The maximum range a ship
running silent with engines shut down can be detected with current technology is:
Rd = 13.4 * sqrt(A) * T2
where:
Rd = detection range
(km)
A = spacecraft projected area
(m2 )
T = surface temperature
(Kelvin, room temperature is about 285-290 K)
If the ship is a convex shape, its projected area will be roughly one quarter of its surface area.
Example: A Russian Oscar submarine is a cylinder 154 meters long and has a beam of 18 meters, which would be a good
ballpark estimate of the size of an interplanetary warship. If it was nose on to you the
surface area would be 250 square meters. If it was broadside the
surface area would be approximately 2770. So on average the projected area would be 1510 square meters
([250 + 2770] / 2).
If the Oscar's crew was shivering at the freezing point, the maximum detection range of the frigid submarine would be 13.4 * sqrt(1510) * 2732 =
38,800,000 kilometers, about one hundred times the distance between the Earth and the Moon, or about 129 light-seconds. If the crew had a more comfortable room temperature, the Oscar could be seen from even farther away.
To keep the lifesystem in the spacecraft at levels where the crew can live, you probably want it above 273 K
(where water freezes), and preferably at 285-290 K
(room temperature). Glancing at the above equation it is evident that the lower the spacecraft's temperature, the harder it is to detect.
"Aha!" you say,
"why not refrigerate the ship and radiate the heat from the side facing away from the enemy?"
Ken Burnside explains why not. To actively refrigerate, you need power. So you have to fire up the nuclear reactor. Suddenly you have a hot spot on your ship that is about 800 K, minimum, so you now have even more waste heat to dump.
This means a larger radiator surface to dump all the heat, which means more mass.
Much more mass. It will be either a whopping two to three times the mass of your reactor or it will be so flimsy it will snap the moment you engage the thrusters. It is a bigger target, and now you have to start worrying about a hostile ship noticing that you occluded a star.
Dr. John Schilling had some more bad news for would be stealthers trying to radiate the heat from the side facing away from the enemy.
Besides, redirecting the emissions merely relocates the problem. The energy's got to go somewhere, and for a fairly modest investment in picket ships or sensor drones, the enemy can pretty much block you from safely radiating to any significant portion of the sky.
And if you try to focus the emissions into some very narrow cone you know to be safe, you run into the problem that the radiator area for a given power is inversely proportional to the fraction of the sky illuminated. With proportionate increase in both the heat leakage through the back surfaces, and the signature to active or semi-active (reflected sunlight)
sensors.
Plus, there's the problem of how you know what a safe direction to radiate is in the first place. You seem to be simultaneously arguing for stealthy spaceships and complete knowledge of the position of enemy sensor platforms. If stealth works, you can't expect to know where the enemy has all of his sensors, so you can't know what is a safe direction to radiate. Which means you can't expect to achieve practical stealth using that mechanism in the first place.
Sixty degrees has been suggested here as a reasonably "narrow" cone to hide one's emissions in. As a sixty-degree cone is roughly one-tenth of a full sphere, a couple dozen pickets or drones are enough to cover the full sky so that there is no safe direction to radiate even if you know where they all are. The possiblility of hidden sensor platforms, and especially hidden, moving sensor platforms, is just icing on the cake.
Note, in particular, that a moving sensor platform doesn't have to be within your emission cone at any specific time to detect you, it just has to pass through that cone at some time during the course of the pre-battle maneuvering. Which rather substantially increases the probability of detection even for very narrow emission cones.
(Somebody suggested using a continuous blinding barrage of nearby nuclear detonations in order to hide thrusting)
The timescale of the radiant emission from a nuclear detonation in vacuum is measured in milliseconds. The recovery time of a good CCD array is measured in microseconds. You'll need to detonate nuclear explosives at a hundred hertz, minimum, to cover an accelerating ship. That's going to get expensive.
It also rather clearly indicates where the enemy should start looking...
Ken Burnside said:
The problem with directional radiation is that you have to know both where the enemy sensor platforms are, and you have to have a way of slowing down to match orbits that isn't the equivalent of swinging end for end and lighting up the torch. Furthermore, directing your waste heat (and making some part of your ship colder, a related phenomena) requires more power for the heat pump - and every W of power generated generates 4 W of waste heat. It gets into the Red Queen's Race very quickly.
Imagine your radiators as being sheets of paper sticking edge out from the hull of your ship. You radiate from the flat sides. If you know exactly where the enemy sensors are, you can try and put your radiators edge on to them, and will "hide". You want your radiators to be 180 degrees apart so they're not radiating into each other.
Most configurations that radiate only to a part of the sky will be vastly inefficient because they radiate into each other. Which means they get larger and more massive, which reduces engine performance...and they still require that you know where the sensor is.
The next logical step is to make a sunshade that blocks your radiation from the sensor. This also requires knowing where the sensor is, and generates problems if the sensor blocker is attached to your ship, since it will slowly heat up to match the equilibrium temperature of your outer hull....and may block your sensors in that direction as well.
If you are actually trying to apply thrust, the
upper equation comes into play, and they can see you all over the solar system. What's worse, they can measure the spectrum of your drive to estimate the thrust and use a telescope to observe your acceleration. Simple division will reveal the mass of your ship.
"Well fine!", you say,
"I'll just burn once and drift silently"
But now you will be months in getting to your target. The extra time increases the chance that the enemy will spot you. It will be harder to keep your directional radiator aimed away from any enemy observers. And if you are spotted, so much of your ship mass will be radiators instead of weapons, so that the enemy ships will out-gun you by an obscene margin.
Not to mention the fact that once your initial burn is spotted, the enemy will be able to calculate your future position anytime in the future. They can set a computer controlled telescope to track your current calculated position, and will quickly spot any future course correction burns.
