Stealthy orbital insertion?

[sevenperforce]

45 minutes ago, Shpaget said:

Well, as soon as somebody notices that the Pe is raising, the game is up.

Well, unlike in KSP, there's no "track object/find Pe/Ap" option, so they'd have to get an exact measurement of the apoapse velocity and then do the same thing again the next time around. Aerocapture would look like a small meteor skipping off the atmosphere, and then you'd be able to "go dark" all the way around to apoapsis. 

Another option would be to raise your apoapsis only slightly, so it was still in the upper atmosphere, but only barely. Then you'd just look like you were a meteor that had aerocaptured and was slowly decaying, when in reality you could be snapping pictures or whatever else you wanted to be doing. Eventually you'd re-enter altogether.

 

[John Cochran]

Slightly off topic, but related. Some years ago there was a study on how to destroy every geosynchronous satellite in orbit. The idea was simple enough, launch a satellite in the same orbit path as that for geosynchronous, but have it retrograde. And the satellite itself would be simply a whole lot of sand and a small explosive charge to disperse the sand into a cloud. After a few orbits, all the geosynchronous satellites would be nothing but junk after being sand blasted by sand with a relative velocity of 6.14 km/sec a few times. The real trick was how to launch the satellite so that the attack wouldn't be expected and to reduce the amount of delta V required. The planned mission would look like a science mission to lunar orbit and while in the vicinity of the moon, it would take advantage of a gravity assist and perform a transfer into a retrograde geosynchronous mission. And the maneuver in lunar space would have been undetectable. So anyone observing the mission would see the launch to the moon, but wouldn't see the return until it was too late and the geosynchronous resources were being sand blasted.

So taking a page from that, have your stealthy vehicle look like a remote science mission, and perform your orbital adjustments while outside of view. 

[p1t1o]

So, about this "cold expanded hydrogen" exhaust cooling system for stealthy ships:

It is stated that to cool gas from 3000K to 20K requires an expansion ratio of 5:1 or thereabouts.

However, I have just been reading about the Space Shuttle Main engines, whose combustion chamber temperatures spiked around 3300K. The SSME's had an expansion ration even greater, of around 9:1 (10 inch throat, 90 inch bell exit).

So how come the space shuttle didnt have a cryogenic exhaust?

[MatterBeam]

13 hours ago, p1t1o said:

So, about this "cold expanded hydrogen" exhaust cooling system for stealthy ships:

It is stated that to cool gas from 3000K to 20K requires an expansion ratio of 5:1 or thereabouts.

However, I have just been reading about the Space Shuttle Main engines, whose combustion chamber temperatures spiked around 3300K. The SSME's had an expansion ration even greater, of around 9:1 (10 inch throat, 90 inch bell exit).

So how come the space shuttle didnt have a cryogenic exhaust?

3000/20: 1500. The gas must expand to a volume 1500 times it's initial volume before it exits the nozzle.

For a straight nozzle, this would require an exit area 1500^0.5: 38.73 times wider than the throat. For a pulsed expansion chamber, the chamber needs to be 1500^0.333:11.44 times wider than the propellant's initial volume.

The shuttle SSME's nozzles should expand a 3000K exhaust at the throat to about 37K at the exit.

The actual relationship between exhaust gas temperature and expansion ratio is not so straightforward, as evidenced by this page of NASA equations: https://www.grc.nasa.gov/www/k-12/airplane/rktthsum.html

Edited August 30, 2017 by MatterBeam

[p1t1o]

20 minutes ago, MatterBeam said:

3000/20: 1500. The gas must expand to a volume 1500 times it's initial volume before it exits the nozzle.

For a straight nozzle, this would require an exit area 1500^0.5: 38.73 times wider than the throat. For a pulsed expansion chamber, the chamber needs to be 1500^0.333:11.44 times wider than the propellant's initial volume.

The shuttle SSME's nozzles should expand a 3000K exhaust at the throat to about 37K at the exit.

But it doesnt?

If it did, then an SSME would resemble gods-own snowblower, even at 1bar.

??

 

Edited August 29, 2017 by p1t1o

[MatterBeam]

4 hours ago, p1t1o said:

But it doesnt?

If it did, then an SSME would resemble gods-own snowblower, even at 1bar.

