starship-design: Re: Suggestion for starship fuel cycle (fusion)

[KellySt@aol.com]

Dear Michael,
Thanks for your interest in the site, and your comments.  I'm afraid I don't
know what you mean by a "proton-catalyzed carbon cycle reaction" or any type
of "regenerative fusion cycle".  All fusion cycles we could find consume fuel.
Some like the Lithium-6 cycle can use the reactants from the first cycle to
power the second cycle, but you can't just recycle the same fuel mass over and
over.

Also its important to note that we chose anti-nutronic fuel cycles that
release their energy in the kinetic energy of the fusion products. No
radiation.  Virtually no heat.  So they make highly effective rocket fuels.

However the basic limitations of the power a given mass of fusion fuel can
generate, becomes a staggering limitation.  As you noted, the fuel loads
required would be massive!  Grossly dwarfing the starship.

As to you other main points:

The Explorer-class vehicles Bussard ramjet idea was abandoned after the
diagrams and web site were worked up.  It looks pretty unlikely that a
ramscoop could even scoop up its own weight in interstellar material, assuming
you could even make it work at all.  So the Explorer concept now uses large
fuel canisters (tankers?) boosted up to it to refuel it as it boosts out of
the star system.  Effectively it boosts out using fuel feed to it in a just-
in-time system, and decels using internal fuel reserves.

We toyed with the idea of using a scoop like system to brake the ship, but
were skeptical that it would save us any fuel.

Explorers hardware, living quarters designs, and such, are the nominal
reference design for the other concepts.



As to the M.A.R.S.-class
It relies on beamed power from our Solar system, but we could never figure a
solid idea on how we could use that to decelerate, nor were we optimistic at
getting it to the ship over interstellar distances.  To put it mildly this
would be a staggering challenge.  Thou your inverse square issue can be
avoided by a maser beam, it would still defuse and the aim would degrade at
such distances.

The financial and political investment in Sun-orbital photoelectric collector
arrays would be a problem.  MARS and Fuel/Sail would both need vast emitter
systems, with astronomical costs.  Unless their was some major advance in
automated, low cost manufacturing of such platforms, these systems would be
undoable.  Explorer has a similar problem with its fuel launcher system.


Your security issue for the emitter array is also a common problem.  Thou
Fuel/Sail and Explorer only need the transmitters for a few months at boost,
they also depend on them being their to slow them down on return to Earth.  If
for example that can't ABSOLUTELY prove they pose no contamination threat, the
array will likely not be fired up to break them on their return.

Since Alpha Centauri isn't in the ecliptic plane, Sols planets wouldn't cross
the beam's path.

We assumed their would be few if any mid flight maneuvers, so tearing the sail
didn't seem an issue.

I'm afraid your mistaken, the sail can not be decelerated by tacking into the
beam.  This only works in our star system due to orbital mechanics.  Useless
on a fast interstellar mission.

Sail erosion at high velocities would be a major issue.  Fuel/Sail could furl
its Sail after a couple month, but MARS would take it hard!


Like all our designs, MARS is unaffordable with out major technical
improvements in manufacturing.  A society that could attempt the degree of
space based construction needed for the emitter arrays, would have taped the
huge resources of space, so raw materials wouldn't be an issue, and power
would have to be common through out the developed world or its space based
colonies.  But that's a very big if!

Not being able to change course isn't much of a problem though.  If you look
out in the night ski,  you'll notice there is little if anything near any
given course to or from any star.



Fuel/Sail seems the most promising at the moment.  Offering fairly high speed
(.4c) and less extreme technical challenges, but it still is unaffordable.


Thanks for all the time you put into your response.  I'll forward it to the
group.  If you'd like, you might join the group if you wish to make frequent
comments.  Thou I'm afraid we haven't had a lot of activity the last year or
two.  We frankly ran out of new ideas, and developed the old ones about as far
as we could.  Without a major break through in technology or physics, these
designs are undoable.

Thanks again.


