Spacecraft heat management
Matthias Warkus
OK, if there's anything common sense and the perennial threads about
the topic in this group teach us about travel and combat in space,
it's that any spaceship, and especially a spaceship that wants to use
weapons, will need a radiator to dissipate excess heat. With a space
warship, obviously the aim of radiator development is to have the
radiators dissipate as much heat per unit of area as possible: To keep
the hull area taken up by the radiator low; to minimise the
performance hit caused by losing a fraction of the surface in combat;
etc.
As a laymen, I imagine a radiator as a simple grid of tubes on the
outer hull, with a fluid running through them and a reflective foil
behind them.
My questions: Which is the most effective radiator design you can
imagine for a future spaceship? If possible, it shouldn't be
handwaved, so neutrino beams are out :)
Furthermore, what is the best medium to transport heat inside a future
spaceship? Liquid helium (as used by Star Trek, ugh) sounds sexy, but
might not be too sensible. Sodium seems to be quite popular with heavy
nuclear equipment, but I don't know whether running a grid of pipes
full of liquid sodium all through a spaceship is all that sensible.
Gaseous media need to circulate at quite a rate to accomplish
anything, but they've got their advantages (i.e. you can use an inert
gas and there won't be much trouble from leaks or corrosion).
You get bonus points if you cannot only give me breakthrough designs
for a heat dissipation system, but also reasonable estimates for how
many joules per square metre of radiator surface it will actually be
able to radiate off :)
A space warship might also prefer to keep its radiators retracted,
collapsed or under armour (no discussions about spaceship armour
please, I know the issue) for less vulnerability and store excess heat
in a "heat sink" during combat to dissipate it later. Here again, as a
layman the best I can imagine is something like melting a huge lump of
ice or (more contemporary) a pool of special salts, as done in solar
power plants to store energy.
Big thanks to anyone who helps me. As you can guess, I'm busy at
scientifically hardening a couple of stories.
mawa
--
Verba volant, scripta manent!
Christopher Neufeld
Matthias Warkus <ma...@iname.com> wrote:
>
>My questions: Which is the most effective radiator design you can
>imagine for a future spaceship?
>
large physical radiator, heat is dissipated by spraying a non-volatile,
high temperature molten working fluid between pairs of spars mounted on
the outside of the ship. The fluid loses heat while flying through the
vacuum, and is collected for recirculation at the far end. This has
numerous advantages:
- the radiators can be "retracted" almost instantly
- a physical hit to your radiator doesn't actually damage it, unless it
hits one of the (small) spars
- you can carry extra working fluid to replace losses in combat, or liquid
which misses the recovery spar because of unanticipated accelerations
- you can replace a huge radiator area by simply pushing out spare spars
I don't recall if Forward described the actual material used as the
working fluid. Ideally it should be something which is liquid not too far
from room temperature (to avoid the extra cost of pumping heat up a
hill), but which can be heated to incandescence without acquiring a
significant vapour pressure.
--
Christopher Neufeld neu...@caliban.physics.utoronto.ca
Home page: http://caliban.physics.utoronto.ca/neufeld/Intro.html
"Don't edit reality for the sake of simplicity"
John Schilling
>OK, if there's anything common sense and the perennial threads about
>the topic in this group teach us about travel and combat in space,
>it's that any spaceship, and especially a spaceship that wants to use
>weapons, will need a radiator to dissipate excess heat. With a space
>warship, obviously the aim of radiator development is to have the
>radiators dissipate as much heat per unit of area as possible: To keep
>the hull area taken up by the radiator low; to minimise the
>performance hit caused by losing a fraction of the surface in combat;
>etc.
The down side is that the heat flux scales as the fourth power of
temperature, so "as much heat per unit area as possible" means "as
hot as possible". And the thermodynamic efficiency of whatever it
is you are radiating waste heat away from, scales as the ratio of
the input and output temperatures - so making the radiator (i.e.
the output end of your engine, weapon, whatever) as hot as possible
means making the overall process as inefficient as possible.
You don't want the smallest radiator possible, because that drives
your cycle efficiency to zero. And you don't want the most efficient
system possible, because that drives your radiator size to infinity.
Somewhere in between, exactly where we can only guess, is optimal.
>As a laymen, I imagine a radiator as a simple grid of tubes on the
>outer hull, with a fluid running through them and a reflective foil
>behind them.
Something along those lines.
>My questions: Which is the most effective radiator design you can
>imagine for a future spaceship? If possible, it shouldn't be
>handwaved, so neutrino beams are out :)
An array of heat pipes connected by metal webs. The heat pipes can
run directly from the engine or other heat source, or from a central
heat exchanger. The metal web conducts heat from the heat pipes to
a large surface area for radiation - you balance the heat pipe and
web dimensions so that neither one chokes the other.
For ~room-temperature operation, ammonia heat pipes and anodized aluminum
webbing work best. If you want something hotter, use liquid metal heat
pipes (sodium, potassium, lithium) and copper or silver webbing. Black
surface mandatory, of course.
If the required radiator surface area cannot be readily accomodated on
the surface of the spacecraft, you'll need radiator "fins" or "wings".
Preferablt two, not more than three or four or your radiators no longer
have a clear field of view - you can't radiate to another radiator :-)
Plan B, if you want a lightweight, high-capacity cooling system and don't
mind losing a bit of coolant, is a liquid droplet radiator. Pump your coolant
through a showerhead to produce a fan-shaped spray directed at a trough-like
collector. The droplets will radiate and cool during the trip, and you don't
need to provide pipes and radiating surfaces. You do need to make up for
evaporative losses when you spray a liquid into vacuum, of course. But
these can be reduced to surprisingly tolerable levels if you use high-MW
synthetic oils for moderate temperatures or liquid lithium metal up to
maybe a thousand degrees - very low vapor pressures.
>Furthermore, what is the best medium to transport heat inside a future
>spaceship? Liquid helium (as used by Star Trek, ugh) sounds sexy, but
>might not be too sensible. Sodium seems to be quite popular with heavy
>nuclear equipment, but I don't know whether running a grid of pipes
>full of liquid sodium all through a spaceship is all that sensible.
If you haven't mastered reliable sealing technology, you have no business
building a spaceship :-)
>Gaseous media need to circulate at quite a rate to accomplish
>anything, but they've got their advantages (i.e. you can use an inert
>gas and there won't be much trouble from leaks or corrosion).
You probably want to use the same material in your internal cooling system
that you do in the radiator heat pipes or droplet streams. Ammonia at
moderate temperatures, liquid metal for high-temperature radiators, oil
or lithium for vapor-droplet systems.
>You get bonus points if you cannot only give me breakthrough designs
>for a heat dissipation system, but also reasonable estimates for how
>many joules per square metre of radiator surface it will actually be
>able to radiate off :)
5.67x10^-8 T^4 Watts per square meter, for T in Kelvin.
>A space warship might also prefer to keep its radiators retracted,
>collapsed or under armour (no discussions about spaceship armour
>please, I know the issue) for less vulnerability and store excess heat
>in a "heat sink" during combat to dissipate it later. Here again, as a
>layman the best I can imagine is something like melting a huge lump of
>ice or (more contemporary) a pool of special salts, as done in solar
>power plants to store energy.
Molten salts are your best bet here, but they are only good for half a
megajoule per kilogram or so, and that's not all that much by space-travel
standards. Ice water is easier to handle and works at lower temperatures,
but holds even less heat. These will do fine for soaking up waste from
your life support systems, computers, and whatnot, at least for a while.
They won't do a damn thing where the propulsion or weapons are concerned.
--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
*schi...@spock.usc.edu * for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *
Luke Campbell
> OK, if there's anything common sense and the perennial threads about
> the topic in this group teach us about travel and combat in space,
> it's that any spaceship, and especially a spaceship that wants to use
> weapons, will need a radiator to dissipate excess heat. With a space
> warship, obviously the aim of radiator development is to have the
> radiators dissipate as much heat per unit of area as possible: To keep
> the hull area taken up by the radiator low; to minimise the
> performance hit caused by losing a fraction of the surface in combat;
> etc.
The drives are likley to produce at least as much heat as the weapons,
probably a lot more, and missiles do not heat the warship up
significantly when fired.
> As a laymen, I imagine a radiator as a simple grid of tubes on the
> outer hull, with a fluid running through them and a reflective foil
> behind them.
Yeah. Paint the tubes black and you're good to go.
