Project Gemini

Gemini Program Objectives:

• The purpose of NASA′s Gemini program was to develop the technologies and techniques needed to fulfill President Kennedy′s goal of landing a man on the Moon by 1970. The major objectives of the program were:
  • Demonstrate that humans and their equipment can survive up to two weeks in space
    
  • Demonstrate rendezvous and docking techniques in orbit
    
  • Demonstrate the technology and techniques needed to perform EVAs (Extra-Vehicular Activities)
• Meeting all of these objectives was necessary if the Apollo lunar program were to be successful.
  
  

  Diagram showing the major systems of the Gemini spacecraft. (McDonnell)

  A cutaway drawing of the Gemini spacecraft. (NASA)

  
  
• Gemini was a two-man spacecraft that was roughly conical in shape with a base diameter of 3.3 meters which stood 5.8 meters tall.  Built by the McDonnell Aircraft Corporation (which merged with Douglas in 1967 to become McDonnell Douglas which subsequently merged with Boeing 30 years later), it consisted of two major sections. The first section was the reentry module which housed the crew, their equipment, food supplies and so on in orbit as well as the recovery systems needed to safely return them to Earth. The nose of this module also contained an L-band radar system for orbital rendezvous operations. Unlike today′s crewed spacecraft, the Gemini crew cabin was pressurized with pure oxygen at about one-third standard atmospheric pressure to save weight. The next section, the adapter section, connected the reentry module to the launch vehicle during ascent and housed equipment needed to support the crew while in orbit. It consisted of a retrograde section which held a set of four solid retrorockets used to start the descent to Earth from orbit and an equipment section which housed the in-orbit propulsion system called OAMS (Orbital Attitude and Maneuvering System), life support, power systems and all other equipment not needed for the return to Earth.
• With a typical launch mass of up to about 3,700 kilograms or more, Gemini needed the largest operational rocket available at the time to get into orbit: a modified Titan II ICBM built by Martin Marietta (which is now part of Lockheed Martin). A number of modifications were made to simplify the operation of this rocket, smooth out its ride and improve its reliability to support crewed missions. With the Gemini spacecraft mounted on top, the Titan II GLV (Gemini Launch Vehicle) was 33 meters tall and had a fully fueled launch mass of about 154 metric tons.

Rendezvous and docking:

• The earlier flights in the series had begun to address in a careful step-wise fashion all of the objectives of the program save for one: docking in orbit. The docking target chosen for the Gemini program was a modified Agena D upper stage known as the Gemini Agena Target Vehicle (GATV) launched into orbit using the SLV-3 (Standard Launch Vehicle-3) version of the Atlas booster. Built by the Lockheed Missile and Space Company (which is now part of the aerospace giant, Lockheed Martin), development of the Agena had started back in 1956 for use as an upper stage with the Thor and Atlas missiles initially to orbit defense-related payloads such as the Corona spy satellites and the MIDAS early warning satellites. In addition to serving as an upper stage for these missions, the Agena remained attached to the these payloads (as well as others) while in orbit to provide support functions such as power and attitude control. Later the Atlas-Agena was chosen to launch a variety of NASA missions including the Ranger lunar missions and the first Mariner spacecraft to Venus and Mars.
  
  

  Diagram showing the major components of the early model Agena B. (NASA)

  The modified Agena D in its role as the Gemini Agena Target Vehicle (GATV). (NASA)

  
  
• Although it had experienced a number of problems since its first launch in 1959, the Agena had developed into a versatile and increasingly reliable tool to support a wide range of missions. The Agena D was specifically engineered to be a standardized production model which, like the earlier models, was 1.5 meters in diameter and used a Bell 8000-series rocket engine which employed UDMH (unsymmetrical dimethyl hydrazine) and IRFNA (inhibited red fuming nitric acid) as propellants. These hypergolic propellants ignite on contact thus simplifying engine design. The standard, 6.3-meter long Agena D included options for four modules containing guidance, beacon, power, and telemetry systems, a standard payload console, and a rear rack above the engine for plug-in installation of optional equipment such as solar panels and piggyback payloads including deployable subsatellites. The standard Agena D of this time employed a 71-kilonewton Bell 8096 engine for its primary propulsion system which was capable of up to three starts for a total burn time of 265 seconds. This highly adaptable stage was ideal for serving as a docking target for Gemini.
• Modifications made to the production model Agena D to support its new role in the Gemini program included replacement of the standard Bell 8096 engine in the primary propulsion system with the 8247 model which was capable of up to 16 starts in space. Such a capability was required not only to insert the stage into orbit and perform subsequent trajectory modifications needed for rendezvous, but also to support the option of sending the combined Gemini-Agena into higher altitude orbits after docking. The ullage propulsion system normally carried in the rear to keep the propellants settled at the bottom of their tanks prior to reigniting the main engine was replaced with a pair of new secondary propulsion modules. Each of these modules was equipped with 71 and 890-netwon engines burning UDMH and a nitrogen oxide mixture. These engines were capable of multiple restarts to perform small orbit adjustments in addition to ullage functions. When the propulsion systems were not operating, attitude was controlled using a set of cold gas thrusters.
• The forward end of the modified Agena D was fitted with an auxiliary rack holding special rendezvous and telemetry equipment. Also added were strobe lights and an L-band radar transponder to aid in rendezvous operations as well as command equipment to allow the GATV to be controlled from the ground or by the Gemini crew. A cone shaped target docking adapter (TDA), which was under a shroud during launch, was added to the forward end of the stage to allow the nose of the Gemini reentry module to dock with the Agena and mechanically lock the two spacecraft together. Once in orbit, the GATV was 9.7 meters long with a mass of about 3,175 kilograms.
  
  

  Diagram showing the details of the forward end of the GATV with its target docking adapter (TDA) (NASA)

  
  
• The original objective of the two-day Gemini 6 mission was to rendezvous and dock with the first flight-ready GATV, Agena GATV 5002. Unfortunately, the Agena was lost during launch on October 25, 1965 when its main engine failed upon ignition. Without a replacement GATV available, a new “Gemini 6A” mission was quickly devised where the spacecraft would rendezvous with Gemini 7 during its already planned two-week mission. Gemini 6 successfully performed its rendezvous with Gemini 7 on December 15 but no docking was attempted because of the lack time to build and qualify an appropriate interface on the crewed target vehicle

Astrophilately covers:

Project Gemini. Cancel Cape Canaveral

Project Gemini. Cancel Cape canaveral