feld
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Post by feld on Oct 5, 2016 18:41:24 GMT
Dear qswitched,
Just got the game. Very good start! Hopefully more will follow. I for one would willingly pay the same price for DLC or new versions which expanded your base space warfare model.
In one of your forum posts you mentioned that you included in early development a sensor model and quickly became convinced of the no stealth in space hypothesis. Such models are a hobby of mine and I wanted to ask a question about yours: what background did you use for it? I.e. when deciding whether a detection was made or not?
Thanks and v/r feld
PS - please be as technical as needed. I love your blog posts for their clarity at simplifying complex technical issues. But I'm a 21 year nuclear submarine veteran with multiple degrees in spacecraft engineering (long story) and I crave the details or pointers to the references you used.
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Post by RA2lover on Oct 5, 2016 19:32:05 GMT
Side question: Does decoy temperature matter regarding detection distances?
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Post by peridot on Oct 5, 2016 21:44:22 GMT
Most of the radiation - most of the photons in the Universe - would be 2.7K blackbody photons, the cosmic microwave background. There will be oodles of point sources, mostly stars, peaking in the near IR or optical, a modest radio background, and a decent number of variable point X- and gamma-ray sources. The distinctive feature of ships is that they'll move against the background of stars, so monitoring with an array of space-based telescopes distributed around the Solar System should spot any realistic ship. Exactly what one would have to look for depends on ship design (how hot do you let your radiators get, for example). Precision tracking of ships can use active radar, particularly from ships in combat, but a collection of space telescopes would get a decent trajectory just from the optical/IR, given time.
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feld
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Post by feld on Oct 6, 2016 22:20:28 GMT
Peridot, Your statement (..."most of the photons in the Universe...") is technically correct but displays a sadly common misconception about the solar system space environment that has stymied this silly stealth/no-stealth in space debate from the beginning. Most of the background visible/IR near the plane of the ecliptic is scattered sunlight or thermal radiation from the interplanetary medium in the plane of the ecliptic. Look up the "zodiacal light" for more information on the visible part of this phenomenon. en.wikipedia.org/wiki/Zodiacal_lightThis is the visible component of that scattered sunlight. The ecliptic dust disk is pretty thick. It covers most of the real-estate in the solar system I can see people fighting over. And the space visible/IR background is NOT the common 2.7K if your target is in a band around the plane of the ecliptic. Now, astronomers routinely subtract out that background light from images they take ... that's probably why many people don't realize it's there. But it has real effects on IR telescopes...like creating pretty big patches of sky that they just don't look in because their detectors will be saturated. Furthermore, subtracting out the light is subtracting one measured quantity with uncertainty (the "known" background radiation) from another measured quantity with uncertainty (the total observed light coming from that chunk of sky possibly containing a spacecraft you're trying to detect). The first question that needs to be answered in any stealth in space problem is: can I reduce the signal the spacecraft presents to the detector to a low enough fraction of background that it is statistically likely to be indistinguishable from that background? I've talked to lots of the people who've done calculations regarding the "Stealth In Space" question: I've not come across ANYONE who has actually used the actual background radiation environment that exists in space in their math when they "convinced" themselves that they had "the Answer". I was asking qswitched to find out if they've used a better background model or if they just went with 2.7K blackbody. And I *think* I've read all of the spacecraft detection calculations floating around the net and not one of them has used the space background that actually exists. They all use the 2.7K blackbody everywhere. That model is only good when you're in interstellar space or if both sensor and target are not in the ecliptic. Please don't get me wrong. I'm not saying "qswitched is wrong" or "stealth in space is easy". It's not like that at all. I just know for a fact that the question isn't as simple as any of the models I've seen in the SF conflict simulation community make it out to be and (frankly) I'm really hoping someone else has figured that out and done the math for me. If they haven't I just need to make time to do it myself and pass it to Winchell Chung on the Atomic Rocket website. But the life of an active duty navy officer is not exactly full of free time and I'd need to devote a lot of effort to do that. Sorry for the wall of text ... you presented as someone who might be interested in more detail. v/r feld
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Post by peridot on Oct 6, 2016 23:15:22 GMT
Did I say visible? Indeed the CMB is not very bright at visible frequencies. And yes, zodiacal light is a fact of life for optical astronomers, as the Galactic radio background is for radio astronomers, and the poorly-understood X-ray and gamma-ray backgrounds are for those of us who look at those frequencies. But the hardest kind of radiation for a ship to control is its thermal emission, so the real question is about thermal sources at the temperatures ships radiate. Exhaust plumes are a bit special because you're probably stuck with spectral lines, which have (effectively) a much lower background. But for the ships' thermal radiators you probably want to focus on near-IR and optical, maybe extending down to far IR for really determined attempts at stealth.
Can a source be seen against the zodiacal light? It's going to be very hard to answer that without knowing more about the instrument. A big enough light bucket can measure the zodiacal light in the region of interest as well as you want. GAIA, with its 0.7 m^2, has a limit of G magnitude 20, which is about 60 MW at one AU. That's G, of course, so visible light, where the zodiacal light is probably a worse problem than down in the IR (the sun peaks in the visible, after all). And if getting things to space is cheap, putting big mirrors on telescopes is also not a big challenge.
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feld
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Post by feld on Oct 7, 2016 1:13:49 GMT
I did not think you meant visible light. I only showed you the visible phenomenon because it is the quickest way to show the extent of the area the disk takes up.
Completely agreed that the best way to see a s/c will be IR (either emitted from onboard generation or retransmitted solar heating) but the dust disk I'm talking about predominantly radiates in the IR. It's a bunch of little tiny "blackbodies" (really graybodies) in solar orbits in a disk extended out (if memory serves) as far as the asteroid belt. The dust absorbs solar radiation and re-radiates it at IR frequencies. So the background I'm talking about peaks around 10 microns (ref 1). This corresponds to a blackbody temperature around 290 K.
I also completely agree that we have to know about the instrument! But I wanted to start just by comparing the signal to noise for a crew module to see if running dark made any sense at all.
Anyway, I don't expect to convince anyone of anything without math. I myself am unconvinced. But I hope to have at least shown why more analysis might be interesting. Just need to get time to do all the work.
v/r feld
ref 1. Glass, I.S., Handbook of Infrared Astronomy, Cambridge University Press, 1999.
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Post by blothorn on Oct 7, 2016 2:56:38 GMT
I have not fully thought this through, but I would think that the impossibility of cooling a spacecraft without a visible heat source is relative to the background---if you are only emitting background levels of radiation your net emissions of heat are zero. Variability of the background does give you a bit of room to play with (particularly at single-pixel distances); I suspect the big question is "how much".
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feld
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Post by feld on Oct 7, 2016 22:54:02 GMT
I fear that I have inadvertently started a debate...that was not my intent. The author(s) of the game have been very clear that they're not interested in stealth. They also mentioned that they had done some math though. Given their thoroughness in other areas - I was just hoping to learn what background they used.
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