On the glider world, that is called a sustainer engine.. :) And here is my first attempt at an electric one.. https://www.youtube.com/watch?v=zAwpi7VPQTk

That's a cool idea. Though I see no need to make it VTOL. A simple tug plane, like a glider tug but for passenger aircraft. Really nice thought!

The issue is that if you cannot take off yourself, you cannot abort a landing.

So if you have to keep it, you're trading a bit of efficiency for a lot of complexity.

The tug could be more like a powerbank. It would not physically pull the aircraft up but assist by providing power. The aircraft could launch on its own but it is advantageous to use a tug because this increases the range or energy reserves that the aircraft itself has.

(outline of a launch: aircraft and tug are connected by cable before launch, both lift-off, gain altitude, cable is detached, aircraft moves on and tug returns to launch site)

Except you probably have another aircraft already landing, the slots are tight. Maybe you could have another shorter landing strip just for the tug, but again, that seems like a lot of complexity (i.e risk) for maybe a 20% increase in efficiency at best.

Like in the other comment in this subthread, I had vertical take-off and landing vehicles in mind.

You can even ground tow to altitude then go flying.. Glider have been doing that for ages..

That's an interesting and unconventional idea. If I remember Lilium's [0][1] numbers correctly, most of the motor power is needed for vertical take-off and landing and only a small fraction (1/10 to 1/5) is needed when they have transitioned to wing-borne lift. This means that supporting the aircraft for just 1 minute during take-off would give 5-10 minutes of extra vertical flight, which translates to 25km-50km of extra range at a cruising speed of 300km/h.

[0] https://www.lilium.com [1] https://en.wikipedia.org/wiki/Lilium_Jet

Good idea, you essentially have a craft that only brings a plane up to altitude and then goes to land to recharge.