Stars, planets and other Sci-Fi peeves

[Darat]

Read some earlier posts and I am always amused when you read about two ships being able to meet and I'm amused because despite the terrible "science" involved in e.e. Doc Smith lensmen universe FTL (back in the 1950s) he actually made a lot of use of "intrinsic" velocity, in other words two of his ftl craft could on FLT zoom to almost touching distance but then to actually transfer across the master pilots had to fight the rockets to match the ships " intrinsic" velocity ( their velocity before they switched to FTL). It was decades later that other science fiction writers started to think about this, and many of them still just ignore it.

 

[Darat]

You must really hate January sails.

 

[HansMustermann]

The other question though that I'm not clear on is actually how far away you'd be able to track a spacecraft that is tens or even hundreds of meters long? Sure, it's giving off thermal radiation, but then everything in the solar system is giving off reflective light but as I understand it we are still unaware of the vast majority of smaller bodies.

The problem is that those bodies are still relatively cold. If your ship is only 300k or so, you're shining like a beacon against the cosmic microwave background.

And then you want to make course corrections too? Well, those thrusters put out some almost completely ionized plasma, one way or another. Not only it emits its own EM spectrum, and a LOT of it at that, but it's opaque to light so it's absorbing and re-emitting the light from the sun too. So it's essentially like a solid and rather reflective trail by itself PLUS its own photons.

How easy it is to track that? Actually more than just position.

That trail tells an observer both the exhaust energy, temperature (via Stefan-Boltzmann) AND exhaust speed (via Stefan-Boltzmann and Doppler), so they can know your exact thrust. Divide by observed acceleration, and now they know the exact mass of your craft too.

The latter also meaning that decoys utterly won't work. You can't fake a battleship unless you're moving an asteroid that actually is as heavy as a battleship.


Finally, let's look at WW2 flak training, because it's relevant to what you propose there. There were two kinds of flak fire:

- directed, as in maybe one battery, two tops would fire at you while in flight to your target, and they'd try to predict where you'll be by the time their projectiles get there. This was somewhat countered by changing course all the time, like you propose.

- saturation. This happened over important objectives. These guys wouldn't even try to aim at any particular plane. They'd just try to fill a box at your flight height with as many explosions as they can. There is NO known counter to that. Pilots were just told to fly in a straight line and get out of that box as fast as possible.

Thing is, the latter is actually trivial in space, because your own high speed works against you. Once they have an even vague probability cone of where you'll be, and some missiles that can reasonably get within light seconds in front of you, they only need to explode bags of sand in your path. Passing through that cloud at the kind of speeds that would even make such a fight have any point, will shred your fleet real good.

Or if you want to be extra nasty, or the enemy is extra armoured, make it sacks of 1mm sized Tungsten balls instead of sand.

Basically, have you seen Gravity? Yeah, that kind of scenario.

 

[Bikewer]

In Greg Bear’s “Anvil Of Stars”, which features VERY advanced space combat, one of the weapons proposed is clouds of very dense “needles”....

Occupants of the ships are protected from “super-G” maneuvers by “volumetric fields” which actually penetrate the body and keep you from moving....

 

[Roboramma]

You're answering that to my objection that you'd need a collector that's a 12 km sail floating behind your ship, you know?

Sure, but I don't think that's actually true. You seemed to assume that the coolant would evaporate, but note for instance that the actual experts who looked into the idea (at least according to dasmiller) weren't planning on that.

You want a smaller vessel, here's a simpler idea: according to Messrs Josef Stefan und Ludwig Boltzmann, the heat you radiate is proportional to the 4'th power of the temperature of your radiator. So you just put a heat pump between your craft and the radiator. Hell, deck the radiator in Peltier plates.

Yeah, absolutely. I assumed you'd be doing that already. But you should be able to combine both approaches.

That might make avoiding evaporation difficult, but imagine your coolant runs through radiators that are brought into contact with a solid that absorbs the heat. That solid is then thrown out into space toward a small collector craft. It radiates heat as it travels toward the collector. It's then launched back to the original craft and taken back aboard. Repeat. At all times you can have a stream of these solid objects travelling from your main spacecraft to small collector and back. They're not dissipating into a mist because they are not a fluid, but their total surface area will be quite high (and we've heated to a high temperature with your heat pump so they are radiating heat at a high rate).

