Serious Scientific Answers to Absurd Hypothetical questions

[DDE]

56 minutes ago, p1t1o said:

Even a knife would be useful in a hand-to-hand between two astronauts. Although now that I think about it I think a mace would be more effective (spacesuits ought to be highly resistant to blades, due to their durable construction).

Aramid fibre is not particularly resistant to stabbing, and there have been glove pucture cases on EVAs.

[ARS]

1 hour ago, p1t1o said:

Can we use ICBMs or does it need to be able to fit into a briefcase? Or a small vehicle? Space Shuttle?

What is the target? Is it armoured/hardened? Is it a swarm of small targets? What are its maneuvering capabilities.

The size of any of those 3 types of weapons (bullets includes machine guns, gatling, cannons, autocannons, railguns) would be roughly around current day weapons for fighter aircraft, aka attachable to one man fighter. The target itself is (assumed to be) a swarm of space based drones (soccer ball- sized spherical ball with omnidirectional RCS thrusters on 6 sides, able to make sudden movement on one direction and coordinated movement) the target's protection is a simple ball shaped titanium frame around 10mm thick around it

[p1t1o]

59 minutes ago, DDE said:

Aramid fibre is not particularly resistant to stabbing, and there have been glove pucture cases on EVAs.

Are not "stab vests" made of aramid?

*google*google*google*

Turns out you can make stab vest and bullet proof vest from aramid, but one is not necessarily the other. Apparently depending on the construction, properties can vary quite wildly. Learn something new every day

 

25 minutes ago, ARS said:

The size of any of those 3 types of weapons (bullets includes machine guns, gatling, cannons, autocannons, railguns) would be roughly around current day weapons for fighter aircraft, aka attachable to one man fighter. The target itself is (assumed to be) a swarm of space based drones (soccer ball- sized spherical ball with omnidirectional RCS thrusters on 6 sides, able to make sudden movement on one direction and coordinated movement) the target's protection is a simple ball shaped titanium frame around 10mm thick around it

 

So, generally speaking, a one-man craft of some kind, vs. a swarm of smaller targets? With a tough but not impregnable shell?

Sounds like a job for a standard Phalanx CIWS - they use tungsten discarding-sabot penetrators and have a proven ability to intercept targets closing at speeds around 1km/s. They are supposed to be good at "swarm" attacks, but this ability is debatable and has not been tested. We can assume however, that if you are building a spacecraft that we can up-rate it with some off-the-shelf sensors and software to better deal with longer ranges, different ballistics, easier sensor environment (no sea clutter) and possibly higher closing speeds.

There are similar weapons which use this kind of ammunition as well (those are little tungsten blocks):

But Im not sure how effective those fragments will be against 1cm of titanium, but for small, fast, agile targets its a pretty neat solution. The round can fragment at a chosen distance/time so you can set up "clouds" of defensive projectiles.

 

Going to be hard to hit a maneuvering target that small with a missile or larger gun.

Unless of course you want to just lob a nuke in their general direction:

https://en.wikipedia.org/wiki/AGM-69_SRAM

There's loads of calculations you can try to see how big a nuke you need to kill something through 1cm of titanium, or how far away it can be, I dont have time at the moment but you can have a browse of project rho for lots of data + examples.

 

[DerekL1963]

3 hours ago, DDE said:

Lasers still suffer from target coupling - it’s more efficient to punch through targets than to melt them

The damage mechanism from big lasers isn't melting.  It's shock - the laser vaporizes the surface of the target, and the rapid expansion of the vaporized material is the equivalent to setting off a high explosive charge.

[A Soviet Tank]

Would it be possible to make a gun that could shoot a shell to the moon, with our current technology? If not, could we ever do it? 

I don't mean a railgun or anything, just a normal gun. 

[sevenperforce]

1 hour ago, A Soviet Tank said:

Would it be possible to make a gun that could shoot a shell to the moon, with our current technology? If not, could we ever do it? 

I don't mean a railgun or anything, just a normal gun. 

Limiting factor on the muzzle velocity of a normal combustion gun is the pressure wave pushing the projectile up the barrel. In a gun, the highest pressure and acceleration are at the very moment of combustion; both drop immediately.

[DDE]

2 hours ago, A Soviet Tank said:

just a normal gun

How much deviation from "normal" are you willing to tolerate? Light gas guns are limited by speed of sound in hydrogen, so their muzzle velocity is somewhere in the range of 6 km/s. The Voitenko compressor can achieve a full 67 km/s, but is blown up in the process. At which point... what would you say to a CASABA-HOWITZER?

[A Soviet Tank]

55 minutes ago, DDE said:

How much deviation from "normal" are you willing to tolerate? Light gas guns are limited by speed of sound in hydrogen, so their muzzle velocity is somewhere in the range of 6 km/s. The Voitenko compressor can achieve a full 67 km/s, but is blown up in the process. At which point... what would you say to a CASABA-HOWITZER?

Snippy snip snip

That's normal enough for me. I didn't even know that that Voitenko compressor (holy crap 67km/s!?) existed, but I guess gas operated guns aren't too nonsenseical. 

