Weight. Every pound your rocket weighs versus your thrust is a pound that you cannot have in payload. A rocket made in the '50s would weigh so much they would have to add fuel, which is also weight. There's a point where it can become a runaway cycle where the weight and extra fuel don't make the rocket better. In the 50s when he said this he was 100% right. But the innovation and engineering that the American space program did to create the specialized launch vehicle Saturn 5 was amazing. Up until then every single astronaut was sent into space atop an ICBM ballistic missile repurposed for the space program. They developed Saturn 5 and it didn't need to be the size of the space program because it had 7.5 million pounds of thrust
Weight. Every pound your rocket weighs versus your thrust is a pound that you cannot have in payload. A rocket made in the '50s would weigh so much they would have to add fuel, which is also weight. There's a point where it can become a runaway cycle where the weight and extra fuel don't make the rocket better. In the 50s when he said this he was 100% right. But the innovation and engineering that the American space program did to create the specialized launch vehicle Saturn 5 was amazing. Up until then every single astronaut was sent into space atop an ICBM ballistic missile repurposed for the space program. They developed Saturn 5 and it didn't need to be the size of the space program because it had 7.5 million pounds of thrust