The Delta is a expendable medium capacity launch vehicle first
flown in 1960. The Douglas Aircraft Company was commisioned to build
the Delta in April of 1959, as a civilian launch vehicle based on the
Thor Intermediate Range Ballistic Missile. It was originally called
the Thor-Delta, since it was the fourth modification of the Thor
missile.
The first Delta launch failed due to an attitude control problem in
the second stage, but the next 22 launches were successful. The
original Delta spacecraft could place up to 100 pounds in geostationary orbit or
500 pounds into low earth orbit, but in 1962 Douglas Aircraft started
a series of modifications and enhancements that would increase Delta's
capacity tenfold over the next nine years. Since then dozens of
additional configurations have been flown.
The most powerful current configuration is the Delta III, which is
capable of boosting 18,280 pounds to low earth orbit or 6,000 to
geosynchronous orbit. The Delta III is about 130 feet tall and uses a
Rocketdyne RS-27A motor in its first stage, which burns liquid oxygen
and kerosene to produce 244,100 pounds of thrust, assisted by nine
Alliant solid-fuel boosters (six of which are ignited at lift-off and
three are ignited in-flight) that produce 1,244,100 pounds of thrust.
The second stage uses a Pratt & Whitney RL10B-2 engine, which burns
liquid oxygen and hydrogen, which has a push of 24,750 pounds and can
be turned off and re-started.
The Deltas have been the workhorse of satellite deployment. Many
telecommunications satellites, most weather satellites, all of the
Global Positioning System satellites and a variety of scientific and
explorational satellites and probes rode into service atop Deltas. As
of this writing, Delta has seen over 40 years of service, with 292
flights, only 15 of which were total failures (a success rate of over
94 percent).
A Delta IV is being developed by Boeing, but it's a complete redesign
with almost no similarities to the existing Delta line.
Sources:
http://kevin.forsyth.net/delta/
http://pao.gsfc.nasa.gov/gsfc/lithos/delta/delta.htm