Sincerely,

Kelly Starks


>Dear Kelly,
> 
>   Have you considered using a regenerative fusion cycle, such as a
> proton-catalyzed carbon cycle reaction, which uses ordinary boron as a
> "seed" fuel? I mention this to you because, after looking over your
> design summaries, I have some misgivings about each of your propulsion
> approaches.
>   To begin with the Explorer-class vehicle, which uses a lithium-6 fuel
> cycle:
> 
>    * Though Li-6 is relatively plentiful and inexpensive, the cycle is
>      non-regenerative (once the fuel is "burned", it is not recycled
>      into "new" fuel); therefore, any practical starship would be a
>      massive construction, nearly all of which is fuel tankage/storage.
>    * The engine system (essentially a 1960-vintage Bussard ramjet) has
>      several enormous engineering hurdles to overcome, not least in the
>      area of field generation and control. Even if these challenges
>      could be met, in the end you are left with a tremendously huge and
>      complex vehicle which self-destructs whenever it is throttled up to
>      full power. (A slight modification to the system might, however,
>      serve as a usable braking system for a starship)
>    * Assuming one could get the engine up to full power without
>      destroying itself and/or the collection field, the efficiency of
>      the system is limited by the hydrogen density in the area(s) of
>      operation. Thus, it would be a much more practical propulsion
>      system if used near a galactic core than in the environs out here
>      in the Orion Arm.
> 
> The next type of mission vehicle described is the M.A.R.S.-class
> starship, which relies on beamed power from an outside source
> (presumably located within the Solar system):
> 
>    * The energy requirements for the microwave emitter are, indeed, very
>      large, but are not, in and of themselves impossible given a large
>      enough financial and political investment in Sun-orbital
>      photoelectric collector arrays.The chief problem in this respect is
>      in the received power fall-off at the starship as it moves farther
>      into space (and away from the emitter array).due to the inverse
>      square law regarding signal propagation. Though this can, in
>      principle, be compensated for by continuously increasing the
>      emitter's ouput, at some point in the process a limit would be
>      reached wherein the emitter is transmitting at full capacity and
>      further increases in power output are beyond the physical
>      capabilities of the device. Additionally, this system demands that
>      the entire emitter infrastructure be in place and operational prior
>      to the first mission launch.
>    * The emitter system requires near-absolute reliability and security
>      during operation; security is required not only to protect the
>      infrastructure from deliberate sabotage, but also from damage
>      caused by debris, comets, meteoroids, etc. which also inhabit the
>      space around our star. Needless to say, security in this case also
>      depends, perhaps critically, on guaranteed (or at least assured)
>      funding throughout the mission cycle, which is an iffy thing at
>      best, especially during the extrasolar stages of the mission when
>      it will be difficult to gather public enthusiasm for the mission.
>      Another aspect of security, which can be partly planned for, is the
>      security of the beam itself.: It would probably have to be directed
>      out of the ecliptic plane to prevent periodic interruptions from
>      planets, etc. which would cross the beam's path; similarly, an
>      exclusion zone of some sort would be necessary to prevent other
>      spacecraft from wandering into the path of the beam (if that
>      happened, the effect on the beam would probably be negligible at
>      worst, but from the vehicle's point of view, it would be a very bad
>      thing).
>    * The chief difficulty for the starship will be to maneuver the
>      (very) large sail during in-flight maneuvers without tearing the
>      sail in the process. (By the way, the sail can be decelerated by
>      the expedient of tacking into the beam). Aside from this is the
>      problem of sail erosion by the interstellar medium, especially at
>      high velocities.
>    * The Solar system facilities, especially the photoelectric collector
>      arrays, must be so designed as to permit changeout of the array
>      cells during operation without severely degrading performance. This
>      is necessary as the collector arrays (located, most probably, in
>      intra-Mercurial orbits for efficiency) will be subjected to high
>      thermal loading and intense radiation exposure, which degrades the
>      collector cells over a (short) period of time.
> 
>   In short, the M.A.R.S. concept, while technically feasible in the
> context of the base scenario (i.e. the 2040-2060 timeframe), it is
> probably not politically feasible, especially in a society which would
> be all too eager to divert some (or all) of the power from the emitter
> array to increase the energy supply (hence, wealth) of its members (not
> to mention the fact they would probably not be too eager to divest
> themselves of the resources required to build the infrastructure,
> especially if there are perceived to be greater needs on Earth). And, of
> course, the M.A.R.S. starship is not autonomous, in any sense of the
> word; it cannot, for example, change course to investigate some
> previously unknown phenomenon, at least not without extensive
> consultation with Earth and major modifications in the flight plan.This
> alone suggests that the best use for the system might be point-to-point
> automated cargo transport, which would depend on predictability of the
> mission profile. Any missions of exploration, however, would be limited,
> at least during the cruise phase, of strictly fly-by types of profiles.
> 
>   The Argosy-class starship design is not detailed on your website, so I
> cannot comment on that concept. The Fuel/Sail-class vehicle addresses
> some, though by no means all, of the drawbacks to the M.A.R.S.-class
> starship, though it is still limited by fuel availability, as is the
> Explorer-class design.
> 
>   My humble suggestion is that a new class of vehicle be investigated,
> one that uses most (if not all) of the non-propulsive elements in the
> Explorer-class vehicle, but uses a regenerative carbon-cycle fusion
> reactor for propulsion and on-board electrical power. A side benefit to
> this reaction is that its primary waste product is helium-4, which can
> be readily liquified for use in the engine's cooling systems, thus
> reducing the necessary area of the ship's radiator arrays. Similarly,
> another output of the reaction cycle is a stream of positrons, which can
> be reacted with electrons to provide added thrust. The primary benefit
> of this reaction cycle is of course, its reduced need for fuel, thus
> freeing up a larger fraction of the vehicle for useful
> payload/supercargo.
> 
>   I apologize in advance for having taken so much of your valuable time
> on this matter. If you have any questions on the material above, or if
> you wish my assistance in further development of this concept, please do
> not hesitate to contact me at mjones@dzn.com.
> 
> 
> Sincerely,
> 
> Michael E. Jones