Stealthy orbital insertion?

[KiwiTyke]

Hi all, I am working on a science fiction novel set in an advanced society (think human +50-75 years). One group wants to stealthy insert a human spy into the enemy's planet, and I want to explore the possibility of doing this by disguising a small spacecraft as an asteroid / comet and having it enter the atmosphere and apparently breaking up. In fact, the break-up is the ejection of a rocky outer layer so a heavily stealthed atmospheric craft can then get to the surface (preferably at an oceanic coastline) safely in an unpopulated area. I know this is way beyond current human tech capability!

So my key questions are:

1. What degree of precision could you get in terms of the correct angle of approach to atmospheric re-entry without using highly visible manoeuvring propulsion systems to avoid getting burned to a crisp? It needs to look and behave as much like a dead rock as possible. So, could you (theoretically) calculate the orbital mechanics to sufficient accuracy that you could simply launch secretly from around 300-400,000km and assume it's going to be OK? At present, I'm toying with the idea of using a comet disguise to enable a few small manoeuvring jets to be disguised as pockets of liquid / gas being evaporated explosively from the surface. 

2. What other major issues would crop up in such a scenario that aren't immediately obvious?

Thanks in advance for any ideas and constructive criticism of this idea!

[Shpaget]

Hi and welcome to the forums!

There have been quite a few discussion here about warfare in space all of which eventually come up with the idea of stealth in space. Perhaps you should read those topics for additional info, but the consensus is that there is no stealth in space.

Even with our current technology, if we spot a spaceship we could easily differentiate it from a space rock by it thermal signature. Basically anything artificial will be warmer than asteroids and stand out in infra red. There is a possibility of active cooling and directional thermal radiation, but that would highly depend on your story setting and the capabilities and resources of the spied-upon side.

Another issue is that couple of hundred thousand km is very close and anything lighting their engines that close would register on sensors, especially if you have two warring sides that are concerned with each other and monitoring their neighborhood. If your spy containing ship is detected (but not recognized as a spy ship) and tracked, any change in velocity not consistent with ordinary orbital movement would, without doubt, be instantly suspicious.

Before the Chelyabinsk meteor hit Earth in 2013, it was undetected, mostly because it came from an unusual direction (roughly from the direction of the Sun). Perhaps you could use that in your story.

After atmospheric entry, anything dropping from the sky will experience significant forces and heating. While that may be survivable, it can not go unnoticed. What happens then? Your spied upon nation will go out to try to find the meteor. Once they do find it they will realize it's a spaceship.

Trajectory calculation can easily be done and you could perform your trajectory correction maneuvers behind a nearby moon to hide the exhaust, but it will still change the trajectory and if tracked it will be obvious that something is going on.

From the storytelling you might want to have a system breakdown period that offers a window of opportunity for spy insertion, or have the spy ship approach from a non monitored direction (not in plane of ecliptic). That makes the maneuver a lot more complicated and expensive, but more realistic for stealth.

[KiwiTyke]

Thanks Shpaget, I have been browsing the other forums since joining up and will persist! Really nice ideas in there, and I will investigate to see what I can manufacture to make the scenario work; or, of course, simply abandon the idea as not practical :-) I like its drama and the technical challenge, but ultimately it just might not work.

Just to pursue a couple of avenues of thought you have opened up:

1. The thermal signature is the killer issue, and probably one I was sub-consciously ignoring!

2. The craft was envisaged as actual asteroidal rock hollowed out, so there would be bits of real rock out there that could be found post-landing. Of course there would be a lot of mass missing, even if it was seen to disintegrate! Having that happen over the ocean would probably mean nobody bothered to go and find the bits.

3. Very nice idea on having it approach from an unusual direction. I have toyed with the idea of the proposed system having a considerably more densely "populated" asteroid belt than the Mars-Jupiter belt we have here, so asteroid activity is greater and so less interesting.

 

[shynung]

Atomic Rockets page on stealthy ships

To sum up, it is possible to make a spaceship nearly invisible. Spaceships are detected mainly by their thermal signature emitted from radiators, engines, exhaust plume, etc. A design mentioned in the linked article discusses a stealthy ship that minimizes its thermal signature by using liquid hydrogen/helium as single-use heatsinks - it is boiled to absorb heat from the ship itself, then expanded in an expansion chamber (to lower its temperature), the resulting gas being used as a cold-gas thruster. The ship itself is shaped like a long, thin cylinder, one end constantly facing the sun, to reduce sunlight reflection. A combination sun-shield and concentrating lens (here using fresnel lens), both to shield the ship from the sun's heat (keeping it cold enough to be stealthy), and use it to power a solar-thermal rocket, using the gas from heatsink boiloff as coolant.

The solar thermal rocket pulses the propellant ejection, instead of letting it flow freely. This is to ensure that the propellant is almost as hot as the heating element before it is released to the nozzle, to improve specific impulse.

