Re: Because this went over so well on livejournal

On Dec 15, 5:39 pm, David Johnston <da...@xxxxxxxxx> wrote:
On Mon, 15 Dec 2008 16:59:07 GMT, fairwa...@xxxxxxxxx (Derek Lyons)
wrote:





David Johnston <da...@xxxxxxxxx> wrote:

On Mon, 15 Dec 2008 03:25:42 GMT, fairwa...@xxxxxxxxx (Derek Lyons)
wrote:

wjtingle <wjtin...@xxxxxxxxxxx> wrote:

Derek Lyons wrote:
David Johnston <da...@xxxxxxxxx> wrote:

Why fragile?
Because sturdiness adds mass, reducing delta-v.  

By that logic, every powered craft on Earth should be flimsy so as not
to introduce extra mass and consume extra fuel.

They are all built to the economic limit of flimsiness. Flxible produced
aluminum framed buses in the 90s(?). They had to refit them a lot, since
aluminum frames aren't sturdy enough for transit buses. There are other
considerations, but in space travel*, delta-v and payload mass fraction
are so dominant that almost all other considerations are far behind.

Airplanes were once like that.  So were automobiles.  So were
[seaborne] ships.  So were [horse powered] carriages...

The difference with airplanes and automobiles is that you reach a
point of diminishing returns past which further drive power makes them
uncontrollable or starts to rip apart the structure of the vehicle
unless you increase sturdiness.

Except that automobile and [seaborne] ships are subject to the same
limits.

I wasn't suggesting otherwise.



That isn't going to happen with anything designed for interplanetary
travel because the working accelerations are minute and you aren't
dealing with noticeable friction at any speed you can ever reach with
onboard delta-v.    

In other words - nothing will ever change when it comes to spaceships.

Space will always be a big environment which isn't at all dense, yes
and delta-v will always be more important than acceleration once you
have achieved escape velocity.  Things will change.  Not those things,
barring the invention of Star Trek propulsion.  

This will always be true for chem-fuled reaction drives. I haven'
triued to run numbers, but it might be a given for fusion-powered
recation drives. I'm not so sure about fustion powered drives where
the reation mass emerges as photons only, leading to no practical
limits on speed, and relatively few on delta-V, but definite limits on
acceleration. Matter/anti-matter reacion powered drives whre the
reaction mass emerges as photons are even less limited, because they
generate energery from their fuel supply far more efficiently.

If you have a non-reaction drive (and many written SF works postulate
one, although no one knows of even a theoretical basis for one AFAIK),
then the above changes completely.

-JM

.

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