What is the principle of ramjet?

Answer 1

Compressed air is heavier and richer in oxygen than air under ordinary pressure, especially at high altitudes. Jet engines compress air to make the burning of fuel more efficient and add more mass of the air to the ejection that pushes the airplane. Most jet engines actively compress the air using turbines, but ramjets use the forward motion of the airplane to "ram" (force) air into an opening in front. Thus a degree of air compression (not as great as active compression) is achieved. Since ramjets depend on forward motion to work, an assist of some sort is required for takeoff.