The two orbital maneuvering 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 monomethyl 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!
Either one or both of the OMS engines can fire, depending upon the orbital maneuver. Each OMS engine can produce 6,000 lb (26,400 N) of thrust. The OMS engines together can accelerate the shuttle by 2 ft/s2 (0.6 m/s2). This acceleration can change the shuttle's velocity by as much as 1,000 ft/s (305 m/s). To place into orbit or to de-orbit takes about 100-500 ft/s (31-153 m/s) change in velocity. Orbital adjustments take about 2 ft/s (0.61 m/s) change in velocity. The engines can start and stop 1,000 times and have a total of 15 h burn time.
Now let's put these pieces together to lift off!
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Profile of shuttle launch and ascent into orbit
As the shuttle rests on the pad fully fueled, it weighs about 4.5 million pounds or 2 million kg. The shuttle rests on the SRBs as pre-launch and final launch preparations are going on through T minus 31 seconds:
- T minus 31 s - the on-board computers take over the launch sequence.
- T minus 6.6 s - the shuttle's main engines ignite one at a time (0.12 s apart). The engines build up to more than 90 percent of their maximum thrust.
- T minus 3 s - shuttle main engines are in lift-off position.
- T minus 0 s -the SRBs are ignited and the shuttle lifts off the pad.
- T plus 20 s - the shuttle rolls right (180 degree roll, 78 degree pitch).
- T plus 60 s - shuttle engines are at maximum throttle.
- T plus 2 min - SRBs separate from the orbiter and fuel tank at an altitude of 28 miles (45 km). Main engines continue firing. Parachutes deploy from the SRBs. SRBs will land in the ocean about 140 miles (225 km) off the coast of Florida. Ships will recover the SRBs and tow them back to Cape Canaveral for processing and re-use.
- T plus 7.7 min - main engines throttled down to keep acceleration below 3g's so that the shuttle does not break apart.
- T plus 8.5 min - main engines shut down.
- T plus 9 min - ET separates from the orbiter. The ET will burn up upon re-entry.
- T plus 10.5 min - OMS engines fire to place you in a low orbit.
- T plus 45 min - OMS engines fire again to place you in a higher, circular orbit (about 250 miles/400 km).
You are now in outer space and ready to continue your mission.
Now, let's look at where and how you will be living while you are in space.