Why are NERVAs not yet used?

[shynung]

It must be noted that SEP is barely useful past Mars orbit, due to the inverse-square law. When we go to Jupiter's moons, we would be well-advised to have a more potent propulsion system.

[Streetwind]

I see, time and time again, claims in threads like these that the main problem lies with treaties. Can I get a detailed source on that, please? I've never seen a single conclsuive proof over the years - though if I can get one, I'll happily read up on it.

 

Edited February 8, 2016 by Streetwind

[Frozen_Heart]

A: People don't want nukes in space as they're all scared of them.

B: Research on them hasn't been funded since the 70s

C: Technologies like ion and vasmir are much better compared to solid core rockets. Though a nuclear reactor may be needed for big ones.

[Elthy]

Those things radiate like crazy when being used and afterwards, not something where you want people closeby. Radiationshielding is heavy...

[kunok]

44 minutes ago, Frozen_Heart said:

B: Research on them hasn't been funded since the 70s

In EEUU yes, but in the USSR they continued funding in the 80's, and they even got an operative model the rd-410 (with I was doing as mod and I will retake when I have again 3d tools) with had better TWR and isp than the nerva, but it never flow to the space. This engine could be remade in only a few years if there was political support. Source in Spanish: http://danielmarin.naukas.com/2010/11/23/cohetes-nucleares-a-la-conquista-del-sistema-solar/

[Streetwind]

Okay, I spent some time to do some math, because for my part, I think that there's a much simpler reason than treaties why we're not using NERVAs right now: because they're bad engines. Or rather, more precisely: because they require bad tanks.

 

A typical liquid fuel rocket stage is between 90% and 95% fuel by mass (source: check the stats of your favorite orbital launch vehicle). That means, the actual tankage has mass ratios around 20:1, since there are parts on any given rocket stage that add mass but are not fuel tanks (such as rocket engines). There is very little difference between different fuel types here, since even hard cryogenic fuel like liquid hydrogen doesn't need to be stored for more than an hour or two.

However, try to store liquid hydrogen for any greater length of time, and you run into problems. Not only is boiloff a concern, but hydrogen also has this annoying tendency to simply drift through solid tank walls (and damage them in the process, too). Now, this problem has been largely solved; NASA has drafted up prototypes for "zero boil-off" ("ZBO") tanks. By means of reinforced walls and active cryocooling, they can store liquid hydrogen for years at a time with only negligible loss. Unfortunately, the mass ratio of these tanks is, at best... 3.25:1. (PDF Source)

So let's say you're trying to push around 20 tons of payload/misc dry mass, and you have 1 ton worth of chemical engines or a 10 ton NTR (which is a flattering lower-bound estimate, given the tables in the above source). Let's also add, I dunno, 2 tons of radiation shielding for the NTR variant. The chemical propulsion system would use storable hypergolics, and let's give it 340s worth of Isp - a realistic number, as such an engine already exists (Source). The hypothetical NERVA-derived NTR that the document suggests assumes 900s Isp (a number the real NERVA never reached, but apparently the Russians managed, see kunok's link). Let's also say you want a dV of about 6.5 km/s, for a Earth-Mars-Earth roundtrip (number also pulled from the PDF above). According to the rocket equation, that amount of dV with those specified Isp's can be achieved with the following mass ratios:

- Chemical: dV = 9.807 * 340 * ln(7.1) = 6535 m/s with a mass ratio of 7.1:1
- Nuclear: dV = 9.807 * 900 * ln(2.1) = 6548 m/s with a mass ratio of 2.1:1

Looking pretty good for the NTR here, but let's actually do the math - factoring in the mass ratio of the tankage.

- To achieve a total vessel mass ratio of roughly 2.1:1, the NTR spacecraft needs to pack roughly 195 tons of fuel, adding about 60 tons of dry mass to the 32 tons (payload + engines + shielding) already present. Total spacecraft mass is 287.0 tons.
- To achieve a total vessel mass ratio of roughly 7.1:1, the chemical spacecraft needs to pack roughly 232 tons of fuel, adding about 11.6 tons of dry mass to the 21 tons (payload + engines) already present. Total spacecraft mass is 264.6 tons.

So the almighty NTR ends up the worse option of the two! And this is already a fairly high dV mission for a single stage, where the high Isp is supposed to shine.

 

TL;DR - The NERVA is, nor ever was, anywhere near as good as most space hobbyists think it is; it was probably killed for a good reason. Nuclear thermal rockets in general would need a much higher Isp, or a much more easily stored fuel choice, to even begin to be considered a viable choice over decades-proven, dead simple hypergolics.

