Wednesday, July 19, 2017

Kuiper Belt Navigation and Mining


Whether, What, and Why  1

Project Categories  2

Thousand-Year Projects  2

Relay Satellite Infrastructure  2

Prospecting Craft  3

Tugs and Maintenance Craft  4

Rockrider Craft  4

Manned and Unmanned Craft in Such Projects  4

Target Objects and Objectives  4

Riding the Rocks  6

Blinkered Apes  7

Bull Markets for Competent Bulls  7

 

Whether, What, and Why

How might we steer objects from the Kuiper belt to achieve objectives in the inner orbits of the solar system? This question arose from my proposal to alter the rotation of the planet Venus for purposes of exploitation and habitation. That proposal is discussed in the article "Small Fetters" in which I express doubt that humanity has the right stuff to achieve any such objective. http://fullduplexjonrichfield.blogspot.co.za/2013/12/small-fetters.html

Some critics denied that it could be practicable to achieve anything of the kind as a realistic engineering project at all, even in principle. Some simply said that the scale of the project was too large ever to be done in principle. Some said that it could not be worth doing even if it were possible. Some of the more open-minded suggested that, assuming it were done when 'tis done, then 'twere well I should explain how it were to be done and why.

In a thread I pointed out that Kuiper Belt objects, that authorities assume to number in their millions — some estimates suggest billions — should bear kinetic energy more than adequate for practical exploitation. I did not at that time bother to discuss the detailed engineering, taking it for granted that navigating rocks from trans-Pluto orbit to intercept Venus suitably was not intellectually challenging in principle, however challenging the project might appear from political or engineering perspectives.

However, contrary to my expectations, some participants in the thread impugned this, and since some aspects of the topic are of intrinsic interest, I here discuss approaches to obtaining and navigating comets, asteroids, and assorted Kuiper Belt meteorites as required.

I still skirt currently irrelevant engineering issues of course.

Project Categories

Several other ideas and approaches might prove relevant, but here I concentrate on projects such as the engineering of Venus and Mercury and assume that the scale of the project would involve something of the order of 100,000 Kuiper Belt objects of perhaps 10 km diameter on average. Please do not bother to point out that not all Kuiper Belt objects of that order of magnitude are spherical; this is strictly a spherical cow exercise.

Obviously there are many types of approach to such potential projects, and in this discussion I ignore all that are not aimed at a long-term commitment (probably 1000 years or more, possibly even several thousand years; gaining a new planet with all its resources would be worth a good deal more than that in any rational scheme of things). The discussion also has nothing to do with steering any single object, of either gigantic or trivial size, nor bringing it to some sort of relative stop or storage trajectory at some arbitrary position in space. Instead I consider perhaps tens to hundreds of thousands of objects of worthwhile size, small enough to steer economically, and large enough to achieve particular objectives.

Nothing of the kind would be worth discussion without an appropriate infrastructure, but at such a distant remove, details of that infrastructure are hardly worth discussion. Anyone who doubts this should read Arthur C. Clarke's original proposal for communications satellites; he proposed that they should be manned! His proposal was in no way stupid; in fact, given our current applications of communications satellites, if they really had to be manned, they still would have been worth it; it is just our luck that advances in technology since World War II free us from any requirement for such an extravagance. Similarly, it is practically certain that anything I propose here would seem ludicrous to engineers of the 22nd century.

Instead I merely brush over some plausible requirements of such an infrastructure in discussing principles.

Thousand-Year Projects

It is well not to be too casual about beginning a highly technological 1000-year project. I suggest that for reasons of navigation and communication we should start by installing at the very least a few hundred permanent unmanned satellites in strategic orbits in the solar system. That no such project has yet been undertaken is a blot on our record already. However, the solar system equivalent of a GPS system plus communication relay system plus near space (meaning solar system in this context) astronomic, planetary, and cosmological observation system, would be necessary as the first step in the project. Call such craft the Relay Satellites.

