How would you make "Sensors" for a starship?

[Morilore]

I can't do the math myself, but I'm pretty sure that orbital mechanics is highly deterministic, so as long as you had a good track on the ships you could plot out their courses fairly precisely. Somebody with more knowledge about the subject can probably address it better though.

That's the problem right there. Sure, anyone can see an intense IR signal from Pluto. But as most of the posts in this thread have been discussing, you might not be able to see what kind of source the signal is coming from, or what it's trajectory is, or how many ships are producing it: you'll see a few shining pixels. You need a certain level of resolution to figure out detailed information. Check out Mike's optical calculator.

But the enemy still sees a bunch of rockets headed toward a critical installation. He'd be an idiot to just brush that off. Maybe you could get him wondering about how to divide his forces since he doesn't know where the attack will come, but that gets into a second issue:

The problem with decoys in general is that you can deduce a ship's mass from how fast it's accelerating and how much energy is going into the drive plume. The decoys all have to be the same mass as your ships, and the same engines as your ships, in which case you probably almost might as well make them ships as they'll probably cost a good chunk of the cost of a ship. If you can send decoys off to "attack" twenty of the enemy's bases to confuse him, for that kind of money you could probably have built real ships and attacked 10 of his bases for real instead of just one.

You aren't understanding decoys. The idea is a small drone with a powerful IR emitter, not an actual spaceship. At a certain level of technology, you could even make an IR lamp strong enough to swamp the signal from the ship's engines, the same way you wouldn't be able to see a flashlight's signal if it was pointed at you from the direction of the sun.

Also, what if there are many independent and non-belligerent powers in this solar system? How would you know that the fleet being launched is even an attack fleet at all? Suppose Major Power A is attacking Minor Power B in retaliation for pirate raids or whatever, but an outpost for the neutral Major Power C sees one of the decoys pointing at nothing, and through computer or human error, thinks it's a fleet heading for them? There could be considerable uncertainty of this type.

[Darth Wong]

The problem with decoys in general is that you can deduce a ship's mass from how fast it's accelerating and how much energy is going into the drive plume. The decoys all have to be the same mass as your ships, and the same engines as your ships, in which case you probably almost might as well make them ships as they'll probably cost a good chunk of the cost of a ship. If you can send decoys off to "attack" twenty of the enemy's bases to confuse him, for that kind of money you could probably have built real ships and attacked 10 of his bases for real instead of just one.

How much of this thread have you even bothered to read?

[Junghalli]

That's the problem right there. Sure, anyone can see an intense IR signal from Pluto. But as most of the posts in this thread have been discussing, you might not be able to see what kind of source the signal is coming from, or what it's trajectory is, or how many ships are producing it: you'll see a few shining pixels. You need a certain level of resolution to figure out detailed information.

You don't need to see more than pixels to determine acceleration and heading. All you need to do is track the movement of those pixels against the background stars.

You aren't understanding decoys. The idea is a small drone with a powerful IR emitter, not an actual spaceship.

This "lamp" will still need to be generating the same amount of heat as the actual ship's engines are, so I don't really see how that's much better. Oh, I suppose you could try to make it like a flashlight and shine the light only in the direction of your enemy to make it look brighter, but that has the same problem with trying to hide your burn behind a giant cool umbrella: it only works if you know where all your enemy's observation platforms. In fact a lamp may very well be worse, because many rocket designs benefit from being able to use the propellant as coolant, or in the case of the most high energy drives like Orion try to move the propulsive reaction outside the ship, whereas your IR lamp will need ridiculously huge radiators, so it may very well be more expensive than a replica engine.

There's also the fact that the enemy can tell what sort of gasses your drive plume is made up of through spectroscopy (and spectroscopy won't require you to be able to see more than one pixel either). Can your IR lamp convincingly fake the emission lines?

Also, what if there are many independent and non-belligerent powers in this solar system? How would you know that the fleet being launched is even an attack fleet at all? Suppose Major Power A is attacking Minor Power B in retaliation for pirate raids or whatever, but an outpost for the neutral Major Power C sees one of the decoys pointing at nothing, and through computer or human error, thinks it's a fleet heading for them? There could be considerable uncertainty of this type.

So send your fleet at one guy's facility, send a bunch of decoys at a facility belonging to a third party, and hope Guy # 1 assumes the fleet headed towards him is made up of decoys? I don't see how that really accomplishes much - he'd be stupid not to at least anticipate the possibility that he's a target. I suppose maybe it might work, if the enemy commander wasn't too bright and the circumstances were right, but it's definitely something that should only work once.

How much of this thread have you even bothered to read?

I haven't seen anything addressing the problem of convincingly faking a much bigger vessel's drive plume in a decoy. I may have missed it though; where was it addressed?

Edit: are you talking about the fact that you won't get more than pixel resolutions at great distances? As far as I know you don't need more than that to check whether the drive plume looks "kosher". You take the amount of light and heat you're seeing, determine the distance, determine how much energy the thing must be putting out for it to be this bright from that distance, track its movement against the background stars and hence the rate of acceleration, and you should be able to figure out the ship's mass from that. I don't see why any of that requires being able to see more than a point source of light.

[Darth Wong]

This "lamp" will still need to be generating the same amount of heat as the actual ship's engines are, so I don't really see how that's much better.