 

(Disclaimer: I am not a rocket scientist. This example relies on some rough over-the-thumb estimates I made while looking at the linked sources. It shouldn't be taken as gospel, but rather as food for thought in support of my point. If you can disprove me, I am interested to see your approach!)

 

Edited February 8, 2016 by Streetwind

[tater]

1 hour ago, Frozen_Heart said:

B: Research on them hasn't been funded since the 70s

This is just 100% incorrect. I know people working on this, and they're not spending their own money, lol.

http://www.nasa.gov/centers/marshall/news/news/releases/2014/ntrees.html

[Temstar]

@Streetwind, only the Mars return fuel tank needs to be zero boil off, the earth departure tank can be regular LH2 tank since it will be used straight away. This tank and its content is by far the biggest proportion of the vehicle and can be 20:1 tank.

In fact once you use up the Earth Departure LH2 tank it can be jettisoned straight away, there's no need to carry this empty tank back to Earth or even brake it into Martian orbit:



Generally the design for NTR ships have this "saddle strut" in the middle where the earth departure tank sits so it can be floated off out the side once empty.

Edited February 8, 2016 by Temstar

[Streetwind]

14 minutes ago, Temstar said:

@Streetwind, only the Mars return fuel tank needs to be zero boil off, the earth departure tank can be regular LH2 tank since it will be used straight away. This tank and its content is by far the biggest proportion of the vehicle and can be 20:1 tank.

In fact once you use up the Earth Departure LH2 tank it can be jettisoned straight away, there's no need to carry this empty tank back to Earth or even brake it into Martian orbit:



Generally the design for NTR ships have this "saddle strut" in the middle where the earth departure tank sits so it can be floated off out the side once empty.

The Earth departure tank is designed for ZBO in the most recent version of the Mars DRM. Probably because the whole thing needs to hang around a while for orbital assembly to happen, but I'm just guessing here.

But that's not even the point I'm trying to make. You can also make the chemical variant multi-staged, and get a similar benefit to using a drop tank with the NTR. It doesn't change the picture in the end And the mere fact that you have to resort to such measures to make the NTR valid is a testament to how underwhelming it performs running on LH2 in a realworld scenario.

Edited February 8, 2016 by Streetwind

[kunok]

1 hour ago, Streetwind said:

Okay, I spent some time to do some math, because for my part, I think that there's a much simpler reason than treaties why we're not using NERVAs right now: because they're bad engines. Or rather, more precisely: because they require bad tanks.

 

A typical liquid fuel rocket stage is between 90% and 95% fuel by mass (source: check the stats of your favorite orbital launch vehicle). That means, the actual tankage has mass ratios around 20:1, since there are parts on any given rocket stage that add mass but are not fuel tanks (such as rocket engines). There is very little difference between different fuel types here, since even hard cryogenic fuel like liquid hydrogen doesn't need to be stored for more than an hour or two.

However, try to store liquid hydrogen for any greater length of time, and you run into problems. Not only is boiloff a concern, but hydrogen also has this annoying tendency to simply drift through solid tank walls (and damage them in the process, too). Now, this problem has been largely solved; NASA has drafted up prototypes for "zero boil-off" ("ZBO") tanks. By means of reinforced walls and active cryocooling, they can store liquid hydrogen for years at a time with only negligible loss. Unfortunately, the mass ratio of these tanks is, at best... 3.25:1. (PDF Source)

So let's say you're trying to push around 20 tons of payload/misc dry mass, and you have 1 ton worth of chemical engines or a 10 ton NTR (which is a flattering lower-bound estimate, given the tables in the above source). Let's also add, I dunno, 2 tons of radiation shielding for the NTR variant. The chemical propulsion system would use storable hypergolics, and let's give it 340s worth of Isp - a realistic number, as such an engine already exists (Source). The hypothetical NERVA-derived NTR that the document suggests assumes 900s Isp (a number the real NERVA never reached, but apparently the Russians managed, see kunok's link). Let's also say you want a dV of about 6.5 km/s, for a Earth-Mars-Earth roundtrip (number also pulled from the PDF above). According to the rocket equation, that amount of dV with those specified Isp's can be achieved with the following mass ratios:

- Chemical: dV = 9.807 * 340 * ln(7.1) = 6535 m/s with a mass ratio of 7.1:1
- Nuclear: dV = 9.807 * 900 * ln(2.1) = 6548 m/s with a mass ratio of 2.1:1

Looking pretty good for the NTR here, but let's actually do the math - factoring in the mass ratio of the tankage.