Relay Satellite Infrastructure

We should be able to install a profitable, workable and worthwhile foundation within a few decades, but the planned lifetime of the Relay Satellite system would be measured in millennia rather than years and there would be no question of committing to naive uniformity of design during such a period. That infrastructure would be an indefinite project, adapting to our needs and technologies for the foreseeable future. In practice of course, certain apparently arbitrary standards might remain constant, but such are merely practical details that are not immediately relevant to us. The principle is well understood and tolerated and there is a fair discussion of a traditional example at: http://www.snopes.com/history/american/gauge.asp

'Nuff said on that point.

As I see it the Relay Satellites should have modest navigational capabilities, just about enough for indefinitely maintaining station and attitude in their appropriate trajectories. Some of them in near solar orbit might use solar power of one sort or another, but by far the majority would occupy orbits beyond Mars, and some perhaps beyond Pluto.  

Whether to power them with beamed energy, solar energy, solar-wind energy, isotope energy, or on-board fission or fusion power generators is an example of a question I leave to future generations. They should need considerable power for communication at least, plus very sensitive reception equipment, because they would be communicating partly with tiny craft that could not carry giant antennae to capture faint signals.  

Whether there would be relatively few multi-function satellites or relatively many specialist function satellites, and whether a satellite that outlives its fuel supply would be parked and catalogued, or scrapped, refurbished, recycled, or destroyed, I also leave to future interests.  

Personally I like the idea of large-scale, modular satellites to be serviced and upgraded by specialist unmanned craft, but I do not insist upon that.

Note that there is no suggestion that such a fleet of Relay Satellites should be dedicated to the Kuiper Belt object navigation project. There would be plenty of function for it without that. It is quite possible that a Kuiper Belt initiative would affect the scale and details of parts of the fleet, but that is not especially relevant here.

Prospecting Craft

The second class of craft, Prospecting Craft, in contrast to the Relay Satellites, would be specialised for ranges of function, dealing with exploration and prospecting in the Kuiper belt. To this end they would be capable of extremely long range, long-term navigation beyond the orbit of Pluto. Their communication and navigation capabilities would be powerful, but specialised for their role.  

They would rely on the permanent Relay Satellites for most of such functions, and partly for keeping track of the prospecting craft. Of course, each satellite would have its own intelligence and a very large memory, probably petabytes rather than terabytes, enough not only to manage the data that it accumulates, but the parameters of the infrastructural system as well. They would have sufficient intelligence for routine tasks, including some fairly complex ones, because apart from the question of how far robotics would have advanced in the next century or two, such tasks would be, if not highly stereotyped, at least confined to a small universe of discourse. They also would have great redundancy of function and capacity to ensure resilience in the face of predictable radiation and unpredictable accident. None of your single-drive hard disks and the like!

Prospecting craft would be exceptional in the amount of reaction mass and energy that they would have to carry, because they not only would have to reach the Kuiper Belt, but would have to do considerable amounts of unpredictable navigation within it. At such a distance from the sun, solar power would be practically worthless, probably including solar wind power. It also would not be practical for such craft to rely on isotopic thermal energy; they would need fission power generators at least, which is the good news; the bad news is that they also would need large amounts of reaction mass, even if they used ion thrusters. Much as they would rely on the relay satellites for control and communication, they might have to rely on rendezvous with tugs and maintenance vehicles for refuelling and upgrading.

Their function would be to locate and characterise as many Kuiper Belt bodies as possible, determining their nature, mass, trajectories and the like by any practical means, whether optical, radar, infrared, gravitational, theoretical or generic, to name but four... err... or so...  

Bodies that either pose a threat to the inner solar system, or that seem to be potentially valuable to the main project, probably would be visited physically to obtain all relevant information. For example, bodies that are rich in ice or ammonia might either be particularly valuable or not usable for the purposes of the project. Similarly, bodies that amount to aggregations of gravel might be valuable if their trajectories were particularly suitable for gentle manoeuvring, but hopelessly dangerous to use otherwise unless they could be cemented, say by combination with an iceberg or ammonia-berg.

This is a large subject, at this point not worth exploring in any depth. Suffice to say, some such functional craft would be needed to locate and evaluate the objects to be selected for navigation in the project.

