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

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Afamily of launch vehicles derived from the Thor
ballistic missile for the purpose of placing intermediate-mass
civilian payloads into orbit. First introduced
in 1960 and manufactured by Douglas Aircraft,
the Delta was conceived as a short-term solution to
NASA’s launch needs of that era, but it continued to
evolve and remains in heavy use today.
The Thor had already been adapted for satellite
launches using a variety of upper stages, giving rise to
the Thor-Able, Thor-Able Star, and Thor-Agena A. The
intended name, Thor-Delta, reflected the fact that this
was the fourth upper-stage configuration of the Thorbased
space launch vehicle. However, since the vehicle
was designed mainly as a civilian satellite launcher, the
military name Thor was dropped. The same naming
principle was applied in the late 1950s as the Redstonebased
and Jupiter-based rockets were renamed Juno I
and Juno II, respectively, for civilian satellite launching.
But as far as extant missile-descended rocket families
are concerned, the Delta is unique: the Atlas and Titan
retained the names of their military ancestors.
The original Delta was a three-stage launch vehicle
consisting of a Thor first stage mated to improved
Vanguard upper stages. The first Delta, launched on
May 13, 1960, and carrying Echo 1, failed due to a
second-stage attitude-control problem. However, the
Echo 1 reflight three months later was a total success.
In 1962, Douglas Aircraft began a series of upgrades
and modifications that would increase the Delta’s
capacity tenfold over the next nine years.
The Delta A was almost identical to the original
Delta but had an upgraded Rocketdyne first-stage
engine. The Delta B, with an improved guidance and
electronics system, longer second-stage propellant
tanks (to increase fuel capacity), and an upgraded Aerojet
second-stage engine, launched the world’s first geosynchronous
satellite, Syncom 2, in 1963. The Delta C
was similar to the B but with a bulbous fairing to house
bigger payloads and a new, more powerful third-stage
solid-rocket motor developed for the Scout rocket.
With the Delta D—originally known as the Thrust
Augmented Delta (TAD)—came a major improvement
that became a Delta trademark. Three Castor 1 Solid
Rocket Boosters (SRBs) were added to augment the
thrust of the first-stage engine (which was itself
upgraded) and thus the overall payload capacity. This
Delta
strap-on configuration had already been applied successfully
to the Thrust Augmented Thor-Agena D.
Ignited at liftoff and jettisoned during flight, the SRBs
gave the Delta the extra thrust needed to place operational
Syncom satellites into geostationary transfer
orbit (GTO).
The Delta E—originally called the Thrust Augmented
Improved Delta (TAID)—came with a further
upgraded first-stage engine and more powerful Castor
2 SRBs. The second stage was made restartable and its
propellant tanks widened. The third-stage motor was
from either the Delta D or a more powerful Air
Force–developed motor, and the payload space was
made still larger using an Agena fairing. A two-stage
version of the E, known as the Delta G, equipped with
a reentry vehicle, was specially made to carry Biosatellite-1
and -2 into LEO (low Earth orbit). Delta J featured
a new third-stage Star motor.
Referred to as a Long Tank Delta (LTD) or a Long
Tank Thrust Augmented Delta (LTTAD), the Delta L
used the new Long Tank Thor first stage with its lengthened
propellant tanks. The L, M, and N models were
identical except for their third-stage configurations.
The Delta L used the same third-stage motor as the E.
The Delta M used the same third stage as the J, while
the M-6 variant was the same as the M but with the
addition of three more Castor 2s, for a total of six. The
Delta N was a two-stage version of either the L or the
M, and the N-6 was a two-stage version of the M-6.
With the 900 series came an important evolutionary
step in the Delta program and a vital link to the
Deltas that followed. The Delta 900, which came in
two-stage (900) and three-stage (904) versions, featured
nine strap-on Castor 2s and an improved guidance
and electronics package. The second stage used a
more powerful Aerojet engine, the AJ-10, previously
flown as the Titan Transtage.
All subsequent Deltas used a four-digit numbering
system for model identification. The first digit indicates
the first stage and SRB type. For example, all 6000-series
models employ the Extra Long Extended Tank Thor
first stage with a Rocketdyne RS-27A main engine and
Thiokol Castor 4A SRBs. The second digit tells how
many SRBs are used—3, 4, 6, or 9. The third digit indicates
the type of second stage—0 (Aerojet AJ10-118F), 1
(TRW TR-201), 2 (Aerojet AJ10-118K), or 3 (Pratt &