Cruiser-Feeder Concept - feasible? The people from cruiser-feeder started a project called RECREATE. 


  However, for the concept with fuel transfer from feeder to cruiser (civil air-to-air refuelling operations), the results of our collaborative research indicate a fuel burn reduction potential on isolated aircraft level between 11% and 23 % for a typical 6000 nautical miles flight with a payload of 250 passengers.


That sounds very good, doesn't it? I answered this question about civil air-to-air refuelling with the cruiser feeder concept. A User said:


  with the Breguet equation it is easy to see that the savings are modest.


And:


  This concept will never be certified


They even created a high-realistic simulation environment in which they tested the refuelling-process with real pilots as you can see in this video.

The feedback from the pilot is very good and promising that the concept might become real.

I'm curious why the savings are modest, because the planes can reduce the fuel burn from 11% to 23% and why this concept will never be certified.

Can the cruiser-feeder concept become real? That a feeder pumps fuel to a cruiser after he took of?

When does the use of the feeder burn more fuel than it saves?

What could stand in the way to make the RECREATE Project become real?
The system is highly impracticable for a number of reasons.


The referred paper acknowledges that the fuel savings due to existing tankers are negligible and a new design is required. I doubt anyone is going to develop this. Even the military uses modified civil airliners for its tankers needs. Development of the tanker as outlined in the paper will cost billions of dollars and no one is going to do that for a handful of tankers.
Even if such a tanker is built, the existing aircraft will require significant modifications for in flight refueling (assuming that the tanker is standardized from day one, which is practically not possible; even the US armed forces use two different type of tankers). Basically, the structural weight goes up,which reduces performance and increases fuel consumption.
Then the pilots have to be trained for operations of both the aircraft. In flight refuelling is potentially dangerous.See http://foxtrotalpha.jalopnik.com/aerial-refueling-hooking-up-is-hard-to-do-1559495801. Nobody's going to take that risk with civil operations.
The study is extremely simplistic. modern civil aircraft fly in a number of 'flight levels', separated by set altitude. So, for refueling, either the tanker or receiver should climb or descend the difference in altitude, and that burns up more fuel.
If carrying extra fuel in the aircraft is dead weight, it makes no sense to carry it in another aircraft and pay for its operations and maintenance.
It is not correct to compare the military and civil operations. Their priorities, methods and end goals are different. What looks good on paper may not work on real life. Remember Concorde?