Tugs and Maintenance Craft

The third class of craft would be the tugs and maintenance craft. They would be a job lot, and I do not discuss their design, which would be variable at all stages of all the projects.  

All the other craft would require updating, modification, refuelling, repair, and possibly even retrieval. The Relay Satellites might well require being transported to their stations as well.  

No one tug or maintenance craft would be suitable for all such functions. However, each one probably would be versatile and each one would have powerful thrusters of appropriate kinds. However, they might need less fuel than the prospectors, because they would have shorter missions, and more closely defined.

Rockrider Craft

How many different classes of craft would be needed, I cannot say; the only other one worth discussing here would be the Kuiper Belt object navigation craft. Let’s call it the Rockrider craft. It is the one at the cutting edge, or the coalface if you prefer. It's job would be to rendezvous with a selected object, prepare it for transport, and steer it to the objective.

What possibly, just possibly, could be simpler?

A lot of things of course, but not many are as worthwhile.

Manned and Unmanned Craft in Such Projects

Very well. Notice that I have said practically nothing so far about manned craft. I do not say there would be none such, but for the purposes so far discussed, I cannot see any being required, and frankly I cannot in the short term see any manned craft being practical for transient applications beyond say, the orbit of Mars.  

We are after all speaking of Rockrider craft undertaking voyages lasting decades at least, and commonly centuries. Even if we condemned convicts to such voyages, it is hard to imagine what we would want them to do out there in space, even if we could trust them there.  

I leave such distasteful speculations to readers with the appropriate distastes.

Target Objects and Objectives

Now, each Rockrider craft would have the task of rendezvous with a nominated object that had been identified, located, and characterised by the prospector craft. Apart from a few prototypes, probably none would be launched before some thousands of target bodies had been selected as having suitable masses, constitutions, neighbours, and trajectories for the project. Many, possibly the majority of such Kuiper Belt objects, might be perfect for launching in a few hundred thousand years, but useless for short-term projects of 1000 years or so. However, there are assumed to be many millions of bodies out there, so I do not feel too defensive about assuming that we would be spoilt for choice of suitable objects.

A suitable Kuiper belt object would have to be one that could profitably be adjusted in its attitude and trajectory, so that a Rockrider craft could manipulate and navigate it down from say 40 astronomic units, to rendezvous as required with Venus or Mercury at less than 1 AU.  

For example, if an otherwise suitable 100 gigatonne object were spinning at a rate of several hertz, the very task of de-spinning it strikes me as discouraging; I would rather go on to look for something friendlier. Nor would we be interested in 10-tonne or peta-tonne objects, or at least that is what I assume.  Again, we would prefer to deal with objects whose orbit we could adjust most economically in terms of energy and time. Exactly which variables would be most important in a given case, I do not much speculate upon.

I suspect that very circular orbits would be expensive to adjust, whereas elliptical orbits that approach, or could be coaxed near to Neptune, could be adjusted drastically at modest cost. Difficult decisions would be the business of the human orbital engineers, but generally most decisions could be handled programmatically.  

Possibly one could use nuclear explosions for crude preliminary adjustment of some kinds of orbits of suitable bodies, or even to persuade some bodies to collide usefully.

Using collisions might seem a bit optimistic, given that even millions of bodies so far out would have a considerable mean separation, but it’s just an example of the kind of consideration that might arise.

The point in general is that we would select bodies with orbits that could be adjusted with minimal investment of energy and material, whether by bombs or by thrust.

However, we could afford a reasonable investment, bearing in mind that a typical energetic profit for dropping from say 40 AU to the orbit of Venus would be about sixty-fold, and to Mercury, 100-fold. Those already are attractive figures. And if we could gain useful momentum by slingshotting past the major planets, we could increase that profit dramatically.

Slingshotting would be important for more than just the increased yield of energy; it would be vital for steering large bodies. Any adjustment of the trajectory of a billion- to trillion-tonne mass would be so expensive that we would care less about the factor of profit, than whether we could afford the project at all. As a result we might well be happy to work at a trajectory for a few centuries to get a finally profitable result by nibbling at the gravitational field of one planet after another.  

