Ammo For Space Combat

[DB]

I'd heartily recommend C.J.Cherryh's science fiction works.  She writes a pretty good tale and her space combat takes into account relativistic effects in I believe quite a realistic way.

140921[/snapback]

 

I concur, and it's exactly the recommendation I made the last time this came up.

 

Where she's particularly strong is in the analysis of the available informaiton - effectively her characters understand world lines and the like.

 

High-c weapons, if they can be produced, and in her universe they can, are crippling to fixed defences. There is no effective defence against something that closely follows the lightwave telling you it's on the way.

 

On the inverse-square law: my calculations indicate that a four meter aperture can theoretically generate a beam with a central lobe just under 50 m wide at 5 light seconds, assuming that the focussing is "perfect" and using 100 nanometer wavelength. That wavelength is a bit shorter than extreme UV. I would hope that by this sort of time we'd have diode lasers with 100% efficiency emitting 1 nanometer wavelength, at the "hard" end of "soft" x-rays, which would give a 50cm wide beam.

 

The latter would be good enough for government work.

 

(The relevant equation is beam width = range * 1.22 * wavelength / aperture size, unless I've forgotten my physics.)

 

Of course, pointing it is a very different and rather difficult problem.

 

David

[Smitty]

As to dodging -- I just really doubt the return on invested energy.  You're travelling through space at 25,000 kph, in a 40,000 kg craft.  How many kgs of fuel will it take to change your trajectory by 1 degree?  I don't know the math, but I am expecting it would be quite a bit.  And ... what difference does it make for you to change your trajectory by only 1 degree?

 

Hmm. If I'm in orbit around Jupiter and you're in orbit around Mars, it could take MONTHS or even YEARS for your "shotgun pellets" to reach me. (the Mars Express mission took seven months at an absolute top speed of over 100,000kph)

 

During that time, even a minute change in my orbit will cause your entire 100km wide field to miss me by thousands of kilometers.

 

 

Consider your challenge.  You and I have equal shooting skills, equal shooting technologies.  I shoot at you with my shotgun.  You try to shoot each and every one of my shotgun pellets from the sky.  Good luck!

 

I don't have to shoot each one out of the sky. I only have to shoot the ones that may hit me. And even then, I don't have to destroy them. I could use a laser to just nudge them out of the way. And at interplanetary distances, I have a long time to do this.

 

 

And the real clincher is that, in space, all I have to do is hit you with the debris anyways.  When we have a closing trajectory of 50,000 kph, or 200,000kph, or perhaps much more, a single BB is likely to do terrible damage to a 40,000 kg space craft.  Split that BB into 16 tiny pieces, and it is STILL just as lethal.  Liquifiy it and it is STILL just as lethal.  Vaporize it and it is STILL just as lethal.  You have no likely means of reducing its mass, only of affecting the organization of its mass.  But it does not need to be coherent.  It only needs to be mass at velocity.

 

Actually, this is not true. Take a look at the design of Transhab's skin.

 

http://www.universetoday.com/html/special/transhab.html

 

It is designed to stop high velocity particles by first breaking them up and then absorbing the fragments.

 

Undoubtably a warship would significantly improve on this capability.

 

16 tiny pieces of BB are NOT as effective as a single projectile when it comes to penetration.

 

For a simple analogy, your 12 guage with buckshot will penetrate far less than your 12 guage firing slugs.

[Guest aevans]

Mk 1:

 

As RL used to say, "Let's run the numbers".

 

Assuming that an opponent could deploy an array of destructive projectiles 100 km in diameter at predicted point of impact, closing at a relative velocity of 25 km/sec. What do we have to do to dodge it? Well, just to make things simple, let's assume that our dodging maneuver is to move 100 km normal to the approach vector of the incoming projectiles. (That should provide plenty of safety margin.) If we only have the delta-v to apply a 10 m/sec (36 km/hr) translation impulse, then we have to detect the incoming projectiles 10,000 seconds prior to impact, at a range of 250,000 km. Guess what -- I think we've got the radar technology to do that right now.

