Stealthy orbital insertion?

[p1t1o]

15 hours ago, MatterBeam said:

The specific hydrogen steamer design on my blog can stay undetected for decades at a time. 

This is the rub. "Undetected". You mean "Less detectable". The degree to which an object can be made "less detectable" is what is in question, and may not be significant, dependant on sensor technology.

I get how it is supposed to work, but a lot of your hypotheses depend on 100% efficiency here and there, you cant just shunt energy around wherever you want like that. Nobody is saying you cant make a spaceship harder to detect, only that there are hard limits to what you can achieve. No amount of hydrogen cooling will render the skin of your craft literally undetectable. 

For example, you have cooled your reactor with LH2. Now you have very high pressure (because you cant allow it to expand yet) lines running through your ship at 3000K. Those are an internal source of heat now too. You havnt sequestered the heat, just moved it. How do you insulate that plumbing from leaking heat into your ship? More cooling right? 

Your maths all adds up but it illustrates a very simplified approximation. You are using active (energy-consuming) techniques to cool the surface of your ship, collect heat from other parts of your ship and dump it overboard. No part of that is going to be 100% efficient (and worse, there will be feedback loops - you didnt take into account the heat produced by your heavy duty refrigeration equipment.) meaning that the skin of your ship is going to be a lot warmer than merely equalising both sides of an enthalpy equation suggests. 

I dont doubt that an LH2 heatsink can make the skin of your ship nice and cold, but its not going to be as cold as you make out, ergo, not as undetectable as you make out.

And the enormous gas cloud is still the elephant in the room if you ask me.

As far as stealth in space goes though, its probably one of the more convincing regimes I've heard of, you should send it in to the guy at project rho, see if he has anything to say about it.

[ARS]

Does it have to be stealthy? In space you can't treat stealth like here on earth. Space is massive, cold and so empty that anything that produce heat is gonna be shining like a beacon for every sensors to see. If your subject requires life support, then inevitably, it will produce heat. You can stealthily (well, not quite) if you just use drone for spying, attach it behind an asteroid and send it towards the target planet with the burn phase is hidden behind the asteroid while shadowed by nearby star. The asteroid will burn on the atmosphere and impact the surface of the planet. Either your drone ejects from asteroid mid-air as it breaks apart or wait until it impacts and start crawling from smoldering crater. Assuming your drone survived the whole ordeal, then you are good to go (I'm not a hardcore scientist, but I know the stealth in space is pretty much a discussion that goes nowhere because the law of physics and thermodynamic conspire to prevent it, can someone confirm the viability of this method? Will the drone becomes undetectable using this method?) personally, though, the closest that you can get to stealth in space is by disguising as something innocent instead trying to hide something completely. If both sides are able to go to space and wage space warfare, then smuggling a spy inside their cargo ship is much more easier than trying to insert the spy directly

[shynung]

@p1t1o Atomic Rockets already have a section on MatterBeam's design (see my 1st post on this thread). That's where I got the references on that design.

[p1t1o]

12 minutes ago, shynung said:

@p1t1o Atomic Rockets already have a section on MatterBeam's design (see my 1st post on this thread). That's where I got the references on that design.

Yeah, apologies, got the sources mixed up a bit. It still only contains MatterBeams own analysis.

Edited August 17, 2017 by p1t1o

[MatterBeam]

5 hours ago, SinBad said:

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.

This is actually a good point. Anti-ballistic missile systems try to catch their targets during the slow ascent phase (like ABMs in South Korea) or during the terminal phase (over the US). Since project Star Wars (SDI) was never implemented, I don't think anyone has the capability of intercepting re-entry vehicles over the ocean. 

Of course, if you have stealth ships, regular space travel and settlements on other planets, then creating this capability might be an unintended consequence. 

5 hours 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.

I can give more details on how I arrived at certain numbers if you wish.

The proposal should be applicable to the pre-orbital-insertion ("disguising a small spacecraft as an asteroid / comet") part of the scenario being described. The exact design on my blog was a strategic weapon, like a modern submarine. If you only plan to use it for a few days, a much smaller liquid hydrogen reserve, and a smaller overall size, could be used. 

5 hours ago, shynung said:

@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.

Exactly! 

The steamer can stealthily deliver anything from secret agents to nuclear missiles. 