The radiators could still be on the external surface of your ship, with these solids put on a rail system or something that can come into contact with them, and might only be used when extra cooling is needed.

 

[grmcdorman]

[snip[

That might make avoiding evaporation difficult, but imagine your coolant runs through radiators that are brought into contact with a solid that absorbs the heat. That solid is then thrown out into space toward a small collector craft. It radiates heat as it travels toward the collector. It's then launched back to the original craft and taken back aboard. Repeat. At all times you can have a stream of these solid objects travelling from your main spacecraft to small collector and back. They're not dissipating into a mist because they are not a fluid, but their total surface area will be quite high (and we've heated to a high temperature with your heat pump so they are radiating heat at a high rate).

The radiators could still be on the external surface of your ship, with these solids put on a rail system or something that can come into contact with them, and might only be used when extra cooling is needed.

That seems to be just a complicated large-area radiator with an odd working fluid. A more conventional design that unfolds in some fashion - in your case, long hoses with fins or texture to increase the surface area - would probably work much better. Just as vulnerable to attack, though.

 

[catsmate]

Assuming the detection isn't the problem I'd see them using intelligent ordinance, the attacking ship throws it out based on a rough prediction of where the enemy will be, then as it gets closer it begins to fine tune its approach. Or even splits into different parts to increase the chance of a hit.

It would be extremely wasteful and any sudden large change of vectors would probably save the shop being attacked.

I am reminded of a short story where a person, in a transhuman setting, was being investigated for a data breach; he'd unknowingly been having virtual sex with a smart missile.

You must really hate January sails.

Light sails?

 

[HansMustermann]

Sure, but I don't think that's actually true. You seemed to assume that the coolant would evaporate, but note for instance that the actual experts who looked into the idea (at least according to dasmiller) weren't planning on that.

I would assume at least some of the stuff would evaporate anyway, just because, you know, it's at zero ambient pressure. Most liquids, including water, for which I had done the calculation, will outright BOIL if dropped into space at room temperature. Rather explosively, at that, because it's suddenly some 90K above its boiling point. In fact, water won't even have a liquid phase in a void. It will turn to solid when it gets below 210K.

So now in addition to needing a huge sail behind you to collect it, you have to deal with the impacts of ice needles at bullet speeds against that sail. Did I mention the movie Gravity yet?

Mercury, btw, would do much the same, if you use that instead. There may be some liquid out there that doesn't, but I don't remember any off the top of my head.


And if you want to combine that with the heat pumps, i.e., dump liquid a few hundred degrees above room temperature, well, your choices just narrowed considerably, compared to, you know, just keeping it in a hose.


And again, do you even gain anything? Given that you get ALL the kinetic energy back for whatever you collected, and the only thing you really lose any energy from is the stuff you lose, then essentially you've gained nothing compared to just venting a smaller quantity and forgetting about it. Any extra cooling will have to be done by the collector, PLUS the extra cooling for its own thrusters. So then why not just tether it, and save yourself the overhead of those thrusters?

 

[HansMustermann]

In Greg Bear’s “Anvil Of Stars”, which features VERY advanced space combat, one of the weapons proposed is clouds of very dense “needles”....

Well, the only problem with needles is that you need to orient them to actually gain anything. And it being in a void, fins wouldn't work. So either you can give each needle some kind of propulsion and on-board computer, or you can just take the cheap version and dump a bunch of tiny balls instead.

Edit: I'll grant though, that if you're writing SF, some nanotech smart micro-rocket flechettes sound a heck of a lot more interesting than just exploding a sack of sand.

 

[HansMustermann]

BTW, one more thing about detection: think of the spectrum of black body radiation.

Any inert and cold stuff out there will reflect sunlight, i.e., have the peak of the spectrum at Sun's temperature. Any energy you EMIT, however, is going to have the peak for your own temperature. So basically if you have good enough sensors for a long range space fight to be even possible, much less sensible, you probably can track the thing that has a spectrum for 300K.

 

[jimbob]

Light sails?

That was what prompted me to correct it from sales, yes.

