Using __repr__ or __str__ for own printable class?

Erik Max Francis max at
Tue Apr 15 06:36:27 CEST 2003

Andrew Dalke wrote:

> I hadn't heard of a bielliptic transfer before, but then again, nearly
> everything I know about this topic comes from reading Heinlein.  ;)
> (Okay, and reading magazines like Nature and various articles
> about space flight.)

I wouldn't worry about it too much, bielliptic transfers are an obscure
element of orbital dynamics which are very rarely used.

> Still, a gravitational assist can get things out faster and with less
> fuel
> than either of those two.  But they are non-trivial to compute, and
> Heinlein sure hadn't heard of them, given especially the constraint
> of having to work it all out on paper and sliderule.

Direct propulsive capability (which will be much more easily available
in the era when the game opens) will be a much better way to get from
place to place, either because you want to make a transfer off nominal
time (i.e., I want to take a Hohmann-like transfer from Earth to Mars
but I don't want to wait for the right opportunity), or just to use a
dead-run thrust from point A to point B.  When you have advanced fusion
or even antimatter drives, your exhaust speed is high enough that you
get some serious bang for your buck (momentum divided by reaction mass,
or thrust divided by fuel consumption rate).

Gravitational assists are certainly a possibility, but they sort of get
in the way of the approximations of the model (namely that the objects
in question are far too small to "accidentally" bump into unless you are
deliberately making a transfer away from or toward one of them). 
Furthermore, favorable arrangements for gravitational slingshots are
much rarer than Hohmann transfer opportunities (to make a Hohmann
transfer, you have to wait for the right alignment between your source
and destination points -- expressed as a relative central angle between
them).  So for the purposes of the model I've decided to ignore them. 
Another possibility, straddling the difference between letting gravity
doing the work and direct propulsive capability, would be to allow
solar/magnetic sails; these take a considerably long time, but
essentially require zero deltavee (you're letting the Sun's photon
flux/solar wind and gravity do the work for you).  Again, if you really
want to go from here to there quickly, it's a lousy solution, but if
you're just ferrying bulk materials as part of a trading network it
doesn't matter how long at takes as long as the "train" keeps going.

 Erik Max Francis / max at /
 __ San Jose, CA, USA / 37 20 N 121 53 W / &tSftDotIotE
/  \ They have rights who dare defend them.
\__/ Roger Baldwin
    EmPy /
 A templating system for Python.

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