> Furthermore, what is the best medium to transport heat inside a future
> spaceship? Liquid helium (as used by Star Trek, ugh) sounds sexy, but
> might not be too sensible. Sodium seems to be quite popular with heavy
> nuclear equipment, but I don't know whether running a grid of pipes
> full of liquid sodium all through a spaceship is all that sensible.
> Gaseous media need to circulate at quite a rate to accomplish
> anything, but they've got their advantages (i.e. you can use an inert
> gas and there won't be much trouble from leaks or corrosion).
Liquid helium would be largely unworkable. It has a very low heat
capacity, so it could not carry much heat anyway, and would boil off once
you put any heat into it. The only thing you would want to use liquid
helium for would be to cool things off than need to be kept at only a few
degrees above absolute zero.
My vote would be water. It has a high heat capacity, remains liquid at
temperatures that much of the ship will be working at and can stay a
liquid at high temperatures if kept under pressure, it is non-toxic,
non-caustic, and non-explosive (so it is safer for the crew and the
equipment is easier to repair), the equipment to pump, store, and
transport it is cheap compared to more exotic alternatives, the
technology of working with water is well developed, water is common so it
can be easily replaced, and if you really need to dump a lot of heat you
can boil it off into space and make use of water's anomalously high heat
of vaporization.
Luke
Luke Campbell
> My vote would be water. It has a high heat capacity, remains liquid at
> temperatures that much of the ship will be working at and can stay a
> liquid at high temperatures if kept under pressure, it is non-toxic,
> non-caustic, and non-explosive (so it is safer for the crew and the
> equipment is easier to repair), the equipment to pump, store, and
> transport it is cheap compared to more exotic alternatives, the
> technology of working with water is well developed, water is common so it
> can be easily replaced, and if you really need to dump a lot of heat you
> can boil it off into space and make use of water's anomalously high heat
> of vaporization.
A few notes on how good this is - at 1 atm, water boils at 100 C (373.15 K),
so at atmospheric pressure, your radiatiors, using liquid water, will be
radiating away about 1.1 kilowatt per square meter. You could pipe the
water to the radiators as steam and then condense it out on the radiators,
maintaining a constant radiator temperature in the process.
Alternately, you could pump high pressure water around. If your pipes can
safely hold 100 atmospheres, the boiling point is about 580 K and your
radiators can get rid of about 6.6 kilowatts per square meter if you are
working with radiators which are heated by steam condensing to water. If an
enemy ship puts a hole through your cooling pipes, though, the water vapor
jetting out at 100 atmospheres and 580 K will make this system much more
dangerous to repair durring battle, which is when you will be needing the
cooling system the most.
Luke
Captain Button
[ water cooling for spaceships ]
> Alternately, you could pump high pressure water around. If your pipes can
> safely hold 100 atmospheres, the boiling point is about 580 K and your
> radiators can get rid of about 6.6 kilowatts per square meter if you are
> working with radiators which are heated by steam condensing to water. If an
> enemy ship puts a hole through your cooling pipes, though, the water vapor
> jetting out at 100 atmospheres and 580 K will make this system much more
> dangerous to repair durring battle, which is when you will be needing the
> cooling system the most.
But will the latter case be more dangerous to repair than the liquid
sodium system mentioned previously?
:-)}
--
"You may have trouble getting permission to aero or lithobrake
asteroids on Earth." - James Nicoll
Captain Button - [ but...@io.com ]
MA Lloyd
"Luke Campbell" <lwc...@u.washington.edu> wrote in message
news:39A45095...@u.washington.edu...
> Luke Campbell wrote:
> A few notes on how good this is - at 1 atm, water boils at 100 C (373.15
K),
> so at atmospheric pressure, your radiatiors, using liquid water, will be
> radiating away about 1.1 kilowatt per square meter. You could pipe the
> water to the radiators as steam and then condense it out on the radiators,
> maintaining a constant radiator temperature in the process.
You can use water for *most* of the system and add an extra heat pump
between the water loop and the molten tungsten carbide or whatever you
insist on using for the radiator. The radiator temperature needs to be
high, the heat pipe system needs to be below room temperatures,
after all you want it to absorb machinery generated heat *spontaneously*
not need a refrigeration system to pump heat into it. Water's actually
a pretty common choice for moving heat around at these sorts of
temperatures, ammonia's nice to, especially since it gives you a nice
phase change at reasonable temperatures and pressures to help in the
heat pump design, ditto for freons and other chloroflurocarbons.
Incidentally the best radiators are the ones with the maximum surface
area - the droplet radiator earlier in the thread is an example, but
anything in droplet form will be volatile enough you can expect to
lose a lot of it to boiloff - worse if it's only liquid cause it's hot the
boiloff will be condensing all over your hull. The more typical space
design for this uses a stream of fine dust.
Paul F. Dietz
> but
> anything in droplet form will be volatile enough you can expect to
> lose a lot of it to boiloff -
This is not true. There are substances that have negligible
vapor pressure at their melting points.
Paul
MA Lloyd
"Paul F. Dietz" <di...@interaccess.com> wrote in message
news:39A47CA3...@interaccess.com...
Like? I suppose something fairly polar with a very high
molecular weight might come close to negligible, but since
the idea is to get a *lot* of surface area, it doesn't take a
great deal of loss per unit surface to be a problem.
Paul F. Dietz
> > This is not true. There are substances that have negligible
> > vapor pressure at their melting points.
>
> Like? I suppose something fairly polar with a very high
> molecular weight might come close to negligible, but since
> the idea is to get a *lot* of surface area, it doesn't take a
> great deal of loss per unit surface to be a problem.
Gallium. Lithium. Tin.
Paul
Jonathan Cresswell
MA Lloyd <mall...@io.com> wrote in message news:39a47...@206.30.194.5...
>
> "Luke Campbell" <lwc...@u.washington.edu> wrote in message
> news:39A45095...@u.washington.edu...
> > Luke Campbell wrote:
>
> > A few notes on how good this is - at 1 atm, water boils at 100 C (373.15
> K),
> > so at atmospheric pressure, your radiatiors, using liquid water, will be
> > radiating away about 1.1 kilowatt per square meter. You could pipe the
> > water to the radiators as steam and then condense it out on the
radiators,
> > maintaining a constant radiator temperature in the process.
>
> You can use water for *most* of the system and add an extra heat pump
> between the water loop and the molten tungsten carbide or whatever you
> insist on using for the radiator. The radiator temperature needs to be
> high, the heat pipe system needs to be below room temperatures,
> after all you want it to absorb machinery generated heat *spontaneously*
> not need a refrigeration system to pump heat into it.
Assuming you don't need cryogenic superconductors and so forth, could you
run part of the ship relatively hot (reactors, rocket motors and so forth)
in order to to dump its waste heat efficiently, while keeping a portion of
it much cooler with a separate radiator system, under the condition that it
not produce anywhere near as much waste heat (life support, computers,
cryogenic fuels)?
>Water's actually
> a pretty common choice for moving heat around at these sorts of
> temperatures, ammonia's nice to, especially since it gives you a nice
> phase change at reasonable temperatures and pressures to help in the
> heat pump design, ditto for freons and other chloroflurocarbons.
>
> Incidentally the best radiators are the ones with the maximum surface
> area - the droplet radiator earlier in the thread is an example, but
> anything in droplet form will be volatile enough you can expect to
> lose a lot of it to boiloff - worse if it's only liquid cause it's hot the
> boiloff will be condensing all over your hull. The more typical space
> design for this uses a stream of fine dust.
>
A warship needs at least two levels of cooling -- normal operations (either
drive-on, or drive-off), and combat operations (drive-on, weapons-on, and
whatever hits it receives that generate sudden surges of waste heat). If
water is being using to move the heat to the radiator, then under emergency
or combat conditions, could it be boiled off as a sort of 'afterburner',
allowing rapid heat rejection but only for a short period of time?
--
Jonathan C
John Park
variety.
--John Park
Jeroen
black-body radiator. The best design is the one that provides the most
radiator surface per square meter (or ft, depending on what side of the
water you live on).
Let's brainstorm a little about the design: For maximum area surface, a
fractal design would be cool (literally in this case!). Imagine a radiator
with 10 fins, each fin has on its surface 10 smaller fins, which in turn
have 10 even smaller fins on their surface etc etc. I'm not sure if fins are
useful in space though. There's obviously no medium for heat transport, so
all waste heat is dumped by radiation. Two parallel fins next to each other
also heat each other, which reduces effectiveness. Maybe something spherical
is better: Start with a hemi-sphere, covered with smaller hemispheres,
covered with....etc.
