Delta, Improved
Eighth in a Series Reviewing Thor Family History
by Ed Kyle, Updated 7/26/2009

Delta 34, the first TAID, at LC 17A

Delta D, the Thrust Augmented Delta, had employed strap-on solid motors to nearly double Thor-Delta liftoff thrust.  The next logical step was to take advantage of that power to lift a bigger, heavier, more capable second stage.  That step, taken in 1965, created Thrust Augmented Improved Delta (TAID).   

Hypergolic propellant innovator Aerojet built both the Improved Delta second stage and its AJ10-118E engine.  The stage used 1.389 meter diameter stretched Ablestar tanks, which held about 5.2 tonnes of hypergolic propellant and helium pressurant.  This roughly doubled the burn time compared to the previous Vanguard-based Delta stage.    

The 3.58 tonne thrust pressure-fed AJ10-118E engine was restartable, a first for Thor-Delta.  The restartable second stage substantially improved mission flexibility.

An MB-3 Block 3 Rocketdyne engine powered Improved Delta's Thor first stage.   MB-3-3 provided only slightly more thrust (79.4 tonnes at liftoff), but used more reliable components.  Thor still retained its original tank dimensions but was topped by a different adapter for the wider second stage. 

Three Thiokol Castor strap on solid motors provided the "Augmented" thrust.  Both Castor I and Castor II motors could be used with the TAID series.  Castor II provided slightly less liftoff thrust than Castor I (23.68 tonnes thrust each versus 24.94 tonnes), but provided a longer-lasting thrust profile that extended burnout until T+39.3 seconds. 

Third stage options included the ABL-258 and FW-4D spin-stabilized solid motors that had seen use atop Delta C, C1 and D.  With ABL-258, Improved Delta was called "Delta E".  With FW-4D it was "Delta E1".  The rocket was identified as "Delta G" when no third stage was used. 

Improved Delta was topped by a 1.651 meter diameter Agena payload fairing.  With the fairing, the rocket stood 27.985 meters to the bottom of the MB-3-3 thrust chamber.  Delta E could lift 735 kg to low earth orbit (LEO) or 204 kg to geosynchronous transfer orbit (GTO).  The E1 variant could boost 205 kg to GTO.
 

Delta E

Improved Delta Second Stage at Douglas Santa Monica in 1965

Delta 34 was the first Improved Delta.  Using Thor 457, the rocket launched from Cape Canaveral LC 17A on November 6, 1965 with Explorer 29, also known as GEOS 1 (Geodetic Earth Orbiting Satellite).  The three-stage Delta E boosted 175 kg Explorer 29 into a 1,120 x 2,269 km x 59.4 deg orbit.  It was the first of six Delta E launches, all during 1965-67. 

LC 17A sported a new fixed umbilical tower for the occasion, giving the launch pad a more substantial look.  The tower provided umbilical connections for the payload, second stage, and for the upper part of the Thor booster.   Previous Thor-Delta types had used smaller, retracting type umbilical towers.  

Delta 34 was adorned, as usual, with the Delta vehicle number ("34"), but for the first time the number appeared within a triangle - a Greek upper case letter "Delta".  The rocket started a three year run of 25 consecutive Delta successes (all Delta types), a NASA record that would stand into the mid-1970s.

The second Delta E (Delta 35) flung NASA's Pioneer 6 into solar orbit from the Cape on December 16, 1965.  The 63 kg spin-stabilized satellite helped map the solar wind, in coordination with subsequently launched Pioneers 7-9.  Pioneer 6 became NASA's longest lived spacecraft when controllers contacted it in 2000 to commemorate its 35th anniversary.

Delta 41, the First Vandenberg Delta

The last three Delta E rockets were the first Thor-Deltas to fly from California's Vandenberg AFB, giving NASA a dedicated polar orbit launch facility.  On October 2, 1966 Delta 41 became the first west coast Delta, lifting off from Space Launch Complex 2 East (SLC 2E), the former Complex 75-1-1.  ESSA 3, a 145 weather satellite entered a 1,283 x 1,493 km x 101.06 deg sun synchronous near-polar earth orbit.  Delta Es orbited two more ESSA satellites from Vandenberg in 1967. 

