Orbital airship

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The orbital airship is a proposed space transportation system that carries payloads to and from low Earth orbit. It is intended to achieve orbital altitude and speed using low thrust rocket propulsion and during its ascent to fly in the manner of an airship rather than a rocket, employing both buoyancy and aerodynamic lift rather than vertical thrust.[1]

JP Aerospace’s Airship To Orbit[edit]

In the Airship To Orbit (ATO) design envisioned by JP Aerospace, there are three components. A conventional airship (“Ascender”) lifts payloads up to 30 to 43 kilometers above the ground – roughly the maximum altitude a conventional airship can achieve. At this altitude the second component, a floating docking station (“Dark Sky Station”), acts as a resupply station for the third stage. The third stage is an orbital airship (“Orbital Ascender”), which takes payloads to low earth orbit (i.e., it accelerates itself horizontally to orbital velocity and gains an altitude in excess of 100 km) over several days.[2][3]

Multiple vehicles are needed because any airship made strong enough to survive the relatively turbulent lower atmosphere would be too heavy to lift payloads to space. An orbital airship would need to be built larger to improve its buoyancy-to-weight ratio, with thinner walls, and designed to operate at notably lower pressure. Even in the outer fringes of the atmosphere, helium is still lighter than air.

Both the conventional and orbital airships will be V-shaped for aerodynamics. The orbital airship wings will be shaped to function as hypersonic airfoils and can be angled upwards to help generate lift. As the airship gains altitude, drag will reduce, allowing the vehicle to accelerate with increasing altitude. According to JP Aerospace, there is a wide margin of drag-to-power ratios within which an orbital airship can attain orbit.[4]

Early development stages of the station and the airships will be powered by fuel cells. In the long term, the surface of these objects can be sprayed with a thin-film solar cell, which, while inefficient in energy conversion, would benefit from light weight, simplicity, and the large surface area. The final version of the orbital ascender can also employ refractory materials on the wing leading edge to reduce thermal wear. JP Aerospace’s US patent #7614586 identifies the orbital ascender’s propulsion system as chemical and/or electric rockets. John Powell’s Floating to Space cites several candidate propulsion systems. JP Aerospace is currently developing a hybrid chemical/electric rocket engine.

Their estimated marginal costs for cargo are one dollar per ton per mile of altitude, as quoted to Dr. Jerry Pournelle at the 2004 Space Access Conference[5]

Design issues[edit]

A practical orbital airship design must deal with multiple engineering challenges for both high altitude balloons and spacecraft. The altitude record for balloons is only 53 kilometres (33 miles).[6] To achieve significantly higher altitudes with a useful payload, very large volumes and/or very strong but lightweight materials are required.

For example, air density at 51 km in the mesosphere is estimated at 0.00086 kg per cubic meter according to the International Standard Atmosphere model. To be lighter than air at this altitude, the airship's total density must be less than this. By comparison, the ISAS BU60-1 scientific balloon, holder of the world altitude record for an unmanned balloon as of 2009, flew to 53.0 km. With an inflated mass of 39.77 kg and a maximum volume of 60,000 cubic meters, the total density of BU60-1 was 0.00066 kg per cubic meter.[7]

The material needed to contain a given space increases as the square of its dimensions, while the volume of the space increases as per the cube of its dimensions. Thus, the larger the airship the lower its theoretical minimum density. In theory one can create a lead zeppelin and have it fly if it is of sufficient size. In practice the craft must be robust enough to be re-usable.

For example JP Aerospace’s proposed first-stage Ascender airship would be the largest airship ever constructed to date, with an expected volume (57 million cubic feet) greater than seven times that of the Hindenburg. The size of this vehicle would pose unique problems for design, construction, maintenance, deployment and storage. But the Ascender would be dwarfed by the later stages, with volumes among the largest inflatable structures of any type ever constructed. These vehicles would hopefully operate in less structurally demanding environments, however size related problems of design, construction and maintenance remain.

An orbital ascender faces additional issues. Hypersonic gas dynamics will create high temperature flow across the envelope, and heat transfer must be controlled. The vehicle also faces some of the same harsh environmental conditions as a space elevator, such as elemental oxygen, radiation and space debris.[8]

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