Sea Dragon internal and external views. Both show the ballast tank attached to the first-stage engine bell. An Apollo CSM-like spacecraft is mounted on top.
The Sea Dragon was a 1962 conceptualized design study for a reusable two-stage sea-launched orbital super heavy-lift launch vehicle. The project was led by Robert Truax while working at Aerojet,one of a number of designs he created that were to be launched by floating the rocket in the ocean. Although there was some interest at both NASA and Todd Shipyards,the project was not implemented.
With dimensions of 150m (490ft) long and 23m (75ft) in diameter,Sea Dragon would have been the largest rocket ever built. As of 2026,Sea Dragon is generally regarded as the tallest launch vehicle ever formally proposed. It has mistakenly been given the title of the largest and most powerful vehicle seriously considered,with a payload of 550 tons. It falls into the category of other unbuilt concepts such as Convair's Nexus and Super Nexus,Boeing's Large Multipurpose Launch Vehicle,Phillip Bono's series of reusable single stage launch vehicles,and various NOVA Post-Saturn launch vehicles.
Design
Truax's basic idea was to produce a low-cost heavy launcher,a concept now called "big dumb booster." To lower the cost of operation,the rocket itself was launched from the ocean and constructed in a shipyard with more standard materials. A large ballast tank system attached to the bottom of the first-stage engine bell was used to orient the rocket vertical for launch. In this orientation the payload at the top of the second stage was just above the waterline,making it easy to access. Truax had already experimented with this basic system in the Sea Bee[1][NB 1] and Sea Horse.[2][NB 2] To lower the cost of the rocket,he intended it to be built of inexpensive materials,specifically 8mm (0.31in)steel sheeting. The rocket would be built at a sea-side shipbuilder and towed to sea for launch. It would use wide engineering margins with strong simple materials to further enhance reliability and reduce cost and complexity. The system would be at least partially reusable with passive reentry and recovery of rocket sections for refurbishment and relaunch.[3][4]
The first stage was to be powered by a single pressure-fed 36,300,000kgf (356MN;80,000,000lbf) thrust engine burning RP-1 and LOX (liquid oxygen). The tank pressure was 32atm (3,200kPa;470psi) for the RP-1 and 17atm (1,700kPa;250psi) for the LOX,providing a chamber pressure of 20atm (2,000kPa;290psi) at liftoff. The first stage would also be equipped with an asbestos-based recovery heatshield for reuse of the vehicle.[5] As the vehicle climbed the pressures dropped off,eventually burning out after 81 seconds. The vehicle would be 25 miles (40km) up and 20mi (32km) downrange,traveling at a speed of 4,000mph (6,400km/h;1.8km/s) before staging. The normal mission profile would see the first stage land in a high-speed splashdown some 180 miles (290km) downrange.
The noise of the first stage engine,which would have a SPI of 184dB[6] at liftoff,would have created a extremely challenging sonic and vibrational environment for a traditional land-based launchpad. This was a common point of issue for vehicles around the scale of Sea Dragon. Other solutions to this problem included the idea of the "water-filled acoustic limiter,"[7] a parabolic dish placed under a launchpad,which Phillip Bono proposed for use. This would have added significant cost to launchpad construction and design due to the size and amount of infrastructure required due to the materials required to construct it. Instead,the Truax design team intended reduce infrastructure construction costs by launching Sea Dragon from the ocean.
The second stage was also equipped with a single very large pressure fed hydrolox engine,in this case a 6,404,000kgf (62.80MN;14,120,000lbf) thrust engine,fed at a constant lower pressure of 7atm (710kPa;100psi) throughout the entire 260second burn,at which point it was 142mi (229km) up and 584mi (940km) downrange. To improve performance,the engine featured an expanding engine bell which covered most of the first stage tankage. during the events of staging,this expandable nozzle would go from a linear to a more conical shape,improving the second stage's expansion ratio.
