Hybrid Jet Engine

[fechter]

This is something just in an idea stage. Ever since I was a little kid, I've always been facinated by jet engines, but never got to play with one. This may be a way to make it simple.

Basically, take an electric ducted fan and attach something like an afterburner to it. The idea is to heat up the air enough so it expands to several times it's original volume, increasing the velocity coming out of the tail, increasing thrust. With a typical military jet, the afterburner nearly doubles the thrust. If starting with cold air instead of hot jet exhaust, the afterburner might give more than 2x increase. Modern ducted fans are pretty impressive by themselves.

A glow or spark plug would be used to ignite the fuel. Fuel flow would need to be controlled such that the optimum rate was given depending on ducted fan speed. The burner could be turned off and restarted mid flight. Maybe a good idea to place a pyrometer somewhere to throttle back fuel if it gets too hot.

A coaxial tube design with a fair amount of bypass air between the tubes would keep the outside temps down (hopefully) as it would tend to get red hot or melt. All the parts that get hot would be fairly simple sheet metal pieces. No high temperature turbine blades or bearings to worry about. Where can I get titanium beer cans?

This design could use nearly any type of fuel. You'd try to carry just enough fuel to last as long as the batteries.

I think this could be scaled over a wide size range. World's smallest jet? Personal jet pack?

Even if the burner part didn't add enough thrust to make up for its weight with fuel, it would look and sound really impressive, especially at night.

So, is there some obvious reason I've overlooked why this couldn't work?

 

[Arlo1]

I think I saw this a while back where someone made one but I can't remember if it made much more thrust or not.
[youtube]7BkdKQRemso[/youtube]

 

[bigmoose]

Here is a nice little contractor report from the days where a lot of data was published. Sort of a how to for supersonic cruise duct burning... the design is more complicated than you would need as they were after a very low NOx signature. You could likely get by with "V gutters" for flame stabilization like older afterburners. Good luck! Make video's. Neat idea.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800008814_1980008814.pdf

 

[Xrain]

Well this is certainly an interesting idea.

The biggest problem you would have with ducted fans, is first the exhaust velocity of an EDF is pretty low, and second they are built to produce a large volume of airflow at a pretty low static pressure.

I think what would likely happen is when you lit the afterburner, it would reverse the thrust of the EDF and blow hot gasses backwards through your EDF, since the EDF doesn't have enough static pressure to overcome the gas expansion of your afterburner.

You would probably be better off making it into a pulse jet. But it is possible to make this idea work as you intend it, however I don't think you will save much on weight.

These are just my gut feelings though so I would still recommend looking more into it.

 

[fechter]

I think you're right about the pressure, but it may not be that bad. If the exhaust side is wide open, there won't be much back pressure. Still, more compression may be needed to optimize it, so possibly two ducted fan units in series (counter rotating might be good) would be able to increase the pressure. Otherwise it might work to simply change the blade pitch on a single one.

I don't think it would flame back to the fan unit unless you dumped in way too much fuel. It should be continuous combustion.

The modern small jet engines are very impressive, but also super expensive. Most of them seem to have centrifugal compressors. Maybe an electric one?