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I have been poking at the idea of an eventual re-emergence of the PACER concept, where energy is obtained by detonating nuclear explosions... but not exactly in the same form. After humans become space-faring, the main technical and economic limitations of PACER (related to managing a large containment chamber, or massive underground cavities) are lifted, because there exists a large industrial base in microgravity. In such a world, building sufficient containment and radiation management becomes essentially trivial with scaling up the yield of the explosions. Nuclear explosions, for the record, have tremendous scaling ability. Twice the yield does not require twice the cost. Because of this, PACER eventually (I can not say exactly when) becomes outrageously cheap energy compared to all alternatives.

While this establishes the technical and economic argument, I am completely empty-handed regarding what social, political, and historical (accidental) factors could enable an organization to re-embark on program for nuclear explosives. Considering the shifts that humanity will undergo as we become interplanetary, what changes would be important for the re-enablement of PACER-type projects?

EDIT:

Making vague references to "technical" elements, I left out an important detail of how I picture these nuclear explosions being used. In order to extract the heat to run engines, it must still occur in a closed space. This could be satisfied by a pressure vessel in microgravity. This pressure vessel, if built in microgravity, could be miles across, instead of a few 100 of meters, as nuclear containment structures are necessarily limited to on Earth. That does incur an additional construction cost. Chambers could also be built below the surface of a planet, moon, or asteroid. On the moon, with 1/6th the gravity, the chamber could be built approximately 6 times as large as on Earth, with possibly 6^3=216 scale up in yield. These are small potatoes. Ceres and Vesta are large, but sufficiently small such that robotic equipment can have pressure tolerance to operate in its very center. Sealing is needed in these cases, but no rigid vessel. The upper limit on yield you could detonate (saving more and more money) would be mind-numbingly large. A Ceres society could extract energy efficiently from the largest, continent-destroying, bomb that have very rarely even been discussed in human history.

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  • $\begingroup$ Where are the PACER chambers located? You mention space-faring, are they sited on other planets or here on Earth? $\endgroup$ – a4android Oct 2 '16 at 8:20
  • $\begingroup$ @a4android in space $\endgroup$ – MolbOrg Oct 2 '16 at 17:41
  • $\begingroup$ @a4android I should have elaborated on that point. Because of the level of complexity of that answer, I will leave it as a belated edit. Would have been better to write that when I submitted it. $\endgroup$ – AlanSE Oct 2 '16 at 19:17
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What would revitalize this concept in your scenario is the need for high ISP/high thrust drives to reach planets in a reasonable time and to carry massive payloads (including radiation shielding and life support consumables) to the destination. This argues for the revival of the ORION pulsed nuclear drive concept.

While launching 4000 ton ORIONs from Nevada is out of the question in almost any scenario not involving the imminent destruction of the Earth, ORION craft built in space or on the Moon would be acceptable. Indeed, the possibility of ships with hundreds or thousands of nuclear explosives aboard might mean that they must be assembled far from Earth, to remove any lingering doubts that these are warships.

Once at the destination, the nuclear pulse units can be quickly repurposed to tunnel into asteroids or the surfaces of moons, allowing the ships crew to move into a large cavern away from much of the danger of the space environment (the energy of the plasma is used to drill the tunnel, while the nuclear explosion is allowed to happen in space away from the asteroid, or using a timed detonation above the surface of the moon, dispersing the radiation away from the work area).

The people in space are now quite familiar and comfortable using nuclear devices as tools rather than weapons, so lining a chamber drilled into the surface of a moon or large asteroid and using a nuclear device to "energize" the chamber and heat the working fluid isn't a large step for most people to take.

The real question for you is why PACER is a better solution than orbiting solar mirrors, nuclear reactors (fission or fusion) or tapping the giant magnetosphere of Jupiter for energy instead?

