Re: Fwd: starship-design: Rex Finke's "Starflight Mechanics Notes"

[DotarSojat@aol.com]

Hi Kelly (et al)

Thanks for the review of "Starflight Mechanics Notes."  In my
3/7 cover letter that went with the original transmittal of the
paper to starship-design I wrote, "I would be receptive to any
suggestions on how to improve it or how to adapt it for any
purpose you might see fit to use it."

Speaking of my cover letter, it is not among the correspondence
that Kelly forwarded.  On the possibility that it has fallen by
the wayside, I reproduce it here:

>Hi all
>
>As a token of my appreciation for the privilege of participating
>in Group discussions from March '96 to July '97, I have
>assembled some of the Group email notes from that period on
>starflight mechanics and propulsion issues into an integrated
>paper, or monograph, or whatever, entitled "Starflight Mechanics
>Notes."
>.....
>If the paper gets a positive response, I will send the Appendix
>"An Engineering Review of Relativity for Interstellar Flight"
>as an attachment to a separate email.
>
>From the Preface to the paper:
>"This document is intended to gather in one place the answers
>to the questions about starflight mechanics and propulsion
>technologies raised by Group members in the first three years
>of the Starship Design project.  (The emphasis is not so much
>on providing answers as it is on showing how to find answers.)
>
>"The intent of the documentation here has been to use only
>selections from notes as originally posted and to reproduce
>them as closely as possible, except where they were incorrect
>or incomplete, or where they were inconsistent with other
>postings.  (A few useful facts have been added here and there
>to make the write-up more comprehensive.)  It was of primary
>interest to capture and preserve the feeling of discovery that
>accompanied the interactions of the members."
>
>I would be receptive to any suggestions on how to improve this
>compilation or how to adapt it for any purpose you might see
>fit to use it.
>
>I am not on the starship-design mailing list, so please direct
>any comments to <DotarSojat@aol.com>.
>
>Best Regards
>Rex Finke

In a message dated 3/13/01 9:50:52 AM, kgstarks writes:

>Rex,
>Just read your LIT starflight paper.  Thanks for collecting it
>together.

I enjoyed doing it and reminiscing.

>I noticed you sometimes forgot to include units.  For example
>in the MeV to speed table you didn't list the units for the
>resulting speed, nor in all the definitions of the letters in
>the equations.

I hope you have a better example than this.  (I wonder how many
others don't know what the units of "beta" are, especially when
there is a definition of "beta" about five lines above the table
[p. 12].)  The v and V on p. 13 are not end-product variables
that need units; only the E1 and E2 are.

>I'm not sure why you didn't include reaction rate numbers for
>all the fusion reactions on page 14?

Rather than my saying "...with the reaction rates...given for
the first five reactions," I guess I should have said "...with
the reaction rates given [in the Plasma Formulary only] for the
first five reactions..."  I didn't foresee this confusion.

>You might want to include full names in quotes?

(?) I don't know what "full names" you're talking about.  (The
"full names" of the contributors are given in the Preface.)

>Quotes came with " >" at the start of each line; but since you
>didn't add hard returns when you added them to the paper, they
>now sometimes float in the sentences.

I didn't see this in either the WORD or PostScript versions.  I
considered going to quotation marks at the beginning and end of
each quote, but decided to keep the original email format so that
I could distinguish quotes within postings from stand-alone
quotes.  What would you suggest (hard returns mess up WORD)?

>I'ld argue with some of your implications that there's a
>physics limit to the project.  A nasty engineering to cost
>limit, yes; but that would require a very differnt set of
>solutions.

OK, I'll concede; understatement is always better.  Change
"Human interstellar flight in a human lifetime is far beyond
current physics/economics." to "Human interstellar flight in a
human lifetime is far beyond current engineering/economics."
(Hmm, the former version is a direct quote of the original
posting, to which you didn't object at that time.)
 
>For example my fuel sail concept (which you didn't mention)
>with a assumption of greatly reduced automated systems costs.

I never understood what distinguished it from the "Deceleration
of a Sail Pushed by a Constant-Power Beam."

>Or refueling at the target system to eliminate the need to
>carry round trip fuel.

I guess my primary consideration in the paper was "getting
there."  The problem of getting there is hard enough, without
compounding it now with the complexities of getting back, too.
(I'm not arguing here for a "suicide mission," I'm just saying
that when we figure out how to "get there" within then-existing
engineering/economics limitations, we'll probably have a better
handle on how to "get back.")

>Or using a lithium base fusion fuel to eliminate the need for
>a tank.