Nonsense. You only need to produce the same radiation signature. You haven't even established that this ship is going to have this extraordinarily bright plume of yours in the first place. Hot gases in space will emit radiation through one of two mechanisms: braking radiation and line radiation. Braking radiation is irrelevant at any significant distance from the cone because the gases rapidly dissipate in space, and the particles stop magnetically interfering with each other. Line radiation would not even apply to, say, an ion drive.

Oh, I suppose you could try to make it like a flashlight and shine the light only in the direction of your enemy to make it look brighter, but that has the same problem with trying to hide your burn behind a giant cool umbrella: it only works if you know where all your enemy's observation platforms.

First, it's not necessarily unrealistic to know where the enemy's observation platforms are. Second, if we're attacking a static facility, then the whole idea of hiding drive plumes is pointless because a ship would coast in. After all, you're trying to attack something whose location is known well in advance. Third, if you're attacking such a system, you would probably hit the outlying observation posts first.

In fact a lamp may very well be worse, because many rocket designs benefit from being able to use the propellant as coolant, or in the case of the most high energy drives like Orion try to move the propulsive reaction outside the ship, whereas your IR lamp will need ridiculously huge radiators, so it may very well be more expensive than a replica engine.

See above. If you're trying to sneak into a system, you won't be doing full burns. Your decoys won't be trying to simulate full burns either.

There's also the fact that the enemy can tell what sort of gasses your drive plume is made up of through spectroscopy (and spectroscopy won't require you to be able to see more than one pixel either). Can your IR lamp convincingly fake the emission lines?

See above. As for being able to fake the emission lines, I don't know. But that would be a pretty minor conceit compared to most of the things people do in sci-fi, such as having interstellar combat warships at all.

PS. One obvious use for decoys is to sniff out things like orbiting telescope arrays. In order to track a suspicious dot, they will have to fire up the maneuvering jets on such an array and try to track the object as it moves through space. That makes the array itself visible.

[aimless]

On the point of masking approaches, if we're talking about our solar system or really standard solar systems, the example of seeing burns on Pluto from the inner system is a bit extreme.

Aren't there a few ways to maybe cold launch a fleet? How detectable is laser launching something?

[Darth Wong]

On the point of masking approaches, if we're talking about our solar system or really standard solar systems, the example of seeing burns on Pluto from the inner system is a bit extreme.

Aren't there a few ways to maybe cold launch a fleet? How detectable is laser launching something?

People seem to be thinking of a violent high-powered burn. I see no reason why an attacker would not use a low-thrust engine running for weeks in order to accelerate, rather than a big BOOOOOM rocket like we're used to thinking of.

[Morilore]

You don't need to see more than pixels to determine acceleration and heading. All you need to do is track the movement of those pixels against the background stars.

If you're starting at Pluto, a few arc-seconds of error here could make the difference between heading for Earth and heading for Mars.

This "lamp" will still need to be generating the same amount of heat as the actual ship's engines are, so I don't really see how that's much better. Oh, I suppose you could try to make it like a flashlight and shine the light only in the direction of your enemy to make it look brighter, but that has the same problem with trying to hide your burn behind a giant cool umbrella: it only works if you know where all your enemy's observation platforms.

This "lamp" will still need to be generating the same amount of heat as the actual ship's engines are

same amount of heat

heat

The primary function of engines is to generate kinetic energy, not heat. Heat is inefficiency, and any engineer would want to minimize inefficiency for reasons that don't have anything to do with combat. A decoy, built entirely to simulate or drown out IR signals, would not need nearly as much power as an engine that generates heat as a byproduct of accelerating massive spaceships.

In fact a lamp may very well be worse, because many rocket designs benefit from being able to use the propellant as coolant, or in the case of the most high energy drives like Orion try to move the propulsive reaction outside the ship, whereas your IR lamp will need ridiculously huge radiators, so it may very well be more expensive than a replica engine.

Ridiculously huge radiators? The entire thing is a radiator! It's designed to radiate! What is waste heat for a spaceship is useful energy for a decoy. I don't understand why you think that duplicating waste heat takes the same kind of engineering as duplicating an entire engine.

So send your fleet at one guy's facility, send a bunch of decoys at a facility belonging to a third party, and hope Guy # 1 assumes the fleet headed towards him is made up of decoys? I don't see how that really accomplishes much - he'd be stupid not to at least anticipate the possibility that he's a target. I suppose maybe it might work, if the enemy commander wasn't too bright and the circumstances were right, but it's definitely something that should only work once.

That was an example to demonstrate confusion arising from multiple data sources in a politically complex world. It wouldn't be so simple as "totally dark sky, someone launches, BOOM you know everything about who he is and what kind of ships he has and where he's going and what he intends."

[Junghalli]

Nonsense. You only need to produce the same radiation signature.

The way you deduce engine power of a rocket from a distance would be (I think) determining how much energy the rocket would have to be putting out to be that bright from that distance. So wouldn't your decoy need to put out that much energy? Although I guess it could be much more "efficient" if it's only emitting photons, not shooting out a giant stream of gas from which the light and heat is just a byproduct. I suspect that's probably what you're getting at there, and I guess you're right about that.

It does occur to me though that practicality of that would depend a lot on what sort of ships are standard in-universe. If most ships are nuclear thermal rockets or something relatively low energy like that, then I suppose it could work. On the other hand if you're using something like an Orion, would it be at all realistic to try simulating the energy emissions of small nuclear bombs going off without destroying the decoy in the process?

