Re: Japans Moon Ambitions - They're KIDDING, Right ?

From: Will (mclean1382_at_aol.com)
Date: 03/09/05 

Date: 8 Mar 2005 19:50:15 -0800

Rand Simberg wrote:
> On 6 Mar 2005 17:54:52 -0800, in a place far, far away, "Will McLean"
> <mclean1382@aol.com> made the phosphor on my monitor glow in such a
> way as to indicate that:
>
> >> >A reusable vehicle helps with the first category, but not
> >necessarily
> >> >with the second or third.
> >>
> >> A flawed design, at least one that leads to extreme unreliability,
> >> will generally show up any well-panned flight test program.
> >>
> >
> >These are all relative terms. Is a 1% or 2% failure rate extremely
> >unreliable for an orbital vehicle? Not by current standards.
>
> That is an indictment of current standards more than anything else.
>
> > Is 100
> >test flights without payloads a well planned (or panned) test
program?
>
> By my standards, it would be a reasonable test program.
>
> >Not by current standards. If you hope to ultimately achieve better
than
> >98% reliability, you will achieve 90% reliabilty many flights before
> >that. At that stage, flying empty is leaving money on the table.
>
> Only because the current market (which won't support a highly
> reliable, low-cost system) accepts it.
>
> >> >The last two add failure modes that aren't
> >> >present with an expendable vehicle.
> >>
> >> No, but there are ways to mitigate component failure, by designing
> >the
> >> system for regular inspections.
> >
> >Yes. But it is an additional failure mode that reusable vehicles
must
> >address.
>
> Why do you think that they wouldn't?
>

Do you think that regular inspections will reduce failures from wear
and fatigue to a negligable level? That hasn't worked for manufacturing
errors.

> >And the last one isn't a different
> >> category of failure--it's just part of the overall mission profile
> >> that has to be designed for and tested.
> >>
> >
> >It is a different category in that it is only relevant to reusable
> >vehicles. It's only part of the mission profile if you recover the
> >vehicle.
>
> I guess I'm failing to see the relevance, if you're saying that this
> will somehow result in a lower reliability than low-rate expendables,
> given a sensible flight test program.
>

What I am is disputing is how much of a reliability improvement you get
from reusability alone. If you specify that the expendable must fly
fewer flights, then *that* would effect reliability. But it doesn't
necessarily follow that the expendable will fly fewer flights, as a
comparison of STS and Soyuz shows.

Consider a hypothetical mature expendable with more than 100 flights,
like Delta II, with a failure rate of ca 2%. Half of that is
manufacturing error not caught in ground tests. The other half is other
issues, such as processing errors like ice in the propellant lines,
tape not removed on stage separation switch, error in loading flight
software, or wrong commands from ground control. Also design flaws that
haven't shown up in the first 100 flights, weather, lightning, etc.

Even if you find all manufacturing errors in test flights, and
replacement parts never have defects, you still have a failure rate of
1% on ascent. The reusable has to come back intact, so unless you
achieve return reliability of better than 99% you haven't improved
reliability.

> >> > Also, by needing systems that an
> >> >expendable vehicle doesn't, a reusable adds potential
opportunities
> >for
> >> >design and manufacturing failure. If you don't have a wing, you
> >don't
> >> >need to worry about foam striking the wing. If you don't have
> >landing
> >> >gear, you don't have to worry about someone not servicing the
> >landing
> >> >gear correctly.
> >>
> >> Yes, I guess that's why the airline industry has been working so
hard
> >> to eliminate wings and landing gear in the name of reliability...
> >
> >The airline industry isn't working very hard on anything that isn't
> >winged and subsonic, using only airbreathing engines and a single
> >stage.
> >
> >The optimal solution for the airline industry has limited relevance
to
> >reaching space, and even less to reaching orbit.
>
> I think that you're deeply mistaken. It has the utmost relevance in
> doing so affordably and reliably. Sadly, the industry has ignored
it,
> because the market hasn't demanded it.

Some of it does, some doesn't. The tricky part is knowing which is
which. Surely you aren't suggesting that since air-breathing engines
are optimal for airlines, they are the best way to get to orbit?

Will McLean

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