# How is the D8 Double Bubble aircraft by NASA so efficient?

How does the 'double bubble' fuselage cross-section of the "Double Bubble D8" aircraft (developed by MIT for NASA) help improve overall efficiency by around 70%?

Image credit: NASA/MIT/Aurora Flight Sciences

• Do you have a reference that shows that it actually does improve overall efficiency? – BSteinhurst Jun 3 '15 at 13:44
• nasa.gov/content/variation-on-a-theme The efficiency increase is claimed. I just wish to know if the fuselage cross section has any specific contribution to that – Victor Juliet Jun 3 '15 at 13:45
• @VivianjeetSinghSudan, you have to scroll through about 30 articles to find one on the D8. Got anything better? – egid Jun 4 '15 at 5:03
• Efficiency improvement compared to what? – Manu H Nov 20 '15 at 16:10

The main advantage is its lower flight Mach number of 0.74. This allows it to use minimum sweep, which in turn reduces wing area, structural masses and thrust requirements. Now pick the right definition for efficiency (one that neglects speed), add the engine fuel consumption expected in 20 years, and the concept looks like a winner. If fuel prices go to \$200 or \$300 a barrel, this type of aircraft will lower the cost of flying substantially. The double-bubble fuselage will contribute only a little to the overall savings, though. Most will come from the (hypothetical!) engines and the high aspect ratio wings with little sweep.

Note that the lower flight speed is compensated by the claim of much shorter boarding times, so the double bubble comes out ahead in block times. The only reasoning is that passengers will board more quickly because they can choose between two aisles to get to their seats. Very questionable!

Below is page 105 from this report which shows the fuel consumption improvement of the separate steps from a Boeing 737 reference design to the D8.1 concept. The biggest contribution is from lowering cruise Mach (and increasing cruise lift coefficient, which allows to reduce wing area) and from engine optimization.

As it looks now, the marginal cost of fracking will keep oil prices below \$70 for the next years, and airlines will keep ordering planes which look like those they operate today.

Looking through the proposals in the PDF linked in your comment, most of them make me cringe. Since the Seventies we see the same concepts for supersonic travel, and here they are again, only presented in different colors. Some subsonic designs are sound, but again we see hopeless concepts like the boxed wing which is kept alive by a stream of NASA grants, only because it looks so different. I am equally skeptical of those blended wing concepts which pop up with every new generation of aircraft designers.

• It sounds like a straighter wing, lower cruise speed and more efficient engines would get similar gains from a 'traditional' cross-section 737. Am I hearing you correctly? – FreeMan Nov 20 '15 at 21:16
• @FreeMan: Yes. Look at turboprops for proof. – Peter Kämpf Nov 20 '15 at 22:21

Looking at MIT's presentation, more specifically pages 13 - 15, they're counting on the fuselage being a lifting body, among other things.

For most aircraft, the fuselage encompasses the passenger cabin, cargo cabin, fuel tanks etc. and is designed to impose as little drag as possible in the process. With a lifting body, the fuselage actually provides lift, instead of just adding minimal drag.

The concept is also counting on

• cleaner aerodynamics
• higher-efficiency engines
• less wing area
• with the fuselage providing some lift, the lighter weight and less fuel needed for the projected mission, less wing area is needed

All of these, in combination, provide the estimated 70% greater efficiency.

• Due to the way induced drag works, fuselage providing lift will add more drag than if the lift generation is left up to the wings. – Jan Hudec May 20 '17 at 21:17
• The engines require less power, it requires less runway to take off on (about 5000 ft)

• flying shorter and more direct routes, for cost-efficiency.

• Reliance on promised advancements in air traffic management such as the use of automated decision-making tools for merging and spacing enroute and during departure climbs and arrival descents.

overall it essentially improves most aspects of the aircraft, (I have oinly listed a few things, their are loads more on the NASA website)

• I've edited the question, as my specific question is regarding the fuselage cross section contribution in improving the overall efficiency. Sorry for the inconvenience. – Victor Juliet Jun 3 '15 at 13:40

Their presentation on page 87 specifically covers reasons for the fuselage design.

Additional lift and more cabin room in a smaller fuselage seem to be the main reasons they give. But there are a lot of other details in the presentation too.

But note: the fuselage shape is only part of the overall claimed efficiency improvements. There are many other features such as a lower cruise speed, reduced sweepback of the wings leading to weight saving, etc.