You haven't even established that this ship is going to have this extraordinarily bright plume of yours in the first place.

Well, I was going off Atomic Rockets there. Maybe they're wrong.

First, it's not necessarily unrealistic to know where the enemy's observation platforms are.

Well, it would probably be a lot easier to have an observation platform change orbits silently than to do the same for a ship. Put them in the outer system and have them use a propulsion system much too low thrust to be practical for a big manned vessel. Stealth, if applied, is a game that should favor the little unmanned platforms over the big warships pretty heavily.

Also, even if you knew where they are, they'd be observing from a bunch of different angles, although this is more of a problem for ideas like "hide your burn behind a cooled shield" than "make a decoy that saves energy by broadcasting in narrow cones at known enemy observation posts".

Second, if we're attacking a static facility, then the whole idea of hiding drive plumes is pointless because a ship would coast in.

But it will still need to do an initial burn to put itself on an intercept course for the fixed facility, and that should be detectable.

Third, if you're attacking such a system, you would probably hit the outlying observation posts first.

If the enemy is smart the observation posts will be widely scattered and many of them will be farther away than the facility. You could try taking them all out all over the solar system, I suppose, but you might not be able to find them all, and the enemy might be able to throw up new ones before you can finish the job. If they're all over the solar system they'll probably take weeks or months just to reach even with a very energetic drive system.

See above. If you're trying to sneak into a system, you won't be doing full burns. Your decoys won't be trying to simulate full burns either.

Ah, I thought we were talking about intra-system warfare, not attacks from beyond the solar system.

Hmm, question for the board in general, how "quiet/loud" would a coasting relativistic missile be?

PS. One obvious use for decoys is to sniff out things like orbiting telescope arrays. In order to track a suspicious dot, they will have to fire up the maneuvering jets on such an array and try to track the object as it moves through space. That makes the array itself visible.

You could just give the arrays reaction wheels.

[Junghalli]

If you're starting at Pluto, a few arc-seconds of error here could make the difference between heading for Earth and heading for Mars.

Well, that's mostly a question of how good your computers are. I'd expect the sort of computers we'd have by the time we're actually fighting space wars to be very good.

The primary function of engines is to generate kinetic energy, not heat. Heat is inefficiency, and any engineer would want to minimize inefficiency for reasons that don't have anything to do with combat. A decoy, built entirely to simulate or drown out IR signals, would not need nearly as much power as an engine that generates heat as a byproduct of accelerating massive spaceships.

True, conceeded.

[Morilore]

Well, that's mostly a question of how good your computers are. I'd expect the sort of computers we'd have by the time we're actually fighting space wars to be very good.

Actually, it's a question of how good your sensors are. Every instrument in the history of the universe has error bars, and the most powerful Godputer imaginable wouldn't be able to negate that. And every additional gravitational effect where you have to measure the position of an object is another error bar. Multiple sensors at multiple locations could ameliorate that through comparison, though.

[Junghalli]

Multiple sensors at multiple locations could ameliorate that through comparison, though.

And I'd expect any competently designed monitoring grid to have exactly that.

Can't we already track and predict orbits of Kuiper belt objects with fairly high precision now? We can send a space probe to Pluto, after all, and that pretty much requires knowing exactly where it will be years ahead. Granted we've had a long time to refine our calculations.

[Darth Wong]

We have a pretty big advantage when tracking rocks in space, because they were not designed with any special attention paid to minimizing reflections. Sunlight reflects off them, and we can see that. A ship with engineered reflection angles and a low-e surface would be a lot harder to detect for that reason.

As for outlying observation posts, yes, I agree that a story based on such things might involve a constant battle of targeting, blinding, destroying, and replacing remote observation devices. I don't see why that nullifies the idea: you seem to be acting as if something isn't worth doing if it isn't Total Win.

[Darth Wong]

The way you deduce engine power of a rocket from a distance would be (I think) determining how much energy the rocket would have to be putting out to be that bright from that distance. So wouldn't your decoy need to put out that much energy?

For the umpteenth time, no. That is a completely unwarranted assumption on your part.

It does occur to me though that practicality of that would depend a lot on what sort of ships are standard in-universe. If most ships are nuclear thermal rockets or something relatively low energy like that, then I suppose it could work. On the other hand if you're using something like an Orion, would it be at all realistic to try simulating the energy emissions of small nuclear bombs going off without destroying the decoy in the process?

If you're using something extraordinarily violent and short-duration, you would probably do it behind some obstacle, so that the enemy cannot see. What are you going to do next, assume that this is intra-system warfare and that your own territory is littered with the enemy's observation posts, so you can't do anything without being observed from multiple widely divergent angles?

You haven't even established that this ship is going to have this extraordinarily bright plume of yours in the first place.

Well, I was going off Atomic Rockets there. Maybe they're wrong.

That's not much of an answer to my point.

First, it's not necessarily unrealistic to know where the enemy's observation platforms are.

Well, it would probably be a lot easier to have an observation platform change orbits silently than to do the same for a ship.

And how the fuck did the platform get there in the first place? Did it click its heels three times and say "there's no place like Io orbit?"