[ARS]

I get it that "stealth in space" is pretty much impossible. But with so many space debris out there, is it possible to "disguise" a satellite as a debris/ defunct satellite? (Cut all systems, go into passive sensor and keep watch of surrounding area)

[MatterBeam]

11 hours ago, DDE said:

Missiles, full stop. Effectively unlimited range, higher impact velocity than regular guns. Lasers still suffer from target coupling - it’s more efficient to punch through targets than to melt them - and they don’t actually have perfect accuracy.

Thus far, lasers haven’t been tested in orbit. There has been a 23 mm cannon trial (R-23M in Kartech-1 gun pod on Almaz stations) and de facto missile strikes in the form of air-to-space missiles (fired by F-15s and possibly MiG-31Ds) and coorbiting kamikadze satellites (various versions of IS, operational since 1967, over two dozen intercepts).

Just to nitpick, most missiles, even ASAT weapons, have very limited maneuverability once it is in space. It is fine if the targets (satellites) follow predictable trajectories and do not move, but in a warfare situation, any satellite with thrusters will be able to go toe-to-toe against an ASAT weapon.

What you need is anti-ballistic missile warheads designed to intercept MIRVs during their above-atmosphere coating phase. Those have powerful omnidirectional thrusters.

 

9 minutes ago, ARS said:

I get it that "stealth in space" is pretty much impossible. But with so many space debris out there, is it possible to "disguise" a satellite as a debris/ defunct satellite? (Cut all systems, go into passive sensor and keep watch of surrounding area)

You can coat the spacecraft in radar absorbent materials, add a layer of non-reflective protection such as VantaBlack, and then circulate tube of liquid helium against the inside of your hull. Too cold to emit IR, invisible to radar, doesn't reflect sunlight. Decent enough protection, no? 

Edited November 15, 2017 by MatterBeam

[ARS]

17 minutes ago, MatterBeam said:

You can coat the spacecraft in radar absorbent materials, add a layer of non-reflective protection such as VantaBlack, and then circulate tube of liquid helium against the inside of your hull. Too cold to emit IR, invisible to radar, doesn't reflect sunlight. Decent enough protection, no? 

Sounds like a good idea

[DerekL1963]

6 hours ago, MatterBeam said:

Just to nitpick, most missiles, even ASAT weapons, have very limited maneuverability once it is in space. It is fine if the targets (satellites) follow predictable trajectories and do not move, but in a warfare situation, any satellite with thrusters will be able to go toe-to-toe against an ASAT weapon.

Only if someone is observing the satellite and its vicinity and can command it to execute an avoidance burn when an incoming ASAT is detected.  Or, to put it more simply, it's much more complex than just outfitting the satellite with thrusters.  They're just the end effectors of a much larger and more complex system.

[Oiff]

6 hours ago, MatterBeam said:

You can coat the spacecraft in radar absorbent materials, add a layer of non-reflective protection such as VantaBlack, and then circulate tube of liquid helium against the inside of your hull. Too cold to emit IR, invisible to radar, doesn't reflect sunlight. Decent enough protection, no? 

It would need to stay on the night side of the planet or the black silhouette would stand out to anything in a higher orbit or it could even end up between the Earth and the Sun and be picked up by any amateur with a solar telescope.

Staying in the Earth’s shadow would also help with the issue of solar heating.  Vantablack works by absorbing the vast majority of UV, visible and IR light that hits it, doing this in space would require you to be able to dissipate the heat produced by absorbing all those photons.  Heat is lost through radiation in space, if you cover your sat in a material that absorbs this you will have a problem.

I think a small patch would need to be exposed to radiate heat through, an exhaust if you will.  This could be directed to reduce likelihood of detection but would still prove to be a weak spot in the camouflage.

An alternative would be to store the heat chemically in batteries or fuel cells.  This would have the added bonus of providing a power supply (obviously the heat still needs to be dealt with if the power is used).

 

Vantablack reflects longer wavelengths so it would be susceptible to discovery by a search at these wavelengths. Since the Vantablack is on top of the RADAR absorbent material it will reflect the radio waves before they reach it.  This is probably the biggest weak point as there are already ground based RADAR installations that scour space for unknown targets.

 

 

Possibly the best camouflage would be to hide in plain sight, attach to a commercial satellite and it will be unlikely that anyone would give it a second glance.

[p1t1o]

Oh not stealth in space again! 

***

2 hours ago, Oiff said:

Possibly the best camouflage would be to hide in plain sight, attach to a commercial satellite and it will be unlikely that anyone would give it a second glance.

Probably one of the more effective approaches.

***

Its an oft-ignored point that your enemy could well have sensors all over the place, looking for you from all sorts of angles, including (for targets in LEO) with the Earth as a backdrop where a cold, black body will stand out. This also makes the tactic of "I'll just radiate in this direction only" less effective.

