Conference noted::sf

Perhaps it has something to do with the energy it takes to collect the "fuel".
Imagine the scoop slamming into the molecules, forcing them into the intake.
The faster the scoop, the harder the slam.

Just a guess...


John B.

Another thought is this: the closer one comes to relativistic speeds, the
field will be affested.  Is a magnetic-field "scoop" moving at near-light
speeds going to behave similarly to one moving much slower with relation-
ship to the molecules it's going to scoop up?

I haven't done the math on this, but I'll bet it would have a significant
effect.

Steve Kallis, Jr.

Re .2:

I believe the remark I saw did use relativistic effects to make its point.
However, it probably used the ship itself as a frame of reference, which is
easier.  This makes the question not "How does the scoop behave?" but "How do
the incoming and outgoing particles/mass/energy behave?".


				-- edp

GENTLEMEN et al

My first job out of college was at Goddard Space Center, which has one
of the most extensive technical libraries I have ever haunted ! This
went along with my college hobby of relativity (& sf).

Now to business; I have copies of R. Bussard's original paper on the
interstellar ram, and the article discussing the very topic of an 
upper limit velocity attainable with the Bussard ram. I (very, very
fuzzily) seem to recall a limit around .25c ; I have not reviewed
the articles in years (no-one to discuss such niffty topics with).

If anyone is interested in copies contact me at:

	FRSBEE::Farrington

HOPEFULLY, I CAN STILL DIG THEM OUT...

Dwight Farrington sent me copies of papers discussing the Bussard Ramjet,
including R. W. Bussard's original paper, "Galactic Matter and Interstellar
Flight". 

Bussard's article discusses the basic equations of motion for a ramjet --
energy, momentum, apparent time and acceleration, and so on.  Two other
articles discuss limitations due to such mundane things as the nature of
magnetic scoops, structural materials, galactic matter, and so on.

I read Bussard's paper and skimmed the others.  Bussard's equations allow a
ramjet to continue accelerating forever.  The later papers discuss rather
depressing limitations on ramjets but do not show there is a maximum
theoretical speed below the speed of light for an ideally constructed ramjet.

The basic equations of motion are quite clear (if anybody would like, I will
enter them here).  This leaves me not knowing what it was I saw which I thought
placed a limit on ramjets.  Bussard's equations account for energy, momentum,
density of matter in space, and the efficiency and energy yield of nuclear
reactions -- which covers pretty much all of what we were speculating on.


				-- edp 

While tracking down the Bussard article I ran across a number of articles
addressing the topic of interstellar flight (NASA's Goddard Space Center)
and limitations on velocity due to fundamental laws and/or technology.
I also vaguely recall an upper limity on the Bussard ram; I thought it 
was related to limits on vessel dimensions (in an attempt to maintain a
grasp on **~realistic** vessel designs) due to materials' science limitations
and power plant design. After all, a ram throat 10 km across seems a bit
much.  But then, at the time I was young (read as 'naive') and was reluctant 
to photo-copy the few hundred or more pages...

But, there is also the issue of the actual make-up of the interstellar
medium; is it truly manipulable, to a useful degree, by electromagnetic fields ?
Something along the lines of a preponderance of electrically nuetral particles,
or ions (non-fusible) in sufficient concentration to prevent proper burn.
Is the density sufficient to maintain fusion burn, as well as thrust (shades
of LENSMAN series and the first intergalactic excursion by CIVILIZATION) ?

The issues are endless and intriguing; not to mention entertaining !!!

Anyone out there interested in pursuing a cooperative effort to research
and ** "design" ** a DEC star drive ????

NOTE: "non-fusible" meaning our current technologies requiring a light nucleus
fusion reaction, as being sufficiently ENERGETIC to supply the densities
required to be of any engineering use... We humans cannot economically utilize
the processes of the stars (Fe fusion, et al); economies of scale are a bit
beyond us at present.


dwight

re 211.6, Fe fusion:  I thought the thing about iron was that you couldn't
fuse it or fission it.  I seem to recall some kind of curve showing available
energy as a function of atomic weight, with a minimum at Fe, so there wasn't
anyplace to go if you started with Iron.

len.

I am thinking of some discussions in the basic theory of nuclear fusion
versus the engineering of a useable fusion generator...

Apparently the environment in a stellar heart is such that ~heavy nuclei
may be invovled in a fusion reaction; however, the reaction is of absolutely
no economic benefit or interest to humans (ignoring the purely theoretical).