The computing load would be heavy, but routine. Much of it would be done Earthside many years in advance. There would be plenty of time to seek out the most obscure scenarios for each Kuiper Belt object, where each major improvement in handling a single rock would be worth billions of dollars, ignoring inflation.

Slingshotting would be important in two different ways: energetic gain, as mentioned, and steering. Energetic gain would work essentially by parasitising the orbital momentum of a planet. This is no novelty; it already is a routine technique of long standing in spacecraft navigation.

Steering is another aspect. Obviously passing a sufficiently large planet in a suitable trajectory can change the course of a body almost arbitrarily within the ecliptic. What is more, by passing the planet at greater or lesser distance, one can affect the angle of change practically as much or as little as one likes. In passing close to the planet, one is in a position to adjust one’s exit direction greatly by adjusting one’s incoming course by only a few tens or hundreds of kilometres. To achieve such a difference would require only the gentlest of nudges or persistent pressure a few years in advance. However, assuming such a delicate requirement one can see why we would want such an elaborate infrastructure of navigation satellites.

The other steering requirement that slingshotting offers, is one’s position relative to the ecliptic. By adjusting one’s position so that we pass to the north or south of a planet, we can adjust our approach so as to move out of the ecliptic. We could for example use such techniques to hit the north or south pole of Mercury practically vertically, or either limb of Venus grazingly, with an enormous bang in either case.  

We would deliver many times more energy and momentum than we had invested. It would achieve either excavation on the target or adjustment of rotation as required. 

Riding the Rocks

Well then, we know that if we can steer a planet into a good starting position and apply a bit of adjustment at critical points, and can stop our rock against a target instead of having to stop it in space, we have it made.

This is critically important, please note. If we got no more energy out of a rock than we had put into it, we might as well save ourselves the trouble and go and shove at the planet directly.  And if we did that, we could not nearly afford the energy. This whole exercise is predicated on the idea that we might manage to get away with three or four orders of magnitude less energy, by application of a little brainwork, commitment and patience. And being apes rather than termites, we can easily manage that can’t we? 

The way we always do?

All the same, there still is the requirement to apply that fraction of a percent of energy, or nothing special will happen.

Let us then consider a hypothetical project. Imagine a typically peanut-shaped, teratonne, predominantly rocky body, spinning about a short axis, but not so fast that any part of it is travelling much faster than escape velocity for this rock. OK?

Still, the spin is unacceptably high. Our Rockrider selects a suitable spot, based on the prospector’s information, instruction from Earth, and its own calculations, lands there, using tethers as necessary, checks the details, and drills into the body of the rock near one end, probably using plasma or laser drilling for the most part.  

Together with adjustments calculated in the light of what it finds on the way down, it carefully places a multi-megatonne nuclear bomb ordered in advance in the light of the prospecting report decades before, covers the hole nicely like a cat, retreats to a safe distance a few hundred kilometres away in space and on the sheltered side of the rock, and when the attitude and position are right, it blasts a tidy slice off one end of the rock.  

The resultant vector of the blast both kills the rotation or very nearly, and accelerates the rock into an improved, more elliptical, trajectory. You see, the depth of the bomb was such that several thousand tonnes of material were blasted off at a modest velocity, imparting a really efficient delta-V in the desired direction.

Waste not, want not! Eat your heart out, Saturn V!

Having checked how well the blast had worked, perhaps while it waited for the blast site to cool, the Rockrider lands again, possibly on the blast site, and anchors itself nicely. It begins to run its nuclear generator and plasma drills and to excavate more material in the form of vaporised rocky or sooty material that it condenses as an impalpable powder in very intense atmospheres of energetic electrons and ions in separate  chambers. The cooled particles become powerfully charged microscopic electrets. An electret wouldn't have to retain its charge for more than a few seconds, but in practice probably would retain it for years or indefinitely.

Meanwhile the Rockrider has unlimbered its main thrusters, which are specialised twin (or multiple?) linear accelerators, whether electrodynamic, electrostatic or laser. Details, details... It feeds them charged dust particles that they accelerate to modest velocities, roughly two thirds of the delta-V of the whole system as calculated for the entire trip. Pretty well optimal for the energy utilisation, which is one of the limiting factors for the project.  

