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starship-design: Re: Solar sail breaking

At 8:49 PM 10/20/96, Nick Tosh wrote:
>Hello everybody.
>You may not remember me - I haven't contributed anything for nearly a year
>now. I had computer problems that kept me off the net for over six months,
>and for various reasons I've been very busy since. Anyway, I'd like to
>'rejoin' LIT. I'm a student in England, and I'll be going to study physics
>at university next year. You already have me on your mailing list, so I'm
>pretty much up to date.

Hi Nick, welcome back.  Are you on the


List?  Its become our central list rather then addressing each other

>>From what I've gathered from reading past mailings, stopping a starship
>powered by a Sol based particle/EM rad beam is a major problem. The (fairly
>obvious) idea that occurred to me was that a solar sail would be much more
>attractive as a braking device when approaching a target star at high
>relativistic speed than as an accelerating device for leaving Sol. The
>doppler shifts that are such a pain in the latter case, or for the Sol
>based microwave beam, become a significant help if the sail is moving
>towards the energy source (i.e. Tau Ceti, or even better, the multi-star
>-triple?- Centauri system). If the starship is moving towards the target
>system at velocity v, expressed as a fraction of c, then the the impulse
>imparted per reflected incident photon of frequency Fo measured in the
>star's frame is:
>                  _____
>2(h Fo)    /  1 + v
>----------   /  -------- (by the way, do any of you use Mathcad? I hate
>typing out maths like this and would much rather e-mail Mathcad files).
>    c        \/    1 - v

Sorry.  Macs come with a free (oldish) version of Mathmatica.  Would the
files be compatible?

>(The photons is reflected, so its change in momentum is twice the magnitude
>of its initial Doppler shifted momentum. I think that the reflected photon
>is of the same -shifted- frequency of the incident photon, since otherwise
>the solar sail would have to absorb more energy than it radiated, and this
>surely cannot be a theoretical requirement; in any case, how would the sail
>know what the 'proper' non-shifted frequency was meant to be?) The above
>expression obviously shows that the impulse imparted to the ship by each
>photon approaches infinity as the velocity of the ship approaches 1c.
>Unfortunately, the mass of the ship also approaches infinity, so the
>decellerating effect remains finite. However, the mass increase factor is 1
>/ sqrt(1-v^2), while the impulse increase factor is sqrt[(1+v)/(1-v)]. If
>we divide the impulse factor by the mass factor we get (1+v). So the
>impulse per photon increases faster than the mass does - the decelleration
>(in ship's frame) provided by the sail increases with speed. The limiting
>case, v approaching 1, results in double the 'rest' decelleration. Since
>for most other forms of propulsion relativistic effects make decelleration
>more difficult at higher speeds, might not a sail be a useful additional
>'brake' for the starship? (I wouldn't even suggest it as the only one). The
>main disadvantage of this system would be that to reap the benefit of the
>sails high velocity performance, the starship would have to still be moving
>very fast while near enough to the target star to get a fairly intense
>photon flux. Chances are it couldn't slow down in time. Some compromise
>would be necessary. Could the ship purposely overshoot the star, so as to
>get some big delta-V from the sail while VERY close to the star? As it
>passed by, it could furl the sail to avoid getting accelerated away from
>the star (I think I like that idea. Has it been considered before? Making
>such a long journey even a little longer is an odd idea, but it could be
>useful. I'll do some maths on it later).

The stellar radiation would be too breif and weak to help us much.

>I need some help on the following point. Does time dilation result in the
>high velocity starship intercepting more photons per unit time than an
>observer in orbit around the star sees the star emit in the ship's
>direction? Does relativity really increase both the impulse gained from
>each photon AND the photon flux (in the ship's frame)? I don't know enough
>to answer this, and I'm too tired to think about it now. If the aswer to
>this is yes, than I think some serious modelling of the sail brake should
>be done immediately. Please could someone answer this for me as soon as

Don't look at me.  I'm the fusion guy.  ;)

>Could people tell me if this mail gets through?


Kelly Starks                    Phone: (219) 429-7066    Fax: (219) 429-6859
Sr. Systems Engineer                                     Mail Stop: 10-39
Hughes defense Communications
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Email:  kgstar@most.fw.hac.com