Re: Mid-term Interplanetary Drives (long)

On Feb 5, 5:01 pm, phoe...@xxxxxxx (Damien Sullivan) wrote:
CharlesRCap...@xxxxxxxxx wrote:

the assumptions is that fusion is only recently been made reliable
enough to use for power generation, it is large, expensive, massive,
fragile, and twitchy which means that it is prone to shutting down
unexpectedly and occasionally tends to melt itself spectacularly. The

Fusion plants don't have "meltdowns", though I could see a magnetic
bottle failure damaging the interior of the reactor, requiring
replacement. Not that spectacular, except to the accountants.

[...]

(Since fusion is still considered experimental for power generation,
such installations are not emplaced on planetary surfaces, and instead
placed in high orbit.)

That's totally unnecessary.

Has hollywood lead me astray again!?!
In all reality, that's fine. I was never really wedded to the idea.


behind your craft for thrust would be a bad idea. Finally, the
propulsion needs to be able to send the craft to the outer system and
back in a reasonable timeframe without refueling. Keeping the fuel to

Define "reasonable timeframe".

6 months to Saturn, 1 year to inner Oort cloud objects with refueling
at destination. Let's say double or at most triple that without. This
means that it would be unlikely for anyone to send warships to the
outer system without a guarantee of friendly refueling at some
location. (For example an attack on an installation in Saturn orbit
might overfly the attack point and turnover for refueling somewhere
further out.) I'll go with that as reasonable. Shorter would be better
of course.

You'll want to attend to the likely thrust/acceleration of ion drives,
though I don't have numbers handy.

They are miniscule. Those same back-of-a-bar-napkin calculations I
mentioned and cannot find at the moment suggested very fractional g
accelerations. I think the highest with xenon fueled ion drives was
something like 0.05 g assuming a reasonable warship with a fair amount
of mass devoted to drives. (I wish I knew what I did with those
calculations.) The VASIMR drive has much better acceleration numbers I
understand.

right angles from the ecliptic.) Of course sending sensor satellites
into orbits that take them high and low over and under the ecliptic
would be a normal precaution to detect such craft, but they would also
be the target of close scrutiny of the opposing side to track their
positions. This would result in warship captains needing to turn off

I'd suggest that if you hope to be able to stealth a large 300 K manned
warship, you can probably stealth a small unmanned low-temperature
satellite.

On balance I agree with you. However, I'm thinking that there is
strategic stealth, operational stealth, and tactical stealth. The
other side is probably going to know that you have launched such a
satellite, and has all the time in the world to look for it and plot
it. After all, it's not going anyplace unexpected once you find it. On
the other hand, the satellites could be far more capable, all but
mobile ships in their own right. I would think then that they would be
harder to hide (needing more power for more powerful drives) and also
much more expensive. Or just try to flood the north and south with
cheap satellites. Both of these routes are good and have plenty of
story possibilities. I'm leaning towards there being a very hot "cold
war" going on where each side takes out the other side's satellites as
they detect them. Also there is the concept of tactical stealth. If
the other side is relying on the intel from these satellites in
distant orbits to the north and south, how old is their data once it
gets to them? Hours perhaps? That's certainly good enough for tactical
purposes. It's hard to aim at something if you only know where it was
an hour ago.

As for scrutiny, once a warship has revealed itself -- in combat, or
when first launched -- how does it ever reacquire stealth? You've got
one set of ranges for a wide-angle camera scanning the sky for warm
bodies; you've got another range for a telescope trained on a ship and
tracking anything it does. And telescopes should be cheaper than ships,
so a big military could afford one for every ship in the system.