In principle such electret propulsion could be far more efficient than ion thrusters. The reaction mass is cheap. In navigating a teratonne rock we could afford to use thousands of tonnes as charged reaction mass without serious regret.

Right. The Rockrider and its Earth support have calculated not only the best things to do, but the best ways to adjust the intensity and direction of the acceleration in feedback to the response of the rock to the thrust. That is what we call steering, right?

Now it gets boring.

That Rockrider is going to sit there for a long time, rendezvousing with a few planets for slingshot purposes during the next few centuries. Possibly it gets a few more charges of fuel from visiting tugs and maintenance craft.

“Oh, it’s you again is it?”

“Yeah chatterbox. Who did you expect? Goldilox? Eat up!”

A few days before impact, the Rockrider kisses its mount goodbye, and goes off for some maintenance and its next trip, which had been chosen for it before it even started on this one.

Coasting Rockriders could kill time by acting as incidental observers of conditions and events wherever they pass or pause, or by relaying signals wherever convenient.

Notice that there are major differences between any viable options for this kind of navigation and the Buck Rogers stories. There is no question of fast turns and dramatic accelerations (except for the occasional nuclear blast of course.) Everything is worked out years or centuries in advance and gently nudged for centuries en route. We cannot afford the energy or the risks of abrupt manoeuvres, but we can trade energy for time, which we have plenty of if we are to aspire to the dignity of termites rather than apes. 

Another objection that might occur to cavillers is that if it is going to take us hundreds of years to ride a single rock home, and we need 100000 rocks, we will take a lot of millions of years for the project. But that is a blinkered point of view. We would have thousands of Rockriders, all working in parallel, and sometimes in teams. It might be centuries before more than a few of the first rocks began to splash down, but then it would be a flood of hundreds per year. Although it would generally be the intention that each Rockrider would bring in more than one rock, even one typical rock would pay a generous profit for its Rockrider.

Blinkered Apes

A really serious objection for a race of monkeys is that we would be labouring for human benefits thousands or millions of years after we were multiply recycled dust and dregs, and monkeys don't do that sort of profitable venture; they want it fast, cheap, for themselves, and now.  

The nature of such projects is at odds with human nature.

But all is not lost.  

All we need do is change human nature.

For a start, do a bit of genetic modification, so that new generations have unlimited lifespans, together with the mental equipment and social competence to handle them. That might sound greedy, but actually, at the rate our biological knowledge is advancing, we might crack the associated problems in a century or two. . .

If no dino-killer beats us to the punch.

Bull Markets for Competent Bulls

Though the ultimate bull market benefits are far in the future, there is plenty of bull for the developers en route. Whole dynasties of companies could profit hugely from running the projects, improving the technology, applying the information gained much as we have profited immeasurably from satellite communications, weather observations, and Earth science and mapping, even while moaning bitterly and persistently about the costs of space technology and research.

Preventing just one single dino-killer collision with Earth (never mind turning it into a profitable collision with Venus or Mercury) would pay for the whole initiative.

What could be simpler? Or easier?

Or more gratifying?

 

 

Saturday, April 1, 2017

Heavier Duty Banking -- Appendix & Supplement



Heavier Duty Banking

Appendix & Supplement

Shortly before commencing to write this article I wrote on the topic of energy storage by means of suspending masses that could release usable power as they yielded up their potential energy, which in all cases amounted to a maximum of mass times height.See:
http://fullduplexjonrichfield.blogspot.co.za/2017/02/heavy-duty-energy-banking.html

The topic of storage of potential energy was well worn, and I only got into thinking it over during a discussion in which the idea of suspending huge pistons in fluid-filled cylinders sprang to mind. In my previous article on the topic a considerable range of options and variations emerged. Subsequently a friend showed me that the idea was not as novel as I had imagined, and in fact online exploration revealed that some companies had already been floated to implement some of the ideas I had mentioned.

Oh well, whatever has been original before can be original again...