The hot hydrogen gas from the solar thermal rocket is further cooled by a large nozzle assembly (not depicted in the diagram) by expansion.

Quote

Using perfect gas laws (PV=nRT), dropping the propellant from 3000K to 20K temperature requires a decrease in pressure or volume of a factor 150. An isobaric nozzle keeps the pressure constant, so volume must increase for the temperature to drop. A hydrogen pulse is roughly spherical. A 150x reduction in volume entails a 5.3x increase in diameter. Therefore, the nozzle must be at least 5.3 times wider at the opening than at the throat.

To make the ship even harder to detect, it can be coated with Vantablack, a special substance which absorbs up to 99.965% of light in the visible spectrum.

Edited August 16, 2017 by shynung

[1101]

I'm not going to claim to be an expert, but if you have sufficient life support and time isn't too much of an issue, then you could drop off the lander/rock a lot further out.  If your lander can be captured or slingshotted by a moon toward the needed atmospheric trajectory, the last impulse you need give it could be weeks before it gets close to the target.  You would need a lot of processor power to calculate the trajectory, and well mapped out gravity well, but it could work, I think.

Alternatively, if the enemy planet knows it is being watched, pretend it's a failure.  Have a small lander system (like MOOSE) in among a much bigger decoy craft that disassembles during the entry sequence.  Then, your manned bit looks just like a bit of debris and no one is any the wiser.

[kerbiloid]

1. Burst a bunch of nukes in high orbit.
2. While all are watching this, land.

1. Launch thousands of inflatable foil ducks in low orbit.
2. While all are watching and trying to distinguish "old" and "new" objects, land.

1. Collide two rocks in high orbit.
2. The same.

Edited August 16, 2017 by kerbiloid

[Bill Phil]

If you want stealth in space, you need to either change the laws of physics, or use magitech. If you can use warp drive like tech to create a sort of pocket dimension, you could leave something the size of a proton in this universe. That's magitech, though.

[p1t1o]

@shynung 

Im not sure what use Vantablack would be. Why would you want your ship to absorb incident visible radiation? That will just raise your temp and people are not going to be looking for you by shining torches! Vantablack is very black, but it will still emit IR if its hot enough - Good absorbers are by definition good emitters. If anything, you want to coat your ship with something reflective - so you dont absorb as much incident radiation and lowering your emissivity. (NB: "reflective" does not necessailry mean "silvery", it could mean "white" or any number of other shades, like that grey reflective tape you get on safety clothing) but in any event, "holding your heat in" will only get you so far. Once your heat sink is tapped, you will have to radiate every erg of generated heat, or die. And big heat sinks are themselves hard to hide.

The laws of thermodynamics also make pretty much every mechanism that you describe less-than-perfectly-efficient, meaning that with the cold of space as a backdrop, your stealth gains are incremental at best. I think "harder to detect" is achievable and closer to the truth than "nearly invisible", just like contemporary radar stealth.

For example, your expanded-gas cooled exhaust. Expanding the gas does not actually get rid of any energy. The gas is now cooler so is emitting less intensely in the IR spectrum, but now the cloud is much larger, so the radiation it does emit is less intense but coming from a larger source. And large clouds of gas can be detected in numerous other ways. Not to mention that the base of the nozzle and the attached combustion chamber will of course be at operating temperature.

 

The decoys approach is a good idea, the greatest weakness in detection is likely the capabilities of the sensors looking for your. When you cannot disguise your own signature, you attack the sensors, passively (decoys, chaff, flares, drones) or actively (jamming, dazzling, physical assault). 

 

Im not certain, but I strongly suspect that an approach to a planet with the sun behind you, would render your IR signature enormously difficult to detect (point-of-view limited of course). Attaining this position undetected would have the usual difficulties though. 

[YNM]

Make it small enough. If you could chop your fellow man to be small enough, you could probably just drop it in. Some bits should survive.

Or why go to such hassle ? Just drop a drone if they're smart enough...

Or most likely, you should exploit the geography or something. I'm sure something that falls over some remote landscape shouldn't be noticeable.

Edited August 16, 2017 by YNM

[shynung]

1 minute ago, p1t1o said:

Im not sure what use Vantablack would be. Why would you want your ship to absorb incident visible radiation? That will just raise your temp

To reduce glints of sunlight reflection. Also to absorb stuff like LIDAR emissions. A radio-wave-absorbent coating directly underneath the Vantablack takes care of the RADAR emissions absorption.

The ship's skin would have cooling ducts inside, to keep temperatures down. The ship is double-hulled, so that the cold outer skin does not receive thermal energy via conduction from e.g. hab modules.

4 minutes ago, p1t1o said:

The laws of thermodynamics also make pretty much every mechanism that you describe less-than-perfectly-efficient, meaning that with the cold of space as a backdrop, your stealth gains are incremental at best.