As I recall, the reaction was "slow", lacking the economically required
energetic outputs, as well as needing a great deal of energy input to get
started and/or to be maintained. These are trivial concerns when the scale
involved is that of even a small star such as Sol.

By all means, if I am wrong in this LET ME KNOW !! Any excuse to do some
digging in a library...

Dwight

I think Iron's the end of th eline for fusion.  There's an article in
a recent Scientific American on supernovas, and if I remember correctly
(no promises made or implied), the step from silicon burning to iron
"ashes" takes a few seconds or so and then the star collapses.  I think
you can't do anything with iron from a fission/fusion standpoint.  It's
the bottom of the barrel.  Anybody out there who really knows what they're
talking about?  (edp?  you've bailed us out before...)

Re .9:

The amount of energy that can be gained from a fusion reaction does
decrease as the elements progress through the periodic table, and it does
become zero at some point.  Although I do not recall which is the first such
element, iron sounds reasonable.  I will look it up.

It should be noted that although no energy can be gained, it is still
possible to go beyond the no-return point by using energy.  This even has a
practical purpose, such as creating a universe (notably, ours).


				-- edp

There is a book _The Science in Science Fiction_ by Peter Nicholls
(published by Knopf), a large format paperback whose subject is self
explanatory.  It raises the following five objections to Busssard ramjets:

	1) the required magnets are beyond any engineering technology
	   available to us, in terms of absolute field strengths, field
	   densities, and current densities.  This argument can be exposed
	   (sorry, that should be DISposed) of by assuming a technological
	   deus ex machina somewhere in the future.

	2) the deuterium content of the interstellar medium is so low that
	   the bulk of the stuff in front of the ramjet will be nonfusable
	   junk, creating more drag than the fusion reaction can overcome.
	   I'd like to see some numbers for this one.

	3) Deuterium fusion produces neutrons, which cannot be shielded
	   against via electromagnetic techniques.  Hence the mass needed
	   to keep the crew from being fried by the neutrons produced is
	   prohibitively large.   Again, some numbers would be useful.

	4) At relativistic velocities, the radiation produced by the impact
	   of the interstellar medium on the ship would also fry the crew.

	5) The energy needed to compress the interstellar medium enough to
	   cause it to fuse is aas large as or larger than the energy
	   released by the fusion reaction.  Again, this ought to be
	   amenable to analysis.

I'm not saying I believe these arguments, just reporting them.  They're the
only ones I've seen.  I've got to admit, I'd sure like Bussard ramjets to
be feasible.  I think there's a similar set of wet-blanket arguments that 
disallow Dyson spheres.

len.

Re: .9

The Scientific American article was in the May 1985 issue, titled _How a 
Supernova Explodes_ by Hans A. Bethe and Gerald Brown.

To confirm your premise about Fe,

"One last cycle of fusion combines silicon nuclei to form iron, 
specifically the common iron isotope 56Fe, made up of 26 protons and 30 
neutrons.  Iron is the end of the line for spontaneous fusion.  The 56Fe 
nucleus is the most strongly bound of all nuclei, and further fusion 
would absorb energy rather than releasing it."

Type I and II Supernovae create the heavier elements.  Some suns never 
even get that far...

"A star the size of the sun gets no further than helium burning, and the 
smallest stars top with hydrogen fusion."

As far as the time line goes, it varies with mass.  A graph was given 
which I've placed in the table below:

Mass=25 solar masses	Time span		Temperature
						(Million degrees Kelvin)
--------------------	---------		------------------------

Hydrogen burning	7 million years		~ 10
Helium burning		500,000 years		~ 30
Carbon burning		600 years		~ 90
Neon burning		1 year			~175
Oxygen burning		6 months		~230
Silicon burning		1 day			~400
>Boom< :-)


	 [} Mike {] 

Re .11:

1)	Technology will improve.

2)	The deuterium content of interstellar space is still subject to
	speculation.  Fusion of regular hydrogen is also a possible source
	of energy.

3)	The ship is going to be very large anyway; put the crew far away
	from the reactor.

4)	Some shielding will be automatically provided in the process of
	funneling particles into the reactor.  Additional shielding may
	be necessary for what remains.  However, this is a technological
	limitation, not a theoretical one.

5)	There are two ways matter needs to be compressed.  First, it must
	be gathered together.  Second, it must be subjected to pressure to
	cause fusion.  This second compression yields more energy than is put
	into the process for light elements.  The first compression requires
	energy only because of the laws of thermodynamics, and can
	theoretically be made as small as desired.