Also, even if you knew where they are, they'd be observing from a bunch of different angles, although this is more of a problem for ideas like "hide your burn behind a cooled shield" than "make a decoy that saves energy by broadcasting in narrow cones at known enemy observation posts".

So you're talking about intra-system warfare rather than extra-system warfare, yet you are basically assuming that the enemy controls the entire system and has observation posts in front of, beside, and behind you? Why?

PS. One obvious use for decoys is to sniff out things like orbiting telescope arrays. In order to track a suspicious dot, they will have to fire up the maneuvering jets on such an array and try to track the object as it moves through space. That makes the array itself visible.

You could just give the arrays reaction wheels.

Yes, of course! And coupled with perfect batteries and perfect motors and a mechanism which is perfect in every other way, there would be no heat, right? And of course, there would also be no vibration introduced by this kind of attitude control system which would fuck up the telescope's shit.

[Junghalli]

Edit:

As for outlying observation posts, yes, I agree that a story based on such things might involve a constant battle of targeting, blinding, destroying, and replacing remote observation devices. I don't see why that nullifies the idea: you seem to be acting as if something isn't worth doing if it isn't Total Win.

Well, it'd probably be worth doing, but I suspect if you manage to get to the point where you've effectively blinded the enemy and can do the kind of stealth tricks that require all the enemy's sensors to be concentrated in one area you've probably basically already won anyway.

And how the fuck did the platform get there in the first place? Did it click its heels three times and say "there's no place like Io orbit?"

It doesn't have to stay in the orbit it was initially put into. The rocket to initially place it in orbit might be highly visible, but then it could use some ultra low thrust drive to do a small plane change or something. It's certainly much more feasible to do this with little observation platforms than huge manned ships.

So you're talking about intra-system warfare rather than extra-system warfare, yet you are basically assuming that the enemy controls the entire system and has observation posts in front of, beside, and behind you? Why?

Because it'd be rather difficult to effectively claim and control an area as vast as the Kuiper belt, unless you had serious military superiority over your enemy anyway. It's not like the surface of a planet where you can set up and defend borders; you'd be able to effectively defend the areas close to your bases because your ships could get there quickly, but you'd have no such advantage in some random trans-Neptunian orbit that's AUs from any of your or their bases. It'd effectively be neutral territory.

I imagine in a war you'd end up with a race where each side was trying to destroy the other's observation posts, and trying to lay new ones as old ones were found and blown up.

Yes, of course! And coupled with perfect batteries and perfect motors and a mechanism which is perfect in every other way, there would be no heat, right? And of course, there would also be no vibration introduced by this kind of attitude control system which would fuck up the telescope's shit.

It doesn't have to be perfect, it just has to be quiet enough that it wouldn't be noticed from millions of km away. Many of the stealth tricks that have been suggested would work a lot better for a tiny unmanned observation platform than a rocketship.

[TheLostVikings]

PS. One obvious use for decoys is to sniff out things like orbiting telescope arrays. In order to track a suspicious dot, they will have to fire up the maneuvering jets on such an array and try to track the object as it moves through space. That makes the array itself visible.

You could just give the arrays reaction wheels.

Yes, of course! And coupled with perfect batteries and perfect motors and a mechanism which is perfect in every other way, there would be no heat, right? And of course, there would also be no vibration introduced by this kind of attitude control system which would fuck up the telescope's shit.

That last one there is a completely BS straw man argument considering the Hubble has been using them for years and years without them "fucking up it's shit" in any way, shape or form.

And as for the heat issue: give them an "umbrella" (like you yourself proposed) that covers it from behind and it would be pretty much impossible to detect the majority of the enemy's sensors as most of them would be covered by their "umbrellas" at any given time.

Hell, since what it's using to turn is just electric engines (which are pretty efficient even with current tech) you are releasing no exhaust plumes at all. Automatically making every single stealth suggestion in the entire tread much more feasible for a sensor drone than for an actual warship.

So if you can make your warships/missiles invisible, you sure as hell can make the sensor drones a whole fucking lot more invisible in comparison.

[Beowulf]

Big, high thrust engines like would be used for a short burn at the beginning of a journey are generally low-Isp engines. It's likely that advanced warships would user relatively high Isp engines, like VASIMR or NSWR, instead of chemical engines. Thus, they would tend to have relatively long burns.

Also, nearly all engines operate through heating up a working fluid and expelling it through a nozzle. The purpose of an engine may not be to create heat, but that's how it works. One of the few exceptions that I can think of are ion engines, which are inherently thrust limited.

Lastly, space telescopes are pointed using gyroscopes today. RCS can deposit residues on mirrors.

[Darth Wong]

That last one there is a completely BS straw man argument considering the Hubble has been using them for years and years without them "fucking up it's shit" in any way, shape or form.

Don't be a goddamned idiot. I wasn't saying it would destroy or damage the telescope. But the Hubble can afford to track slowly, and people can wait for it to settle. If you're tracking something that's moving at an appreciable rate, it's not easy to do, especially when you need to maintain nanometre-scale accuracy between array elements (or you need to move a large heavy telescope).

And as for the heat issue: give them an "umbrella" (like you yourself proposed) that covers it from behind and it would be pretty much impossible to detect the majority of the enemy's sensors as most of them would be covered by their "umbrellas" at any given time.

A masking umbrella between yourself and the target pretty much completely nullifies the purpose of a telescope.

