The Gemini Spacecraft is a conical structure nearly 19 feet high, 10 feet in diameter at its base and weighing over 7000 pounds.
The Spacecraft is designed to endure the aerodynamic pressures, temperature loading, vibration and acoustical noise of launch; the temperature and vacuum of orbital flight; and the extreme heat of reentry, and the impact forces of water landing while providing life support for two astronauts and the necessary equipment for planned missions and experiments.
Gemini's design reflects the knowledge obtained from the development, manufacture and flight operation of the Mercury Spacecraft which, like Gemini, was produced for NASA by McDonnell Aircraft Corporation, St. Louis, Missouri.
Gemini is launched by an Air Force Titan II launch vehicle built by the Martin Company.
The spacecraft consists of two major parts, a reentry module and an adapter module. The reentry module is designed to withstand the extreme heat of reentry. Its side walls are protected by heat resistant shingles and the large bulkhead by an ablative heat shield. The sections of the adapter module remain in decaying orbits and are burned up during reentry.
The spacecraft is primarily "skin-stringer" construction. Ring stabilized stringers carry near all axial loads. Structural materials and construction methods exhibit the influence of Gemini engineers' search for optimum strength-to-weight ratios. Titanium and magnesium are the principal metals used.
REENTRY MODULE
The reentry module (the dark portion of the spacecraft) has three primary sections: rendezvous and recovery (R&R), reentry control system (RCS), and a cabin section. Integral to the reentry module is a heat shield attached to the large end of the module. An aerodynamic covers horizon sensors at the midpoint of the reentry module.
Materials used in the conical section of the reentry module vary greatly because of the effects of reentry heating. Super alloys such as Rene 41 and L-605 are used for the outer skin and skin attachments, which are thermally isolated from the inner structure by Johns-Manville MIN-K, Fiberglas and thermoflex RF insulation. The basic load- carrying shell is titanium. Aluminum is used Inside the cabin where heat is not a structural problem.
RENDEZVOUS AND RECOVERY (R&R) SECTION
The R&R section provides sufficient volume for a rendezvous radar system and a parachute landing system. Structural rings, stringers, and bulkheads are made of titanium. The external surface is covered with beryllium shingles. The nose fairing is reinforced, plastic and Fiberglas-laminate. The R&R section is attached to the RCS section with 24 frangible bolts which are fractured to jettison the R&R section upon deployment of the pilot chute following reentry.
Under the beryllium shingles are Thermoflex RF blankets held in place by a titanium mesh attached to the stringers. The outer surfaces of the rings and stringers are insulated with 0.0015 in Inconel-foil-encased Min-K held in Fiberglas channels.
REENTRY CONTROL SYSTEM (RCS) SECTION The RCS section houses reentry control system fuel and oxidizer tanks, and thrust chamber assemblies. This section is located between the rendezvous and recovery section and the cabin section. The cylindrical RCS section is an inner skinned ring-stringer titanium structure with an outer skin of beryllium shingles. External heat protection is essentially the same as for the rendezvous and recovery section.
CABIN SECTION
The cabin of the Gemini Spacecraft is a truncated cone (a conical enclosure with approximately the top one-third cut away) which houses the Gemini crew, electrical and life support equipment, and various experimental devices. The pressure vessel (crew compartment) provides adequate space for the two-man crew plus instrumentation and life support equipment.
The pressure vessel has a fusion-welded titanium frame attached to side panels and fore and aft bulkheads. The side panels and pressure bulkheads are double thickness, thin-sheet titanium (0.010 inch) with the outer sheet beaded for stiffness. A hatch is provided over each astronaut for entering and leaving the spacecraft.
Equipment bays, which contain a variety of electrical and electronic equipment, are located outside the pressure vessel. Unlike the Mercury Spacecraft which had nearly all systems inside the pressure shell, the Gemini Spacecraft has most system components located in unpressurized equipment bays. These components either require no pressurization or are internally pressurized. Since equipment is normally only one layer deep within the compartments, launch crews can remove a hatch, quickly pull out a malfunctioning unit and insert a new one, reinstall the hatch and proceed with the launch.
Experiments for NASA and the U.S. Air Force are installed in bays on the "bottom" of the reentry module, in the adapter section, and in the pressurized crew compartment.
PERSONNEL HATCHES
Two hatches, contoured to the shape of the conical cabin, are located in what is the top of the spacecraft during orbital flight. A hatch is located over each astronaut and is manually operated by handles inside and outside the spacecraft. The latching mechanism is mechanical. The hinge is on the outboard side of the door.
Each hatch incorporates an observation window consisting of one outer and two inner panes of glass with an air space between each pane. The outer panes are high-temperature 96% silica glass. The innermost pane is of temper-toughened alumino-silicate glass for structural strength. The surface of each pane, with the exception of the outer one, is coated to reduce reflection and glare and to aid in attenuating ultraviolet radiation.
Skin and beam construction make up the structural design of the personnel access hatches. A silicon rubber seal around each hatch sill and around the two inner panes of glass in the window prevents loss of Cabin pressure when the hatches are closed.
A hatch curtain is stowed alongside the hinge of each hatch which, after a water landing, prevents water from entering the canin when the hatches are opened. In emergency situations the hatches open by a 3-sequence operation actuated by a pyrotechnic (explosive) device. When initiated, these actuators unlock the mechanical latches, open the hatches, and finally supply a hot gas that ignites a seat-ejection catapult rocket.