Liquid hydrogen boils at 22 K, that of helium is 4 K. Detecting a 22K object against space radiation background (2.73 K) is very difficult - 4 K, nearly impossible.

15 minutes ago, p1t1o said:

For example, your expanded-gas cooled exhaust. Expanding the gas does not actually get rid of any energy. The gas is now cooler so is emitting less intensely in the IR spectrum, but now the cloud is much larger, so the radiation it does emit is less intense but coming from a larger source. And large clouds of gas can be detected in numerous other ways. Not to mention that the base of the nozzle and the attached combustion chamber will of course be at operating temperature.

The expanded-gas exhaust is expanded just enough so that their temperatures when exiting the nozzle is comparable to the ship skin's temperature. The heating chamber is closed off at most times by a shutter with a cooled outer skin. The nozzle base will only touch hot gas intermittently, and are cooled by the heatsink.

Also needed mentioning is that the heatsink is only used until it reaches its boiling point. Spent heatsink is ejected overboard at its boiling temperature (22K for hydrogen, 4 K for helium). The main exception is the propulsion system, which does release puffs of gas at high temperatures, but this is mitigated by the exhaust gas being cooled by expansion in the nozzle. The main point is, it does not try to 'hold the heat in', but rather release it by boiling a cold liquid, using the enthalpy of vaporization to cool down without raising the coolant's temperature.

Also, the process is not very efficient on the heatsink, as noted. A 1kW source of heat needs 8 kg/hour of liquid hydrogen to cool down to 22 K. Consequently, the ship's mass at launch will mostly be heatsink.

[kerbiloid]

1. Call ufologists and tell them that you are going to land at (precise time and location). Do not lie!
2. Wait a week until they inform everybody about the coming contact.
3. Land at (precise time and location).

No self-respecting person will happen to appear inside direct visibility range.
Nobody will even read a police report.

NB 1.
Best use this if wearing greenish-gray tights and red cape with a golden emblem.
Then you may even allow them to make photos.

NB 2.
Best date - April, 1st. Best place - any sacral place at your choice.
(Pyramids and Stonehenge are not advised due to being real extraterrestrial activity sites).
Also a comet or an asteroid flyby date is nice. Tell them that you will arrive on a comet.

Edited August 16, 2017 by kerbiloid

[p1t1o]

@shynung

I hate to be a downer but this is going to boil down to the usual arguments for/against stealth-in-space...

No matter how clever your cooling setup, thermodynamics will get you. Even if your heatsink was stealthy (which I have my doubts about) you wont be able to shunt all of your heat into the coolant. Yet more rules of thermodynamics mean it is impossible to cool said surface to the temperature of the cold coolant, only to an intermediate temperature, so straight of the bat, the skin of your ship is way hotter than 22K and there is nothing you can do about it.

I dont have the maths, but I cant see creating ginormous clouds of hydrogen as stealthy. Not for nothing but clouds of cold hydrogen are already specifically looked for with even todays tech. What happens when hydrogen gas is ionised by solar radiation for example? And what happens to the light of background stars shining through a gas cloud? Hard to detect, sure, but a lot easier to detect than the nothing that was there before.

And one other thing - were you intending on using expelled heatsink as your sole propulsion? Because you wont get much thrust or dV from it if we are talking a few hundred kilowatts (lets face it, if you need to lose much more, the amount of heatsink required quickly gets untenable). You will need a hot engine to get anywhere with any speed.

And of course, once you are out of coolant (gotta admit I hadnt considered your method before) you will be 110% boned.

All those techniques would help, but I think "nearly invisible" is still out of reach.

 

[DDE]

Thing is, you don't probably need to hide the mothership, you need to just hide the launch from the defending side that is free to take potshots at the "low altitude recon flyby".

[MatterBeam]

Hello guys. Might I weigh in?

8 hours ago, KiwiTyke said:

Hi all, I am working on a science fiction novel set in an advanced society (think human +50-75 years). One group wants to stealthy insert a human spy into the enemy's planet, and I want to explore the possibility of doing this by disguising a small spacecraft as an asteroid / comet and having it enter the atmosphere and apparently breaking up. In fact, the break-up is the ejection of a rocky outer layer so a heavily stealthed atmospheric craft can then get to the surface (preferably at an oceanic coastline) safely in an unpopulated area. I know this is way beyond current human tech capability!

So my key questions are:

1. What degree of precision could you get in terms of the correct angle of approach to atmospheric re-entry without using highly visible manoeuvring propulsion systems to avoid getting burned to a crisp? It needs to look and behave as much like a dead rock as possible. So, could you (theoretically) calculate the orbital mechanics to sufficient accuracy that you could simply launch secretly from around 300-400,000km and assume it's going to be OK? At present, I'm toying with the idea of using a comet disguise to enable a few small manoeuvring jets to be disguised as pockets of liquid / gas being evaporated explosively from the surface. 

