English: How does the Orbital Maneuvering System work? At main engine cutoff for the Space Shuttle, it doesn't have sufficient speed to stay in orbit. So it must use its Orbital Maneuvering System, as well as its Reaction Control System, or 'OMS RCS' as we call it. These are hypergolic systems. What that means is, you have two chemicals -- monomethyl hydrazine and nitrogen tetroxide -- who really, really hate each other. I don't need a spark-ignition device, they're not cryogenically stored. I just open up two valves,they flow into the combustion chamber, boom_ I have rocket propulsion; a very, very simple system.The downside to these chemicals is, they're very, very toxic. Whenever you see people working with them here on the ground, you'll see them essentially wearing a spacesuit to protect them from these chemicals. Because if you got a deep enough whiff of these things, you would have to go to the hospital and you could have some significant lung damage.So we trade that ease of use on orbit for the problems we have handling it here on the ground. Now you use the Orbital Maneuvering System to give you that tiny, extra boost of speed to get you up into space, and then you also use it on the day you come home in order to slow the orbiter down just enough so that it will reenter the Earth's atmosphere.And that's how we use the OMS RCS system.