Lasers

[Carlobrand]

Queries:

The CT bulkhead, by multiple CT sources, takes 1000 points of damage to cut a man-size hole in (which I interpret as a roughly circular hole of 2-3 feet diameter). The CT room partition takes 100 points to do the same thing. The CT bulkhead takes 100 points to penetrate, putting a hole in it that will equalize pressure on both sides.

The CT laser rifle (TL9, presumably) does 5d6 damage: average about 18 points, max about 30. 4 to 6 hits from the rifle will poke a hole in the bulkhead or cut a man-size hole in the partition.

(Striker elaborates that the laser has a penetration of 9 at effective range: 2.25 cm of steel. Implies 3 hits from the laser should penetrate 6-7 cm of steel. Now, mixing Striker and CT doesn't always work well - Striker introduces tank-rated ship hulls - but it's the best we got. But we'll put that to the side for now.)

Question is this:
You aren't going to penetrate a man's armor by applying a laser beam over several seconds. You aren't going to cut a man-size hole in a partition by punching 4 to 6 quarter-second-or-less laser pulses through it. Is it possible that the laser has an adjuster setting that allows it to be used as tool as well as weapon, that the duration of the beam can be lengthened so that the same energy consumption gives a weaker, longer-duration beam for cutting through walls?

 

[BlackBat242]

No, that would not be practical.

The key issues here are heat and energy, both of which also affect weight of the weapon/tool.

Producing the laser beam requires energy... and beams transfer that energy less efficiently than pulses. This is because as the material vaporizes it creates a vapor cloud that continues to absorb energy from the beam, reducing the amount of energy that reaches the un-vaporized metal. The vapor continues to heat up... eventually reaching a "superheat" plasma state, which creates "blowback" or "flash-out", which damages surrounding material and/or personnel. Pulse lasers give the vapor time to dissipate before the next pulse, thus delivering more energy to intact metal. And then there is the fact that there is actually a shock-wave created in the material when the beam/pulse first hits that also weakens the metal... a pulse-laser produces multiple shock-waves while a beam creates only one.

Therefore, a pulse-laser requires less energy to penetrate a given material than a beam-laser... and given the self-contained nature of the Traveller laser-weapon/backpack-powerpack combination, efficiency is highly important. The more energy is needed the heavier (and larger) the powerpack needs to be.


Generating the laser pulse/beam generates heat within the weapon/tool... so the weapon/tool can easily overheat if used for too long a time without allowing the device to cool. The more heat is generated the more material is needed in the "business end" of the weapon/tool to absorb and/or dissipate the heat, which adds more weight.



A laser welder/cutter can be designed completely differently from a weapon... the power can be provided by a power-lead/umbilical attached to a power source in the ship/vehicle itself, eliminating the backpack-powerpack, and vastly increasing the amount of power available.

A combination tool-cooling/vapor-dissipating feature can also be incorporated... the power-lead/umbilical can include a hose supplying an inert gas (such as nitrogen) which passes through the tool to carry off heat, reducing the amount of material needed in the "business end" of the tool to absorb and/or dissipate the heat.

The inert gas can then be directed out the "business end" of the tool at the material being welded/cut, to blow the vaporized material away. This eliminates the possibility of "plasma blowback" by preventing superheating of the vapor... and since you do NOT want shockwaves in something you are doing precision welding/cutting on, the "low-shock" beam is preferred for these applications. All of this allows a continuous beam to be very efficient.


You could install blowers with a directional exhaust nozzle on a pulse-laser weapon to both cool the device and deal with the vapor issues (to allow a long-pulse or even beam operation)... but this adds weight to your "man-carried" weapon, and...


Considering all this, the beam-laser tool can be light and efficient... but is both designed and functions in a different manner than a laser weapon.

 

[Carlobrand]

Okay, but clearly the game lasers are doing something that the lasers you describe cannot. How do we rationalize a weapons laser being used as a cutter? Is there any way to justify this, or do we just say it's magic-tech?

 

[HG_B]

Okay, but clearly the game lasers are doing something that the lasers you describe cannot. How do we rationalize a weapons laser being used as a cutter? Is there any way to justify this, or do we just say it's magic-tech?

I just don't allow it. You can poke holes (possibly) but not slice.

 

[Carlobrand]

I just don't allow it. You can poke holes (possibly) but not slice.

That presents a dilemma. In essence, we are not allowing something that the game repeatedly allows. That's not a new thing - we all do our own IMTU things to correct for this or that odd bit. I for one am really, really bothered by sandcasters (with respect to lasers). However, I was hoping to avoid going that route.

 

[HG_B]

That presents a dilemma. In essence, we are not allowing something that the game repeatedly allows.

I haven't allowed it since '78. It is no biggie. Most players will understand when explained to them. Plus, I have had it come up only 2-3 times.

 

[atpollard]

The ability of a laser to punch/cut a hole is based on the energy applied and the area over which it is applied and the focal distance from the weapon to the target.

A laser weapon that does 3D6 at 300 meters might spread that beam over about 1 square centimeter for a fraction of a second (say 1/100 of a second). The spread of the beam is inverse to the square of the range. At 3 meters, the spread is only 1/100 of a centimeter, the area 0.0001 square centimeters.

Over that area at that range, the laser will penetrate armor with the effectiveness of a 30,000 D 6 laser striking 1 square centimeter at 300 meters range. So what if each pulse could be adjusted to offer 100 micro pulses spread out over a full second rather than 1/100 of a second. At 300 meters, that would be a wimpy 0.03 D 6 weapon with 100 bursts. However at 3 meters, that is equivalent penetration to 300D6 times 100 pulses.

