I was very impressed with those little jet engines. I wanted to buy one myself for my own little ultralight. But after doing some research I discovered, to my dismay, that they are not practical for ultra light applications. The price of around 4000 Euros wasn't the major drawback.
Both of those engines in that photo produce enough power to get the cri-cri airborne and in stable flight. The wing on the cri-cri allows that thrust to produce a speed of around 150 mph. Unfortunately, your gas tank will run dry in minutes. Each engine gobbles up about a 1/2 liter of fuel per minute! The compressor ratio on the biggest engine is only about 4 to 1 which means that it produces little thrust for the amount of gas going in the combustion chamber. A jet engine on an modern airliner, or example, has a compressor ratio more like 30-40 to 1 and most of the thrust comes from the cold air induction fan with only about 25% coming from the exhaust gas flow. On these tiny model jet engines, however, the only thrust produced is from exhaust gases, which isn't much, especially if you're not compressing the air more than 4:1 to begin with.
These model jet engines are weak, noisy, and fuel guzzling for their anemic thrust output. If they could only get the compressor ratio up there, and perhaps an induction fan on a shaft, then this type of engine would revolutionize the ultra light market. Hopefully it's only a matter of time before we can get an affordable turbojet engine writ small. I'd love to get rid of my prop.
An optimist robs himself of the joy of being pleasantly surprised