In itself this congruence of great minds was nothing to be astonished at and I am sure that the items I saw were the merest samples of what is being explored in practice. This essay is just an appendix to my previous effort; to refer to the major premises I promote it is necessary to read that essay as well, preferably before continuing to read this one. The text below is not intended to supplement my previous ideas with suggestions from external sources, which would be pointless anyway; it is to add some thoughts in the light of the sheer scale of some of the proposals I have seen and to emphasise some points and proposals that to me seem to have been neglected elsewhere.

Let us begin by recapitulating some of the essential features of my original suggestions, and developing a few more principles.

  1. The fundamental principle is to use pistons in vertical cylinders as masses to be raised by fluid pressure as a medium for storing energy. Letting down the masses to drive the fluid through power generators would deliver the power on demand.

  2. The extent to which the cylinders are to be built above or below ground level is not essential to the principle of the device; for any scale of unit and choice of  materials the economic output is a function of height plus depth; the longer the path up and down the cylinder and the greater the mass to follow that path, the better. Accordingly the ideal cylinder in any realistic situation should be determined by the relative costs of the piston material, and of the above-ground and subterranean construction at various depths and heights. Each of these costs is significant in calculating the trade-offs and the general economics of any such project. This point of cost justification will be taken largely for granted in the following discussion; specific figures would be too speculative at this point.

  3. The pistons are to be functionally “dumb”, inert in themselves, with no internal mechanism. They are raised and lowered by pumps that control fluid pressure through ports in the cylinder wall. When a piston is neither charging nor discharging its stored energy, detents that retract into the cylinder wall can hold it passively suspended at suitable heights without relying on friction or expenditure of power. This aspect of the design differs radically from some other designs that use brakes or active devices.

  4. The choice of working fluid is a matter of choice to suit local needs and approaches; I like the idea of non-drying, non-gumming oil where that may be practical, but obviously water, possibly brine, has its own advantages, partly depending on the design and circumstances.

  5. Notionally each piston should be monolithic in function and as dense as may be practical. I still see lead as the most desirable material, followed by various forms of iron, though I have seen alternative suggestions such as concrete, which to me seem inferior in several respects. As I explain below, the fact that the pistons are monolithic in function need not imply that they are monolithic in structure.

  6. One problem with lead is that it is softer and more deformable than rival materials such as steel or even cast iron. It accordingly is vulnerable to damage in accidents during installation and maintenance. Such damage could compromise a piston’s precision and options for sealing the contact between the piston and the cylinder wall.

  7. Another problem with the piston idea is that the mass would need to be enormous. Various designs assume piston masses of hundreds of tonnes or even hundreds of thousands of tons. Installation and handling of such large objects and great masses as indivisible units could be a bad idea if there is any practical alternative.

  8. Accordingly instead of simple, solid slugs of lead, each piston should be jacketed with a suitable material. The most obvious design might be a hollow box to be filled effectively solidly with suitably packable lead segments. Originally I assumed a steel box, but I am increasingly interested in the possibility of polymer jackets, probably with steel-reinforced floors designed to hold the vulnerable lead segments and to rest on detents safely and without harm.

  9. Each segment of lead inside a piston box should be suitably shaped and coded with a unique, machine-readable identification number so as to be suitable for installation and removal on site by intelligent gantries. There are so many possible alternative designs for such segments that I do not discuss the details here, beyond remarking that any design needs be easy to place into a stable and precise configuration, probably in an oil medium. The essential effect is that the jacket then could be designed for assembly and manipulation on site by gantry, installed and packed with lead, with no need to handle the total piston mass by any means other than fluid pressure, and only after it is installed.

  10. The lead segments might be designed so that once installed, their weight could exert some outward force on the piston walls to enforce proper sealing against the cylinder walls, though this is not an essential feature. One way to achieve that would be by fitting the slabs of lead at an angle of probably less than one degree from the horizontal on an oil surface, thereby resting a slight fraction of their mass against the outer wall of the jacket.

