
Starship Radiators - user_235711
http://www.icarusinterstellar.org/starship-radiators/
======
Sharlin
One of the small things I appreciated in _Avatar_ was the huge red-hot
radiator arrays clearly visible on the starship ISV _Venture Star_ :

[http://static3.wikia.nocookie.net/__cb20100127043621/jamesca...](http://static3.wikia.nocookie.net/__cb20100127043621/jamescameronsavatar/images/1/1f/Isv.jpg)

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Udo
This reminds me of a SETI idea: looking for infrared signatures that could
match the thermal waste output of interstellar ships.

This comes with an interesting model that has far-reaching implications, such
as "should we have detected these a long time ago?" Of course the method comes
with all the usual Fermi caveats, namely that space is big and the statistical
likelihood of the effect happening nearby enough for us to see it.

~~~
dalke
A "thermal waste signature" is black-body radiation. We have infrared
telescope which can pick up red dwarf and brown dwarf stars, even down to a
surface temperature of 300K.

The primary difference would be the lack of spectral bands. That would be
pretty easy to pick out, and so I don't think there's an advantage to having a
SETI search with this as the primary goal.

~~~
Udo
I'm sorry, I should have been clearer. When I talk about signature, I don't
only mean the radiation itself but also position data and other markers that
would help isolate the signal from astronomical sources, and - as you said -
spectral information.

 _> That would be pretty easy to pick out, and so I don't think there's an
advantage to having a SETI search_

So, because you say it's an easy search that's an argument why we shouldn't do
it? It's not at all clear that we should have discovered this already by
accident, if that's your point. I say it's incredibly hard, and also that it's
worthwhile.

~~~
dalke
You left out the rest of the quote "with this as the primary goal."

Any search which can pick out a small hot interstellar spaceship will be very
effective at searching for brown dwarfs and other low temperature objects in
space. ("Low" means "not a fusing star".)

I doubt there's anything you can do to optimize a SETI search which makes the
identification of brown/red dwarfs more difficult. In fact, since you don't
know what the background low-temperature IR sources are, you would need the
brown dwarf survey to help characterize what's normal vs. a possible
spaceship.

There's already funding for dwarf studies, and it's an area of astronomy which
is relatively new and unexplored. (The first brown dwarf was not discovered
until 1995, for example.) So we know that's a proposal that many can get
behind.

Identification of possible non-stellar objects falls out necessarily and
almost trivially (compared to the survey itself) from the data.

Position and velocity data is also important for brown dwarf observations, so
you'll get that anyway.

So you can tell me - why should we prioritize a very low probability SETI
search when there's a high probability that a brown dwarf survey will be
scientifically useful, and end up with useful SETI information?

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ackydoodles
From the FAQ:

Q: Why did you choose fusion based propulsion for Project Icarus?

A: Well, we thought about it and we realized that the only appropriate power
source for an imaginary starship was a likewise imaginary one. And while there
are certainly many options in the field of imaginary power generation, we
rejected some of the main contenders for a variety of reasons.

"Maxwell's Demons" would have alienated both the new-agey spiritual sci-fi
crowd and also the fundamentalists because it sounds medieval and just evil at
the same time.

"Dilithium crystals" was taken. The zero-point people are bunch of wack jobs
and we certainly didn't want to be tarred with that brush. "Fairy wings" just
sounds too wussy for our tastes.

So, you see, there aren't all that many choices of power left for an imaginary
starship. Fusion sounds just about right, especially if you say is slowly,
with gravitas, like "fuuuuuuusion".

~~~
ctdonath
Tangent: fusion will require carrying...how much hydrogen? I once calculated
that trying to gather it along the way (interstellar hydrogen) is pointless,
as a 1m^2 swath from here to Alpha Centauri would net 0.01g of the stuff.

ETA: Yup, I was considering the Ramjet design. Scale of the scoop overwhelmed
itself trying to acquire enough to function. I was just startled at how
_little_ interstellar hydrogen there is; I knew it wouldn't be much, but 0.01g
in ten million cubic kilometers was even less than I'd assumed.

~~~
lutusp
> ... how much hydrogen? I once calculated that trying to gather it along the
> way (interstellar hydrogen) ...

FWIW this is an old sci-fi idea / plot device called a Bussard ramjet:

[http://en.wikipedia.org/wiki/Bussard_ramjet](http://en.wikipedia.org/wiki/Bussard_ramjet)

But this isn't meant to dispute your calculations. The linked article comes to
the same conclusion: it's not feasible, too little interstellar hydrogen.

~~~
ackydoodles
Most notional implementations of the Bussard Ramjet depend upon some sort of
heretofore undiscovered force field rather than a physical structure to
collect hydrogen, because size matters.

Naturally, a very large force field ramjet will deplete interstellar hydrogen
along the main routes between stars, leading to a situation known as "peak
hydrogen", and possibly prompting the invasion of developing hydrogen-rich
planets on some sort of flimsy pretext.