Hell, since what it's using to turn is just electric engines (which are pretty efficient even with current tech) you are releasing no exhaust plumes at all.

The exhaust plume is severely overrated. People tend to assume that it has to be like a Saturn V rocket.

Automatically making every single stealth suggestion in the entire tread much more feasible for a sensor drone than for an actual warship.

The sensor drone has to communicate with the home base in order to be useful, remember?

So if you can make your warships/missiles invisible, you sure as hell can make the sensor drones a whole fucking lot more invisible in comparison.

There is absolutely no reason why a sensor drone should be smaller or quieter than a missile in space. In fact, given the telescope size, it would need to be considerably larger. I would grant that it's harder to detect than a large ship, but if we're talking about hard sci-fi, I can't imagine manned ships do most of the attacking.

[Junghalli]

Since we were discussing decoys earlier, I thought it might be interesting to try to figure out what sort of drive systems a decoy might or might not be feasible for.

The decoy has to fool spectrography to be convincing, so it basically has to be a super high energy colored lightbulb.

I've taken thrust and exhaust velocity figures off the Atomic Rocket engine page. I've arbitrarily assumed a ship mass of 1000 tons, a mass ratio of 2, and an energy efficiency of 2/3 (1/3 becomes waste heat). For thrust calculations I use the median mass between an empty tank and full tank ship.

Chemical MAX

Exhaust velocity is 4500 m/s, therefore total energy of exhaust is 1/2(1,000,000)(4500^2) = 1 X 10^13 joules.
Waste heat = 3.4 X 10^12 joules.
Thrust is 1,669,000 newtons. 1,669,000 newtons/1,500,000 = 1.113 m/s^2
Delta V of this spacecraft is 4500ln2 = 3119 m/s, divided by 1.113 m/s^2 is 2802 seconds.

3.4 X 10^12 joules / 2802 seconds = 1.2 gigawatts.


Nuclear Thermal Rocket NERVA (H2)

Exhaust velocity is 8093 m/s, therefore total energy of exhaust is 1/2(1,000,000)(8093^2) = 3.27 X 10^13 joules.
Waste heat = 1.1 X 10^13 joules.
Thrust is 49,000 newtons. 49,000 newtons / 1,500,000 = .033 m/s^2
Delta V of this spacecraft is 8093ln2 = 5610 m/s, divided by .033 m/s^2 is 169,989 seconds.

1.1 X 10^13 joules / 169,989 seconds = 64.7 megawatts


Nuclear Thermal Gas Rocket MAX

Exhaust velocity is 98,000 m/s, therefore energy of exhaust is 1/2(1,000,000)(98,000^2) = 4.8 X 10^15 joules.
Waste heat = 1.6 X 10^15 joules.
Thrust is 3,000,000 newtons. 3,000,000 newtons/1,500,000 = 2 m/s^2
Delta V of this spacecraft is 98,000ln2 = 67,928 m/s, divided by 2 m/s^2 is 33,964 seconds.

1.6 X 10^15 / 33,964 seconds = 47 gigawatts


Hydrogen-Boron Fusion

Exhaust velocity is 980,000 m/s, therefore energy of exhaust is 1/2(1,000,000)(980,000^2) = 4.8 X 10^17 joules.
Waste heat = 1.6 X 10^17 joules.
Thrust is 61,000 newtons. 61,000 newtons / 1,500,000 = .04 m/s^2
Delta V of this spacecraft is 980,000ln2 = 679,284 m/s, divided by .04 m/s^2 is 16,982,106 seconds.

1.6 X 10^17 / 16,982,106 = 9.4 gigawatts


Nuclear Salt Water Rocket

Exhaust velocity is 4,700,000 m/s, therefore energy of exhaust is 1/2(1,000,000)(4,700,000^2) = 1.1 X 10^19 joules.
Waste heat = 3.7 X 10^18 joules.
Thrust is 13,000,000 newtons. 13,000,000 newtons / 1,500,000 = 8.7 m/s^2
Delta V of this spacecraft is 4,700,000ln2 = 3,257,792 m/s, divided by 8.7 m/s^2 is 374,459 seconds.

3.7 X 10^18 / 374,459 = 9.88 terawatts


Project Daedalus (Inertial Confinement Fusion)

Exhaust velocity is 10,000,000 m/s, therefore energy of exhaust is 1/2(1,000,000)(10,000,000^2) = 5 X 10^19 joules.
Waste heat = 1.7 X 10^19 joules.
Thrust is 7,540,000 newtons. 7,540,000 / 1,500,000 = 5 m/s^2
Delta V of this spacecraft is 10,000,000ln2 = 6,931,472 m/s, divided by 5 m/s^2 is 1,386,294 seconds.

1.7 X 10^19 / 1,386,294 = 12.3 terawatts

With realistic tech I'd say decoys might be possible if you're using chemical, NERVA, or hydrogen-boron fusion engines. I'm fairly dubious about being able to practically build a 47 gigawatt light bulb without magitech, and I think it's safe to say it's not an option for the nuclear salt water rocket or ICF, which have energy outputs comparable to small nuclear bombs and only manage to not melt because the propulsive reaction takes place outside the body of the ship.

Basically it's feasible with very low thrust ships, but not higher thrust ones.

