starship-design: Lots O Stuff...

[wharton@physics.ucla.edu (Ken Wharton)]

Here are some responses to recent ideas...

Timothy writes:

>> Even if we're
>>assuming some pretty amazing technological advances, I find it hard to
>>imagine storing huge quantities of anti-matter on a ship. I don't know
>>if this has been discussed already, but I would guess there would be
>>some sort of theoretical minimum matter/antimatter ratio, just from
>>containment considerations.  Anyone want to tackle that one?

>A minimum ratio? Why would do you think that?

Also, Lee says...

>Theoretical efficiencies don't look promising...we won't even get into
>the problem of producing antimatter on an industrial scale...

Given you already have the antimatter, how much can you fit on a ship?
What IÕm thinking of here is a twofold problem: 1) we have to contain
the antimatter, and 2) we have to accelerate it with the rest of the
ship.  I assume itÕs a given that the fuel will be antihydrogen; (if you 
think fusion is tough, think of what antimatter fusion would be like!)  
Solid antihydrogen would be the obvious storage choice, and a spherical 
geometry would allow the minimum surface area with the maximum fuel.  
One problem, though, is that solid hydrogen isnÕt very dense; for a lot 
of antimatter weÕre talking about a huge container, with every point 
capable of generating fields to isolate the anti-H (Can solid hydrogen 
be repulsed by a superconducting field?) Then, when the ship 
accelerates, something has to accelerate the anti-H as well; and think 
of how much the anti-H will ÒweighÓ in 1-g acceleration.  WeÕll need 
equipment capable of making huge fields to transfer the force, as well 
as huge structural supports to handle the necessary forces on this 
equipment: all that will take matter. Adding this structural matter to 
the equivalent amount of regular hydrogen matter that we need for fuel, 
and you start to see why I was talking about a ÒminimumÓ 
matter/antimatter ratio.

On the flywheel subject, Timothy writes:

>I've thought of a flywheel before, but never mentioned it because it
>seemed too unpractical:

>- Its estimated weight was too high.
>- Containing/guiding a huge fly wheel that rotates with super high
>  velocities seemed almost infeasable.

>>So already, for a 200 meter radius flywheel,
>>we're talking about an energy storage of 10^10 J/Kg.  We'd probably
>>need at least 10^14 J/Kg to make a decent spaceship.

>Actually I think that 1E16 J/kg is the absolute minimum if you want to
>get into the direction of relativistic velocities.

I suppose a 20-kilometer flywheel is out of the question.  Anyone know 
how strong perfect diamond is, anyway? Or what about a two-stage engine; 
a flywheel to get the ship moving fast enough where a ramjet-type drive 
could take over.  I know you guys have talked about ramjets, but IÕm not 
sure what the current consensus is...

Lee also writes:

>Hmm, you mean like a rotating black hole? I think Robert Forward has
>already proposed this one. Check out a few of his books. Bear in mind 
>that now you have to accelerate the mass of the black hole also...and 
>anything spinning at relativistic speeds is going to have the same mass 
>moments as a black hole anyway...

Very Interesting.  But what is the mass moment of a black hole?  DoesnÕt 
the mass get sucked into a singularity at the center?  (Or is it also
spinning infinitely fast?)  But if we do have a black hole handy, I 
think it would be easier to use SteveÕs idea:

>On the other hand, if you can get the right evaporation rate and
>maintain it, you also get the advantage that the evaporation products
>will be statistically 1/2 matter and 1/2 antimatter (except for the
>photons).  So if it is possible to keep a quantum black hole just on
>the edge of evaporation at a rate useful for propulsion and power
>generation, you also get near-total matter-to-energy conversion by
>recombining the evaporation products.

But keep in mind, before the black-hole conversation gets too deep, that
small black holes will need an accepted theory of quantum gravity, which 
doesnÕt currently exist.  

The idea of using a catalyst to turn mass into energy leads straight
into monopoles.  Timothy writes:

>>       Idea #2:  Catalyze nucleon decay using captured magnetic
>>monopoles.

>The mean idea of using a monopole is probably it's heigh energy:weight
>ratio. Maybe we can find other ways to store energy.
>For example storing photons in a perfect mirror sphere.

If we donÕt need too many monopoles the energy:weight ratio wonÕt matter
too much; the main fuel source will still be nucleons.  I like the 
photon sphere idea, but my main concern is that for our purposes the 
photon pressure would probably drive the sphere unstable.  HavenÕt
looked at any numbers, though...

Also, Steve writes:

>Ken also claimed that the decay results would be "pure energy".  My
>initial reaction is that conservation of spin and charge would preclude
>complete conversion; not all of the products would be photons.

>Anyway, what we want is the best matter-to-momentum conversion, not the
>best matter-to-energy conversion.  Converting matter to photons does
>get you the most momentum for a given amount of matter, but it's
>probably not feasible to convert matter completely to photons.

I did look at the numbers for this one.  Turns out you get a proton 
decaying into a highly energetic positron (roughly 800MeV) and a slow
neutral pion (or several pions...)  The pions will decay into usable
energy (Photons, I think, adding up to the remaining 130MeV) , and the 
positron can either be captured or used as propellant.  Also, assuming 
we have 100% efficiency energy collectors and accelerators, doesnÕt the 
energy/momentum problem go away?  WeÕll simply convert our given amount 
of energy to momentum in the way that best accelerates the ship.  
But once you start talking about efficiency losses; LeeÕs right.  WeÕre 
better off using the end products as propellant.  Anyone want to figure 
out what a ship could do if we could convert protons into 800MeV 
positrons and use them as propellant directly?

Finally, Lee writes:

>>       Idea #2:  Catalyze nucleon decay using captured magnetic
>> monopoles.

>I think Forward beat you to this one also...

>The biggest problem with interstellar propulsion is finding a method 
>that is simultaneously capable of generating lots of thrust (ISP) and 
>not adding tremendous amounts of mass to the vehicle. The trick here is 
>to find a mechanism that like a solar sail, relies upon an external 
>source of power which we can tap directly or indirectly for propulsion. 
>This source needs to be large, inexhaustible, and NATURALLY produced, 
>i.e. we don't have to make it, correlate it, or concentrate it as in 
>beamed power.

What book is this in?  IÕve never read any Forward, but IÕd be 
interested to see it.

One last thought:  Although Lee is certainly correct about wanting to 
find an external source of power, there is another option:  Find an 
external source of Inertia.  If we can find something heavy to push off 
of itÕll be a whole lot easier to convert energy to ship-momentum.

Ken