As a weapon against a person, 100 holes at a hundredth of a centimeter (micrometer) in diameter is ineffective. (Imagine trying to stab someone to death with the smallest bore needle available.) On the other hand, moving even closer and cutting a 1 micrometer slice through a wall from less than a meter, might be handy as a tool.

[Most of this is extrapolated from FF&S - TNE - and only as accurate in the details as my leaky memory ... But the CONCEPT behind it is sound and at least technically plausible.]

 

[Carlobrand]

Okay, let's play with the lasers a bit more. The Traveller ship laser is a 250 megawatt beam or pulse laser. In other words, it puts out 250 million joules per second - assuming it's on for a second. Striker seems to think so, so we'll go with that for the moment. Might be less, we don't know how much is lost in inefficiency, but it can't be too much less or the laser unit itself is at risk of taking damage.

I'm going to say, completely arbitrarily and to make the math a bit simpler, that it's a 10^15 Hz laser. Puts it in the UV range, if I have it right. Physics says the energy of a photon of that frequency is the frequency times Planck's Constant. Means a photon in that frequency is 6.6x10^-19 joules. Means a 1-second pulse out of the ship laser's putting out, umm, 3.8x10^26 photons. (ballpark, assuming close to 100% efficiency)

Here's the question: can you backtrack from that figure to calculate how wide the emitter must be? What variables are involved that woud confound that calculation?

If I kick the frequency up to 10^16, is it better at penetrating that fog of debris the initial laser energy's producing?

Striker says a ship laser can punch through 625 cm of steel. We're assuming we're dealing with a 1 second blast, 'cause later it talks about pulse lasers and batteries with storage measured in megawatt-seconds. There's some problems with that, but we'll put that to the side for the moment and say the beam laser's delivering pretty close to 250 million joules on a hit. Iron has a heat of vaporisation of 340 thousand joules per mole. That laser blast could optimally vaporize - umm - 735 moles of iron? Iron has a molar weight of 55.845 grams per mole, and a density of 7.87 grams per cubic centimeter. The blast could optimally vaporize - 5215 cc of iron? Steel's supposed to be hardier, takes about 10% more heat to vaporize if the somewhat obscure and questionably reliable source I have is any good. So, maybe 4700 cc steel? Means the laser hit's vaporizing a column 625 cm deep and maybe 3 cm in diameter?

Many many unknowns and other considerations, I know, but is that a reasonable upper end?

 

[HG_B]

I'm going to say, completely arbitrarily and to make the math a bit simpler, that it's a 10^15 Hz laser. Puts it in the UV range, if I have it right.

At those power levels you will go with a FEL as it is tunable.

 

[tjoneslo]

Most of the descriptions of weaponized lasers I've ever seen involve the laser generating a sequence (say 10-20) very short (order of 10 femtoseconds) pulses of laser light separated by a few milliseconds. This gives the laser a "drilling" action, but allows time for the plasma generated by the impacting laser pulse to clear.

So, laser pulse 1 arrives on target and vaporizes some small amount of material. In the common vernacular, a superheated quantity of material in a confined space is called an explosion. So depending upon the material you'll get a different kind of explosion happening at the impact site.

So If I wanted to have a laser gun which would also be used as a cutting tool (rare but possible for a boarding weapon), I'd add a hardwired specialized computer with a spectral analysis sensor. And a switch for "safe", "kill things" or "drill".

In drill mode, the laser, under the control of the computer, would be sending out fewer short pulses (1 to 3), and using the analysis sensor to determine if the set punched a hole in the wall. It then waits for the human operator to swing the weapons targeting point by the width of the beam (a few milliseconds), and repeats the process.

This won't work at more than a few dozen meters, but as a boarding weapon, it doesn't need to.

 

[atpollard]

Here's the question: can you backtrack from that figure to calculate how wide the emitter must be? What variables are involved that would confound that calculation?

Just FYI:
FF&S identifies Ship's Lasers as being Extreme X-Ray (1 angstrom), with a 20% efficiency and most of the remaining energy going into the 'magic tech' of Grav Focusing.
Grav focusing makes the 'effective focal array diameter' roughly* equal to the 'Actual Diameter' squared. A 1 meter Grav Focused Array would be equivalent to a 360 meter optical array. Under FF&S this 1 meter array would focus its beam on an area of 1 square centimeter at an effective range of 36 million kilometers.

A TL 15 non-grav focusing laser, for comparison, would be 85% efficient and a 1 meter diameter array would focus the Extreme X-ray beam on 1 square centimeter at an effective range of 100 thousand kilometers.

A TL 15 non-grav focusing small arms laser with a 2.5 centimeter diameter array would focus the Extreme UV/Soft X-ray (100 angstrom) beam on 1 square centimeter at an effective range of 25 kilometers (in vacuum) or 250 meters in standard atmosphere.

I was just offering this as a potential 'additional explanation' to add to the Striker data, use or ignore as appropriate.


* I say 'roughly' because no calculation in FF&S is that simple ... for a heavy x-ray laser, first you convert the diameter in meters to decimeters; then you multiply it by x4 at TL 13, x5 at TL 14 and x6 at TL 15; then you square the value; then you convert the decimeters back to meters; now you have the effective diameter with grav focusing.