That... kind of makes sense. But (the way I* see it) modern large aircraft aren't very overpowered (otherwise they would be less efficient), so their engine power is just a little bit above what's needed to get them into the air with a "usual" runway size in ~ the worst possible conditions ("hot & high"). Which means that they would need ~ double the runway length to take off with a glider in tow? So this works with the small aircraft they're currently testing with (they just have to use a longer runway), but if they want to extend it to larger cargo jets, they would risk exceeding the limitations of existing airport runways?

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* - someone who has an interest in aviation, but no professional background or training therein