				-- edp

    re IRON as the "end of the line":
    
    all nuclear reactions, fission and fusion, release (or absorb) energy
    as a result of the difference of the binding energy of the nuclei
    before the the reaction and after. uranium has more binding energy
    than lead which is why energy is given off as it slides down the
    binging-energy curve. two hydrogen atoms posses more binding energy
    than one helium. thus energy is produced when hhydrogen is fused
    into helium. the real reason fusion is so much "better" than fission
    is that the slope of the binding energy curve is very much steeper
    on the lighter side than it is on the heavier side. Iron is at the
    minimum of the curve. Stars create the heavier elements only during
    a supernova when there is enough energy around to be absorbed into
    pushing nuclei "up" the binding energy curve.
    
    sm

    	The following is quoted from T.A. Heppenheimer's 1977 science
    book, COLONIES IN SPACE:
    
    	"In 1960 Robert Bussard proposed the interstellar ramjet.  This
    idea involved a starship which wouldscoop up the incredibly thin,
    diffuse hydrogen of interstellar space and heat it and compress
    it to conditions found in the center of the sun.  It would undergo
    thermonuclear fusion, yielding helium, which would exhaust out the
    back.  Bussard's concept had the advantage of being ingenious. 
    It had the disadvantage of being wrong.
    	He assumed a drag-free starship, which in his context is a little
    like assuming a drag-free airplane.  If such a plane could be built,
    it could fly at high speed just by burning atmospheric smog in its
    own ramjet.  Such a smog-burner, of course, would give very weak
    thrust (smog is never THAT bad) and so would Bussard's ramjet. 
    Hydrogen, at the conditions found in the solar interior, yields
    energy at the rate of two ergs per second per gram.  A cooling bathtub
    releases energy a thousand times more rapidly.  In terms of
    power-to-weight ratio, a Roman galley fully of slaves would be more
    efficient.  Even when the slaves were asleep you could get more
    energy from their body heat."
    
    	Larry
    

re: .15

Solar fusion may give off only two ergs / second / gram of hydrogen, 
but nothing (that I know of) says that our fusion plants need be like that.  
I assume that the fusion researchers are already working on this, 
since that low an energy density would make a fusion reactor 
impractically large.

The limit on a ramjet's speed is when the drag equals the thrust. Drag
goes up as the cube of the velocity (square because the energy of the
impinging medium, and one more power because more hits you as you go
faster), and thrust only goes up linearly with the speed (since you're
getting more material to burn).  At some point the cubic will beat the
linear and the ramjet won't accelerate.  (This is all ignoring
relativity).  Don't know where that point is, but it probably rules
out "Tau Zero" type stories where the ramjet can get arbitrarily close
to c. 

/jlr

    I don't follow that the drag is really a cube factor.  Every gram
    of hydrogen that hits the forward collector takes energy to accellerate
    to the ramships speed- but where does that energy go?  Into heating
    and compressing the hydrogen.  You get this energy back when you
    allow the hydrogen (now dirtied with a bit of helium :-) ) to expand
    and cool in the exit nozzle.
    	
    ( I say collector and nozzle as though they were material objects,
    even though they should be magnetic fields)
    	
    So, the energy balance for the ramjet with the fusion fire turned
    off is correct in the perfect case- energy absorbed in compression
    and accelleration is regained in the expansion and cooling.
    	
    All the fusion fire does is to add energy to the already compressed
    hydrogen, so that the energy obtained by allowing it to expand against
    the sides of the nozzle is GREATER THAN, rather than equal to, the
    energy expended in gathering the hydrogen.
                                              
    Does my lens need cleaning?

The energy goes into just accelerating the hydrogen, not into heating 
and compressing it.  Heating takes still more energy.
/jlr

    But you get the work back from the expanding hydrogen when you let
    it expand....

    The compression mode generates the heat.  Too, the Bussard ram
    envisions an electromagnetic 'scoop' of truly gigantic proportions
    (with respect to the dimensions of the ship).  While I am not
    sure of Heppenheim's credentials in this area (re: Scientists
    Against Nuclear 'something'; most are _not_ involved in the physical
    sciences, nuclear, physics, et al) a number of learned papers
    were released discussing, with full math analyses, the theoretical
    and practical limitations of Bussard's ramjet.  This included
    some discussion setting conditions; material or generator technology
    advances...

    Remember that you can't fuse all the incoming hydrogen; some of it is
    not going to make it into the constriction. (e.g. it will be merely
    deflected by the scoop). This represents an energy loss that's going to
    get worse as you go faster (relative mass/energy of the protons
    increases and the time you have to deflect them decreases).
    