Once you have revealed yourself, yes it's going to be mighty hard to
hide yourself again. It would take something like going to "silent
mode" (minimal settings on the reactor or running off batteries) using
lower energy ion drives to try and put enough distance between where
you ought to be and where you are. Could take weeks or longer or you
might never be able to regain stealth on that opperation. I was
considering (before I posted) how to launch such an operation without
the other side watching every move. I came up with something that
might be workable. Ships are docked and charge batteries from a
stationary installation. They are then launched away from the station
using an electromagnetic catapult, putting distance between themselves
and the station before they need to light off their reactor and start
putting out enough heat to be detectable. It would be slow and might
not fool them at all if they have spies in the area, but it's better
than nothing.

the dispersal pattern.) Long range is the domain of the missile. With
ranges of up to 10 light seconds, missiles are large, expensive, and

How does a missile in space have a range?

The missile needs to be able to make course adjustments to keep up
with where the target ought to be when it gets there. This location
where the target ought to be happens to change a lot (and is even more
problematic if the missile can't see the target and must then try to
anticipate where the target will be with data that could be
considerably older than 10 light seconds if it has to rely on intel
passed in from a satellite.) The missile needs enough fuel to keep up
with the target's evasions as well as generate enough delta-v to make
itself a difficult target to hit on terminal approach. The 10 light
second range is rather glib and depending on the circumstances could
change quite a bit. I'm willing to entertain ranges much longer than
10 light seconds if the missile becomes larger with even more fuel.
However we're dealing with small ships here which are fairly good at
shooting down missiles. So you need to carry a reasonable number of
them. I'm thinking somewhere in the vicinity of 20 to 40 at most (for
a capital ship) with a maximum effective range of something like 10
light seconds (considerably shorter in a stern chase, and more in a
head on encounter.) Now, this is just an off the cuff guestimate (on
the 10 light second range) I don't have any specific figures to back
it up. I'm a computer programmer by trade and education, not a rocket
scientist or physicist. =) So I'm certainly willing to accept the
advice of more knowledgeable folks. If say something the size of a
trident missile (59 tones, 13.5 meters long 2.1 meters diameter) would
have a longer effective range using a chemical drive, then let me
know.

Engagement Range: From 1/2 light second to 10 light seconds.

How is a projectile or missile (speed: 20 km/s) useful at a range of
half a light-second (150,000 km, needing 30,000 seconds to traverse)?

"150,000 km"

Off by 3 orders of magnitude on C, but then again so was I. I put
300,000 km/s as C into my calculator and not 3,000,000 km/s. So it
should be 1/2000 light second. That's a pretty hefty logic error in a
book. Glad you caught me on it.

Limited self guiding projectiles would have a maximum effective range
of around 150,000 km. Crossing that distance in 125 minutes. Even with
limited self guiding this is a low probability proposition. They make
up for their inaccuracy with numbers. A capital ship might have only
20 to 40 missiles, but it will have enough rails to launch hundreds or
more likely thousands of railgun projectiles.

I'm looking for "tooth and nail range" to be the range that a 2 meter
aperture can focus a 5 megajoule laser to deliver 1 megajoule per
square cm. (I don't have the formula to work that out handy, but
that's I'm thinking about for the range.) (also, is that going to melt
the mirror? I used to know, I can't remember anymore though.) Knife
fighting range would be the range that same laser could be focused to
deliver 10 kilojoules per square cm. (or whatever value would be
guaranteed to burn out sensors) short range begins where the chemical
driven projectiles end their useful range, or about 2,500 km. (which
now that I think about it, might not be longer than the laser range...
I really need to find that formula again.) The chemical fired
projectiles would have a flight time of a little over 5 minutes. Not
good odds to hit a small, evading missile, but they can put a lot of
projectiles down range, so they might get lucky at that extreme range.
So short range would have a flight time of 125 seconds for railgun
rounds, which along with harder burns from their self guiding system
and near real-time data, pretty much ensures mutual destruction.
Medium range would begin where such rounds could be reliably dodged
(maybe 18,000 km? 15 minute fight time.) and end at around 150,000 km.
At which range missiles come into their own. Not that you can't fire a
missile at a target closer than 150,000 km, but it would be unlikely
that you would get to that range without expending your missiles
first.
.

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