- To achieve a total vessel mass ratio of roughly 2.1:1, the NTR spacecraft needs to pack roughly 195 tons of fuel, adding about 60 tons of dry mass to the 32 tons (payload + engines + shielding) already present. Total spacecraft mass is 287.0 tons.
- To achieve a total vessel mass ratio of roughly 7.1:1, the chemical spacecraft needs to pack roughly 232 tons of fuel, adding about 11.6 tons of dry mass to the 21 tons (payload + engines) already present. Total spacecraft mass is 264.6 tons.

So the almighty NTR ends up the worse option of the two! And this is already a fairly high dV mission for a single stage, where the high Isp is supposed to shine.

 

TL;DR - The NERVA is, nor ever was, anywhere near as good as most space hobbyists think it is; it was probably killed for a good reason. Nuclear thermal rockets in general would need a much higher Isp, or a much more easily stored fuel choice, to even begin to be considered a viable choice over decades-proven, dead simple hypergolics.

 

(Disclaimer: I am not a rocket scientist. This example relies on some rough over-the-thumb estimates I made while looking at the linked sources. It shouldn't be taken as gospel, but rather as food for thought in support of my point. If you can disprove me, I am interested to see your approach!)

 

¿Why an 10 ton nerva? ¿Do it need a good TWR? IIRC the motor itself it's shielded, at least the rd-410, and the fuel provides extra shielding.

 

20 minutes ago, Temstar said:

@Streetwind, only the Mars return fuel tank needs to be zero boil off, the earth departure tank can be regular LH2 tank since it will be used straight away. This tank and its content is by far the biggest proportion of the vehicle and can be 20:1 tank.

And you can drop tanks as they got empty, you don't need one big tank.

I will do the numbers with the rd-410 data later. I'm curious.

Edited February 8, 2016 by kunok
Typos always typos

[tater]

29 minutes ago, Streetwind said:

Okay, I spent some time to do some math, because for my part, I think that there's a much simpler reason than treaties why we're not using NERVAs right now: because they're bad engines. Or rather, more precisely: because they require bad tanks.

(other stuff snipped for space)

They'd only need the higher dry-weight tanks for the return trip, so you could assume half the propellant is used immediately for the trans-Mars injection burn, and the low/zero boil-off tanks only for the return trip propellant. You can also simply include extra propellant carried for boil-off (15%?) using better mass ratio tanks. Note that the DRA assumes LH₂ for both types of rocket anyway, so the boil off issue is the same (though there is less with chemical, since part of the mass is O₂.

Bottom line is that NASA seems to think they are at least competitive with each other.

[DerekL1963]

1 hour ago, Elthy said:

Those things radiate like crazy when being used and afterwards, not something where you want people closeby. Radiationshielding is heavy...

There are three ways to reduce exposure:  Time, Distance, Shielding.

- Time is obviously not an option for a Mars journey, since it's years long.

- Distance is a big part of exposure reduction, as can be seen in pretty much any NTR craft design they put the crew as far away as humanly possible.  (Radiation follows the same inverse square law as visible light.)

- Shielding, well you don't need as much as you might think.   Not only does the vehicle's tankage and structure act as shielding, you only need a disk on the front of the reactor sufficient to put the rest of the vehicle into it's "shade". 

[wumpus]

Trying to get a non-trivial boiloff for the Earth departure won't be easy.  ISS resupply missions tend to take *days* to dock with the ISS (then again, nothing there is boiling off).  A low boiloff mission pretty much means launch, circularize, rendezvous, dock, light the engines and go to Mars in as few orbits as possible (preferably one).  This just doesn't seem to fit the slow count down and launch that would charecterize such an operation (although I'd assume that the full checkout less fuel would be done prior to fuel launch).  I don't think Apollo 11 did more than a single orbit in their "parking orbit" (it was pretty low, they wanted that Obereth effect), so there is presumably some precedence for such a quick check and go (which was boiling off H2 as well, wasn't it).

You could presumably do such a "last minute fuel launch", but it would be a white knuckle thing.  Steely eyed missile men need only apply.

Anybody know what the Chinese think about nuclear power?  NASA isn't the only game in town, and I've never heard of congress planning on giving them a budget to Mars, nor a president willing to follow a previous president's space plan (Johnson got a lot of mileage out of following the "martyred" Kennedy in other areas, so presumably was willing to maintain Apollo.  Nixon put a halt to things as soon as possible).

