Harry Julian Allen (April 1, 1910 – January 29, 1977), also known as Harvey Allen, was an aeronautical engineer and a Director of the NASA Ames Research Center, most noted for his "Blunt Body" theory of atmospheric entry which permitted successful recovery of orbiting spacecraft.

[1] Allen was interested in the full range of aerodynamics research, and made contributions to the study of subsonic, transonic, supersonic and hypersonic flow.

[2] When the United States became interested in the design of ballistic missiles, Allen began research in the dynamics and thermodynamics of atmospheric reentry, as well as the effects of radiation and meteorites on space vehicles.

[2] Earlier ballistic missiles, developed by both the United States and the Soviet Union, featured long nose cones with very narrow tips, which had relatively low drag when entering the atmosphere at high speeds.

Allen's theory led to the design of ablative heat shields that protected the astronauts of the Mercury, Gemini and Apollo programs as their space capsules re-entered the atmosphere.