  11. The jacket of the piston itself, if of steel and of really large scale design, must also be modular. In sizes up to say, two metres diameter and ten metres long, the jacket fairly routinely might be transportable and installable, but some of the designs I have seen online seemed to imply sizes of about ten metres diameter and 125 metres long (and eight of those above each other in a single cylinder...)  Though I do not here deny the feasibility of such units, nor even their possible desirability once installed, I do not think it would be worth trying either to transport or install them as finished units, whether empty or with their internal mass installed. The very notion of assembling the jackets on installation is challenging and intriguing, bearing in mind the many technologies for doing so, ranging from welding to bolting and gluing; it is as sobering as the challenge of the required precision on the required scale. The whole idea strikes me as a charming example of an engineering project in its own right.
    But not trivial.

  12. Nowadays we have options other than steel jacketing of the piston modules. Some modern polymers, probably fibre-reinforced, might be equally suitable for the jacketing, either as a one-piece structure, or floored with steel, or with just a steel rim around the bottom to accommodate detents that might be designed for extension inward from the cylinder wall to hold the pistons stationary where necessary. One advantage of such jackets over steel, is that they could be cast or welded in place from the bottom up, creating an essentially perfect fit with the cylinder wall. Depending on the nature of the polymer, they could be cured with the aid of ultraviolet or gamma ray sources as they are fabricated in place on site, though for my part I rather favour thermoplastics instead of thermosets.
    But those are details that one should not force on the polymer engineers in advance.

  13. Brakes or detents of some sort clearly are necessary for a number of purposes. Pumping fluid beneath or between pistons would achieve nearly all required positive, powered movement, either raising pistons, usually to store energy, or lowering them, usually to generate power. However, when it is necessary for a piston to remain in one position indefinitely, such as while the energy reservoir is full and the power demand is very low, then under constant pressure one must expect undesirable leakage past the piston. It then would be desirable to apply positive static control. Or such controls might become necessary in dealing with a damaged piston. Some schemes proposed in other discussions suggest brakes for holding the piston in place, but I reject that idea partly because of the constraints it would place on the piston's density and complexity, and partly because of the tendency to damage the cylinder wall, not to mention the problem of brakes slipping on the wall lining, very likely damaging the wall in the process and jeopardising the integrity of the seal.

  14. Instead I prefer the use of detents. There are many design options, but what I like offhand is the idea of detents that fit into gaps in regions in which fluid can be pumped into or out of the spaces between stacked pistons. The detents could be cantilever bars recessed into the cylinder walls. They could be deployed by control machinery when no piston either is in the way or needs to pass. The detent assemblies might perhaps include some sort of shock-absorbing mechanism; even at the immensely slow speeds in question, one does not simply say "whoa!" to a 10000-tonne mass. These details too are for the engineers to decide. Still, valving the fluid flow should offer very fine control indeed, so it might be possible to rely on direct control rather than shock absorbers.
    Still, shock absorbers might be an important feature in the event of a catastrophic control failure. I like to have a passive fail-safe option; I was horrified to discover that the Fukushima nuclear reactors had needed active controls to prevent disaster, which could have been prevented by passive controls that, expensive or not, would have been a lot cheaper than cleaning up the mess afterwards.
    O
    ne attractive idea is to use bi-stable detents designed to remain passively engaged or retracted until once again activated, but mono-stable detents that only let pistons pass when actively permitted, might be still safer.

  15. At the same level as each ring of detents there would be one or more input/output ports in the cylinder wall, through which fluid could be forced in by pumps or drawn off either to generate power or to lower a piston.

  16. The seal between the piston and the wall would be of self-lubricating polymer on either the surface of the piston or of the wall, or both. The cylinder's internal wall might be of polished steel or lined with hard silicone or other appropriate polymer surface of a type that could readily be repaired or serviced when necessary. In a steel cylinder jacket the seal could be cylinder rings extending entirely round the piston without any gap, and made of solid, self-lubricating polymer. If the piston's entire jacket were itself of polymer, it could be fabricated in place to fit the cylinder, providing its own seal with no piston rings. To exploit the flexibility of the polymer in such a jacket, lead segments inside the piston could be designed to exert some small fraction of their weight outwards, forcing the piston wall snugly against the cylinder wall. In either case the width of the sealing surfaces should exceed the width of any interruptions in the wall, such as inlet-outlet openings or detent recesses, so that passing such gaps would not present any seepage problems whenever the piston passes over.