2. What other major issues would crop up in such a scenario that aren't immediately obvious?

Thanks in advance for any ideas and constructive criticism of this idea!

In 50-70 years, unless an arms race comes along or a revolutionary technology like the Epstein Drive is developed, we will be unlikely to move far past the asteroid belt. The pessimist in me tell me we still won't have a permanent presence outside of low earth orbit, but that's another story.

What this means is that there won't be constellations of thousands of satellites pointed outwards to spot anything in detail. There'll be sensors tracking spaceships going to and fro from known destinations, but most will be pointing inwards at Earth to detect cheap launch vehicles from being used in nefarious ways. 

Using methods detailed below, you can keep a spaceship undetectable until it gets close to Earth. Staying above the planetary plane and using eccentric polar orbits will avoid the bulk of the sensors. There's no simple way to get to the ground because re-entry is a very violent event that reveals a lot of information about what is coming in. The radio reflection on the re-entry vehicle can distinguish between a lumpy rock or a smooth metal container. The trajectory reveals whether it is a chaotic fragmentation with the g-force meter jumping up and down, or the human-survivable descent of a reentry vehicle. Either way, we've had the sensor technology and dozens of spy satellites watching the atmosphere from above watching for nuclear re-entry vehicles for decades. It'll be hard to escape their gaze. 

Your best option is to manoeuvre your stealthy spaceship near a automated cargo pod and either latch on to it or hide inside of it. These will exist because in 50 years, we will still be dependent on Earth-based manufacturing for high technology components, which needs rare metals obtained from asteroids to run. Once you are in the lower atmosphere, deploy wings and fly away from the landing point before the recovery team swoops in. Swoops, yes, because you don't want tons of platinum to be floating unattended in the Pacific ocean (a likely landing zone). 

7 hours ago, Shpaget said:

Hi and welcome to the forums!

There have been quite a few discussion here about warfare in space all of which eventually come up with the idea of stealth in space. Perhaps you should read those topics for additional info, but the consensus is that there is no stealth in space.

Even with our current technology, if we spot a spaceship we could easily differentiate it from a space rock by it thermal signature. Basically anything artificial will be warmer than asteroids and stand out in infra red. There is a possibility of active cooling and directional thermal radiation, but that would highly depend on your story setting and the capabilities and resources of the spied-upon side.

Another issue is that couple of hundred thousand km is very close and anything lighting their engines that close would register on sensors, especially if you have two warring sides that are concerned with each other and monitoring their neighborhood. If your spy containing ship is detected (but not recognized as a spy ship) and tracked, any change in velocity not consistent with ordinary orbital movement would, without doubt, be instantly suspicious.

Before the Chelyabinsk meteor hit Earth in 2013, it was undetected, mostly because it came from an unusual direction (roughly from the direction of the Sun). Perhaps you could use that in your story.

After atmospheric entry, anything dropping from the sky will experience significant forces and heating. While that may be survivable, it can not go unnoticed. What happens then? Your spied upon nation will go out to try to find the meteor. Once they do find it they will realize it's a spaceship.

Trajectory calculation can easily be done and you could perform your trajectory correction maneuvers behind a nearby moon to hide the exhaust, but it will still change the trajectory and if tracked it will be obvious that something is going on.

From the storytelling you might want to have a system breakdown period that offers a window of opportunity for spy insertion, or have the spy ship approach from a non monitored direction (not in plane of ecliptic). That makes the maneuver a lot more complicated and expensive, but more realistic for stealth.

I beg to differ!

http://toughsf.blogspot.com/2016/03/stealth-in-space-is-possible.html

http://toughsf.blogspot.com/2016/03/stealth-in-space-is-possible-ii.html

http://toughsf.blogspot.com/2016/03/stealth-in-space-is-possible-iii.html

http://toughsf.blogspot.com/2016/04/stealth-in-space-is-possible-iv.html

The point of stealth is not to hide completely, but to reduce your detectability from sensors. Learning how sensors works allows you to defeat them.

Infrared sensors measure photon 'hits' on a Charge-Coupled Device. Enough photons striking the same spot trigger a voltage-controlled switch, illuminating a pixel. Over time, enough pixels are collected to form an infrared image of the target. The objective here is to reduce the energy of these photons and their number down to levels where the sensor cannot distinguish incoming photon strikes from self-triggering 'noise'. The big sources of noise are quantum inefficiency in the CCD's pixels/cells, and the temperature of the device emitting photons onto itself.

Future technology might increase quantum efficiency to extreme levels (99.99% of photon hits add to the trigger voltage instead of the current 85%). What technology cannot do is beat thermodynamics. A sensor cannot detect a target colder than the device itself, nor a target the same temperature as the universe's background radiation.

