Monday, May 25, 2015

About The Hot Equations

Ken Burnside makes a brave attempt to discuss the actual implications of thermodynamics in space warfare. It is on the 2015 Hugo ballot for Best Related Work. Unfortunately, he gets a lot wrong.
In space, the horizon assumption is almost always wrong. The one exception is Low Earth Orbit (LEO), where the limb of the earth can temporarily obscure something for roughly an eighth of an orbital period; this is about a 15-minute window, tops. Detection range is never limited by terrain for militarily significant increments of time. 
Not true for sufficiently distant observers. For an observer on Mars or Ceres, a ship in LEO is going to be eclipsed almost half an orbital period. In a hostile environment, this is exactly when the Earth ship would choose to make major delta-v changes.
With an emissions spectrum on your drive flare, plus distance and proper motion, they can determine the mass pushed by that drive flare. Making your spacebattleship look like a space rowboat doesn't work, and neither do decoys, which need the same drive signature, apparent motion, and mass as the ship they're duplicating. 
You can’t make a battleship look like a rowboat, but you can make a rowboat look like a battleship. A rocket engine is designed to convert as much of the energy used as possible into accelerating propellant. A mechanism designed to simply produce the same amount of heat and lighter will be lighter, simpler, cheaper and use less energy. Compare, for example a welding torch to a rocket engine with the same thermal output. Similarly, a craft with electric propulsion could route electricity directly to radiators to simulate the heat signature of a much more massive craft.
The usual counter-argument made is "I'll just drift in, with engines cold and go undetected." Your life support system and power plant will be a detectable signal once your engine turns off, and they'll know where to look. 
Again, a decoy can have a heat source to simulate a manned ship running without thrust. And unmanned ships can hibernate while not under thrust, with very low power output. We’ve already shown that unmanned craft can be lethal weapons platforms, even when operating in the unpredictable environment of an atmosphere with weather.
The ion thrusters used by NASA's probes to Pluto have ISPs of around 10,000 seconds with a thrust of around 4milligees. 
NASA’s one probe to Pluto, New Horizons, does not use ion thrusters. The author is evidently thinking of Deep Space 1 and Dawn, both asteroid missions.
The combat actions won't be naval in nature, at least in the conventional Battleof Jutland sense. They'll be closer to anti-piracy actions in the Sea of Cebu or the Gulf of Aden; a pirate will lay in wait at a point where a ship must make a course correction – and where missing that correction by a few hours can result in everyone aboard dying of starvation – and capture the ship to hold for ransom.  
This shows a profound misunderstanding of orbital mechanics. First, most cargo missions won’t need a crew and won’t have one. Second, capturing a ship at interplanetary speeds is much easier said than done.

Consider a specific scenario: the asteroid pirates in Poul Anderson’s 1966 The Moonrakers. Robot freighters travel on Hohmann Orbits between Mars and the Jovian Moons, and space pirates from the asteroids match courses and loot them as they pass through the asteroid belt. There are several problems with this concept.

Simply matching courses takes a lot of delta-v, even if the most efficient course is chosen, and the most efficient course is a very long haul for the pirate crew. Getting away with the loot requires still more delta-v, and another long haul for the pirate crew. For most goods, it’s probably cheaper to buy honestly in Mars orbit and ship to the belt on a robot freighter.

Second, if Burnside is correct that plausible space drives are visible at great distances, it will be quite difficult for the pirates to either achieve surprise or get away without being tracked and targeted. 

Third, reliably disabling enough of the freighter’s systems to make it safe to board without damaging the cargo will be tricky, even if the pirates can achieve surprise. And I can imagine a lot of ways a bloody-minded owner could booby trap a ship so that unauthorized boarding becomes too risky for any rational pirate.

1 comment:

Matt Picio said...

"Unfortunately, he gets a lot wrong" - actually, not as much as you think.

"Not true for sufficiently distant observers. For an observer on Mars or Ceres, a ship in LEO is going to be eclipsed almost half an orbital period." - that's a good point, and a scenario Burnside didn't cover. The section you quoted is still correct, however - Burnside states "Detection range is never limited by terrain for militarily significant increments of time." This remains true. If you're "sufficiently distant", the 45 minutes a craft spends on the other side of a planet isn't a militarily significant increment of time.

"You can’t make a battleship look like a rowboat, but you can make a rowboat look like a battleship" - no, you can't, without giving that rowboat the same mass and emissive characteristics as the battleship. Also, if it's producing the same amount of heat, then it's using the same amount of energy.

"Again, a decoy can have a heat source to simulate a manned ship running without thrust." - it can, but the originating vector and lack of maneuverability might give it away. If it can maneuver, then it has to have the same energy output and mass or it will be obvious as a decoy.

"And unmanned ships can hibernate while not under thrust, with very low power output." - That depends. If it's using any kind of nuclear fission, or any significantly-sized power source which requires any kind of "idle" setting, then no. The residual heat from nuclear decay or from idling with eventually radiate out from the ship. If it uses any sort of electronics, then also no - electronics need a significant temperature to maintain operation (in absolute terms, from a human perspective it would still be very very cold), and that heat will eventually radiate out from the ship. You'd need something which could operate very very close to background temperatures (7 Kelvin) and still function and react. Unless someone devises something which violates or bypasses known science, that's not going to work.

"This shows a profound misunderstanding of orbital mechanics. First, most cargo missions won’t need a crew and won’t have one. Second, capturing a ship at interplanetary speeds is much easier said than done" - Since I've had discussions with Burnside on sfconsim-l and other venues, I guarantee he doesn't misunderstand orbital mechanics. We don't know yet whether cargo missions would be manned or unmanned. Unmanned is cheaper to run - no life support needed, less "non-paying" mass, etc, but you lose the ability for a crew to make repairs if something fails, you lose reaction time if the flight is controlled remotely, etc. I think the point was that the thermodynamics arguments make it more likely to be single-ship actions rather than fleet actions, and I'd agree with that contention. Your basic point about intercepting a ship "in flight" is definitely valid - matching vectors and intercepting is a non-trivial problem.

The basic premises that Burnside outlines in the essay are evident in the Tabletop wargame Attack Vector which he published in 2004. Other writers have outlined the basic weapon types used: beams, kinetics, and missiles; and employment generally results either in a mission kill, or constraining movement of your opponent until they no longer have enough Delta-V to satisfy their objectives. Ultimately, though, the point of the essay is to make people think and contemplate exactly the items you outline here. Rick Robinson and others have outlined a number of reasons which make piracy scenarios problematic - most of them are economic rather than science or engineering concerns.