A RARE letter written by the Derbyshire inventor who created the bouncing bomb used by the Dambusters is to be sold at auction.

The note by Sir Barnes Wallis is about the technical details of a plane on which he was working as an engineer in 1971.

The Ripley scientist is best known for designing the bouncing bomb, which was used by Second World War pilots to destroy three German dams.

And he went on to develop grand slam and earthquake bombs which were used on Nazi targets such as missile launch sites and submarine pens.

The letter, written eight years before his death at the age of 92, does not name the aircraft he was working on.

It is written to "Frank" and includes notes such as: "By flying at about 125,000 feet up, the aircraft will get no hotter than the 'Concorde' at 60,000 ft at only M=2 (Mach 2)."

The letter came from a Cumbria man's private collection of aircraft memorabilia which also includes the autographs of Orville Wright, Charles Lindbergh and Amelia Earhart.

It will be sold in London on Saturday alongside another smaller note from Sir Barnes and a framed autograph and photograph of the inventor.

The second note thanks a correspondent for their letter and returns some photographs.

The director of organisers International Autograph Auctions, Richard Davie, said the longer letter was expected to fetch between £150 and £200.

He said: "Sir Barnes' signature is fairly common but it's unusual to find a letter from him about the technical side of aviation.

"He's obviously a very important figure in the Second World War whose great mind was utilised by our air force, so we expect lots to do with him to do well."

Mr Davie said the shorter note would sell for between £100 and £150 and the autograph and picture for between £80 and £100.

Sir Barnes worked for Vickers and its successor companies, including the British Aircraft Corporation, from 1913 until his retirement in 1971.

His bouncing bomb design was immortalised by the actions of the Dambusters raiders who destroyed the Moehne, Sope and Eder dams on May 16, 1943. Only 11 out of 19 Lancaster bombers returned from the mission.

Saturday's auction will take place at the Radisson Edwardian Hotel, near Heathrow Airport. Bids can be placed either through the IAA website at www.autographauctions.co.uk or by calling Mr Davie on 01158 451010.