??

 

Because I'm guessing the SSME has different design requirements than our cooling nozzle. For example, it needs to maintain a minimum back-pressure to work in the atmosphere and aims for an optimal expansion ratio which is a balance between maximizing thrust, minimizing nozzle length and preventing flow separation due to over-expansion. Also, there's the factor that the thrust chamber contains a superheated mix of hydrogen, oxygen and water/water ions, while the nozzle ejects water vapor and excess hydrogen, so we can't use a simple expansion ratio to temperature relationship.

In our nozzle, we're expanding to near zero pressures, we only have hydrogen to deal with and we're aiming for the lowest temperature possible instead of the best thrust-to-weight ratio. 

For example, here is how Children of a Dead Earth calculates the nozzle exit temperature of a 1:1000 expansion ratio:

The tiny dot on the left is the nozzle throat. Temperature drops by a factor 33. Even more extreme expansion ratios can allow for higher starting temperatures. 
 

[Iskierka]

On 16/08/2017 at 1:14 PM, 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.

Important note that this is a true but naive description that assumes flat surfaces and directly what is absorbed/reflected by the material. Vantablack is not a (very) good absorber as a material itself, as it is just carbon nanotubes. Vantablack is so black because the nanotubes are stood on end and any light they reflect bounces inward, into the forest of nanotubes, and will eventually be absorbed by something far more often than it manages to find its way back out again. Because of this, Vantablack is a major exception to the rule and is also a very bad emitter - any light it tries to emit is also likely to get trapped in the forest of tubes, and so be re-absorbed, and the energy is never lost.

This does not, however, actually give anything you could really call an advantage. With normal materials that do follow that rule, all will equalise at the same temperature, as they emit as well as they absorb, but black materials do it better so will equalise faster. This would mean white materials are preferable for something you have to cool, as the rate of heating from the sun that you have to fight is lower, if you want to maintain a different temperature from that equilibrium. Vantablack is very bad here, because it does not equalise at the same temperature - it is an excellent absorber, but a terrible emitter, so it captures light, and never lets it go, getting hotter and hotter and hotter.

Because of this, by the time it equalises, it will be having to dump a LOT of light at a very unusual and noticeable high-temperature wavelength, and if you're trying to manage the heat actively, you need HUGELY powerful systems to do it. Purely optically, Vantablack might technically work well, but it creates an engineering nightmare that is hard to work with and very visible to technology.

[WinkAllKerb'']

interesting ^^

[MatterBeam]

3 hours ago, Iskierka said:

Important note that this is a true but naive description that assumes flat surfaces and directly what is absorbed/reflected by the material. Vantablack is not a (very) good absorber as a material itself, as it is just carbon nanotubes. Vantablack is so black because the nanotubes are stood on end and any light they reflect bounces inward, into the forest of nanotubes, and will eventually be absorbed by something far more often than it manages to find its way back out again. Because of this, Vantablack is a major exception to the rule and is also a very bad emitter - any light it tries to emit is also likely to get trapped in the forest of tubes, and so be re-absorbed, and the energy is never lost.

This does not, however, actually give anything you could really call an advantage. With normal materials that do follow that rule, all will equalise at the same temperature, as they emit as well as they absorb, but black materials do it better so will equalise faster. This would mean white materials are preferable for something you have to cool, as the rate of heating from the sun that you have to fight is lower, if you want to maintain a different temperature from that equilibrium. Vantablack is very bad here, because it does not equalise at the same temperature - it is an excellent absorber, but a terrible emitter, so it captures light, and never lets it go, getting hotter and hotter and hotter.

Because of this, by the time it equalises, it will be having to dump a LOT of light at a very unusual and noticeable high-temperature wavelength, and if you're trying to manage the heat actively, you need HUGELY powerful systems to do it. Purely optically, Vantablack might technically work well, but it creates an engineering nightmare that is hard to work with and very visible to technology.

While these complications might hold true for a Vantablack face sitting in the sun, we are dealing with active cooling. Heat comes in one end, liquid hydrogen carries it away the other end. Vantablack might be very easy to deal with if its thermal superconductivity holds true. 

