Date: 2 Feb 90 18:27:55 GMT
From: zaphod.mps.ohio-state.edu!wuarchive!mailrus!jarvis.csri.toronto.edu!utgpu!utzoo!henry@tut.cis.ohio-state.edu  (Henry Spencer)
Subject: Re: Spacecraft drives and fuel efficiency

In article <7007@mentor.cc.purdue.edu> f3w@mentor.cc.purdue.edu (Mark
Gellis) writes:

>A friend of mine pointed out that the Daedelus design is a highly
>inefficient fusion engine.  It is, he claims, based on the bomb-version
>of fusion propulsion.  He suggested that you could get a much higher
>Isp by using fusion power to superheat reaction mass--hydrogen, probably--
>and then spew it out at very high velocity...

Um, what does he think Daedalus was doing?  It was using fusion reactions
to superheat reaction mass, mostly hydrogen, and then spew it out at very
high velocity.  It happened to be doing this in a rather bursty way rather
than with a continuous reaction.

The Daedalus engine was not a terribly efficient one, but that's because
it was designed with the constraint of being as close to current technology
as possible.

>... Actually, if you can get
>controlled fusion at all (meaning you can control magnetic fields and
>deal with multi-million degree temperatures) it sounds like it would be
>more efficient this way...

Much depends on details.  Remember that a big continuous-fusion reactor
is likely to be heavy, and that matters.  Practical interstellar missions
need not only very high exhaust velocities, but also respectable acceleration,
otherwise the acceleration phase simply takes too long.  Accelerating to
0.1c at 0.001G takes most of a century; a substantial fraction of one
gee is pretty much a requirement.  (The rule of thumb for such calculations,
incidentally, is that the speed of light is roughly one gee-year.)

>I know that, in theory, you cannot get an Isp
>better than 30,000,000 (because it means you have an exhaust velocity of
>the speed of light), and I am curious as to how much you could get...

My recollection is that fusion peters out at a few million.  There is an
inherent limit because the fusion reaction requires a specific mass of
fusion fuel to produce a given amount of energy.  The net mass consumption
has to include that fuel mass.  Phrased another way, the exhaust includes
the "burned" fusion fuel -- keeping it on board just adds dead weight --
and this limits the exhaust velocity.  Unless I've goofed up on the math,
it turns out that dumping the spent fuel at low velocity while using the
fusion energy to accelerate other mass to high velocity always gives a
net loss in exhaust velocity.  The theoretical limit for fusion is when
all of the fusion energy goes into accelerating the fusion products.

If you want really high exhaust velocities, antimatter is better.  The
idea of antimatter rockets is now being taken very seriously.  We
(probably) know how to make antihydrogen cheaply enough to make them
viable.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu

Date: 6 Feb 90 16:52:01 GMT
From: zaphod.mps.ohio-state.edu!samsung!cs.utexas.edu!jarvis.csri.toronto.edu!utgpu!utzoo!henry@tut.cis.ohio-state.edu  (Henry Spencer)
Subject: Re: Spacecraft drives and fuel efficiency

In article <466@sixhub.UUCP> davidsen@sixhub.UUCP (bill davidsen) writes:
>| If you want really high exhaust velocities, antimatter is better... We
>| (probably) know how to make antihydrogen cheaply enough...
>
>  I think that's correct, but do we know how to store the antihydrogen
>well enough to make this interesting in any current timeframe.

We can't order antimatter storage facilities off the shelf from General
Antimatter Inc. :-)  Substantial engineering work would have to be done.
But the physicists store very small amounts for days at a time at high
velocity, and there doesn't appear to be any big problem in dealing with
larger amounts at lower (i.e. zero) velocity.  It looks to be practical.
Current vacuum technology is adequate, the physicists already have designs
for deceleration to near-zero velocity, and non-contact handling techniques
of several kinds can be demonstrated in the laboratory.  It could probably
be done in a few years if someone felt like funding it in a major way.

(There was a serious attempt, a few years ago, to get SDI to fund a big
push on antimatter space propulsion.  Three phases:  (1) engineering;
(2) pilot plant producing enough antimatter to test-fire an engine;
(3) production plant.  Too long-term for them, alas -- no payoff for
10-15 years.)
-- 
SVR4:  every feature you ever |     Henry Spencer at U of Toronto Zoology
wanted, and plenty you didn't.| uunet!attcan!utzoo!henry henry@zoo.toronto.edu

Date: 6 Feb 90 16:39:08 GMT
From: mailrus!jarvis.csri.toronto.edu!utgpu!utzoo!henry@tut.cis.ohio-state.edu  (Henry Spencer)
Subject: Re: Spacecraft drives and fuel efficiency

In article <48777968.20b6d@apollo.HP.COM> rehrauer@apollo.HP.COM (Steve Rehrauer) writes:
>>We (probably) know how to make antihydrogen cheaply enough...
>
>A question or three.  Given all the flack aimed at NASA, who is "we"?