40 minutes ago, p1t1o said:

This is the rub. "Undetected". You mean "Less detectable". The degree to which an object can be made "less detectable" is what is in question, and may not be significant, dependant on sensor technology.

I get how it is supposed to work, but a lot of your hypotheses depend on 100% efficiency here and there, you cant just shunt energy around wherever you want like that. Nobody is saying you cant make a spaceship harder to detect, only that there are hard limits to what you can achieve. No amount of hydrogen cooling will render the skin of your craft literally undetectable. 

For example, you have cooled your reactor with LH2. Now you have very high pressure (because you cant allow it to expand yet) lines running through your ship at 3000K. Those are an internal source of heat now too. You havnt sequestered the heat, just moved it. How do you insulate that plumbing from leaking heat into your ship? More cooling right? 

Your maths all adds up but it illustrates a very simplified approximation. You are using active (energy-consuming) techniques to cool the surface of your ship, collect heat from other parts of your ship and dump it overboard. No part of that is going to be 100% efficient (and worse, there will be feedback loops - you didnt take into account the heat produced by your heavy duty refrigeration equipment.) meaning that the skin of your ship is going to be a lot warmer than merely equalising both sides of an enthalpy equation suggests. 

I dont doubt that an LH2 heatsink can make the skin of your ship nice and cold, but its not going to be as cold as you make out, ergo, not as undetectable as you make out.

And the enormous gas cloud is still the elephant in the room if you ask me.

As far as stealth in space goes though, its probably one of the more convincing regimes I've heard of, you should send it in to the guy at project rho, see if he has anything to say about it.

Sensor technology must still obey physical limits. We are reaching for those physical limits, so no matter how advanced the technology or how sensitive the sensors, we will not be detected. Absolute emissions stealth can be obtained by further cooling the hull to 2.71K. You will be indistinguishable from background radiations. It's just very hard to do and of little benefit compared to 4K or 22K temperatures, since they are so hard to detect already.

What do you mean by assuming 100% efficiency? Which part of the design should be 'inefficient'? Sunlight hits a hull surface. A lossless process. Vantablack absorbs over 99.99% of the sunlight. 0.01% inefficiency? Vantablack conduct heat. A lossless process, only the rate matters. Liquid hydrogen channels at 20K are run through the hull panels. The pumps consume energy? The hydrogen warms to 22K and boils. A lossless process, only the rate matters. The boiling gasses are run through tubes to the exit point. 

That is the core process which allows for stealth, and it is only a ramped-up version of open-cycle cooling. Replace the sunlight by any hot component, and absorb the waste heat of secondary systems (coolant pumps ect.) by simply using more liquid hydrogen.

I agree that there are limits to what can be achieved. The important fact is that these limits are high enough that useful levels of stealth (undetected at 100km for example) can be achieved. 

The example I gave in the previous post had two reactors: one small one to power the craft, another to propel the spaceship. The first reactor operates in a closed loop with something like sodium or heavy water moving heat from the nuclear reactor to a heat exchanger, then steam or CO2 to move heat from the heat exchanger through a turbine (to generate electricity). The final loop emits waste heat at 1300K. Normally, the heat goes through a second heat exchanger that shunts it into a radiator's coolant loop. Instead, we replace the radiator with liquid hydrogen and just throw the boiling gasses in the final expansion chamber. 

The second or propulsive reactor works with all the boiling hydrogen produced by cooling the spaceship's equipment and hull. This hydrogen is used like a propellant. From its current 1300K temperature, it is further heated by a nuclear core to 3000K. A de Laval nozzle takes this high temperature, high pressure, low velocity gas and expands it into a low temperature, low pressure high velocity gas. 

The conduction rate between gaseous hydrogen and the tube carrying it is very low, because the thermal conductivity of hydrogen gas is quite low. Nevertheless, I agree, insulation and a vacuum gas can prevent the hottest hydrogen being handled (1300K from the electric reactor's waste heat loop) from heating the interior. Then, whatever heat leaks back into the ship is dealt with by simply using more liquid hydrogen. Yes, more is always the answer.  

Thankfully, all cooling is done passively, because heat naturally moves out of the 300 to 1300K environments into the 20/22K heatsinks. No heat pumps are needed for this part. This is how cryostats work. The active cooling only comes into play of you want to cool the hull (nothing else) below liquid hydrogen's evaporation temperature. 