  17. The bottom edge of the otherwise cylindrical piston should be chamfered into a recessed rectangular rabbet all round the edge, deep enough to accommodate the detents, and high enough to enable the matching input-output ducts to work at full capacity even if the head of the piston immediately below is in actual contact. See figure. In the design of large pistons this might demand that at least the floor of the piston be of steel. The surface of the head of each piston should not be such as to permit pistons to contact each other too closely; it should always be possible to inject fluid between, and never be possible for them to damage each other or get stuck together.

  18. The foregoing designs would place all the fluid- and energy-handling resources and also all the controls outside the pistons, and in fact outside the cylinders as well. Ideally, once the cylinder and pistons are constructed and installed, they should never need maintenance apart from occasional inspection every few decades. All the rest of the attention could be devoted to the piping, the fluid, the pumps etc. None the less, the design should permit inspection and maintenance at any time without dismantling, and as far as may be, without interrupting operation. This might affect the choice of fluid, giving preference to transparency etc. Clear water with traces of harmless corrosion inhibitors and preservatives such as zinc compounds, might have advantages.

  19. Some of the designs described by other parties online put multiple pistons into a single cylinder. This entails both advantages and disadvantages. Most importantly it makes it possible to limit the size of any mass to be handled as a unit at any point in an operation; it also reduces the pressure that individual pumps must work against, and so on.

  20. Multi-piston cylinders do introduce complications, such as the need for fluid to bypass pistons, and they complicate the design of semi-open units that either have mushroom-headed pistons, or that elevate large fractions of the working mass above the top of the cylinder. But for very large systems multiple piston designs probably are unavoidable for this approach at least.

  21. In the multi-piston approach considered in this essay the pressure pipe or pipes run up the outside of at least one side of each cylinder, with inlets or outlets for the fluid at least at each level where the chamfered rabbet around the bottom of a cylinder may be brought to rest. At each such level, there also should be a set of detents.

  22. Assuming that we use the multi-piston approach, I propose that the workable pressures within the system be at least somewhat greater than twice the pressure exerted by any one piston.

  23. In withdrawing power from a multi-piston cylinder, first insert the detents into the bottom of the gap through which the lowest piston selected to deliver power must pass. Then insert enough fluid beneath the pistons selected for immediate power delivery, to raise them enough to remove the load from their detents. When the force on the detents stops, retract the appropriate detents so that the piston can exert its downward working pressure. Thereafter begin to release the fluid supporting the selected pistons,  permitting it to drive the selected power turbines, then re-inserting their fluid into the space above the highest of the pistons providing power at this time.  As each piston delivering power comes close to the end of its intended course, the detents at the bottom of that gap already having been inserted, the next piston above gets released as  required, and the procedure continues.

  24. Cylinders could in principle be daisy-chained in series to maximise output pressure, or combined in parallel to maximise power output at a given lesser pressure. In some cases it also could permit power input to some gaps, while extracting power output from others.

  25. In any closed or semi-closed system in which the fluid is driven back up above the moving piston or pistons as they sink, the mass of the fluid itself does not contribute to the total output energy, because it has to be raised during power output.

  26. To raise any combination of multiple pistons for accumulating energy, first raise the top piston to just above its highest intended detents, probably inserting intermediate detents as the piston passes, as a precaution against emergencies. Stop just above the top detents and  engage them. Then proceed downwards, raising the next pistons in turn.

  27. Note again especially, that all these design variants lend themselves to economical and reliable safety measures, in particular, passive economical safety measures. In this they are in contrast to say, batteries that, if in any way shorted, can release disastrous heat, or even fires. As long as the working fluids can be automatically drained or contained, they are safe. If the towers are say, a city block away from vulnerable objects, they will not be a major threat even in a major earthquake.   

Note that this essay discusses just one class of design options. The choice of design detail would depend on many variables such as the scale, the materials, the desired duty cycles, the respective costs of available materials and so on.