In short, there is a temperature and a range at which it takes an inordinate number of hours of observation to collect enough photons to confirm a target's detection. 

This situation occurs for 22K (hydrogen-cooled) objects further than 10000km away, and for 4K objects a few hundred kilometers away. A 2.7K object sitting right in front of an infrared sensor is indistinguishable from background space. In practice, the sensors won't be perfect and won't be able to maintain 10^-19W/m^2 sensitivities in a real world situation. Anything colder than 22K will likely slip right through entire networks of sensors because even if they can detect it over time, their target is quickly moving across and out of their field of view.

Asteroids at Earth orbit are actually sitting in very strong naked sunlight (1.3kW/m^2) and their exposed surfaces can reach toasty 200K+ temperatures. This is why the ices on their surface vaporize or sublimate. The problem is detecting the far away asteroids that sit at 150K, 70K or cooler. We have trouble detecting even these 'hot' objects in the asteroid belt. 

6 hours ago, shynung said:

Atomic Rockets page on stealthy ships

To sum up, it is possible to make a spaceship nearly invisible. Spaceships are detected mainly by their thermal signature emitted from radiators, engines, exhaust plume, etc. A design mentioned in the linked article discusses a stealthy ship that minimizes its thermal signature by using liquid hydrogen/helium as single-use heatsinks - it is boiled to absorb heat from the ship itself, then expanded in an expansion chamber (to lower its temperature), the resulting gas being used as a cold-gas thruster. The ship itself is shaped like a long, thin cylinder, one end constantly facing the sun, to reduce sunlight reflection. A combination sun-shield and concentrating lens (here using fresnel lens), both to shield the ship from the sun's heat (keeping it cold enough to be stealthy), and use it to power a solar-thermal rocket, using the gas from heatsink boiloff as coolant.

The solar thermal rocket pulses the propellant ejection, instead of letting it flow freely. This is to ensure that the propellant is almost as hot as the heating element before it is released to the nozzle, to improve specific impulse.

The hot hydrogen gas from the solar thermal rocket is further cooled by a large nozzle assembly (not depicted in the diagram) by expansion.

To make the ship even harder to detect, it can be coated with Vantablack, a special substance which absorbs up to 99.965% of light in the visible spectrum.

Quite right! The fresnel lens is not technically necessary - it is only an accessory meant to collect more sunlight to power the solar-impulse drive. Without it, it can only move by a few mm/s per day.

If we want a powerful yet stealthy rocket, use a nuclear thermal drive. Run the exhaust through a massive exhaust nozzle. The nozzle starts off closed. Hot exhaust is injected into the nozzle... it is hidden while expanding and cooling by the shutter. Just as it reaches the nozzle's end, the shutter opens and allows it to leave the rocket. From the outside, the rocket is just emitting 20K puffs of gas. 

Or, find a way to curve the nozzle without shock-heating of the exhaust and you can run it at full output. We're talking gigawatts of undetectable drive power here.

6 hours ago, 1101 said:

I'm not going to claim to be an expert, but if you have sufficient life support and time isn't too much of an issue, then you could drop off the lander/rock a lot further out.  If your lander can be captured or slingshotted by a moon toward the needed atmospheric trajectory, the last impulse you need give it could be weeks before it gets close to the target.  You would need a lot of processor power to calculate the trajectory, and well mapped out gravity well, but it could work, I think.

Alternatively, if the enemy planet knows it is being watched, pretend it's a failure.  Have a small lander system (like MOOSE) in among a much bigger decoy craft that disassembles during the entry sequence.  Then, your manned bit looks just like a bit of debris and no one is any the wiser.

Quite a good plan. Make most of the trip on a registered civilian ship, then drop off a 'stealth pod' that finds its way to the ground without being detected. Any N-body physics simulator can handle the calculations needed, like Children of a Dead Earth. 

5 hours ago, Bill Phil said:

If you want stealth in space, you need to either change the laws of physics, or use magitech. If you can use warp drive like tech to create a sort of pocket dimension, you could leave something the size of a proton in this universe. That's magitech, though.

Or use an understanding of detection techniques and work against those!

4 hours ago, p1t1o said:

@shynung 

Im not sure what use Vantablack would be. Why would you want your ship to absorb incident visible radiation? That will just raise your temp and people are not going to be looking for you by shining torches! Vantablack is very black, but it will still emit IR if its hot enough - Good absorbers are by definition good emitters. If anything, you want to coat your ship with something reflective - so you dont absorb as much incident radiation and lowering your emissivity. (NB: "reflective" does not necessailry mean "silvery", it could mean "white" or any number of other shades, like that grey reflective tape you get on safety clothing) but in any event, "holding your heat in" will only get you so far. Once your heat sink is tapped, you will have to radiate every erg of generated heat, or die. And big heat sinks are themselves hard to hide.