As for the 3000K you mention, it is inside the nuclear thermal rocket's core. Because you are matching your reactor output to what the hydrogen can absorb, no heat should leak out of the reactor. If the reactor walls still get hot... more LH2!

22K for a ship's hull is pretty darn cold. It would be radiating away less than 13.2 milliwatts per square meter and is undetectable by sensors not themselves cooled to below 22K. For comparison. back when the Spitzer infrared telescope was running at 5.6K cooling, it had a sensitivity of about 2e-18W/m^2. This means that the 22K hull cannot be detected unless the $720 million telescope was pointed right at it within 46000km for days on end.... which is unlikely to happen because any spaceship would cross that distance in a matter of minutes to hours. A 4K hull would allow for a 915-fold reduction in detection distances.

[p1t1o]

@MatterBeam

I did say this would boil down to the usual arguments for/against stealth-in-space!

Dont get me wrong, its probably the most effective concept for stealth-in-space that I have ever heard, but it has too many holes, and stealth-in-space butts up against too many physical laws for me to think of it as anything other than another increment in detection difficulty. That stealth can be incrementally improved was already a given.

The physics and maths of the cooling you describe, I’ll gladly agree that is a significant increase to stealth, though in reality perhaps not as great as one might wish.

The core tenet still stands – there is no [absolute] stealth in space!

I mean, when you look at it typed out like that, what does “absolute stealth” mean? It means total invisibility, which everybody already knows is impossible - at least in terms of macroscopic amounts of normal matter.

What the core tenet is supposed to mean is something more fundamental – that due to physical law, stealth will always be imperfect and thus open to detection. Even if todays IR detectors approach maximum possible sensitivity, your target will always be radiating and thus in principle at least, open to detection.

And Im still really skeptical about the invisibility of that gas cloud…

 

Anyhoo, as someone else pointed out, we are way off-topic. OP is asking about a stealthy atmospheric re-entry.

[KiwiTyke]

My original post has created a fascinating discussion, and it's all been useful. Suffice to say it has given me much to consider!

I am re-thinking some of the fundamental tenets of the space combat I depict, looking to balance the demands of narrative with my own compulsive need to get the science right. I am also re-assessing some key elements of the way the story unfolds, so that the story makes the most of the science rather than butts up against it.

Thank you all for the input, and keep it coming! Clearly there are some important nuances of how antagonistic societies might interact in space :-)

[wumpus]

I suspect that "stealthy orbital insertion" on the Earth might be unlikely.

My "most stealthy guess" would be connected to a meteor that would burn up in the atmosphere, but the thing would break up and eject a "probe".

Obvious Gotcha: you can't create an orbit by aerobraking that won't quickly decay without an obvious burn.  That includes any means of ejecting the probe as well.

Possibly, it could have a sufficiently eccentric orbit that angling the (conveniently colored) rock in *just* the right way will allow it to act as a solar sail in an extremely eccentric orbit and raise the perigee enough to maintain orbit.  This coloration would be the means for any maneuvers afterward, but they would mainly have to mostly rely on gravity-chaos to avoid tracking.  No idea if you can have a sufficiently discrete solar panel (perhaps disguised with a light rock coating over it) and a nearly "magical KSP" gyroscopic alignment system (you really want to avoid using RCS to desaturate your gyros.  That's a giveaway that you aren't natural).

I'm guessing that in any society where anyone would build/launch such a thing, all bodies orbiting a planet (especially anything inhabited) are assumed non-natural and tracked.  You aren't going to do a stealthy insertion.

[DDE]

19 hours ago, MatterBeam said:

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. 

Can mere expansion really "dump" that much heat without the nozzle being heated up and thus likely shaking the system loose?

[magnemoe]

15 hours ago, shynung said:

@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.

Problem is getting an huge ship into orbit undetected.
You can hide an small pod falling free, or at least show it off as an comet fragment, not an huge ship with powerful engines. 

Easiest solution would be to get smuggled in, pay some freighter captain to take you in, you could parashute out before landing to avoid border control you could even do an moose deorbit if going to space station first. 
Have captain say he dumped some barrel of stuff illegal in the system he found one client tried to get him to transport for the fun factor. 

[Nothalogh]

It seems largely doable, especially coming in on a trajectory over the south pole, into one of the southern oceans.

