NASA Allows RTG Proposals Again
“From: James Green, Director, NASA’s Planetary Science Division via Lunar and Planetary Institute: After analysis and consultation with the Department of Energy, NASA’s Planetary Science Division is pleased to announce that the ban on the use of Radio-isotope Power Systems (RPS) by proposers responding to the upcoming Discovery 2018 Announcement of Opportunity (AO) has been removed. Proposers will be able to include the use of up to two (2) Multi-mission Radio-Isotope Thermal Generators (MMRTG’s) to enable or significantly enhance their mission concept. Costs to be borne by proposers for the MMRTGs, the related environmental impact assessments, and the required Nuclear Launch Approval process will be announced once determined.”
Good for a potential Enceladus life sampling mission! It beats having to include some truly massive solar panel arrays just to eke out a tiny amount of power.
I’m not so sure how much this will help outer solar system Discovery missions. At the end, they say, “Costs to be borne by proposers for the MMRTGs, the related environmental impact assessments, and the required Nuclear Launch Approval process will be announced once determined.”
They had the same language in the New Frontiers 4 AO (Dec. 2016). It’s $77 million for one MMRTG and $94 million for two. That’s a big chunk of the $495 million cost cap. Dragonfly (one of the two NF4 proposals selected for further study) would use one. But the cost cap for New Frontiers is about twice that for a Discovery.
It would be interesting if, say, a substantially cheaper launch vehicle might give a little more leeway under the Discovery cap….
Not directly. Budgets for Discovery missions are a little convoluted. The formal cap ($495 million) is what the PI is responsible for up until launch. Other things, like operations after launch and the cost of using the Deep Space Network antennas are estimated and considered in selecting the proposals, but they aren’t within the cap. That includes launch services.
Proposals which need “high performance” vehicle or special launch services are penalized by having their cost cap lowered; proposals which only require a “low performance” launch are allowed a higher cap. But other than that, the cost of the launch is NASA’s problem.
At the same time, you can solve lots of problems by throwing mass at them. If someone develops a launch vehicle can lift more and still cost acceptable to the Discovery program, there are lots of things you could do. For example, radiation hard electronics often draw way more power than modern, commercial equivalents. With mass to spare, you could use the commercial parts and not need an RTG due to the lower power (or need fewer RTGs.)
I thought the actual issue was lack of the fuel itself?
I would agree. A quick search finding nothing about a usage ban on Radio-isotope Power…on this site or google. This was the known issue I remember reading. http://nasawatch.com/archiv…
It was a bit obscure. The “ban” was in an email to a mailing list. NASA uses it for scientists who want to keep up with the status of current and future opportunities. (Usually, it’s things like, “due to a blizzard, we are extending some proposals deadlines by one week.”) This one gave the planned release date for the next Discovery AO and a few of the planned details. That included things like the cost cap and the fact that they don’t want a extrasolar planet mission this time, as well as a no-RTG rule.
That sort of rule isn’t uncommon. RTG’s aren’t cheap or easy to come by and giving one to a small mission might delay a flagship mission. Saying so means people know not to waste their time proposing an unselectable concept. Saying one or two RTGs but not more is also helpful to people putting together proposals.
The Department of Energy has Pu238 back in production. But at the current production rate, they can only make enough for one MMRTG every year and a half. That provides 110 W at launch, and drops by 4-5% per year. New Frontiers 4 might use one (if Dragonfly is selected), Mars 2020 will use one, and if they go ahead with a Uranus or Neptune mission, it would almost certainly use a few (three to five, based on recent studies.) That’s potentially a decade of Pu238 production. NASA is being careful about how they commit what the DoE can produce.
It seems that perhaps the production rate could be increased.
Oak Ridge has only been producing Pu-238 for a couple of years now and can only make small amounts. From what I have read they will eventually be able to increase production rates but that is expected to take a few more years.
Maybe the Iranians will help?
I wonder if an increase in production will be related to updating of weapons.
I don’t think so. The raw material for Pu-238 production is a byproduct of making weapons grade material, but there is plenty of the raw material left over from the Cold War. The only reason that Pu-238 production stopped was because Savannah River where it was previously produced shut down most of their operations after the Cold War ended. Oak Ridge recently picked up the Pu-238 mantle, however they are using completely different equipment and it takes time to get production up to speed as it involves improving efficiency in what is a quite complicated process.
