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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?

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jklingler
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    the video as well as the website are just marketing tools to hopefully get venture capitalists to give them money which will then go who knows where, but most likely into the pockets of the company owners right before they jump ship. – jwenting Aug 14 '15 at 06:52
  • @jwenting Can you provice any source on that assumption? Isn't that assumption a bit extreme? I mean the project is founded by the EU's Seventh Framework Programme for research, technological development and demonstration... – jklingler Aug 14 '15 at 07:42
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    The EU funding should be the red herring here. Sorry to be blunt, but EU bureaucrats are not exactly known for their prescient use of public money. The numbers simply don't add up. Please read and understand what I have answered here - those 23% are totally incredible. – Peter Kämpf Aug 14 '15 at 09:29

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The system is highly impracticable for a number of reasons.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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?

Federico
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The savings for the cruiser/feeder concept come from two main areas:

  1. Cruiser fuel saving as it does not have to carry the fuel for the second leg of the journey during the first leg (first leg is before refuelling, second leg is after). The feeder is much smaller and flies a shorter distance than the cruiser, this is what makes the process efficient.
  2. Smaller cruiser as you do not need as large an aircraft to fly the same distance. E.g. rather than a 6000nm cruiser you can design an aircraft for a design range of 3000nm. It there has a much smaller operating empty weigh etc. and is therefore more fuel efficient.

So the answers talking about using simple calculations such as the Breguet equation are misguided, as they have completely ignored one of the main areas of fuel saving, the fact that you use a smaller and more fuel efficient aircraft to fly the same distance.

The main issue is that you can largely get the same effect with staging the journey. Rather than having a feeder aircraft, you simply land and refuel. This obviously eliminates the feeder, but has its own problems (longer journey times, more take off/landings at busy airports).

There are clear fuel savings to be made with the cruiser/feeder concept for a specific subset of journeys. It just seems a large investment when the same effect can be implemented with staging much more easily.

ace
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  • how would you save fuel by landing and taking off again when this operations are the most expensive under the fuel consumption point of view? – Federico Aug 14 '15 at 10:37
  • The aircraft is not carrying the fuel for the second leg of the journey during the first leg. Over long distances e.g. 6000nm this fuel weight is substantial. You also have a much smaller aircraft i.e. rather than having an aircraft that is designed for 6000nm you design an aircraft for 3000nm, it weighs much less and is more fuel efficient. The extra fuel burnt in taking off and landing one more time is a lot smaller compared to the saving you would see when staging a long flight. – ace Aug 14 '15 at 10:44
  • I am unconvinced. Do you have any source or computation done? Or is it speculation done by you? – Federico Aug 14 '15 at 11:09
  • ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070032063.pdf – ace Aug 14 '15 at 11:12
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    But if the cruiser doesn't take off and land then the feeder has to. Either way you can't avoid a takeoff/ landing process. Sounds dubious to me. – TomMcW Aug 14 '15 at 20:37