I flew my Lego Technic "Death Star Booster" rocket on an Estes C6-5 model rocket motor today. It flew to ~300 feet before the ejection charge popped off the Lego Death Star "11x11 Hemisphere" parts #98114 + #98115 (from the current Lego 9676 set).
About this creation
FLIGHT #1 (Version 1)
The rocket consists of only 36 Lego pieces (no gluing). Eight #2780 "Technic Pins with Friction" hold the #64782 "Technic Panel Plate" Box Fin assembly together. Using 8 more "Technic Pins with Friction", four #32278 1x15 Technic Liftarms clip onto the gray #32324 "4x4 Technic Brick with Open Center" (placed upside-down to accommodate the black #3709b "2x4 Technic Plate with 3 Holes"). The Death Star hemispheres have studs on the top and bottom to press onto the Technic Plate.
I had to carve the cardboard rocket motor casing with a knife a bit, to enable it to fit inside the "4x4 Technic Brick with Open Center". Remove just enough (about 1/4") on four sides of the top of the motor casing, to make a square-sided shape that is fairly loose. You want the motor's ejection charge to deploy the Death Star (via the holes in the Technic Plate) AND push itself off of the rocket frame at apogee. The motor hangs down; make sure it is aligned well for straight thrust.
The black rocket frame and Death Star both impacted the earth hard, but everything just pops together for more flights! Nothing was cracked or broken. You can remove the minor black soot on the Death Star and Technic Plate by using White Vinegar.
The rocket can also fly on an 18mm C6-3 motor (with a 3-second delay). Holes in the edge of the 5x11 Technic Panel Plates allow you to place the rocket on a standard 1/8" launch rod (as shown in the picture).
I rebuilt my Lego Technic Death Star Booster Rocket's body, to make it lighter. You can see how I carved off the top of the cardboard motor casing so that it fit into the 4x4 Technic Brick with Open Center. Instead of 1x15 Technic Liftarms, I used four #2637 1x16 Technic Links. The thinner profile and lighter weight of the link allowed the rocket to climb to ~400 feet (~120 meters) on an Estes C6-3 motor. The 3-second delay is more suited to this rocket, and the Death Star ball was deployed at rocket apogee.
You can see how I carved the top of the cardboard Estes rocket motor casing, to enable it to fit inside the 4x4 Technic Brick with Open Center.
The flexible Technic Links absorbed the shock better than the liftarms. Even though I lowered the rocket's "Center of Gravity" by 3 studs (via a lower attachment point on the Box Fins), the booster still came down in a "ballistic" manner. I will fly it again using shorter, #6247 1x11 Technic Links. I want the booster to "tumble" down, so I need to make the rocket "unstable" AFTER it spits its motor at apogee.
FLIGHT #3 (Version 3)
For its third flight, I flew a shortened Lego Technic "Death Star Booster" rocket on an Estes C6-5 motor. I used four #6247 1x11 Technic Links to move the weight downwards. The total number of Lego parts is now 28. The YouTube video displays what I wanted it to do -- tumble back to earth softly. The "Center of Gravity" is now close to the "Center of Pressure". See this YouTube video to see the launch .