 

[Roboramma]

And again, do you even gain anything? Given that you get ALL the kinetic energy back for whatever you collected, and the only thing you really lose any energy from is the stuff you lose, then essentially you've gained nothing compared to just venting a smaller quantity and forgetting about it. Any extra cooling will have to be done by the collector, PLUS the extra cooling for its own thrusters. So then why not just tether it, and save yourself the overhead of those thrusters?

The point was never to use it as a means of thrust. That would be stupid as it would defy conservation of momentum.

ETA: Okay, wait, I think you are saying that when it radiates heat you'll get that energy back in the form of the kinetic energy when it impacts the collector? That's just false. The radiation is going in all directions, it's not speeding this stuff up relative to the ship, it will impact as the same velocity that you launched it at. You don't get that heat energy back somehow.

 

[Roboramma]

I would assume at least some of the stuff would evaporate anyway, just because, you know, it's at zero ambient pressure. Most liquids, including water, for which I had done the calculation, will outright BOIL if dropped into space at room temperature. Rather explosively, at that, because it's suddenly some 90K above its boiling point. In fact, water won't even have a liquid phase in a void. It will turn to solid when it gets below 210K.

Did you miss the whole discussion of using a solid instead of a liquid? The solid could be ball bearing, for instance, though I'm sure there are better ideas.

So now in addition to needing a huge sail behind you to collect it, you have to deal with the impacts of ice needles at bullet speeds against that sail. Did I mention the movie Gravity yet?

Why are they "moving at bullet speeds"? They are being launched from your ship to your collector. You obviously control the speed at which they are launched, and I don't see any reason that you'd want them going at such a high velocity.

 

[HansMustermann]

The point was never to use it as a means of thrust. That would be stupid as it would defy conservation of momentum.

ETA: Okay, wait, I think you are saying that when it radiates heat you'll get that energy back in the form of the kinetic energy when it impacts the collector? That's just false. The radiation is going in all directions, it's not speeding this stuff up relative to the ship, it will impact as the same velocity that you launched it at. You don't get that heat energy back somehow.

I was assuming liquid.

The only latent heat of vaporization you'd lose would be in the form of the kinetic energy of the molecules going off. You collect them, you get it back when they impact the collector.

Ditto for bullet speeds. If that gas goes off in all directions at average speeds of many hundreds of metres per second, it solidifies when most of it locally is going at more or less the same speed in the same direction. You know, less local chaos, less temperature. The resulting ice needle would then be on the average going at the speed the gas molecules were going when they went off.

Since, you know, we started from THIS post:

I started a thread once about the idea of just spraying some sort of coolant into space (and then collecting it once it had cooled*).

"Spraying" and "coolant" tend to suggest liquid, you know.

Using a solid, well, that sounds just... not worth it, to say the least. It introduces a whole bunch of complications to even make it work, not the least being that then you'd need some kind of propellant if you want to stick to spraying it. So now you're still losing matter, but for no added benefit.

An even bigger problem is that any solid granules you eject are a lethal hazard for any other ship flying through it. See my space-FLAK comment earlier. So if you want to have a whole fleet, like in epic space battle movies, with half a dozen carriers, hundreds of fighters and bombers buzzing around, etc, congrats, you just made it a minefield for your own side. The WW2 equivalent would be using flak where your own fighters are.

AT BEST, now you've created the problem that everyone needs to be permanently aware of where everyone else's deadly cloud of solid granules is, which is a nightmare.

And you still have extra points of failure when you essentially have two spacecraft instead of one.


And anyway, having a collector following a couple of minutes behind negates the whole advantage of a finite light speed. Because they know the collector has to follow your exact trajectory. So whatever probability cone they would normally have for your ship, they have a MUCH smaller one for your collector. Because from where it's now, you know one point it's going to pass through, and at what moment.

While the end point may still be ahead of that known point, you reduced the time that you need to guess where it's going to end. The probability cone just got a lot smaller.

So now if I want to disable you, I don't even have to flak YOU, I can get a much more accurate prediction to flak your collector.

So, really, why bother?

 

[HansMustermann]

Besides, exactly how many kilos are you proposing to have in transit at any given time? Solids tend to be heavy, if you want something that can stay solid at hundreds of degrees.

As in, how often do you want to get it back?

A couple seconds worth at most? Then you're making the collector move back and forth like a yoyo, just to bring it back that often. And spending a lot of energy on that, and producing more heat on the collector, and now THAT one has to dissipate it.

