Reply To: T-shirt designers unite and take over

[Jolly Roger]

Sailing on the Solar Wind

Tasmodevil44 wrote: Or for greater versatility of propulsion, you could use a combination of focus fusion techniques, instead of just one. I still don’t know exactly what advantages such a system might have right off – hand, but what if you employed both a focus fusion laser for solar sailing, as well as an additional onboard focus fusion rocket or ion engine?

For a spacecraft, mass is our enemy. We want our spacecraft to have as little mass as possible. We won’t take a heavy fusion engine onboard if lighter thrusters will do. We want to leave the more massive components back at the space dock. That is where the fusion-powered lasers will be. However, an onboard fusion engine would provide additional thrust for acceleration and for deceleration at the destination and the thrust for the return trip. Just keep in mind that the engine, fuel and propellant all have mass that has to be pushed too.

Every time your solar sailing craft wandered out of the path of the fusion – powered laser beam, onboard fusion thrusters could also propel it back into the laser beam for additional thrust.

A 5 MW fusion engine would be overkill if used strictly as maneuvering thrusters, but if you have it onboard for additional acceleration and deceleration, you might as well use it. Also, remember that the laser beam travels in a straight line (usually), but the light sail craft travels in a curved path around the Sun. The beam will have to be continually aimed where the spacecraft is supposed to be when the beam gets there. Light is fast, but it is not instantaneous. It takes more than a second for light to travel from the Moon to the Earth.

Also, in sailboat sailing, there’s such a thing called tacking, where you can actually propel a craft upwind and against the wind. To do this, the sailboat has to travel in a zig – zag fashion back and forth. In a similar way, onboard fusion engine thrusters might be able to propel a craft back and forth in such a way as to catch the solar wind easier for traveling upwind against it. This way, you would have two methods of propulsion working together and assisting each other.

Solar sailing is not quite the same as marine sailing. For one thing, solar sails are not propelled by the Solar Wind (magnetic sails are), but by the pressure of sunlight.

For another thing, an unpowered craft is not motionless; it is in an elliptical orbit around the Sun. Adding power either speeds it up, causing it to spiral outward away from the Sun, or slows it down, causing it to spiral inward toward the Sun.

This can be done with thrusters, but for a solar sail, it is much easier to just change the angle of the sail to the Sun; one way speeds it up, the other way slows it down. This is how “tacking” will be done with a solar sail. There will be no “zigzagging”.

A solar sail has some serious disadvantages. It must be large, as big as a square kilometer, and made of a very light material. That material will be punctured by micrometeorites, which will reduce its ability to reflect sunlight.

The power of sunlight drops off with the square of the distance from the Sun. @ 2 AU it is 1/4 of what it is @ 1 AU, @ 3 AU it is 1/9, @ 4 AU it is 1/16, etc.

I much prefer a magnetic sail (Winglee’s M2P2, not Zubrin’s wire loop) to a solar sail. It is a HUGE magnetic bubble (10-20 km) inflated with plasma and propelled by the charged particles of the Solar Wind. It is impervious to punctures; an asteroid could pass through it with no problem.

It expands as it gets farther from the Sun, so the power it derives from the Solar Wind does not diminish with distance.

It is, however, dependent on the density and velocity of the Solar Wind, which can vary from 300 to 800 kps. It also leaks plasma.

A magnetic sail can be used in conjunction with a Plasma Beam, just as a solar sail can be used with a laser. One difference though is that a plasma beam will bend to follow a magnetic sail that drifts off course. A plasma beam may also be able to re-inflate a leaky magnetic balloon.