Also, why would Vantablack radiate its absorbed heat at unusual high-temperature wavelengths when the laws of thermodynamics says it cannot radiate more energy that it receives (so 1300W/m^2 in sunlight) or shine at a higher temperature than the emitter (so 6000K for the sun's surface)?

[WinkAllKerb'']

at matter beam so; you assuming you quantificatation method are right, but what if they are wrong ? i don't intend to mind bug you but well ... who know ^^

Edited August 30, 2017 by WinkAllKerb''

[MatterBeam]

34 minutes ago, WinkAllKerb'' said:

at matter beam so; you assuming you quantificatation method are right, but what if they are wrong ? i don't intend to mind bug you but well ... who know ^^

I could always be wrong on the numbers and ratios, yes. The basic science and facts, that would be less likely. However, I always welcome new information and corrections and if I cannot find better data, then I'll accept it as the new facts.

Edited August 30, 2017 by MatterBeam

[p1t1o]

21 hours ago, MatterBeam said:

3000/20: 1500. The gas must expand to a volume 1500 times it's initial volume before it exits the nozzle.

For a straight nozzle, this would require an exit area 1500^0.5: 38.73 times wider than the throat. For a pulsed expansion chamber, the chamber needs to be 1500^0.333:11.44 times wider than the propellant's initial volume.

The shuttle SSME's nozzles should expand a 3000K exhaust at the throat to about 37K at the exit.

The actual relationship between exhaust gas temperature and expansion ratio is not so straightforward, as evidenced by this page of NASA equations: https://www.grc.nasa.gov/www/k-12/airplane/rktthsum.html

I KNEW it! All credit to you though for doing the actual legwork!

I knew thermodynamics wasn't being uncharacteristically kind!

I havnt been through it exhaustively, but what it boils down to is that you get less cooling than previously expected.

The exit temp is dependant on the Mach number of the exhaust (relative to the speed of sound within the exhaust) which is dependant on expansion ratio and the specific heat ratio of the exhaust (for hydrogen and air it equals about 1.4).

If I am running my numbers correctly, then due to the specific heat ratio of water vapour, the SSME nozzle cuts the temperature of the combustion products roughly in half with an expansion ration of 9:1 and NOT by a factor of a thousand or so. And further to that, in order to drop the temperature by such a high factor, the exhaust velocity would have to be double that of what an SSME already achieves - which would require a monstrously long nozzle, if it is possible at all.

So as I suspected, whilst it is possible to cool your spacecraft in this way, the gains and losses are not so perfect. 

***edit***

FYI: the specific heat ratio of hydrogen is 1.41 and water vapour is 1.33, so expect fairly similar behaviour.

 

Edited August 30, 2017 by p1t1o

[MatterBeam]

9 hours ago, p1t1o said:

I KNEW it! All credit to you though for doing the actual legwork!

I knew thermodynamics wasn't being uncharacteristically kind!

I havnt been through it exhaustively, but what it boils down to is that you get less cooling than previously expected.

The exit temp is dependant on the Mach number of the exhaust (relative to the speed of sound within the exhaust) which is dependant on expansion ratio and the specific heat ratio of the exhaust (for hydrogen and air it equals about 1.4).

If I am running my numbers correctly, then due to the specific heat ratio of water vapour, the SSME nozzle cuts the temperature of the combustion products roughly in half with an expansion ration of 9:1 and NOT by a factor of a thousand or so. And further to that, in order to drop the temperature by such a high factor, the exhaust velocity would have to be double that of what an SSME already achieves - which would require a monstrously long nozzle, if it is possible at all.

So as I suspected, whilst it is possible to cool your spacecraft in this way, the gains and losses are not so perfect. 

***edit***

FYI: the specific heat ratio of hydrogen is 1.41 and water vapour is 1.33, so expect fairly similar behaviour.

 

Agreed. We might have to use... bigger nozzles. 

[shynung]

This was kind of an interesting take on hard scifi take on the stealth ship concept, and I'd like to continue the topic a little further.

Assuming it is actually possible, what would be an ideal expansion ratio for a 3000K thermal rocket to expel the exhaust at 22K?

Also, is there any other way to make a high-performance propulsion system that does not expel hot exhaust? First thing that came to my mind was a coilgun/magnetic accelerator using 22K iron pellets as propellant, but would that be enough?