It's worth noting that a low thrust engine has some big tactical disadvantages; it makes your ship much less manueverable on a tactical level. Which, for starters, might be quite bad since it would likely give the enemy's missiles a much larger effective range against you than yours would have against him. I'd much rather be an Orion-powered warship than an HB-fusion powered warship when it came to combat. So whether or not it would be worth it would probably depend on a variety of factors.

The sensor drone has to communicate with the home base in order to be useful, remember?

That would still almost certainly create a hell of a lot less waste heat than a warship's various systems would. Just the fact that it doesn't have to keep the interior at a human-habitable temperature gives the unmanned platform a huge advantage. As for communication, you could use a narrow-beam transmission like lasers.

[TheLostVikings]

And as for the heat issue: give them an "umbrella" (like you yourself proposed) that covers it from behind and it would be pretty much impossible to detect the majority of the enemy's sensors as most of them would be covered by their "umbrellas" at any given time.

A masking umbrella between yourself and the target pretty much completely nullifies the purpose of a telescope.

Not at all, in fact, exactly the opposite.

You yourself earlier asked why we always assume that the enemy has sensor drones arrayed around all around you, THIS is exactly why. An umbrella placed to cover the back of the sensor means that anybody passing trough the system will only be able to possibly locate the drones happening to point in that general direction at the time.

This obviously means that you will never be able to pinpoint all the enemies possible sensor platforms at any given time, and as a logical extension of which means an incoming ship/missile won't ever be certain in which direction it it's safe to point its shielding umbrella (or when it is safe to use thrusters) as there could easily be a drone that you didn't spot yet in that direction.

Which means any method of stealth relying on pointing your emissions in a specific direction , or putting up umbrellas to shield yourself, will be like a carefully calculated game of Russian roulette where you cross your fingers and pray to the flying spaghetti monster that you managed to spot all the carefully shielded sensors of your opponent.

Hell, since what it's using to turn is just electric engines (which are pretty efficient even with current tech) you are releasing no exhaust plumes at all.

The exhaust plume is severely overrated. People tend to assume that it has to be like a Saturn V rocket.

Actually being able to spot a space shuttle firing its maneuvering thrusters as far away as Pluto seem to be the general cliche, but whatever. It still changes nothing, emitting no exhaust while turning is still a huge advantage of you are trying to stay hidden.

Automatically making every single stealth suggestion in the entire tread much more feasible for a sensor drone than for an actual warship.

The sensor drone has to communicate with the home base in order to be useful, remember?

And in today military reality submarines needs to surface to radio to their superiors because they cant do it while submerged. So what? Sure an orbital platform like this wouldn't be able to move around freely, so once it's spotted it's not useful anymore, but if it's spoted something with a high potential to be hostile it has already proved itself worthwhile. (and unless all your enemies are allied it'll still be of use)

[Morilore]

Big, high thrust engines like would be used for a short burn at the beginning of a journey are generally low-Isp engines. It's likely that advanced warships would user relatively high Isp engines, like VASIMR or NSWR, instead of chemical engines. Thus, they would tend to have relatively long burns.

Which simply means that There Is No Such Thing As A Free Lunch, and a spacecraft built for stealth might have (relatively) low Isp engines for purposes of (relatively) shorter, more powerful burns while (relatively) further away from the target, whereas a spacecraft for which stealth is not as much of a concern would use (relatively) more efficient engines.

Also, nearly all engines operate through heating up a working fluid and expelling it through a nozzle. The purpose of an engine may not be to create heat, but that's how it works. One of the few exceptions that I can think of are ion engines, which are inherently thrust limited.

I know that, but it's irrelevant to my point, which was that it takes less power to make heat and do nothing else than it is to accelerate a massive spaceship using a mechanism that generates large amounts of waste heat.

[Junghalli]

Something that occurs to me about the physics behind this IR lamps as decoy idea. This is what Atomic Rocket says about detecting and measuring exhaust plumes:

Dr. John Schilling discusses why the exhaust plume of a decoy will have to have the same thrust as a real ship:

Problem is, the rate (i.e. velocity) at which the plasma is coming out, manifests itself as a doppler shift in the characteristic emission lines of the plasma. As soon as a dedicated tracking sensor focuses on the target for a second or two, the game is up. If the plasma is coming out fast, it can't help but produce thrust proportional to mass flow rate (manifested as luminosity) times velocity (doppler). If the plasma is coming out slow (or fast but in opposing directions), it will be seen to be coming out slow and thus be recognized as not a real engine.

Now, I'll be the first to admit I know next to nothing about physics, so maybe I'm interpreting this horribly wrong. But the doppler shift is that objects moving toward you will appear bluer, while objects moving away will appear redder. So it sounds like part of the deal is that if you're seeing the drive plume from the front of the ship its light will appear more reddish (as the exhaust is moving away), while from the back it will appear more blue-ish.

Obviously, depending on where your sensor is located, it'll see a different level of red or blue shifting. I think this may be a bit of a problem for the IR lamp decoy idea.

The obvious solution would seem to be to have the decoy shine redder light from the front, transitioning to bluer light in the back. Problem is, an observer on the side is going to be seeing light from the entire facing half object, right? I mean, when you look at Mars from Earth, you're seeing light from that entire hemisphere of Mars (for example). Likewise, somebody looking at this decoy from the side is going to see light from the entire half of the decoy facing him. So he's going to see the redder light coming from the front of the decoy as well as the bluer light coming off the back

Wouldn't this make the drive plume look quite weird and "wrong" when viewed from the side? It'd be redder in the front and bluer in the back, which would probably be a real odd pattern for a rocket's drive plume viewed side on.

