CNN article about nuclear power on space probes

Here's an article that CNN has about nuclear power on space probes. I
list some questions about it below

http://www.space.com/businesstechnol..._focus_040218-
1.html
(Note the line wrap on link.)

One thing that kind of bothers me is the statement that, "The Energy
Department, working with industry, is designing a space-qualified nuclear
fission reactor capable of generating 100 kilowatts of power -- about
1,000 times more than most solar-powered space probes have available
today." Is it true that the average space probe today only uses about
100 Watts? I'm pretty sure that thin film collectors can produce up to
several kilowatts per kilogram of PV panel (at Earth orbit insolation
levels) and that fresnels or other concentrators can improve this power
to weight ratio further. OTOH, the thermal energy produced by a kg of
PU238 is about 500 Watts and a stirling may only allow them to get 30% of
that converted into electricity. Furthermore, new technologies are
allowing for even lighter weight PV and light weight solar thermal
concentrators. I'd think that you'd have to be very far from the sun or
doing fairly exotic things before an RTG only approach would be the best
or the cheapest.

Also, I was wondering about this. It seems to me that rather than using
just Stirlings or just thermal diodes that it should be possible to use
the Stirling as the primary source and the thermal diodes to top off.
The diodes and thermionics are very light weight. They would remove some
of the heat that would otherwise be available to the stirling engine, but
if they didn't drop the overall temperature inordinately then I would
think that they could still produce net power beyond what the Stirling
alone could achieve. Modern thermal diodes can actually be quite
efficient. But, of course, like I said above, these same technologies
could also be used with solar concentrators on many space probes.

--
__________________________________________________ ___
Quibbler (quibbler247atyahoo.com)
"It is fashionable to wax apocalyptic about the
threat to humanity posed by the AIDS virus, 'mad cow'
disease, and many others, but I think a case can be
made that faith is one of the world's great evils,
comparable to the smallpox virus but harder to
eradicate." -- Richard Dawkins

By looking at the MERs the solar panels have no future in robotic
exploration. They need to be faced towards the sun to produce anything and
they require lots of space when deployed. You also need heavy onboard
batteries to cover the blackouts. I seriously doubt solar panels can come
anywhere near RTG in watts per kg comparison, especially if they operate on
the planet with night/day cycles.

quibbler wrote:
Here's an article that CNN has about nuclear power on space probes. I
list some questions about it below

http://www.space.com/businesstechnol..._focus_040218-
1.html
(Note the line wrap on link.)

One thing that kind of bothers me is the statement that, "The Energy
Department, working with industry, is designing a space-qualified nuclear
fission reactor capable of generating 100 kilowatts of power -- about
1,000 times more than most solar-powered space probes have available
today." Is it true that the average space probe today only uses about
100 Watts? I'm pretty sure that thin film collectors can produce up to
several kilowatts per kilogram of PV panel (at Earth orbit insolation
levels) and that fresnels or other concentrators can improve this power
to weight ratio further. OTOH, the thermal energy produced by a kg of
PU238 is about 500 Watts and a stirling may only allow them to get 30% of
that converted into electricity.

They're talking about a nuclear fission reactor when they are discussing
the 100 KWs of power. This is different from an RTG, which just uses the
heat due to natural decay.

quibbler wrote:

I'd think that you'd have to be very far from the sun or
doing fairly exotic things before an RTG only approach would be the best
or the cheapest.

General rule of thumb is... Mars is PV, Jupiter is nuke. Trade off in
the asteroids.

--
Scott Lowther, Engineer
Remove the obvious (capitalized) anti-spam
gibberish from the reply-to e-mail address

In article ,
says...
By looking at the MERs the solar panels have no future in robotic
exploration.

Sorry, but the MERs show quite the opposite. Those solar panels are
extremely cheap and lightweight compared to RTGs. They produce quite
respectable amounts of power and could obviously produce more if they had
been made to track on one or two axes. For that matter, a solar thermal
concentrator could be used to drive a direct heat engine during the day,
making solar an extremely efficient option.

They need to be faced towards the sun to produce anything

Well most all solar collectors and PV will produce around double the
power if they track. However, actually many systems including amorphous
PV and flat panel collectors can work quite well with diffuse light.

and
they require lots of space when deployed.

Perhaps. I'm not sure that this is a problem in most cases.


