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Orbital airship

By Wikipedia,
the free encyclopedia,

http://en.wikipedia.org/wiki/Orbital_airship

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 to 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 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.

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 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.


An alternative might use Laser propulsion (Lightcraft) to power the early stages and the airships. A ground based laser or maser could be used to provide energy. Furthermore, a similar system could be used to heat the helium inside the first stage from the ground. This will naturally cause it to expand - the pressure can be released through nossels in the bottom of the craft providing additional lift. Heated helium would allow for lower internal pressures to be achieved while maintaining the overall volume of the gas and it's craft (a semi-rigid structure might help). (PV=nRT) This allows for the first stage of the craft to achieve higher altitudes - maintaining "lighter-than-air" equilibria at lower surrounding thin atmosphere densities.[ref-1]

The platform could be used to store fuel for the second stage which could resemble more conventional(rocket)(or hybrid) design. It is important to recognize the oxygen represents a large proportion of the mass of a conventional rocket. A large part of the cost of lifting a conventional rocket involves lifting this oxygen. A high altitude platform powered from the ground could be used to HARVEST oxygen from the surrounding low density air over time, condense and store it for the second stage. Oxygen is present at high altitudes - even if it is a very low density. [ref 1]

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.

Issues regarding Lighter than air craft in the mesosphere (or above) [1]

One limitation however is the weight of the material used to contain the airship's gas. For example, air density at 51 km in the mesosphere is estimated at 0.00086 kg per cubic meter. To be lighter than air at this altitude, the airship's total density - the weight of it's gas plus its walls divided by its total volume, must be less than 0.00086 kg per cubic meter. As an initial estimate (back of the envelope calculation), consider an airship made of 1/8 mm thick mylar (similar thickness of mylar used in early balloon satellites in the 1960s). Mylar's density is approximately 1.4 g/cubic centimeter. A cubic balloon - 2 kilometers on a side - would contain approx 4.2 million kg of mylar. It would be able to support (if fully inflated) an additional 2.68 million kg at 51 km. This 2.68 million kg would need to contain the weight of the gas in the airship, and any payload. As long as the weight of the gas in the balloon were less than approx 38% of that of the gas outside, this would support High_Altitude_Platforms as lighter than air at this altitude. (This should be readily achievable with hydrogen, helium &/or with heated gas inside the balloon, &/or with partially rigid supports). (A platform of mostly reflective material of this size could potentially have multiple uses).

It is important to recognize that a smaller platform - say 1 kilometer on a side - would NOT be lighter than air at this altitude - if it were made of mylar - even ignoring the weight of the gas contained in the balloon. This is essentially due to the square-cube rule - common in many engineering calculations. 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. (In theory one could create a lead balloon, or a concrete canoe, (or an ironclad ship) if it is of sufficient size) and have it float, although this may not always be practical.

To achieve significantly higher altitudes, one needs either very large volumes, &/or very strong materials that are less dense than mylar that are affordable in bulk. Nevertheless, a mesosphere based high altitude platform could offer many potential advantages. One could harvest oxygen and store it for further stages - which might resemble a more conventional launch vehicle. Conditions in the mesosphere are very different than those at lower altitudes in the stratosphere or higher in the Thermosphere. Such a platform might also serve as a relay point for maser or laser energy from the ground as per Lightcraft. It is unclear at what point (plasma-based / Magbeam) magnetic sails or solar sails could be effectively deployed to help subsequent stages achieve higher altitudes. However, the low air density in the mesosphere may start to make this feasible.

A mesosphere based high altitude platform could also increase it's altitude temporarily - in a non-lighter-than-air manner - using energy from ground based or solar sources.

Other potential issues

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

Nobody outside JP Aerospace seems to know how the problems of high drag and low lift/drag ratios that are very typically found at hypersonic speeds might be overcome in such a vehicle; and a large degree of skepticism exists.

Of course, escape velocity is typically based on a single initial velocity without thrust. which potentially modifies some of these issues.

Nevertheless, an orbital (thermosphere) (or even mesosphere) airship would face many practical and theoretical challenges and would represent a remarkable technical achievement.


[1] Holt, G. 2009. "Approaches to Improving High-altitude airships to assist first stage to orbit." Journal of premature and hypothetical ideas in science and engineering. Vol 1. Issue 1. ghol7955@gmp.usyd.edu.au

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  1. ^ What is an Orbital Airship?


See also

External links




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Published - July 2009














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