    That energy loss is going to come in either reduction of the magnetic
    field stength or loss of velocity.
    
    I've heard that calculations based on this argument give a figure
    of either .25c (deuterium fusion only) or .40c (proton-proton fusion).
    
    As far starting a ramjet goes, one could use "cold" fusion, that
    is,  catalyzed by muons. The muons replace an electron and reduce
    the size of the electric field surrounding the protons which
    dramatically lowers the fusion temperature...
    
    Regarding unreasonable materials: I would call ceramic room-temperature
    superconductors "unreasonable materials", yet they exist now. I
    won't buy the unreasonable materials argument.
    				- HBM

    In an article in an old magazine (Scientific American?  Analog?
    Fantasy & SF?) I read a truly excellent discussion of space travel
    in general.  
    
    Of Bussard ramjets, the author said (and worked out the math) that the
    speed (momentum?) limit is equal to the speed (momentum?) of the
    exhaust.  He estimated something on the order of 0.12 c for
    proton-proton fusion. 
    
    It would take matter-antimatter drives to approach relativistic speeds. 

    I'll see if I can dig this article up, but no promises.  Also, please
    forgive memories some six or seven years old if I am totally off.
    
    				=ELB=

    One thing I have always wondered is why does a Brussard ramjet
    have to be a fusion reactor?  I mean we know a Brussard ramjet will
    require significant technical advances, perhaps by that time we
    will be able to perform total conversion.  With total conversion
    the speed limit isn't a puny .12C and you don't "waste" heavier
    elements.  Also, you probably don't need as big a ramfield.
    
    Tony 

    	Bussard, not Brussard.
    
    	Larry
    

    No one knows how to do total conversion of ordinary hydrogen.
    You can do total converion of a mixture of hydrogen and 
    anti-hydrogen, but there probably isn't much anti-hydrogen floating
    around in interstellar space.  
    
    There's some thought that protons
    might actually be unstable, with half-lives on the order of
    10^30 years.  If you could find something that catalyzed their
    decay, then you might be able to get more energy out of hydrogen
    then fusion provides.  But as far as I know, no one has demonstrated
    the instability, much less a way to enhance it.  
    
    /jlr

    < Note 211.25 by DEADLY::REDFORD "It's 0700 in America" >
    
>    There's some thought that protons
>    might actually be unstable, with half-lives on the order of
>    10^30 years.  If you could find something that catalyzed their
>    decay, then you might be able to get more energy out of hydrogen

    Theory suggests that magnetic monopoles will do this.
    				- HBM

    Sorry, Bussard, not Brussard 
    
    Incidentally, aren't monopoles what Niven postulates to make 
    his magnetic fields?
    
    Also, I am aware that total conversion of hydrogen is a practical
    impossibility, even beyond theory.  However, let me suggest that
    since a Bussard (I almost always say or write Brussard and I know
    it makes me a bad boy) ramjet is at least 50-100 years away, our
    theories of the universe, time, energy and matter will be completely
    different and may allow monoploes and total conversion.  Thus, I
    feel that given the technical advances required to develop a B.
    ramjet some parallel advances in science should enable the same
    concept to perform at a higher level.  
    
    The advantages are truly awesome.  The energy per h atom is some
    1000X greater than for fusion.  This requires collecting fewer atoms,
    less energy is expended in accelerating the atoms to the speed of
    the starship and smaller/less powerful collection fields
    
    Tony

    > [...], our
    >theories of the universe, time, energy and matter will be completely
    >different and may allow monoploes and total conversion.
                
    Why are we allowed to think that just because our theories change,
    the universe will suddenly 'allow' us to do things that (as far
    as we now understand) are impossible?  Isn't it just as likely that
    we will discover that we were right, and it's really impossible?
    
    Willie

    A nit:
    
    I don't think you would get a 1000x gain over hydrogen fusion with
    matter-antimatter drives.  I once did the math and if my memory
    serves, we are talking something like a 140x gain.  Still impressive.
    
    				=ELB=

    Re: .28
    
    Perhaps the problem was my poor choice of the word completely. 
    
    What I meant was a fuller understanding of the universe may allow
    us to make monopoles and manipulate matter to achieve total conversion.
    
    By this I mean that, just as Newtons Laws are a subset of Einsteins
    relativity work, further development of our present science may
    lead to such marvels.
    
