Date: Tue, 1 Dec 1998 00:18:46 -0500 From: "Robert P. Stefko" Subject: Re: [BLUE PLANET] - Fill 'er up with Premium! Warning! This is a very long message. >All this discussion of delta-v has reminded me of something -- we're >depending on fusion reactors, which require material to fuse (H), to >provide the energy to accelerate the reaction mass (H). Where's all >this hydrogen coming from? Jupiter and Saturn. The atmospheres of these jovians are almost pure hydrogen and helium. The subjovians, Uranus and Neptune, have lots of impurities in their atmospheres—methane, ammonia, hydrocarbon ice crystals, and other "cometary slush"—it gives them their bluish tint. Mining the jovians is simple: throw an asteroid into close orbit, drill a warren of tunnels and chambers into it, add a fusion reactor and life support equipment, install special fuel processors (for the helium and trace impurities), and drop a hose into the atmosphere. Of course, a couple of problems arise with this model. First, you'd be operating deep in the gravity well of a gas giant. It'd be more efficient to just set up an electrolysis station on Europa; however, the jovians have lots of helium, which means they have at least some helium-3 (probably more than can be had on the moon), making them a source of potentially abundant fusion fuel. Second, Jupiter (but not Saturn) has a strong magnetic field. This means its radiation belts are intense and very deadly. An asteroid station would probably have enough shielding to keep its occupants safe, but ships would need extra shielding (and therefore mass) to survive the trip down the gravity well and back. Further out, near the Wormhole, spacecraft could snag comets from the Kuiper Belt and extract water for electrolysis into H and O. Deep space fueling stations leading up to the Wormhole could alleviate some of the reaction mass problems that have plagued the discussion on propulsion systems for BP, since spacecraft would not need to carry the full amount of reaction mass necessary to reach the Wormhole from the onset of their trip. (Bing! Was that a light bulb I saw go on over my head? I think that's a viable option we would do well to explore: a trail of refueling stations leading from Earth to the Wormhole.) >Water seems like the easiest way to get it, but we're talking huge volumes of Hydrogen here, (here I defer to Leif, whose physics appears to be WAY more practiced than mine, for exact numbers) and the two water bearing planets we know of probably don't want to part with that much, and I'm assuming that Europa is used as a water source for Mars (when it's anywhere nearby) and the Belt. That leaves Titan as the handiest ice-bearing body, and it still has to be refined (electrolocized) to get Hydrogen.< Actually, Titan is more attractive for its hydrocarbons. Methane exists as a liquid on Titan, possibly precipitating as rain or snow and forming oceans, rivers, and lakes on the surface. Other hydrocarbons exist in the atmosphere, giving the moon a hazy orangish color. These chemicals could be extracted and refined into all kinds of synthetic materials, decreasing our dependence on terrestrial fossil fuels (which would be beneficial to the environment). >Why not go for the pure stuff then? Neptune, or possibly Persephone sound like good ideas, since Neptune is mostly Hydrogen, and presumably Persephone is too, as it wasn't mentioned in the book as a planetological oddity (heavier elements tend toward the center of the system, leaving outer gas giants mostly Hydrogen). Getting it could be a problem too, as diving would be hard on ships. It might be possible to steer a couple of asteroids out there and make a skyhook anchored at the bottom in Neptune's atmosphere (say in the 5 atm range -- saves you having to pump it up to the upper asteroid), but that's a whole other can of worms.< As I mentioned before, Neptune has considerably more impurities in its atmosphere than Jupiter or Saturn. This is because of its location near the outer edge of the original planetary accretion disc. At this distance, temperatures are low enough that hydrocarbons crystalize, forming ice particles. These particles condensed to form comets and were also captured by the subjovians. These hydrocarbon ice crystals did not form at the distance of Jupiter and Saturn, which explains why those planets have almost pure hydrogen-helium atmospheres. Information on the atmospheric composition of Persephone does not seem to be provided in the book, so I can't comment on its suitability as a source of reaction mass and fusion fuel. >So if it's coming from so far outsystem, where is it refined? You're still going to have impurities. WH 1 (Neptune) and 2 (Persephone) don't sound bad, as they're going to be the fueling stations anyway (I'm assuming that when a ship shuts down it's drive near the WH stations, it's nearly dry. Negotiate the wormhole at low velocities, and then dock on the other side to fuel up for the trip insystem.) Getting the raw fuel to the stations could be a problem decades at a time, though, as the planets can end up in the most damnably inconvenient places (like on the other side of the sun). Not having my copy of Redshift with me, nor my copy of BP, I can't really guess how much of the time that's going to be a problem, compounded by the fact that the wormholes themselves are orbiting...