The laws of thermodynamics also make pretty much every mechanism that you describe less-than-perfectly-efficient, meaning that with the cold of space as a backdrop, your stealth gains are incremental at best. I think "harder to detect" is achievable and closer to the truth than "nearly invisible", just like contemporary radar stealth.

For example, your expanded-gas cooled exhaust. Expanding the gas does not actually get rid of any energy. The gas is now cooler so is emitting less intensely in the IR spectrum, but now the cloud is much larger, so the radiation it does emit is less intense but coming from a larger source. And large clouds of gas can be detected in numerous other ways. Not to mention that the base of the nozzle and the attached combustion chamber will of course be at operating temperature.

 

The decoys approach is a good idea, the greatest weakness in detection is likely the capabilities of the sensors looking for your. When you cannot disguise your own signature, you attack the sensors, passively (decoys, chaff, flares, drones) or actively (jamming, dazzling, physical assault). 

 

Im not certain, but I strongly suspect that an approach to a planet with the sun behind you, would render your IR signature enormously difficult to detect (point-of-view limited of course). Attaining this position undetected would have the usual difficulties though. 

Vantablack's purpose is to not reflect any incoming radiations. 99.99% absorbance means 0.01% reflectivity, or better.... the heat you absorb, mostly from sunlight, is them passed by a heat exchanger into a liquid hydrogen heat sink. 

The heat sink boils, taking the heat you absorbed away with it. LH2 boils at 22K, so just like an ice cube stays at 0C until it has completely melted, LH2 will stay at 22K while it evaporates. 

If you want even lower temperatures, use liquid helium at 2-4K and use a heat pump to move the absorbed heat from the helium into liquid hydrogen. 

Supercooling the vantablack this way prevents it from emitting nearly anything. If it doesn't reflect or emit anything, how do you detect it?

The perfect gas law gives a linear relationship between pressure, mass (moles), volume and temperature. If you expand a gas inside a de Laval nozzle, pressure drops and volume increases while mass stays constant. Consequently, temperature must drop. A real-world example of this effect is... real-world nozzles. The highest temperatures are near the throat, so that's where we see the active coolant loops concentrated. The lower half and the rim are much cooler. If we used a ludicrously big nozzle, the gasses will expand and expand to undetectable temperatures. 

Approaching from the sun against a well-designed sensor is a bad idea. It can adjust its sensitivity of multiple scales to make out your very cold outline against the very hot and bright background. 

4 hours ago, shynung said:

Also, the process is not very efficient on the heatsink, as noted. A 1kW source of heat needs 8 kg/hour of liquid hydrogen to cool down to 22 K. Consequently, the ship's mass at launch will mostly be heatsink.

The 4K temperature ship needs a heat pump to move heat up the 4K to 22K gradient. The pump itself must also be powered by a reactor, and that reactor must also be cooled. Altogether, the small drop in surface temperature will cost you up to 20 times more hydrogen expended per hour!

2 hours ago, p1t1o said:

@shynung

I hate to be a downer but this is going to boil down to the usual arguments for/against stealth-in-space...

No matter how clever your cooling setup, thermodynamics will get you. Even if your heatsink was stealthy (which I have my doubts about) you wont be able to shunt all of your heat into the coolant. Yet more rules of thermodynamics mean it is impossible to cool said surface to the temperature of the cold coolant, only to an intermediate temperature, so straight of the bat, the skin of your ship is way hotter than 22K and there is nothing you can do about it.

I dont have the maths, but I cant see creating ginormous clouds of hydrogen as stealthy. Not for nothing but clouds of cold hydrogen are already specifically looked for with even todays tech. What happens when hydrogen gas is ionised by solar radiation for example? And what happens to the light of background stars shining through a gas cloud? Hard to detect, sure, but a lot easier to detect than the nothing that was there before.

And one other thing - were you intending on using expelled heatsink as your sole propulsion? Because you wont get much thrust or dV from it if we are talking a few hundred kilowatts (lets face it, if you need to lose much more, the amount of heatsink required quickly gets untenable). You will need a hot engine to get anywhere with any speed.

And of course, once you are out of coolant (gotta admit I hadnt considered your method before) you will be 110% boned.

All those techniques would help, but I think "nearly invisible" is still out of reach.

We're working well within the confines of basic thermodynamics.

An example would be the infrared telescopes being used to observe very cool stars. They have reserves of liquid helium on-board that they use to flush the CCD sensors and reduce the noise level until the stars are detectable. Once the helium is all used up, the CCD heats up and those stars become invisible, like what happened on the Spitzer Space Telescope: "Without liquid helium to cool the telescope to the very low temperatures needed to operate, most of the instruments are no longer usable. "

We're only trying to do the same on a much larger scale - instead of just cooling a small CCD sensor, we're cooling the entire hull of a spaceship using hundreds of tons of liquid hydrogen.