I suppose the effect might be so insignificant that it would take an incredibly sensitive sensor to see it. Still, it does seem to offer at least one possible means of picking the decoys out from the actual ships.

Edit: and since any competent space navy would do its best to put a whole lot of observation platforms all over the solar system, I think the odds that there would be something in position to see the decoys side on would probably be pretty high.

[Morilore]

Since we were discussing decoys earlier, I thought it might be interesting to try to figure out what sort of drive systems a decoy might or might not be feasible for.

The decoy has to fool spectrography to be convincing, so it basically has to be a super high energy colored lightbulb.

I've taken thrust and exhaust velocity figures off the Atomic Rocket engine page. I've arbitrarily assumed a ship mass of 1000 tons, a mass ratio of 2, and an energy efficiency of 2/3 (1/3 becomes waste heat). For thrust calculations I use the median mass between an empty tank and full tank ship.

Do you have a rationale for these assumptions? 1000 tons, fine, why not. Mass ratio of 2... wouldn't different engine types potentially have different mass ratios? Efficiency of 67% - do you have any reason at all for picking this number? Did you estimate the sum of the integrals of the power of line radiation and braking radiation for an exhaust plume as a fraction of its kinetic energy? Did you look up an empirical statistic?

maths

Those are some nice maths, but they ignore a couple things. For one thing, exhaust heat radiates in all directions, but enemy sensors will not be present in all directions. They may be present in many directions, but not everywhere. For another thing, the further away a decoy is from a ship the less power is necessary to generate the same apparent magnitude at a target. Yes, two separated sensors would be able to tell a given decoy apart from the ship it's trying to hide through parallax. That doesn't make the whole exercise worthless. The enemy has had to work harder to figure you out, introducing additional probabilities for error. And finally, there are a few weird drives, like mass-drivers-out-your-ass, that have no exhaust plume at all, and all the inefficiency in that engine would manifest as heat in the accelerator mechanism (or the driven mass). (Macroscopic mass drivers would probably have hideously bad mass ratios, though.)

Basically it's feasible with very low thrust ships, but not higher thrust ones.

Which is pretty basic, really - don't use very hot engines if you're trying to be sneaky.

It's worth noting that a low thrust engine has some big tactical disadvantages; it makes your ship much less manueverable on a tactical level. Which, for starters, might be quite bad since it would likely give the enemy's missiles a much larger effective range against you than yours would have against him. I'd much rather be an Orion-powered warship than an HB-fusion powered warship when it came to combat. So whether or not it would be worth it would probably depend on a variety of factors.

And I'd rather be in an HB-fusion powered warship than an Orion-powered warship for sneaking up and bushwacking somebody. TINSTAAFL.

Now, I'll be the first to admit I know next to nothing about physics, so maybe I'm interpreting this horribly wrong. But the doppler shift is that objects moving toward you will appear bluer, while objects moving away will appear redder. So it sounds like part of the deal is that if you're seeing the drive plume from the front of the ship its light will appear more reddish (as the exhaust is moving away), while from the back it will appear more blue-ish.

While this is true, it's not the point of the quote. The point of the quote is that the author of that paragraph either didn't understand the concept of or wasn't discussing radiator lamps simulating thrust emission lines, he was thinking of a decoy with an actual thruster, firing an actual exhaust plume.

Obviously, depending on where your sensor is located, it'll see a different level of red or blue shifting. I think this may be a bit of a problem for the IR lamp decoy idea.

The obvious solution would seem to be to have the decoy shine redder light from the front, transitioning to bluer light in the back. Problem is, an observer on the side is going to be seeing light from the entire facing half object, right? I mean, when you look at Mars from Earth, you're seeing light from that entire hemisphere of Mars (for example). Likewise, somebody looking at this decoy from the side is going to see light from the entire half of the decoy facing him. So he's going to see the redder light coming from the front of the decoy as well as the bluer light coming off the back

Wouldn't this make the drive plume look quite weird and "wrong" when viewed from the side? It'd be redder in the front and bluer in the back, which would probably be a real odd pattern for a rocket's drive plume viewed side on.

Glue the back ends of two flashlights together, so that they are pointing in opposite directions. Tint one red and the other blue, turn them both on, then look at the side of this Darth Maul flashlight. Do you see blue light in one eye and red in the other? No, you see no colored light, because neither of the bulbs are pointed at you!

To simulate an engine burning sideways relative to a sensor, a decoy would radiate light that was neither significantly red-shifted nor blue-shifted.

I suppose the effect might be so insignificant that it would take an incredibly sensitive sensor to see it. Still, it does seem to offer at least one possible means of picking the decoys out from the actual ships.

Edit: and since any competent space navy would do its best to put a whole lot of observation platforms all over the solar system, I think the odds that there would be something in position to see the decoys side on would probably be pretty high.

