What was the Sea Dragon rocket, and what would it have been used for?

In the very last scene of the final episode of the first season of “For All Mankind,” the Apple TV Plus series about an alternate space race, an immense rocket emerges from the ocean, hurtling into space, carrying a cargo of plutonium nuclear fuel to the Jamestown moon base in 1983. It turns out that the rocket was a Sea Dragon, a real-world proposal of what would have been the mightiest launch vehicle ever built. The Sea Dragon never flew in real-life history. Had it been built, its capabilities would have inspired awe.

The Sea Dragon was first proposed by Robert Truax, an aerospace engineer working for Aerojet, in 1962. A study produced by Aerojet the following year demonstrated that there were no technical obstacles for building such a launch vehicle, even considering the state of rocket design in the early 1960s.

As conceived, Sea Dragon would have consisted of two stages. It would have been around 400 feet tall, 75 feet in diameter at its widest point, and it would have weighed 40 million pounds. It would have been capable of putting 550 metric tons into low Earth orbit.

For comparison’s sake, the Saturn V stood at 333 feet, had a diameter of 33 feet and could put just 140 metric tons into low Earth orbit. Elon Musk’s Starship/Super Heavy rocket ship will be 387 feet high, be 30 feet in diameter, and able to launch 100 metric tons into low Earth orbit.

The Sea Dragon would have been constructed at a dry dock, much like a ship. When finished and fueled, it would have been towed out to sea to a safe distance. A ballast stage would have been filled to bring the rocket to an upright position. The Sea Dragon would have been launched while partly submerged.

The genius part of the Sea Dragon’s design, besides its immensity, was that it was totally reusable. The first stage would have used something called an inflatable aerodynamic decelerator to slow its descent and allow it to splash down in safety. The second stage would have been recovered in a similar fashion. The two stages would have been towed back to the offshore facility to be refurbished for the next flight.

The Sea Dragon never made it out of the concept stage in the 1960s. It was not needed for anything that either NASA or the military was doing in space at the time. The Saturn V was quite sufficient for the moon program. The Air Force has an inventory of expendable rockets such as the Atlas and the Titan that it used for satellite launches. The cutbacks that occurred to the space program starting in the mid-1960s forestalled any notion of actually building a Sea Dragon. Had NASA been given the greenlight to send humans to Mars after the Apollo program, a super-heavy launch vehicle like the Sea Dragon might have been justified.

Could something like the Sea Dragon be developed today, given modern rocket technology? An analysis by Citizens in Space suggests that the price of building a modern Sea Dragon would be $22 billion in modern dollars. The trick to making such a rocket economical would be a flight rate of somewhere between 12 and 24 launches per year. Unfortunately, no conceivable space effort would justify a super rocket capable of putting 550 metric tons into low Earth orbit in a single launch flying that often.

The Sea Dragon could loft the equivalent of one International Space Station in a single launch. It could deploy all or most of a ship designed to send humans to Mars and back at once. The number of people and amount of cargo it could send to the moon is staggering to imagine. The rocket could take a number of huge structures to space.

The problem is that the design of space infrastructure is trending toward launching things in pieces and assembling them in low Earth orbit, much as the International Space Station was built. Eventually, factories in space will use materials mined from the moon and asteroids to build space stations, lunar bases and perhaps even spacecraft.

Thus, the Sea Dragon is likely to remain a spaceship of the imagination, a reality only depicted in science fiction such as “For All Mankind.” Still, such a rocket would have been glorious to see emerging from the ocean and shooting to the heavens.

Mark R. Whittington, who writes frequently about space and politics, has published a political study of space exploration entitled “Why is It So Hard to Go Back to the Moon?” as well as “The Moon, Mars and Beyond.” He blogs at Curmudgeons Corner.