I included the Plasma Formulary reaction rates precisely to
indicate the increasing difficulty in achieving controlled
fusion with nuclei having higher atomic number (higher Coulomb
barrier).  I believe controlled fusion with any nuclei other
than deuterons and tritons may be almost as far beyond the
horizon of the Starship Design project as antimatter.  (And the
mass ratios for fusion rockets [see p. 37] are pretty daunting
[e.g., 3,600 for a 24-yr one-way trip to an 8 lt-yr destination]
without making any additional allowance for tankage weight.)

>Or suplying power/fuel in flight from earth to eliminate having
>to carry it.

An encounter with a fuel pellet at a relative velocity upwards
of a km/sec or so constitutes a hypervelocity impact, i.e.,
instant conversion of kinetic energy to heat.  (I presume the
ship must be going faster than the fuel pellets in order to
catch up with them.  Maybe the ship just scoops up puffs of
gases?  The details of how you proposed to do this must have
been in some correspondence that I didn't access.)

>I have a serious problem figuring out the beam power to thrust
>relationships for something like a microwave sail system.

On pp. 25, 31 and 53, I write: "2,940 megawatts of beam power
per kg of thrust" for 100% conversion of beam power to thrust,
and p. 31: "Note: a 'photon rocket' with the same light-beam
power would have the same thrust [as a relativistic electron
thruster]."

>Even more problems understanding how the beamed energy can be
>consentrated into a conversion system for retro thrust, and get
>more retrothrust - then forward thrust from the sail?  A
>critical point since it seems counter-intuative.

Ahh.  GREAT point!  Alarm bells ringing: INCONSISTENCY!
INCONSISTENCY!  (Where were you 4 years ago when we needed you?)
Either (1) the 2,940 Mw of beam power per kg of thrust can't
apply to all beams (all three pages: 25, 31 and 53),
or (2) there is something wrong with the analysis on p. 49 of
the retrothruster for deceleration of the beam-pushed sail.
Fortunately for the beam-driven-sail-concept, number (1) turns
out to be the case: While 2,940 Mw of photons or 2,940 Mw
of 1,000-MeV, say, electrons give 1 kg of thrust, 2,940 Mw of
1,000-MeV protons give 1.6961 kg of thrust.  The number of kg of
thrust for 2,940 Mw of proton beam power is given by the value
of [factor] on p. 49.  The expression [factor] should be replaced
with the name "RBE" (Relative Beam Effectiveness [with apologies
to the radiological hazards people, to whom RBE means Relative
Biological Effectiveness if I remember right]).
The value of RBE is 1.0000 for photons and relativistic electrons,
but for protons RBE increases as the exhaust velocity Vexh
(in lt-yr/yr) is reduced, according to the following table:
     Vexh     RBE
     0.9    1.5954 (the example in the text on p. 49)
     0.8    2.0000
     0.7    2.4488
     0.6    3.0000
     0.5    3.7321
     0.4    4.7913
     0.3    6.5131
     0.2    9.8990
     0.1   19.9499
which makes the deceleration of the beam-pushed sail easier and
easier to achieve as the exhaust velocity of the protons is made
lower and lower.  Remember, "deceleration is possible only if the
efficiency 'eta' of conversion of received power to exhaust power
is greater than" 1/RBE.  (I hope Kevin is still around to be
pleased by this.)
As the Vexh is reduced, however, the deceleration rate is
increased, the deceleration distance is shortened and the
required mass ratio is increased (to be confirmed).  Following
this trend, the argument in the text regarding selection of the
turnaround point should be extended to considering tradeoffs of
choosing turnaround points later than the halfway point.
(The value of the RBE for the 1,060.5-Mev-proton deceleration
beam required to bring the sail/ship to a stop in the second
half of the 7.95 lt-yr trip on p. 51 is 1.6640, so I should have
used 2,940/1.6640 = 1,767 Mw/kgf, instead of 2,940 Mw/kgf, to
calculate the required current for the retro-thruster on p. 53.)
  
>(Oh, did anyone consider converting the microwaves to
>electricity in the sail [rectena sail?] rather then reflecting
>and concentrating it?  Does this help?)

I'd be surprised to learn that microwaves are converted to
electricity in a rectenna, and not at its focus after having
been reflected and concentrated.

>You might want to look up conversino numbers for free-electron
>and LED lasers.  I beleave their conversion efficencies are
>higher.  Not sure though.

This does not strike a responsive chord in my memory.

>In general though a great summary.  If I still had access to
>the web site I'ld convert it to HTML and link it it.

Thanks.  Would HTML fix the floating ">"s?

>Kelly

After I have an appropriate number of reviews from list members,
I'll incorporate the agreed-upon revisions in the text and
resubmit the paper.

Regards
Rex