Orbiter Assembly

Print out Figs. 2 (wing lay-out sheet) and 3 (glider recovery harness). The wing lay-out sheet will probably need to be slightly expanded for the scale dimensions to be accurate.

Cut out the two of the wing halves shown in the pattern sheet from 1/8" balsa stock (Fig. 2a). Cut out and save elevons from wing halves. Stack together and sand the edges so that they are identical. Leave all edges square for now.
Spread aluminum foil for a work surface. Mix and apply a small amount of epoxy to the root edge of one wing half, the butt it against the root of the other half. Lay them together, flat, on the foil. Add a layer of foil over them, weight the stack flat with a couple of large books.
Round one end of the 3/8" dowel (Fig. 2b). Sand about 1/8" off of one side of the dowel to form a flat surface. Peel the top layer of foil and books off the wings. Align and glue the dowel along the centerline of the wings, just far enough forward that it doesn't hang over the balsa point.
Peel the wings off of the aluminum foil, or vice versa. Sand the point of balsa extending past the rounded dowel tip back to where the dowel's rounding joins the wing's balsa; make this joint smooth. Round all leading edges, but leave the wingtips and trailing edges square.
Cut 6.3 cm of BT-20. Mark a straight line down one side, then draw a tic mark inside the ends of the tube, where the outside line meets the ends. Glue this down to the center line of the wings, with the back edge of the tube aligned with the back edge of the wings.
Cut out the two body side panels from 3/32" balsa (Fig. 2d). Taper by roughly sanding the shaded areas of the two panels, to make a right side and left side. Test fit against the body, aligning the outer edges with the inside edges of the deepest notch in the wing's trailing edge. They should lean slightly inward, with their tops slightly lower than the top of the center tube. Glue in place.
Cut a piece of card stock 6.3 cm x 2 cm. Apply 5 minute epoxy to the top edges of the two side panels, and position the card stock so that the back edge is square with the ends of the center tube and slightly overhanging the side panels. Let this overhang stick straight out from the orbiter body sides; it can be trimmed later.
Cut out nose wrap using pattern from Fig. 2e. The paper's grain should run perpendicular to the narrow end, which will be the forward end. Curl the paper slightly along the grain; be careful not to crink it. Test fit to the orbiter nose, curling as needed. There will be a lot of paper overhanging the top and side edges, but these will be trimmed in later steps. Also remember that any small gaps, present now or caused later by excessive trimming, can be easily filled. When you feel comfortable with the fit of the wrap, apply epoxy to all edges where the body wrap meets the other orbiter parts: front edge of the card stock top, tapered front edges of the side panels, top and leading edges of the wings in front of the side panels, and the top of the nose dowel where the hemispherical end meets the unrounded sides of the dowel. Hold in place by hand until glue sets.
Trim all excess glue and paper edges to make a smooth outline of the orbiter nose and top.
Cut out and shape the 2 vertical tail surfaces from 3/32" balsa (Fig. 2c). Stack and sand the two pieces so that they're identical. Round the leading and top edges, but leave the other edges square. Glue them to the tops of the wingtips, as shown in Fig. 2c, with their trailing edges square with the trailing tips of the "booms" left when the elevons were cut from the wings.
Once all glue joints are thoroughly set, carefully round the leading edges of the wings and tail surfaces. Also round the outer edges where the wingtips meet the vertical tails (not too much though; leave enough balsa for structural integrity). Sand down any rough areas where the card stock wraps meet the body. Be a little brutal in this to get a nice smooth shape. At this stage, the orbiter outline is almost being sculpted from the rough balsa and paper form. Leave all trailing edges square. Remember, this is a semi-scale model of a mid-'60s hypersonic vehicle, and that's how they built them then.
Apply a heavy fillet of body putty to the joints where the body panels and vertical tails meet the wing tops. Allow to fully dry.
Fill any gaps around the card stock wraps with Fill-N-Finish (FnF). Important: leave the gaps between the BT-20 center tube's trailing end and the surrounding balsa open.
Bend some of the music wire into the twisted-Z shapes shown in Fig. 2f. Make them mirror images of each other, to have a left side and right side part. Insert these into the trailing edges of the elevons as shown, so that when fitted into place on the wings the outer leg of each wire will catch the underside of the wing tip to limit the elevon's upward range to about 1/4" at the trailing edge. Remove the wires, add a dab of epoxy to the end, and re-insert into each elevon.
Give a rough sanding to all parts filled in step 13. Thin some FnF with a little water, to about the consistency of thick paint. Apply to all external surfaces of the orbiter, everything you can see but the inside of the BT-20 center tube and the gaps between the tube and the side panels. Allow to dry, sand, and repeat as needed -- at least once. Diluted FnF is a water-based sealer that can cause balsa to warp, so apply a little to the top side, then a little to the bottom side of the wings to balance out this tendency. Also, it can and will permanently warp the wrappers, but this can be fixed with subsequent sandings and coats. Give the as-yet unmounted elevons the same treatment.
When you're finally happy with the smoothness of the surface, prime and then give the orbiter and elevons several coats of gloss black. Apply a star & bar to the left wing top, about centered with the front of the vertical tail. Apply 'USAF' or 'U. S. AIR FORCE' in white lettering to each side panel (white "USAF"s are hard to find; model R.R. dry transfer lettering works here). Either designation is reasonably authentic, since both were shown at various stages on mockups and artist's conceptions. Apply a triangular window, made from metallic foil or simply painted, on either side of the cockpit. Because the X-20 was to have a heat shield over the front windows throughout flight until just before landing, and the rounded paper nose shroud has much the same shape, don't put any forward windows on; the prototype wouldn't have shown them at launch. For added detail, give the orbiter a light spray of matte finish, and carefully pencil in a few wing and body panel seams. Finish out with another coat of matte spray. Give the elevons the same matte coat.
Attach the elevons with strips of clear packing tape. Caution: this stuff grabs paint and decals, and doesn't give second chances. If you need to reposition something after it's taped down, chances are you'll be touching up the paint job. Be sure the elevons can flip up and down freely, and lay flush with the wings when in the down position.
Build the recovery harness shown in Figure 3 from light monofiliment fishing line and two 1" pieces from thin rubber bands Black rubber bands are best here if you can find them, but the harness removes for display and this isn't critical. Slip the two end loops over the elevon bars, and the big central loop over the nose as shown.
Screw an eye into the NB-20 nose block, and tie a piece of thread to it. Slip-fit the nose block into the center tube, sanding or adding tape as needed. It should slide freely when the thread is pulled or the block is pushed, but shouldn't slide when the entire orbiter is suspended by the thread.
Test glide the orbiter. The elevon angle is fixed by the wire bars, so trimming is accomplished by sliding the internal block backward or forward to change the CG. This glider tends to tail-heavy, so screw one or more washers to the front of the nose block as needed to shift the CG forward. When you're sure that the orbiter is well-balanced for gliding, mark how far the nose block is pushed into the center tube, and glue it in place. Be sure that there's enough room for the front end of the balsa adapter to fit into the center tube. Add some fixed weight up in the nose if needed, and re-trim. Now the reason for leaving the gaps between the body and the body tube is clear: it's room for rear "fudge weights," if you somehow mess up the balance after gluing in the balsa block. Finally, snip off the thread.

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