Sea Dragon (rocket)

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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.

Saturn V to a similar scale. Its second stage would fit inside the first stage engine and nozzle of the Sea Dragon.

The Sea Dragon was a 1962 design study for a fully reusable two-stage sea-launched rocket. 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, nothing ever came of the design as NASA's Future Projects Branch was shut down in the mid-60s. At 150 m long and 23 m in diameter, Sea Dragon would have been the largest rocket ever built.

Truax's basic idea was to produce a low-cost heavy launcher. To lower the cost of operation, the rocket itself was launched from the ocean, requiring little in the way of support systems. A large ballast tank system attached to the bottom of the first-stage engine bell was used to "hoist" the rocket vertical for launch. In this orientation the cargo 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] and Sea Horse^[2] designs. To lower the cost of the rocket itself, he intended it to be build of inexpensive materials, specifically 8 mm steel sheeting. The rocket would be built at a sea-side shipbuilder and towed to sea for launch.

The first stage was to be powered by a single enormous 36 million kgf thrust engine burning RP-1 and liquid oxygen. The fuels were pushed into the engine by an external source of nitrogen gas, which provided a pressure of 32 atm for the RP-1 and 17 atm for the LOX, providing a total pressure in the engine of 20 atm (~300 psi) at takeoff. As the vehicle climbed the pressures dropped off, eventually burning out after 81 seconds. By this point the vehicle was 25 miles up and 20 miles downrange (40 km x 33 km), traveling at a speed of 4,000 mph (1.8 km/s). The normal mission profile expended the stage in a high-speed splashdown some 180 miles (290 km) downrange. Plans for stage recovery were studied as well.

The second stage was also equipped with a single very large engine, in this case a 6 million kgf thrust engine burning liquid hydrogen and LOX. Although also pressure-fed, in this case the nitrogen kept the system running at a constant lower pressure of 7 atm throughout the entire 260 second burn, at which point it was 230 km up and 940 km downrange. To improve performance, the engine featured an expanding engine bell, changing from 7:1 to 27:1 expansion as it climbed. The overall height of the rocket was shortened somewhat by making the "nose" of the first stage pointed, lying inside the second stage engine bell.

A typical launch sequence would start with the rocket being refurbished and mated to its cargo and ballast tanks on shore. The RP-1 and nitrogen 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, raising the rocket to vertical. 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 metric tons into low earth orbit. Payload costs were estimated to be between $59 to $600 per kg, which is much less than today's launch costs. TRW conducted a program review and validated the design and its expected costs, apparently a surprise to NASA. However, budget pressures led to the closing of the Future Projects Branch, ending work on the super-heavy launchers they had proposed for a manned mission to Mars.

[edit] Notes and references

^ 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 9,000 kgf 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.