Are RBOD spaceships practical or not?

[Spacescifi]

 

Some have read about the infamous RBOD (Ravening Beam Of Death) from project rho (Winchell Chung's site).

 

I just wanted to know what kind of power drain this would put on a spaceship if it had and fired one.

 

Guesses: The more radiator fins the more it can zap. The less it has the less it can zap. Period. Does not matter what is powering it... whether nuclear reactor or antimatter.

 

I have for sometime doubted the practicality of mounting a RBOD on a spaceship, due to the mass needed for cooling likely making the said spaceship slow as molasses.

Meaning it will zap all day until it is ganked by clouds of projectiles, debris, and missiles it has no way of evading.

 

 

Here us a 'quick' refresher:

 

The ever useful Atomic Rockets site has a great section on laser weaponry, but the conclusion is far different than what you seem to be implying. Rather than have a multitude of laser weapons or optical systems, the ultimate aim is to create a Ravening Beam of Death (RBoD) and attack targets from as great a distance as possible.

For practical reasons, this turns out to be one light second (just under the distance from the Earth to the Moon), since you can see the target, aim and make corrections in such a short time frame that the target cannot move an appreciable distance. The massive Free Electron Laser (or actually Xaser, since it is fired in the x-ray frequencies) near the end of the section can vaporize metal, ceramic and carbon in milliseconds at that range, and if you are on an unpowered orbit or on an asteroid, the beam is still lethal at a light minute and dangerous even a light hour away.

Let's take a 10 MW ERC pumped FEL at just above the lead K-edge. This particular wavelength is used because lead is pretty much the heaviest non-radioactive element you can get, and at just above the highest core level absorption for a material you can get total external reflection at grazing angles - so no absorption or heating of a lead grazing incidence mirror. We will use a 1 meter diameter mirror. The Pb K-edge x-ray transition radiates at 1.4E-11 m. This gives us a divergence angle of 1.4E-11 radians. At 1 light second, we get a spot size of 5 mm, and an intensity of 5E11 W/m2.

Looking at the NIST table of x-ray attenuation coefficients, and noting that 1.4E-11 m is a 88 keV photon, we find an attenuation coefficient of about 0.5 cm2/g for iron (we'll use this for steel), 0.15 cm2/g for graphite (we'll use this for high tech carbon materials) and 0.18 cm2/g for borosilicate glass (a very rough approximation for ceramics). Since graphite has a density of 1.7 g/cm3, we get a 1/e falloff distance (attenuation length) of 4 cm. Iron, with a density of 7.9 g/cm3, has an attenuation length of 0.25 cm. Glass, density 2.2 g/cm3, has an attenuation length of 2.5 cm.

At 1 light second, therefore, the beam is depositing 2E12 W/cm3 in iron at the surface and 7E11 W/cm3 at 0.25 cm depth; 1.2E11 W/cm3 in graphite at the surface and 5E10 W/cm3 at 4 cm depth; and 2E11 W/cm3 in glass at the surface and 7E10 W/cm3 at 2.5 cm depth. Using 6E4 J/cm3 to vaporize iron initially at 300 K, we find that iron flashes to vapor within a microsecond to a depth of 0.9 cm. The glass, assumed to take 4.5E4 J/cm3 to vaporize (roughly appropriate for quartz) will flash to vapor within a microsecond to a depth of 4 cm within a microsecond. Graphite, at 1E5 J/cm3 for vaporization, will flash to vapor to a depth of 0.7 cm within a microsecond (the laser performs better if we let it dwell on graphite for a bit longer, we get a vaporization depth of 10 cm after ten microseconds).

Net conclusion - ravening death beam at one light second.

Now lets look at one light minute. The beam is now 30 cm across. This is much deeper than the attenuation length in all cases, so we will just find the radiant intensity and the equilibrium black body temperature of that intensity. We have an area of 7E-2 m2, and an intensity of 1.4E8 W/m2. You need to reach 7000 K before the irradiated surface is radiating as much energy away as heat as it is receiving as coherent x-rays. The boiling point of iron is 3023 K, the boiling point of quartz is 2503 K, and the sublimation temperature of graphite is 3640 K. All of these will be vaporized long before they stop gaining heat. At this range, the iron is subject to 5.6E8 W/cm3 at the surface, the graphite to 3.3E7 W/cm3 at the surface, and the glass to 5.6E7 W/cm3 at the surface. Using the above values for energy of vaporization, we get about 0.1 milliseconds before the iron starts to vaporize, 0.8 milliseconds before the glass starts to vaporize, and 3 milliseconds before the graphite begins to vaporize (because of its long attenuation length, once it begins to sublimate, graphite sublimates rapidly to a deep depth, while you essentially have to remove the iron layer by layer).

Net conclusion - still a ravening death beam at one light minute.

What about at one light hour? The beam is 18 meters across. The equilibrium black body temperature is 900 K. This is well below the melting point of most structural materials. Ten megawatts, however, is a lot of ionizing radiation. Any unhardened vehicle will be radiation killed at these ranges.