< The reaction mass/fusion fuel will be refined where it is extracted. Since refining equipment consumes energy, you'll want the gasses to be as pure as you can get them from the onset (hence the argument for Jupiter and Saturn and against Neptune). If you establish a network of refueling stations scattered all throughout the system—some in orbit around gas giants, some on icy moons of those giants, and lots riding piggyback on comets—you should always have at least one or two stations along your route to and from the Wormhole. As an aside, there are other ways to conserve reaction mass. Gravity assists (to gain velocity) and aerobraking (to lose it) are two methods not yet mentioned. And if you're not in a hurry, you could always use solar sails (for inner system transit). *************************************************************************** To unsubscribe from this list send mail to majordomo@mpgn.com with the line 'unsubscribe blue_planet' as the body of the message. Date: Tue, 1 Dec 1998 10:24:46 +0100 (NFT) From: =?ISO-8859-1?Q?Leif_Magnar_Kj=F8nn=F8y?= Subject: Re: [BLUE PLANET] - Fill 'er up with Premium! On Tue, 1 Dec 1998, Robert P. Stefko wrote: > Warning! This is a very long message. > > >All this discussion of delta-v has reminded me of something -- we're > >depending on fusion reactors, which require material to fuse (H), to > >provide the energy to accelerate the reaction mass (H). Where's all > >this hydrogen coming from? > > Jupiter and Saturn. The atmospheres of these jovians are almost pure > hydrogen and helium. [snip] Or by electrolysing water from captured comets, or whatever. > Further out, near the Wormhole, spacecraft could snag comets from the Kuiper > Belt and extract water for electrolysis into H and O. Deep space fueling > stations leading up to the Wormhole could alleviate some of the reaction > mass problems that have plagued the discussion on propulsion systems for BP, > since spacecraft would not need to carry the full amount of reaction mass > necessary to reach the Wormhole from the onset of their trip. Yeah, this is the method I assume is being used, since the ship has to slow down in order to transit the wormhole (you would *not* want to try to pass through a kilometer-sized hoop while moving at a few percent of lightspeed). I assume refueling stations on both sides of the wormhole, and electrolysing water from KBOs and the like seems like the sanest way to get hydrogen; fusion-powered refineries could feed themselves by fusing some miniscule fraction of the hydrogen they refine. > that a light bulb I saw go on over my head? I think that's a viable option > we would do well to explore: a trail of refueling stations leading from > Earth to the Wormhole.) No, I don't think so. You'd have to not only get the fuel *out* to the speeding ship, but you'd also have to match velocity with it somehow. That requires accelerating the fuel so it has the same speed that the ship has when they rendezvous, which will cost *exactly* as much energy as just carrying it on the ship in the first place would cost (and what's more, the only practical way of *getting* the fuel up to that kind of speed would require launching it on tankers with their own fusion rockets and burning a lot of it just to get the rest up to speed; these tankers would *either* be emptied completely during refueling and thus have to be disposable, *or* they'd transfer only part of the fuel and keep what they needed to slow back down and return to be reused, in which case the amount of fuel the original ship could get would be drastically reduced -- either way it sounds like a horrible way of inflating travel costs). The alternative would be for the ship to actually *stop* at each refueling station, which would be insane since the trip would take much longer (accelerate-decelerate, accelerate-decelerate, accelerate-decelerate etc. rather than accelerate-coast-decelerate). Icy bodies would probably be transported mostly intact to the vicinity of the refinery and only "cracked" when the hydrogen will soon be needed (it's a lot easier to store frozen water than liquid hydrogen). For any given icy body, the trip might take