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Tupolev ANT-25 was designed specifically for setting flight range records. In 1937 two flights from Moscow, Soviet Union to USA via the North Pole happened - one covered 9130 kilometers and finished in Vancouver, Washington and another covered 11500 kilometers and finished near San Jacinto, California.

ANT-25 had a single V-12 piston engine. This sounds kinda not very redundant - should a single engine fail while the plane was crossing Arctic there was good chance the flight would end there and maybe the crew would not be rescued because simply reaching there would be problematic.

Why was a single engine airplane selected for this specific task where reliability was very important?

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    $\begingroup$ In a long range flight record, the most important thing is not the reliability, is the fuel consumption! And the less engines you have, the lower the weight and the consumption. $\endgroup$ – Gianni Alessandro Jun 27 '17 at 10:24
  • $\begingroup$ @GianniAlessandro There's no record if the plane disappears in Arctic. $\endgroup$ – sharptooth Jun 27 '17 at 10:27
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    $\begingroup$ The Soviet Union had a pragmatic approach to these things, if the plane disappeared then there was no record attempt, if they did then they'd let the world know. $\endgroup$ – GdD Jun 27 '17 at 11:48
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Fuel efficiency. One engine can run more efficiently than two, even if the two have the same power output combined than the one. Friction losses, inertia losses, etc...

The same thing can be seen in commercial airliners today. Now that gas turbines have reached a very reliable state, and the regulations for overwater flight recognize this, we are seeing large two engine widebodies (777) replacing four engine widebodies (A340), partially due to lower maintenance costs (cheaper to rebuild two large engines than four smaller ones) but mainly for reasons of fuel consumption.

The Voyager that made the round the world unrefueled flight had two engines, but only used both for takeoff and initial climb. After that, the one was shut down for the duration of the flight, for reasons of fuel consumption.

Obviously, two engines are more reliable than one, and yes, a single engine long range flight is risky.

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  • $\begingroup$ Two engines have twice the chance of a failure, amd are therefore less reliable than one engine. It's the effect of a failure that is less in a twin. $\endgroup$ – Koyovis Jun 27 '17 at 14:04
  • $\begingroup$ True, but the effect of the loss is what matters. While single engine operation on a dual engine aircraft can be difficult, no engine operation on a single engine aircraft is going to end very quickly. Best to consider the reliability of an aircraft in total, not just one component. $\endgroup$ – tj1000 Jun 27 '17 at 14:12
  • $\begingroup$ Reliability as in chance of a failure? $\endgroup$ – Koyovis Jun 27 '17 at 14:14
  • $\begingroup$ Reliability as in the flight coming to an unexpected end. $\endgroup$ – tj1000 Jun 27 '17 at 14:16
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As said in the comments, in a distance record aircraft you want to keep the drag and mass of your aircraft as low as possible. The previous and next record were flown in single engines too and risk assessment was a bit different back then.

Using a single engine for such long distances wasn't the only ANT-25 characteristic you wouldn't see in a normal airplane: the engine was underpowered (only Russian production engine available?) and for record flights the whole aircraft was even lack-stripped in order to weight less.

For emergency landings in the arctic they had skis, tents and other gear stored in the wings.

In the case of having to land on water they would jettison part of the fuel and inflate some kind of balloons which were inside the wings. That way the wings could be used as floating bodies and prevent water from entering. After water-ditching with retracted landing gear the aircraft could apparently stay afloat for a long time.

If you're interested in further details this biography of Pavel Sukhoi describes the project thoroughly in one of the chapters.

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