    We know scientists can be surprised, that is the reason for the
    SSC (superconducting super collider).  By the work of Glashow, etc.
    that won the Nobel prise for predicion of the particles involved
    in the weak magnetic force, there should be no new particles of
    interest at the energy levels of the SSC.  However, there are hints
    (just a few reactions) from CERN that this slightly higher energy
    level may demonstrate scads of newe particles.  Also, there is some
    notion that monopoles could be created with the appropriate energy
    level in a collider.
    
    Finally, I am sure that our present theories are wrong!  The recent
    development of 90K superconductors, when >30K was considered impossible
    less than 4 years ago is just the latest example of scientific
    chauvinism.  Theories are just that, theories, not facts.  I don't
    presume to say we know it all.  Change is a part of science, within
    100 years I believe that most of our present theories will be
    supersceeded.  That is not to say that what we theorize is wrong,
    merely that the fabric is incomplete
    
    Tony 

    I wasn't saying that we know everything now and that nothing will
    change in the future, and I agree that we are probably wrong in
    our understanding of how the universe goes together, what I was
    reacting to was your assumption that new theories and understandings
    will lead to {monopoles, total conversion, ramjets, etc}.  You can't
    know what these new theories will lead to, and counting on sufficiently
    advanced technologies solving specific problems that we currently
    'know' to be impossible is (IMHO) overreaching.
    
    Willie

    What is IMHO?
    
    I must admit that I am an optomist.  While I agree that the specific
    inventions/scientific wish list items that I mention may not be 
    achieved, others that I cannot imagine will be achieved (and in fact
    thoser that cannot be readily percieved may be the most profound). 
    
    No one can say that they know what the future will hold, yet I want
    to believe that interstellar travel will be possible and a space
    ship that collects interstellar hydrogen as its fuel and converts the
    entire mass of that collected fuel to energy, mostly used for forward
    thrust, seems like the most reasonable technology to accomplish
    interstellar travel
    
    Tony

    Don't forget, there are alot of things that were once considered
    impossible that are quite possible now.
    
    I just started reading an edition of 'Tau Zero' with an intro by A.C.
    Clarke.  He briefly discusses the feasibility of the ramjet. I'll try
    to remember to look up and post his comments. 

    I, too, hope our understanding of the universe is wrong, and that
    we can have FTL flight and superconducting steam or whatever.
    
    Of course, there's always the nasty possibility that we'll find
    out our theories were wrong/incomplete... but that we still can't
    beat c.   :-(
                 
    - Richard

    Re: .36
    
    It is true that we may never be able to beat good ole C, however
    a ramjet is not a FTL spaceship.  It is a significantly smaller
    leap of faith to postulate the technologies necessary for a total
    conversion (or even a fusion) ramjet than for a FTL spaceship.
    
    FTL or near-C speeds may not even be necessary if advances in the 
    biological sciences either prolong life or enable hybernation.
    
    Tony

        There is a new book now available on actual starship designs in 
    most mass-market bookstores.  It is titled THE STARFLIGHT HANDBOOK: 
    A PIONEER'S GUIDE TO INTERSTELLAR TRAVEL, by Eugene F. Mallove and 
    Gregory L. Matloff, John Wiley & Sons, Inc., New York, 1989, 
    ISBN 0-471-61912-4 (hardcover - $19.95).

        The authors share extensive backgrounds in aerospace engineering 
    and astronomy.  The book gives a good study of the various and varied 
    ways we may one day reach other star systems, and there is an adequate 
    supply of diagrams and mathematics to back up the engineering plans.  
    It is definitely worth a read for those who want to know what it will 
    really take to attain the 'final frontier'.

        Larry

    OK... I won't pretend I understand what you people are discussing here,
    mono-poles, matter-anti-matter etc... but I do just about understand the
    principles involved in the Bussard Ramjet ( well... almost ).

    Now, the question I have had for years and I've never actually seen 
    answered, is this.

    Given that you have this huge scoop, schlurping in whatever, you stuff it
    into your reactor at a rate of knots and let the ensuing exhaust squirt
    you about the galaxy, just how do you slow this thing down. The only things
    I can think of ( bear in mind my original sentence ) are...

	o You blank off the scoop and rely on drag ?
	o You whop a big parachute out the back ?
	o You somehow turn around the exhaust or intake and blast the exhaust
	  out the front ?

    Silly ideas I know, but then I'm a layman. So... in laymans terms, how WOULD
    you slow the ship down ?