Most scientists working on imaginary spacecraft projects therefore eschew the
Bussard, correctly anticipating the moment when, shortly after the leader
gives a speech on the deck of a starship in front of a huge "Hydrogen
Accomplished" banner, the price of hydrogen rises to over $5 a gallon and
starships are all left to rust in the front yard.

~~~
lutusp
Yep. And shortly after his victory speech, the leader is heard to loudly
complain that he's been "misunderestimated".

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arethuza
"the waste power from the fusion reaction can be 15,000 GW"

That reminds me on a project I worked on where we were getting the operations
of the Hunterston B AGR explained to us and I asked an incredibly stupid
question - given the 1,500 MW thermal output of each reactor and ~700 MW
electrical output, where does the rest go. The answer is, of course, "to heat
the Clyde".

15,000 GW is a _lot_ of waste power!

[Edit - the power figures are per reactor - with each AGR station having two].

~~~
glimcat
Waste power is heat to first order.

If there's a lot of it and you're not making it go elsewhere, assume that the
system and it its environs are going to get a bit toasty.

Sometimes you also get things like radiation and vibration. These _mostly_
show up in meaningful quantities in predictable places - nuclear systems,
mechanical systems, high-power A/C or RF systems, high-power transducers.

You'll usually do okay if you guess that the system uses energy at around ~60%
efficiency, which is enough to give you some idea of the heat to expect if you
know the input power.

Or more generally, assume all input power goes to heat and plan to handle
operation at that limit - while being aware that your expectations of power
consumed by the system may well be low.

~~~
davedx
Is it possible to use thermocouples to convert waste heat to electrical
energy?

Edit: Or maybe one of these bad boys?
[http://physicsworld.com/cws/article/news/2013/dec/09/new-
gen...](http://physicsworld.com/cws/article/news/2013/dec/09/new-generator-
creates-electricity-directly-from-heat)

~~~
glimcat
Some of it, yes.

But they tend to have low efficiency, thermally-dependent efficiency, and the
need for a temperature differential. They operate in response to a temperature
gradient, not just heat.

Using Wikipedia for a quick-and-dirty figure, you can expect efficiencies to
peak around 5-8%. And you can't just stack infinite thermocouples until system
efficiency reaches 99.99% because of the thermal gradient issue.

~~~
ForHackernews
Temperature differential should be easy to come by in interstellar space.

~~~
glimcat
You need _mass_ that's at a lower temperature. Vacuum is an insulator.

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outworlder
For anyone wanting to play with fictional starships and radiators, I'd like to
suggest Kerbal Space Program with the Interstellar Mod. It adds fission,
fusion and antimatter reactors with all their drawbacks( _), generators (even
gives the carnot efficiency), and of course radiators, with the generator
efficiency depending on how hot the reactor is vs how cold the radiators are.

(_) fission produces waste subproducts, have to be shutdown for maintenance
and cool down, fusion requires an electrical generator to sustain the fusion
(also fuel, have to centrifuge deuterium etc) and of course antimatter have to
be collected and stored in magnetic containment devices which require power to
maintain. etc.

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ackydoodles
If you want to build a functioning starship along the lines of Icarus by the
year 2100, there is a lot of basic science that has to be done. It is very
difficult to predict the course of basic science, because you don't know what
you don't know, and it is therefore hard to anticipate how far away the
frontiers might be.

The main problem with Icarus is that it aims to create a ship that can do the
trip on the scale of a human lifetime. With space travel, the amount of energy
required, and therefore the difficulty of the project, is related almost
exclusively to how fast you want to get there, the delta-v, if you will.

Therefore, it is unreasonable for Icarus to set a timetable.

If you really want to set a timetable, there is an easier way: stop worrying
about how long it takes. Concentrate all of your effort into curing death, and
then build a starship with a solar-sail, or some other sort of
environmentally-friendly, but slow, technology.

The advantages to this approach are many: easier to get funding, direct
economic payoff, no difficulty convincing people that your effort is
worthwhile, and, oh yes, you get to cure death.

Then, while you are aboard your starship, you can spend the hundreds of years
(say) that it takes you to get to Alpha Centauri further advancing science. If
you are adequately equipped, as a nuclear-pulse propulsion vessel would likely
be, you can invent better propulsion technology as you go along.

In a sense, the problem of human senescence and its attendant illnesses and
the problem of interstellar travel are one and the same. If someone asked me
to solve the latter as quickly as possible, I would begin by solving the
former.

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keenerd
I'm pretty sure any sort of practical space based radiator system would not
use circulating coolant but would probably use heat pipes. No moving parts and
for large systems you generally use many small pipes in parallel. Combine
these and you get an extremely reliable system that fails gracefully. Which
sounds like a good idea for any sort of nuclear reactor safety system.

The Shuttle used a combination of heat pipes and circulating freon (and dumped
the waste heat during ascent into the cargo bay, poor cargo!), but also did
not have a nuclear reactor on board.

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bane
Could this thermal waste be focused and directed? If so couldn't it radiate
out in a useful way, like maneuvering?

~~~
icegreentea
To focus and direct it, you would have to interact with it with real physical
things, which gets you right back to square one - now you have this hot thing
and you don't want it hot.

~~~
bane
Well yeah, but let's say you radiate out as planned, but near the front of the
spacecraft you expand a very large reflective dish, like a lightsail, say a km
across. Now you're slightly biasing the direction the heat is radiating and
the spacecraft will have to move in the opposite direction. Small changes in
angle of the dish will move the vector of the reflected radiant heat.

In other words, you could use the main engines to get to Alpha Centauri, then
use the radiant heat for the precision maneuvers around the system.

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onion2k
I wonder how you'd bleed it?

:)

