FSP's Jet Engine Kit
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Thrust and how to get it with our simple design
Thrust is derived from Newton's Third Principal; that for every action there is an equal and opposite reaction. So the harder you blow, the more thrust produced. To blow "harder," more air can be blown, or the air can be blown faster.
To blow air faster, a nozzle is attached to the end of the engine. This acts like a venturi to speed up the air. The nozzle essentially takes the energy of the molecules in the air and changes it to be useful. When you speed up the air it's pressure decreases (Bernoulli’s principal.) But the faster speed means greater thrust.
Now the air is shooting out the nozzle at a high speed and low pressure. Here an augmenter can be attached to increase thrust even more. An augmenter is a simple device that uses the low pressure of the jet stream to suck in cool outside air, mix it with the hot exhaust air, and direct this warm air back to create additional thrust.
At first this seems like you're getting something for nothing. But what this simple device is doing is utilizing the energy that is being lost in the form of heat. All that incredibly hot exhaust is blowing away and the energy just heats up the atmosphere instead of creating propulsion. The augmenter uses the exhaust's own properties of hot, low pressure air to warm and move extra air in a rearward direction and create extra thrust. Super efficient augmenters have an airfoil shape and can nearly double the amount of thrust produced by the engine.
The augmenter also has the benefit of changing the small amount of fast moving air into a large amount of slow moving air. Isn't slow moving air bad though? Not necessarily, now that we've added extra air to our jet stream. A large quantity of slow moving air is akin to first gear in a car. This is the best gear for accelerating. This is why propeller driven aircraft have shorter take-off distances than jet aircraft. The fast moving jet stream is more like 5th gear; it's great for cruising long distances at high speeds. (Just like commercial jets.) Adding an augmenter to the jet makes it more useful to power and accelerate a go-kart, boat, or hovercraft.
To summarize, the exhaust nozzle and augmenter work together to create an efficient jet geared to work best on a vehicle that needs good accelerating characteristics.
Augmenter plans and completed sets should be available soon from FSP.
Of course, another way to get the jet to blow more air is to use a bigger blower. When this is done, a special inner combustion chamber needs to be used to keep the mixture burning rich to lean. (See our combustion chamber page.) FSP's QB554HC blower is rather monstrous, and the jet it could power puts a little sparkle in our eyes. FSP is currently designing a jet-powered hovercraft using this blower. Instead of the jet bleeding some air off the hovercraft's blower, we plan on designing a hovercraft that will bleed some air off the jet's blower.
