1/72 scale D-21 drone as an Unmanned Combat Air Vehicle drone.

[seadude]

I'm really not much in a modeling mood right now after hearing about the recent passing of a friend that I've known for about 40 years. But I can't sit around the apartment all day and twiddle my thumbs either. Somehow, I need to keep occupied and keep my mind stable so to speak.
So..........I had a leftover 1/72 scale D-21 drone from an old Revell SR-71 Blackbird kit. I'm thinking and hoping that it shouldn't take too long to build this up as the parts count is very small.
My goal is to convert it into a unmanned combat air vehicle with an internal weapons bay. A pic below shows two types of missiles I could potentially use. The yellow tape on the lower fuselage half shows approximately where the weapons bay may go. When it comes time to paint this small model, I'm not sure what to go with. Maybe just standard USAF gray colors I guess? I don't know. Any thoughts on anything, folks?

[sandiego89]

Gray is boring, and the black was better to dissipate heat, reduce radar cross section and reduce visibility in some conditions, so practically black or the natural titanium with black leading edges and high heat parts like the original D-21 and A-12's is more realistic. 

[Rick Lowe]

Gray is boring, and the black was better to dissipate heat, reduce radar cross section and reduce visibility in some conditions, so practically black or the natural titanium with black leading edges and high heat parts like the original D-21 and A-12's is more realistic. 

Sounds good, and it'll confuse people who think it's a standard D-21... "Hang on a bit..."

[steelpillow]

Depending on how realistic you want to be, and not fill up the engine ducting with gubbins Thunderbird 2 stye, how about giving it a bit of a pregnant tummy for the payload? It has a slight "chin" near the front for the camera bay, under the rear half of the chines. You could stretch that further back, under the CG at approx 50% of wing root chord.
"Blackbird" blue-black overall, with burnt metal hot bits and a few bright orange ID markings might look cool.

[seadude]

Alright, so I'll think about the painting for later. It's not important right now.
But I am curious on people's thoughts for a small internal weapons bay.
I don't know what Lockheed's thoughts were, if any, on designing/converting the D-21 to carry any sort of weapon payload. I've got plenty of books on the history of the SR-71. But none of them give any information as to other design considerations for the D-21.
But I was actually thinking of building this model as a subject if it were built in today's timeframe/era instead of the 1960's.
What other improvements, if any, should I make?

[kerick]

This looks very interesting. I'll be checking in to see the progress.
As for modernization changes, I don't see much opportunity for that. It was a design of its times and changes could lead to a lot of drastic work to make it look in the 2026 era. Some GPS antenna, get rid of those sensor pointy bits up front, weapons bay underneath. That's about it for me. Just my $0.02.
This does give me an idea what to do with the F-117 bodies that someone gave me.

[steelpillow]

building this model as a subject if it were built in today's timeframe/era instead of the 1960's.

Given the general configuration, not much would change simply due to the advance of technology. Modern materials and optimisations to engine and aerodynamic subtleties might allow a modest speed increase, with higher combustion temperatures and, say, slightly more rounded leading edges and cone tip to limit heating there. Reprofile leading edge to taste: increase sweep with shorter span would reduce drag and suggest faster cruise, at the expense of reduced lifting area. But then, you could fatten the root to further reduce weight at the expense of that saving in drag, so it wouldn't matter.

Any more significant changes would arise only due to changes in operational requirements, such as from camera to weapons fit, Mach4+ hypersonic speeds, increased payload-range, low-speed launch/takeoff or reusability, etc. These of course may conflict.
Hypersonics require a scramjet, which is much shorter and typically with belly intake. The shorter span and fatter roots would be appropriate here. You'd also have to find a new nose and fiddle with the chines.
Any additions heavier than the camera will reduce maximum speed and range, but maybe the engine/aerodynamic enhancements would compensate. The addition of a weapons bay, as opposed to simply replacing the camera with an explosive charge, suggests either multiple small ordnance or reusability. Multiple ordnance suggests reasonable loiter time, which militates against hypersonics, maybe even for a dual-cycle turbo-ramjet Blackbird J-58 style (no visible difference though). For reusability, undercarriage would challenge the payload space, maybe the deeper root mentioned above, or widen the belly so that it no longer matches the upper surface. Or instead, some kind of air- or water-borne recovery. 2D vertical-vectoring exhaust would help with the trim changes over the speed range; or, flip-out "moustache" canards retracting into the chines.

Or, throw engineering reality to the winds and just go where the flow takes you. No harm in that.

[seadude]

Made a little bit of work yesterday for the internal weapons bay. Pics below.

[kerick]

Interesting! Keep at it!

[seadude]

And more progress today. Glued the top and bottom fuselage halves and also did a bit of puttying.
Hope to start painting sometime this week as soon as I figure out what colors to paint it.

[kerick]

There is that USAF HAVE GLASS paint option if you are going totally modern. Someone had a recipe for mixing a couple colors for it including the sparkle look.

[seadude]

I think I've finally figured out the paint for this drone. For those who have been painting their SR-71 Blackbird models all black.........you're doing it wrong.
According to various sources, the SR-71's and/or D-21 drones were painted "anthracite" which is a very deep dark blue or blue/black.
It comes close to matching Vallejo # 71.115 Blue Grey (RAL 7016).
In Kelly Johnson's biography book "KELLY: More Than My Share Of It All", on page 144 he mentions that the Blackbirds take their name from the dark blue-black paint. The color of the Blackbirds becomes blue as temperatures increase at high speed and altitude.

[kerick]

I think the blue black color is a good choice. The HAVE GLASS is a future option depending on your next project.
It appears I was behind the times as there several paint companies offering this color.

[steelpillow]

On the ground, the paint is almost black, with just a hint of deep indigo blue. In flight it gets a more pronounced blue, especially where it is hottest around the leading edges and jetpipe. Over time, it fades and well-used craft ended up with a patchwork of repaint and replaced composite panels along the leading edges, as they also degraded under heat.
The "iron ball" paint is a fascinating technology. Both high thermal emissivity and stealthy radiation-absorbing material (RAM). It had to be constantly agitated when spraying to stop it separating, and then cured with both heat and magnets while it dried. Totally toxic, so all done in protective suits.
The F-117 Nighthawk had a low-temperature derivative that was very fragile. My Vulcan B Mk.4 has the British equivalent, applied as a powder coating so it was easier to control the thickness and more durable.  Got the first coat of European Night Sky on the notorious "chocolate triangle" now, must post some more pix.

[seadude]

I've been doing a deep dive on the Internet the last few days regarding the paint of the SR-71/D-21's. It ain't easy nor is it exact. 
The only REAL people who know the color of the Blackbirds are the people who created the formula for the paint, mixed the paint, and sprayed/applied it to the actual aircraft.
I've asked several questions during Google searches and the below screenshots are what I got back.