***

TL;DR - like all stealth, its a matter of degrees and context. It is always possible to make something hard-ER to see, but  true "invisibility" is next to impossible in all situations. A radar absorbent coating does not make you invisible to radar!

 

[Oiff]

True enough, RADAR absorbent coatings tend to be too efficient in that they create a black hole in the noise pattern that is easy to spot if you know what you are looking for.  Even if they are exactly right there will be an odd gap between the target and the next relfection behind it.

This is how ships spot icebergs with RADAR, the berg itself has a very low RADAR profile, lower than the water around it.  You just turn the gain right up and look for the gaps in the noise.

[p1t1o]

1 hour ago, Oiff said:

True enough, RADAR absorbent coatings tend to be too efficient in that they create a black hole in the noise pattern that is easy to spot if you know what you are looking for.  Even if they are exactly right there will be an odd gap between the target and the next relfection behind it.

This is how ships spot icebergs with RADAR, the berg itself has a very low RADAR profile, lower than the water around it.  You just turn the gain right up and look for the gaps in the noise.

Im going to go ahead and politely challenge that idea, any supporting data?

I can accept that it can work that way for icebergs, but I have never heard of RAM being "too efficient" and RAM-protected vehicles being visible because of the "hole" they leave in returns. It doesnt even make sense for aircraft or spacecraft as there is no reflective background. I even have a hard time believing that RAM protected ships, with a sea background, do not still have a significant radar return.

Radar stealth has never been about invisibility, it has always been about merely reducing the range at which the return you give is reasonably detectable.

Why do stealth craft have "stealth shapes"? Because they still reflect radar and they want to reflect as much of it as possible away from the emitting sensor.

Edited November 15, 2017 by p1t1o

[ARS]

1 hour ago, p1t1o said:

Radar stealth has never been about invisibility, it has always been about merely reducing the range at which the return you give is reasonably detectable.

Why do stealth craft have "stealth shapes"? Because they still reflect radar and they want to reflect as much of it as possible away from the emitting sensor.

True enough. Most countermeasure against radar detection isn't about making the target invisible, it's about minimizing the return signal towards the radar. Stealth aircraft often have weird or unnatural shape in order to create optimum deflection angle against incoming radar signal

[Oiff]

Yes you need a background to be able to paint a shadow, this is an inherent weakness in stealth technology.

For ships the background is the sea, the returns that you would expect to get from the water the ship displaces and the water directly behind it are being either absorbed or reflected in other directions by the ship.  This results in a shadow.

 

For aircraft this is more difficult, it may be possible if there are clouds behind the aircraft though or if you can get above them then you will see a shadow against the ground reflections.

 

Spacecraft will be even more difficult than aircraft, you would almost certainly have to get above it to take advantage of this effect unless it is moving slowly (relatively) enough for you to be able to detect it passively against the background.

[wumpus]

2 hours ago, Oiff said:

Spacecraft will be even more difficult than aircraft, you would almost certainly have to get above it to take advantage of this effect unless it is moving slowly (relatively) enough for you to be able to detect it passively against the background.

Isn't this a standard procedure to detect the size of celestial bodies (comets, asteroids, and the like)?  I seem  to remember Scott Manley describing the procedure (for either our recent celestial visitor or perhaps New Horizon's new target).  You basically measure how long they occlude the known stars behind them and  work out the size of the object.

This makes spacecraft a bit harder to stay "dark".  While they might be hard to detect, the "background" is almost completely known.

[Oiff]

Indeed, although spacecraft are very small so detecting a dark spot against the background is very difficult.

Satellites are frequently caught in Astro images taken from the ground, this is the sunlight reflected off the sat though rather than it being picked out of the background.  If the sat is moving fast across the star field I don’t know how you would pick it up in an image as it would be a slightly darker streak across the image rather than the more obvious bright streak.

If you ever see an Iridium flare you will notice that they can suddenly vanish, that is the satellite going into the Earths shadow.

[p1t1o]

17 minutes ago, Oiff said:

 If the sat is moving fast across the star field I don’t know how you would pick it up in an image as it would be a slightly darker streak across the image rather than the more obvious bright streak.

If it occludes a bright object, like a star, it will be as obvious to a telescope as an iridium flare.

You wont get much data from a single occlusion, but what you would do, if you were so inclined, is look very intensely at that area with all of your other sensors. 

[DAL59]

10 hours ago, p1t1o said:

Oh not stealth in space again! 

TANSIS.  

You guys should read the project rho page on it.  

[MatterBeam]

29 minutes ago, DAL59 said:

TANSIS.  

You guys should read the project rho page on it.  

What is TANSIS?

[DAL59]

There Ain't No Stealth/Sound In Space

[ARS]

No no no, this isn't about making something stealthed in space. It's about disguising an object to look like an orbital debris. Stealth is pretty much impossible in space, but a disguise COULD work, right? Making something invisible in space to avoid detection is not gonna work, but making something appear like stuff people won't take a second glance is still possible. The main question is, could we use a satellite with a passive sensor to monitor the surrounding area (no active usage at all) while it's disguised as a debris or defunct satellite?