Nuclear Powered Spacecraft

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Am-241 has a half-life of approximately 433 years. At equal mass, the power output of Am-241 fuel is only about one-fifth that of Pu-238, with 1kg of americium dioxide producing only about 74W of thermal power. In addition, Am-241 emits gamma rays during decay, which could cause unforeseen issues in missions lasting several decades.

I did not realize that the power output was that low. Yeah, that's a problem. Somebody who studied this issue extensively over many years (but did not actually work in the Pu-238 production field) told me that one benefit of Pu-238 was that it did not dissolve in water, so a launch accident meant that the fuel would stay intact in the ocean and could be retrieved. See the recent article on The Space Review about the recovery of an RTG off the California coast in the 1960s.

What that goes to show you is that there are a lot of different issues to consider for RTG fuel sources. It's not just production cost, or energy density, or experience and equipment for handling the material. It's a whole lot of other things too, like safety.

That said, there are private companies (at least one) that want to produce Am-241 RTGs. I am in favor of adding a new source of material. However, I am also wary. What tends to happen when new companies come along with new solutions is that they then lobby the government to kill the old solution so that they can have a clear playing field. But that can result in getting rid of a good solution and creating a new monopoly.

 

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A few days ago, Isaacman said that he would like to create Space Reactor 2, 3, and 4 programs to follow on SR-1, and to enable outer planets missions to places like Enceladus. Alas, this is not going to happen. It's not a good idea. SR-1 is already estimated to cost $2 billion. Does anybody think that SR-2, 3, 4 will be cheaper? (Inflation alone will drive up their costs.) Robotic planetary missions, including outer planets ones, already have a cost problem. Adding a propulsion system to them that will cost billions of dollars won't help. It will only make them less affordable, and means that they will never get funded. It's a downward spiral.

Also, I don't for a second believe that SR-1's price tag will stay at $2 billion. It's going to go up. Wait until the first half of next year when NASA reluctantly admits to a new price tag, and that the schedule is not achievable.