Voyager probe RTG energy saving

[Bob Masta]

There have been stories in the news lately about NASA
switching the Voyager 1 and 2 probes to using backup
thrusters to save energy. This is supposed to save 12 watts
by allowing the primary thruster heaters to be turned off.

<www.space.com/13538-nasa-voyager-spacecraft-backup-thrusters.html>
<www.time.com/time/health/article/0,8599,2099245.00.html>

The articles claim that this energy saving will allow
extended lifetime of the power supply, which is a
radioisotope thermoelectric generator powererd by decay of
plutonium 238.

Huh? How does the electrical load on the thermocouples have
any effect on the decay of plutonium?

No explanation is offered. Assuming the life extension is
correct, my only guess is that it is some strange
second-order effect, like maybe the thermocouples have some
sort of electromigration failure mode that is sped up at
higher currents... or something.

Anyone know the true story?

Best regards,


Bob Masta

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[fungus]

On Nov 30, 2:23 pm, N0S...@daqarta.com (Bob Masta) wrote:

The articles claim that this energy saving will allow
extended lifetime of the power supply

Not quite...it will extend the life of the probe, which
isn't the same thing.

The output of the power supply is steadily dropping.
It was 470 Watts at launch (according to Wikipedia)
and the JPL article says it's now at 270. Reducing
power consumption by the thrusters will allow other
systems to function instead.

 

[BeeJ]

fungus explained on 11/30/2011 :

On Nov 30, 2:23 pm, N0S...@daqarta.com (Bob Masta) wrote:
The articles claim that this energy saving will allow
extended lifetime of the power supply


Not quite...it will extend the life of the probe, which
isn't the same thing.

The output of the power supply is steadily dropping.
It was 470 Watts at launch (according to Wikipedia)
and the JPL article says it's now at 270. Reducing
power consumption by the thrusters will allow other
systems to function instead.

You have to squeeze plutonium really hard to get juice and then it
doesn't taste all that good.

 

[Chiron]

On Wed, 30 Nov 2011 13:23:05 +0000, Bob Masta wrote:

<snip>

Huh? How does the electrical load on the thermocouples have any effect
on the decay of plutonium?

No explanation is offered. Assuming the life extension is correct, my
only guess is that it is some strange second-order effect, like maybe
the thermocouples have some sort of electromigration failure mode that
is sped up at higher currents... or something.

Anyone know the true story?

I'm not sure how the reactor in space is managed; on earth, the rate of
fission is controlled through the use of moderators that can cause it to
speed up or slow down. If they can save some energy, that means they can
reduce the rate of fission, extending the life of the fuel. But as I
said, I don't know whether they use controllable fission or whether the
rate is fixed.

--
Life is the living you do, Death is the living you don't do.
-- Joseph Pintauro

 

[Fleetie]

Chiron wrote:

I'm not sure how the reactor in space is managed; on earth, the rate of
fission is controlled through the use of moderators that can cause it to
speed up or slow down. If they can save some energy, that means they can
reduce the rate of fission, extending the life of the fuel. But as I
said, I don't know whether they use controllable fission or whether the
rate is fixed.

I think (as in 99% sure) that there is no such form of control. It's a warm lump
of plutonium that gets cooler as the decades go by and it decays. It is in
thermal contact with the warm junctions of a set of thermocouples, I think,
and the cold junctions radiate to cold space. In the usual way, the temperature
difference causes a voltage to develop across the thermocouples.


Martin

 

[krw@att.bizzzzzzzzzzzz]

On Thu, 05 Jan 2012 21:22:21 +0000, Fleetie <fleetie@fleetie.demon.co.uk>
wrote:

Chiron wrote:

I'm not sure how the reactor in space is managed; on earth, the rate of
fission is controlled through the use of moderators that can cause it to
speed up or slow down. If they can save some energy, that means they can
reduce the rate of fission, extending the life of the fuel. But as I
said, I don't know whether they use controllable fission or whether the
rate is fixed.

I think (as in 99% sure) that there is no such form of control. It's a warm lump
of plutonium that gets cooler as the decades go by and it decays. It is in
thermal contact with the warm junctions of a set of thermocouples, I think,
and the cold junctions radiate to cold space. In the usual way, the temperature
difference causes a voltage to develop across the thermocouples.

Correct. It's not a "reactor" at all in that there is no "chain reaction" to
control/moderate. It's simple radioactive decay of a lump of PU. As the PU
decays it generates less heat, less delta-T = less power generated. The
"power management" that's going on is simply choosing which instruments to
shut down so the thing operates the most interesting experiments (and of
course the radios) with the available power left.

 

[bw]

<krw@att.bizzzzzzzzzzzz> wrote in message
news:f2gcg75569redhm74fdm0r28cvockfcf7f@4ax.com...

On Thu, 05 Jan 2012 21:22:21 +0000, Fleetie <fleetie@fleetie.demon.co.uk
wrote:

Chiron wrote:

I'm not sure how the reactor in space is managed; on earth, the rate of
fission is controlled through the use of moderators that can cause it to
speed up or slow down. If they can save some energy, that means they
can
reduce the rate of fission, extending the life of the fuel. But as I
said, I don't know whether they use controllable fission or whether the
rate is fixed.

I think (as in 99% sure) that there is no such form of control. It's a
warm lump
of plutonium that gets cooler as the decades go by and it decays. It is in
thermal contact with the warm junctions of a set of thermocouples, I
think,
and the cold junctions radiate to cold space. In the usual way, the
temperature
difference causes a voltage to develop across the thermocouples.

Correct. It's not a "reactor" at all in that there is no "chain reaction"
to
control/moderate. It's simple radioactive decay of a lump of PU. As the
PU
decays it generates less heat, less delta-T = less power generated. The
"power management" that's going on is simply choosing which instruments to
shut down so the thing operates the most interesting experiments (and of
course the radios) with the available power left.

PU must mean some chemical combination of P and U
Plutonium is Pu
Phosphorus is P
Uranium is U

 

[krw@att.bizzzzzzzzzzzz]

On Thu, 5 Jan 2012 20:02:40 -0600, "bw" <bwegher@hotmail.com> wrote:

krw@att.bizzzzzzzzzzzz> wrote in message
news:f2gcg75569redhm74fdm0r28cvockfcf7f@4ax.com...
On Thu, 05 Jan 2012 21:22:21 +0000, Fleetie <fleetie@fleetie.demon.co.uk
wrote:

Chiron wrote:

I'm not sure how the reactor in space is managed; on earth, the rate of
fission is controlled through the use of moderators that can cause it to
speed up or slow down. If they can save some energy, that means they
can
reduce the rate of fission, extending the life of the fuel. But as I
said, I don't know whether they use controllable fission or whether the
rate is fixed.

I think (as in 99% sure) that there is no such form of control. It's a
warm lump
of plutonium that gets cooler as the decades go by and it decays. It is in
thermal contact with the warm junctions of a set of thermocouples, I
think,
and the cold junctions radiate to cold space. In the usual way, the
temperature
difference causes a voltage to develop across the thermocouples.

Correct. It's not a "reactor" at all in that there is no "chain reaction"
to
control/moderate. It's simple radioactive decay of a lump of PU. As the
PU
decays it generates less heat, less delta-T = less power generated. The
"power management" that's going on is simply choosing which instruments to
shut down so the thing operates the most interesting experiments (and of
course the radios) with the available power left.

PU must mean some chemical combination of P and U
Plutonium is Pu
Phosphorus is P
Uranium is U

What a moron.