[DavT]

Wow, it's like watching other people do my thinking for me. I'm trying to rewrite the d20 Future space combat system for me and my friends and this stuff is pretty close to gold. We're going with unrealistic engine and armour technology, but I thought I'd at least try to keep the weapons as realistic as possible. I know those two factors completely skew the picture, but it's not fun for people when they can't shoot on a regular basis and expect to live.

 

Just as a note, lasers will go through diffraction in space, which can further limit their potential effective range. However, nailing something 300,000km away a second after you pull the trigger and with a minimal miss chance still has a lot to offer.

[UN-Interested Observer]

If I'm travelling 300,000 km per second towards your system, and you're travelling 300,000 km per second towards my system, would we even see the bullets that hit us?

[Ssnake]

If I'm travelling 300,000 km per second towards your system, and you're travelling 300,000 km per second towards my system, would we even see the bullets that hit us?

Of course you would - if you have X-ray sensors, since incoming light would be frequency-shifted (but still arrive at the speed of light). That's assuming that the "bullets" that we shoot are emitting or reflecting electromagnetic radiation so that you could "see" them at non-relativistic speeds as well.

[Ssnake]

... we have to detect the incoming projectiles 10,000 seconds prior to impact, at a range of 250,000 km. Guess what -- I think we've got the radar technology to do that right now.

Provided

1. that you switched on your radar,
   
2. the projectiles are of a radar reflecting technology
   
3. you know where to look from where the projectiles are coming
   

. Picking up a dispersed cloud of fiberglass particles or plastic pellets at the latest of 250,000km could be a challenge (and don't forget that your spacecraft needs both the energy source to power that radar emitter as well as house the antenna (and shield the astronauts inside your craft).

Edited February 3, 2005 by Ssnake

[Guest aevans]

Provided

1. that you switched on your radar,
    
    
    
    
   
2. the projectiles are of a radar reflecting technology
    
    
    
    
   
3. you know where to look from where the projectiles are coming
   

. Picking up a dispersed cloud of fiberglass particles or plastic pellets at the latest of 250,000km could be a challenge (and don't forget that your spacecraft needs both the energy source to power that radar emitter as well as house the antenna (and shield the astronauts inside your craft).

141052[/snapback]

 

The US Army was bouncing relatively weak radar signals off of the moon and reading their reflections almost sixty years ago. No metalic reflective materials there either. I really don't think people's faith in "stealth" materials is very well placed. Also, any thing made of plastic, if it is detectable, could easily be vaporized by laser fire. To be effective, projectile would almost certainly have to be dense and resistant to energy weapon attack.

 

As for the power source for my radar, what about the power source for your projectile weapon? to have a chance of multiple hits against a likely sized spacecraft, I'd think your projectile pattern would have to have a cross sectiontional dispersion of one projectile every 1,000 or so square meters. Doing the math once again, that's 7.85 million projectiles. Let's say that an effective projectile only has to mass one gram. That's still almost eight tons of mass, for a single shot -- that you have to find the energy to accelerate that mass to 25 km/sec. If you can do that, I think I can find the electrical power to run my radar.

[Josh]

One thing I was thinking about even before this string is the effectiveness of bases in asteroids and planettoids. I remember discussing using solar cells on the moon's surface to power the earch--now that idea seemed hard to work given the fact that the moon could only point at one side of the earth at any given time, plus lunar eclipses. But for closer in planetoids or asteriods, you could use ample sunlight to power eletromagnetic cannon and lasers to your hearts content. Alternatively you could use modern day nuclear power if you could figure out how to sufficiently cool it. Your weapons and your ammunition would be near limitless and you could bury many of your essential systems under a hard surface of meters of rock that would be largely impervious to small KE, laser, and hard radiation. True you suffer from the fact that everyone knows your position, and potentially limited arcs of fire, but you have shielding the likes of which no vessel could approach, unlimited firepower, and the ability to continuously actively sweep as far as your detection apparatus can sweep. Trying to push asteriods into strategic orbits and equiping them with 360 degree weapondry and power source (either heavily redundant solar or nuclear--admittedly cooling is going to be a problem but no more so than for a ship, with more room for redudancy due to lack of weight considerations) seems like a better policy for engaging ships in system than trying to go toe to toe with the equivalent of tissue papper. Whatever range the engagements end up at based on the discussions below, both sides will have similar range limits for a given technology level and both will be equally vulnerable--and due to hte light lag, not only would a ship have to fire first, it would have to destroy its target completely before it could fire back. Hard to do assuming todays telescopes and computers and reaction drives. Nothing can leave the earth without being detected and its safe to say any offworld settlement would be equally observed, and no coating is going to cover up a remotely effective reaction drive regardless of radar transparency.

