# RE: starship-design: still some hope

```Ken,

> Sure, light pressure can't do it alone, but there's more than just light.
> There's also the solar wind and gravity.  Here's a way we could maybe use
> all three:

My point was that if we use every trick in the book to accelerate away from the Sun, and we are approaching another Sun with similar mass, luminosity, etc. then we will have to use all the same tricks to decelerate. All other things being equal, the maximum cruise velocity we can achieve using every trick ever thought of for solar sailing is only 0.003 c which is woefully inadequate. Timothy's point was that there wasn't sufficient time to decelerate from 0.42 c using a sail alone - even if we use every trick in the book, and he is right.

The problem here is a little complicated. The only area where we can get sufficient useful solar light pressure is too close to a yellow dwarf to be usable. We CAN get up to higher velocities by increasing the "lightness ratio" of the sail. This means using a larger sail area or decreasing the payload to thrust ratio or both, etc. Only, when we do this the acceleration force quickly leaves the bounds of what we can survive.

For example, the Icarus mission which was proposed several years ago used a powered perihelion maneuver, a one kilometer radius sail with a frontal density loading of 2 x10^-5 kg/m^2 and weighed only 63 kg. It would be able to reach 0.012 c but would experience a maximum acceleration of over 400 g's to get there.
We would be paste. Even at this august velocity it was expected to take 350 years to get to Alpha Centauri.

The "solar wind", as someone pointed out in my slip of the tongue, is basically useless. Gravity was already figured in, it is an integral part of the powered perihelion maneuver, and again, if we use it to get out of solar space we will have to employ it to decelerate also, or find something else just as good. Charging the sail during deceleration does make quite a difference, but it was allowed for (and used) in Tsu's colony ship proposal, which is where most of the math I used came from.

> I don't have time to look at numbers, and I'm not even sure where I'd
> start.  The first question to answer, though, would be: how close to a
> star can we get without frying the ship?  From that we can compute the plasma
> density, and I should be able to give some braking numbers.  (I'm supposed to
> be a plasma physicist, after all...)

The distance from the Sun (or any sun for that matter) to get full potential acceleration has been considered to be 1 to 1.5 solar radii. I don't know where they came up with this number, but most of the acceleration schemes I have seen required the use of an occulter in order to unfurl the sail at perihelion. Besides Tsu, Forward and others have devoted quite a bit of study to this particular concept, chiefly because it is the only one which is currently technically feasible, and pronounced it doable.

On a further note, my apologies to Timothy for so many short cuts in my math, but I really do hate ASCII math/art, do you have an equation editor? Can you embed files from it in e-mail? The sail area was figured in, it was the lightness number represented by Lf in at least one of the equations.

Lee Parker

(o o)
------------------------------------------------------oOO--(_)--OOo---------
"Imagination is more important than knowledge. For knowledge is limited,
whereas imagination embraces the entire world, stimulating progress, giving
birth to evolution."

-- Albert Einstein

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