Sea Dragon was a 1962 design study for a two-stage sea-launched orbital super heavy-lift vehicle. It is the largest launch vehicle ever conceived and the second largest in terms of low Earth orbit payload capacity.
Artist's depiction of a Sea Dragon night launch (youtube.com)
The concept spawned from Robert Truax, an engineer at Aerojet, who wanted to produce a low-cost heavy launcher. Sea Dragon falls into the class of launch vehicle known as "big dumb booster", which comes from the idea that it is cheaper to launch large rockets of simple designs than smaller rockets of more complex designs regardless of payload. Sea Dragon is so unique because it was to launch from the ocean: The water would suppress the sound of the engine as well as take away the need of a new launch complex and support structures. Such a large rocket would not be able to launch from Cape Canaveral, as Launch Complex 39 at the Kennedy Space Center would later be built to support Saturn V launches.
This concept of sea-launching was tested using two much smaller vehicles, Seabee and Sea Horse. Seabee and Sea Horse were not brand new rockets but modified existing surplus rockets. Seabee was an Aerobee rocket modified so that it could be fired underwater. Later tests were made to determine the reliability of reusing the rocket. The cost of turn-around ended up being only about 7% of the cost of a brand new unit. Sea Horse was a modified Corporal missile. Its engine was first fired above the water, then lowered and fired deeper and deeper every time. All tests of the engines and their corresponding rockets proved to be successful.
Sea Dragon would have been 490 feet (150 m) tall and 75 feet (23 m) in diameter. It would have weighed 40 million pounds (roughly 18 million kg) and would have been able to put up to 1.2 million pounds (544,000 kg) of payload into low-Earth orbit. For comparison, the Saturn V stood 363 feet (110 m) tall with a diameter of 33 feet (10 m) and could put 261,000 pounds (118,000 kg) into low Earth orbit. SpaceX's Starship has a height of 390 feet (119 m), a diameter of 29.5 feet (9 m), and a payload capacity of 330,000 pounds (approx. 150,000 kg) into LEO . Large counterweights were to be attached to the bottom of the first stage engine bell to stabilize the rocket vertically for launch. They also served as a cap protecting the engine. Most depictions of the rocket include an Apollo command/service module-like spacecraft at the top with a launch escape system. The payload was housed at the top of the second stage, which was just above the waterline when ready for launch. The "nose" of the first stage was pointed, fitting inside the second stage engine bell and actually reducing the height of the rocket. The first stage would have used a single 79 million pound (36 million kg) thrust engine expected to burn out after 81 seconds. The second stage also had a single engine that produced 13 million pounds (6 million kg) of thrust. It featured an expanding engine bell to improve performance. Both stages would have been reusable. Sea Dragon would be built at a seaside shipbuilder and then towed out to the water for launch.
Depiction of Sea Dragon being configured for launch (wikipedia)
The expected launch sequence was somewhat different from the launches people are familiar with: First, the rocket would be mated to its cargo and weight tanks on the shore. The propellant and nitrogen would be loaded into the fuel tanks. Then the rocket would be towed to its launch site, a predetermined designated spot in the water, where liquid oxygen and liquid hydrogen would be loaded as well. A nuclear-powered aircraft carrier would be used as a power supply. The ballast tanks would then be filled with water, orienting the rocket vertically.
There was some interest at NASA and Todd Shipyards but the project never made it past the conceptual stage. Todd Shipyards concluded that construction on such a vehicle was within their capabilities and their 8 millimeter thick maraging steel, a type of steel that possesses superior strength and toughness. NASA sent the designs to TRW, an aerospace corporation, for review and it fully confirmed Aerojet's expected costs and engineering for Sea Dragon. Aerojet was considering purchasing Sudden Ranch, a stretch of coastline between Santa Barbara and Vandenberg Air Force Base in California, as a launch site for their rocket. This was the only site on the continental United States that could launch directly into polar orbit without flying over populated areas.
Schematic of Sea Dragon (neverworld.net/traux)
Sea Dragon, despite its massive size and very impressive capabilities, was not needed by NASA nor the military during this time. The Saturn V was in development to send men to the moon and a vehicle as big as Sea Dragon was just not necessary. NASA dissolved their Future Projects Branch when the Apollo program began receiving budget cuts. Had NASA sent humans to Mars following Apollo, a super-heavy launch vehicle like Sea Dragon would have been justified. It is still to this day the largest rocket ever conceived.
The Apple TV+ series For All Mankind features a Sea Dragon launch on a resupply mission to a fictional US lunar colony.
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 This is in Starship's fully reusable configuration. Its fully expendable configuration has a payload to LEO capacity of 550,000 pounds (approx. 250,000 kg)
Grossman, David. “The Enormous Sea-Launched Rocket That Never Flew.” Popular Mechanics, Popular Mechanics, 15 Feb. 2018, www.popularmechanics.com/space/rockets/a25915/sea-dragon-history-curious-droid/.
Whittington, Mark R. “What Was the Sea Dragon Rocket, and What Would It Have Been Used for?” TheHill, The Hill, 6 Jan. 2020, thehill.com/opinion/technology/476898-what-was-the-sea-dragon-rocket-and-what-would-it-have-been-used-for.