Your assumptions about thermodynamics are quite wrong. The only way that the spaceship's hull heats up is if the incoming radiations are not conducted to heat exchanger fast enough. That's a design problem, not a physics problem. Thankfully, Vantablack is just vertically-aligned carbon nanotube, which makes them thermal superconductors in the horizontal direction. Heat transfer rates will be incredible. 

Again, you need the maths to make those claims. A large cool hydrogen is exactly what the far away galaxies are. Why isn't the sky entirely lit up end-to-end in the infrared spectrum? Because those 'gas clouds' are simply too faint. And, even if stable H2 molecules manage to get ionized by sunlight (when actually 15.84eV is needed, 183815K temperature), they'd just point out that a hydrogen-propelled rocket was used within this hundred thousand kilometer volume within the past few minutes. A 9km/s hydrogen exhaust out of a 10m nozzle, released at 20K, would diffuse into a 42 billion m^3 volume within a single minute, more if it had any lateral diffusion. 

The specific hydrogen steamer design on my blog can stay undetected for decades at a time. The more you scale it up, the more endurance you get out of each kilogram of hydrogen on-board. 

Feel free to ask more questions!

[DerekL1963]

4 hours ago, MatterBeam said:

Supercooling the vantablack this way prevents it from emitting nearly anything.

OK, what happens to the heat you absorb in order to cool it?  Like the rest of your cooling schemes, you forget that one all important detail.  You have to get rid of it somehow.

 

4 hours ago, MatterBeam said:

Your assumptions about thermodynamics are quite wrong. The only way that the spaceship's hull heats up is if the incoming radiations are not conducted to heat exchanger fast enough.

And there, once again, is the hole in the scheme the amateurs miss - you are radiating heat, that heat can be detected.

 

[SinBad]

save the hiding for after reentry. come in fast and hot. fast and hot enough that interception during re-entry is near impossible. then hit the deck (under the radar) over a large ocean and do a few minutes at high speed to get away from the projected landing site. then you can slow down and start using more terrestrial stealth solutions. but at some point you are going to have to land and hide or dispose of the craft. thats probably the fiddly part as the farther it is from eyes, the farther you have to travel overland. the farther you need to go, the higher the need for a vehicle, which you either need to bring with you (making your craft larger) or have arranged to be waiting for you by some local agent, complicit or otherwise.

if you want to be fancy you could re-enter at noon and use an ablater material that will emit the same spectrum as the sky, or do it over a storm (also good cover for sonic booms).

the point is, they know something arrived, but they don't really know what, or where it is. after all, you have a whole planet to get lost in. even better if there is a high population that is used to strangers visiting from other cities.

[MatterBeam]

1 hour ago, DerekL1963 said:

OK, what happens to the heat you absorb in order to cool it?  Like the rest of your cooling schemes, you forget that one all important detail.  You have to get rid of it somehow.

And there, once again, is the hole in the scheme the amateurs miss - you are radiating heat, that heat can be detected.
 

It has been explained many times, maybe we weren't clear. The heat that the Vantablack surface absorbs is absorbed by a liquid hydrogen heat sink. The liquid hydrogen boils. The boiling hydrogen is then expelled. 

The transition from liquid to gaseous state consumes energy. This energy is the heat we are handling. The scientific term is heat of vaporization. For liquid hydrogen, it is is 450kJ/kg. 

This means that each kilogram of liquid hydrogen onboard will absorb 450kJ of heat and boil away. The Vantablack, the gasses and the remaining liquid are all at 22K, which is about the boiling temperature of liquid hydrogen.

There are several extra steps in the hydrogen steamer design made to maximize the amount of heat the hydrogen steals away before it is ejected. One of them is heating the gaseous hydrogen even further. By concentrating sunlight onto a heat exchanger, we can turn it into propellant for a rocket. Hydrogen gas heat capacity rises from 14kJ/kg/K to 22kJ/kg/K as it is heated from 22K to 3000K. This is a lot of extra heat we can absorb. The hotter the component we need to cool down, the more efficiently the hydrogen is used. A hull surface absorbing sunlight needs to stay cool at 22K, so we can only use the 450kJ/kg vaporization energy as the heat sink. A crew habitat at 300K allow for a additional heat capacity of (300-22)*14: 3892 kJ/kg for a total of 4342kJ/kg. A hot laser operating at 700K allows for a total waste heat absorbed of 10MJ/kg. Even nuclear reactors can be cooled: at 2000K, over 36MJ/kg can be absorbed.

The best thing is that your 2000K superheated hydrogen gas can then be run through a rocket nozzle and expanded until it is a very cool but fast propellant stream, no warmer than your hull surfaces. 

Let me give you an example:

A thin cylinder with a cold frontal area of 10m^2 absorbing sunlight at 1.37kW/m^2, a 5 crew habitat emitting 1kW of waste heat at 300K, a life support system emitting 10kW of waste heat at 500K and a 1MW reactor powering all the systems at 1300K. 

