SeaDragon - Water Launched RLV

Sea Dragon is a water launched HTHL TSTO RLV concept. The concept focuses on the first stage (booster) segment, based on low-cost, existing technology. Analysis in regards to very large Horizontal Takeoff Horizontal Landing (HTHL) RLVs has shown that systems with Gross Take Off Weights (GTOW) above 1.5 million pounds may face significant technological and operational challenges with respect to standard runway operations (large number of gear, jet engines, etc.). Many of these issues can be addressed if the vehicle is launched from a body of water instead of a land based runway.

Historical Background

In the early days of aviation, the largest aircraft developed were flying boats. This was due to the fact that most cities were located next to a body of water and there was a fundamental lack of runways. In addition, the landing gear technology was immature to handle the large gross weights of the aircraft, and by eliminating the landing gear the aircraft could carry more passengers. The ultimate culmination of the flying boat was the Spruce Goose - The Worlds Largest Aircraft ever built - constructed in 1947 by Howard Hughes. It had eight engines, a gross mass of 400,000 lb and a 11,430 sq-ft wing area (twice the size of a Boeing 747).

Today, flying boats are still used for a range of fire-fighting and search and rescue (SAR) missions. In addition, the Russian's built a number of amphibious jet powered aircraft that fly in ground effect to transport people and supplies across the Caspian Sea. The largest of these vehicles was the Caspian Sea Monster, which flew in 1964 and weighed 1.2Mlb.

System Characteristics

Horizontally launched RLVs have significant advantages in simplified operations, and a strong potential for increased reliability when compared to vertically launched systems. However, a conventional runway is vulnerable to damage from rocket exhaust, necessitating the use of air-breathing propulsion. Due to the maximum size of jet engines currently available, their thrust limitations, and the maximum Mach number at which they can operate, this forces the design to a subsonic first stage with a large number of engines, or a first stage with multiple propulsion systems. If the vehicle is instead launched on water, the following advantages are realized:

• The runway is "self-healing" and thus impervious to damage from rocket engine exhaust. Thus the first stage can take full advantage of the high thrust of rocket engines, in both subsonic and supersonic flight regimes.
• Because the first stage is rocket powered, it can be staged at a Mach number much closer to the ideal delta-v split, reducing overall system size while maintaining the same payload capability.
• Since each individual engine rocket engine is more powerful than any available air-breathing engine, the number of engines is greatly reduced, simplifying vehicle configuration and reducing the number of failure modes.
• Higher take-off speeds are achievable, reducing wing size and thus system dry-mass. For a given level of structural complexity, lower dry mass is generally equivalent to lower cost.
• Flexible Operations - The World is covered by 70% water and, since the vehicle takes off while "heading out to sea", there are very few overflight issues.
• The first stage can be fully rocket powered, with one and two-engine out capability, and additional abort scenarios designed for down-range emergency landings.
• The first stage can still be equipped with a reduced number of jet engines to partially lift the craft out of the water prior to take-off, for controlled flyback and landing after staging.

A water-launched RLV has all the advantages of horizontal take-off, without the limitations imposed by runway lengths, vulnerability to rocket exhausts, or landing gear loading. The concept of a large, rocket powered "flying boat" RLV is not without challenges (water corrosion, fuselage shape for hydrodynamics, etc.) but has strong indications of being the superior solution based on simplicity of technology and cost, when compared to a land based horizontal take-off RLV of equal payload capability.

Configuration

The configuration of the SeaDragon booster was originaly derived from the aerodynamic shape of the X-43 hypersonic demonstrator, and the under carriage of a hydroplane race boat. Jet engine inlets are placed on the forward edge of the top surface to minimize the likelihood of water spray being sucked into the engines. The jet exhaust points below underneath the center section to help pressurize the center cavity and raise the vehicle out of the water prior to take-off. The center cavity can be closed with a body flap located on the underside of the center section. The vehicle has two main sponsons which also contain the propellant tanks for all engine systems. The flight deck is located at the center of the forward section of the center body. A possible design is shown in the illustration.