Obviously, the ideas of "close, medium and far" ranges have very different meanings in a space war context. The only way to effectively deal with a weapon like that is to have several weapons of similar power in your constellation, or be prepared to fill the sky with tens of thousands of kinetic kill vehicles (referred to in Rocketpunk Manifesto as Soda Cans of Death or SCoDs). With an overwhelming number of targets, the individual laser will eventually not be able to track and kill every target, and of course other factors like the service cycle (how often you might have to stop and cool down the system), or the speed the laser mirror can swivel to track incoming targets reduces the absolute number of targets you can service even with a RBoD.

 

Your thoughts?

Edited July 10, 2021 by Spacescifi

[Shpaget]

11 hours ago, Spacescifi said:

 

I just wanted to know what kind of power drain this would put on a spaceship if it had and fired one.

The quote you posted says 10 MW, so let's go with that number?

[razark]

6 minutes ago, Shpaget said:

The quote you posted says 10 MW, so let's go with that number?

But why?

Let's just make stuff up and use that, instead!

[Scotius]

To sidestep cooling issues: use a planetoid-sized, thermonuclear Casaba Howitzer. It will destroy everything you want gone, and doesn't need any radiators.

[Codraroll]

Let me just nab a few applicable responses out of the previous couple of threads:

On 6/30/2021 at 11:32 PM, Terwin said:

In fiction, the author defines the laws of the universe to suit the story.

In historic fiction or hard sci-fi, most of the changes are social rather than playing around with physical laws.

On the other hand, if the author wants giant transforming humanoid mecha spamming missiles, then they add some sort of change that makes such things practical(reflex technology in the case of Robotech/Macross for example).

If the Author wants fireball spells, dragons, and magic swords, then they add Magic to the mix.

In short, you identify what you want for your story, then choose if you want to just hand-wave it into existence with no explanation, create a cultural or technobabble reason for what you want, or use existing period-appropriate technologies(does not work beyond technologies currently in-use).  Anything else is pretending expertise and foreknowledge that you do not have and which will be transparently stupid to those who know better.  For examples look at the science fiction hall of shame thread.

 

On 7/1/2021 at 11:27 AM, Hannu2 said:

It is completely impossible to say. With you assumptions it is probably true but they seem to be very arbitrarily chosen combination of scifi and modern technology. If you use some other assumptions it may be very feasible (...)

Story can have whatever solution. All them may be criticized against reality but I think it is the whole idea of stories.

 

On 7/4/2021 at 9:37 AM, Scotius said:

Unless you have magical fusion drives, pulling extra power out of subspace via handwavey sorcery

https://www.sarna.net/wiki/DropShip

Then... you can have everything you want and be happy forever.

 

On 7/5/2021 at 12:23 AM, Terwin said:

I have difficulty thinking of a single sci-fi trope that is not at least 50% nonsense, and the vast majority of them are 100% nonsense.

So, yes, that specific trope is also nonsense.

(most of the sci-fi tropes that are less than 75% nonsense have already been made into reality in some form or fashion, such as flip-phones being a real-live alternative to Start Trek TOS flip-open communicators, or submarines and the Nautilus; and thus not really a sci-fi trope anymore)

 

On 7/6/2021 at 7:51 AM, sevenperforce said:

Does your fiction need (a thing) for a plot point? If so, make it so. If not, don’t.

 

That should cover the gist of it. You tend to ask the same type of question over and over, so giving the same type of answer over and over seems only appropriate. 

Edited July 11, 2021 by Codraroll

[DDE]

Well, it's not without historical precedent

 

[magnemoe]

Wow an laser like that would easy make the inner solar system an kill box with interlocking fire zones. 
Note that 10 MW is not much here, aircraft carriers generate 500 MW, typical hard scifi fusion drives even more. 
And for bases you could scale this up to 100 MW and an 10 meter mirror. 
With this I think you could destroy targets around Jupiter from LEO / moon. 

Yes you can have laser duels but they would be over interplanetary distances. 
It however leaves one interesting weapon. The laser bomb. One laser, an turbine running gas oxygen and methane, LOX and LNG tanks, targeting system.
Do an hard burn, drop an spreed of these who then falls free, cold and stealthy. They fall for weeks or months, bleed off some cryogenic to be cold. 
Ones inside light minutes of target they are activated and open laser and target system. use cryogenic to cool the laser and the gas turbine to power it, use the exhaust to random walk. Optional also hot gas thrusters as you probably need more cooling than power.

Or how the Kuiper alliance crippled the UNN battle fleet at L3, no it did not work better than Pearl Harbor for the same reasons. 
 

8 hours ago, DDE said:

Well, it's not without historical precedent

 

Or I want to be an battleship then I grow up. 
Seriously can this thing fire sideways without tipping over. 

Also Norwegian IKEA does not sell artillery, guess this is an US only thing, and you are overcompensating. Something cruiser style is way more good looking.