years; that doesn't matter much as long as there's a sufficient stream of new arrivals at the refinery. [snip] > As an aside, there are other ways to conserve reaction mass. Gravity assists > (to gain velocity) and aerobraking (to lose it) are two methods not yet > mentioned. Gravity assists aren't very meaningful for fast spacecraft. There are two types of gravity assist; unpowered ones merely slingshot around some celestial body, and only serve to change the direction you're travelling (you do go faster for a while during the pass, as potential energy in that body's gravity well is converted to kinetic energy, but you lose *precisely* the same amount of kinetic energy as you recede toward "infinity"). Powered ones take advantage of the fact that you can bring fuel with you into the gravity well and burn it while you're close to whatever celestial body you're using; using this method, you can actually gain more velocity than you could if you were burning the same fuel in "flat" space. However, the amount of extra kinetic energy your ship can get is limited by the escape energy of the planet (or sun) you're using, and by how much fuel you can actually burn while you're making the pass. This *is* good for slow, chemically-propelled craft like the space probes etc. that we're using today, but not for a ship that intends to move at thousands or tens of thousands of km/s since what you could gain would be negligible compared to the velocity you're able to attain on your own, *and* in order to gain anything at all you'd have to go through contortions that would add months or years to your travel time.... Aerobraking is similarily useless for a really fast spaceship; in fact it's worse than useless since you'd either lose a negligible amount of speed (passing *very* far from the planet, where the residual atmosphere is so thin that it's better modelled as individual atoms zooming around in space than as a gas), or else you'd die (if you pass through dense enough atmosphere to lose a meaningful amount of velocity; take a close Earth flyby, for example, at ten thousand km/s: It would take you about 1.3 seconds to pass one Earth diameter, and quite apart from the fact that the Earth is a spheroid and not a disk so you wouldn't be close to the surface for more than a small fraction of that time, you'd need something like 100 g of deceleration to shed even *one* km/s in that time). Not to mention the fact that the locals wouldn't want anyone buzzing their planet at that kind of speed, so they'd try to blow you up 10 or 20 AU away, etc. > And if you're not in a hurry, you could always use solar sails > (for inner system transit). Yeah, but that sort of makes the idea of a fast spaceship moot again. And actually, I think that magsails might be better than solar sails for leisurely in-system travel (particularily if you have high-temperature superconductors that approach the theoretical limits for energy-storage density); they'd get a better mass/thrust ratio, as well as being less vulnerable. Both solar sails and magsails are somewhat irrelevant (except perhaps for some specialized missions) if you have fusion rockets that are as good as we need them to be in Blue Planet, however. *************************************************************************** To unsubscribe from this list send mail to majordomo@mpgn.com with the line 'unsubscribe blue_planet' as the body of the message. Date: Tue, 1 Dec 1998 11:23:10 +0100 (NFT) From: =?ISO-8859-1?Q?Leif_Magnar_Kj=F8nn=F8y?= Subject: Re: [BLUE PLANET] - Spacecraft Propulsion in BP On Mon, 30 Nov 1998, Robert P. Stefko wrote: > >Boarding actions are unlikely against any ship moving at high speed. First > of all, any physically reasonable drive needs a lot of time (and reaction > mass; I do not think reactionless drives are physically > reasonable) to get up to speed. If you want to intercept a moving ship > you have to be able to make your location *and* your vector coincide, and > you have to make that happen before the rogue ship reaches its target; this > is probably not going to be possible.