The major task however would be to transfer heat from the ship to the
radiator. The second law of thermodynamics puts a hard limit at the
efficiency a heat-engine/pump process can reach. The most efficient process
possible is a Carnot-cycle (check
http://oldsci.eiu.edu/physics/DDavis/1150/14Thermo/engines.html), which is
cyclic process. Not the way to go about it.
A bit more fancy: Have a grid of thermally (super-)conducting material
running through the ship. Link every piece of heat-producing equipment to
this grid as well as the radiators. The gridlines will have to be thermally
isolated from the rest of the ship by puting them in internally reflecting
vacuum tubes, which they don't touch (Or very little). For the gridlines,
come up with a material that has an extremely high coefficient of thermal
conductivity (Joules per second per meter per degree temperature difference)
For a heatsink, this depends on the operating temperature you want to keep
your equipment on. Ice is alright if you want to keep your equipment near 0
degrees C (32 Fahrenheit). You need a BIG block of ice for some serious
equipemtn though. Find a material with a very high specific heat (Number of
joules it takes to heat a kg of the material 1 degree) and a melting point
near your equipment's operating temperature.
The amount of heat radiated by a black body per square meter is easy: P =
q*T^4, where q = 5.67*10^-8, T is the temperatur of your radiator and P is
the amount you're looking for.
Jeroen
"Matthias Warkus" <ma...@iname.com> wrote in message
news:slrn8q7t9...@audrey.my.box...
>
> OK, if there's anything common sense and the perennial threads about
> the topic in this group teach us about travel and combat in space,
> it's that any spaceship, and especially a spaceship that wants to use
> weapons, will need a radiator to dissipate excess heat. With a space
> warship, obviously the aim of radiator development is to have the
> radiators dissipate as much heat per unit of area as possible: To keep
> the hull area taken up by the radiator low; to minimise the
> performance hit caused by losing a fraction of the surface in combat;
> etc.
>
> As a laymen, I imagine a radiator as a simple grid of tubes on the
> outer hull, with a fluid running through them and a reflective foil
> behind them.
>
> My questions: Which is the most effective radiator design you can
> imagine for a future spaceship? If possible, it shouldn't be
> handwaved, so neutrino beams are out :)
>
> Furthermore, what is the best medium to transport heat inside a future
> spaceship? Liquid helium (as used by Star Trek, ugh) sounds sexy, but
> might not be too sensible. Sodium seems to be quite popular with heavy
> nuclear equipment, but I don't know whether running a grid of pipes
> full of liquid sodium all through a spaceship is all that sensible.
> Gaseous media need to circulate at quite a rate to accomplish
> anything, but they've got their advantages (i.e. you can use an inert
> gas and there won't be much trouble from leaks or corrosion).
>
> You get bonus points if you cannot only give me breakthrough designs
> for a heat dissipation system, but also reasonable estimates for how
> many joules per square metre of radiator surface it will actually be
> able to radiate off :)
>
Jeroen
> >imagine for a future spaceship?
> >
> large physical radiator, heat is dissipated by spraying a non-volatile,
> high temperature molten working fluid between pairs of spars mounted on
> the outside of the ship. The fluid loses heat while flying through the
> vacuum, and is collected for recirculation at the far end. This has
> numerous advantages:
Innovative, but not very practical: A ship cannot accellerate while this
proces is going on, or the drops will miss the target-spar. Also, these
drops are hardly effective radiators. Assuming the drops are spherical, they
radiate roughly half their heat back to the ship's exterior.
Jeroen
Isaac Kuo
Matthias Warkus <ma...@iname.com> wrote:
>My questions: Which is the most effective radiator design you can
>imagine for a future spaceship? If possible, it shouldn't be
>handwaved, so neutrino beams are out :)
It depends upon the sheer power level we're talking about and
the desired operating temperature. Generally, the more
power you are using the more waste heat you need to reject.
The more waste heat you need to reject, the more you will
need to use high temperature radiators.
The problem with high temperature radiators is that you need
to use even more power to pump waste heat from the relatively
cooler equipment and crew compartment to the heat rejection
system.
In other words, you want high temperature radiators if and
only if you have high power equipment. For a typical
near future space craft, the ideal radiator designs are
mundane solid radiators involving pipes of working liquid
and/or heat pipes. These can be on the outer hull or in
heat radiator vanes.
For higher power requirements, a slightly higher temperature
loop radiator might be ideal. This is a large loop of ribbon
material which is spun into a somewhat stable circle many
times larger than the ship. The ship is at one point along
the loop, and it uses rollers to transfer heat to the loop.
Unlike liquid droplet radiators, this radiator system does
not have significant evaporative losses. However, it is
not suitable for use while the ship is engaged in high thrust
maneuevers.
Another heat radiator possibility is a large transparent
balloon tube in a donut shape. The working fluid is a
low pressure low temperature gas circulated throughout
the donut. Unfortunately, this possibility is probably
impractical due to vulnerability to micrometeorite impacts.
Others have mentionned liquid droplet radiator systems.
These are suitable for relatively high power systems.
A more radical radiation system is a plasma radiation system.
The ship creates a magnetic field around itself which
contains a cloud of plasma--potentially dozens or hundreds
of kilometers across. The plasma radiates over a huge
area, but at relatively high temperature and with quite
significant losses due to leaking. This radiation system
is suitable for very high power systems, and can double
as a propulsion system against the solar wind and/or
external particle beams and/or Orion-like nuclear blasts
(detonated far away enough so that it doesn't blow away
the plasma). The magnetic field and plasma cloud may
severely complicate the design of other ship systems,
possibly necessitating an extremely long boom to separate
the power/heat rejection module from other systems.
Another radical radiation system is an electron radiation
system. This is similar to the plasma radiation system
except it uses an electron cloud, which is contained by
electrostatic force as well as a magnetic field. This
is suitable for a very high power system and requires
a high radiator temperature--the radiators are essentially
cathode ray emitters. This has the highest rejected waste
heat power to mass ratio of all. You can radiate over a
huge area at extremely little mass cost. The electric
and magnetic fields as well as the electrons themselves
will complicate design of ship systems, of course.
>Furthermore, what is the best medium to transport heat inside a future
>spaceship? Liquid helium (as used by Star Trek, ugh) sounds sexy, but
>might not be too sensible. Sodium seems to be quite popular with heavy
>nuclear equipment, but I don't know whether running a grid of pipes
>full of liquid sodium all through a spaceship is all that sensible.
This depends upon the operating temperature of the equipment
being cooled. Ideally you want a liquid with a boiling
temperature just under the desired equipment temperature
(under reasonable pressure conditions).
It may even be desirable to use different heat transport
mediums for different areas of the ship. A human crew
compartment needs to operate around 300K, but a high
power laser might satisfactorily operate at a much higher
temperature.
Also, keep in mind that heat pipes offer a lot of advantages.
They are self contained zero maintainance units which can
be mounted redundantly in parallel to provide excellent
damage/failure resistance.
--
_____ Isaac Kuo k...@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
__|_)o(_|__ ICQ 29055726
/___________\
\=\)-----(/=/ "Why does Mini-Bigglesworth not have fur?"
Isaac Kuo
Jeroen <j.bo...@chello.nl> wrote:
>[someone else wrote]
>> I really like Robert Forward's idea in _Rainbow Mars_. Instead of a
>> large physical radiator, heat is dissipated by spraying a non-volatile,
>> high temperature molten working fluid between pairs of spars mounted on
>> the outside of the ship. The fluid loses heat while flying through the
>> vacuum, and is collected for recirculation at the far end. This has
>> numerous advantages:
>Innovative, but not very practical: A ship cannot accellerate while this
>proces is going on, or the drops will miss the target-spar. Also, these
>drops are hardly effective radiators. Assuming the drops are spherical, they
>radiate roughly half their heat back to the ship's exterior.
No, the ship's exterior can be reflective so it absorbs a minimal
amount of heat radiated onto it.
Also, these spars presumably extend far away from the ship so the
sheet of fluid spray is along a plane with the ship body aligned
with an edge. Thus, most of the radiated photons would not
intersect the main ship.
Assuming the ship is roughly a long cylinder with a rocket
nozzle on one end, the spars could be set up to spray liquid
from the spars mounted on the "front" end to be collected
by the spars mounted on the "rear" end. If the ship
accelerates along its main axis, the liquid will still
hit the "rear" spars.
Still, thin spars seem a challengingly small target even
without any motion.
