Editing Delta II (section)

=== Solid rocket boosters ===
For additional thrust during launch, the Delta II used solid boosters. For the 6000-series, Delta II used Castor 4A boosters (sometimes stylized as "Castor IVA"), while the 7000-series used GEM 40 [[Graphite-Epoxy Motor]]s manufactured by [[Alliant Techsystems]] (ATK). Like its predecessors, the Delta II 6000-series was only offered in nine-booster configurations. However, with the arrival of the 7000-series, variants with three and four boosters were introduced to allow Delta II to fly small payloads for cheaper.<ref name ="Guide"/> When three or four boosters were used, all ignited on the ground at launch, while models that used nine boosters would ignite six on the ground, then the remaining three would ignite in flight after the burnout and jettison of the first six.<ref name="data sheet"/>

The Castor 4A boosters were an improvement over the Castor 4 motors flown on the earlier [[Delta 3000]] rocket, replacing the propellant with more modern [[Hydroxyl-terminated polybutadiene|HTPB]]-based propellant and providing an 11% increase in performance.<ref name="Castor4A">{{cite web |url=http://www.astronautix.com/c/castor4aengine.html |title=Castor 4A engine |first=Mark |last=Wade |website=Encyclopedia Astronautica |access-date=2024-02-27 |archive-url=https://web.archive.org/web/20221220153857/http://www.astronautix.com/c/castor4aengine.html |archive-date=2022-12-20}}</ref> The GEM 40 boosters on the 7000 series further improved Delta II's performance by each featuring {{convert|2.5|t|lb}} of extra propellant than the Castor 4A thanks to a lengthening of {{convert|3|m|ft|sp=us}}. In addition, the GEM boosters also boasted a lower dry mass than the Castors due to the former's carbon composite construction.<ref name="gem40">{{cite web |url=http://www.astronautix.com/g/gem40.html |title=GEM 40 |first=Mark |last=Wade |website=Encyclopedia Astronautica |access-date=2024-02-27 |archive-url=https://web.archive.org/web/20221223211757/http://www.astronautix.com/g/gem40.html |archive-date=2022-12-23}}</ref>

In 2003, the Delta II Heavy debuted, featuring larger GEM 46 motors from the abandoned [[Delta III]] program. These new motors allowed the vehicle to carry over {{cvt|1,000|kg}} of extra payload into low-Earth orbit. Only Cape Canaveral Space Launch Complex 17B was capable of flying the Heavy configuration, as it was previously reinforced to handle the Delta III.<ref name ="Guide"/>

The Castor motor nozzles were angled 11° from vertical to aim their thrust into the vehicle's center of gravity, while the GEM motors had a slightly lower 10° tilt.<ref name="Castor4A"/> On the nine-engine configuration, the three air-lit motors featured longer nozzles to allow the boosters to perform better in the upper atmosphere.<ref name="D2PG"/> All solid motors that flew on the Delta II featured fixed nozzles, meaning the first stage was solely responsible for vehicle control during the initial portions of the flight.<ref name ="Guide"/>