Astro Jr

SEMROC Model #KA-4


The Astro-Jr was one of the first model rocket kits to incorporate the “Quick-Change” engine mount that became a standard in model rocketry.
Designed to be an excellent beginner’s kit, the Astro-Jr became the first rocket for thousands of new model rocketeers.
The “rake-back” fins were so different that many were built incorrectly with the fins glued
on to resemble the Estes Alpha. The Astro-Jr is a smaller version of the Astro-1.

Additional Information

  • Length: 14.90″
  • Diameter: 0.91″
  • Weight: 0.70 oz.
  • Fin: Balsa
  • Fin Span: 4.10:”
  • Recovery: Parachute
  • Recommended Motors:
    • 1/2A3-4T, A10-4T, A3-4T


Photo Gallery

Build History

  • 10/27/2004 – Purchased Semroc #KA-4 Astro-Jr from Semroc for $8.00 (Retail $8.00)
  • 01/14/2007 – Took pictures of package and components.
  • 01/15/2007 – Scanned in components. Measurements are located on page 52 Book #3.
  • 01/16/2007 – Tied a loop into one end of Kevlar cord. Looped it around the engine clip and pushed the clip through motor mount tube slot. Pushed one centering ring over the tube and clip to to the loop. Pushed the other centering ring 3/4″ from the rear. Glued the thrust ring into the forward end. Filleted one side of each centering ring joints. Marked the entire length of the body tube fin guidelines using a door jamb.
  • 01/17/2007 – Filleted other side of centering ring joints. Glued motor mount assembly inside body tube so the tube ends are even.
  • 01/18/2007 – Glued three fins along guidelines. Filleted fin joints. Glued screw eye into base of nose cone.
  • 03/14/2022 – Spray painted entire model with Red Primer.
  • 02/28/2023 – Spray painted fins with Krylon #51601 gloss black.
  • 05/12/2023 – Removed nose cone and masked body tube and nose cone. Spray painted fins with Rust-oleum #254146 gloss black and nose cone with Krylon #8803 gloss banner red. Removed masking.
  • 05/14/2023 – Applied water slide decal susing picture as placement guide. Took pictures of completed model. This completes construction of Semroc #KA-4 model rocket.








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