Given this sort of ship configuration, I'd be more comfortable
with a wide net-like belt radiator. The two spars are large
rollers transfering heat to the belt by conduction. The
belt is a fine net of thin wires rather than a solid film to
save mass.
jti...@my-deja.com
"Jeroen" <j.bo...@chello.nl> wrote:
> A short answer is that the best material is the one that comes
closest to a
> black-body radiator. The best design is the one that provides the most
> radiator surface per square meter (or ft, depending on what side of
the
> water you live on).
>
> Let's brainstorm a little about the design: For maximum area surface,
a
> fractal design would be cool (literally in this case!). Imagine a
radiator
[snip of really complicated design]
Of course, you could get most of the theoretically possible benefit of
any system with a big flat aluminum plate with some water-glycol
circulating tubes brazed to it in a zig-zag pattern painted with common
black paint and held edge on to the sun. Cheap too.
Most of the information needed to design low temperature radiation heat
transfer systems was codified in the 70's and early 80's for the design
of flat plate solar collectors (the system is symmetrical and
bidirectional: radiator=collector). If you want the ultimate in
performance (maybe 10% better), copper plates with integral tubes and a
high emissivity coating are available at a slightly higher cost in your
choice of designer housings. For space use, NASA has some dandy light
weight versions at an even higher cost per pound.
Regards,
Jack Tingle
Sent via Deja.com http://www.deja.com/
Before you buy.
Jeroen
> >proces is going on, or the drops will miss the target-spar. Also, these
> >drops are hardly effective radiators. Assuming the drops are spherical,
they
> >radiate roughly half their heat back to the ship's exterior.
>
> No, the ship's exterior can be reflective so it absorbs a minimal
> amount of heat radiated onto it.
Hm, that's true. Still, given a high operational temperature, the spectrum
of the emitted radiation is very wide. All materials I know of have large
dispersion across such wavelength ranges, which means they will transmit at
least part of the radiation. Of course, this is true for all designs. A
possible solution is coating a surface with a stack of thin optical layers
(10's of nanometers). This way you can optimize reflection for certain
wavelength-ranges.
> Also, these spars presumably extend far away from the ship so the
> sheet of fluid spray is along a plane with the ship body aligned
> with an edge. Thus, most of the radiated photons would not
> intersect the main ship.
That is not beneficial for a fast retraction, but assuming it's fast enough
for a warship, that is indeed true.
> Assuming the ship is roughly a long cylinder with a rocket
> nozzle on one end, the spars could be set up to spray liquid
> from the spars mounted on the "front" end to be collected
> by the spars mounted on the "rear" end. If the ship
> accelerates along its main axis, the liquid will still
> hit the "rear" spars.
It's not very likely a warship will follow a predictable striaght line
course. Also, the system would have to be shut down during _any_
maneuvering, even during, say, docking operations.
> Still, thin spars seem a challengingly small target even
> without any motion.
As long as you can accurately control the spray at the source-spar, this is
not a problem. It's not as if the droplets will fan out like they do in an
atmospheric environment. That's the advantage of space :).
> Given this sort of ship configuration, I'd be more comfortable
> with a wide net-like belt radiator. The two spars are large
> rollers transfering heat to the belt by conduction. The
> belt is a fine net of thin wires rather than a solid film to
> save mass.
It saves mass, but it also reduces the radiation capability of the system.
Jeroen
John D. Gwinner
getting rid of waste heat seems to always come up. If it's that big of an
issue, why doesn't any of our current spacecraft have a heat management
system? I guess the shuttle has some, but I'm not sure I remember anything
else having anything. Or am I missing something?
So how is it done now?
== John ==
P.S. Presumably weapon systems generate heat which may make non-civilian
craft need more heat management.
"Matthias Warkus" <ma...@iname.com> wrote in message
news:slrn8q7t9...@audrey.my.box...
>
> OK, if there's anything common sense and the perennial threads about
> the topic in this group teach us about travel and combat in space,
> it's that any spaceship, and especially a spaceship that wants to use
> weapons, will need a radiator to dissipate excess heat. With a space
> warship, obviously the aim of radiator development is to have the
> radiators dissipate as much heat per unit of area as possible: To keep
> the hull area taken up by the radiator low; to minimise the
> performance hit caused by losing a fraction of the surface in combat;
> etc.
>
> As a laymen, I imagine a radiator as a simple grid of tubes on the
> outer hull, with a fluid running through them and a reflective foil
> behind them.
>
> My questions: Which is the most effective radiator design you can
> imagine for a future spaceship? If possible, it shouldn't be
> handwaved, so neutrino beams are out :)
>
> Furthermore, what is the best medium to transport heat inside a future
> spaceship? Liquid helium (as used by Star Trek, ugh) sounds sexy, but
> might not be too sensible. Sodium seems to be quite popular with heavy
> nuclear equipment, but I don't know whether running a grid of pipes
> full of liquid sodium all through a spaceship is all that sensible.
> Gaseous media need to circulate at quite a rate to accomplish
> anything, but they've got their advantages (i.e. you can use an inert
> gas and there won't be much trouble from leaks or corrosion).
>
> You get bonus points if you cannot only give me breakthrough designs
> for a heat dissipation system, but also reasonable estimates for how
> many joules per square metre of radiator surface it will actually be
> able to radiate off :)
>
> A space warship might also prefer to keep its radiators retracted,
> collapsed or under armour (no discussions about spaceship armour
> please, I know the issue) for less vulnerability and store excess heat
> in a "heat sink" during combat to dissipate it later. Here again, as a
> layman the best I can imagine is something like melting a huge lump of
> ice or (more contemporary) a pool of special salts, as done in solar
> power plants to store energy.
>
>
Jonathan Cresswell
John D. Gwinner <7516...@compuserve.com> wrote in message
news:8obksi$k5h$1...@sshuraab-i-1.production.compuserve.com...
> Interesting thread, but I want to ask a basic question. This whole topic
if
> getting rid of waste heat seems to always come up. If it's that big of an
> issue, why doesn't any of our current spacecraft have a heat management
> system? I guess the shuttle has some, but I'm not sure I remember
anything
> else having anything. Or am I missing something?
>
>
> So how is it done now?
The shuttle has radiator panels that are exposed when the bay doors open (if
the doors can't be opened for some reason, the mission must be aborted due
to overheating problems).
Current spacecraft don't have major power systems aboard (like fusion
reactors, etc.). Since the only really 'high-powered' systems are rockets,
which eject waste heat in their plumes, it hasn't really come up yet in a
significant way, I suppose. The space station will have a heat-rejection
system, but I don't know what kind.
--
Jonathan C
Paul F. Dietz
> Interesting thread, but I want to ask a basic question. This whole topic if
> getting rid of waste heat seems to always come up. If it's that big of an
> issue, why doesn't any of our current spacecraft have a heat management
> system? I guess the shuttle has some, but I'm not sure I remember anything
> else having anything. Or am I missing something?
>
> So how is it done now?
Thermal management is a problem in all spacecraft.
Some are small enough that simple conduction and
radiation suffice.
The shuttle has at least two systems. The primary system
are radiators on the inside of the payload bay doors.
This is why the shuttle must open its payload pay
as soon as it reaches orbit. The secondary system
(used when the doors are closed) employs evaporation
of a liquid. It can only be used for a short time
before the liquid is exhausted.
Paul
Jeroen
"John D. Gwinner" <7516...@compuserve.com> wrote in message
news:8obksi$k5h$1...@sshuraab-i-1.production.compuserve.com...
if
> getting rid of waste heat seems to always come up. If it's that big of an
> issue, why doesn't any of our current spacecraft have a heat management
> system? I guess the shuttle has some, but I'm not sure I remember
anything
> else having anything. Or am I missing something?
As others said, the shuttle has radiators built in the payload bay doors.
The station will have a significant heat-dump system. Take a look here:
http://spaceflight.nasa.gov/station/assembly/flights/2000/4a.html
You see the solar-panels in a horizontal ("flat") position. The vertically
oriented panel is a radiator. The finished station will have a lot more of
these. Just look at pictures of the finished station. There are at least ten
radiators that I can see. Six in two sets of three pointing "aft" attached
to the central truss, and four single panels pointing "down" also attached
to the central truss. But there may be more that I'm missing.
