Saturn V
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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] F-1 engine
First, a description of the engine. A large combustion chamber and bell have an injector plate at the top, through which, RP-1 fuel and LOX are injected at high pressure. Above the injector is the LOX dome which also transmits the force of the thrust from the engine to the rocket's structure. A single-shaft turbopump is mounted beside the combustion chamber. The turbine is at the bottom and is driven by the exhaust gas from burning RP-1 and LOX in a fuel-rich mixture in a gas generator. After powering the turbine, the exhaust gas passes through a heat exchanger, then to a wrap-around exhaust manifold which feeds it into the periphery of the engine bell. The final task for these hot gases is to cool and protect the nozzle extension from the far hotter exhaust of the main engine itself. Above the turbine, on the same shaft, is the fuel pump with two inlets from the fuel tank and two outlets going, via shutoff valves, to the injector plate. A line from one of these 'feeds' supplies the gas generator with fuel. Fuel is also used within the engine as a lubricant and as a hydraulic working fluid, though before launch, RJ-1 ramjet fuel is supplied from the ground for this purpose. At the top of the turbopump shaft is the LOX pump with a single, large inlet in-line with the turboshaft axis. This pump also has two outlet lines, with valves, to feed the injector plate. One line also supplies LOX to the gas generator. The interior lining of the combustion chamber and engine bell consists of a myriad of pipework through which a large portion of the fuel supply is fed. This cools the chamber and bell structure while also pre-warming the fuel. Lastly, an igniter, containing a cartridge of hypergolic fluid with burst diaphragms at either end, is in the high pressure fuel circuit and has its own inject point in the combustion chamber. This fluid is triethylboron with 10-15% triethylaluminium. (ap15fj)
[edit] F-1 engine start
At T minus 8.9 seconds, a signal from the automatic sequencer fires four pyrotechnic devices. Two cause the fuel rich turbine exhaust gas to ignite when it enters the engine bell. Another begins combustion within the gas generator while the fourth ignites the exhaust from the turbine. Links are burned away by these igniters to generate an electrical signal to move the start solenoid. The start solenoid directs hydraulic pressure from the ground supply to open the main LOX valves. LOX begins to flow through the LOX pump, starting it to rotate, then into the combustion chamber. The opening of both LOX valves also causes a valve to allow fuel and LOX into the gas generator, where they ignite and accelerate the turbine. Fuel and LOX pressures rise as the turbine gains speed. The fuel-rich exhaust from the gas generator ignites in the engine bell to prevent backfiring and burping of the engine. The increasing pressure in the fuel lines opens a valve, the igniter fuel valve, letting fuel pressure reach the hypergol cartridge which promptly ruptures. Hypergolic fluid, followed by fuel, enters the chamber through its port where it spontaneously ignites on contact with the LOX already in the chamber.
Rising combustion-induced pressure on the injector plate actuates the ignition monitor valve, directing hydraulic fluid to open the main fuel valves. These are the valves in the fuel lines between the turbopump and the injector plate. The fuel flushes out ethylene glycol which had been preloaded into the cooling pipework around the combustion chamber and nozzle. The heavy load of ethylene glycol mixed with the first injection of fuel slows the buildup of thrust, giving a gentler start. Fluid pressure through calibrated orifices completes the opening of the fuel valves and fuel enters the combustion chamber where it burns in the already flaming gases. The exact time that the main fuel valves open is sequenced across the five engines to spread the rise in applied force that the structure of the rocket must withstand.
As fuel and LOX flow increase to maximum, the rise in chamber pressure, and therefore thrust, is monitored to confirm that the required force has been achieved. With the turbopump at full speed, fuel pressure exceeds hydraulic pressure supplied from ground equipment. Check valves switch the engine's hydraulic supply to be fed from the rocket's fuel instead of from the ground. (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] J-2 engine
The S-II stage carries five J-2 uprated engines which burn LH2 and LOX to produce up to 1,041 kN (234,000 pounds) thrust each. They are capable of being restarted in flight but this feature is only implemented in the engine used in the S-IVB.
The thrust chamber and bell of each engine is fabricated from stainless steel tubes brazed together in a single unit. Supercold LH2 is pumped through these tubes to cool the thrust chamber and simultaneously prewarm the fuel. The engine carries two separate turbopumps, both powered in turn by the exhaust from a gas generator which burns the stage's main propellants. The hot gas exhaust is fed from the gas generator, first to the fuel turbopump, then to the LOX turbopump before being routed to a heat exchanger and finally into the engine bell. The fuel and LOX outputs of both turbopumps are fed, via main control valves, to the thrust chamber injector via the LOX dome. Unlike the solid steel injector of the F-1, the J-2 injector is fabricated from layers of stainless steel mesh sintered into a single porous unit. A solid LOX injector behind this carries 614 posts which pass LOX through the injector and into the combustion chamber. Each post has a concentric fuel orifice around it and these orifices are attached to the porous injector. The fuel delivery is arranged to ensure that about 5 percent seeps through the injector face to cool it, the rest passing through the annular orifices.
The ASI (Augmented Spark Igniter), fed with propellant and mounted to the injector face, provides a flame to initiate full combustion. Valves are provided to bleed propellant through the supply system well before ignition to chill all components to their operating temperatures otherwise gas would be formed which would interfere with the engine's use of propellant as a lubricant in the turbopump bearings. A tank of gaseous helium is fabricated within a larger tank of gaseous hydrogen. This is the Start Tank. The helium provides control pressure for the engine's valves while the hydrogen spins up the turbopumps before the gas generator is ignited. A PU (Propellant Utilization) valve on the output of the LOX turbopump can open to reduce the LOX flowrate. This adjusts engine thrust down to 890 kN (200,000 pounds) during flight to optimise engine performance. (ap15fj)
[edit] J-2 engine start
To start the J-2 engine, spark plugs in the ASI and gas generator are energised. The Helium Control and Ignition Phase valves are actuated. Helium pressure closes the Propellant Bleed valves, it purges the LOX dome and other parts of the engine. The Main Fuel valve and the ASI Oxidiser valves are opened. Flame from the ASI enters the thrust chamber while fuel begins to circulate through its walls under pressure from the fuel tank. After a delay to allow the thrust chamber walls to become conditioned to the chill of the fuel, the Start Tank is discharged through the turbines to spin them up. This delay depends on the role of the engine. A one second delay is used for the S-II engines. Half a second later, the Mainstage Control Solenoid begins the major sequence of the engine start. It opens the control valve of the gas generator where combustion begins and the exhaust supplies power for the turbopumps. The Main Oxidiser valve is opened 14° allowing LOX to begin burning with the fuel which has been circulating through the chamber walls. A valve which has been allowing the gas generator exhaust to bypass the LOX turbopump is closed allowing its turbine to build up to full speed. Finally, the pressure holding the Main Fuel valve at 14° is allowed to bleed away and the valve gradually opens, building the engine up to its rated thrust. (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)