Re: starship-design: We need to get on the same (pellet) track first

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

Timothy van der Linden wrote:
>Hello Isaac,
>>>That is only so if you keep the exhaust velocity constant.
>>>You can also keep the mass ratio constant and increase the exhaust velocity.

>>No you can't.  Like it or not, there's a top exhaust velocity available
>>just barely acceptable if we're using fusion power.

>Then we agree. All that I should have added is "for low cruise velocities
>with a fusion design":

Okay, assuming you're talking about less than .1c, which is only barely
acceptable for a manned interstellar mission in the time frame this
mailing list is for (a couple hundred years might not be enough to
develop long human lifespans).

Above .1c, you have to assume very high exhaust velocity low loss
fusion drives for them to be practical at all.

>For low cruise velocities one needs just as much energy when using the same
>amount of mass for a ramjet design as for a self-fueled-fusion design.

Untrue, because at such low cruise velocities it's doubtful a fusion
ramjet design will operate at all (without shoving the pellets at it
at some high velocity, costing extra energy).  However, a chemical
pellet ramjet design could work.  The energy required is no longer a
useful method of comparison because the costs of that energy depends
upon the energy source.

Also, at these intrasolar speeds, the costs of the fusion drive will
likely outweigh its entire lifetime's cost in fuel.

[about the rocket equation being used with various exhaust/destination
velocities]
>>>I should note that anything above 0.3c doesn't apply to pure fusion designs,
>>>but only for designs that use a more energy rich fuel.

>>Which means antimatter.  (Or something even more bizzare, like a black
>>hole.)

>Or a partial beamed design...

Not exactly.

Above .3c, you're definitely using an exhaust velocity of .2c or
more.  At such high exhaust velocities, any beam providing power
for that beam will have a large fraction of momentum compared
to the exhaust it powers (even assuming on board fusion providing
a theoretical maximum boost).

That means the rocket equation can't be used with such an externally
powered rocket.  In the extreme case, external power is provided
by laser to a .99+ %c exhaust.  In this case, the momentum of the
laser light is the same as the momentum provided by the exhaust,
assuming 100% efficiency!

>>You simply can't ignore the realities of particular methods of
>>propulsion.  For instance, you can't ignore the fact that without
>>anti-matter power, maximum exhaust velocity is _the_ limiting
>>factor in traditional interstellar rocket designs.

>I wasn't discussing the realities of a particular method, but of a
>self-fueled design in general. Now that we cleared things up, I guess we can
>indeed go to the particular cases.

Unfortunately, the only criteria you discuss, energy consumption,
isn't always the driving factor.  In fact, it rarely is directly
significant, because the cost per energy unit varies so greatly
from one source to another (cost and availability are the true
important factors).

If the only important factor were energy consumption during the
starship's flight, then we could just say the ideal interstellar
rocket is an antimatter rocket with a propellant/payload mass
ratio of 4 with a matter/antimatter ratio appropriate to give
an exhaust velocity of 1.2 times the cruising velocity.

After all, we can make atoms of antimatter today.  So what if it's
astronomically expensive to produce per calorie?

>>For instance, it's really pointless in discussing the potential
>>energy efficiency of launching something solid at relativistic
>>velocities via an electromagnetic mass driver because you'd
>>never be able to build one long enough.

>True, but before you can discard a method, you have to determine what the
>limits are.

If you'd like, I can rehash my calculations on the potential muzzle
velocities and other limitations of mass drivers.  The overall
conclusions are actually pertinent to this discussion, since EM mass
drivers would be used to fire pellets from the fuel packets in my
stardrive proposal.

>I've found that there are few numbers available about the
>designs we are talking about. To avoid needless calculation I approach the
>designs in a general way. Then after having looked at the results, I'll
>discard a particular case.
>I guess I was put a little bit off balance by your cooked-&-ready approach,
>just as you probably were by my step-by-step approach.

Yes, and me disagreeing with the basic assumptions in your steps
isn't helping either.  In particular, the basic assumption that
you can just look at any exhaust velocity you feel like is one
I just can't agree with.

When it comes to exhaust velocity, the harshest fact of rocket design
is that the limiting factor is _availability_.  You can't just wish
for a 50,000sec Isp rocket and get it.  You have to live with whatever
maximum Isp is available (after R&D).
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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