Editing Joshua Pawlicki's Explanation of How It Works (section)

===''Atmospheric Flight''===
The problem with fast in-atmosphere (atmo) flight is overcoming the resistance of the air against your hull. Anybody can fly a boat slow. Anyone can drive a car slow. But you never see anyone doing that. There are several solutions to generating efficient propulsion in atmo and beating the resistance of the air. Aerodynamicy will reduce the resistance of the air because you can channel it well so that it's bouncing off you less. The trick with the flight is staying up there. Modern aircraft use airfoils on the wing to keep them up (which, in small detail, uses air flowing over the wing to push up on the underside of the wing while also creating a lack of air that pulls up on the top of it). You may choose to include these in your boat or not. The further you get from the fuselage (the main body part of an airplane), typically the less distorted air you get, and the more stability. Thus, typically, a larger wingspan means more force directed properly on the airfoil, and better lift. Properly designed airfoils have minimal drag, slowing you down less than poorly designed airfoils. This is, however, limited by materials. The force exerted on the tips of the wings of a too-far-reaching boat will literally rip the wings right off, and then you're in trouble. Of course, the "screening" ability of the gravity drive makes this totally unnecessary.
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So right now you're probably thinking that you didn't want to talk about a plane, instead a spaceship. Airfoils are a neat thing for a mechanic to know, but your boat probably has big huge engines, not wings. In the Firefly/Serenity universe, most boats have large engines that swivel and provide lift to gain effects similar to a VTOL. Well, the design of these engines is (with modern materials) rather difficult to make reliably, although given perfect strength materials it could be done easily. The primary engine, known as a thruster or a pod, is a four-part engine. The basic principle relies on bringing in air, compressing it, superheating it, then letting it out the back of the engine. Since hot materials expand, it leaves at a speed considerably greater than the speed at which it entered, producing a force which then, by Newton's Third Law of Motion, pushes the engine forwards. This concept is called a ramjet engine. Given an constant source and concentration of fuel, a ramjet engine will accelerate until the forces of the air rip it into shreds. The trick is to thus limit the fuel that heats the air. Ramjet engines are extremely efficient, but will only work if you're already traveling at a certain speed roughly equal to half the speed of sound (around 620 km/h or 380 miles per hour), and are inefficient unless going even faster.
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The question, then, is how to get up to that speed, which is where the turbine comes in. In the engines of the 'Verse, a turbine exists in each engine, powered electrically from the electricity generated by the fusion reactor (we'll get to that later). Anyways, once you have this turbine (basically a big fan) rotating, you can "suck" in air at sufficient pressure to run a ramjet-like process on it and do the same thing. The problem is that you can only go so fast before the force of the air pushing on the turbine rips it apart. Also, this requires a lot more fuel and electricity, and thereby should be used as little as possible. Once you get up to a sufficient speed, the turbine blades fold back or retract depending on the design and cease to turn. In theory you could reverse engineer a turbine to rotate at high speeds as an electrical generator (I.E. a windmill/wind power), but they're not designed to do so, and fusion power systems really negate the need. Either way, this is when the engine becomes a ramjet engine. The speed varies depending on the design and power of the turbine, but is usually around 600 miles per hour or 1000 kilometers per hour.
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Once you pass Mach 1 or so (depending on design), forces of compressed air start bouncing around inside the ramjet engine due to the compression (it compresses the air to subsonic speeds). This can generate unwanted stress just about everywhere and eventually lead to all kinds of bad. The engines reconfigure somewhere within the transonic range to become scramjet engines (the method of how they reconfigure themselves depends on design, although the end products are similar if not the same). Scramjet engines are merely ramjet engines with a slightly different shape so as to allow supersonic air to pass through efficiently. They can usually get you up to Mach 28 or so (modern scramjets are capable of Mach 20-24, and it can be assumed that those in the 'Verse are constructed with better materials), which is more than enough to break the gravitational pull of the planet (you can orbit a planet from a low space orbit at Mach 22 safely, although of course scramjets don't work in space).
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The final configuration of the engines of Serenity/Firefly ships is a more optional use, at least in atmosphere. When in the rocket mode, the front of the engine closes, and superheated gases (likely helium exhaust from the fusion generator, although some designs use a fission reaction on the hydrogen fuel) are shot out the back. The upside of this is that it'll work in unusual conditions, such as a vacuum. The downside is of course that it burns tons and tons of fuel very quickly and, when compared to the ramjet-family, is extremely inefficient. But it is useful to know.
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