How are the boosters on the Delta II numbered?
Close observers have noted that each of the GEM strap-on boosters has a large number stenciled near its base (as well as a much smaller number just below each nose cone). These numbers are not in sequence. They go in threes, 1–3, 4–6, 7–9, equilaterally spaced. In other words, boosters 1, 2 and 3 mark the points of an equilateral triangle, as do 4–6 and 7–9. Boosters 4–6 are the air-lit boosters, and have larger nozzles for improved efficiency at altitude. On Delta III, which used the larger GEM-46 motors, boosters 7–9 had TVC nozzles. (Delta II-Heavy also uses GEM-46 motors, but this author speculates that TVC is not employed.
Close observers have noted that each of the GEM strap-on boosters has a large number stenciled near its base (as well as a much smaller number just below each nose cone). These numbers are not in sequence. They go in threes, 1–3, 4–6, 7–9, equilaterally spaced. In other words, boosters 1, 2 and 3 mark the points of an equilateral triangle, as do 4–6 and 7–9. Boosters 4–6 are the air-lit boosters, and have larger nozzles for improved efficiency at altitude. On Delta III, which used the larger GEM-46 motors, boosters 7–9 had TVC nozzles.