Mankind.

>And roughly what would be "cheaply enough"?  "Viable" means "doable for
>one program", or "an alternative to conventional propulsion for evermore"?

Antimatter would have to be spectacularly cheap to be viable for launches
to orbit.  For in-space propulsion, however, at $50M/mg (yes, fifty million
dollars per milligram) it is cheaper than chemical fuels lifted from Earth.
At $20M/mg, it is cheaper than fission rockets.  At $10M/mg it beats fusion.
How much it would actually cost is open to debate, since the high-energy
physicists (who run the only current antimatter factories) have different
priorities and their equipment isn't ideal for volume production.  There
appear to be no fundamental barriers, last I heard, to bringing it in at
$10M/mg or less.
-- 
SVR4:  every feature you ever |     Henry Spencer at U of Toronto Zoology
wanted, and plenty you didn't.| uunet!attcan!utzoo!henry henry@zoo.toronto.edu

Date: 7 Feb 90 00:31:17 GMT
From: samsung!cs.utexas.edu!jarvis.csri.toronto.edu!utgpu!utzoo!henry@think.com  (Henry Spencer)
Subject: Re: Spacecraft drives and fuel efficiency

In article <7147@mentor.cc.purdue.edu> f3w@mentor.cc.purdue.edu (Mark Gellis) writes:
>... anti-matter sounds as if it involves technical
>problems that make fusion engines look like tinker toys...

The fun part is, it may actually be the other way around!  Antimatter
handling technology obviously needs a lot more work if antimatter rockets
are to be practical, and somebody needs to invest heavily in production
facilities.  However, there are few really basic problems involved, and
little doubt that the result would work.  The problem with fusion is that
it is fiercely difficult to make it work in the first place, and *then*
you have to make it light enough for a rocket.  (That last is not a trivial
issue; few of the existing controlled-fusion designs look like they could
be turned into rocket engines with useful thrust:weight ratios.)  Fusion
has a large head start, but antimatter research has been making far more
rapid progress in recent times.

>... (as I recall, you make He3 by  
>fusing deuterium, so if you have d-d fusion, you can make all the He3
>you want).

Unfortunately, making useful amounts of He3 this way requires truly vast
fusion plants.  The Daedalus study planned to mine its He3 out of the
atmosphere of Jupiter, because it looked easier!  They did look at making
it by transmutation, and at gathering it from the solar wind.  Both those
schemes lost out because they were far more difficult.  Making large
quantities by transmutation involved such staggering power outputs that a
civilization capable of doing that probably has better ways to power
starships.  Mining it out of the solar wind involved such huge collecting
areas that a civilization which can do that can probably build a Bussard
ramjet instead.
-- 
SVR4:  every feature you ever |     Henry Spencer at U of Toronto Zoology
wanted, and plenty you didn't.| uunet!attcan!utzoo!henry henry@zoo.toronto.edu

Date: 7 Feb 90 16:55:19 GMT
From: pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!uwm.edu!cs.utexas.edu!jarvis.csri.toronto.edu!utgpu!utzoo!henry@tut.cis.ohio-state.edu  (Henry Spencer)
Subject: Re: Spacecraft drives and fuel efficiency

In article <1990Feb7.024413.9969@cs.rochester.edu> dietz@cs.rochester.edu (Paul Dietz) writes:
>>>... anti-matter sounds as if it involves technical
>>>problems that make fusion engines look like tinker toys...
>
>>The fun part is, it may actually be the other way around!
>
>Really?  But we already have a proven method of releasing unlimited
>amounts of fusion energy -- hydrogen bombs.  Dyson designed an H-bomb
>propelled interstellar spacecraft years ago.  It had low acceleration,
>but it could reach .01 c.  I believe it used hydrogen bombs pushing
>against a copper plate, which was cooled by radiation.  A magnetic
>nozzle scheme might have better performance.

If we want to propel something with about the size and mass of Chicago,
Dyson's scheme works pretty nicely (although 0.01c is only marginally
viable as an interstellar velocity).  Unfortunately, it does not scale
down too well.  Dyson was figuring on 100MT bombs, as I recall.  Smaller
nuclear bombs are an expensive source of energy; very small ones are very
expensive.  When you get down into the range of interest for current
in-space propulsion, bombs basically aren't practical at all unless
you have a better way of igniting the reaction.  Which gets you back to
the problems of fusion, complicated by severe weight constraints.  A
spacecraft probably cannot use laser-ignited fusion unless lasers improve
vastly -- they are too heavy.  That's why Daedalus used electron beams,
which solves that problem at the expense of creating others.

Actually, a little bit of antimatter makes a dandy igniter for a fusion
reaction...!
-- 
SVR4:  every feature you ever |     Henry Spencer at U of Toronto Zoology
wanted, and plenty you didn't.| uunet!attcan!utzoo!henry henry@zoo.toronto.edu