How They Work: “The Space Shuttle (Cont’d)”

The orbiter has three main engines located in the back of the fuselage of the spacecraft. What is amazing is that, each engine is 4.3 metres long by 2. 3 metres in diameter at its widest point and weighs about 3039 kg. The main engines provide the remainder of the thrust of 29 percent to lift the shuttle off the pad and into orbit. The engines burn liquid hydrogen and liquid oxygen, which are stored in the external fuel tank (ET), at a ratio of 6:1. They draw liquid hydrogen and oxygen from the ET at an amazing rate, equivalent to emptying a family swimming pool every 10 seconds! The fuel is partially burned in a pre-chamber to produce high pressure, hot gases that drive the turbo-pumps (fuel pumps). The fuel is then fully burned in the main combustion chamber and the exhaust gases (water vapour) leave the nozzle at approximately 10,000 km/h. Each engine can generate between 1,668,083 to 2,090,664 N of thrust; the rate of thrust can be controlled from 65 percent to 109 percent maximum thrust. The engines are mounted on round bearings that control the direction of the exhaust, which controls the forward direction of the rocket. As mentioned above, the fuel for the main engines is stored in the ET. The ET is 48 m long and has a diameter of 8.4 m. When empty, the ET weighs 35,455 kg. It holds about 719,000 kg of propellant with a total volume of about 2 million litres. The ET is made of aluminium and aluminium composite materials. It has two separate tanks inside, the forward tank for oxygen and the aft tank for hydrogen, separated by an inter-tank region. Each tank has baffles to dampen the motion of fluid inside. Fluid flows from each tank through a 43 cm diameter feed line out of the ET through an umbilical line into the shuttle’s main engines. Through these lines, oxygen can flow at a maximum rate of 66,600 litres/min) and hydrogen can flow at a maximum rate of 179,000 l/min. The ET is covered with a 2.5 cm thick layer of spray-on, polyisocyanurate foam insulation. The insulation keeps the fuels cold, protects the fuel from heat that builds up on the ET skin in flight, and minimizes ice formation. When Columbia launched in 2003, pieces of the insulating foam broke off the ET and damaged the left wing of the orbiter, which ultimately caused Columbia to break up upon re-entry into the earth’s atmosphere. How does the Space Shuttle Lift off? The two orbital manoeuvring systems’ (OMS) engines are located in pods on the aft section of the orbiter, one on either side of the tail. These engines place the shuttle into final orbit, change the shuttle’s position from one orbit to another, and slow the shuttle down for re-entry. The OMS engines burn mono-methyl hydrazine fuel (CH3NHNH2) and nitrogen tetroxide oxidizer (N2O4). Interestingly, when these two substances come in contact, they ignite and burn automatically (i.e., no spark required) in the absence of oxygen. The fuel and oxidizer are kept in separate tanks, each pressurized by helium. The helium pushes the fluids through the fuel lines (i.e., no mechanical pump required). In each fuel line, there are two spring-loaded solenoid valves that close the lines. Pressurized nitrogen gas, from a small tank located near the engine, opens the valves and allows the fuel and oxidizer to flow into the combustion chamber of the engine. When the engines shut off, the nitrogen goes from the valves into the fuel lines momentarily to flush the lines of any remaining fuel and oxidizer; this purge of the line prevents any unwanted explosions. During a single flight, there is enough nitrogen to open the valves and purge the lines 10 times! (To be Cont’d) eddie@afrowebs.com