Beam Weapons

Use House Rules

Beam weapons emit some form of radiation, such as light or energetic particles, which travel in nearly a straight line from the weapon. Typically, the radiation will be concentrated at the target to high inetnsities, causing local heating that can burn or blast away the target material. There may also be other, secondary, effects, such as radiation sickness or flash blindness.

High TL engineering will produce effects from their equipment in ways that we cannot envision today. This may include damaging beams of various sorts, with much higher powers and energies than can be produced by modern devices. However, once the beam has left the device it will propagate through the intervening medium and interact with the target via basic physical processes which are known, and thus the effects of such beams can be predicted. The descriptions here are meant to simulate realistic behavior of beam weapons for campaign settings with a hard science or hard superscience feel. Hard science would be for settings without superscience. Hard superscience allows devices that cannot be explained by modern science (such as force screens or teleportation), but attempts to treat the effects of such technology and their interaction with the environment in a consistent manner (for example - if you can see through a force screen, it obviosuly lets light through so it will not protect against visible light lasers). This will not be appropriate for all game settings - many players and GMs have plenty of fun ignoring the strictures of scientific plausibility. In this case, these rules may not be for you - you may be better served using the GURPS Ultra-Tech blasters that shoot little glowing bolts while making a "pew-pew" sound.

The beam weapons given here are meant to give the GM a variety of choices to equip his setting. The GM should not feel obligated to allow all beam weapons listed that are said to occur at a given TL. For example, a GM running a game with a TL 11 setting may like laser blasters, but feel that ray guns are too powerful, electron beamers are too wierd and fit too specialized a niche, and relativistic plasma beamers include far more radiation effects than he wants to deal with. Another GM may want to run a TL 10 game that is firearms heavy, and thus restricts lasers to heat rays which are primarily mounted on vehicles and spacecraft and eliminates particle beamers altogether.

The varieties of beam weapon described here include

Variable Power Beams

Beam weapons can fire reduced power beams. Changing beam settings takes a ready action (exception - laser heat rays do not have to change beam settings, they can just keep the fire button pressed for a shorter time or sweep the beam around to increase RoF). There are two ways to do this:

Burning Through Armor

Beam weapons are sometimes used to burn through armor over an extended period of time to reach the vulnerable bits inside. To use a beam weapon in this way, the beam needs to be held steady on the same spot. Make an attack roll using the rapid fire rules without any bonus for the number of shots fired. All shots that hit are treated as if corrosive for subsequent shots: 1/5 of the damage is removed from the DR against following shots from the burst. If all shots hit and none are dodged, this reduction to DR will carry over into subsequent turns. A beam weapon using this option cannot be increasing its ROF using the rules for reduced power beams. Beam weapons aimed by a mechanically actuated turret directing a beam pointer telescope and controlled by a fire-control program can automatically hit with all shots in the beam on any successful attack roll, but a successful dodge will avoid all the shots. Living beings are unable to achieve this kind of accuracy. If the GM is using any optional blow-through rules (e.g. High Tech pg. 162, or my house rules), the maximum damage for all cumulative shots is limited by blow through.

In addition to drilling (putting a single hole in something), players may occasionally wish to use a beam weapon for cutting (making a slice through something, for example to cut open a bulkhead or door). If used as a torch for cutting, a beam weapon can cut through
0.35 × dice of damage × ROF centimeters
DR / armor divisor
of material every second. If the cover DR (see below) exceeds DR / armor divisor, use the cover DR in the equation instead (keep track of fractional DR, or else you can get silly results). A successful roll against a relevant skill, such as Forced Entry, Demolitions, or Machinist, will double this cutting rate. Critical success will triple the cutting rate. Putting a beam weapon in cutting mode or taking it out of cutting mode takes a ready action.

It is easy to see that using decreased power variable power beams (see above) results in faster cutting (as long as the weapon is not a heat ray) because the ROF increase overcomes the damage decrease. The main limit on this is that you can quickly get to a point where the decreased depth of focus (see Focused Beams, below) is less than the thickness of the thing you are trying to cut through. If you do not have the depth of focus to get your full armor divisor, use the armor divisor appropriate to the worst focus within the armor slab. Often, you know the DR but not the thickness, in that case you can divide the DR by the DR per cm to get the thickness in cm:
Material DR ⁄ cm Notes
Armor, ablative, TL 9 300/50* Woven
Armor, ablative nanoplas, TL 9 210/35*
Armor, energy cloth, TL 12 250* Woven
Armor, bioplas, TL 10 150/50*
Armor, monocrys, TL 11 250/80* Woven
Armor, nanoweave, TL 10 125/45* Woven
Armor, reflex, TL 9 50/18* Woven
Armor, rigid, TL 9 75
Armor, rigid, TL 10 100
Armor, rigid, TL 11 150
Armor, rigid, TL 12 200
Brick 1
Concrete, reinforced 4
Glass, plate 2
Iron or Bronze 20 Conductive
Leather 2 Combustible
Linen or Hemp 2 Combustible, Woven
Steel, mild 22 Conductive
Steel, armor plate 28 Conductive
Stone 5
Wallboard 0.4 Combustible
Wood 0.4 Combustible
(Note that not all armors are the same density. Even though one may provide more DR/cm, that does not mean it provides more DR per weight.)