Delta G and J

Two Delta G launches occurred.  These 2.5 stage Improved Deltas orbited Biosat 1 and Biosat 2 from Cape Canaveral on December 14, 1966 and September 7, 1967, respectively.  The "Biological Satellites" used GE reentry vehicles developed to return film from Corona spy satellites.  For Biosat, the reentry vehicles returned biological samples rather than film. 

Although both Biosats were successfully orbited (by Deltas 43 and 51, respectively), the Biosat 1 RV retrorocket failed, leaving the capsule in orbit.   Biosatellite 2 successfully returned samples, but the reentry had to be initiated about one day early because of a storm approaching the recovery area. 

A unique Improved Delta, identified as a "Delta J", flew once, in 1968.  Delta J, topped by a Star 37D third stage, was theoretically capable of lifting 263 kg to GTO.  Delta 57, the lone "J" launched on July 4, 1968 from Vandenberg AFB SLC 2E, carried 193 kg Explorer 38 into a 5,851km x 5,861 km x 120.6 deg orbit.

Delta E1

Delta 61, an E1 variant, Launches ESA's HEOS 1 on December 5, 1968

Seventeen Delta E1 vehicles flew during the 1966-1971 period.   In addition to Pioneers 7, 8, and 9, the "E1s" launched five NASA Explorers, two ESSA weather satellites, three international science satellites, and four Intelsat 2 communication satellites. 

Hughes built the Intelsat 2 spin-stabilized spacecraft, which improved upon the Early Bird series.  Improved Deltas lifted the satellites, which weighed 192 kg with their apogee kick motors, into GTO.  Delta 42 successfully put Intelsat 2-1 into GTO on October 26, 1966, but the satellite's built-in kick motor failed, leaving the satellite in a useless orbit.   Intelsat 2-2, 2-3, and 2-4 were all successfully launched in 1967, and all successfully boosted themselves into operational geostationary orbits. 

Six of the 17 Delta E1 flights took place from Vandenberg AFB.   A second Delta pad, SLC 2W (former 75-1-2), entered service with the launch of Delta 69, an E1 carrying Explorer 41 on June 21, 1969.    Explorer 41 entered a highly inclined, highly elliptic orbit to measure energetic particles, magnetic fields, and plasma in "cislunar" space.

Delta 50, an "E1" model launched on July 19, 1967 from Canaveral's LC 17B, boosted NASA's Explorer 35 toward an eventual lunar orbit.  A new "Delta" logo adorned this rocket.  It consisted of a solid blue triangular "Delta", outlined by two red lines.  A white number "50" was centered in the triangle, as were 17 white stars. 

Since the previous 16 consecutive Delta launches had succeeded, the star field seemed to present a "fingers crossed" hope for a 17th success, which Delta 50 dutifully provided.  Subsequent Deltas carried increasing numbers of stars on the logo as the successes accrued.  When a failure finally occured, of a Delta M model in 1968 after 25 consecutive successes, Delta logos carried 25 stars until the record was finally bested during the mid 1970s. 

Delta 84 was the final Delta E1, and the final Thrust Augmented Improved Delta.  It carried ISIS 2, a 264 kg Canadian ionospheric observatory into a 1,358 x 1,428 km x 113.6 deg orbit from Vandenberg AFB SLC 2E on April 1, 1971.  ISIS 2, and similar ISIS 1 launch in 1969 by another Delta E1, operated until 1990 when they were purposely deactivated. 

Twenty six TAID launches took place.  Every single one succeeded.  Though successful, further improvements in the Thor-Delta family finally outmoded the type.  The "Improved Delta" upper stage continued to fly after 1971, but now it flew atop "Long Tank" Thor first stages.