 

The only way I see a planetary base being vulnerable is if its weapons could not be brought to bear due to some orbital concern (rotation, out of the plane of ecliptic, friendly fire onto a populated planet, etc) or if weapons of truly relativistic speeds could be employed--a fair chuck of the speed of light with a heavy enough projectile and even an asteriod wouldn't be safe, and the incoming projectile will be give less reaction time and far less vulnerable to shoot downs than say a 50 megaton nuke at much slower velocity.

Edited February 3, 2005 by jua

[DF1]

I guess that by the time we actually have deep space combat between ships we will also have ships that have high delta v and high thrust levels. I think the real problem here is to accurately guess what they are fighting over.

 

For your typical sci-fi space combat i.e. two ships slugging it out in deep space for no real reason and assuming that the radiation shielding is so effective that it actually has to be penetrated. I would suggest a X-ray laser at around 0.10 angstrom with a 1 meter aperture. This will theoretically produce a beam with a diameter of approximately one cm at one lightsecond. The aim would be to produce rapid very short, very high energy bursts to maximise penetration and try to kill or disable the opposing ship by a large number of small penetrations as a single one cm hole will probably not affect the ship too much unless it is in a critical component or a crewmember.

[DB]

Of course you would - if you have X-ray sensors, since incoming light would be frequency-shifted (but still arrive at the speed of light). That's assuming that the "bullets" that we shoot are emitting or reflecting electromagnetic radiation so that you could "see" them at non-relativistic speeds as well.

141051[/snapback]

 

If we stipulate that both sides are actually travelling just *below* the speed of light rather than just *above* it, then the ability to "see" the incoming opfor fleet at whatever wavelength they're emitting would likely be moot. You wouldn't have the ability to fit in a reaction in the time between detection and arrival.

 

(It's a nit-pick, but the speed of light isn't 300,000km/s, it's officially defined to be 299792458 m/s exactly.) So I'm picky for less than 0.07%, so sue me

 

 

David

[DB]

As for the power source for my radar, what about the power source for your projectile weapon? to have a chance of multiple hits against a likely sized spacecraft, I'd think your projectile pattern would have to have a cross sectiontional dispersion of one projectile every 1,000 or so square meters. Doing the math once again, that's 7.85 million projectiles. Let's say that an effective projectile only has to mass one gram. That's still almost eight tons of mass, for a single shot -- that you have to find the energy to accelerate that mass to 25 km/sec. If you can do that, I think I can find the electrical power to run my radar.

141128[/snapback]

 

If he has that level of power, he'd be glowing in the IR, or maybe even visible, too. I've touched on this before - in space, heat dissipation is a very serious problem.

 

Running your radar wouldn't be necessary, you'd just have to have a wide aperture IR detector looking for new brown dwarfs

 

David

[DB]

I guess that by the time we actually have deep space combat between ships we will also have ships that have high delta v and high thrust levels. I think the real problem here is to accurately guess what they are fighting over.

 

For your typical sci-fi space combat i.e. two ships slugging it out in deep space for no real reason and assuming that the radiation shielding is so effective that it actually has to be penetrated. I would suggest a X-ray laser at around 0.10 angstrom with a 1 meter aperture. This will theoretically produce a beam with a diameter of approximately one cm at one lightsecond. The aim would be to produce rapid very short, very high energy bursts to maximise penetration and try to kill or disable the opposing ship by a large number of small penetrations as a single one cm hole will probably not affect the ship too much unless it is in a critical component or a crewmember.