As far as the hostility to nukes dying down, I can only hope that it has something to do with the fact that nukes replace coal plants and don't contribute (significantly) to global warming.  I don't really believe that, but the idea of giving guys from Enron the keys to a nuke plant does not give me the warm fuzzies either.

[DerekL1963]

1 hour ago, wumpus said:

 A low boiloff mission pretty much means launch, circularize, rendezvous, dock, light the engines and go to Mars in as few orbits as possible (preferably one).  This just doesn't seem to fit the slow count down and launch that would charecterize such an operation (although I'd assume that the full checkout less fuel would be done prior to fuel launch)

There's nothing preventing a full checkout followed by immediate fueling and a swift departure thereafter.
 

1 hour ago, wumpus said:

 I don't think Apollo 11 did more than a single orbit in their "parking orbit" (it was pretty low, they wanted that Obereth effect)

Apollo 11 was in parking orbit for two and a half hours.   And the parking orbits were low not because of the Oberth effect, but to minimize the fuel spent reaching orbit and to maximize the fuel available for TLI.

 

1 hour ago, wumpus said:

Anybody know what the Chinese think about nuclear power?

China isn't going anywhere fast.  Despite the fever dreams of the space fanboi community, the reality is that they have just enough of a space program to be thought of as a Major Country and not a fen's worth more.

[sevenperforce]

Additional option: ditch the liquid hydrogen and use something with slightly lower Isp that won't boil off. Liquid methane doesn't boil off nearly as badly as liquid hydrogen and it's much easier to store in cryo. You can even go really extreme and choose something like diesel, which lets you fit a ton of hydrogen into really small tanks, at the cost of reduced Isp. I don't much feel like calculating the Isp of a NTR that uses diesel fuel as a propellant but it ought to be dramatically better than dragging LOX along to burn with the diesel fuel in a conventional chemical rocket.

Although there are more exciting options...

It's probably unlikely that we'll ever build a true NSWR, for a myriad of reasons. However, what about a hybrid NTR/NSWR?

For fuel, use a standard solid plutonium or uranium cylindrical core, encased in something that is neutron-transparent but very heat-resistant. Put this inside a combustion chamber made out of a neutron reflector:

So far, it makes sense, except there's no way to control it, right?

That's where the propellant comes in. Pump ordinarily light water through solid lithium-7 hydride, producing OH-, Li+, and H₂, then send that stuff straight into your combustion chamber.

The fast fission neutrons from the fissile mass will break lithium-7 into tritium, helium, and a neutron. Lithium-7 fission is itself endothermic, as the neutron released has lower energy than the neutron absorbed, but that's not going to matter in this situation because the effective reflection of neutrons will cause the core to immediately go critical, heating the mixture very quickly. Ideally, the temperature will be sufficient to disassociate the OH- into O and H, but even if it doesn't, you've still got a mixture which is mostly very lightweight elements which can achieve extraordinarily high exhaust velocities.

The exhaust contains no fissile material (the main problem with NTRs) but you'd still have great Isp. You could "throttle" by changing the water pressure leading into the combustion chamber; the more lithium-7 that ends up packed in around the fissile mass, the faster the reaction will go. It's also inherently safe, because the faster the reaction progresses, the greater the pressure in the vessel rises and the faster the propellant is ejected, lowering the reaction rate.

Any idea what kind of performance you could get out of something like this? Any major problems I haven't anticipated?

[wumpus]

5 hours ago, DerekL1963 said:

Apollo 11 was in parking orbit for two and a half hours.   And the parking orbits were low not because of the Oberth effect, but to minimize the fuel spent reaching orbit and to maximize the fuel available for TLI.

Two orbits then.  Sounds like they weren't leaking LH2 all that badly.

[fredinno]

On February 7, 2016 at 10:10 AM, tater said:

A lunar station would need many resupply flights, and NASA can't really afford 1 SLS launch per year, much less several.

NTP is still on the table for manned Mars, so like other propulsion candidates it deserves testing. I want to say the current design is only about 1600kg.

SLS needs missions, though, and Lunar space stations offer reasonable science for low development time and costs.

On February 7, 2016 at 11:09 AM, Bill Phil said:

You still have some gravity losses. Albeit not much at all, but they are present.

ISP of ION still trumps it.

22 hours ago, DerekL1963 said:

No they don't, not anymore at least.   Over time the noise and activity of the usual protesters have markedly declined.   (There was an especially noticeable drop after 9/11 and the start of Desert Storm II, the cynic in me wants to the said drop was due to the usual protesting types being distracted and busy elsewhere.)

