Re: we dont do other planets anymore

In sci.physics.relativity, heckuva
<jg85jt85h57@xxxxxxxxxxxxxx>
wrote
on 28 Jan 2006 14:45:54 -0800
<1138488354.592507.64370@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:
> whay arent we doin other planets any more
>
> whay not investing in the future of other fathers
> as they say, goen for instance to other planets
> and mars, with men in spacecrafts
>
> not just becus it is easy, but becus it is dificult, and
> we bulive that the men of earth shud comit themself
> makin comitments
>
> where are all that space travel they bullshited us in
> tha near past?
>

The best I can do is 32 or so years to visit a star 8
light-years away (say, Sirius), and that's with some
*very* optimistic assumptions.

Assume, for the nonce, that we do have an Ultimate Engine,
with v_e = 0.115 c, and using Newtonian physics. (At this
v_e the error isn't too bad.) We want to get to Sirius,
which is 8 light-years away, in 32 years. (We also want
to actually land there, which has some ramifications
fuel-wise.)

Tsiolkovsky to the rescue. Well, sort of; in any event
v_f = v_i + v_e * log(M_i/M_f). (There's an alternative
definition in Relativistic space, which I'd have to
look up, though someone did post it not too many days ago.)

..25 = 0 + .115 * log(M_i/M_f)
log(M_i/M_f) = .25 / .115 = 2.174
M_i = M_f * 8.793.

Bear in mind that this is acceleration. Deceleration requires
more fuel; in that case M_i = M_f * (8.793)^2 = 77.31 . Or,
put another way, the spacecraft is going to have to be 98.7% fuel.

The good news: at 0.25c relativistic effects will shave about
1 year off the journey. Also, the engine needn't be all that
powerful -- just have a very good v_e. One might even enjoy
normal gravity (9.805 N/kg) for about 3 months.

The bad news: there's no real good way for them to get back. :-)
Also, the destination system will get peppered with rocket parts,
if one assumes a multistage design with detachable fuel tanks.

And then there's the actual design of the Engine. I'm assuming
that it is possible to combine 4 protons using the equation:

4p => 2He4 + 25MeV

(a *very* questionable assumption, as Uncle Al pointed out
when I originally proposed this Engine) then propelling
the helium nucleus out the rear. Since an alpha particle
has 3727.37917 MeV I get a gamma correction factor of

(3727.37917 + 25) / 3727.37917 = 1.0067071

which corresponds to a v of .116 c.

This is arguably the best we can ever do; one can get
a higher v_e by ejecting protons or even electrons but
the helium becomes an issue.

With contemporary rockets v_e = 30000 m/s for an ion thruster
(a la Deep Space 1) or 10^-4 c; a VASIMR rocket might get
10^-3 c. Assuming M_i = M_f * 30 (a more reasonable assumption
given contemporary construction), and a VASIMR rocket, we get

v_f = 1/2 * 300000 * log(30) = 510180 m/s = 1.7018 * 10^-3 c

or a transit time of about 4700 years.

http://en.wikipedia.org/wiki/Specific_impulse

--
#191, ewill3@xxxxxxxxxxxxx
It's still legal to go .sigless.
.