rocket nozzle

Ion Propulsion…What Is It?

Ion thrusters are being designed for a wide variety of missions – from keeping communications satellites in the proper position to propelling spacecraft throughout our solar system. But, what exactly is ion propulsion and how does an ion thruster work? Great question! Let’s take a look:

Regular rocket engines: You take a gas and you heat it up, or put it under pressure, and you push it out of the rocket nozzle, and the action of the gas going out of the nozzle causes a reaction that pushes the spacecraft in the other direction.

Ion engines: Instead of heating the gas up or putting it under pressure, we give the gas xenon a little electric charge, then they’re called ions, and we use a big voltage to accelerate the xenon ions through this metal grid and we shoot them out of the engine at up to 90,000 miles per hour.

Something interesting about ion engines is that it pushes on the spacecraft as hard as a single piece of paper pushes on your hand while holding it. In the zero gravity, frictionless, environment of space, gradually the effect of this thrust builds up. Our Dawn spacecraft uses ion engines, and is the first spacecraft to orbit two objects in the asteroid belt between Mars and Jupiter.

To give you a better idea, at full throttle, it would take our Dawn spacecraft four days to accelerate from zero to sixty miles per hour. That may sounds VERY slow, but instead of thrusting for four days, if we thrust for a week or a year as Dawn already has for almost five years, you can build up fantastically high velocity.

Why use ion engines? This type of propulsion give us the maneuverability to go into orbit and after we’ve been there for awhile, we can leave orbit and go on to another destination and do the same thing.

As the commercial applications for electric propulsion grow because of its ability to extend the operational life of satellites and to reduce launch and operation costs, we are involved in work on two different ion thrusters of the future: the NASA Evolutionary Xenon Thruster (NEXT) and the Annular Engine. These new engines will help reduce mission cost and trip time, while also traveling at higher power levels.

Learn more about ion propulsion HERE.

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Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)

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This numerical simulation gives a glimpse of flow inside an unsteady rocket nozzle.  The nozzle is over-expanded, meaning that the exhaust’s pressure is lower than that of the ambient atmosphere. A slightly over-expanded nozzle causes little more than a decrease in efficiency, but if the nozzle is grossly over-expanded, the boundary layer along the nozzle wall can separate and induce major instabilities, as seen here. In the first segment of the video, turbulent structures along the nozzle wall boundary layer are shown; note how the boundary layer becomes very thick and turbulent after the primary shock wave (shown in gray). This is due to the flow separating near the wall.  The second half of the video shows the unsteadiness this can create. The primary shock wave splits into two near the wall, creating a lambda shock wave, named for the shape of the lower case Greek letter. This shock structure is indicative of strong interaction between the boundary layer and shock wave. (Video credit: B. Olson and S. Lele)

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Aerial fireworks are essentially semi-controlled exploding rockets. Here Discovery Channel shares high-speed video of fireworks taking off. The turbulent billowing exhaust on the ground is reminiscent of other rocket launches. The tube-launched firework clip is a great example of an underexpanded nozzle. The pressure of the gases in the tube is higher than the ambient air, so when the gases escape, the exhaust fans out to equalize the pressure. And, finally, the explosion that propels the colorful chemicals outward forms jets that can affect the final form of the display. To my American readers: Happy 4th of July! And be safe! (VIdeo credit: Discovery Slow-Down)