Be prepared to Thor Heyrdahl your way to land, though 

[shynung]

5 hours ago, magnemoe said:

Problem is getting an huge ship into orbit undetected.
You can hide an small pod falling free, or at least show it off as an comet fragment, not an huge ship with powerful engines. 

I think you missed the point. MatterBeam's ship design constantly boils off liquid hydrogen to keep the ship's hull at very cold temperatures (22 K, 4 K if liquid helium is used instead of hydrogen). This makes the ship very difficult to detect. The ship's propulsion system also releases its exhaust at the same temperature (post-expansion in a large nozzle bell), so the ship can get itself into orbit nearly undetected.

The only way to detect cold-running ships like this is to have the sensors/telescopes be cooled down to the same temperature. This works out in the stealth ships' favor - keeping a sensor cold for a long time requires either enormous stores of liquid hydrogen/helium on that sensor itself or a power-hungry heat pump, which probably needs a fission reactor to power it. This would make cold sensors expensive to build and run, limiting their numbers and their up times. In contrast, a cold-stealth ship only needs enough hydrogen stores to last its mission, can choose to launch when most of the cold sensors are in their down times, avoid known cold sensor positions and scout ships, etc - they have better tactical choices than their opponents.

[p1t1o]

5 hours ago, shynung said:

The only way to detect cold-running ships like this...

Not exactly. IR works well in space, especially at long ranges, but approaching a planet you'd be vulnerable to detection by radar, it'll take more than a coat of radar absorbent material to disguise a large ship.

[shynung]

@p1t1o Of course. Avoiding detection by radar means, in addition to radio-absorbent coating, needing ugly angular shapes like the ol' F-117.

Even so, there are tactical advantages to being detectable at closer ranges (say, Earth-Moon distance) as opposed to being detectable at half a solar system away. You can arrange for multiple stealth ships to attack from different approaches without defenders not knowing where they'll come from until the attack has already begun, for instance. Covert transport of 'important individuals' is also possible, as well as covert mine-laying and other similar 'hush-hush' activities, as long as distance is kept.

[MatterBeam]

11 hours ago, p1t1o said:

Not exactly. IR works well in space, especially at long ranges, but approaching a planet you'd be vulnerable to detection by radar, it'll take more than a coat of radar absorbent material to disguise a large ship.

It may be possible to use carbon nanotubes to absorb wavelengths spanning into the Low Frequency (1Gz) range. Shorter wavelengths (higher frequencies) have been laboratory-demonstrated as being absorbed. Longer wavelengths... I can't find the data, but it might not be possible to use the nanotubes for this purpose. Thankfully, radio waves just bend around objects that are much smaller than their wavelength as if they weren't there. This means all military radio bands cannot be used to detect the spaceship.

[sevenperforce]

12 hours ago, shynung said:

@p1t1o Of course. Avoiding detection by radar means, in addition to radio-absorbent coating, needing ugly angular shapes like the ol' F-117.

Common misconception, actually. Ugly angular shapes are no better at scattering radar signals than curved ones. The shape of the F-117 was due to the fact that computers of the era were not powerful enough to solve the equations for proper geometry of a smooth radar-scattering surface, and so they had to make do with a multiplanar solution.

[MatterBeam]

25 minutes ago, sevenperforce said:

Common misconception, actually. Ugly angular shapes are no better at scattering radar signals than curved ones. The shape of the F-117 was due to the fact that computers of the era were not powerful enough to solve the equations for proper geometry of a smooth radar-scattering surface, and so they had to make do with a multiplanar solution.

As far as I know, the F-117's shape was centered entirely around preventing radars protecting its target from detecting it before it dropped its bomb load and was gone. 

This lead to a shape that was very good at preventing ground-based radio waves from being reflected back to their source. You are correct that the flat panels do nearly nothing to reduce the strength of the signal returned, but are effective at bouncing it off in another direction.

I'm pretty sure the Nighthawk has the radio signature of a brick house if seen from an angle it wasn't designed for, like over the right wing or from directly underneath. 

[sevenperforce]

10 minutes ago, MatterBeam said:

As far as I know, the F-117's shape was centered entirely around preventing radars protecting its target from detecting it before it dropped its bomb load and was gone. 