< > > Well, first of all, ships moving in-system are not going to travel at such > high velocities because there isn't enough time to accelerate. Even at 1-G, > it would take roughly 17½ days to reach .05c, during which time the craft > would travel 76.4 AU—well outside the orbit of Pluto. It would then somehow > have to turn around and head back in-system, without losing velocity, before > it could threaten to collide with an inhabited planet. Um, bad assumption. Imagine this scenario instead: Some misanthrope has a ship, and is willing to spend a few years preparing to die if he can take the Earth with him in a blaze of glory. He takes the ship out to the Kuiper Belt in a "normal" fashion (i.e. travelling rather more slowly than a worhole express, taking perhaps a year or two just to get out to where he can find a nice juicy KBO with lots of ice). Then he sits there for however long it takes to refine as much fuel as he needs. *Then* he invokes Cthulhu (or whatever) and lights the drive, starting the acceleration from a relative near-stop perhaps a hundred AU out from Earth, and accelerating for most of that distance. None of that going out at high speed and then "somehow turning around without losing velocity" (which you can't do anyway; the *only* way to turn around in deep space *is* to lose all the velocity you don't want any more and then accelerate in the direction you do want to go; gravity assists are right out since you'd need something like a long series of black holes in just the right places, which aren't there, and even if they were and you *could* use them to slingshot around you wouldn't survive the maneuver). > This means that only > spacecraft emerging from the Wormhole will be travelling at a significant > fraction of c, and then it will be months before they come near any > settlements. And months is what you will *need* in order to intercept them. Do not ignore the possibility of some "ordinary" wormhole-express spacecraft going rogue in midflight, either. When going from the wormhole to Earth, say, approximately a month might be spent accelerating inwards, followed by a few months of coasting and then a final month of decelerating -- that might give you less than one month's warning about a coming kamikaze attack, if the first thing you know about it is that a ship that's supposed to start decelerating does the opposite thing instead. > >Furthermore, assuming that the chaser and the chasee have about the same > capabilities wrt. drive performance and fuel, then they're both going to > have about the same maximum delta-v -- for instance, say most ships are able > to make a total delta-v of 5% of lightspeed, then that can mean going up to > 2.5% of c and then back down to a relative stop, *or* going all the way up > to 5% of c with no hope of ever slowing back down (except by crashing). A > kamikaze ship could exercise the latter option, heck, it would do so by > *choice*; a similar ship trying to catch it would necessarily be signing its > own death warrant since -- even if the interception and boarding action were > successful -- they'd then be stuck going at 5% of lightspeed with no fuel > left and thus no way to slow down; nor any way of getting help since nobody > *else* can get up to that speed and back down again either.< > > But why assume similar capabilities? Because in order to travel between Earth and Poseidon in six months, you already *need* to stretch the capabilities of fusion rockets close to their theoretical limits. There just isn't that much room for improvement over the fastest civilian ships, and you *must* be prepared for the possibility of one of those going rogue (hijackers, deliberate use by the *owner* as a weapon, whatever). You *can* get dramatically improved acceleration for a ship with a fusion rocket of some given power, by introducing a significant amount of inert (i.e. non-fusing) reaction mass to be simply *heated* by the fusion process. However, this will reduce the exhaust velocity of the rocket by the same factor as you increase thrust by, and reaction-mass consumptio rate will increase as the *square* of that factor (ten times higher thrust will mean ten times *lower* exhaust velocity so that the same amount of fuel will give you only one-tenth the delta-v; *and* you'll use up the fuel 100 times faster). This makes it possible to have high-acceleration interceptors that can catch fast ships *if* the prey doesn't have much of a lead in either distance or speed (so you can catch someone who's trying to flee from your space station, for instance -- it doesn't help him much that he can go ten times as fast as you, if it takes him a month to reach that speed but you can catch him within the first couple of days). But it's absolutely no good for deep-space, high-speed interception. > No nation I know of has Coast Guard > ships the size of supertankers, just a lot of little patrol boats. > Interceptor spacecraft will be small and fast, with most of their mass as > fuel Which is already the case for the fastest civilian transports (most of their initial mass *is* fuel, they spend something like 1/3 of the Earth-WH1 trip burning that fuel and the rest in coasting, and piling on more fuel is pretty pointless due to exponential growth in fuel requirements). > and just enough life support to deliver a team of space cops to their > target and bring them back. The little ship would be like a remora attaching > itself