Estes SR-71 Blackbird (Clone) Build Thread

[grapetang]

Fleet rebuild - adding another to the many great blackbird build threads here.
This one is a clone that I built with my kid in about 1.5wks.

Many thanks to Lockheed, Estes, JimZ, and the people who've posted prior.
Special thanks to LUNAR for the spare parts and Sandy H. for the Estes nose cone!

Helpful links:

• http://spacemodeling.org/jimz/est1942.htm
  
  
• sr-71-blackbird-build-thread-finished
• estes-sr-71-i-need-fins
• cg-on-a-stock-build-sr-71
• estes-sr-71-nose-weight
• papering-sr-71-fins
• estes-sr-71
• another-sr-71-blackbird-build

 

[grapetang]

Some changes:

• D-engine mount, re-inforced, with 18mm adapter
• 3/16 launch lugs
• lower ventral fins "corrected" with the larger edge forward
• balsa fins papered with tissue paper
• smaller 14" or 15" parachute w/ snap swivel

Problem areas:

• front of paper shrouds are weakening
• super tight parachute packing + holes in chute --> need more wadding
• the chute will jam or the tube in front of the engine/motor *WILL* burst

 

[grapetang]

Fins patterns from PDF being glue-stuck to cereal box cardboard.
Good thing for the scanned companion ruler as I had problems printing at scale.


Tracing patterns onto pre-sanded 3/32" balsa (2x sheets). So many pieces to cut...
Note the lower ventral fin and outer wing grain direction.
One sheet was denser & heavier than the other so we tried balancing the parts.
Also adjusted angles and lengths to line up.


Gluing multipart fins with Elmers wood glue, pressed flat in plastic sheets between two large tiles while drying.

 

[grapetang]

Gluing inner wings to main body tube with Elmers wood glue & double-glue joints.
The paper jigs work but it's good to weigh down the fins (front & rear) while they dry.


Papering the outer wings + stabs with Elmers Glue-All and scrap tissue paper from Digikey packing.
Papered both sides and pressed flat between plastic sheets and the tiles again while they dried.
Tried to leave 3/32"-1/8" exposed balsa at the root edge but that didn't seem to happen consistently.

We used tissue paper because it's thin & light. It had a shiny side and a matte/fibrous side.
In tests, I found better adhesion to balsa if I white glued the shiny side to the balsa.
We put glue on the balsa first and then applied the tissue paper.

Not sure how the matte side will paint & finish.


Fins dried and trimmed with scissors, blade, then light sanding.

 

[Cape Byron]

Problem areas:

• front of paper shrouds are weakening
• super tight parachute packing + holes in chute --> need more wadding
• the chute will jam or the tube in front of the engine/motor *WILL* burst

Got room for a baffle or laundry shelf in there?

 

[grapetang]

Got room for a baffle or laundry shelf in there?

Baffle: probably, right above the engine mount.
Shelf: probably not. It's really tight with the shock cord + 15" chute and I'm worried about deployment. The original called for an 18" chute. I really do see a chute jam or tube rupture at some point. Ugh...

Thanks for suggestions and for reading! We may try the baffle.

 

[Cape Byron]

Baffle: probably, right above the engine mount.
Shelf: probably not. It's really tight with the shock cord + 15" chute and I'm worried about deployment. The original called for an 18" chute. I really do see a chute jam or tube rupture at some point. Ugh...

Thanks for suggestions and for reading! We may try the baffle.

Maybe thin mil nylon for the chute? They pack small.

A baffle really close to the MMT will have a hard and short life. #askhowiknow

Maybe work out your packed chute size and shove the baffle up as high in the airframe as you can, given the space.

 

[grapetang]

D-engine mount (not E). Note the paper reinforcement under the front hook.
Engine block too and a sliver of engine block aft of the front hook.
The hook is on the top so as not to interfere with the launch lugs on the belly.


Shrouds traced and cut. Note the slot on the top/dorsal shroud for the D-engine hook.


Gluing the shrouds on with Elmers Glue-All. Using an angle bracket to help form the curve.
Per everyone's suggestion, the nose cone and engine mount were temporarily installed to support the main tube here.
(Ignore the cat butt in the picture that rolled a natural 20 on attack).

 

[grapetang]

Maybe thin mil nylon for the chute? They pack small.
A baffle really close to the MMT will have a hard and short life. #askhowiknow
Maybe work out your packed chute size and shove the baffle up as high in the airframe as you can, given the space.

Will look for the thin mil ones. Thanks!

Re: baffle next to MMT: Uh oh, I can guess... Hope your rockets with this fared well before the farewell.
This is why I'm afraid of a tube rupture above the MMT.

What have other people used for this rocket's recovery?

 

[grapetang]

Not shown: glued the nacelles on with Elmers Wood Glue. The center tube is levelled with the jigs under the front & rear fin edges.
Note - the nacelles (only) should be elevated by the shroud thickness to line up with the main fuselage.
Slide extra shroud cardstock under the nacelles only.

