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Orbital airship

From Academic Kids

The orbital airship, also called the space blimp, is a proposed space transportation system that carries payloads to and from low Earth orbit.

In the Airship To Orbit (ATO) design envisioned by JP Aerospace, there are three components. A conventional airship (Ascender) lifts payloads up to 30-40 kilometers above the ground - roughly the maximum altitude a conventional airship can achieve. At this altitude the second component, a 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 via an ion engine in three to nine days (i.e., it accelerates itself horizontally to orbital velocity and gains sufficient altitude). Their estimated marginal costs are one dollar per ton per mile of altitude, and their development costs thus far have been under one million dollars.

Two stages are needed because any airship made strong enough to survive the atmosphere would be too heavy to lift payloads to space. An orbital airship would need to be built larger to improve the volume/surface area 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. Consequently, airships can continue lifting payloads, but the same airship cannot make the entire trip.

Both the atmospheric and orbital airships will be V-shaped for aerodynamics. However, the orbital airship will be angled upwards to help generate lift. As the airship gains altitude, drag will reduce. According to JP Aerospace, there is a wide margin between the thrust that ion engines can provide the airship and the amount of drag the airship would experience in the outer fringes of the atmosphere. Early 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.

Several potential problems exist in the design, the largest of which is the threat of micrometeorites. As these will frequently impact the airship, it must have an effective self-healing mechanism, without gaining much weight. Still, additional helium will need to be continually added to the airship to help keep it buoyant. It also faces some of the other risks that face a space elevator, such as elemental oxygen and space debris.

JP Aerospace believes the problems can be solved, and has already begun tests of the Ascender. They hope to test a 30 meter wide prototype station at 9 kilometers altitude by the end of 2005, and have been funding their operations thus far with contracts for development of military communication and spy airships designed to hover over battlefields at altitudes too high for conventional anti-aircraft systems. They also point out that, unlike getting to space on a rocket, if something goes wrong on an airship, nothing bad will happen to you or your payload.

Skepticism

Several prominent posters on the sci.space Usenet groups, of which many knowledgeable amateur, and some professional, rocketry and space enthusiasts discuss such technologies, remain highly sceptical of JP Aerospace's claims. They believe that existing propulsion technologies for the airship are heavy enough, and the lift-to-drag ratios of lifting bodies at hypersonic speed poor enough, that without a fundamental breakthrough in one or both of these areas, there is no way the craft will be able to accelerate itself to orbital velocity and gain sufficient altitude. Their consensus is that either the company has made a mistake, or that their plans revealed to date omit some vital technical point.

JP Aerospace's design proposal was found to be technically flawed in an independent analysis by Robert Pickar. Specifically, the design assumes that electric propulsion, powered by solar cells, are sufficient to power the vehicle into orbit. In fact, the thrust generated by electric propulsion is insufficient to overcome atmospheric drag. Thus, the ATO craft would not accelerate beyond a low velocity. This is not a matter of technology, it is a matter of fundamental physics.

Electric propulsion is inherently power-limited, that is, the thrust generated by an electric rocket engine is limited by the power of the electric power source supplying it. Given the intensity of sunlight on solar cells (the solar constant), there is a finite amount of electric power that can be generated. For ATO, this would amount to some 10 MWe. Electric rocket engines can produce about 25 Newtons (6 lbs.) per MWe of power. Thus, a maximum of 60 lbs or so of thrust could be produced. Atmospheric drag on the ATO vehicle comes to about 11,000 Newtons (2,500 lbs). The thrust is far lower than atmospheric drag, and the vehicle will not accelerate.

Additionally, the ATO vehicle would have power cut off during the night. JP Aerospace claimed that this would be handled with regenerative fuel cells. However, regenerative fuel cell systems have a specific mass of 600 W/kg. Replacing the solar cell power with fuel cell power would result in a very large fuel cell. The system would be 100 times the vehicle mass itself.

These are all reasons why the ATO concept is unworkable from the physics standpoint in its present design. According to the skeptics' claims, a different kind of propulsion, not limited by the available sunlight, must be used, and it is difficult to make any of the current technologies light enough for an airship design.

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