Specifically, we have a stockpile (or radioactive waste storage problem) of about 300 kg of Np237. That’s got a 2 million year half-life, and we’re not in a position to convert more than a few kilos of it to Pu238 per year. I’m hopeful that we’ll have a better power source before that runs out.
Maybe by a factor of two. But anything more would take a significant investment. I think we’d get more for our money by developing better RTGs. The current MMRTGs only turn 5.5% of the energy into electricity. The rest is radiated as waste heat. Worse, the converters degrade, so the output goes down at 4 or 5% per year. (The half-life of Pu237 only accounts for about 1% of that.)
NASA (Glenn Research Center in particular) is working on an enhanced MMRTG, which should have a conversion efficiency of 7 or 8%, output dropping at under 2.5% per year, and ready for flight by 2022. And a next generation RTG with over 10-15% efficiency, decreasing at under 2% per year and ready by 2028. If funding for that work continues, within a decade, the same amount of power will require two or three times less Pu237 _and_ require a lower mass system.
This is something NASA should be partnering with DOE and going “all in” on this. I really like what this may provide for SMD. We need to continue to develop this technology and make it better.
It requires a significant investment but it is worth it.
NASA is partnering with DoE. Pu238 is back in production, and work on improved RTGs is in progress. But the budget and available resources only support so much. The RTG rules are about managing that and making sure the mission which really need and can make good use of RTGs get them. Note that a Discovery proposal asking for three or more MMRTGs is still ruled out; the change is just their saying, “we thought about it some more and we can spare one or two.”
That is the problem. Everyone talks about it but we never really fund it like we should. Honestly I would be all for scrapping a lot of filler and getting this going properly to where we could have missions using RTGs at a cadence of one per year… for starters.
Instead we dole out the crumbs and don’t make the progress we should.
JMHO
That would make sense if NASA had funding to fly one mission per year that needed a RTG. NASA isn’t even flying one planetary mission per year. In a very optimistic world, NASA might be able to manage a Discovery every two years (there is a stated intention to select two for flight every four years), a New Frontiers mission every five years and a flagship every decade.
Nothing in the inner solar system really benefits from RTGs, other than lunar and martian landers. Even Jupiter is marginal. Solar has improved to the point where there isn’t a real difference in mass, but the large size of solar arrays at Jupiter is cumbersome. It’s hard for me to see how NASA would need more than ten MMRTGs per decade, unless its budget went way up or the cost of missions went way down.
Ten MMRTGs per decade is less than the production rate, but they are ramping up. And, with the higher efficiency of eMMRTGs (ready for flight in 2022), we may end up with more RTGs than missions to use them.
Don’t get me wrong. I’d love to see a mission to the outer solar system launch every year. But until there is funding for that, cranking up RTG production isn’t really a good idea. The money would be better spent on things like Deep Space Network upgrades (which is also a resource limiting planetary missions.) Or improved instruments. Or better on-spacecraft processing and autonomy.
Hmmm… …is it feasible to reprocess Apollo ALSEP RTGs in any way – there are five sitting on the Moon doing nothing? I don’t know the production process for Pu238, but if feasible then this might be a use for a long-life unmanned rover. Even if they can’t be reprocessed they should still be a heat source during those long Lunar nights.
According to Dr. Crary above, they lose 4-5% of available output per year. Over the course of 40 years or so those puppies are barely warm…
Most of the 4-5% is actually from the system that converts heat to electricity. The SNAP-27s Apollo used were actually worse (and the GPHS-RTG used from Galileo to New Horizons were better.) The original plutonium should still be putting out about 2/3s of the initial heat. But trying to use that probably wouldn’t be a good idea. Some of the things Pu238 decays into are nasty. And some of oxygen in the plutonium oxide gets activated and turns into a neutron emitter.
The real problem is the thermoelectric converters, which turn the heat into electricity. I’m looking at a plot of that, and the ALSEP SNAP-27s were down to 50% output within five or ten years. You’d have to pull the plutonium heat source, install it in a new RTG, and do all of that while dealing with the radiation from daughter products and activated parts. That sounds like a lot of work for limited return.
Interesting timing. Europe has started to consider RTGs after eschewing them for the longest time. Seems to be congruent with a focus on outer planet missions. I believe that the work is UK based (as opposed to general ESA programs) and is focusing on Americium and not Plutonium.