This lead to a shape that was very good at preventing ground-based radio waves from being reflected back to their source. You are correct that the flat panels do nearly nothing to reduce the strength of the signal returned, but are effective at bouncing it off in another direction.

I'm pretty sure the Nighthawk has the radio signature of a brick house if seen from an angle it wasn't designed for, like over the right wing or from directly underneath. 

From Wikipedia: "The F-117A's faceted shape (made from 2-dimensional flat surfaces) resulted from the limitations of the 1970s-era computer technology used to calculate its radar cross-section. Later supercomputers made it possible for subsequent aircraft like the B-2 bomber to use curved surfaces while maintaining stealth, through the use of far more computational resources to perform the additional calculations."

The B-2 was far more radar-invisible and has completely smooth surfaces. 

[sevenperforce]

Of note -- the F-35B has the radar cross-section of a 2 cm steel marble, if I recall correctly.

[kerbiloid]

(Or at least its manufacturer states this.)

[KSK]

13 hours ago, sevenperforce said:

Common misconception, actually. Ugly angular shapes are no better at scattering radar signals than curved ones. The shape of the F-117 was due to the fact that computers of the era were not powerful enough to solve the equations for proper geometry of a smooth radar-scattering surface, and so they had to make do with a multiplanar solution.

So basically the F-117 is a low poly stealth aircraft?

[Rune]

On 16/8/2017 at 10:40 AM, 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!

So yeah, ignoring all the tangents about stealth in space, this is very feasible just as you envision it, because it ain't stealth in space. At least, not in the usual sense.

This is, in fact, being a rock and surviving reentry. No one would look twice at a rock reentering, if it was small enough. We miss ~10m impacts all the time, because they are over uninhabited areas. I doubt we will have a good enough early alert system in 50 years to catch them. Make it ~5m across to give you some margin, and you can still fit a small spherical capsule inside (Dragon is three and something meters in diameter).

And the 'tech' is dirt simple, the "disguise" of rock would make a pretty good heatshield over your heatshield, until it burned/blew away. Small, cold-gas maneuvering thrusters for small orbit correction could indeed be explained away as comet-like emissions if they are small and far away, if anybody is caring enough to look. During reentry, no one is really looking, so you can pretty much do what you want, as long as it isn't much more (or less) energetic than a typical piece of rock burning up (and there is a wide variety of rocks, so 'typical' is a very loose term). Then pop chutes at the last second, or even better, do a soviet-style parachute jump from a falling capsule, and let it sink in the ocean to hide the evidence.

Of course, the long trip to another planet, especially if it is ballistic all the way, on a tiny armored tin can with a limited thermal budget... yeah, not very comfortable (but some small powersource would be feasible as long as it didn't change the IR emission dramatically, after all it could be an albedo thing). But magic away something like suspended animation, and/or drop it close to the target when no-one is looking (say, form a legitimate freighter doing a legitimate cargo run), and you are set.

 

Rune. But people sure like to discuss 'Stealth in Space', which is in itself an interesting data point if you write sci-fi.

[sevenperforce]

If you want to do a stealth orbital insertion (as opposed to a stealth entry), then you can make a very very small correction burn far away, placing you on an aerobraking trajectory just low enough to aerocapture into an unstable high orbit. Then, any sufficiently low-thrust engines permit any number of correction burns to adjust your orbit to wherever you want it.

[Shpaget]

31 minutes ago, sevenperforce said:

If you want to do a stealth orbital insertion (as opposed to a stealth entry), then you can make a very very small correction burn far away, placing you on an aerobraking trajectory just low enough to aerocapture into an unstable high orbit. Then, any sufficiently low-thrust engines permit any number of correction burns to adjust your orbit to wherever you want it.

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

[Eklykti]

If you are at war with some civilization which is approximately at your technological level, they will most probably try to watch the sky with all sorts of instruments to detect incoming objects. So any sort of big enough target will be tracked from as early as they first detect it, and from this point any non-ballistic behavior (course corrections, speed changes, possibly even attitude/rotation changes) will mark it as suspicious and they will watch you even closer. So you must be a ballistic target starting from the point that they can theoretically detect you. Even approaching from the sun will not guarantee you from detection if they have additional telescopes that are monitoring the sky from Lagrange points. So you must be a rock and behave like a rock up to the point at which you enter the atmosphere, and then hide very quickly in the terrain before they realize where you can be going.