You also need heavy onboard
batteries to cover the blackouts.

Yes, for things like rovers perhaps. For normal probes they can rely on
sun virtually all the time. It is also worth noting that one could use a
small 1 or 2 kilo RTG as a backup power source, rather than a real
battery. It will provide plenty of waste heat in cold areas like mars
and as well as reasonable amounts of standby power. As you probably know
the MERs do have about 8 tiny radio-isotopic heaters on board.


I seriously doubt solar panels can come
anywhere near RTG in watts per kg comparison,

I gave the figures. RTGs standardly produce 500W thermal per kg of fuel.
That is standardly only converted at around 10% efficiency or less with
thermal diodes. OTOH, PV can easily get several KW electrical per pound
of solar panel, especially if the panels are equipped with tracking
capabilities. Even cutting this capacity factor by about 3 for 8 hours
of sunlight and assuming the lower insolation of mars means that PV and
other solar technologies come out ahead.

However, I think that a combination of solar and RTG could work quite
well. Things like amorphous PV can be coated onto many external surfaces
and provide basic DC power needs during the day. Like I said before,
you'd have to be very far from the sun, or in a very special environment,
before an RTG only approach would be the best one.

especially if they operate on
the planet with night/day cycles.

Of course there are appropriate technologies. It would be nice to have
an RTG so that a rover could keep operating at night. But I don't see
any fundamental technology breakthrough on the horizon that will make
RTGs substantially cheaper. OTOH, solar continues to improve in
efficiency and drop in cost virtually every five minutes.

--
__________________________________________________ ___
Quibbler (quibbler247atyahoo.com)
"It is fashionable to wax apocalyptic about the
threat to humanity posed by the AIDS virus, 'mad cow'
disease, and many others, but I think a case can be
made that faith is one of the world's great evils,
comparable to the smallpox virus but harder to
eradicate." -- Richard Dawkins

In article , says...
quibbler wrote:
Here's an article that CNN has about nuclear power on space probes. I
list some questions about it below

http://www.space.com/businesstechnol..._focus_040218-
1.html
(Note the line wrap on link.)

One thing that kind of bothers me is the statement that, "The Energy
Department, working with industry, is designing a space-qualified nuclear
fission reactor capable of generating 100 kilowatts of power -- about
1,000 times more than most solar-powered space probes have available
today." Is it true that the average space probe today only uses about
100 Watts? I'm pretty sure that thin film collectors can produce up to
several kilowatts per kilogram of PV panel (at Earth orbit insolation
levels) and that fresnels or other concentrators can improve this power
to weight ratio further. OTOH, the thermal energy produced by a kg of
PU238 is about 500 Watts and a stirling may only allow them to get 30% of
that converted into electricity.

They're talking about a nuclear fission reactor when they are discussing
the 100 KWs of power. This is different from an RTG, which just uses the
heat due to natural decay.

Yes, I suspect as much. Obviously a fission reactor can produce a lot
more power with a lot less weight, though it will require more shielding
mass. My question was really just about their claim that 100KW was 1000
times more than todays space probes. I find that to be rather dubious.
I would be quite easy for the average space probe to unfurl a couple KW
worth of solar panels or even a dozen KW. In the region of the outer
planets solar becomes less attractive, of course. I just think they
might have fudged their figures a bit with the three orders of magnitude
claim. But I'd like to see if anyone knows where they derived that
particular number.


--
__________________________________________________ ___
Quibbler (quibbler247atyahoo.com)
"It is fashionable to wax apocalyptic about the
threat to humanity posed by the AIDS virus, 'mad cow'
disease, and many others, but I think a case can be
made that faith is one of the world's great evils,
comparable to the smallpox virus but harder to
eradicate." -- Richard Dawkins

In article ,
says...
quibbler wrote:

I'd think that you'd have to be very far from the sun or
doing fairly exotic things before an RTG only approach would be the best
or the cheapest.

General rule of thumb is... Mars is PV, Jupiter is nuke. Trade off in
the asteroids.