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  • $\begingroup$ tapping the giant magnetosphere of Jupiter for energy instead a bit elaborations about that, some concepts, numbers, links. I'm pretty close to downvote or upvote can't decide )) although probably good answer for op's purposes. $\endgroup$ – MolbOrg Oct 2 '16 at 17:46
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    $\begingroup$ I have referenced this on several answers on this board (worldbuilding.stackexchange.com/questions/53921/… for example), but you can also look for other information. en.wikipedia.org/wiki/Magnetosphere_of_Jupiter gives you an overview, and tapping the two trillion watts of energy in the flux tube between Io and Jupiter is going to be one of the engineering challenges of the 22nd century. $\endgroup$ – Thucydides Oct 2 '16 at 20:29
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    $\begingroup$ There's going to be a downside to any energy source. Renewables are diffuse, needing large physical collectors. Some finite sources are too limited in quantity (whale oil, hydrocarbons), and others are just too expensive (fission power as we know it). PACER in microgravity offers fusion with essentially no price floor, and does so without any wishful thinking or assumptions about future tech. It doesn't matter if another option is a little less cheap, at sufficient scale, PACER wins as a mathematical fact. But I will read up on that Jovian magnetic flux anyway. $\endgroup$ – AlanSE Oct 2 '16 at 20:35
  • $\begingroup$ This could become an interesting comments thread. Renewables are not an option in space (which is how I interpreted the OP's question), and in microgravity, I can build collectors of essentially unlimited size. Solar mirrors of arbitrary size can focus sunlight to generate power by boiling water and using steam turbines, or energizing MHD generators. Ordinary fission reactors are quite compact, a PEEWEE NTR test rocket generated 4000MW of thermal energy in a package about the size of a modern van, to use one example. The failing of PACER was the high cost of the "fuel". $\endgroup$ – Thucydides Oct 2 '16 at 20:49
  • $\begingroup$ Ok it look like it boils down to something like this paper-numbers yes pretty significant amount of power in pretty compact form. No, I'm not excited with two trillion watts, because it is not much for space in general and as we jump there out energy consumption jumps by order of magnitude just for food production(10kW, average energy we produce today is something 0.7kW per person), and I was more interested in W/mass ratio of possible solutions. Not sure what u mean under flux tube between Io and Jupiter. $\endgroup$ – MolbOrg Oct 3 '16 at 1:32
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Operation Plowshare and Nuclear Explosions for the National Economy were American and Soviet projects, respectively, to harness nuclear weapons for some positive good. You can read at the links about the various things attempted with said weapons. Some of the highlights were using the weapons for tunneling, canal building, and for using the vibrations from a nuclear blast for oil and gas exploration.

If you are taking this technology to space, then I could see alot of scope for using nuclear explosions to conduct large scale construction projects. Blowing asteroids into usable pieces, digging canals for Martian water to flow through after terraforming melts the poles, blasting one side of Enceladus to get its cryovolcanoes to spit out more water for easy collection. Plenty of uses.

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what changes would be important for the re-enablement of PACER-type projects?

fail in achievement thermonuclear and energy demand on far orbits 3 a.u. and more. Pretty unrealistic as for my opinion.

Although as easy substitution for thermonuclear reactor - it may work. What unlock that option are space habitats, low tech, rich sources of materials for bombs, more peoples in space then on earth, more them in places where sun collectors are not viable option.

Probably for some reason it may be urgent solution. Let say bugs captured everything at 3 a.u. distances, no one left on earth, 1000000 peoples who need energy and know how to make bomb and have no resources for other solutions. So amount of peoples could be pretty low.

Actually some fear of using bombs is partially from cold war inheritance and current situation and not much useful results here on earth of using them as tool, ecology fears.

Although in time of better excitement with nuclear power and nuclear solutions and dreams about cars with nuclear reactors(totally possible as for now, but even I would say no, for obvious misuse of that, and high demand for such misuse) - there was conducted tests, at leasе few uses of bombs for peaceful purposes. One of them was for sealing gas leakage on oil field, old short kinda docu about, was kinda similar mishap as with mexican gulf, but easier one in some sense.

But eventually it leads to some problems we can't control atm and long-lasting side effects we can't control atm.

If such control would be possible and for use cases and for results probably there would be no reasonable objections to use them, but also probably would be no reasons to use them too(may or may not).

In space there are less objection for that, and probably if planetary defense exists, which is reliable and prevents of entering unknown bodies to earth, which you might wish to have anyway for different reasons, do use people nuks in space or not, it might unlock use of nukes in space, technically.

But there is another problem technology transfer. So you have to have pretty stable situation on earth, or you have to have peoples(which are probably independent) who do not care about that or it have to be same peoples who have nukes already atm.
Not caring people may use nukes are the accepted and welcomed on earth or not, and have advantages because potential barrier to use them or not will be only having technology(which means guys which had them, failed to keep vital components, or there is less advanced maybe more bulky solutions which is easy to discover)

Or some countries got into space too, easy enough and cheap enough, where they can do what they want and have no control over action from other political forces.

So there are plenty options and combination of factors which may allow and unlock use of nukes, but none of them are certain. They are not easy, they are set of factors - so no golden bullet there, for plot device.

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You do like to gloss over some of the high points. I particularly liked

This could be satisfied by a pressure vessel in microgravity.

A pressure vessel made of what? Keep in mind that rock works fine in compression, but sucks in tension. This is made worse by the existence of flaws in natural rock which act as stress concentrators, and which will exacerbate the effect of shock waves in the internal medium. In other words, rock pressure vessels will tend to fall apart under pressure. And materials such as foamed metal will, as you might expect, take an enormous amount of material, which ain't cheap. Plus, the bigger the chamber the greater the heat losses due to thermal radiation. And note that the existence of vacuum to make thermos bottle construction relatively easy doesn't help with radiative losses. And it's hard to make a good reflective material for near IR, particularly on the scale you're imagining.

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