanup and we'll be ready to spray some primer!
View attachment V-2 Hatches copy.pdf
Awesome work as usual. I need to find myself a light table...
Awfully handy to have around. IIRC, I paid about $20 for this one almost twenty years ago using a 30% off coupon at either Michael's or Hobby Lobby.
Edit: They seem to have gone up a bit over the past couple of decades.
This is the exact light box that I have, but it is now $60. With a 40% off coupon that would only be $36, though.
Time to build the staging piston for the WAC sustainer. This will be a sacrificial, replaceable unit, the aft end of which will insert into the forward end of the avionics sled. The aft/motor end of the WAC sustainer will then fit into the forward end of the extensible piston. In addition to providing a secure mount for the sustainer, the piston contains most of the ignition blast, providing a degree of protection for the forward end of the booster.
We'll need sections of BT-20 and BT-5 tubing, along with 520 and 2050 centering rings.
A standard 1/4"/6mm hole punch is used to punch a hole hear the aft end of the BT-20 section. This hole will be the exit for the igniter leads.
The BT-20 section is cut so that the forward end stops roughly 1"/25mm below the aft end of the WAC sustainer. This dimension will vary according to your specific installation, but on my model the BT-20 tube was ~ 7"/180mm long.
A 520 centering ring is glued to the aft end of a section of BT-5 tubing to form the piston. The outer layer of paper is peeled from the centering ring to allow the piston to slide easily within the stationary BT-20 tube.
The BT-5 piston tube is slid into the BT-20 tube, but not glued.
The inner surface of a 520 centering ring is peeled so the BT-5 piston tube will slide easily through the ring. This will be glued into the forward end of the BT-20 tube, and will serve as a forward stop for the piston tube.
More in a future post,
A pair of 2050 centering rings are mounted on the aft end of the BT-20 tubing that forms the stationary piston base.
A bulkhead is glued into the back of the BT-20 piston tube, backed by a 520 centering ring.
Here are two completed pistons, one in the retracted position, one extended.
Here's how the system works. Normally this would be mounted in the base off the V-2 nose cone, but that has been omitted for clarity.
A Q2G2 igniter is soldered to long leads cut from fine wire wrap wire, and the igniter is snaked through the hole at the base of the piston base, through the entire piston assembly, emerging from the forward end of the piston tube. The piston base is then inserted into the forward end of the electronics sled.
The igniter is inserted into the sustainer motor, which is then secured into the piston tube. Turn on the electronics and you're ready to fly!
Minor updates, none of which are worthy of photos...
The V-2 airframes and nose cones have been primed with a product I'm trying for the first time, a Rustoleum white primer intended for metal. Yeah, I know it was risky, but life is short. I was a little concerned at first because the primer took a few days to fully cure, but the results are fantastic. The primed parts were sanded to 600 grit, and the excess primer came off in talc-like clouds. This may be my go-to primer for all projects going forward. You can purchase the stuff at Home Disappointment:
The WAC sustainers have also been primed and sanded, and the next task will be to add the conduit and spin motor details.
Time to add some details to the WAC sustainers...
There are two significant extras found on the WAC sustainer: a pair of cable/pipe conduits and a pair of spin motors. The conduit is created by cutting a section of .080" half-round styrene strip to length, rounding off the ends, and gluing them in place in line with two fins on opposite sides of the airframe.
Next up are the spin motors, which are a complex tetrahedral structure. Given the relatively simple nature of this build were going to go with a flat representation of the component. (If this model were bound for a museum or an FAI contest we would probably whip up a 3d model of the part in a CAD program and have Shapeways print it.)
A drawing of the part is created in Illustrator, printed, and taped to a scrap of .010"-thick styrene sheet. After punching the locations with an awl the spin motors are drilled out, and the parts are then cut. A pdf of the Illustrator file is attached at the end of this post.
After the edges are lightly sanded two simulated spin motors are glued into place on opposite sides of the sustainer, between the conduit details.
Here's how everything looks with the spin motors in place. Sorry, I neglected to snap a photo before a layer of Tamiya primer was applied. Next, we'll paint the sustainer white with a grey nose cone.
View attachment WAC Spin Motors copy.pdf
James, FYI if you have not seen this info that links the Bumper WAC to the HoJo:
Via this photograph, the Corporal had rail buttons (apparently 3 on each quadrant) for apparent rails inside the V-2.
Also, the Corporal appears to have a single bay of spin motors in the housing, whereas the Honest John had two.
(Hi-res link: http://nalfl.com/wp-content/uploads/2014/07/5b-WAC-mate-July-27-1950-G-0630.jpg)
I knew about the rail buttons, but blew them off because they are hidden within the V-2 nose before launch. The spin motors are news to me, but the background story you shared makes perfect sense. Thanks for the link!
This is a really awesome build thread! Thanks for sharing James...watching with great interest!!!
Time to paint this puppy! With the primer coat sanded down a few coats of Rustoleum gloss white are applied to the airframe and nose cone. Tamiya pure white lacquer was also applied to the completed, primed, and sanded sustainers.
After the Rustoleum was allowed to dry a couple of days (during which my daughter and I slid over to Houston to see U2), the upper and lower circumferential stripes were masked with Tamiya 10mm tape. The Bumper in the back is my 1/24 scale model flown at the 2012 World Championships, which I used as a reference for determining the position of the upper circumferential stripe.
The stripes were then airbrushed using Tamiya Semi-Gloss Black acrylic. Why do I keep using Tamiya products? There are no hobby products out there that are higher quality and more consistent, that's why. They are a little more expensive to purchase up front, but still cost far less than the time one usually wastes redoing something as a result of using sketchy products.
After the airbrushed acrylic dries for a few minutes the masking comes off, revealing perfect stripes!
Next, we'll paint the chevrons on the airframe.
Separate names with a comma.