The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

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(GEMs) for improved performance. It bears little
resemblance to the original Thor-Delta. With a height
of 41 m, the Delta 7925 stands over 10 m taller than its
ancestor and has a GTO payload capacity of about
1,870 kg—more than 40 times that of the Thor-Delta.
Delta III and IV
Almost four decades of continual development have
increased the Delta’s payload capacity by steady incre-
Delta A Delta II launches an
Iridium communications satellite
from Vandenberg Air Force
Base. Lockheed Martin
ments. The Delta III, however, represents a sudden leap
in capacity to over twice that of the largest Delta II. A
major part of this improvement is due to the Delta III’s
second stage, powered by a Pratt & Whitney RL10B
engine derived from the RL10 that has been the basis
of the Centaur for over 30 years. Burning liquid hydrogen
and liquid oxygen, the RL10B has a world-record
specific impulse rating (an efficiency measure) of 462
seconds and represents the first use of a high-energy
cryogenic engine in a Delta. Additionally, new SRBs