Gemini Program – A Brief Overview

[Information taken from,

In December of 1961, NASA announced a plan to extend manned spaceflight. The goal was a manned mission to the moon and to do that, NASA needed to learn everything. What were the effects of weightlessness on a human body? Could humans function in space at all? What where the psychological effects of being confined to a (VERY) cramped spacecraft for a long period of time? Though the math supported the idea, could humans actually effect docking with another spacecraft? What would happen to a human’s perceptions outside a spacecraft? Communication and tracking needed to be refined. None of what we know today was anything other than a theory in 1961. If the US was to fulfill Kennedy’s mandate of a man on the moon by the end of the ’60s, all of this had to be ascertained first.

There were a series of twelve launches, two unmanned for testing and ten crewed launches.

Gemini layout

It was an ambitious and successful program (even when things went wrong). Aside from the obvious difference of two crew and a larger capsule, I think the major difference between the Mercury program and the Gemini program was in the basic paradigm. Mercury was primarily ground-controlled research and development vehicle and the astronaut’s ability to control the capsule could be effected, though limited (however Gordon Cooper’s manual re-entry after the onboard computer crashed really stretched the limits of what the astronaut could do!). The Gemini capsule was designed to be primarily controlled by the crew, though ground-control could effect back-up control. The Gemini program intended to develop procedures astronauts would need to get to the moon and back.

Another major difference was how the capsule itself was constructed. Mercury capsules were essentially built from the inside out. If something inside broke, everything outside that was in the way had to be removed before it could be repaired. The Gemini capsule offered modular construction enabling just the affected module to be replaced.

About the capsule:

The Gemini spacecraft was a cone-shaped capsule consisting of two components, a reentry module and an adapter module. The adapter module made up the base, or rear, of the spacecraft. It was a truncated cone 228.6 cm high, 304.8 cm in diameter at the base and 228.6 cm at the upper end where it attached to the base of the reentry module. The spacecraft consisted of a truncated cone which decreased in diameter from 228.6 cm at the base to 98.2 cm, topped by a short cylinder of the same diameter and then another truncated cone decreasing to a diameter of 74.6 cm at the flat top that housed the radar. The reentry module was 345.0 cm high, giving a total height of 573.6 cm for the Gemini spacecraft. The adapter module and radar module were detached in orbit and the capsule then became the reentry module.

The adapter module was an externally skinned, stringer framed structure, with magnesium stringers and an aluminum alloy frame. The adapter was composed of two parts, an equipment section at the base and a retrorocket section at the top. The equipment section held fuel and propulsion systems and was isolated from the retrorocket section by a fiberglass sandwich honeycomb blast shield. The retrorocket section held the re-entry rockets for the capsule.

The reentry module consisted mainly of the pressurized cabin which was to hold the two Gemini astronauts. The unmanned test flights had instrumentation pallets holding cameras, accelerometers, batteries, and other devices situated in the astronaut area for those missions. Separating the reentry module from the retrorocket section of the adapter at its base was a curved silicone elastomer ablative heat shield. The module was composed predominantly of titanium and nickle-alloy with beryllium shingles. At the narrow top of the module was the cylindrical reentry control system section and above this the rendezvous and recovery section which holds the reentry parachutes. The cabin held two seats equipped with emergency ejection devices, instrument panels, life support equipment, and equipment stowage compartments in a total pressurized volume of about 2.25 cubic meters. Two large hatches with small windows could be opened outward, one positioned above each seat.

The program started with the first unmanned launch on April 8, 1964, and concluded on November 11, 1966.

About this build:

I’m looking to replicate the capsule used on the Gemini VIII mission. That was an interesting mission and certainly showed how adaptable a crew could be when the alternative was death. Here’s the highlights of that mission:

Gemini VIII March 16, 1966

Crew: Neil Armstrong (Command Pilot), David Scott

Mission: Three-day mission. Planned rendezvous and dock with an Agena target vehicle and perform one EVA (Scott). Mission aborted after 10 hours, 41 minutes, and 26 seconds due to thruster malfunction.

Problems: After a successful dock the stack went into a tumble. Armstrong attempted unsuccessfully to use the OMS (orbital maneuvering system) to stabilize the stack. He undocked and the capsule went into an even worse tumble, turning one revolution per second, causing the crew to black out for a short period of time.

Out of radio and radar contact with Ground Control, Armstrong decided to abort the mission. Without confidence in the OMS, Armstrong switched to the Landing System, took manual control, and stabilized the capsule, using most of the fuel required to de-orbit.

Managing a successful de-orbit burn, Gemini VIII splashed down in a different ocean than had been planned for. Instead of the intended splashdown site off the east coast of the United States, the capsule came down a thousand miles south of Japan in the Pacific Ocean. The USS Leonard F. Mason successfully retrieved the crew and capsule.

No conclusive reason for the thruster malfunction was determined*. It has been assumed that an electrical short, probably due to a static electricity discharge, allowed power to flow to the thruster, even when switched off. Subsequent designs changed the circuits so that each thruster would have an isolated circuit. The thruster problem never repeated.

*The reason for the lack of determination of what caused the problem is due to the fact that all the hardware associated with the problem had to be detached in orbit in order for the capsule to reenter successfully.

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