Jeroen
Christopher M. Jones
"John D. Gwinner" <7516...@compuserve.com> wrote:
> Interesting thread, but I want to ask a basic question. This whole topic
if
> getting rid of waste heat seems to always come up. If it's that big of an
> issue, why doesn't any of our current spacecraft have a heat management
> system? I guess the shuttle has some, but I'm not sure I remember
anything
> else having anything. Or am I missing something?
Yes, quite a bit actually. All space craft need to take special
care of their heat budgets. In many cases however, the bounds
are loose enough and the heat sources weak enough so that
"extreme" measures do not have to be taken. Nevertheless, all
spacecraft have heat management systems of some sort, typically
they have internal heaters that keep equipment warm enough to
function properly as well as various heat dissipation systems.
Usually this doesn't require anything more than "louvers" that
can be closed to trap in radiated heat, or opened to let out the
heat (in the form of EM radiation) and cool the inside of the
spacecraft. With much larger spacecraft that use a lot of power,
more sophisticated systems usually need to be used, such as
radiators and the like. For example, on the International
Space Station (which will use quite a lot of power, 100 kW)
large radiators (Thermal Control System arrays) will be used to
rid the station of heat.
Captain Button
> Interesting thread, but I want to ask a basic question. This whole topic if
> getting rid of waste heat seems to always come up. If it's that big of an
> issue, why doesn't any of our current spacecraft have a heat management
> system? I guess the shuttle has some, but I'm not sure I remember anything
> else having anything. Or am I missing something?
> So how is it done now?
Well, some stuff about how they did it then, IMS.
I recall something from the Apollo era where the spacecraft were set
to rotating very slowly to spread the solar heating evenly.
I recall from some documentary about the Apollo 13 accident, that the
problem was that it was getting quite cold without power for heating.
Again, running from memory, when Skylab had problems resulting in the
loss of one of the solar panel assemblies one of the fixes was installing
a big sunscreen/tarp thing.
John D. Gwinner
"Christopher M. Jones" <christ...@uswest.net> wrote in message
news:RDjq5.1440$NR6.3...@news.uswest.net...
>
> "John D. Gwinner" <7516...@compuserve.com> wrote:
> > Interesting thread, but I want to ask a basic question. This whole
topic
> if
> > getting rid of waste heat seems to always come up. If it's that big of
an
> > issue, why doesn't any of our current spacecraft have a heat management
> > system? I guess the shuttle has some, but I'm not sure I remember
> anything
> > else having anything. Or am I missing something?
>
You should have added a <G> behind that ,then it would have been funny.
== John ==
P.S. ;-)
John D. Gwinner
Thanks .. I guess my question would be that if Apollo could 'get away
with' not having large radiator panels, what's the BOTE numbers for what
kind of power sources or distance to the sun do require some form of heat
disipation?
I mean, if we run the numbers is it that big of a deal? My rough guess
tells me that if the shuttle has to have radiator panels, then probably
other larger craft would also.
== John ==
"Jonathan Cresswell" <jcressw...@netrover.com> wrote in message
news:xOcq5.16696$Z2.2...@nnrp1.uunet.ca...
>
> John D. Gwinner <7516...@compuserve.com> wrote in message
> news:8obksi$k5h$1...@sshuraab-i-1.production.compuserve.com...
> > Interesting thread, but I want to ask a basic question. This whole
topic
> if
> > getting rid of waste heat seems to always come up. If it's that big of
an
> > issue, why doesn't any of our current spacecraft have a heat management
> > system? I guess the shuttle has some, but I'm not sure I remember
> anything
> > else having anything. Or am I missing something?
> >
> >
> > So how is it done now?
>
Erik Max Francis
> I mean, if we run the numbers is it that big of a deal? My rough
> guess
> tells me that if the shuttle has to have radiator panels, then
> probably
> other larger craft would also.
If you look at it as a general physics problem, it's a question of
equilibrium. There are two things adding heat to the spacecraft --
solar insolation and internal heat generation -- and two things removing
heat -- natural thermal radiation and active heat dissipation. Solar
insolation is a function of the luminosity of the Sun, distance from
Sun, spacecraft albedo, and exposed surface area; natural radiation is a
function of the emissivity of the ship, its total surface area, and its
temperature. If
(solar insolation) + (internal heat) >
(natural radiation) + (active dissipation)
then the ship will get progressively hotter and hotter; if it's <, then
it will get cooler and cooler. (In actuality internal heat and active
dissipation would be functions, and would be tunable, but they'd have
limits.)
--
Erik Max Francis / m...@alcyone.com / http://www.alcyone.com/max/
__ San Jose, CA, US / 37 20 N 121 53 W / ICQ16063900 / &tSftDotIotE
/ \ You and I / We've seen it all / Chasing our hearts' desire
\__/ The Russian/Florence, _Chess_
Polly Wanna Cracka? / http://www.pollywannacracka.com/
The Internet resource for interracial relationships.
George William Herbert
>"John D. Gwinner" wrote:
>> I mean, if we run the numbers is it that big of a deal? My rough
>> guess
>> tells me that if the shuttle has to have radiator panels, then
>> probably
>> other larger craft would also.
>
>If you look at it as a general physics problem, it's a question of
>equilibrium. There are two things adding heat to the spacecraft --
>solar insolation and internal heat generation -- and two things removing
>heat -- natural thermal radiation and active heat dissipation. Solar
>insolation is a function of the luminosity of the Sun, distance from
>Sun, spacecraft albedo, and exposed surface area; natural radiation is a
>function of the emissivity of the ship, its total surface area, and its
>temperature. If
>
> (solar insolation) + (internal heat) >
> (natural radiation) + (active dissipation)
>
>then the ship will get progressively hotter and hotter; if it's <, then
>it will get cooler and cooler. (In actuality internal heat and active
>dissipation would be functions, and would be tunable, but they'd have
>limits.)
Sort of. Remember, (natural radiation) and (active dissipation)
both are T^4 laws, so there will be an equilibrium temperature for
any given sum of insolation and internal heat generation.
The challenge is to keep the value of T within a reasonable range
for vehicle performance, and/or to have differential cooling so
that parts of it which are heat resistant can heat up or cool
down but other parts with say people or computers stay in a more
tolerable zone.
-george william herbert
gher...@retro.com
Jonathan Cresswell
George William Herbert <gher...@gw.retro.com> wrote in message
news:8ofdbi$kt$1...@gw.retro.com...
Right. I've noticed that satellites and even space probes pay a great deal
of attention to thermal control (in a laid-back sort of way :), but in the
case of ones like Voyager or Pioneer, too-low temperatures are an issue as
well as too-high ones.
It doesn't seem to be a big issue *if* the technology is handwaved so that
items which produce large amounts of waste heat (ie, fusion reactors) can be
run at high temperatures, allowing them to radiate the heat rapidly with a
manageable size of radiator surface. If one's fusion reactor has built-in
computer chips, so that it has to be kept at 50 degrees C or else
malfunction, then it will present quite a challenge to radiate away its
waste heat. The surface area required quickly goes into square kilometers.
--
Jonathan C
John D. Gwinner
You're missing the point, I'm wondering if someone has 'run the numbers'.
Instead of running around and arguing over powdered metal vs liquid spray
bars and the like <G> lets get a rough idea of how much cooling is required.
I realize that without knowing what technologies our future dreadnaughts
will be using, this is hard, but some quick BOTE I think would be a
worthwhile exercise.
Whats' the shuttle got that generates so much heat, anyway? Is it just
better insulated than Apollo?
== John ==
"Jonathan Cresswell" <jcressw...@netrover.com> wrote in message
news:y7Nq5.17207$Z2.2...@nnrp1.uunet.ca...
George William Herbert
>Folks:
> You're missing the point, I'm wondering if someone has 'run the numbers'.
>Instead of running around and arguing over powdered metal vs liquid spray
>bars and the like <G> lets get a rough idea of how much cooling is required.
>I realize that without knowing what technologies our future dreadnaughts
>will be using, this is hard, but some quick BOTE I think would be a
>worthwhile exercise.
Without knowing what technologies our "future dreadnaughts" have it's
beyond hard, it's not a solveable equation.
Determine effective surface area of "future dreadnaught".
Determine power dissipated internally by dreadnaught engines
and electronics and life forms and life support systems and
waste heat from weapons and the like.
Pradiated/unit area = emissivity (range very small to 1.0) times
T^4 times ... durn it, where's my sophomore physics text,
not under the pile of nuclear proliferation books...
times the boltzman constant I think, but could be wrong.
Pradiated/unit area * surface area = Pinternal
solve for T
> Whats' the shuttle got that generates so much heat, anyway? Is it just
>better insulated than Apollo?
