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RE: starship-design: What is safest?
On Saturday, December 20, 1997 5:58 PM, Timothy van der Linden
> But what is their goal with setting such high standards? Clearly not to
> their equipment longer (which is our SD goal). More likely these higher
> standards are a byproduct of needing equipment that works in a wider
> of external variations. (Eg. Their computers should work in freezing
> conditions but also in a hot and humid jungle.) Such variations are
> not what most equipment of our starship has to deal with.
Yes, you are right that the missions of military systems and starships are
driven by different philosophies. However, they both end up producing the
same set of requirements for hardware. The military doesn't necessarily
expect its systems to function at the designed lifetimes on a day to day
basis. In fact, most systems are replaced or refurbished well short of the
point at which they would fail. This is because the military needs to be
certain that a certain percentage of its striking forces are ALWAYS
available at instant notice. Consequently, the systems designs promulgated
are tougher, more robust, and generally last longer than civilian designs.
You can easily cancel that flight to Albuquerque if the jumbo jet breaks
down. The military can cancel that battlefield reconnaissance flight
because of a bad spark plug.
Similarly, we, on a starship can only take a limited amount of supplies.
This is our "payload fraction". This payload fraction includes the crew,
our air, water and food; recycling machinery, living quarters, command and
control areas, exploration equipment, auxiliary vehicles, spare parts, and
repair facilities among other things. We cannot simply provide spares for
everything. Some of the parts likely to require maintenance are part of the
ship, not the payload fraction and as such, providing even one spare engine
(for instance) would equal the total payload fraction.
So we have to compromise. We build the major ship systems such as
structural components, engines, fuel tanks and delivery systems with high
levels of reserve capacity. I would think double the mission lifetime would
be adequate. Other systems which are important but harder or to expensive
to build so tough can engineered with some redundancy built in as in your
example of computers (BTW, I know what you meant, but it doesn't work that
way with parallel computers). Other systems such as light bulbs, etc. just
have to be replaced or done without. Properly designed, these systems would
have little if any effect on the eventual completion of the mission, their
failure would just make things uncomfortable.
> While cost is an issue, there may not be such a huge difference in
> the majority of equipment to last 40 or 80 years.
> Anyhow, we started this discussion because Kelly said it was impossible,
> which is quite different from expensive.
Well, maybe. If it becomes too expensive to design for the lifetimes we
require then it is effectively impossible to build a starship. No one is
going to pay for it...
> Hmmm, I thought he was trying to say that some systems can more easely be
> made to last longer.
He did, and some can. We have just recently made advances in lubrication
that will enable bearing lifetimes to more than double. So things like
motors and pumps and hard drives will last longer. Some things we still
have trouble understanding what caused them to fail in the first place,
much less how to prevent them from doing it again.
> Yes, several onboard manufacturing facilities are certainly needed (both
> one and two way missions, but one-way likely needs more). Furthermore
> parts of the ship should be constructed so that they can be
> with the manufacturing facilities onboard. The latter may mean that
> things have to be completely redesigned and that uniformity/compatibility
> a main key.
> Rather than to design for efficiency in large numbers we need to design
> efficiency in small numbers and limited equipment.
Which is an excellent point. This is the main reason I used the example of
naval ships several weeks ago. They pretty much do exactly what you just
described, albeit on a smaller scale then what we will have to achieve.
After all, the analogy breaks down when you consider that if all else
fails, a naval ship can call for a tug if it has to...
He who thro' vast immensity can pierce,
See worlds on worlds compose one universe,
Observe how system into system runs,
What other planets circle other suns,
What varied being peoples every star,
May tell why Heav'n has made us as we are.
- Alexander Pope