[MatterBeam]

On 30/08/2017 at 11:23 AM, p1t1o said:

I KNEW it! All credit to you though for doing the actual legwork!

I knew thermodynamics wasn't being uncharacteristically kind!

I havnt been through it exhaustively, but what it boils down to is that you get less cooling than previously expected.

The exit temp is dependant on the Mach number of the exhaust (relative to the speed of sound within the exhaust) which is dependant on expansion ratio and the specific heat ratio of the exhaust (for hydrogen and air it equals about 1.4).

If I am running my numbers correctly, then due to the specific heat ratio of water vapour, the SSME nozzle cuts the temperature of the combustion products roughly in half with an expansion ration of 9:1 and NOT by a factor of a thousand or so. And further to that, in order to drop the temperature by such a high factor, the exhaust velocity would have to be double that of what an SSME already achieves - which would require a monstrously long nozzle, if it is possible at all.

So as I suspected, whilst it is possible to cool your spacecraft in this way, the gains and losses are not so perfect. 

***edit***

FYI: the specific heat ratio of hydrogen is 1.41 and water vapour is 1.33, so expect fairly similar behaviour.

 

I've been looking around for nozzle calculators I got the following results:

Hydrogen, gas constant 4184, input temperature 2330K, ratio between nozzle throat area and exit area: 30000, exit temperature 22K, exit velocity 8.2km/s.

So, a nozzle fitted onto some sort of nuclear thermal would allows for a near undetectable exhaust, while propelling the rocket with an Isp of 835s. If we want a thrust of roughly 100kN, we'd use a nozzle throat 7.6cm wide at a pressure of 1atm. The resultant nozzle exit diameter is 13.1 meters. 

Higher input temperatures are acceptable for moderate increases in exit temperature. 3000K input, with the same parameters, allows for an exit temperature of 28K and 9.39km/s exhaust velocity. Increase the expansion ratio to a ridiculous 100000, and you can even use high temperature 4000K hydrogen for 10.8km/s exhaust velocity and yet only release it at 23K. At that point, you're absorbing over 80MJ of heat per kg of hydrogen.

1 hour ago, shynung said:

This was kind of an interesting take on hard scifi take on the stealth ship concept, and I'd like to continue the topic a little further.

Assuming it is actually possible, what would be an ideal expansion ratio for a 3000K thermal rocket to expel the exhaust at 22K?

Also, is there any other way to make a high-performance propulsion system that does not expel hot exhaust? First thing that came to my mind was a coilgun/magnetic accelerator using 22K iron pellets as propellant, but would that be enough?

I've done the calculations above. Expansion ratios, measured in nozzle exit area over nozzle throat area, will be at least 30000. To convert into a ratio of diameters, divide by 3.14 and calculate the square root - in this case, it'll be (30000/3.14)^0.5: 173

The coilgun/mass driver propulsion option is a good idea. The only downsides are the extremely poor thrust to weight ratio (compare the mass of the magnets, cooling systems, reactor, energy storage and so on, to a simple Nuclear Thermal Rocket) and a much lower amount of heat absorbed per kilogram of hydrogen onboard. 

[shynung]

1 hour ago, MatterBeam said:

The coilgun/mass driver propulsion option is a good idea. The only downsides are the extremely poor thrust to weight ratio (compare the mass of the magnets, cooling systems, reactor, energy storage and so on, to a simple Nuclear Thermal Rocket) and a much lower amount of heat absorbed per kilogram of hydrogen onboard. 

It's possible to compensate the lower TWR by increasing coilgun muzzle velocity. It'd be the ion-drive equivalent for stealth ships. Can also double as defensive weapon/CIWS, so I'd say it's useful to have.

[DDE]

On 8/19/2017 at 7:43 AM, kerbiloid said:

(Or at least its manufacturer states this.)

From the front.

In the X band.

Whereas its JTIDS/MIDS/Link 16 works in the L band.

And http://www.ausairpower.net/APA-2009-06.html

Oops.

Edited November 23, 2017 by DDE

[PB666]

On 8/16/2017 at 5:34 AM, 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.

Good absorbers are also good emitters.  https://en.wikipedia.org/wiki/Thermal_radiation

If it absorbs all the visible light it will have a beautiful infrared signature.

