Cloning and enhancing another classic - Space Shuttle with boosters

OpenRocket simulation of my boosted Space Shuttle in flight

Back when I was a BAR99 (born-again rocketeer in 1999) I built and launched an Estes #1284 Space Shuttle in October 1999.  It was a complicated kit, with 54 steps.  The final rocket flew on a C6-3 to about 200 feet then the shuttle separated and sort of glided back to the ground, with the main rocket recovered with its own 18" parachute.  I flew it successfully a couple times and is still on my shelf.  Estes sold this kit from 1976 to 1998, and it's occasionally available now for exorbitant fees on eBay and elsewhere.

I've been intrigued by something Apogee Components released a couple of years back that allows us to attach booster rockets to a main rocket that will fall away after from the main rocket after their booster engines are exhausted.  The original Estes Space Shuttle used the "boosters", which were glued on, to hold stabilizing fins that could be removed for display but provided nothing to help the Shuttle go higher.  I kept this interest in the back of my mind for months and then revived this idea when I stumbled upon three resources to help me accomplish my project:

1. A user named Jeff Caldwell had posted a detailed RockSim simulation of the original Estes Space Shuttle on RocketReviews.com.  I could use OpenRocket to export and 3D print the Shuttle itself, and any other parts I might need.

2. Separately, a user named ScottDee shared the complete 3D STL files for the original Estes #1284 Space Shuttle on Thingiverse.  He didn't provide the Shuttle (i.e., the glider) itself, but everything else I would need to recreate the kit.

3. Finally, a user named JackHydrazine shared the STL files needed to 3D print a strap-on booster kit for BT-70 main rocket and BT-50 strap-on boosters.  I'll use these for the external "SRB" (solid rocket booster) boosters attached to the main body ("external tank").

I printed ScottDee's parts and quickly discovered two problems:  First, the original Estes shuttle used a “BT-67” body tube for the main external tank and used "BT-46NI" body tubes for the SRB bodies.  Neither of these body tubes are still available, so I decided to use BT-70 (2.175" outer diameter) for the external tank and BT-50 (0.976" outer diameter) for the SRB boosters.  I also wanted to use a 24 mm engine (D/E) instead of the original 18 mm engine (C), so I had to cut a hole in the external tank bottom piece in Tinkercad to accommodate the larger engine.  It took several tries to correctly resize the part, some when slicing the object in Cura and some resized and edited in Tinkercad like when I had to enlarge the engine hole but I finally got them all printed.  Next, I exported just the shuttle from the RockSim file using OpenRocket and printed the shuttle separately, which turned out great and actually glides a bit.  Finally, I printed the files for the strap-on booster kit (BT-50).  I wanted to use clear fins for display and flight, so I printed these separately using clear PLA, then realized I needed to modify the SRB cuffs to accommodate the 1.5 mm fins.  I stretched the cuff part, making it almost an inch tall, to better stabilize the fins.  I'll need to glue these to hold the fins then attach the cuffs + fins to the bottom of the BT-50 tubes.  I printed the SRB nose and cuffs using white PLA, in case I don't paint it before flying, but printed the rest of the parts in grey (except the white shuttle) since I'll want to paint the main body (external tank) orange.  I also needed to print BT-70 cutting guide, since I'd never used that before but had some BT-70s I'd ordered for another project.  Here are some photos of the parts described above:

Shuttle being printed with supports

     

Here's the trimmed but unpainted shuttle, exported from RockSim

Printing the ~transparent fins

Printing the strap-on booster clips and caps

Test fitting a BT-50 tube into one of the "stretched" SRB bases with slots cut for the transparent fins

All the final 3D printed parts laid out for review, including the two centering rings with the 24 mm engine mount.
Note the grey retaining clips sticking into the bottom of the small white SRB nose cones, used to hold it until SRB engine ejection.

After all the printing was complete, I ran simulations in OpenRocket using the corrected body tube sizes, engines in the boosters, and added the final printed weights to account for each component.  The simulations show the final (unpainted) weight should be about 9.5 ounces (the original Estes model was 4.4 ounces), that I need to add 1.5 ounces of weight to the main body's nose cone (external tank), and that it should fly stably to almost 300 feet on a C11 plus two B6 engines, to almost 500 feet on a D12 plus two B6 engines or to over 700 feet on a D12 plus two C6 engines.  Normally, I would want to use 0 second delays on the boosters but am worried the main rocket would lose stability when the (only) fins fall off.  If I use same delay for the boosters and the main engine I should be able to get away without needing fins on external tank!  Here are some simulations:

An OpenRocket simulation showing the center of gravity and the center of pressure on the simulations and the flight summaries for three combinations of engines.

 

OpenRocket simulation of flight on a C11 and two B6s

OpenRocket simulation of flight parameters using a D12 and two B6 engines

Next steps:  It's time to cut the body tubes and start glueing things together.  Aligning the clips and cutting slots to hold the external booster will be tricky (there's a hole under each SRB nose cone that releases from the pin mounted on the main body when the booster engine ejects, causing the booster to fall detach and fall off)  but the whole project is less intimidating than the original's 54 step instruction guide.  Assembling the shuttle, trimming it and getting it balanced for flight in 1999 was a several day project.  Now I just need a few hours to reprint if this one gets lost at launch!  I'll focus on assembly, minimal painting and add decals later if needed.  I even printed SRB caps to cover the bottom of the boosters, though haven't figured out how I'm going to display this (an extended 24 mm engine mount or two 18 mm mounts below the SRBs, even though the boosters are just held on by clips; see the parts on the cutting board, above).  I still need to figure out some things like engine mounts, whether to uses Kevlar to connect the shock cord and whether to use 9" parachutes or streamers to recover the fall-away boosters.  I'll post updates when I finish and (hopefully) launch this soon!