A typical launch sequence would start with the rocket being refurbished and mated to its cargo and ballast tanks on shore. The RP-1 would also be loaded at this point. The rocket would then be towed to a launch site,where the LOX and LH2 would be generated on-site using electrolysis;Truax suggested using a nuclear-powered aircraft carrier as a power supply during this phase. The ballast tanks,which also served as a cap and protection for the first-stage engine bell,would then be filled with water,sinking the rocket to vertical with the second stage above the waterline. Last minute checks could then be carried out and the rocket launched.
The rocket would have been able to carry a payload of up to 550 tonnes (540 long tons;610 short tons) or 550,000kg (1,210,000lb) into LEO. This is enough to comfortably launch the ISS in a single launch (which weighs a "mere" 450 tons). Payload costs,in 1963,were estimated to be between $59 and $600 per kg (roughly $630 to $6,400 per kg in 2025 dollars[8],for comparison the Falcon 9 is estimated to cost ~$3,100 per kg in 2025[9].) TRW (Space Technology Laboratories,Inc.) conducted a program review and validated the design and its expected costs.[5] However,budget pressures led to the closing of the Future Projects Branch,ending work on the super-heavy launchers they had proposed for a crewed mission to Mars.
Principle of the Sea Dragon (Scale is incorrect)Composite of two NASA technical drawings,of the Saturn V rocket and the proposed Sea Dragon rocket,to the same scale. The second stage of Saturn V would fit inside the first-stage engine and nozzle of the Sea Dragon.A composite visualization of the Sea Dragon standing in front of the NASA Vehicle Assembly Building for scale.
Sea Dragon in fiction
The Sea Dragon appears in the first-season finale of the 2019 Apple TV+ series For All Mankind. The series is set in an alternate history timeline in which the 1960s-era space race did not end. In the post-credits scene,which takes place in 1983,a Sea Dragon is depicted launching from the Pacific Ocean to resupply the US lunar colony. An astronaut says in a voice-over that the ocean launch is being used as a safety measure because the payload includes plutonium.[10] The Sea Dragon continues to play a role in season 2;its high payload capacity is used to resupply an expansive lunar base and is the subject of a lunar blockade by the Soviet Union.[11] There are some small changes from the original concept compared to the version in the series,namely a lack of Launch Abort System for the Apollo capsule at the top of the rocket,and the lack of expanding second stage nozzle,instead using a large,more standard rocket engine,with four additional engines surrounding. To increase the visual impact,the series version begins almost entirely submerged at launch,with only the command module above the waterline visible before liftoff. Sea dragon in the show is also shown to have smoky second stage vernier engines,which in reality would've been a faint clean blue color.
Many depictions of sea dragon showcase it with either a non expanding second stage engine bell, or a segmented expanding engine bell, regardless, the expanding engine bell would be crucial for the remaining ascent for the relatively low power hydrolox upper stage. Some others, For All Mankind being the most mainstream example, showcase the four hydrolox vernier engines shutting down after the first stage clears the water line. In the real launch sequence, these would not turn off until orbit. While comparatively small, These four engines provide the only attitude control for the second stage; as well as roll control for the entire vehicle, as it's large center engines would not have been able to provide roll control.
↑Sea Bee was a proof of principle program to validate the sea-launch concept. A surplus Aerobee rocket was modified so that it could be fired underwater. The rocket worked properly the first time in restrained mode. Later tests of repeat firings proved so simple that the cost of turn-around was about 7% that of a new unit.
↑Sea Horse demonstrated sea-launch at a larger scale and on a rocket with a complex set of guidance and control systems. It used a surplus 9,000kgf (20,000lbf; 88,000N) pressure fed, acid/aniline Corporal missile on a barge in San Francisco Bay. This was first fired several metres above the water, then lowered and fired in successive steps until reaching a considerable depth. Firing from underwater posed no problems and there was substantial noise attenuation.
12"Study of Large Sea-Launch Space Vehicle," Contract NAS8-2599, Space Technology Laboratories, Inc./Aerojet General Corporation Report #8659-6058-RU-000, Vol. 1 – Design, January 1963
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