How much hydrogen do we consume? 1.37*10/450 + 1/[(300-22)*14+450] + 10/[(500-22)*14+450] + 1000/[(1300-22)*16+450] = 0.0304 + 0.00023 + 0.0478 = 78.5 grams per second. If we have 1000 tons of liquid hydrogen on-board, the ship can run undetected for five months. If we take that hydrogen at its different temperatures and quantities and run it through a 3000K nuclear rocket engine, we can absorb 3.8MW of propulsive power without needing any more hydrogen. 

I hope this answers some of your questions.

1 hour ago, SinBad said:

save the hiding for after reentry. come in fast and hot. fast and hot enough that interception during re-entry is near impossible. then hit the deck (under the radar) over a large ocean and do a few minutes at high speed to get away from the projected landing site. then you can slow down and start using more terrestrial stealth solutions. but at some point you are going to have to land and hide or dispose of the craft. thats probably the fiddly part as the farther it is from eyes, the farther you have to travel overland. the farther you need to go, the higher the need for a vehicle, which you either need to bring with you (making your craft larger) or have arranged to be waiting for you by some local agent, complicit or otherwise.

if you want to be fancy you could re-enter at noon and use an ablater material that will emit the same spectrum as the sky, or do it over a storm (also good cover for sonic booms).

the point is, they know something arrived, but they don't really know what, or where it is. after all, you have a whole planet to get lost in. even better if there is a high population that is used to strangers visiting from other cities.

Ballistic missile trackers and interceptors are designed to handle MIRVs turning at over 10G, in multiple directions, simultaneously. No human-occupied vehicle can outrun them. 

[YNM]

I know !

Just cover it in SEP Field.

How to make it real :

 

Edited August 17, 2017 by YNM

[mikegarrison]

https://en.wikipedia.org/wiki/Air_Force_Space_Surveillance_System

The original US space fence could detect anything the size of a basketball or bigger that orbited over the US. The new fence will catch things even smaller.

[magnemoe]

Simplest would be to look harmless, for this you need to be pretty small, put an shield towards earth, this has ambient temprature or close to it, pod is behind.
If noticed you look like an 2 meter comet fragment, so small it will break up in atmosphere and it will hit in the wilderness anyway. 
And you break up, on entering, you loose the forward shield if not pulled in and used as heat shield, you loose the living area and life support leaving just an drop pod, probably breaking this to, 
Now problem would be inserting the pod into the trajectory it would need an mothership. This you can not hide, 

[SinBad]

4 hours ago, MatterBeam said:

...

Ballistic missile trackers and interceptors are designed to handle MIRVs turning at over 10G, in multiple directions, simultaneously. No human-occupied vehicle can outrun them. 

true, but are such systems capable of intercepting objects over the oceans? are such technologies in use in this scenario? if you came down over the pacific for example. purely ballistic, pulling out and into an ocean hugging randomly chosen direction as late as you can survive. then get out of the area as fast and as low as you can. would a shore based interception system be able to get ordinance on location fast enough to still find you there? remembering that at this time they need to intercept a supersonic/hypersonic (depending on how advanced 'advanced civilisation ' means) seaskimmer, or if they are really slow they need to find a stealthed sub sonic sea skimmer rather than the expected high speed high altitude target they were designed to locate.

Edited August 17, 2017 by SinBad

[Shpaget]

11 hours ago, MatterBeam said:

snip

Thank you for your effort to write this post. I haven't checked the math, but it's a nice read anyway.

However, your proposal is not applicable in OPs scenario where he wants to inject a single individual (or a small group) undetected onto another planet. The ship with your design would weigh thousands of tons and would be quite a disaster if it struck an Earth like planet.

[shynung]

13 minutes ago, Shpaget said:

Thank you for your effort to write this post. I haven't checked the math, but it's a nice read anyway.

However, your proposal is not applicable in OPs scenario where he wants to inject a single individual (or a small group) undetected onto another planet. The ship with your design would weigh thousands of tons and would be quite a disaster if it struck an Earth like planet.

@MatterBeam's stealth ship isn't supposed to crash into the planet - and it doesn't need to. It can covertly insert itself into orbit, which can then drop the spies via MOOSE pods.

[kerbiloid]

44 minutes ago, mikegarrison said:

The original US space fence could detect anything the size of a basketball or bigger that orbited over the US. The new fence will catch things even smaller.

1. Over the US. (~20 mln km² of 510 total).
2. Total sky at once or inside a known cone?
3. Is anti-missile defense no more oriented along several main directions like it was always?

[Nibb31]

It seems like the OP has his terminology wrong. He seems to be talking about atmospheric reentry, not orbital insertion. He also seems to be talking about camouflage, not stealth.