That was before the Fukisima Meltdown.

[fredinno]

11 hours ago, Frozen_Heart said:

A: People don't want nukes in space as they're all scared of them.

B: Research on them hasn't been funded since the 70s

C: Technologies like ion and vasmir are much better compared to solid core rockets. Though a nuclear reactor may be needed for big ones.

 

9 hours ago, Streetwind said:

Okay, I spent some time to do some math, because for my part, I think that there's a much simpler reason than treaties why we're not using NERVAs right now: because they're bad engines. Or rather, more precisely: because they require bad tanks.

 

A typical liquid fuel rocket stage is between 90% and 95% fuel by mass (source: check the stats of your favorite orbital launch vehicle). That means, the actual tankage has mass ratios around 20:1, since there are parts on any given rocket stage that add mass but are not fuel tanks (such as rocket engines). There is very little difference between different fuel types here, since even hard cryogenic fuel like liquid hydrogen doesn't need to be stored for more than an hour or two.

However, try to store liquid hydrogen for any greater length of time, and you run into problems. Not only is boiloff a concern, but hydrogen also has this annoying tendency to simply drift through solid tank walls (and damage them in the process, too). Now, this problem has been largely solved; NASA has drafted up prototypes for "zero boil-off" ("ZBO") tanks. By means of reinforced walls and active cryocooling, they can store liquid hydrogen for years at a time with only negligible loss. Unfortunately, the mass ratio of these tanks is, at best... 3.25:1. (PDF Source)

So let's say you're trying to push around 20 tons of payload/misc dry mass, and you have 1 ton worth of chemical engines or a 10 ton NTR (which is a flattering lower-bound estimate, given the tables in the above source). Let's also add, I dunno, 2 tons of radiation shielding for the NTR variant. The chemical propulsion system would use storable hypergolics, and let's give it 340s worth of Isp - a realistic number, as such an engine already exists (Source). The hypothetical NERVA-derived NTR that the document suggests assumes 900s Isp (a number the real NERVA never reached, but apparently the Russians managed, see kunok's link). Let's also say you want a dV of about 6.5 km/s, for a Earth-Mars-Earth roundtrip (number also pulled from the PDF above). According to the rocket equation, that amount of dV with those specified Isp's can be achieved with the following mass ratios:

- Chemical: dV = 9.807 * 340 * ln(7.1) = 6535 m/s with a mass ratio of 7.1:1
- Nuclear: dV = 9.807 * 900 * ln(2.1) = 6548 m/s with a mass ratio of 2.1:1

Looking pretty good for the NTR here, but let's actually do the math - factoring in the mass ratio of the tankage.

- To achieve a total vessel mass ratio of roughly 2.1:1, the NTR spacecraft needs to pack roughly 195 tons of fuel, adding about 60 tons of dry mass to the 32 tons (payload + engines + shielding) already present. Total spacecraft mass is 287.0 tons.
- To achieve a total vessel mass ratio of roughly 7.1:1, the chemical spacecraft needs to pack roughly 232 tons of fuel, adding about 11.6 tons of dry mass to the 21 tons (payload + engines) already present. Total spacecraft mass is 264.6 tons.

So the almighty NTR ends up the worse option of the two! And this is already a fairly high dV mission for a single stage, where the high Isp is supposed to shine.

 

TL;DR - The NERVA is, nor ever was, anywhere near as good as most space hobbyists think it is; it was probably killed for a good reason. Nuclear thermal rockets in general would need a much higher Isp, or a much more easily stored fuel choice, to even begin to be considered a viable choice over decades-proven, dead simple hypergolics.

 

(Disclaimer: I am not a rocket scientist. This example relies on some rough over-the-thumb estimates I made while looking at the linked sources. It shouldn't be taken as gospel, but rather as food for thought in support of my point. If you can disprove me, I am interested to see your approach!)

 

 

8 hours ago, Temstar said:

@Streetwind, only the Mars return fuel tank needs to be zero boil off, the earth departure tank can be regular LH2 tank since it will be used straight away. This tank and its content is by far the biggest proportion of the vehicle and can be 20:1 tank.

In fact once you use up the Earth Departure LH2 tank it can be jettisoned straight away, there's no need to carry this empty tank back to Earth or even brake it into Martian orbit:



Generally the design for NTR ships have this "saddle strut" in the middle where the earth departure tank sits so it can be floated off out the side once empty.

That is only used for the Constellation one, I think.

