FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1969
1969 - 0236.PDF
FEET 210 FLIGHT International, 6 February 1969 The three elements which constitute the spacecraft—the service module, command module and lunar module-—are shown here joined together as they will be during the flight to the Moon. The service module is at the extreme right; the conical command module faces left and is joined to the lunar module at left MOON LANDING . . . a cluster of five Rocketdyne F-l engines. Working upwards from the engine is the thrust structure, fuel tank (containing 203,000gal kerosene), inter-tank structure, lox tank (containing 331,0OOgal liquid oxygen) and forward skirt. The thrust structure takes the entire static and dynamic thrust loads and, weighing 24 tons, is the heaviest first-stage component. Four fairings and the same number of rigid fins are attached to the lower end of the structure. The purpose of the half- conical fairings is to smooth the airflow over the engines, while provision of fins reduces the angular rate of rotation caused by "hard-over" failures, giving the crew more chance to take remedial action or abandon the vehicle. Four of the five F-l engines are gimballed for attitude control, the fifth (central) engine being fixed. Each engine is a single-start unit, 19ft long and 12ft 4in in width (the nozzle diameter is lift 7in) delivering 1,520,0001b thrust at sea level, with a rated burn time of I50sec and a specific impulse of 260sec. The total mass flow for five engines (lox-(-kerosene) at launch is 28,4151b/sec. The S-1I second stage, built by North American Rockwell, is the most powerful hydrogen-fuelled launch vehicle at present in production in the western world. It also consists of five stages: inter-stage ring, aft skirt and thrust structure (which supports and houses the five engines), lox tank, liquid hydrogen tank, and forward skirt. As with the first stage, four of the five engines are gimballed, the fifth (axial motor) being fixed. The Rocketdyne J-2 engine is 11ft tin long, with a nozzle exit diameter of 6ft 5in, and delivers 225,0001b thrust at altitude for a rated duration of 500sec and a specific impulse of 424sec. Five solid-propellant ullage motors, each generating 22,5001b thrust for 4sec, provide "artificial gravity" (after first-stage separation) to ensure that the fuel and oxidiser settle in the tanks so that the fuel system remains primed. The stage has a single relatively thin bulkhead which forms both the top of the lox tank and the bottom of the liquid hydrogen tank, and which has to maintain a temperature difference of 90°C between the two liquids. Although the fuel/oxidant content of the STI stage is considerably less than that of the first stage, they differ only slightly in size because the liquid hydrogen occupies such a large volume. Four solid-propellant retro-rockets of 37,5001b thrust each push this stage away from the S-IVB at separation after burnout. The third and final propulsive element of the Saturn V vehicle is the S-IVB stage, designed and built by McDonnell Douglas Astronautics. It consists of a forward skirt, propellant tank, thrust structure, aft skirt, and aft interstage. As with the S-II stage, the fuel and its oxidant—liquid hydrogen and lox—are separated by a common bulkhead. The single J-2 engine is interchangeable with the propulsion units in the second stage, and is gimballed for control purposes. Two solid-propellant ullage motors, each of 3,4001b thrust, accelerate the stage sufficiently to settle the fuel and oxidant after separation from the S-IIC stage. Attached to the upper end of the S-IVB stage is the instrument unit. This ring structure, 36in high and 260in in diameter, is built by IBM. It is the "nerve centre" of the vehicle- and contains the electrical and electronic equipment needed for^guidance, tracking, and telemetering of engineering data for the- Saturn V. It also contains its own environmental- control system which is also used to condition equipment in the forward end of the S-IVB stage. The other component used only during the launch is the launch escapes tower. This is a lattice structure some 33ft high which is attached to the conical end of the command module. The upper end contains a 147,0001b-thrust rocket engine. In the e%ent of an emergency arising during the period T— 30min t<MT+3min (T=lift-off) the crew can abandon the launch vehicle aby operating the escape system. This breaks the connections with the vehicle and energises the rocket motor, causing the tower and command module to separate from the launch vehicle and eject to a safe distance from the launch site. At the top of the trajectory the tower detaches itself and falls away separately, while the recovery parachutes in the command module are deployed. 3-THE COMMAND MODULE The only part of the vehicle and spacecraft which is recovered intact at the conclusion of the flight, the command module houses the three astronauts during their journey to and from the Moon, and consists of a conical chamber about 12ft in diameter and I Oft high. The configuration of the module, used also for the Mercury and Gemini spacecraft, is due very largely to the work of H. J. Allen (until recently director of NASA's Ames Research Centre) in 1952. A hody at a great distance from the Earth, and allowed to fall freely under the influence of the gravitational field of the latter, acquires a characteristic velocity of about 37,OOOft/sec. It is convenient to allow atmospheric drag to dissipate the kinetic energy so acquired in the form of heat, and Allen showed that this could be achieved by using a shape having a blunt face towards the direction of flight. By this means about 90 per cent of the heat load could be transferred to the shock-wave formed ahead of the hypersonic body. Much of the residual heat could then be dissipated in raising the temperature of a special coating on the blunt face, which could then disintegrate in a controlled way, so reducing the heat load which was finally transferred to the spacecraft. During early studies of lifting bodies it was found that uncontrolled entry at a fixed lift/drag ratio was unacceptable for the lunar mission owing to the wide spread of touch down positions, resulting from unavoidable inaccuracies in re-entry path measurement and injection and uncertainties in atmospheric data. The choice of L/D depends on the cone angle of the spacecraft; for Apollo an L/D value of 0.35 was adopted. This allows a range variation of some 3,000 miles from 400,000ft. Another parameter, the ballistic co efficient W/CdA (where W and Cj are the weight and drag coefficient of the spacecraft, and A is the effective area of the re-entry face) was chosen to give appropriate characteristics. This quantity may be thought of as a kind of wing loading. The value chosen for Apollo, 501b/sq ft, provides the appro priate deceleration levels for the design flight path and allows the spacecraft to be manoeuvred for landing over a very wide range. Fortunately the Earth's atmosphere is sufficiently dense that quite low values of the coefficient can be achieved with reasonably small effective base areas. The command module consists of two main structures joined together: an inner pressure shell and an outer heat shield. The first is of aluminium-sandwich construction in which aluminium-honeycomb core is faced both sides with aluminium sheet; the thickness varies from 1.5in at the base to 0.25in at the apex of the spacecraft. The heat shield is made from stainless steel honeycomb brazed between steel alloy facing sheets. It varies between 2.5in and 0.5in thick, and accounts for 25 per cent of the weight of the command module. The principal task of the heat shield that forms the outer cover is to protect the crew and spacecraft systems from the high temperatures—about 2,800°C—experienced during the re-entry stage. The ablative material with which the heat shield is faced is a phenolic epoxy resin, which controls the rate of heat absorption by charring and then melting away. The pressure-tight cut-outs allow entry to the command
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
