Delta III

Due to the continual size and mass growth of commercial satellites in the late 1980s, McDonnell Douglas realized the need for a higher-performance rocket than even their new Delta II. New satellite bus offerings from Hughes required a launch vehicle with a 4-meter diameter payload faring as well as the ability to send 3.5 tons of payload to a geostationary transfer orbit – neither of which Delta II offered. Multiple options for evolving the Delta II to support larger payloads were considered in the late 1980s and early 1990s, namely using higher-performing liquid hydrogen/liquid oxygen upper stages. Eventually, the Delta III was announced in 1995, boasting an evolved Delta II first stage and a second stage based on that of the Japanese H-II rocket. This led to Delta III being similar in size to Delta II, meaning that the existing Delta II infrastructure at SLC-17B could be used after some modifications. Soon after the announcement, Hughes placed an order for 13 Delta III launches. Delta III would only fly three times. The first two launches, both carrying live satellites, ended in failure. The third and final launch, carrying a dummy payload, was only partially successful after the RL-10B second-stage engine shut down prematurely. After commercial interest declined, the Delta III program was officially ended in 2003. Boeing then transitioned their focus to the new Delta IV rocket, which was much more capable than Delta III. Multiple Delta III rockets were already built and would have been unused, but they were cannibalized for parts for both Delta II and Delta IV. == Description ==

Delta III was developed from the Delta II rocket. The new vehicle sported a modified first stage and a new, more efficient upper stage. This led to Delta III having around double the payload capacity of Delta II. However, the consecutive failures of the initial Delta IIIs, combined with the more advanced Delta IV program and the continuing success of the Delta II, left the Delta III as an interim vehicle. The vernier engines were also used for attitude control after the main engine shut down, just before the second stage separated. A further refinement of the 5-meter diameter DCSS, known as the Interim Cryogenic Propulsion Stage, is used on the Block I Space Launch System rocket. Control of the second stage was provided by 4 sets of hydrazine thrusters installed around the bottom of the liquid oxygen tank. During engine burns, these thrusters only provided roll control (as the engine itself could gimbal for pitch and yaw). During coast periods, these would then provide 3-axis control. Star 48B third stage Delta III was offered with an optional Star 48B solid-fueled third stage. It would have been attached on top of the DCSS and contained inside the payload fairing. The Star 48B would have been used for high-energy orbits, like geostationary or interplanetary missions. It was never flown on Delta III but was commonly used on Delta II missions. The Star 48B has also seen use on Delta IV and Atlas V. Payload fairing Delta III's payload fairing was a new composite design, matching the upper stage hydrogen tank's diameter and allowing larger payloads than the Delta II's 9.5 or 10-foot-diameter fairing. Delta III's 4-meter fairing was derived from Delta II's 10 ft composite fairing. This fairing design would later be repurposed on the Delta IV Medium. == Launches ==