At a rate of 98.7%, and 155 launches for nearly 30 years, the Delta II rocket was a reliable workhorse that launched numerous spacecraft away from Earth. Delta II is part of the Delta rocket family and was designed by McDonnell Douglas. The rocket flew its final mission ICESat-2 on 15 September 2018, earning the launch vehicle a streak of 100 successful missions in a row, with the last failure being GPS IIR-1 in 1997.

Delta II 7925 Heavy after ignition with Dawn on Launch pad 17B
Delta II 7925 Heavy after ignition with Dawn on Launch pad 17B. Credits: NASA

The rocket played a significant role in launching NASA missions. Between 1998 and 2010, the rocket delivered nearly 60 percent of the agency’s scientific satellites into space. It has carried many notable payloads including the Mars Rovers — Spirit, Opportunity and Pathfinder.

Design

The first stage of the Delta II is propelled by a Rocketdyne RS-27 main engine burning RP-1 and liquid oxygen. It also has two vernier rocket engines that help provide vehicle roll control during flight, and each contributes more than of 4.45 kN of thrust to the main engine.

For additional thrust during launch, the Delta II used solid boosters. For the 6000-series, Delta II used Castor 4A boosters, while the 7000-series used Graphite-Epoxy Motors manufactured by ATK. The vehicle could be flown with three, four, or, most commonly, nine boosters. 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 in flight after the burnout and jettison of the first six.

Delta II 7425 diagram
Delta II 7425 diagram. Credits: NASA

The second stage is powered by an Aerojet AJ10-118K engine, a pressure-fed engine in which a separate gas supply pressurizes the propellant tanks to force fuel and oxidizer together into a combustion chamber.

Depending on the payload, the Delta II offers an optional third stage, but it can fly with only two stages. The additional spin-stabilized, third-stage motor, the Star 48B motor, is produced by Alliant Techsystems. Payloads bound for higher energy orbits such as GTO or to reach Earth escape velocity for trans-Mars injection or other destinations beyond Earth used finds the use of the third stage.