# Re: starship-design: Hitching a Ride on a Magnetic Bubble

```STAR1SHIP@aol.com writes:
> "The solar wind's force per unit area decreases as the square of the distance
> from the Sun. Doubling the distance, for instance, decreases the solar wind
> pressure by a factor of four. "The solar wind is weaker far from the Sun, but
> the bubble is bigger, too (precisely because the solar wind pressure is
> lower)," he continued. "It so happens that the cross section of the bubble
> increases by the same factor that the solar wind pressure declines. The two
> effects completely cancel." It seems amazing, but the propulsive thrust of an
> M2P2-powered craft remains the same whether the spacecraft is near the Sun or
> in the outer reaches of the solar system.
> " end partial quote
> -----
>
> I would have to examine the math. On the surface It seems to violate
> the laws of conservation of momentum. P=mv, so the momentum of the
> push force must equal the momentum of the accelerating object. I do
> not see how the bubble getting bigger cancels the inverse square law
> that determines the push force in relation to the acclerating object
> bubble field size. In other words, I am not convinced the area
> exposed to the solar wind by increased bubble size is an equal size
> inverse square function as claimed. Bubble size may indeed have some
> linear effect, but not to the exponential extent claimed.

In theory, at least, if the effective area of the magnetic sail is
proportional to r^2 at a distance r from the Sun, then it would be true
that there would be no decrease in propulsion as it moved away.  That
would imply, however, that the magnetic field strength could be
increased without limit, which is clearly not possible.  It's probably
true that up to a certain distance from the Sun while the solar wind
compresses the magnetic field significantly, the weaking of the solar
wind would be counteracted by an increase in the effective cross-section
of the sail.  Eventually, at a great distance from the Sun, the magnetic
field would no longer be compressed by the solar wind and the sail
cross-section would remain at a constant size, so the thrust would fall
off with 1/r^2 as expected.

More simply, though, the magnetic sail could not propel the craft to a
speed greater than that of the solar wind, since once it's traveling at
the speed of the solar wind, there would no longer be any thrust.  That
would be a fairly impressive speed by modern standards, but still a
fairly small fraction of the speed of light.
```