SR-71 or SR-72?

On the NAR Facebook page, Frank Burke posted a picture of his latest R/C Boost Glider:

"YF-12 interceptor version of the A-12/SR-71 family. Hardly ever modeled. This was an interceptor version that wound up being used for testing the aim 47 missile and doing tests for NASA. The chines were shorter on this design because of the large rounded nose cone that held the radar system. The R/C Rocket glider uses 9mm depron, bt80 tubing and is designed for 32mm glider G-12 reloads. Currently 16 oz airframe weight. with electronics, paint, nose weight and motor it'll be about 26 and 1/2 ready to fly so it should be a pretty good performer."




I thought this profile looked familiar. I put together the (much smaller) Apogee SR-72 Darkbird a while back. 
To see the build: CLICK HERE

At the time, I asked Tim at Apogee where he came up with the name. He said: "We wanted to show the next generation of the SR-71, we call ours the SR-72."

Looks like the Apogee kit was based on the same YF-12 Interceptor that Frank built. I never even knew it existed.

Apogee SR-72 Darkbird Build Part 9, Finished

This flat black paint ended up more like a satin finish. The dull black is more "Stealthy" anyway. The design works well, sort of a profile SR-71 style.
The only thing I don't like is the rough surface of the hinge. I'm sure it could be smoothed out, I just didn't take the time.

This small pop-pod glider operates like the Estes Sky Dart and original Citation Bomarc. Yep, the first Estes Bomarc did "glide". Later versions of the kit had parachute recovery.

Not much color and no decals on this one. Test glides and a launch await after they find a new NEFAR field! Fingers crossed.

Apogee SR-72 Darkbird Build Part 8, Paint Problems

I thought I would save some glider weight by not doing a filler primer coat after the CWF was sanded down.

Carpenter's Wood Filler does not seal the balsa wood.
After the first light gloss black was sprayed I was left with a blotchy surface. Even though most all of the Duplicolor Filler/Primer is usually sanded off before color paint is sprayed, it still seals the wood and allows for a smoother paint coat.

The other weird thing was the drying time. The paint on the wood surfaces dried as normal, but the body tubes took four days to dry! Central Florida weather has been mild this week with very low humidity. The paint was a gloss black Rustoleum enamel.

I did some dry sanding to even up the surface.
On the right I used a wide point permanent marker at all the root edges. Sometimes it's hard to get spray paint into the inside edges. I didn't want to add any more paint weight than I have to.

With the surfaces not being as smooth as I would have liked, I switched over to flat black paint. Flat paint can help hide a rough build. Gloss paint shows everything!

The back end of the engine pod got some black from a wide point marker.

Apogee SR-72 Darkbird Build Part 7, Elevon Hinge

The elevon hinge is a fibrous, almost transparent piece of cloth. I've seen this material before but can't recall where.

The instructions say to use CA glue but I was concerned that would make it stiff and possibly crack later on.
Because it is so porous, I knew white glue would work. I applied the glue and spread it out evenly with a Q-tip. I glued half onto the elevon first.

The other side of the hinge is centered on the back edge of the wing.

The picture on the left also shows the elevon stop pieces, the forward ones are glued in place.






I used the dihedral guide piece to find the correct start angle and set the rear stop piece.

The top picture shows the small orthodontic rubber bands hooked over the outside ends of the elevon stop pieces.
This picture has the pop-pod engine mount in place. The stop wing is holding the elevons flat, even with the surface of the wing.

In the lower picture the engine mount and stop ring are slid out the back. The rubber bands have raised the elevons.

Apogee SR-72 Darkbird Build Part 6, Ram Jet Nose Cones

The ram jet nose cone bases were a little wide to fit into the BT-5 tubes.

On the left is the cone base before.

On the right is a cone base that was sanded flat and the sides cleaned up. It didn't take much sanding to get a better fit.

It's going to be hard to get the spray paint inside the ram jet tube with the nose cone glued in place.

I pre-colored the inside of the tube edge and near the base of the nose cones with a wide Sharpie.

A small ring of Fabri-Tac glue was applied inside the tube and the nose cone slid in place.

You'll have to use your eye to center the cone. Both cones should be slid in the same depth.

Apogee SR-72 Darkbird Build Part 5, Pop-Pod Assembly & Streamer

This instruction drawing was a little confusing. It reads: "Glue four large centering rings . . ." The drawing only shows two wide rings.