Ross

                          -< Brakes ? What brakes ? >-

Well as I understand these things .. the idea is that you have
(in effect) a very large funnel that just scoops in hydrogen
gas.. and as the ship is moving forward the gas(matter) is forced
down the funnel.. and gets compressed as it goes down the narrowing
funnel... at some point the gas is compressed to the point that it
undergoes fusion.. thus producing energy.. then driving the ship

I guess the way to slow the ship would be to change the funnel
so that it does not narrow so much.. ie before you had a funnel that
came to (nearly) a point.. but now you change it so that comes
no where near a point.. so now the gas does'nt get compressed as
much.. it does not undergo fusion.. now the gas is not
creating energy.. so now the ship is pushing its large funnel
forward and scooping in gas  "for no return of energy".. ie the
ship is expending energy by pushing the funnel foward .. but
is not getting any energy back.. so then it must slow down

the big question is.. in the case of the gas undergoing fusion
the fusion reaction provides the power to drive the ship
generators for the funnel.. what will provide the power for
the funnel when you are slowing down??  (big storage cells???)

Wouldn't that take an awfull long time to slow the thing down ?....

    
    I may be missing the obvious, but can't you run a Bussard scoop
    "in reverse", as it were ? In other words, modify the fields and
    flow in such a way as to aim the exhaust in the direction of travel.
                                                               
    
    Ian
    

r.e. .42

If you turn the ship round then I would have thought that that would have been
enough.

Slowing down ramjets :

 Openeing up the funnel would work. A competently desinged ship would
 have a seperate powerplant for lifesupport, etc...(and a very well designed one
 would have duplicate/triplicate backup systems....)

 Also, in order for a ramjet to work, it has to be going fast enough when the 
 ramjet is turned on to instantly produce fusion (in other words, you can not
 use a ramjet from a standing start, you need an initial propulsion system.)

 As far as slowing down the ramjet, it will take approxiamately half the 
 voyage to slow down.  Also, keep in mind that ramjets are sublight speed 
 craft, and the faster you want to go, the longer it will take to get to speed.
 therefor they would probably be used as a generation ships's propulsion system.

 The assumption could be made that whatever got you up to operating speed for
 the ramjet was/is internal to the vessel and could be used to slow you down...

 grins,

 clark.

    
    
         Turn it around??  But now the scoop is facing the wrong way.
         You'd have to redirect the exhaust to slow it down or
         develop some other sort of "space brake". 
                                                             
         As a related question, how do you get the thing started up?
         It would have to move awfully fast to be self-propelled.
         
         If you're near a sun you could move towards it and slingshot.
         But what about when you've stopped in deep space? 
                                                            

    Re: .40
    
> the big question is.. in the case of the gas undergoing fusion
> the fusion reaction provides the power to drive the ship
> generators for the funnel.. what will provide the power for
> the funnel when you are slowing down??  (big storage cells???)
    
    Easy.
    
    Kinetic energy of the spacecraft itself. Remember, from the
    spacecraft's point of view, it's not moving at .3c, the charged
    particles around it are moving. Moving charged paricles in a magnetic
    field create a current flow (this is how an MHD tunnel works). This
    current flow can be used to power the magnetic field, which you will
    need moving at .3 lights anyway, since those high-speed particles are
    gonna fry you unless you deflect them.
    
    Re: .45
    [How do we get back up to jet speed in interstellar space?]
    
    Well, you'd better have an "inboard hydrogen cooling tank" and the
    ability to run the fusion reaction from cold hydrogen. Or maybe an
    "Orion"-type pulse-fission drive (using the magnetic field as your
    pusher-plate). In any case, you will need some kind of auxillary drive.
    				- hbm

Well, I'll try to take a shot at it with pictures.

Remember, the cone is NOT physical, it is typically a magnetic field and 
should be configurable.

Also, the rear of your craft in the forward mode, or the back of your 
craft in the reverse mode is either going to be HOT (radioactive) or 
need its own magnetic or unobtainum shielding.

===  is the ship
/ or \ is the magnetic scoop
* is the fusion reaction point
+ is the cold inert gas 
- is the hot fusion products

forward mode:                reverse mode:
                   /+++                         /+++++++++++
                  /++++                        /++++++++++++
                 /+++++                       /+++++++++++++
                /++++++                      /++++++++++++++
     ==========/+++++++                     /==========+++++
-------*+++++++++++++++                     ++++++++*-------
     ==========\+++++++                     \==========+++++
                \++++++                      \++++++++++++++
                 \+++++                       \+++++++++++++
                  \++++                        \++++++++++++
                   \+++                         \+++++++++++

Tony

    Just as a matter of interest....
    