8 hours ago, Streetwind said:

The Earth departure tank is designed for ZBO in the most recent version of the Mars DRM. Probably because the whole thing needs to hang around a while for orbital assembly to happen, but I'm just guessing here.

But that's not even the point I'm trying to make. You can also make the chemical variant multi-staged, and get a similar benefit to using a drop tank with the NTR. It doesn't change the picture in the end And the mere fact that you have to resort to such measures to make the NTR valid is a testament to how underwhelming it performs running on LH2 in a realworld scenario.

I've read the most recent DRM with SLS, and thys just use ION instead.

[tater]

Yeah, SLS might be a gift to Bigelow, as they could make a hab that could need SLS, but still be pretty cheap. For SLS, a billion dollar payload is cheap.

[DerekL1963]

4 hours ago, fredinno said:

On 2/8/2016 at 6:47 PM, DerekL1963 said:

No they don't, not anymore at least.   Over time the noise and activity of the usual protesters have markedly declined.   (There was an especially noticeable drop after 9/11 and the start of Desert Storm II, the cynic in me wants to the said drop was due to the usual protesting types being distracted and busy elsewhere.)

That was before the Fukisima Meltdown.

We launched Curiousity, with her two RTG's, four years after Fukishima and with essentially nary a peep from the usual protesters.

Edited February 9, 2016 by DerekL1963

[wumpus]

11 hours ago, DerekL1963 said:

We launched Curiousity, with her two RTG's, four years after Fukishima and with essentially nary a peep from the usual protesters.

Did you measure crowd levels of protest, or simply media reports of peep level.  NASA could have said "look, shiny!*" and the peeps would have gone for naught.

* or more appropriately "look, blood!" (it bleeds it leads).

[DerekL1963]

11 minutes ago, wumpus said:

Did you measure crowd levels of protest, or simply media reports of peep level.  NASA could have said "look, shiny!*" and the peeps would have gone for naught.

No, NASA could not have simply gone "ooh, shiny" for Curiosity and silenced the peeps, or at least it certainly did not work for any of the previous launches.

For Curiosity, there were no lawsuits challenging the EIS or for repeating the EIS, or for cancelling the launch.  There were no noticeable protesters at the gate (and I have sources on the ground and do not depend on the media for information on these).  The blogosphere was essentially silent.   Etc... etc...   All of these thing characterized previous launches of spacecraft carrying RTG's.

[GreenWolf]

On February 7, 2016 at 10:13 AM, fredinno said:

RTGs are not fissile material. NERVA contains fissile material.

This is patently, absurdly, egregiously false.

Fissile material does not mean "reactor fuel or bomb making material". Fissile materials are those which are capable of undergoing nuclear fission. Plutonium-238, the most common isotope used in spacecraft RTGs, is fissile.

Please do some minimal fact checking before making such bold-faced claims.

[fredinno]

14 hours ago, DerekL1963 said:

We launched Curiousity, with her two RTG's, four years after Fukishima and with essentially nary a peep from the usual protesters.

Good. Now once we get people to stop protesting about nuclear power plants being built and get that Nevadan Nuclear Dump built, we can hope that NERVA will also be publically acceptable.  Remember, the public thinks the exhaust is radioactive (it's not). In that sense, Nuclear-Electric is likely better in the public view.

3 hours ago, DerekL1963 said:

No, NASA could not have simply gone "ooh, shiny" for Curiosity and silenced the peeps, or at least it certainly did not work for any of the previous launches.

For Curiosity, there were no lawsuits challenging the EIS or for repeating the EIS, or for cancelling the launch.  There were no noticeable protesters at the gate (and I have sources on the ground and do not depend on the media for information on these).  The blogosphere was essentially silent.   Etc... etc...   All of these thing characterized previous launches of spacecraft carrying RTG's.

Maybe it was since so many previous launches with RTGs didn't cause release of radiation?

2 hours ago, GreenWolf said:

This is patently, absurdly, egregiously false.

Fissile material does not mean "reactor fuel or bomb making material". Fissile materials are those which are capable of undergoing nuclear fission. Plutonium-238, the most common isotope used in spacecraft RTGs, is fissile.

Please do some minimal fact checking before making such bold-faced claims.

https://en.wikipedia.org/wiki/Plutonium-238

Nope, you're describing Pu 239, not 238, the stuff used in RTGs.

[GreenWolf]

@fredinno No, I'm not. ²³⁸Pu is fissile. The fact that you apparently don't understand what nuclear fission is doesn't make this any less true.