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Subsonic planes (like commercial airplanes) are designed to fly under the speed of sound, while supersonic planes (like fighter jets) fly faster than the speed of sound. 

Subsonic planes are mostly made of aluminum, with steel and titanium alloys to reenforce and strengthen important parts. Supersonic planes, however, encounter problems  like shock waves, or increased drag across their surface. To resolve these issues, some supersonic aircraft are made out of titanium (and/or alloys); but this can be expensive. The Concorde used aluminum alloys to increase its strength.