# Could an electric airliner be aerotowed to cruise altitude by an electric “tug” aircraft?

The low energy density of batteries make electric aircraft need a high ratio of battery to payload. Take off and climb to cruising altitude could account for a substantial fraction of the energy required for a short haul flight. The point of aerotowing would be be to have less battery (and maybe motor?) requirement and more payload in the towed craft for cruise and landing whilst the tug aircraft could return to the take-off point. There would be the downside of needing to handle the tug landing of course.

• At this point in technology, I think it would take more fuel/energy to "tow" the aircraft to altitude than it would to just burn the fossil fuel necessary to do so, meaning the batteries in the tow aircraft have to be recharged from something, nothing is free. The power/density of fossil fuel is still unmatched in the battery world. – Ron Beyer Dec 17 '18 at 16:34
• An alternative would be to have the "tug" give power to the airliner through an electric cable, rather than towing it. Either way, I think this is a really interesting idea; I look forward to seeing the responses. – Tanner Swett Dec 17 '18 at 17:12
• Thanks Ron but the idea is not to use less energy, much less get something free, but to potentially have zero-carbon airliners. – Tony Cooke Dec 17 '18 at 17:22
• Have you seen this recent article discussing plans by Boeing, Airbus, and others for short haul all-electric planes? techcrunch.com/2018/07/08/the-electric-aircraft-is-taking-off I just wonder - what is the fuel source to make all the electricity to charge the batteries? There are losses in making the electricity, losses in charging the batteries, losses in efficiency in cooling the motors, etc. – CrossRoads Dec 17 '18 at 18:20
• And another, Zunum Aero, proposing hybrid to start and full electric in the 2020s "Did you know that short-haul flights produce over 40% of aviation emissions? With our aircraft, we believe these will be largely eliminated within twenty years. Our aircraft are “hybrid-to-electrics” that sip fuel only when they have to, will use even less over time as batteries upgrade, and will one day go completely without — so that flying will be kind to the Earth." zunum.aero/#About-us – CrossRoads Dec 17 '18 at 18:53

If we only look at the energy needed to lift a conventional airliner to its cruise altitude, the magnitude of the problem should become obvious. Here, only the tug is electric while the airliner is a normal A320. We have lots of answers with A320 data, and assuming that less fuel is taken onboard since we get some help in climb, the take-off weight used here is 72 tons. Cruise is in 30.000 ft, so we need to lift 72.000 kg by 9144 m. Energy $$E$$ is mass times height times gravitational acceleration, or 1.8 MWh or 6,456 MJ. Add to that the acceleration to Mach 0.8 which is 240 m/s at altitude and we need $$\frac{1}{2}\cdot m\cdot v^2$$ = 2,074 MJ more.