Re: General Setting/Tech Musings

On 10 Jul., 01:59, Luke Campbell <lwc...@xxxxxxxxx> wrote:

On the other hand, there is plenty of justification for directly
converting matter into energy.<snip>

Oh, great. I never heard of any of those. These sound sweet. I'm quite
tempted to do another about-face. But by now I'm a bit scared of
opening the can of worms that a clean, super-efficient, 9e18J/kg
energy source can be. (If it's not that efficient, you get TWs of
waste heat which would be impossible to handle). For one thing, I
don't want TW lasers frying the living hell out of everything. On the
other hand, you're still going to need propellant so the actual mass
ratios wouldn't change so much, I guess.
If it should turn out that either of these magnetic monopole or Q-ball
annihiliation methods would lend itself to fuelling a drive but not an
electricity plant, it might be just about ideal.

Nevertheless, D-3He fusion should let you get around the solar system
in reasonable times, even if you can't keep continuous maximum
acceleration the entire time.

Let's see -- D-3He has a yield of about 350TW/kg, right? It would make
fuel a bit more of a commodity, since D and 3He aren't quite as easily
available as regular hydrogen. That's not a bad thing, as story
devices go. But the energy yield may be just too small for fast
interplanetary travel.

is MN supposed to be MW?

I meant MN, as in Meganewtons of thrust. IIRC, you need 1MN to
accelerate 100t with 1G.

Anyway, you already included magical FTL. If you also introduce "some
kind of field" you might as well have the spaceships go "whoosh" as
they whiz by and pull banking turns.

Ouch. XP

Note that 25 hours of 1 G acceleration will boost you to a delta V of
900 km/s. This can get you from earth to mars in a time between 20
and 120 hours, depending on their orbital configuration.

When fiddling with the numbers, I discovered that brachistochrone is
always more efficient than an impulse/coasting journey. Rather than
boost at 1G, coast and then brake at 1G, running a brach at some
fraction of a G can get you there in the same time at the fraction of
energy cost, all due to the square in E= 1/2 mv². That's why I want
variable thrust engines.

You can make the drive bell look sexy-looking. It can be stored in a
compact configuration, and unfurled in a delicate filigree of
glittering lacy wires and gleaming drive components which then lights
up with a blinding actinic glare as the craft surges away.

That sounds kinda cool. ^^ But I bet it would be very fragile (think
of kinetic slugs of any sort).

If you want to remove heat quickly, you can use diamond (which has an
anomalously high thermal conductivity and can withstand withering
temperatures before it decomposes into graphite) and active heat
transport with a working fluid pumped through pipes and heat
exchangers.

Sounds fair enough. Concerning that fluid, would liquid Lithium be the
element of choice? I suppose that would mean you have to keep the
whole system on temperature at all times, as letting the Lithium get
cold and solid would clog the pipes and ruin the system.
Let's also talk about droplet radiators for a bit. What would be the
best coolant, also Lithium, or something else?

Once you have GW fusion reactors, you can light up an enclosed reactor
and use it to directly heat air. Use variable bypass turbo-air rams
to scoop up and compress air, heat it with the reactor, then shoot out
a jet at much higher velocities. <snip>

Sweet, thanks for the ideas. You still need of auxiliary engine to
attain the necessary speed for the ramjet to work, right? Or maybe a
bimodal turbine/ramjet engine, like a futuristic SR-71.

Those more versed in economics than I will probably tell you that even
these other cargoes will likely benefit from the faster drives.

You are probably right, I'm going to play with the numbers a bit to
get an estimate. Hydrogen as a fuel is gonna be dirt cheap, so it
would probably pay to burn more of it if that means making more round
trips. My thinking was that slow Hohman transits allow for much lower
mass ratios, like 1.03 as opposed to 2 or 3. However, I just looked at
Hohman synodic intervals and transit times and have to agree that they
are pretty abyssmal and not economic in a world with working fusion
drives.

Note that unless you are in an atmosphere, you will not be able to see
the beams.

I had no idea. Duly noted.

problem is wicking the heat away from the laser crystals/
semiconductors/whatever to the radiators.

So the lasers will also need an active cooling cycle like the engines,
with the difference that you can't use a closed cycle with liquid
metal as coolant because the laser isn't going to produce heat all the
time. So you're limited to either just diamond heat conductors or
plumbing with some other, less awkward coolant. Liquid metal open
cycle cooling should work, however. Which means your laser needs ammo.
If (and I mean IF) you can vaporize the Lithium coolant and "charge"
it up to 3000K before ejecting it, you get rid of about 10GJ per ton.
That figure should also help us to determine workable laser powers.
Say how many tons of expendable coolant you're willing to lug around,
and how many shots you want to fire, the rest is doing numbers. For
instance, if you can spare 10t for laser coolant, and want to fire at
least 1000 0.1s-bursts, and have 50% efficiency, that's 100GJ waste
heat for 100s of fire --> you can have a 1GW laser (if the rest of the
technology plays along, like physical limits on the mirrors and the
like).
If you have to eject the Li right after it evaporates, that figure is
reduced to <6GJ/t at best.

FWIW, I think that due to the military situation (or lack thereof) in
my setting, space lasers are currently produced for two purposes: one,
as point defense against incoming kinetic slugs, natural or otherwise;
and two, if required by the tech chosen, for power generation (as the
laser-induced fusion you mentioned).
There simply won't be powerful dedicated shipkiller deathrays because
there is no space navy. Earth will not be unified, rather arranged
into blocks/nations/alliances, but even in the event of war this will
take place on the ground. I believe planetary defenses will beat
spaceborne attackers every time, due to superior camouflage, power
supply, and above all, cooling.
By the time we enter the setting, the first colonies (Mars or
extrasolar ones) may feel a compelling drive to independence, but the
first independence war will be fought with refitted freighters and
couriers rather than dedicated warships. I also think that's going to
be much more fun.

Again, thanks a lot for your input, Luke, I appreciate it and try to
adopt as much as possible.
.

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