Here's how to do it.

1. Build a reentry ship made of two section, one is to get you to pE which is say 100,000 meter minimum. It has an engine that fires at PE allowing it to escape the the planets orbit and return to interplanetary space, in the fog of re-entry it would be very difficult to probe the ships KE versus altitude. The ship should be fusiform and should point nearly directly at PE as it enters. It should have a diffuse reflectivity and an A of about 0.7 (not shiney).

2. At the rear of the first ship is  a second ship of much lower density with detachable friction brakes that cause this ship to stick into the atmosphere once it detaches (about 140,000 meters) it will have an injection force of 50 a/kg from the first ship and its added drag to weight ration will cause it to reenter. The single person craft has a thin shell, as single person in a minimal suit capable of survival for 20 minutes, a parachute, a light weight dirt bike, a several cans of sealed gasoline (that can be opened on landing), and a shovel. 

Next the reentry will occur over a desert in the mid afternoon period when the that desert is its hemispheres summer (lets say on the border between chad and libya, masking the heat signature of the entering space ship. As the ship slows down the parachute comes out 500 meters above the ground, and once landed the human inhabitant removes the suit, takes 2 days worth of food rations, the gasoline and straps it to the dirt bike.  Using the shovel he buries the heat shield, the parachute, any rcs thrusters used to decelerate during burn, he disassembles the frame of the ship and buries it and finally he buries the shovel with his hands.

Next he travels to the closest body of water, empties the gasoline (or ethanol which can also provide calories) from one can into his tank, he uses a small portion of citruline based soap to clean the container and then filters the water and fills the tank. Having completed this he once again takes off, using hiways or backroads until he reaches a municipality where he can blend in. Within 10 miles of the outskirts he goes off road and then buries the dirt bike and begins to walks. Knowledge of  dump is a good place to find clothes that fit the people in the municipality. He should also have small quantities of platinum or gold which he can see to a Jeweler. (By now knowing the language of the land is a must, and I assume he has plastic surgery to alter his appearance to fit that of the place he landed).

To summarize, absolute stealth is not needed, but low profile and entry somewhere that has low strategic value. Then move from low strategic value area to target areas by stepping into gradually more secure areas. THere are hundreds of objects that burn up every-night, security agencies can't afford to monitor everyone, an ICBM headed down over an uninhabited area (like the explosion that occurred in the South Atlantic in the 70s) are not likely to be of great interest until well after the fact (as in who did this, lets study the issue and find out).

 

 

Edited November 24, 2017 by PB666

[MatterBeam]

1 hour ago, PB666 said:

Good absorbers are also good emitters.  https://en.wikipedia.org/wiki/Thermal_radiation

If it absorbs all the visible light it will have a beautiful infrared signature.

The whole point of the 'hydrogen steamer' is that it uses hydrogen boil off to cool the hull down to temperatures where it emits so little radiation that it is undetectable. Hence, no infrared signature.

[shynung]

1 hour ago, PB666 said:

Good absorbers are also good emitters.  https://en.wikipedia.org/wiki/Thermal_radiation

If it absorbs all the visible light it will have a beautiful infrared signature.

As MatterBeam said, it uses evaporative/phase-change cooling with liquid hydrogen to minimize IR signature. The absorbent coating is there to minimize visibility from active Radar/Lidar sensors.

Also, perhaps 'stealth' is not a completely appropriate word for the hydrogen steamer design. 'Low observable' would fit much better.

Edited November 24, 2017 by shynung

[DDE]

6 hours ago, shynung said:

Also, perhaps 'stealth' is not a completely appropriate word for the hydrogen steamer design. 'Low observable' would fit much better.

It almost always is.

[MatterBeam]

11 hours ago, shynung said:

As MatterBeam said, it uses evaporative/phase-change cooling with liquid hydrogen to minimize IR signature. The absorbent coating is there to minimize visibility from active Radar/Lidar sensors.

Also, perhaps 'stealth' is not a completely appropriate word for the hydrogen steamer design. 'Low observable' would fit much better.

Phase-change cooling seems to me to be more and more effective the closer I look into it. Expect some interesting designs to come from this!

5 hours ago, DDE said:

It almost always is.

