Saturn V stages
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[edit] S-IC
Of the 110.6 metre height of the entire Apollo 15/Saturn V stack, 42.1 metres comprise the S-IC first stage. Five F-1 engines are clustered at the bottom of the stage to provide 34,025 kN (7,700,000 pounds) of thrust in total. The propellants used are RP-1 (Rocket Propellant-1 or highly refined kerosene) as the fuel and LOX as the oxidiser. The lower of two tanks is filled with 810,000 litres of RP-1 at T-13 hours with a final topping up occurring at T minus 1 hour. At nine hours before launch, the larger upper tank has nitrogen gas pumped through it to purge it of air and water vapour contaminants. Six and a half hours before launch, it is precooled to prepare it for the loading of 1.3 million litres of LOX at a temperature of -183° C. Initially, the LOX is fed at a slow rate of 95 litres per second until the tank is sufficiently chilled to retain the liquid up to 6.5% full, then the tank is filled to 98% at a rate of 630 litres per second, a process lasting over 45 minutes. The slow fill rate is reestablished until the tank is full at about 4 hours 55 minutes before launch. From then on, until three minutes before launch, the level is replenished as the volatile LOX boils off. Both of the first stage tanks are then pressurised prior to launch using helium; the fuel tank at T-96 seconds, the LOX tank at T-72 seconds. Original source: (ap15fj)
[edit] S-IC/S-II separation
[edit] Apollo 15
The S-IC/S-II staging sequence for AS-510 differed markedly from earlier missions. On previous flights, the interstage, or skirt, a 4.9-metre tall ring matching the 10-metre diameter of the S-IC and S-II that it sits between, carried solid-fuel rockets which fired shortly after the first stage separated to settle the S-II propellants in their tanks. AS-501 and AS-502 had eight of these ullage rockets, while AS-503 to AS-509 had four. They were deleted from the Apollo 15 launch vehicle, along with four of the eight retro rockets built into the conical engine fairings around the base of the S-IC, in order to save weight and increase payload. Ullage is a brewers' term for the portion of a barrel occupied by air, not liquor. The separation did not quite go according to plan. After the F-1 engines were shut down, the thrust they generated during the tail-off period was greater than expected. The engines don't instantly stop thrusting when they receive their cut-off command. After a quick drop to about 2% thrust, they take over four seconds to decay to zero. As the engines expired, the acceleration imparted to the, now separate, empty and therefore light S-IC stage was above the predicted value. Despite deliberately coasting for longer than usual between separation and S-II ignition, the distance between the two stages was less than engineers had planned and the blast of hot gases from five J-2s against the top of the empty stage disabled a telemetry package with which the S-IC was to be monitored until its impact with the Atlantic Ocean. (ap15fj)
[edit] Dual plane separation
Staging of the S-IC and S-II is technically described as a "dual plane separation," as the vehicle is cut across two geometrical planes. The first plane is between the skirt and the S-IC, with the S-II engines starting 1.1 seconds later. The second plane separation, when the second stage loses the skirt, occurs at 3 minutes, 10.7 seconds; 30.0 seconds after the S-IC separation. This time allows the S-II's attitude to stabilise because if either part of the launch vehicle were to be yawing or pitching excessively, there would be a danger of the engine bells striking the S-IC and skirt as the two great metal cylinders coast along before ignition of the S-II. The skirt provides clearance above the first stage's LOX tank for the five J-2 engines of the S-II stage. (ap15fj)
[edit] S-II
The second, or S-II, stage of Apollo 15's Saturn V vehicle is 24.9 metres tall and is powered by the combustion of LH2 (liquid hydrogen) and LOX in a cluster of five J-2 rocket motors which generate a total thrust of 5,115kN (1.15 million pounds). A million litres of LH2, cooled to -253°C to get it into a liquid state, is loaded into the large, upper tank of the stage while 331,000 litres of LOX is loaded into the smaller, squat tank below. These tanks share a single insulated structure with only an insulated, common bulkhead between them. With both propellants being so cold - LH2 is only 20 degrees above absolute zero - the tanks must be prepared and chilled down before they can be filled.
Air and water vapour is purged from the tanks by repeated pressurisation and venting with helium. Helium is used because nitrogen would freeze in the presence of liquid hydrogen. Once clear of contaminants, the tanks are cooled to accept the propellants by first passing cold gas through the system then feeding propellant at a slow rate and allowing it to boil off, taking heat with it. Seven hours before launch, LOX is fed at 31.5 litres per second until it is 5% full, then the fill rate goes to 315 litres per second to take the tank to 96% full. This takes about 25 minutes and then the tank is topped up at 63 litres per second. Five hours before launch and after purging and cooling, LH2 enters at 63 litres per second, further cooling its tank so that propellant begins to remain liquid and rise in level in a process similar to that for the LOX tanks. Once the level of liquid propellant reaches 5 percent, the fill rate is increased to 630 litres per second until the tank is 98 percent full, when the fill rate reduces again to 63 litres per second to top off the tank's load. To compensate for loss due to boil-off, both tanks are replenished until about three minutes before launch when the tanks are pressurised. Up to the launch, pressurising helium gas is supplied from the ground. After launch, the boil-off of the propellants is enough to maintain pressure until the engines are ignited 2 minutes and 40 seconds into the flight. (ap15fj)
[edit] S-IVB
The third stage of the Saturn V, called the S-IVB for historical reasons, could be described as a smaller version of the S-II stage in that it also consists of a single tank structure with a common bulkhead between the LH2 and LOX compartments. These propellants, which are stored at the same supercold temperatures as for the S-II, are burned in a single J-2 engine which yields a thrust of 890 kN (a shade over 200,000 pounds). The engine's capability for restarting is utilised for the boost out of Earth orbit to the Moon. The construction of the S-IVB's propellant tank differs from the S-II stage by having the insulation on the inside of the tank's metal skin, a detail which made manufacture easier by not having to develop a bonding system which had to work at only 20 degrees above absolute zero. With the insulation between it and the propellant, it would be substantially warmer. About 8 hours before launch, the cryogenic systems of the S-IVB stage are purged, including the engine feeds and pump cavities. At T minus seven and a half hours, the LOX tank is precooled by pumping LOX onboard at 31.5 litres per second and allowing its conversion to a gas to take away heat from the tank. When enough liquid remains to fill the tank to 5%, the fill rate goes to 63 litres per second, the fast fill rate, until the tank is 98% full. Finally the tank's total capacity of 92,350 litres is reached at a slow fill rate of up to 19 litres per second, and after that, replenished at a rate of up to 2 litres per second. The LOX tank filling takes about 25 minutes. Filling of the 253,200 litres of LH2 follows a similar process beginning 4 hours and 11 minutes before launch. Tank precooling and filling to 5% is achieved with a fill rate of 31.5 litres per second, before the fast filling of the tank at 190 litres per second takes the tank's quantity to 98% three and a half hours before launch. The slow rate of fill is reestablished to top off the tank and keep it replenished. LH2 tank pressurization is maintained, during initial flight, by the boil-off of the fuel, then later with helium from a collection of spheres mounted on the exterior of the thrust structure at the base of the stage. The LOX tank is pressurised from heated helium fed from cold storage tanks within the LH2 tank. (ap15fj)