Leave whole tons of it behind and/or in the collector? Then you're hauling a lot of extra weight, and at any given time only a small fraction of it will actually be in space to radiate anything. The rest is in the collector not doing jack for you.

Why not just make some radiators that weigh the same? They may have less total surface, but you have ALL of it in use all the time, so it should more than even out.

 

[Roboramma]

I was assuming liquid.

Right, but I'm talking about using a solid. There may be other solutions that I haven't thought of that would allow some sort of liquid, but your criticisms in that regard seem valid to me (I had considered the issue of evaporation and thought it was a major problem, for instance).

The only latent heat of vaporization you'd lose would be in the form of the kinetic energy of the molecules going off. You collect them, you get it back when they impact the collector.

But I'm not trying to use vaporisation to increase the speed of heat dissipation. I'm still relying on radiant heat loss.

Look, think of the idea like this: it's a radiator without the pipes. It works in exactly the same way as any other radiator, except we're trying to do it without the pipes.

Why? Initially I thought you could save on the mass of the pipes, but that potentially efficiency seems to be lost to other inefficiencies inherent in the design. Okay. The other gain is the possibility that pipes can be a target. If you don't have pipes, no target. If someone hits my stream of ball bearings with a laser, what happens? I lose a few ball bearings. If it was a pipe the pipe ruptures and all the fluid inside starts to spill out into space in all directions. That's the only problem that I'm trying to solve right now. My solution may bring up other problems, but I think I've addressed those at least in so much as it's possible to do this in a way that could be used to radiate heat, though whether the gain from the design is enough to make up for it's inefficiencies is a different issue.

 

[Roboramma]

Besides, exactly how many kilos are you proposing to have in transit at any given time? Solids tend to be heavy, if you want something that can stay solid at hundreds of degrees.

As in, how often do you want to get it back?

A couple seconds worth at most? Then you're making the collector move back and forth like a yoyo, just to bring it back that often. And spending a lot of energy on that, and producing more heat on the collector, and now THAT one has to dissipate it.

I originally suggested sending the collector back, but in another post I suggested just launching then back from the collector. If it's a solid you should be able to target a small enough area that this could be feasible.

 

[Roboramma]

"Spraying" and "coolant" tend to suggest liquid, you know.

That's fair.

Using a solid, well, that sounds just... not worth it, to say the least. It introduces a whole bunch of complications to even make it work, not the least being that then you'd need some kind of propellant if you want to stick to spraying it. So now you're still losing matter, but for no added benefit.

You don't need a propellant to throw something off your spacecraft. We're not talking extremely high speeds here.

An even bigger problem is that any solid granules you eject are a lethal hazard for any other ship flying through it. See my space-FLAK comment earlier. So if you want to have a whole fleet, like in epic space battle movies, with half a dozen carriers, hundreds of fighters and bombers buzzing around, etc, congrats, you just made it a minefield for your own side. The WW2 equivalent would be using flak where your own fighters are.

I have no concept of a huge fleet of spacecraft in small area. That seems likes a stupid idea to me.

 

[HansMustermann]

Well, ok, but you still have some thrusters on the collector if it has to follow you around accurately, the fuel for them, the cooling for the collector, the mechanisms for collecting and sending those things back, etc. That's a lot of mass. We're talking many tons just for the collector. Why not just use that mass as cooling panels on the normal ship?

They don't even have to look like those on the space station. I mean, ever notice how a super star destroyer is like a reverse tardis, a lot smaller on the inside than outside? Just put your panels in the shape of a wedge around your ship and you've got one

 

[Roboramma]

In the end you are probably right. While I like the idea (mainly because I was thinking of it one day and thought "Huh, that's sort of cool"), I never really thought it would be a good idea practically, because, as you are pointing out, the problems that it creates are generally going to be bigger than the ones it solves.

I do think it could work, but you are right that there will almost certainly be better solutions.

But, back to the issue of your radiators being a target for enemy fire, I think that there may be possible creative solutions that could at least make it less of a target.

I do agree that you're not likely to find a way to hide the radiation, though, so it's going to be hard to avoid being found by other spacecraft, though again I think the distances might make a difference here. A far off small, faint, point of light can be hard to find. Once you've found it, it should be easy to continue to track it, though.