141203[/snapback]

 

There has been some discussion here in the past on the damage mechanism of rather less exotic laser wavelengths. I thought at the time that the effect was more analogous to a HESH (HEP) round hitting a target than a melting "death ray" - the flash heating of the armour surface would cause an "explosion" that could induce structural failure. Perhaps a better parallel would be a hammer strike.

 

Incidentally, did you choose that wavelength to make the maths easier, or because it approximates atomic lattice spacing?

 

I believe, anyway, that your ultra short bursts would be dealt with quite effectively by (relatively) thin ablatiive shield layers. You'd punch a bunch of holes in the outer layer(s), and inner layers would mop up the resulting vapour/plasma with little effort.

 

After an engagement, you'd have something like an HMS Pepperpot, but no real internal damage. The same type of shielding would deal with the "buckshot" as well.

 

IIRC, there is some web based evidence that paint flecks have caused ~8mm deep penetrations of the windshield of the space shuttle. These would be expected to be ejecta from commsat boosters, or some such, and one might estimate impact speeds around 7-10km/s

 

Fianlly, my SWAG estimates that a one centimeter hole loses 0.025m3 of air per second at STP, that's about 31 grammes worth. (Assuming no edge effects, you get a column of ejected air 330metres long by one centimetre diameter, that's 0.025 cubic metres...), which is probably negligible, and you might run your warships at slightly lower pressure anyway, thus losing less mass. Nobody, Auric Goldfinger or Alien Resurrection notwithstanding, is going to get sucked out through that sort of hole.

 

David

 

[Edited - wrong Alien movie]

Edited February 3, 2005 by DB

[UN-Interested Observer]

DB, I knew someone would point it out:)

 

Would autonomous craft be essentially landmines, they exist in an area (damn the word area isn't applicable is it?) and attack whatever approaches? Or would AI even be advanced enough to trust it with a weapon and complete control?

 

Could lasers be used to control UAVs?

 

Last question, not so far out, but:

 

Where does body heat go? In space-suits they say there is an air-conditioner, but do those just store up the heat until full? You can't emit heat in space, can you?

[DB]

Last question, not so far out, but:

 

Where does body heat go? In space-suits they say there is an air-conditioner, but do those just store up the heat until full? You can't emit heat in space, can you?

141268[/snapback]

 

Astronauts wear an undergarment that contains a liquid cooling system in thin tubes. Google being my friend, I find out that there is a heat exchanger in the backpack, so presumably there's a radiator surface also.

 

One could use evaporative cooling, but you then permanently lose water.

 

David

[Lentzner]

Did anyone read the "Hypersonic Penetrator" thread in the Armor forum? In a nutshell, there's a point of diminishing returns where a solid penetrator benefits less and less from higher impact speeds. It starts acting like a liquid - going splat instead of thud. Energy transfer effeciency starts to plummet. Small 1g projectiles don't keep getting more and more lethal as a square of their velocity.

 

In spite of this, it would probably turn out that having a light, fast, and menueverable thin-skinned vehicle is more survivable that having a heavily armored and more ponderous vehicle. Thin skins mean little behind armor effects (assuming an inert projectile). You're worse off with armor that can't reliably stop a projectile. Additionally, with all the delicate sensors you would need, armor wouldn't be likely very practical anyway. A mission kill might be fairly easy to get that might leave the ship itself relatively unscathed.

 

The mesh projectile idea is an interesting one, but I think, with all that surface area, it would be too easy to vaporise it with a defensive laser.

 

Matt

[Josh]

Penetration is not necessarily the only goal in a KE colision. Inducing yaw could have very negative effects if taken in conjunction with the main drive still being fired. Also I don't think anyone is going to be using paint flecks as projectiles, and further more even at typical current speeds, if a projectile was fired down the incoming path of a space craft the closing velocities would be such that even failing penetration, the resulting heat and other radiation from the colision could quite well be the kill mechanism. Hard to say without having some better idea of the acceleration of our theoretical warships and rail guns, but we could be bordering on relativistic speeds in the low single percentages of c. Think about something moving at 1% c: thats 3000 km/s. At that speed a kilogram of mass would put the hurt on an aircraft carrier. Admittedly that's insanely fast for any near future discussion, but clearly firepower has a huge advantage of armor in the traditional sense even assuming all important parts of the ship are armored.