Gluing the outer wings on with Elmers Wood Glue and the jigs. Weigh them down while drying.
Again, the outer wing jigs & the nacelles should be raised by the shroud thickness to line up with the main fuselage.
Slide extra shroud cardstock under the jigs & nacelles only.


Glued the vertical stabilizers with Elmers Wood Glue.
The stab guides looked off so I changed them for 3/32" wings with 3/32" stabs at 100 deg (top) and 80 deg (bot) accounting for the wing & stab thicknesses.
Note the corrected lower ventral stab orientation with the larger edge forward.

 

[grapetang]

I don't know what the official stats are but this is what we used :
Patterns attached.

• Nacelle inlet cones: 1.5" long, white glued in with 1.25" extending out the front.
• Nacelle tail cones: 0.75" long, 0.8" aft diameter, white glued in.

Tried to glue the cardstock cones with the lap seams facing inward, but that didn't quite happen.



18mm/B/C engine/motor mount adapter.

Made a mistake where the 18mm engine extended 1/2" past the aft fuselage rather that 1/4".
(1/4" sticking out for D-engine + 1/4" sticking out for B/C engine = 1/2").
Was worried about shifting too much weight back, so:

• ripped the front CR off & repositioned 1/4" aft (adapter is now 2.5")
• added a cutout in front to allow adapter to slide past the D-engine hook
• added the 1/4" popsicle stick extender to the rear (mount is back to 2.75")

 

[grapetang]

Added about 7.5-8g nose weight in clay. This is for the 18mm B/C engine and has not been balanced for a D.
Should've checked earlier but I originally put in 25g rather than 0.25oz and it was a pain to get out.

Added about 3.5ft of 1/8" sewing elastic for the shock cord, white glued in with a G. Harry Stine / Centuri 3-holed shock cord mount.
I originally used 1/4" sewing elastic but there's no room for laundry so I cut down and tied on the 1/8" elastic.

Drilled a hole on one side of nose cone shoulder for a stronger attachment point, and added a snap swivel.



Not shown:
3/16" launch lugs added. We moved them 1.125" up from the bottom and 3/16" down/back from the rectangular cutout.
The D-engine hook is on the top side so as not to interfere.

Dry weight: no engine, no laundry, no paint:



Need to attach the 14" or 15" parachute but she's done for now.

 

[grapetang]

Aug 27, 2022: Flying with LUNAR at Brigantino Park, Hollister, CA.
2x successful flights with C6-3, 400-500ft altitude on the 1st flight.
Angled the rod & arched into the wind on the 2nd.

Drag raced an Apogee SR-72 Darkbird (it won).

It was quite windy but they waited for a calm before launching. Paul P. (I think) from LUNAR had a
great suggestion to turn the blackbird sideways so there would be less area exposed for weathercocking.

Laundry was tight both times but they ejected. There are burn marks on the chute so it needs more wadding.
I'm still worried about melting the chute, burning the elastic, jamming, and/or bursting the main tube.

 

[grapetang]

Flight videos:

Next/TBD:

• fix the front shrouds
• ??? fill the balsa edges?
• paint & decal/detail - hopefully won't be overly heavy
• make adjustable D-engine nose weights or use an appropriately weighted swappable nose cone

Thanks for your interest and thanks reading! Comments, questions, and/or suggestions welcome!

 

[grapetang]

Looks like I have some unexpected time on my hands.

Shaved down the wing extensions even with the nacelle front.


Not shown: Filled fin edges with 2-3 thin coats of Elmers wood glue, filed flat.

Cut off weakened, problematic shroud front.




Fill (rather sloppily) front & rear shroud gaps; front w/balsa, rear w/cardstock.

 

[grapetang]

Rebuild nose cone shroud portion, ala "zipperless" design. 2-layers of cardstock with the cutoff balsa strake fronts, wood glued to the nose.



Of course, wood glue does not stick to polystyrene...

so use hot melt glue (4 blobs applied from the back) to keep the shroud in place.


Not perfect but "That'll do, Pig. That'll do."



Additional weight penalty = ~6g.
Will wait for suitable weather & time to spray bomb.

ps - Found a blueprint drawing with stab angles shown at 15 deg (top) and 11 deg (bot). This model used 10 deg from vertical though 15 deg on both may look better.

 

[dpower]

Interesting clone build, and reinforcing techniques! The nacelle nose/tail cones look really nice.

 

[grapetang]

Interesting clone build, and reinforcing techniques! The nacelle nose/tail cones look really nice.

Thanks! I appreciate the comment!

Does anyone have the decals re-drawn, by any chance? If not, I'll try my hand at it and use white laser decal paper with the surrounding areas blacked out. I found the Amarillo font here: amarillo-usaf.font. My kid has ideas for the tail art so this blackbird may look a little different. Any help would be appreciated! Thanks!