    
    Why would anyone *WANT* to stop in deep space???
    
    Sightseeing??

    re .48
    
    Of course someone wouldn't want to stop in deep space, unless there was
    something interesting there.
    
    But, the beauty of the bussard ramjet is that its fuel is free.
    therefore, to minimize trip time, it would accelerate all the way,
    unlike current manned or unmanned probes which just go on momentum most
    of the time.  Now, in order to stop, you have to begin braking about
    half the way there, assuming of course that it has equal thrust in both
    accelerating and decelerating modes.
    
    Tony

    
    What would be the consequenses of crossing someone else's
    exhaust beam?? instead of nice, clean H and D, wouldn't there be
    a fair quantity of heavier atoms/molecules....??
    
    What would be the effect on the ramjet??
    
    A Backfire could be fatal.....

    
    David

    re:  <<< Note 211.51 by CURRNT::OTTEN "what we want is.. a shrubbery" >>>

    
    >What would be the consequenses of crossing someone else's
    >exhaust beam?? instead of nice, clean H and D, wouldn't there be
    >a fair quantity of heavier atoms/molecules....??
    
    Well, in the first place, the exhaust stream will get pretty stirred
    (it is now moving at some significant fraction of c), so I think we
    only have to worry about the "real soon after" case.  Also, space is
    huge, so I really think someone woud have to intentionally move down
    your exhaust.  
    
    I think there would be an advantage to following someone's exhaust
    beam.  Fusions of the collected h would be incomplete and collection of
    h would be imperfect.  So assuming that you could tap into an exhaust
    beam you would have a richer fuel mixture and it would already be in an
    accelerated state, so parasitic losses on the system (drag from
    accelerating h fuel), would be considerably less.
    
    Also, I think it unlikely that any fusions would be of higher order
    than p-p fusion, still leaving fusionable elements.
    
    Tony

    Re: .51
    
    [Advantages of following another ramjet]
    
    This is covered by Larry Niven in "Protector". The Pak build two-stage
    ramjets that use the exhaust from the leading engine to run the
    trailing engine. Higher-order fusion results. Also, there are combat
    methods described: for example, dropping radon bombs into a following
    enemy's engine. Radon fuses into transuranics, which then fission...
    big neutron flash...
    				- hbm

>    [...] So assuming that you could tap into an exhaust
>    beam you would have a richer fuel mixture and it would already be in an
>    accelerated state, so parasitic losses on the system (drag from
>    accelerating h fuel), would be considerably less.
    
    One problem with this is that the exhaust beam is moving towards you,
    instead of being 'stationary' interstellar hydrogen.  You would have to
    accellerate it up to your speed first, and drag from fuel accelleration
    would be less.  You also couldn't add to the fuel you collected this
    way unless your collector was larger than that of the ship you were
    following.
    
    Willie

    re .54

    Well, I guess you got some point there.  In actuality, the drag on your
    magnetic collector would be greater if you took in an exhaust beam than
    if you took 'stationary' h fuel.  So there is some loss here.

    But, you would need a SMALLER collector, if you were following someone
    rather closely, because the leading ship would have done 'pre'
    collecting for you.  As I understand how a ramjet would work, some
    fraction of the potentially collectible h never makes it into the
    ramjet, it just gets most of the way there.  Also, fusion inside the
    ramjet will not be complete and the exhaust will be FAR richer in h
    than ambient interstellar space.

    Tony
                                                                 

From:	DECWRL::"Ted_Anderson@transarc.com" "MAIL-11 Daemon" 25-JUL-1990 
        10:17:27.38
To:	space-tech@CS.CMU.EDU 
CC:	Dietz@cs.rochester.edu 
Subj:	JBIS Interstellar Studies 

    A few months ago I got into an argument about whether Bussard
Ramjets had any possible basis in fact (perhaps it was in the space
list).  I don't remember being exactly convinced by the ensuing
arguments.  Among the problems were that fusing protons is
ridiculously difficult (even compared to fusing deuterium), and the
implementation of the scoop. 
 
    The easy answer to the first problem is not to bother with fusion
but to bring along antimatter and use the interstellar medium for
reaction mass.  But I've never seen a plausible design for a scoop. 
However, the June issue of Journal of the British Interplanetary
Society, entitled "Interstellar Studies", contains an article by G.D.
Nordley, "Application of Antimatter - Electric Power to Interstellar
Propulsion". His main thesis was that instead of building a magnetic
combusion chamber to exhaust the matter-antimatter reaction products
directly you instead use an MHD generator to make electricity to run
an ion drive.  I apparently missed the argument about why this was an
improvement, however.  You'd think the extra stages of conversion
would just add to the mass and reduce the overall efficency. 
 