Much much more electronics, life support systems (motors running fans,
pumps, etc etc). Its fuel cells put out about 12 kWe.
Speaking of which, Geoff, you left Fuel Cells entirely out of your
spacecraft power chapter in the Human Spaceflight book. Someone needs
to address a lot of the "we didn't have enough space to..." points
with that book in another book.
-george william herbert
gher...@retro.com
jcre...@my-deja.com
>
> You're missing the point, I'm wondering if someone has 'run the numbers'.
> Instead of running around and arguing over powdered metal vs liquid spray
> bars and the like <G> lets get a rough idea of how much cooling is required.
> I realize that without knowing what technologies our future dreadnaughts
> will be using, this is hard, but some quick BOTE I think would be a
> worthwhile exercise.
On the back of a very unofficial envelope, then...
Take a classic space opera warship. Onboard power is generated by one or
more fusion reactors. If the overall power is 2 gigawatts, and the
efficiency is 90% (a pretty generous estimate, since projections I've
seen for MHD power generation are around 60%) then at full power, the
reactors create 200MW of waste heat. At these sorts of power levels the
waste heat of the crew, computers, coffee makers, etc. can be ignored. If
there are energy weapons, assume they too are 90% efficient and use 500MW
of power when fired, generating another 50MW waste heat. Lump in a lot of
other minor systems and you get something like 300MW total waste heat
that has to be gotten rid of at peak.
Where it gets complex, AFAIK, is the question of how hot you can allow
the ship's interior to get. Let's assume that the environmental areas
have heat pumps that allow them to stay a fair amount cooler than the
engineering areas (since they're not generating the majority of the heat
to begin with) and there are no low-temperature superconductors and so
forth to worry about. If the engines and weapons can operate happily at
150 degrees Celsius, that's 423 degrees Kelvin. So that's our starting
point -- the coolant (probably liquid sodium or lithium at that temp)
gets that hot before it's pumped through the radiators to cool off again,
at which point:
Heat lost [watts] = 5.67e-8[Stefan's Constant] * area [m^2] * emissivity
* T^4 [degrees Kelvin].
If the radiators are perfectly black (emissivity of 1), and the coolant
temperature is 423 degrees K, then in order to radiate away 275MW of
heat, the radiator needs be about 150,000 square meters in area (of
course it's double-sided, so the actual fin(s) only need to be 75,000 m^
2). That's a square 275 meters on a side, or roughly a large city block,
simply to deal with the ship's own waste heat at full power. If the ship
needs to radiate away additional heat due to taking in, say, 400MW of
energy from an enemy ship's lasers, you'd probably have to double or
triple that figure (and make darn sure to keep your fins edge-on to the
enemy ship firing at you! :). Of course all this is very crude and
assumes perfect efficiency of a number of things (some of which I'm
probably unaware of :). In reality you might get 80% of that theoretical
performance. Or perhaps less. And the first thing damaged in a battle
would probably be the radiators (big, hard to protect).
The structural mass of a large radiator fin could be a substantial
fraction of the entire ship's mass, and that slows down the acceleration
of the ship, which needs more power for thrust, which gives off more
waste heat, and so on... So the idea of using spray wands and droplet
coolants is attractive.
OTOH, if you need to keep the whole ship at a comfy temperature like 20C,
then it's almost hopeless. The radiating area required is so enormous
that high acceleration isn't practical at all (something like half a
million m^2).
Another alternative is to design the ship to only radiate away normal,
routine power levels, and to boil off propellant to deal with peak loads.
But that goes through a lot of propellant pretty fast at high power
levels. Dreadnaughts become like modern jet fighters -- only good for a
few minutes of intense combat before the fuel runs out. Once it's gone,
you can't crash, but you have to surrender or be boiled...
Parenthetically, the Apollo Service Module included two radiator areas,
one for the "environmental subsystem" and a larger unspecified one,
according to an illustration in "The Encyclopedia of U.S. Spacecraft".
While the SM was operating normally, these were obviously required at
times (presumably in the case of Apollo 13, when the SM was crippled by
an explosion, the only remaining source of waste heat on board was the
crew, which wasn't enough to keep the Command Module very warm).
--
Jonathan C
John Park
>[...]
I think at this point you need to decide how big your ship is: 150,000 m^2
is about the surface area of a 100 m-radius sphere. If the ship was that
big, it wouldn't need fins for cooling (though it might want them to
restrict IR emissions to a few directions).
--John Park
Jonathan Cresswell
John Park <af...@FreeNet.Carleton.CA> wrote in message
news:8oou1s$626$1...@freenet9.carleton.ca...
Good point. So, how big is our dreadnought? The volume of a ship of 100m
radius is about 4 million m^3, so even if it's built as flimsy as Skylab
was, it's going to mass about 400,000 tons. If it's built like a nuclear
sub, more like 2 million tons. In that case the 2GW power figure is way too
low -- assuming drive performance in the region of 1G, anyway, and a decent
delta-v to be able to romp around the Solar System. For comparison, the STS
(Shuttle + boosters) masses about 2,000 tons at liftoff and develops 130GW,
and that's a very high fuel flow rate and very low Isp, both of which push
the ratio down as I understand it (imperfectly). I suspect my initial power
figure was too low for a proper space opera dreadnought :), but if the
warship masses 1000 tons, 2GW seems to be in the ball park for a fusion-type
power/mass ratio for interplanetary cruising. So if it's built like Skylab,
then 1000 tons translates into 11,000m^3, or a cylinder 95m long by 12m
diameter, or a surface area of about 3,700m^2, or one-fortieth what it
needs. If it's built denser, it's even smaller. Here lies the problem:
hanging enough heat radiators off a relatively small hull to make up the
surface area needed, without the radiators massing far more than the ship
itself.
What this may also mean is that the visualization of a "dreadnought" may
need to change a bit -- perhaps it has a high delta-v but continuously
accelerates at 0.01 gravities for weeks instead of a couple hours at 1G, so
the power plant can be far lower ouput (but then it's missiles or nothing --
not much power for energy weapons). Or it's built as a big, very thin disk
with a bulge at the center, in which case laser combat consists of trying to
keep your own ship edge-on to the enemy, while ideally forcing them to
orient flat-on to the sun and soak up more heat that way (although I guess
you'd need two attackers to ensure that). The thing is, heat rejection is so
important to any spacecraft with a major power source on board that it
really should drive a lot of the overall design philosophy. That generally
doesn't come up in SF, it seems (and after all, the story should come first
anyway); but it's a bit like designing a submarine without paying attention
to hydrostatic pressure.
--
Jonathan C
Isaac Kuo
Jonathan Cresswell <jcressw...@netrover.com> wrote:
>>>On the back of a very unofficial envelope, then...
>>>Take a classic space opera warship. Onboard power is generated by one or
>>>more fusion reactors. If the overall power is 2 gigawatts, and the
>>>efficiency is 90% (a pretty generous estimate, since projections I've
>>>seen for MHD power generation are around 60%) then at full power, the
>>>reactors create 200MW of waste heat.
The important issue actually isn't the overall efficiency of the
reactor, it's the ratio between useful energy obtained/used
to the amount of waste heat absorbed by the ship and fixed
reactor components.
Speculative fusion reactor designs I've seen include designs where
the reactor is a cage-like structure where most of the fusion
products aren't ever absorbed by the ship itself. The charged
particle products can directly be used as a rocket exhaust
without generating much waste heat by the use of a superconducting
magnetic nozzle. This would be nowhere near 90% efficient due
to all the products not deflected by the magnetic nozzle and
the imperfect directionality of the exhaust. However, the
important thing is that almost all of the waste heat is in
the ejected products and never absorbed by the ship itself.
>If
>>>there are energy weapons, assume they too are 90% efficient and use
>500MW
>>>of power when fired, generating another 50MW waste heat. Lump in a lot
>of
>>>other minor systems and you get something like 300MW total waste heat
>>>that has to be gotten rid of at peak.
"If" there are energy weapons? Well, if there aren't, what the
heck are you doing with all that power? That's a rhetorical
question--there's another blatantly obvious high power use
(the drive).
However, it's worth thinking about what exactly it is you need
power on the ship for. The ship's drive is a good example of
why it's important. Even an inefficient drive does not
necessarily put much waste heat into the ship itself.