to the belly of a whale: it would match vectors, then either dock at > an airlock or clamp on and burn through the hull. The space cops enter and > do their job. As explained above, this kind of stunt is doable only if you start more or less on the heels of the prey. For deep-space interception at several percent of lightspeed, just getting to the same *place* as the target is going to be difficult enough, nevermind matching vectors at the same time (even if you have maybe twice as much delta-v and acceleration). > If the big craft is out of reaction mass, they fire the > interceptor's rockets to change their vector ever so slightly—at the > distances we're talking about, even a change of a fraction of a degree will > take it out of harms way—sending it toward the sun, possibly using the star > for a gravity brake. No point in gravity "braking" or slingshotting at high speed, nor really any point in aiming for the sun; if you can somehow deflect a fast incoming body enough to make it miss anything important (which probably won't be possible without blowing it up in the process, in which case you get an expanding shrapnel cloud which might only *mostly* miss), you can stop worrying about it since it will just go on into the great beyond and never come back. > As for the whole kinetic weapon discussion, if you're using "dumb" > projectiles, something like buckshot is the best way to go. You deposit more > kinetic energy into the target, and you make a lot of small holes throughout > the hull, making explosive decompression an immediate and deadly problem. Um, holed hulls and decompression isn't really going to be much of a concern with high-speed (as in thousands of km/s) kinetic impactors. Assuming that you're able to dump a significant fraction of the impactor's kinetic energy into the target, what you'll get is more akin to a small nuke blasting the whole thing to vapor and shrapnel. Even one kilogram coming in at ten thousand km/s has the same energy as a 12-kiloton nuke, and that energy has to go somewhere; even if 90% is lost to punch-through, you've still got a kiloton or so going off *right on top of you*. That will not simply leave a neat little entry hole and a neat little exit hole (unless it only hits something like a wispy solar sail or something, rather than the actual ship). *************************************************************************** To unsubscribe from this list send mail to majordomo@mpgn.com with the line 'unsubscribe blue_planet' as the body of the message. Date: Tue, 1 Dec 1998 11:19:38 -0500 From: "Robert P. Stefko" Subject: Re: [BLUE PLANET] - Fill 'er up with Premium! >basically the same thing I was thinking about with the asteroid down in the atmosphere. You could run the hoses up the cable, keep them from drifting. I've got a couple of other ideas that would eliminate the need for pumps, even.< The pressure differential between the tanks and the atmosphere would be enough to force the gasses upward, assuming you drop the hoses down far enough. (By the way, the hoses would probably be semi-rigid, with enough give to survive buffeting in the turbulent atmosphere, but not enough to go lashing around whenever the wind kicks up.) >I'd think the water is needed far more by the belt colonies and anyone working mining in the outer planets or Kuiper belt.< Most of the water in space will probably come from comets moved from the Kuiper Belt. Too, mass drivers on Europa (or Ganymede or Callisto) could hurl chunks of ice into orbit with ease, since the moon has low gravity, and those could be moved like comets to wherever they are needed. However it is done, there'll be plenty of water for all possible uses. >So use Saturn instead. A higher percentage of the planet is hydrogen, anyway.< But a lower percentage of helium, which means less 3He. Hydrogen can be had from a multitude of sources: it's the most common element in the universe. Helium-3 is less common, and far more valuable due to its relative scarcity. >This might work, but not for the Wormhole run. If the fueling stations were stationary, the ships would have to stop and fuel up. That means decel time, then accelerating back up to speed again. The travel times would become unmanageable.< The stations would orbit the sun. And incoming craft would not have to decelerate. Tankers could be sent out ahead of time to match vectors with the approaching spacecraft, attach a fuel probe and reful the craft, and then detach and maneuver back to the station. >The other way of doing this would be to snag comets or small asteroids, and accelerate them to speed, but then you'd use a LOT more fuel, and could really only intersect with ships moving the exact same speed along the exact same path with the exact same acceleration.