Quite right. The distinction is trivial though, when you can only detect these ships at distances of a few km, while combat ranges are measured in thousands of km. In fact, if the engineering constraints make the hydrogen ship a lousy stealth ship than can be detected at 1000km, then it's actually fine as long as your weapon range is 2000km. 

[PB666]

13 hours ago, shynung said:

As MatterBeam said, it uses evaporative/phase-change cooling with liquid hydrogen to minimize IR signature. The absorbent coating is there to minimize visibility from active Radar/Lidar sensors.

Also, perhaps 'stealth' is not a completely appropriate word for the hydrogen steamer design. 'Low observable' would fit much better.

If you saw and asteroid bleeding off hydrogen gas with hydrogen emission spectrum are you going to think 'pitch black comet' or managed stealth system.

Also, to cool that hydrogen to its triple point you need heat radiators. Because of entropy, it takes more energy to hide energy than the energy profile one is attempting to hide.

IOW, not as stealthy as desired.

In coming asteroid travel at up to 30,000 meters per second (even an earth bound escape velocity roid is traveling 11,500 meters per second when it hits the atmosphere. There earth is 6,500,000 meters in radius. if the average approach speed is 5000 meters per second then one earth radius is 1200 seconds. (20 minutes) . This means a fusiform shaped object could avoid detection by have a low profile to the earth to about 2 to 3 hours from earth. A reflective surface would not be detected until it almost reached earth. And if the object entered at the angle of the sun (meaning the suns rays hit from its back side) its reflected light would be difficult to detect at all.

Even if an asteroid is detected with say an hour of impact, predicting were it would land if it had an oblique entry vector would not be easy. So now you are down to the last 15 minutes of flight where a large streak covers the afternoon sky. If this was done in an isolated place with few cell phones or other media. . . . . .it may get noticed but not recognized until days later.

As to the disintegrating asteroid situation. Asteriods are filled with reductants and volatile materials. When they enter the atmosphere the pressure of oxygen at the leading edge increases many fold (Super mach behavior). The nature of super mach is that a pressure density builds up on the object, the boundary layer moves off the object. As we know from Rudolf Diesel's experiment if you compress air 20 times on a reductant it explodes. A asteroid that begins to break up in the lower atmosphere will also explode and the inhabitants in whatever ship inside risk catastrophic damage. This situation can be avoided by separating the lander from the chaff before reaching 100,000 meters.

Edited November 24, 2017 by PB666

[MatterBeam]

40 minutes ago, PB666 said:

If you saw and asteroid bleeding off hydrogen gas with hydrogen emission spectrum are you going to think 'pitch black comet' or managed stealth system.

Also, to cool that hydrogen to its triple point you need heat radiators. Because of entropy, it takes more energy to hide energy than the energy profile one is attempting to hide.

IOW, not as stealthy as desired.

In coming asteroid travel at up to 30,000 meters per second (even an earth bound escape velocity roid is traveling 11,500 meters per second when it hits the atmosphere. There earth is 6,500,000 meters in radius. if the average approach speed is 5000 meters per second then one earth radius is 1200 seconds. (20 minutes) . This means a fusiform shaped object could avoid detection by have a low profile to the earth to about 2 to 3 hours from earth. A reflective surface would not be detected until it almost reached earth. And if the object entered at the angle of the sun (meaning the suns rays hit from its back side) its reflected light would be difficult to detect at all.

Even if an asteroid is detected with say an hour of impact, predicting were it would land if it had an oblique entry vector would not be easy. So now you are down to the last 15 minutes of flight where a large streak covers the afternoon sky. If this was done in an isolated place with few cell phones or other media. . . . . .it may get noticed but not recognized until days later.

As to the disintegrating asteroid situation. Asteriods are filled with reductants and volatile materials. When they enter the atmosphere the pressure of oxygen at the leading edge increases many fold (Super mach behavior). The nature of super mach is that a pressure density builds up on the object, the boundary layer moves off the object. As we know from Rudolf Diesel's experiment if you compress air 20 times on a reductant it explodes. A asteroid that begins to break up in the lower atmosphere will also explode and the inhabitants in whatever ship inside risk catastrophic damage. This situation can be avoided by separating the lander from the chaff before reaching 100,000 meters.