 

Similarly, lasering a ship need not have to have a penetrating or smashing effect. It might be well worth while to simply irradiate the ship broadly in any frequency that would cause the hull to heat up. The ship might well be immobilized or have its weapons incapacitated due to its inability to shed heat rapidly enough. This all would be made worse if the ships is dark colored in the frequency of the laser in order to mask it from vissual or IR sensors. I suspect that combat ships will probably have to carry some kind of emergency cryogens to dump over board for brief periods of intense weapon fire or acceleration; perhaps the system could be integrated with the manuvering thrusters so that heat could be offloaded while manuvering. Otherwise it might be directed into the reaction mass of the main drive. Either way, with the engines shut off any ship would have to radiate out a lot of IR that would be fairly omni directional, unless the ship was large enonugh to mask its own radiators when facing one direction. Or perhaps if there was some way to indefinately store a large amount of heat or convert it back to some other storable form of energy with extreme efficiency. Really, the idea of hiding a ship vissually or thermally, let alone from radar, seems impossible to me even given the current level of detection technology.

[DB]

[snip]Also I don't think anyone is going to be using paint flecks as projectiles, and further more even at typical current speeds, if a projectile was fired down the incoming path of a space craft the closing velocities would be such that even failing penetration, the resulting heat and other radiation from the colision could quite well be the kill mechanism. 

[snip]

141296[/snapback]

 

Neither do I. The point of the paint fleck comment was that it shows the inability of small items to penetrate relatively thin amounts of stuff that isn't armour (more or less pyrex glass) even when speeds are extreme by today's standards.

 

More data:

 

Single stage rockets that use chemical propellants have a delta-v capability limited by the specific impulse of their rocket motor and mass ratio. This means that, as the payload capacity becomes vanishingly small, the limiting delta V (for a single stage rocket!) is around 10 km/s

 

For realistic (ie before the viewers turn over to Desperate Housewives), you're probably going to have to use non-missile technology for the main velocity component.

 

David

[Josh]

Neither do I. The point of the paint fleck comment was that it shows the inability of small items to penetrate relatively thin amounts of stuff that isn't armour (more or less pyrex glass) even when speeds are extreme by today's standards.

 

More data:

 

Single stage rockets that use chemical propellants have a delta-v capability limited by the specific impulse of their rocket motor and mass ratio. This means that, as the payload capacity becomes vanishingly small, the limiting delta V (for a single stage rocket!) is around 10 km/s

 

For realistic (ie before the viewers turn over to Desperate Housewives), you're probably going to have to use non-missile technology for the main velocity component.

 

David

141410[/snapback]

 

I was thinking it would have to be fission in any realistic time frame. Fusion seems a pipe dream and anti mater so hard to produce, ignoring how hard it would be to use, that if you could make it in sufficient quantity you'd probably already have wiz bang tech for FTL

[Stevely]

Similarly, lasering a ship need not have to have a penetrating or smashing effect. It might be well worth while to simply irradiate the ship broadly in any frequency that would cause the hull to heat up. The ship might well be immobilized or have its weapons incapacitated due to its inability to shed heat rapidly enough.

141296[/snapback]

 

ISTM that the energy required to do this would be enormous and so the firing ship would have heat problems of its own long before its laser could deliver enough energy to heat up another ship at range. Actually I can't imagine a laser efficient enough to make this a viable proposition. Heating a spot on the enemy hull long enough to burn through it might work, *hand wave focusing problems* but heating the whole hull enough to cause thermal distress wouldn't. IMO of course.

[Stevely]

Did anyone read the "Hypersonic Penetrator" thread in the Armor forum? In a nutshell, there's a point of diminishing returns where a solid penetrator benefits less and less from higher impact speeds. It starts acting like a liquid - going splat instead of thud. Energy transfer effeciency starts to plummet. Small 1g projectiles don't keep getting more and more lethal as a square of their velocity.