Fair enough. Obviously inner planet missions like Venus orbiters would
also find PV pretty appealing. I think that there is around 2.3 KW/m^2
in the proximity of venus. Off hand, do you know how much solar power
one would get around Jupiter? (I know I could look it up or calculate it
)

--
__________________________________________________ ___
Quibbler (quibbler247atyahoo.com)
"It is fashionable to wax apocalyptic about the
threat to humanity posed by the AIDS virus, 'mad cow'
disease, and many others, but I think a case can be
made that faith is one of the world's great evils,
comparable to the smallpox virus but harder to
eradicate." -- Richard Dawkins

quibbler wrote in message et...
In article ,
says...
quibbler wrote:

I'd think that you'd have to be very far from the sun or
doing fairly exotic things before an RTG only approach would be the best
or the cheapest.

General rule of thumb is... Mars is PV, Jupiter is nuke. Trade off in
the asteroids.

Fair enough. Obviously inner planet missions like Venus orbiters would
also find PV pretty appealing. I think that there is around 2.3 KW/m^2
in the proximity of venus. Off hand, do you know how much solar power
one would get around Jupiter? (I know I could look it up or calculate it
)

The Rosetta mission launching in a few days uses PV and its orbit
perihelion takes it out to Jupiter's orbit. IIRC, it has enough to
get 800W at Jupiter distance from Sun.

For actual Jupiter missions, it would not be just the distance but
also the radiation environment closer to the planet. PV output in
Earth orbit degrades over time. An interesting question is if Galileo
had PV with initial output equal to initial RTG output, would the PV
output degrade faster than RTG output in the repeated passes close to
Jupiter?

quibbler wrote in message et...
Here's an article that CNN has about nuclear power on space probes. I
One thing that kind of bothers me is the statement that, "The Energy
Department, working with industry, is designing a space-qualified nuclear
fission reactor capable of generating 100 kilowatts of power -- about
1,000 times more than most solar-powered space probes have available
today." Is it true that the average space probe today only uses about
100 Watts? I'm pretty sure that thin film collectors can produce up to
several kilowatts per kilogram of PV panel (at Earth orbit insolation
levels) and that fresnels or other concentrators can improve this power
to weight ratio further. OTOH, the thermal energy produced by a kg of
PU238 is about 500 Watts and a stirling may only allow them to get 30% of
that converted into electricity. Furthermore, new technologies are
allowing for even lighter weight PV and light weight solar thermal
concentrators. I'd think that you'd have to be very far from the sun or
doing fairly exotic things before an RTG only approach would be the best
or the cheapest.

The article was about reactors, not RTGs, which are different
kinds of nuclear power sources. Also, Pu238 powered RTGs only
have maybe around 5W/kg for the RTG (about 1% of bulk Pu238
power (heat) output density). They do overstate the difference
between reactors and PV arrays. Most solar powered spacecraft
today use several hundred watts at least, unless they're some
sort of micro-satellite. And commercial satellite power
technology is right now able to deliver tens of kilowatts of
power via PV arrays. I think the current state of the art is
somewhere around 25-ish kilowatts of *delivered*, usable
electric power via PV arrays for commsats. The advantage of
reactors and RTGs is that they're more compact, sometimes more
mass efficient, and quite often more convenient. PV arrays
require proper pointing and only work well in direct sunlight
near the Sun. And even in Watts per kg the PV arrays aren't
all that stellar once you factor in all the necessary subsystems
(power converters, cooling, etc.)

In article ,
Mike Chan wrote:
...Off hand, do you know how much solar power
one would get around Jupiter? ...

The Rosetta mission launching in a few days uses PV and its orbit
perihelion takes it out to Jupiter's orbit. IIRC, it has enough to
get 800W at Jupiter distance from Sun.

No, its solar arrays will deliver only about 350-400W at Jupiter's
distance. (Versus 8700 near Earth.) It will set a new distance record for
solar-powered spacecraft, at around 5AU; if memory serves, the previous
record was set by NEAR at a mere 2.2AU.

Solar-powered operation out near Jupiter is not impossible, merely very
difficult. Rosetta is paying heavily -- in money, in mass, in deployment
worries, in moment of inertia that makes turns difficult -- for those huge
solar arrays that make it possible.

For actual Jupiter missions, it would not be just the distance but
also the radiation environment closer to the planet. PV output in
Earth orbit degrades over time. An interesting question is if Galileo
had PV with initial output equal to initial RTG output, would the PV
output degrade faster than RTG output in the repeated passes close to
Jupiter?

Rather a lot faster, I believe, but I don't have numbers.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |