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Old 09-28-2017, 11:38 PM   #1
Michael Thayne
 
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Default [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

One omission from Spaceships 7 is the heat signatures produced by the drives therein. Here's my best attempt to extrapolate the values from known data, feedback welcome. I'm going to ignore paranormal technologies, for realistic and quasi-scientific designs:
  • Jet engine: My guess is all jet engines should have the same heat signature (+4), which means yes, you can have a fusion-powered stealth bomber that stays in the air for years at a time.
  • Solar Boiler: Same as fuel cell or solar panel (+4).
  • Vacuum Energy Power Plant: Antimatter, fusion, and total conversion reactors all have the same IR signature (+7), giving this power plant the same IR signature is an easy decision.
  • Laser Rocket: Same as HEDM rocket (+5).
  • VASIMR Electric Rockets: I would assume this has the same heat signature as an ion drive, whatever that is. I guess there's no reason for it to be any higher than the power source?
  • Plasma Torch: Same as Fusion Torch (+10).
  • Solar Thermal Engine: No clear analog, but my guess is +4, since it isn't particularly powerful.
  • Radioisotope Sails: Same as lightsail or mag sail (+7) seems like a reasonable guess, but this is so low-thrust that maybe it should be lower?
Areas of uncertainty: maybe the solar boiler and solar thermal systems should be in the same category as lightsail/mag sail due to the size of the required mirrors? Also: maybe fission air ram and fusion air ram should be higher because they're basically flying nuclear reactors? Is anyone more informed about these things than I am?
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Old 09-29-2017, 12:18 AM   #2
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Default Re: [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

I think those are probably mostly close enough, with the possible exception of the vacuum energy power plant. Antimatter, fusion, and total conversion are, unless I'm mistaken, basically all really advanced heat engines (much as modern nuclear power plants are). A vacuum energy plant would, presumably, only produce heat as waste, not as an intrinsic part of its general operation, so I'd expect it to have a smaller signature, if any. I could see vacuum energy plants, depending on fluff, being anywhere from +3 (no difference from the systems it powers) to the +7 of total conversion, or possibly even higher; personally I'd peg it at +4 or so.

For the solar mirror systems, I'd probably have those close to lightsails - just like the latter, solar mirrors are extremely reflective. Now, typically the brightest light source will be directed into the ship's bowels, so it probably won't be quite as bright as a lightsail. +6, perhaps? Note that makes the signature of the system it powers academic, as the mirror itself is far easier to detect.
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Old 09-29-2017, 04:47 PM   #3
Michael Thayne
 
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Default Re: [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

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Originally Posted by Varyon View Post
For the solar mirror systems, I'd probably have those close to lightsails - just like the latter, solar mirrors are extremely reflective. Now, typically the brightest light source will be directed into the ship's bowels, so it probably won't be quite as bright as a lightsail. +6, perhaps? Note that makes the signature of the system it powers academic, as the mirror itself is far easier to detect.
Good point. I like +6.

I'm a little disappointed I haven't gotten more pushback on the fusion-powered stealth bomber. Again, does anyone know enough about these hypothetical proposals to tell me if it's realistic? It has pretty big consequences for the design of aerospace craft! Notably, nuclear air-rams become strong choices for variations on a number of designs in Spaceships 4 if they can be stealthy.
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Old 09-29-2017, 05:21 PM   #4
Anthony
 
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Default Re: [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

Note that the existing rules for IR signature don't bear a lot of resemblance to reality.

The minimum energy output of a drive is 0.5 * thrust * ISp; if we convert to spaceships this works out to 7.2MW * (accelerator/1g) * (delta-V per tank/1 mps) * 10^(SM/2). Looking at a couple of examples:

Chemical: 3G, 0.15 mps/tank, so 3.2MW * 10^(SM/2).
Ion drive (fission reactor): 0.0005G, 3 mps/tank, so 11kW * 10^(SM/2).
Nuclear thermal (TL 9): 0.5G, 0.45 mps/tank, so 1.6* 10^(SM/2).