Similarly, certain energy weapons don't necessarily put much
waste heat into the ship. Chemical lasers with non-recycled
lasing fuel dump most of their waste heat along with the
spent fuel. Nuclear pumped X-ray lasers similarly dump most
of their waste heat outside the ship. These weapons have
rather limitted ammunition compared to what we usually think
of when we think of energy weapons, but their ability to
fire at extremely high power levels without dumping much
waste heat into the ship could be overriding advantages.
>>>If the radiators are perfectly black (emissivity of 1), and the coolant
>>>temperature is 423 degrees K, then in order to radiate away 275MW of
>>>heat, the radiator needs be about 150,000 square meters in area (of
>>>course it's double-sided, so the actual fin(s) only need to be 75,000 m^
>>>2). That's a square 275 meters on a side, or roughly a large city block,
>>[...]
>> I think at this point you need to decide how big your ship is: 150,000
>m^2
>> is about the surface area of a 100 m-radius sphere. If the ship was that
>> big, it wouldn't need fins for cooling (though it might want them to
>> restrict IR emissions to a few directions).
A sphere, of course, is the WORST shape for maximizing your
surface area/volume ratio.
>Good point. So, how big is our dreadnought? The volume of a ship of 100m
>radius is about 4 million m^3, so even if it's built as flimsy as Skylab
>was, it's going to mass about 400,000 tons. If it's built like a nuclear
>sub, more like 2 million tons. In that case the 2GW power figure is way too
>low -- assuming drive performance in the region of 1G, anyway, and a decent
>delta-v to be able to romp around the Solar System. For comparison, the STS
>(Shuttle + boosters) masses about 2,000 tons at liftoff and develops 130GW,
>and that's a very high fuel flow rate and very low Isp, both of which push
>the ratio down as I understand it (imperfectly).
Hmm. I've been thinking in terms of nuclear ablative ice
battleships around this size. A 100m radius ship would have
a mass around 4 million tons. At 10m/s/s and an exhaust
velocity of 10,000m/s, 200TW of power is required. That's
5 orders of magnitude greater than the 2GW figure! Damn
it's a good thing nuclear H-bombs are incredibly powerful
and the nuclear ablative drive I envision is self-cooling.
Still, a 100m radius ship is damn big and 4 million tons
of ship is stupendously massive, even if it's mostly just
ice. A 100m diameter ship at only 500,000 tons sounds
more plausible.
What about the beam weapons power? I envision the main
weapons to be 2m diameter spherical missiles powered
by external pulse lasers ablating layers of it (providing
both thrust power and guidance). Assuming they're made
of a material about as dense as water, this implies a
missile only around 4000kg heavy. With an exhaust
velocity of 10,000m/s, a high acceleration of 1000m/s/s
requires lasers operating at an average power of
20GW. This is a full order of magnitude greater than
the 2GW guestimate.
Therefore, I have to seriously think about this armament.
The pulse lasers would not be self-cooling. OTOH, these
ships are supposed to operate in fleets, and that high
acceleration of 100gees is only necessary during a short
boost phase. It only needs to cruise at 1 gee to react
to the maneuvers of a target evading at 1 gee. Another
thing to consider is that these ships have a massive
built in heat sink in its thick ice hull.
However, I suppose I'd better think of some hefty methods
of heat rejection or come up with some other form of main
weaponry. For example, the missiles perhaps should be
boosted with a Nerva-type rocket bus vehicle, eliminating
the need to lase for the initial boost phase.
>I suspect my initial power
>figure was too low for a proper space opera dreadnought :), but if the
>warship masses 1000 tons, 2GW seems to be in the ball park for a fusion-type
>power/mass ratio for interplanetary cruising.
Hmm. It still sounds too low. Assuming a 100,000m/s exhaust
velocity, 10/m/s/s acceleration requires 500GW. This corresponds
to about 10,000sec Isp, which is low for a fusion drive, and
about 1gee of acceleration.
But that's a heck of a lot of power. A "normal" fusion reactor
might simply not have enough power/mass ratio to do it, so a
much lower acceleration is necessary.
A nuclear bomb powered drive, like Orion, could be that powerful,
and it could be self-cooling, too.
[...]
>What this may also mean is that the visualization of a "dreadnought" may
>need to change a bit -- perhaps it has a high delta-v but continuously
>accelerates at 0.01 gravities for weeks instead of a couple hours at 1G, so
>the power plant can be far lower ouput (but then it's missiles or nothing --
>not much power for energy weapons).
Assuming a 1000ton ship with a 1,000,000m/s exhaust velocity,
and .1m/s acceleration, the required power is 50GW, which is
still 2 orders of magnitude greater than your 2GW guestimate.
This corresponds to 100,000sec Isp, which is a bit high
for an interplanetary fusion drive but still well short of
what's desireable for an interstellar mission.
For a more reasonable 100,000m/s exhaust velocity, .1m/s
acceleration only requires 5GW. That's still a heck of a lot.
Oh--I assume a 100% efficient drive, which isn't going to
happen, of course.
The incredible powers involved makes me think mostly in
terms of self-cooling drives.
>Or it's built as a big, very thin disk
>with a bulge at the center, in which case laser combat consists of trying to
>keep your own ship edge-on to the enemy, while ideally forcing them to
>orient flat-on to the sun and soak up more heat that way (although I guess
>you'd need two attackers to ensure that). The thing is, heat rejection is so
>important to any spacecraft with a major power source on board that it
>really should drive a lot of the overall design philosophy. That generally
>doesn't come up in SF, it seems (and after all, the story should come first
>anyway); but it's a bit like designing a submarine without paying attention
>to hydrostatic pressure.
I'd make an analogy to designing a submarine without paying attention
to oxygen. On board air makes it so you really wouldn't notice the
problem--for a little while, that is.
But the problem of oxygen is a little meaningless unless you
know what you're going to do with it. If you're using a diesel
drive, you're going to need a whole lot of it. Furthermore,
you're not going to have enough power to generate oxygen.
But if you're clever you're going to use batteries, or better
yet nuclear power. With the latter, you can even generate
oxygen for the crew.
Similarly, with a spaceship, you're not going to get far
if you're using an electric drive. It'd be nice to get
rid of all your waste heat by dumping it into the exhaust,
but the exhaust is a really hot stream of ions or plasma.
It's damn tough to try to pump waste heat into that!
If you're clever, you'll use a self-cooling drive which
doesn't bother with converting the kinetic energy of
reaction products into electricity, like Orion. The
waste heat is already in the superhot reaction products,
which you'll dump (the neutral particles/photons) or
deflect (the charged particles).
Jonathan Cresswell
Isaac Kuo <k...@oldbit.csc.lsu.edu> wrote in message
news:8orim3$12og$1...@its1.ocs.lsu.edu...
> In article <bnUr5.19899$Z2.2...@nnrp1.uunet.ca>,
> Jonathan Cresswell <jcressw...@netrover.com> wrote:
> >>>On the back of a very unofficial envelope, then...
>
> >>>Take a classic space opera warship. Onboard power is generated by one
or
> >>>more fusion reactors. If the overall power is 2 gigawatts, and the
> >>>efficiency is 90% (a pretty generous estimate, since projections I've
> >>>seen for MHD power generation are around 60%) then at full power, the
> >>>reactors create 200MW of waste heat.
>
> The important issue actually isn't the overall efficiency of the
> reactor, it's the ratio between useful energy obtained/used
> to the amount of waste heat absorbed by the ship and fixed
> reactor components.
>
> Speculative fusion reactor designs I've seen include designs where
> the reactor is a cage-like structure where most of the fusion
> products aren't ever absorbed by the ship itself. The charged
> particle products can directly be used as a rocket exhaust
> without generating much waste heat by the use of a superconducting
> magnetic nozzle. This would be nowhere near 90% efficient due
> to all the products not deflected by the magnetic nozzle and
> the imperfect directionality of the exhaust. However, the
> important thing is that almost all of the waste heat is in
> the ejected products and never absorbed by the ship itself.
Right. The reactor is stuck on one end of a long boom (or rather, the
reactor is distributed along a long boom) with a thick radiation shield
between it and the rest of the ship. The reactor components only soak up
whatever heat they absorb from the stream of plasma held within them, and
they have their own HR system distinct from the environmental one, which can
run pretty hot and hence efficiently. Not very Hollywood (who prefer beefy,
armoured ships) but it ought to work fine.
> >If
> >>>there are energy weapons, assume they too are 90% efficient and use
> >500MW
> >>>of power when fired, generating another 50MW waste heat. Lump in a lot
> >of
> >>>other minor systems and you get something like 300MW total waste heat
> >>>that has to be gotten rid of at peak.