The simple hydrogen steamer just boils off hydrogen. The hydrogen gas evaporates at 22K, and is released into space as a form of open cycle cooling. No heat radiators involved. Good luck spotting a faint puff (tens of grams per second) sitting just above the temperature you are cooling your infrared satellites' instruments to!
Inefficiencies are dealt with by throwing more hydrogen at the problem.

The expansion-cooling hydrogen steamer takes this a step further. Hydrogen boils, and then is fully heated in steps to the temperatures of the components it is cooling. So 22K for the outer hull, 300K for the habitats, 3000K for your nuclear reactor. The resultant 3000K hydrogen gas has absorbed over 60MJ/kg. You then blow it through a nozzle to release it at 20K temperatures and 9km/s+. Imagine it as a regular nuclear-thermal rocket with a high expansion ratio, using preheated hydrogen as propellant. 

The hydrogen/helium steamer is the ultimate low-signature design. A helium heatsink cools down the hull to 3K. This is indistinguishable from the cosmic background temperature and is physically impossible to detect through emissions alone. The helium is then compressed through a heat pump to a 22K temperature so that it can boil off hydrogen and lose the heat it has absorbed. The heat pump increases power consumption. but like everything else, deal with more heat by throwing more hydrogen at the problem. 

The stealth ship is designed not to be detected at all instead of relying on it being mistaken for an asteroid after detection. Submarines today don't try to pass off as unusually large whales, they are just silent. 

The sides of the hydrogen steamer can also be coated with Vantablack. 

I'm not sure how an asteroid hitting Earth is relevant here. Cool, but nothing similar. 

[PB666]

3 hours ago, MatterBeam said:

The simple hydrogen steamer just boils off hydrogen. The hydrogen gas evaporates at 22K, and is released into space as a form of open cycle cooling. No heat radiators involved. Good luck spotting a faint puff (tens of grams per second) sitting just above the temperature you are cooling your infrared satellites' instruments to!
Inefficiencies are dealt with by throwing more hydrogen at the problem.

The expansion-cooling hydrogen steamer takes this a step further. Hydrogen boils, and then is fully heated in steps to the temperatures of the components it is cooling. So 22K for the outer hull, 300K for the habitats, 3000K for your nuclear reactor. The resultant 3000K hydrogen gas has absorbed over 60MJ/kg. You then blow it through a nozzle to release it at 20K temperatures and 9km/s+. Imagine it as a regular nuclear-thermal rocket with a high expansion ratio, using preheated hydrogen as propellant. 

The hydrogen/helium steamer is the ultimate low-signature design. A helium heatsink cools down the hull to 3K. This is indistinguishable from the cosmic background temperature and is physically impossible to detect through emissions alone. The helium is then compressed through a heat pump to a 22K temperature so that it can boil off hydrogen and lose the heat it has absorbed. The heat pump increases power consumption. but like everything else, deal with more heat by throwing more hydrogen at the problem. 

The stealth ship is designed not to be detected at all instead of relying on it being mistaken for an asteroid after detection. Submarines today don't try to pass off as unusually large whales, they are just silent. 

The sides of the hydrogen steamer can also be coated with Vantablack. 

I'm not sure how an asteroid hitting Earth is relevant here. Cool, but nothing similar. 

liquid hydrogen is extremely difficult to store in long space flights, it needs to be collected from around the storage container, condensed to high pressure, actively cooled and then returned as a liquid back to the tank. This requires energy input and produces energy output in the form of radiation. IN addition the glancing of solar wind produce a 10K temperature on the prograde edge that the vanblack would absorb and emit. While there is very very little matter in space, the matter that is there is extremely energetic (kinetic) and temperature is directly related to the energy gained from collisions. An object that is not absorbing solar and discharging hydrogen would have a noticably cold and hot side, this would clearly identify it as a manmade object. In addition the liquid hydrogen recycler would have a radiator surface that identified it on a cold background, also manmade.

The assumption here is that the target race has a great IR telescope (like J Webb) and could pin point the difference between hot and cold. In addition since SC is intersystemic it would be identified as a local abnormality. Rather than making it less likely man-made, the effforts to hide its radiative heat signature would make it more obvious.