Re: General Setting/Tech Musings

While this can be clean if you only use hydrogen as fuel, it is not
necessarily super efficient in terms of percentages.

Whether that is a problem or not depends on what happens to the unused
energy, and how much usable energy you actually get out of 1kg of
fuel. Unfortunately, I have to admit I didn't understand all of your
treatise.


Thus, you can stop most of the pions by
conducting your reaction in a ball of condensed matter a couple of
meters across.

How dense does this matter actually have to be? We could get a mass
problem here. A ball of Lithium (I presume that's what you had in
mind, correct me if I'm wrong) with the smallest possible radius (1m)
will mass about 2t. That's not too bad I think, but I guess we'll also
lose a lot of pions there. Using a radius of 2m however bumps us up to
almost 18t (so it might be smarter to use 9 1m-balls instead, provided
the remaining radiation won't kill the crew).
If by "condensed matter" you mean something like tungsten, we're
really in trouble. ;)

If you do not use hydrogen as fuel, the nucleon decay creates a lepton
but most of the rest of the energy goes into disintegrating the
nucleus, as if it had absorbed a pion. Fortunately, this means much
less penetrating radiation. Unfortunately, it gives you radioactive
debris.

Since we're talking about space drives here, I'm not overly concerned
about radioactive debris. Who cares if it gets out. Won't harm the
void. Would be a different matter altogether for planetside use.

There are more exotic possibilities. If you can generate really
powerful magnetic fields,

Let's assume this,

unionized fluids

The fluids are organized in unions? ^_^ *scnr*

is perhaps around 10% efficient, if
used as a first stage on a combined MHD/turbine, you can increase your
efficiency a bit.

So, first let's be a bit generous about the efficiencies and give the
heat engine some 70% and the MHD stage about 15%. If I understood the
principle right, that would give us 75% overall efficiency (1 - 0.85
waste * 0.30 waste). That about right?

Pion rockets have an absurd
exhaust velocity, on the order of 95 % the speed of light. You don't
need much propellant, but your 100 MN of thrust becomes 30,000 TW.

Holy rusty metal, batman! I really had to grin about "absurd" (I am
generally easily amused).
We don't need 100MN of thrust in the main stage, though. 10kN per ton
(total mass) will suffice, provided it can be decreased on demand with
an afterburner. So a 500t ship would need about 5MN, but still 1.500
TW sounds quite insane and like nothing you'd let anyone play with who
might possibly get ideas. A one-megaton bomb every three seconds
sounds quite bad enough.

It is probably easier to use an exhaust velocity of 7,600,000 m/s and
a mass flow of 13.2 grams per second at 100,000,000 newtons of thrust
(with an energy output of 380 GW per 100,000,000 N).

Wait. What? I think you have a power error there. 7.600.000m/s *
0,0132g/s = 100,000 N. Would have been too good to be true. ;)
I think the most desirable exhaust velocity would be something like
30-37.000.000m/s. In an ideal world, H-H fusion with 720TJ/kg and
near-1.0-efficiency would burn 0,027kg/s to hype the ash up to
37.000.000m/s for 1 MN of thrust.

Note that both 3He and D are available in gas giants, so you likely
get fuel "mining" operations there.

Okay, good to know.

Heh, this is a tautology. The definition of a brachistochrone is the
path of least time.

I should note that I've been into "hard" SF only for about half a
year, so I never bothered about that kind of thing before and was
quite surprised when I learned all that stuff about Hohmans and
brachistochrones. Anyway, least time yes, but I was surprised to see
that of two journeys of identical length and duration, the
brachistochrone costs just a fraction of the energy (say a fifth) than
the corresponding impulse trip.

That said, I should add that I kind of planned the jump points at
about 10AU distance from the sun, so arriving earthbound ships have to
cover about 9-11AU post-jump (refueling at the jump point may be
possible), and such a trip shouldn't take longer than a couple of
weeks. Hence the need for a brawny drive.

This is easily done, just inject a bit of extra propellant into your
reaction bell and let the x-rays and flying plasma turn it into hot
plasma as well.

That's how I figured that as well.

I had a few further thoughts on this - the drive bell has to support
the entire weight of the ship at 1 G (or at greater than 1G if it is
accelerating harder) - think of a spacecraft under 1 G acceleration as
if it where sitting on its drive bells on a planetary surface under 1
G to see the stresses involved.

Talking about drive bells -- is there no way to get rid of that huge
bell altogether? How would the exhausts of those mass-killer drives
look like? Given a very good control of powerful magnetic fields as
postulated above, could we attain a (possible fold-out) drive bell or
exhaust array just a few metres across?

The diamondoid structure sounds good, though. How would that look
like, literally crystal clear like jewelry diamonds? I'm asking
because I was already off with my envisioning of lasers. ^^
.

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