HEAT SHIELD
The heat shield is a dish-shaped structure that forms the large end of the reentry module.
The ablative substance of the Gemini heat shield is a paste-like material which hardens in standard atmosphere after being poured into a honeycomb form.
Starting with a load-carrying Fiberglas sandwich structure consisting of two 5-ply faceplates of resin-impregnated glass cloth separated by a 0.65inch thick Fiberglas honeycomb core, an additional Fiberglas honeycomb is bonded to the convex side of the sandwich and filled with Dow-Corning DC-325 ablative material. The entire shield is encircled with Fiberite ring. The basic ablative substance of the heat shield was developed by McDonnell and is now being marketed by Dow-Corning.
SHINGLES
The surface of' the reentry module is covered with overlapping shingles which provide aerodynamic and heat protection and hold in place shaped pads of flexible insulation. The composition of the beaded (corrugated) Rene 41 shingle (0.016 inch thickness) on the sides of the cabin is 53% nickel, 19% chromium, 11% cobalt, 9.75% molybdenum, 3.15% titanium, 1.6% aluminum, .09% carbon, .005% boron, and "less than 2.75%" iron. The shingles are identical in composition and manufacturing technique to those used on Mercury. Extra large holes in the shingles at the attachment bolts allow each to expand during aerodynamic and solar heating. Oversize washers cover these holes to minimize heat and air flow penetration.
The R&R and RCS section surfaces are unbeaded shingles of cross-rolled beryllium. The plate is supplied to McDonnell, by Brush Beryllium Company, in sheets ranging in thickness from 0.300 inch to 0.555 inch and is finished by McDonnell to a thickness of 0.090 inch to 0.280 inch. The shingles are attached to the spacecraft by beryllium retainers fabricated from similar plates.
Beryllium was previously used on Project Mercury using sections fabricated from hot-pressed beryllium blocks rather than cross-rolling procedures. The new manufacturing technique permits much higher strength and shock resistance. Gemini rendezvous flights require almost twice the strength and impact resistance available with hot-pressed beryllium blocks.
Both Rene 41 and beryllium shingles are coated with ceramic paint on the outer surface to permit high thermal radiation from the spacecraft. The inner surface of the beryllium shingles has a very thin gold coating to attenuate thermal radiation into the spacecraft.
ADAPTER MODULE
The most obvious structural difference between the Gemini and Mercury Spacecraft is the integral adapter module, which is part of the orbital configuration of Gemini.
The adapter module, 90 inches high and 120 inches across its largest diameter, provides volume for systems and equipments needed for long-duration orbital flights, as well as the mating structure between the reentry module and the launch vehicle.
The adapter is a ring stiffened skin-stringer structure consisting of circumferential aluminum rings, extruded magnesium alloy stringers, and magnesium skin. The T-shaped stringers have a hollow bulbous portion to provide a path for the flow of liquid coolant, which transfers heat to the adapter skin for subsequent radiation into space.
The outer surface of the adapter module is coated with white ceramic paint and the inner surface is covered with aluminum foil to reduce emissivity. The adapter is joined to the reentry module by three titanium tension straps external to the structure of both the reentry module and the adapter section.
RETROGRADE SECTION
At the small end of the adapter module is the retrograde section containing crossed aluminum "I" beams on which are mounted four retrograde rockets. This section makes up the first 30 inches of the adapter module. One retrograde rocket is mounted in each quadrant of the section. In addition to retrograde rockets there are six orbit attitude and maneuvering system (OAMS) thrust chamber assemblies. Four of these assemblies permit orbital translation, up, down, left and right. Non-operating "dummy thrusters" are installed in place of these four assemblies on Spacecraft. No. 3.) Two, whose nozzles face toward the reentry module, provide for the rearward or "backing away" maneuver of the spacecraft.
EQUIPMENT SECTION
At the large end of the adapter module the equipment section provides volume and attach points for several system modules, including orbit attitude and maneuvering system propellant tanks, the environmental control system primary oxygen supply, batteries, coolant, and electrical and electronic components. A honeycomb blast shield between the two sections protects the equipment section and the dome of the Titan launch vehicle from excessive (explosion-causing) heat should it be necessary to fire the retrorockets in an abort condition.
Ten OAMS thrust chamber assemblies are mounted in the equipment section providing for roll, pitch, yaw, and forward maneuvering of the spacecraft during orbital flight.
A Fiberglas cover over the open end of the adapter protects the equipment inside from solar radiation after separation from the launch vehicle. The spacecraft and the Titan II launch vehicle are mated by a forged aluminurn alloy ring, 120 inches in diameter with 20 lugs through which bolts are fastened to secure the mating. When the spacecraft is separated from the launch vehicle, a pyrotechnic charge is fired to sever the adapter section appoximately 1 1/2 inches above the launch vehicle/spacecraft mating point. This charge cuts through the metal skin of the adapter section instantly but in the same way a metal shear would cut through sheet metal.
The adapter structure is constructed as a single unit and the two sections are separated by a shaped-charge pyrotechnic device prior to reentry.
Copyright 1997-2005 by John
Duncan |
