Who provided significant early contributions to understanding rocket propulsion using liquid fuels?

Dr. Hermann Oberth made significant early contributions to understanding rocket propulsion, especially through his work on liquid fuels. He is regarded as one of the founding figures of astronautics and is known for his theoretical and practical advancements in rocketry. His book, "Die Rakete zu den Planetenräumen" (The Rocket into Planetary Space), published in 1923, laid out the fundamental principles of rocketry and the potential of liquid-fueled rockets, influencing future rocket scientists and engineers.

Oberth's insights into thrust, the operation of rocket engines, and the physics of launching payloads into space provided a cornerstone for understanding how rockets could be developed for space travel. His innovative thinking and experimentation in the field of propellants and rocket design set the stage for subsequent advances made by others in the field of rocket science, including pioneers like Wernher von Braun and many others who built on his principles.

Other individuals mentioned, such as Robert Goddard, have also made crucial contributions, but the choice selected highlights Oberth's critical theoretical foundations that helped shape the trajectory of rocketry and propulsion technologies as we know them today.