starship-design: Re: ICAN

[TLG.van.der.Linden@tip.nl (Timothy van der Linden)]

Lee,

Sorry for sending the previous ICAN reply to the list twice. (More about
that at the bottom of this letter.)

>> About the fusion engines:
>> The main reason to keep the firing rate low was because waste heat would
>> otherwise melt the trottle/engine.
>
>Using the baseline ICAN paper - the material (lead!) is well within
>acceptable thermal load regimes for the firing times they require. I really
>don't see any reason that firing durations in excess of 2,000 hours would
>make much difference, but I expect that if we did go to such a design we
>wouldn't be depending PRIMARILY on the material anyway, I would expect some
>advancement in both magnetic and material shielding.

Well this is what they write:

  Heat Dissipation

   Approximately 2 GJ of residual thermal energy remains in the surface of
   the thrust shell within the engine after expansion of the propellant.
   Since this energy is initially concentrated in a hot, thin layer at the
   surface, radiation dissipation of the heat is expected to occur at power
   levels of tens of GW's. At a one hertz firing rate, this should provide
   sufficient cooling to maintain a safe operating temperature. Some active
   cooling of the structural steel and TiC shell components may be necessary,
   and could be used in a power cycle involving other vehicle requirements.
   Liquid droplet shields are under investigation for protection of these
   systems, as well as the ion beam components in the engine.

It indeed doesn't tell about firing durations, but I wasn't discussing that
in the above.
 
>> You suggested to use multiple engines, but that would increase complexity
>> and thus lower lifetime. (But I admit also add a certain redundancy which
>> would increase the time that a certain fraction of the engines keeps
>> working.)
>
>No, you are confusing complexity with quantity. Five screwdrivers are not
>more complex than one screwdriver...similarly, five fusion engines aren't
>more complex than one.

You're right, not more complex but more extensive, having five screwdrivers
means five times more parts which have to be maintained/repaired. (MTBF goes
down, but lifetime stays the same but gets more stable.)

(Unfortunately if one engine fails the others can't correct the acceleration
direction since the engines at one side of the "lever" will push more than
those on the other side of the lever and thus rotate the ship. My guess is
that if one engine fails it means shutting of a second as well.)

So yes, more engines don't decrease or increase lifetime, they only make the
engine system die gradually rather than suddenly but do increase maintenance
needs.

>> This makes my "fears" about complexity even worse.
>
>Gee, that should help alleviate your fears! We would have fifty plus years
>of experience working with the technology and should have thoroughly
>debugged it by then. As for any incremental advances, they shouldn't be as
>critical as the basic technology and would probably be based on solid
>experience as well.

Well if we'd stick to the original ICAN then we'd have debugged it
completely by that time. But since we can't use the original ICAN and have
to increase performance it means a lot of changes, which likely means more
additions.
I guess that performance will go up relatively fast, but that the lifetime
and failure rate do increase much less. (They won't go down, since that
isn't acceptable.)

>> Except of course that cruise velocities over 0.3c still means mass ratios
>> of more than 100 as long as one uses fusion.
>
>Well, nothing is perfect, we agreed not to consider quantum fluctuations :->

Maybe we should consider them, it seems the internet does already use them:

Yesterday evening at 21:42:39 I mailed my first reply from "tip.nl", some 22
minutes later I still hadn't received a copy from our list server at
"darkwing". So I decided to send it again. Well... here are the results:

-----------------------------------------------------------------

First try:

Received: from hengelo-009.std.pop.tip.nl by helium.tip.nl with smtp
	  Thu, 15 Jan 1998 21:42:39 +0100 (MET)
Received: from helium.tip.nl by darkwing.uoregon.edu
	  Thu, 15 Jan 1998 17:23:44 -0800 (PST) = 02:23:44 (MET) next day

Transfer time: +03:41:55 hours

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Second try:

Received: from hengelo-009.std.pop.tip.nl by helium.tip.nl
          Thu, 15 Jan 1998 22:05:11 +0100 (MET)
Received: from helium.tip.nl by darkwing.uoregon.edu
          Thu, 15 Jan 1998 13:03:59 -0800 (PST) = 22:03:59 (MET)

Transfer time: -00:01:12

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Quantum tunneling seems to be the effect that is happening here. There's a
chance that the electrons of my electronic mail arrive before they are sent.
Otherwise they'll be scattered across the internet and if lucky get to their
destination after all. ;-)

Timothy

P.S. The above is a quite twisted view of quantum mechanics.