NASASpaceFlight.com Forum
General Discussion => Q&A Section => Topic started by: Danderman on 05/24/2012 12:54 am
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Since its only 3 years to encounter, I have a question that can't wait. It's actually germane to any spacecraft that has a long mission duration, but a brief encounter period.
The RTG for New Horizons puts out 180 watts, of which 20 or so are allocated to the science instruments. The entire mass budget of the spacecraft is 480 kg.
I am wondering if there is some relatively lightweight power storage system that could be used for the brief encounter period and only for this period so that the science package would have more than 20 watts. Yeah, I know the package only needs 20 watts, but this is a hypothetical question: couldn't a spacecraft use an RTG for the 10+ year cruise phase, but for encounter use some small battery to generate an additional handful of watts? Maybe the RTG could keep the battery charged over the 10 years (I am assuming it would be a battery). A laptop battery could probably deliver 50 watts for 5 hours, no problem, and those barely weigh a couple of pounds.
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A laptop battery could probably deliver 50 watts for 5 hours, no problem, and those barely weigh a couple of pounds.
sure.. commercially available Li-Ions give you about 100 to 250 Wh/kg, at 250 to 340 W / kg.
But isn't the remainder of the RTG power for propulsion? Couldn't they just reallocate it if they don't need both at the same time?
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New Horizons has hydrazine monoprop thrusters for propulsion/attitude control, not something that could use the output of the RTG.
Would the output of the RTG be enough to even matter for an ion propulsion system like that aboard Dawn? The solar arrays that run it are rated at 10Kw.
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I don't know why PNH didn't do that. MSL operates from its battery
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Perhaps they could not find a lightweight battery guaranteed to work after 10 years.
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A laptop battery could probably deliver 50 watts for 5 hours, no problem, and those barely weigh a couple of pounds.
Couldn't they just reallocate it if they don't need both at the same time?
No, the spacecraft housekeeping requirements only provide for 20 watts of power for the science package at encounter.
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Could it be that PNH needs the heat from the relatively large RTG? I recall that Cassini has small
RTG radioisotope heaters at various places.
EDIT: Replaced "RTG" with "radioisotope": the heat from the radioactive decay is used directly, without conversion to electricity.
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Could it be that PNH needs the heat from the relatively large RTG? I recall that Cassini has small RTG heaters at various places.
I am not suggesting that the RTG be replaced by an alternate power system, but rather asking why such a spacecraft would not carry some sort of battery to provide for additional power requirements during the short encounter period at Pluto.
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I understand. What I'm suggesting is that maybe the spacecraft requires more heat than would be provided by an RTG sized to provide adequate electrical power in conjunction with a battery. Perhaps the designers decided that a larger RTG was worthwhile, because it would provide the heat needed and eliminate the need for a battery.
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IIRC the NH RTG was a leftover from Cassini. Maybe 20 Watts was more than they needed for the instruments.
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New Horizons has hydrazine monoprop thrusters for propulsion/attitude control, not something that could use the output of the RTG.
(to sound like Jim) Wrong.
A considerable amount of power is devoted to keeping the hydrazine catalyst beds warm. That's not an issue for Earth-orbit spacecraft, but as Mars Observer found out, hydrazine freezes quick, especially when you're 30 AU from the Sun (and thus get 1/900 the solar heating). There's a reason that Pluto's surface temperature is 40 K.
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New Horizons has hydrazine monoprop thrusters for propulsion/attitude control, not something that could use the output of the RTG.
(to sound like Jim) Wrong.
If you're trying to be cute sounding like Jim, well....fail.
I was addressing QuantumG's comment about using the remainder of the RTG power for propulsion. Keeping the spacecraft systems warm enough to avoid anything freezing up is not the same as using the RTG output for actual propulsion of the spacecraft.
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It's not keeping it from freezing that consumes the power, but rather the heated catalyst beds of the thrusters. Each thruster consumes 2.2W when ready for operations, totaling up to 35.2W when all 16 are ready, such as during the encounter. And that genuinely is power being used for propulsion, as without the heaters, the Isp of the thrusters drops considerably.
For more on the power system (and spacecraft in general): www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf
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heh.. all very interesting but I didn't know any of that :)
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It's not keeping it from freezing that consumes the power, but rather the heated catalyst beds of the thrusters. Each thruster consumes 2.2W when ready for operations, totaling up to 35.2W when all 16 are ready, such as during the encounter. And that genuinely is power being used for propulsion, as without the heaters, the Isp of the thrusters drops considerably.
For more on the power system (and spacecraft in general): www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf
It would be great if such spacecraft could carry a small battery that would only be used at encounter to power science equipment for a few hours - that would certainly increase the power budget for science.
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How significant of a constraint is the power budget on the science payload, compared to the mass budget or the energy budget to send back the data? It could just be that more power around approach just doesn't actually help that much and it's not worth the mass and complexity.