airliner vs ramjet

airliner

noun
  • A passenger-carrying aircraft, especially one of a fleet operated by an airline. 

ramjet

noun
  • A jet engine in which forward motion forces air into an inlet, compressing it (as opposed to having a pump type device compressing the air for combustion with fuel), and where combustion is subsonic. 

How often have the words airliner and ramjet occurred in a corpus of books? (source: Google Ngram Viewer )