On Feb 23, 8:42 pm, "dwight.thi...@[EMAIL PROTECTED]
"
<dwight.thi...@[EMAIL PROTECTED]
> wrote:
> On Feb 23, 8:17 pm, IsaacKuo <mech...@[EMAIL PROTECTED]
> wrote:
> > On Feb 23, 7:56 pm, "dwight.thi...@[EMAIL PROTECTED]
"
> > <dwight.thi...@[EMAIL PROTECTED]
> wrote:
> > > What is this HARD you speak of? Some numbers, please? In fact, an
> > > arrangement that covers 1% of the sky or less should be assumed to
be
> > > doable. Assume the mirror is 99.99% reflective.
> > Why would I assume that? The only mirrors we have which are
> > that reflective are only so reflective at extremely narrow
> > bandwidths.
> Uh-huh. You've an economic power, or powers that can support a huge
> space industry. Possibly parts of the solar system off Earth are
> permanently manned if not outright colonized. You've got all sort of
> toys like nuclear thermal rockets, at the least, but at any rate,
> extremely advanced technologies beyond anything we can achieve today.
I generally don't assume the existence of magic technologies,
even if it's entirely plausible that some sort of science which
today would seem magical will be developed.
> Yet mirror technology improves not a whit. Even though there is
> nothing in the laws of physics that forbid such reflectivities, and
> even though such reflectivities have been achieved for certain
> wavelengths.
Maybe there will be clever designs or new science which gives
us neat broadband highly reflective mirrors. Maybe not. I don't
assume the existence of them.
> And you think this is realistic? Or do you think, as I do, that this
> is swallowing camels and straining at gnats? If not, why suppose all
> these other advances, but not advances in optics(and rather
> pedestrian advances at that.)
What other advances do you think I'm assuming? I tend to be
rather conservative in my speculations on future space
technology. The only specific "toy" you mentioned was
nuclear thermal rockets, which I generally do NOT assume
will be used very much, if at all.
I don't like the general economics and technological
developmental path of nuclear thermal rockets, compared
to solar electric. Solar electric is already a mature technology,
and capable of outperforming nuclear thermal. Nuclear
thermal has never even flown once. Solar electric costs
less, it performs better, it doesn't have political problems,
and it's already flown. The basic technology of solar electric
has extensive civilian terrestrial applications, which means
lots of R&D money and effort to make it even better. In
contrast, the sort of nuclear reactors suitable for nuclear
thermal have no civilian applications and practically no
terrestrial military applications. And what's the potential
eventual payoff? A drive system which underperforms
compared to cheaper solar electric. So no, I don't anticipate
nuclear thermal drives.
This is representative of my approach to future technology
speculations--I like to be conservative. If something exists
today, works well, and has great potential for continued
development, then I like it. If not, then I'll need some
convincing.
> > I've done numbers on various "stealth" radiators before, usually
> > coming from the perspective of trying to design one (i.e. I was
> > "pro-stealth"). I don't remember the specifics, but basically I
> > settled on a design with a 60 degree radiation cone. I wanted
> > to design one with a 15 degree radiation cone, but the numbers
> > never came anywhere close to adding up.
> Instead of saying this, why don't you just show the numbers?
Because I don't feel like looking them up or redoing them. You
don't like that? Well, deal with it. I simply don't feel like going
through the effort.
> > If you have a better design in mind, I'm all ears.
> What? A radiator whose output is redirected by an advanced, actively
> cooled optics system?
> Something along the lines of a paraboloid with the radiating surfaces
> at the focus?
> There's not a whole lot more to say.
That's the design which I had worked on before, and rejected
it on the grounds that the amount of active cooling required
exceeded the amount of power available. Well, I didn't
"reject" it, exactly, I just wrestled with the numbers until I
got something which worked about as well as I could make
it (a 60 degree cone).
If you have nothing more to say, then color me completely
and utterly unconvinced.
> > Just using basic intuition about how long it takes to get around in
> > the outer solar system. Even with 300km/s class drives, it takes
> > decades to get around.
> Really? 20th C Earth did that . . . without even 30 km/s exhausts.
> Your numbers don't say what you think they do.
We used gravitational assists and started off from a nice fast
moving Earth, and the only way to get somewhere fast was
a one way flyby. If you want to get around in the outer solar
system, you need to provide all of the delta-v, and you need
4x the delta-v to do a return journey.
Also, consider that Neptune if five times further away from
the Sun than Jupiter. If you want to get from Neptune to
Uranus, it will take decades even with a high performance
drive.
> > > But so what?
> > > Why shouldn't these sorts of manuveurings take decades?
> > Well, if you don't mind the enemy knowing exactly where you
> > are at all times, because he's suspicious of this rocketship
> > zooming out beyond the sensor network at high speed and he
> > tracks it with active sensors...then fine. You could very well
> > do some "stealthy" maneuver with a heat signature he can't
> > detect because you're beyond the passive sensor network.
> > But he's tracking you with active sensors anyway, so your
> > patience is a wasted effort.
> But now you're invoking something else: what are these 'active'
> sensors you speak of, how do they work, what is their range and
> resolution? Swallowing camels again?
I don't care for your tone. Are you interested in convincing
anyone of your argument or are you more interested in just
insulting others?
The relevant active sensors involved would be UV wavelength
lidar. I've had discussions about active sensors on sfconsim-l,
and it's an interesting interplay between beam size and
photon energy. More energetic photons can be focused
more narrowly, but for a given beam power there are fewer
photons so your get less of a return. Also, low energy
photons can be stealthed against, by directing reflections at
off angles. It seems the ideal photon energy would be
somewhere in the UV range, which is low enough to get
a decent return signal but difficult or impossible to stealth
against (Thomson backscatter).
Now, if I feel like it I'll do the math for a specific long range
sensor design.
The basic design is straightforward enough. You'd have a
free electron laser ship in formation with a fresnel lens
drone several thousand km away. The fresnel lens may
have a radius of perhaps 100m or maybe 1km. The lens
both focuses the outgoing laser beam and focuses the
return only a germanium based detector. It only takes
one or two photons for a detection. From here, it's a
matter of math. That's effort, and frankly I don't feel
like doing that effort if it's for the benefit of a rude bastard.
> > Yes, the enemy knows where the sensor drones are. So what?
> > It doesn't give the enemy any particular capability to do anything
> > about it.
> It doesn't? They can't be evaded then, or taken out, or spoofed, or a
> combination of the three or something else?
They're too far away to be "evaded" in any meaningful
way.
You could perhaps shoot missiles at them, but it'll
take years for the missiles to reach them, during which
time the enemy is going to be launching replacement
sensor drones.
You could perhaps shoot long range interplanetary
X-ray lasers at them, which could be a significant
strategy. However, if the enemy also has
interplanetary X-ray lasers, the enemy will be using
them to shoot at your lasers; your lasers are "wasting"
firepower on sensor drones while the enemy is withering
away your own firepower.
Spoofing is a more interesting possibility, but it's
going to be an interesting challenge to try and fool
all of the sensor drones simultaneously in any way
cheaper than just doing the real thing. For example,
suppose you want to fool them into thinking that a
warship is being launched in some direction. You
want some sort of flare which matches the signature
of the drive, and accelerates the same amount as
the drive. If the enemy only had one or two sensor
drones, then maybe you could do something cheap
and clever by pointing lasers at them. But if the
enemy has dozens or hundreds or thousands of
drones, it may be cheapest just to slap a real space
drive onto a cheap rock and fly it just like a "real"
warship.
Isaac Kuo
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