 

In spite of this, it would probably turn out that having a light, fast, and menueverable thin-skinned vehicle is more survivable that having a heavily armored and more ponderous vehicle. Thin skins mean little behind armor effects (assuming an inert projectile). You're worse off with armor that can't reliably stop a projectile. Additionally, with all the delicate sensors you would need, armor wouldn't be likely very practical anyway. A mission kill might be fairly easy to get that might leave the ship itself relatively unscathed.

 

The mesh projectile idea is an interesting one, but I think, with all that surface area,  it would be too easy to vaporise it with a defensive laser.

 

Matt

141281[/snapback]

 

That's good stuff.

 

Also (I don't know if this has been mentioned already) lots of armor is very unpractical for a vehicle, absent some kind of extremely powerful and efficient engine, total vehicle mass would be subject to very tight constraints.

[Josh]

ISTM that the energy required to do this would be enormous and so the firing ship would have heat problems of its own long before its laser could deliver enough energy to heat up another ship at range. Actually I can't imagine a laser efficient enough to make this a viable proposition. Heating a spot on the enemy hull long enough to burn through it might work, *hand wave focusing problems* but heating the whole hull enough to cause thermal distress wouldn't. IMO of course.

141611[/snapback]

 

Actually I wasn't thinking in terms of lasers so much as perhaps microwaves, (though I did use the term laser in post, sorry) though agreed, unless the efficiency was much greater than 50% in the entire transfer process it would be a loosing proposition. Unless one vehicle was at more of a disadvantage in terms of heat storage and loss--though a more direct way to get a ship to overhead would be to impact it on its radiators, which couldn't be armored to much degree.

[Lentzner]

Actually I wasn't thinking in terms of lasers so much as perhaps microwaves, (though I did use the term laser in post, sorry) though agreed, unless the efficiency was much greater than 50% in the entire transfer process it would be a loosing proposition. Unless one vehicle was at more of a disadvantage in terms of heat storage and loss--though a more direct way to get a ship to overhead would be to impact it on its radiators, which couldn't be armored to much degree.

141729[/snapback]

 

<mr. picky>

LASER is an acronym actually that stands for: Light Amplification by Stimulated Emission of Radiation. It can be of any wavelength. Microwave lasers are certainly possible and have been considered as anti-personnel weapons among other viable uses. Laser does not imply or require radiation in the visible or near visible spectrum.

</mr. picky>

 

No need to apologize.

 

Regards,

 

Matt

 

Edit: When I said no need to apologize I mean that you had apologized for saying laser when you meant microwaves - which was in fact not incorrect; Not that you needed to apologize to me. After I reread this I was concerned that it might seem like I was acting like a smug bastard which was not my intention.

Edited February 4, 2005 by Lentzner

[UN-Interested Observer]

http://en.wikipedia.org/wiki/Maser

 

IIRC light is a special thing, having the properties of a wave and a particle both. I shouldn't say much, because I'm not thoroughly enough versed to argue pedantics.

[DF1]

DB,

The wavelength was chosen based on beam width at one ls and ease of reflection at grazing angles.

It is possible that the wavelength range will not penetrate common materials enough and produce the hammer strike you mentioned. The specific wavelength has to be chosen based on how it will penetrate possible materials it will encounter. An absorption of 50% in 5 to 10 cm of aluminium would probably be a good benchmark for lasers with bursts in the 1-10 MJ range. Unless the aim is to irradiate the crew, but then we are talking about a different concept.

 

Jua,

The heat problem could at least be partialy solved with using energy converters to power your laser and beam away your heat if the laser is reasonably effective.

I do not think that astroids will be a solution as they will be comperativly hard to move into orbits. Much easier to saturate the area with satelites instead of using your lift to move a single astroid.

 

Antimatter drives might not be as far off as you think. While pure antimatter drives will require alot of antimatter, various antimatter fusion/fission hybrid drives requires far less amounts of antimatter. Antimatter Initiated Microfusion drives (AIM) and Antimatter Catalyzed Micro Fission/Fusion drives (ACMF) requires no more antimatter than we can produce with current technology.