[Daumic (OP)]

I already did.  In that message the propellant was the earth.

It is true that the electromagnetic sail takes support on the terrestrial magnetic field.  The movement of the sail generates an opposed force on the geomagnetic field and thus on the Earth itself. But I am not sure that the "propellant" term is adapted in this case.

[Daumic (OP)]

Electromagnetic braking instead of aerodynamic braking

The electromagnetic sail could have another use that a space launcher: it could be a space re-entry vehicle. When an electromagnetic sail follows a downward trajectory towards the Earth's atmosphere, the geomagnetic field induces in the superconductive ring an electric current. The dissipation of this electric energy in a resistive circuit converts the kinetic energy of the vehicle into heat easy to evacuate by a radiator. The electromagnetic sail acts like a brake and should thus allow an atmospheric re-entry at low speed. 

It seems to me that this electromagnetic braking is better than the aerodynamic braking used currently by the space vehicles. Aerodynamic braking uses friction on the atmosphere to slow down the vehicle. This operation is brutal, creates a strong deceleration and a very high heating of the heat shield. So it is a very dangerous phase for the vehicle and its passengers. 

Electromagnetic braking uses the geomagnetic field to slow down the vehicle. The geomagnetic field extends to several thousands kilometers beyond the Earth's atmosphere. Electromagnetic braking can thus start a long time before the contact with the atmosphere. This braking with an electromagnetic sail can be done with a weak deceleration and thus generates moderate constraints on the structure of the vehicle. The contact with the atmosphere can also be done at low speed and thus allow the economy of the heat shield.

[Daumic (OP)]

The electromagnetic sail adapted to the Glaser project

For a few years, the JAXA, the Japanese space agency, have studied a project of space solar power station inspired by the original idea of Peter Glaser (1). This project consists in placing in geostationary orbit a solar power station producing electricity, to convert this electric power into a beam microwave directed towards the Earth, and on the ground to reconvert energy microwave in electricity using an antenna.

The electromagnetic sail seems to me adapted well to the orbiting a space solar power station:

1 - Because elements could be common to both concepts: the antenna microwave on the ground could be used for launching the sail and later for the reception of the space beam microwave. If the superconductive part of the sail, made of MgB2, is inexpensive, it could be sacrificed to each launching. In this case, the reception antenna of the beam microwave of the vehicle would be used as transmitting antenna once in geostationary orbit.

2 - The electromagnetic sail could replace the first stage of a traditional launching by rocket. The structure of the costs of a launching with rocket shows that the first stage accounts for 75% of the total cost of the rocket (2). The replacement of this first stage by an electromagnetic sail could reduce the cost of launching.

3 - Launching by electromagnetic sail should be done at low speed contrary to launching by rocket. Cross the atmosphere at low speed could allow the launching of the entirely deployed solar power station whereas transport by rocket obliges to fold up it under an aerodynamic cap.

(1) [http://spectrum.ieee.org/green-tech/solar/how-japan-plans-to-build-an-orbital-solar-farm]

(2) [http://space.stackexchange.com/questions/8330/what-is-the-cost-breakdown-for-a-falcon-9-launch]

[Daumic (OP)]

Similarities with the concept of space elevator

It seems to me that the electromagnetic sail presents similarities with the concept of the space elevator. 

The concept of space elevator was developed independently in the Sixties by Russian and American researchers. It was especially popularized by the novel “The fountains of Paradise” of Arthur C. Clarke published in 1979. The space elevator consists in deploying a cable between a point on the ground on the terrestrial equator and a mass in altitude. The centrifugal force applied to the mass fixed at the end of the cable maintains it taut. For that it is necessary that the altitude of the mass is higher than geostationary altitude (36000 km). To tighten this very long cable requires a material with an exceptional mechanical strength. Such a material is not discovered yet. Research continues because the space elevator would have two considerable advantages compared to the techniques of launching per rocket:
-   it allows a launching at low power whereas it is the extraordinary power of the rockets, particularly on the takeoff, which makes them so dangerous,
-   in the event of breakdown of the upward system, the payload remains hung to the cable whereas a breakdown on a rocket involves a catastrophic fall.

I summarize the launching per electromagnetic sail presented in the discussion:
-   the launching vehicle is equipped with an electromagnetic sail, a microwave antenna, a small rocket engine of orbital operation and payload,
-   a microwave antenna on the ground supplies energy to the vehicle,
-   the trajectory of the vehicle is vertical; the initial slow speed of the vehicle is maintained all along the flight, by this way the power required to maintain the current in the superconductive ring remains moderate,
-   with altitude, the terrestrial magnetic field decreases more quickly than gravitational attraction; to maintain the same force the electric current in the superconductive ring of the sail must increase to compensate the reduction of the geomagnetic field but does not have to reach the critical intensity,
-   the electromagnetic sail is stopped when an altitude of about 7000 km is reached; the potential energy of the vehicle at this altitude is equivalent to the kinetic energy of an orbiting at low altitude, so the complete orbiting of the payload requires nothing any more but one small orbital engine,
-   the electromagnetic launcher composed of the sail and the antenna separates from the payload and its rocket engine,
-   the electromagnetic launcher returns on Earth at low speed according to the same vertical trajectory; this descent is done without the assistance of the microwave beam because the downward movement in the geomagnetic field generates an electric tension in the superconductive ring; the dissipation of this electric power in a resistive circuit makes it possible to convert the kinetic energy of the vehicle into heat easy to evacuate by a radiator; this descent can be done at low speed thanks to this electromagnetic braking, 
-   the rocket motor of orbital operation is ignited to adjust the orbit of the payload. 

The similarities of the electromagnetic sail with the space elevator are as follows:
-   the electromagnetic sail allows also a launching at low power,
-   in the event of stop of the propulsion and with the condition that the superconductive state of the magnetic shielding and conducting ring is maintained, the fall of the vehicle in the geomagnetic field generates an electrical current in the conducting ring, the Laplace force applied to this current is opposed to the fall of the vehicle: the electromagnetic sail remains "hung" to its support, the geomagnetic field.

[Daumic (OP)]

Magnetic Cavorite

Do you remember the novel of H. G Wells “The first men in the Moon”? In this novel, Wells imagines a material which shields gravity, the Cavorite. 

This material cannot exist because it would create a machine with perpetual motion. Imagine a wheel with horizontal axis; a part of this wheel is protected by a cover made of Cavorite. Terrestrial gravity would act only on the part of the wheel out of the cover and would thus make it turn indefinitely. This perpetual motion is not possible and prohibits the gravitational Cavorite.

A superconductor, by its magnetic susceptibility strictly equal to -1, constitutes a perfect shield to the magnetic fields. Thus a superconductive cover can receive the name of magnetic Cavorite.

The object of this topic, an electromagnetic sail, is a space launcher using magnetic Cavorite. If the electromagnetic sail is possible, the dream of Wells becomes perhaps real.