Here's what they mean.
Four 5/20 centering rings, stacked and glued on the BT-5 pop pod tube. The top ring is even with the end of the tube.

The plastic cast hold down ring was too tight on the rear end of the pop pod tube.

It took some light carving and sanding on a dowel to get a friction fit.

The hold down ring is slid on.
I applied some Fabri-Tac glue fillets.

Fabri-Tac seems to be very good for plastic to tube joints. I wouldn't recommend it for delicate plastic bonding like the Estes Mercury Redstone capsule and tower assembly.

The red Mylar streamer is attached to the low end of the pop pod with a piece of tape.

With a A3-4 engine in place the pop pod assembly was balanced and marked. The taped streamer was centered on the balance point.
The balance point would have been more accurate if I used a spent engine casing.

Apogee SR-72 Darkbird Build Part 4, Small Fins, Launch Lug & Canopy

The smaller top fins are glued onto the BT-5 ramjet tube at an angle.
The approximate angle was sanded on the root edge with my sanding block.

I couldn't find the pattern sheet in the kit. There is some drawings on the instructions I scanned and printed.
The fins were glued on to the correct angle.

The launch lugs are glued onto the bottom of the BT-20. They are not directly centered but set to one side.

TIP: At one time I would use a launch rod to line up two launch lugs. I now use a bamboo skewer. It's about the same diameter and much lighter. The heavier rod can pull lugs out of line.

The canopy is also printed on the instruction sheet. This was also scanned and printed on 65 lb. card stock.

I assembled two canopies. I felt it was a little long so the length was trimmed a bit.

On the right you can see how little white glue was used to adhere it to the body tube.

The canopy isn't very high off the body tube but is easily seen on the finished model.

I slipped the nose cone shoulder into the tube for support when the canopy was pressed in place.

Apogee SR-72 Darkbird Build Part 3, Wing Gluing

As I usually do, all the balsa parts were filled with CWF before gluing the model together.

This balsa is only 1/16" thick. When one side is brushed with CWF it did warp. But after the other side was treated the balsa straightened out again.

The side view shows how straight the three piece wing is after smooth sanding.
To try and save weight for a better glide, I didn't spray the parts with filler/primer. That was probably a mistake as you'll see later on.

TIP: After gluing on the inner wing, I used my aluminum angle to check the alignment of the long wing piece.

Note the wings were glued on 1/2" forward of the end of the BT-20 tube.

The side BT-5 tubes are glued onto the outside edge of the inside fins. These are also spaced 1/2" from the trailing edge of the inside wing.

My sanding block was used as a flat surface to check the alignment.

Apogee SR-72 Darkbird Build Part 2, Wings and Fins

The main fins are long, made up of three pieces.

I set them down on a straightedge. The upper picture shows the main wing before the edges were fitted.


The joint edges got hit with a sanding block until a good fit was achieved.

The outside part of the wing is made up of two pieces. Both were glued up on a straightedge.

The instructions said to use CA glue for most of the rocket construction. I used yellow wood glue.

The instructions don't mention it, but I did round the leading edges of the wings after everything was glued together.

For the first time in a while - The tube marking guides actually fit the tubes. Everything wrapped and lined up. Thanks Apogee!

Why can't all the vendors get this right?

Apogee SR-72 Darkbird Build Part 1, Parts

I bought this rocket a few years back, directly from Apogee.
The SR-72 Darkbird is 14" long, the main airframe is a BT-20.

This pop-pod boost glider operates like the Estes Sky Dart and the original Citation Bomarc. The long engine pod is ejected and returns via streamer. When the pod ejects, the elevons lift and the main body goes into a glide. The elevons are held down during boost. The elevon "up tension" is activated by small orthodontic rubber bands.

I asked Tim VanMilligan why it was called the SR-72. He said it was the next generation of the SR-71, a fantasy stealth bomber.

There are four 5/20 centering rings. The two identical die-cut balsa sheets are 1/16" thick.

To the right, the parts of interest:
I only found one orthodontic rubber band in the bag, I have others.
EDIT: In a comment, Eero Fluge pointed out, the second rubber band was around the streamer! I looked right past it and probably threw it away.
Two BT-5 sized vacu-form ram-jet nose cones.
The red Mylar pop-pod streamer.
At the bottom is the almost transparent elevon hinge material
In the center, the cast elevon hold down ring.