    Anyway, at the end he talks about building a ramscoop to collect
protons for reacting with the antimatter.  His basic design for the
scoop was to use a very powerful laser (at about the Lyman alpha
frequency) directed forward at a levitating mirror which directed the
annular beam towards the axis of the scoop.  The photon pressure
pushes the protons towards the center.  In practice there are many
problems with this, but it's the first more-or-less plausible design
I've seen. 
 
    He also does several mission profile scenarios.  Even with what
seemed like incredibly optimistic assumptions he was getting one-way
trip times of 85 years.  The net result (in my opinion, not his
conclusion) is that even antimatter is not up to the task.  The only
reasonable approach seems to be keeping the power supply at home, such
as Forward's laser powered light sail. 
 
    This entire issue is worth checking out for those interested in
extra solar system activities.  By the way this issue is edited by
(you guessed it) Robert Forward. 
 
        Ted Anderson
  

    The research done by Dana G. Andrews, of Boeing Aerospace, and Robert
    M. Zubrin, of Martin Marietta Astronautics, "has proven that a Bussard
    ramscoop composed of a simple current loop or solenoid would not work
    [; however], it has not ruled out the possibility that more complex
    ramscoop configurations may be functional."
    
    See D.G. Andrews and R.M. Zubrin "Magnetic Sails and Interstellar 
    Travel" (presented at the 39th IAF Congress) in the _Journal of the 
    British Interplanetary Society_, 1990, and R.M. Zubrin and D.G. 
    Andrews "Magnetic Sails and Interplanetary Travel" in the _Journal 
    of Spacecraft and Rockets_, April 1991.

    	See also the May 1992 issue of ANALOG magazine.
    

Article: 80281
Newsgroups: sci.space
From: jackson@snmail.jsc.nasa.gov (al jackson)
Subject: Re:Bussard Ram
Sender: usenet@aio.jsc.nasa.gov (USENET News Client)
Organization: Solar System Exploration Division, NASA JSC
Date: Tue, 21 Dec 1993 15:51:27 GMT
 
Some of the basic papers are:
 
R.W. Bussard, GALACTIC MATTER AND INTERSTELLAR FLIGHT, Astronautica
Acta, vol 6., 179 - 194. (1960) 
 
This is an amazing paper, maybe even more farsightedly than the great
interstellar flight papers by Saenger. 
 
D.P. Whitmire, RELATIVISTIC SPACEFLIGHT AND THE CATALYTIC NUCLEAR
RAMJET, Acta Astronautica , vol 2, 497 - 509. (1975) 
 
I always thought our paper on the laser powered interstellar ramjet
was a nice modification of the straight ramjet. 
 
D.P. Whitemire and A.A. Jackson LASER POWERED INTERSTELLAR RAMJET
Journal of the British Interplanetary Society vol 30, no 6 p 223 - 226
 
Carl Sagan's exposition on the Bussard ram in Communication with
Extraterrestrial Intelligence is quite good, I think there is a
reference to an Icarus paper by him that is good too. 
 
The book BOUND FOR THE STARS and what is the book by Gene Malove...
about interstellar flight...?... have very extensive references. 
 
Al Jackson
 

Article: 80462
Newsgroups: sci.space
From: jackson@sn3.jsc.nasa.gov (JACKSON)
Subject: Re:Bussard Ramjet
Sender: usenet@aio.jsc.nasa.gov (USENET News Client)
Organization: Organization, City, State, etc.
Date: Fri, 24 Dec 1993 11:01:28 GMT
 
This is a follow up for whoever asked about sources of material about
interstellar flight and the Bussard Interstellar Ramjet.  The following
are the best introductions and have the most extensive biography of
technical articles: 
 
THE ROAD TO THE STARS
Iain Nicholson
William Morrow and Company, Inc.
New York, 1978
 
THE STARFLIGHT HANDBOOK
A PIONEER'S GUIDE TO INTERSTELLAR TRAVEL
Eugene F. Mallove and Gregory L. Matloff
John Wiley and Sons, Inc
New York, 1989.
 
And for whoever asked about using antimatter with an interstellar
ramjet see: 

A.A. Jackson
Some Considerations on the Antimatter and Fusion Ram Augmented
Interstellar Rocket, Journal of the British Interplanetary Society
V. 33, PP 117-120, (1980)