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

```In a message dated 10/7/00 10:54:14 PM Pacific Daylight Time, stevev@efn.org
writes:

> STAR1SHIP@aol.com writes:
>   > Partial quote from link
>   > "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.

Hi Steve,
Better stating my point, Any increase in "sail size" follows physics laws.
double the height of a mass object and it's mass increases 4 fold. Any energy
to provide an increased size "magnetic" sail, even though the mass of the
magnetic component is near zero, reguires internal energy from the craft
produced from mass parts that will need to be four times the mass. The gained
momentum (P) is used up as P=MassTimesVelocity. The claimed velocity is
unreachable for the mass inverse square law.

> That would imply, however, that the magnetic field strength could be
>  increased without limit, which is clearly not possible.

True, The law stated causes objects on earth to fall when height exceeds mass
which the foundation can support. There is indeed a practical limit far below
the theoretical limit.

>  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.

The speed of wind is an average velocity made up of light, gravity waves,
heat, electro and magnetic components and also mass parts. A well designed
"light"sail in theory could indeed reach light speed, but practically will
not near c, for the light and electromagnetic energy captured by the sail far
from the effects of the gravity and mass parts is so small in relation to the
mass of the rocket sail, I see little hope of leaving the solar system before
the solar system gravity fields slows and drags the sail back in an
elliptical comet like orbit orbit around the sun.

Regards,
Tom
>
```