IR signature should probably be something like 3 * log10(power) - 7 (+0 at 200W), so at SM +6 we get:
Chemical: 3.2GW, log10(power) = 9.5, signature = +21 (SS1: +11)
Ion: 11 MW, log10(power) = 7, signature = +14 (SS1: +12)
NTR: 1.6GW, log10(power) = 9.2, signature = +20 (SS1: +12)
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Old 09-30-2017, 01:01 PM   #5
Michael Thayne
 
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Default Re: [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

Quote:
Originally Posted by Anthony View Post
Note that the existing rules for IR signature don't bear a lot of resemblance to reality.

The minimum energy output of a drive is 0.5 * thrust * ISp; if we convert to spaceships this works out to 7.2MW * (accelerator/1g) * (delta-V per tank/1 mps) * 10^(SM/2). Looking at a couple of examples:

Chemical: 3G, 0.15 mps/tank, so 3.2MW * 10^(SM/2).
Ion drive (fission reactor): 0.0005G, 3 mps/tank, so 11kW * 10^(SM/2).
Nuclear thermal (TL 9): 0.5G, 0.45 mps/tank, so 1.6* 10^(SM/2).

IR signature should probably be something like 3 * log10(power) - 7 (+0 at 200W), so at SM +6 we get:
Chemical: 3.2GW, log10(power) = 9.5, signature = +21 (SS1: +11)
Ion: 11 MW, log10(power) = 7, signature = +14 (SS1: +12)
NTR: 1.6GW, log10(power) = 9.2, signature = +20 (SS1: +12)
Is it possible for a more efficient drive to have a reduced IR signature? I assumed this was the rationale for higher-TL drives with increased delta-V per fuel tank having the same IR signature. But maybe it doesn't work that way.
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Old 09-30-2017, 01:16 PM   #6
Michael Thayne
 
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Default Re: [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

I'm only learning about this because of GURPS, but as I understand it jet engines can be rated for ISP, which can't be inferred from GURPS stats because they don't carry reaction mass, but nevertheless because ramjets have relatively low ISP, a fission or fusion ramjet probably shouldn't have a higher IR signature than the jet engine in the original Spaceships volume, because that's described as representing a "a turbo ramjet or scramjet".

Am I looking at this right?
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Old 09-30-2017, 01:23 PM   #7
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Default Re: [Spaceships] IR Signatures for the power plants and drives in Spaceships 7

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Originally Posted by Michael Thayne View Post
Good point. I like +6.

I'm a little disappointed I haven't gotten more pushback on the fusion-powered stealth bomber. Again, does anyone know enough about these hypothetical proposals to tell me if it's realistic? It has pretty big consequences for the design of aerospace craft! Notably, nuclear air-rams become strong choices for variations on a number of designs in Spaceships 4 if they can be stealthy.
Note they're crazy expensive...

But...IR stealth for aircraft is a bit different, I think. A fusion reactor for your ramjet might give you considerably more heat to dispose of than a conventional fuel-burning turbine, but the main thing is to keep the externally visible heat diffuse and dissipate it into the air without presenting sharp 'hot spots' for IR sensors, I think. And that probably isn't impossible to do around a fusion reactor.
Quote:
Originally Posted by Michael Thayne View Post
Is it possible for a more efficient drive to have a reduced IR signature? I assumed this was the rationale for higher-TL drives with increased delta-V per fuel tank having the same IR signature. But maybe it doesn't work that way.
To some extent - you can't actually see the energy output of the drive that Anthony is tabulating. Kinetic energy is basically stealthy.

However, for thermal rockets the kinetic energy of the departing particles is proportional to their temperature, and I think there's basically nothing you can do about that.
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Last edited by Ulzgoroth; 09-30-2017 at 01:27 PM.
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