>
> "If" there are energy weapons? Well, if there aren't, what the
> heck are you doing with all that power? That's a rhetorical
> question--there's another blatantly obvious high power use
> (the drive).
>
> However, it's worth thinking about what exactly it is you need
> power on the ship for. The ship's drive is a good example of
> why it's important. Even an inefficient drive does not
> necessarily put much waste heat into the ship itself.
>
> Similarly, certain energy weapons don't necessarily put much
> waste heat into the ship. Chemical lasers with non-recycled
> lasing fuel dump most of their waste heat along with the
> spent fuel. Nuclear pumped X-ray lasers similarly dump most
> of their waste heat outside the ship. These weapons have
> rather limitted ammunition compared to what we usually think
> of when we think of energy weapons, but their ability to
> fire at extremely high power levels without dumping much
> waste heat into the ship could be overriding advantages.
>snip<
> However, I suppose I'd better think of some hefty methods
> of heat rejection or come up with some other form of main
> weaponry. For example, the missiles perhaps should be
> boosted with a Nerva-type rocket bus vehicle, eliminating
> the need to lase for the initial boost phase.
If the NERVA core has many sub-critical-mass components, then when it
detonates it might simply blast all of them together to form a critical
mass -- no separate warhead to carry. You use it twice that way.
>
> >I suspect my initial power
> >figure was too low for a proper space opera dreadnought :), but if the
> >warship masses 1000 tons, 2GW seems to be in the ball park for a
fusion-type
> >power/mass ratio for interplanetary cruising.
>
> Hmm. It still sounds too low. Assuming a 100,000m/s exhaust
> velocity, 10/m/s/s acceleration requires 500GW. This corresponds
> to about 10,000sec Isp, which is low for a fusion drive, and
> about 1gee of acceleration.
I was figuring a much lower exhaust velocity. Think of it as an
afterburner -- you run at high Isp and low accel most of the time, but in
combat you dump raw hydrogen or water into the "burners" and get 1G or 2G
for the same power.
>
> But that's a heck of a lot of power. A "normal" fusion reactor
> might simply not have enough power/mass ratio to do it, so a
> much lower acceleration is necessary.
>
> A nuclear bomb powered drive, like Orion, could be that powerful,
> and it could be self-cooling, too.
>
> [...]
> >What this may also mean is that the visualization of a "dreadnought" may
> >need to change a bit -- perhaps it has a high delta-v but continuously
> >accelerates at 0.01 gravities for weeks instead of a couple hours at 1G,
so
> >the power plant can be far lower ouput (but then it's missiles or
nothing --
> >not much power for energy weapons).
>
> Assuming a 1000ton ship with a 1,000,000m/s exhaust velocity,
> and .1m/s acceleration, the required power is 50GW, which is
> still 2 orders of magnitude greater than your 2GW guestimate.
> This corresponds to 100,000sec Isp, which is a bit high
> for an interplanetary fusion drive but still well short of
> what's desireable for an interstellar mission.
>
> For a more reasonable 100,000m/s exhaust velocity, .1m/s
> acceleration only requires 5GW. That's still a heck of a lot.
>
Yeah, it's hard to get used to, isn't it? :) I keep getting surprised at the
figures...
Of all the times I pick to go on vacation...you guys run with this a bit in
the meantime, OK?
--
Jonathan C
Christopher M. Jones
"Jonathan Cresswell" <jcressw...@netrover.com> wrote:
> Good point. So, how big is our dreadnought? The volume of a ship of 100m
> radius is about 4 million m^3, so even if it's built as flimsy as Skylab
> was, it's going to mass about 400,000 tons. If it's built like a nuclear
> sub, more like 2 million tons. In that case the 2GW power figure is way
too
{...}
I never understand this. Why wouldn't we assume that it would be made
out of lighter and stronger materials than we use today? It makes a
whole heck of a lot of sense to build spaceships out of advanced
composite materials. And yet, you see time and time agains nearly
everyone assuming that they will be built out of thick slabs of some
sort of metal alloy. If you ask me, that's just insane. Especially
so when you consider that any amount of "armor" in the traditional
sense is going to be pretty useless and will in fact be very harmful
since it makes you a lot heavier and a lot slower.
jtingle
>
> "Keith Morrison" <kei...@polarnet.ca> wrote in message
> news:39B419FE...@polarnet.ca...
> > Peter Kwangjun Suk wrote:
>
> > If you can create neutrinos from waste heat, then you presumably
> > can detect neutrinos. If you can detect neutrinos, you've just
> > given the enemy a detection system superior to infrared because
> > now the ship can't hide behind comets, asteroid or planets.
>
> I'm not sure that follows. Substitute infrared photons for neutrinos and
> radiators predate decent infrared sensors by a couple centuries.
Depends on how you want to figure it. The first infrared detectors were
liquid thermometers attached to a black absorber behind a prism: "In fact,
when Herschel discovered the infrared spectrum in the year 1800 he utilized
a simple type of thermal detector, a liquid in a glass thermometer, which is
considered the first thermal detector. The infrared radiation was absorbed
by blackening the bulb of the thermometer." -
http://www.imc.kth.se/rdtec/intro2.htm
or: "In the early 19th century, William Herschel took a prism and saw the
expected 6 colors split out of the white light, just as Newton had. Then
Herschel placed a thermometer next to the red area and noticed that the
thermometer showed increased heat. This he called infrared, and it was the
first detection of a form of light invisible to the human eye." -
http://www.powersof10.com/powers/patterns/patterns/patterns_23.html
Good heat exchanger type radiators date from about the same time. In fact,
decent radiation heat exchange systems didn't really come of age until the
1900's when solar power took off (for the first time), since most
"radiators" in HVAC systems are really mostly convectors.
Bolometric detectors were available in the early 20th century.
I'd say Mr. Suk is substantially right.
Regards,
Jack Tingle
Keith Morrison
> or: "In the early 19th century, William Herschel took a prism and saw the
> expected 6 colors split out of the white light, just as Newton had. Then
> Herschel placed a thermometer next to the red area and noticed that the
> thermometer showed increased heat. This he called infrared, and it was the
> first detection of a form of light invisible to the human eye." -
> http://www.powersof10.com/powers/patterns/patterns/patterns_23.html
>
> Good heat exchanger type radiators date from about the same time. In fact,
> decent radiation heat exchange systems didn't really come of age until the
> 1900's when solar power took off (for the first time), since most
> "radiators" in HVAC systems are really mostly convectors.
>
> Bolometric detectors were available in the early 20th century.
>
> I'd say Mr. Suk is substantially right.
Me, actually.
--
Keith
jti...@my-deja.com
Sorry, I did have a bit of trouble telling who'd quoted who.
Apologies,
Jack Tingle
Keith Morrison
>
> On Wed, 23 Aug 2000 18:00:51 +0200, ma...@iname.com (Matthias Warkus)
> wrote:
>
> [snip]
> >My questions: Which is the most effective radiator design you can
> >imagine for a future spaceship? If possible, it shouldn't be
> >handwaved, so neutrino beams are out :)
>
> Beams would be pushing things a bit. But I like the idea of
> handwaving neutrino emitters, then working out the physical, tactical,
> and strategic consequences, starting with neutrino radiators.
Well, as I said, if someone has managed a more efficient neutrino
detector than we have now, your ship is screwed trying to hide
anywhere.
--
Keith
Keith Morrison
> > If you can create neutrinos from waste heat, then you presumably
> > can detect neutrinos. If you can detect neutrinos, you've just
> > given the enemy a detection system superior to infrared because
> > now the ship can't hide behind comets, asteroid or planets.
>
> So use the neutrino emitter only when you need to dissipate massive amounts
> of heat quickly, ie. in combat - where being detected will probably be among
> the least of your problems anyway.
Oh, I know. I was just replying to the comment that it could be
used for stealth.
--
Keith
Aleksi Liimatainen
"Keith Morrison" <kei...@polarnet.ca> wrote in message
news:39B419FE...@polarnet.ca...
>
> If you can create neutrinos from waste heat, then you presumably
> can detect neutrinos. If you can detect neutrinos, you've just
> given the enemy a detection system superior to infrared because
> now the ship can't hide behind comets, asteroid or planets.
>
So use the neutrino emitter only when you need to dissipate massive amounts
of heat quickly, ie. in combat - where being detected will probably be among
the least of your problems anyway.
--
Aleksi | Some minds are like concrete -
Liimatainen | all mixed up and permanently set.