Augmentor for big pulsejet

[Mike Spike]

Hi, I wonder how much extra thrust I could expect if i put an augmentor on a pulsejet with 100lb of thrust? and has anyone got ready made blue prints for one?
And how fast would I need to go for the augmentor to loose efficency?
Here's the pulsejet http://cgi.ebay.com/ebaymotors/jet-engi ... rtsQ5fGear

[Rocket Man]

An augmentor adds air an increase exhaust volume and converts high velocity exhaust gas to lower velocity exhaust gas. The exhaust gas coming out of a pulse jet can be 500 mph. The high velocity gas does not have a lot of torque for quick take off unless it is a very large engine so the augmentor changes the 500 mph high velocity gas to a lower velocity like 100 mph for example. If you reduce the velocity 5 times it increases the torque 5 times. It is like an AC power supply that produces 1000 watts, 100 volts, 10 amps if you use a transformer to convert that to 20 volts 50 amps you still have 1000 watts. The larger taper on the engine exhaust increases surface area of the exhaust gas every time you double the surface area the high velocity gas speed is reduced by 1/2 and the torque doubles. I may be using the wrong word torque to discribe increase power take off at low speed.

[WebPilot]

... augmenters increase aerodynamic drag at speed. Drag is proportional to the square of the velocity so they need to be jettisoned as soon as the point of diminishing returns is reached.

[tufty]

Pulsejets don't have torque. They have thrust.

Bruce's site gives a pretty good layman's terms explanation of how augmentors work here : http://www.aardvark.co.nz/pjet/augmentors.shtml

Here's some theory : http://www.springerlink.com/content/j14x7n3035076397/ and some more here http://www.lib.ncsu.edu/theses/availabl ... ed/etd.pdf

The latter suggests you should be able to double the thrust of your pulsejet with a well designed augmentor (page 118)

If you assume you're doubling thrust, and you can make a decent approximation of the additional drag of the augmentor, you should be able to work out your break-even point.

[WebPilot]

It's called a pulse jet for good reason -- the thrust pulses with time.

The thrust can be averaged over time, usually one cycle. The thrust can be both +tive and -tive since the gauge pressure acting on the valve plate is so.