Okay, let's get to the heart of the matter: yeah, there are ways and ways around decoy-based deception. Just like there are ways and ways around any stratagem. But bringing it to the point where the enemy needs to have a massive observation platform network to figure you out is an advantage in itself. Every dollar spent tightening a net instead of building warheads, every man-hour spent programming observatory computers, every extra error correction for multiple instruments, every calorie spent barking up the wrong tree is a return on investment. The objective is to confuse the enemy and make him work harder, giving him more room to slip up. Of course, not all returns justify the initial investment. That's why war is filled with judgement calls. There would certainly be cases where stealth of any kind is not at all worth the effort. But it's not so simple as "Technology A beats Technology B, therefore Technology B is useless."

[Junghalli]

Mass ratio of 2... wouldn't different engine types potentially have different mass ratios?

Yes, but I'm lazy and it seemed good enough as a ballpark estimate. Since we know nothing about the mission profiles of the ships whatever more complex numbers I might have made up would be just as arbitrary.

Efficiency of 67% - do you have any reason at all for picking this number?

Where efficiency is given on the Atomic Rockets engine table, it's usually something between 30-90%. 67% seemed reasonable for a ballpark figure.

Those are some nice maths, but they ignore a couple things. For one thing, exhaust heat radiates in all directions, but enemy sensors will not be present in all directions. They may be present in many directions, but not everywhere.

But unless you knew where they all were this wouldn't help. It's not just the ones that are there you have to worry about; it's the ones that might be there.

For another thing, the further away a decoy is from a ship the less power is necessary to generate the same apparent magnitude at a target.

Again, this assumes you know where all enemy observers are. Since it's fairly likely you won't, your decoy better actually look like a real ship.

And finally, there are a few weird drives, like mass-drivers-out-your-ass, that have no exhaust plume at all, and all the inefficiency in that engine would manifest as heat in the accelerator mechanism (or the driven mass). (Macroscopic mass drivers would probably have hideously bad mass ratios, though.)

They won't have an exhaust plume but they'd still be generating quite a bit of waste heat, which must be permitted to escape into space if you don't want your ship to melt.

And I'd rather be in an HB-fusion powered warship than an Orion-powered warship for sneaking up and bushwacking somebody. TINSTAAFL.

I suspect this idea will work much better in a universe where lasers dominate than one where missiles dominate. In a missile dominated universe tactical acceleration is crucially important. In a laser dominated one, not so much.

Glue the back ends of two flashlights together, so that they are pointing in opposite directions. Tint one red and the other blue, turn them both on, then look at the side of this Darth Maul flashlight. Do you see blue light in one eye and red in the other? No, you see no colored light, because neither of the bulbs are pointed at you!

Except real exhaust plumes aren't neat directional lights (photons drives and other exotic rockets aside). Unless you know exactly where all the enemy observation platforms are the decoy will have to be radiating omnidirectionally.

Okay, let's get to the heart of the matter: yeah, there are ways and ways around decoy-based deception. Just like there are ways and ways around any stratagem. But bringing it to the point where the enemy needs to have a massive observation platform network to figure you out is an advantage in itself.

Oh yeah, sure. I won't argue that point.

[aimless]

All these ideas about sensors are just dying to be gamed: decoy cost efficiency vs sensor network costs, and low accel stealthier ships vs faster easy to see ones, fleetformed interferometers, groundside arrays of tremendous size, sky scan speed vs detection range/surety, IR detectability of manned ships vs robotic ones...must make spacewar game including all this

Obviously a game simulating the economic and strategic implications of decoys vs sensor networks is going to have some trumped up guesswork numbers and production models, but it would still be fun to throw in some reasonable estimates and see what players come up with.

And I'd rather be in an HB-fusion powered warship than an Orion-powered warship for sneaking up and bushwacking somebody. TINSTAAFL.

I suspect this idea will work much better in a universe where lasers dominate than one where missiles dominate. In a missile dominated universe tactical acceleration is crucially important. In a laser dominated one, not so much.

Well if your lasers have a range over a light second or two, tactical accel will be pretty important to increase the possibility area of where you could be and thus reduce the chance to be hit. Though even with low accel the chance to hit you will be small at those ranges, fleets would likely do long range laser fire in coordinated volleys to canvass an area and go for a surer kill on 1 ship. Anyways...

[Morilore]

But unless you knew where they all were this wouldn't help. It's not just the ones that are there you have to worry about; it's the ones that might be there.

Again, this assumes you know where all enemy observers are. Since it's fairly likely you won't, your decoy better actually look like a real ship.

Yeah, you guess. It can be a guessing game. In the center of the solar system, you'd have to show decoy light pretty much everywhere because its relatively easy to get 360 degrees around, say, Mars from Earth. If you're attacking from Neptune? Not so much. If you're attacking from Neptune, depending on the level of development of this solar system, the probability of the enemy having scanners behind you is probably low. Same if this is an extrasolar war, and you're attacking from 90 degrees out of the plane of the ecliptic.

They won't have an exhaust plume but they'd still be generating quite a bit of waste heat, which must be permitted to escape into space if you don't want your ship to melt.

Yes, but you can control the direction of that better than you can control the radiation of an exhaust plume, since it starts off in your ship.

Except real exhaust plumes aren't neat directional lights (photons drives and other exotic rockets aside). Unless you know exactly where all the enemy observation platforms are the decoy will have to be radiating omnidirectionally.

Yes yes, but they would only be significantly doppler-shifted along the axis of acceleration. You would shine normal exhaust-plume light to your sides, not just blue to the front and red to the back.