Focused Beams

Beam weapon radiation must be delivered to a tight spot in order to be destructive. The beam must be diffuse enough while it is in the weapon to avoid damaging the device itself. To meet both these criteria, beam weapons produce a wide beam that is focused by optical elements such as mirrors and lenses (for lasers) or quadrupole magnets (for particle accelerators) to a point at the target. The beam weapon must sense the distance to the target (for example, using a laser rangefinder) and adjust the focal length of its optics to deliver the concentrated radiation at just the right range. A fundamental fact of physics is that the wider the beam is initially (and the wider the optics that focus it), the tighter the beam can be focused and the farther it can be kept focused. As a result, focusing beam weapons do not look much like projectile firearms. They have no barrel, no long hollow tubes as are needed for pushing a bullet to high velocities; rather they are short and stubby with a wide aperture at the end, looking something like a military grade camcorder with a pistol grip or rifle stock.

Past the distance where beams lose focus, their damage decreases. However, it never really drops to zero. They have no maximum range. Instead, beyond the ½D range, divide the damage by the multiple of the ½D range or any fraction thereof. That is, at ranges between ½D and 2 × ½D, divide the damage by 2; between 2 × ½D and 3 × ½D divide damage by 3; between 3 × ½D and 4 × ½D, divide damage by 4, and so on. In addition, DR within a given range band is multiplied by the same amount (that is, ×2 between ½D and 2 × ½D, and so on). If the damage was originally tight beam burning, it will become normal burning damage after 3 × ½D range.

Any focused beam weapon can be intentionally defocused, to cause the effect of a given range at any range less than that. Setting this option or turning it off takes a ready action.

Because many beam weapons need to be tightly focused at a specific range to cause damage, if they shoot through a barrier or overpenetrate their target (pg. B 398-390, B408), the beam may expand enough as it propagates past its point of focus that anyone behind the barrier or overpenetrated target that is hit will take less damage. If there is no-one close to the point of focus in the path of the beam, it is probably easiest just to ignore over-penetration altogether. However, if you need to figure it out, you can find the depth of focus multiplier for the beam, abbreviated MDoF. Multiply this by the range to the focused target to find the depth of focus, abbreviated DoF. The depth of focus is treated as the half-damage range for anything behind (or in front of) the point of focus as the beam expands out from (or converges on) the focal point. There are four exceptions:

  1. If using decreased power with variable power beams (see above), the DoF is decreased by the same amount as all the other ranges.
  2. At ranges where the beam is focused at more than ½DMDoF, the beam is no longer focused - it is just shooting straight out in a column and has the normal overpenetration rules.
  3. The beam on the victim can never cause an effect of a range shorter than the actual range to the victim.
  4. If the range to the focal point is less than the longest range of maximum effect of the weapon (the ½D range for heat rays, the range for a (3) armor divisor for pulse lasers, the range for a (5) armor divisor for blasters, and the range for a (∞) armor divisor for ray guns), multiply the depth of focus multiplier by the longest range of maximum effect to find the depth of focus. At all ranges within the range of maximum effect, the depth of focus is constant.
  5. Penetrating radiation beams do not have a ½D range. Instead, the DoF is the distance from the focal point where it does full, unrestricted damage. Other effective ranges are adjusted proportionally.
If there is no listed MDoF, the beam is not focused and normal overpenetration rules apply.

Beam weapons, DR, and cover DR

Many beam weapons have significant armor divisors. Some even ignore armor altogether. However, this usually only applies to DR that is due to material strength and toughness. DR due to the sheer mass of matter in the way is unaffected. In GURPS, this is represented by cover DR. This is treated using the rules on overpenetration (pg. B408). A beam's armor divisor will never drop the DR of a barrier or intervening object below that of its cover DR. Living things give a cover DR equal to HP, Unliving things give HP/2 cover DR, and Homogenous objects give HP/4 cover DR. On Earth, air itself gives cover DR of 1 for every 30 meters the beam has to travel, if the air is not transparent to the radiation that makes up the beam. For exotic atmospheres, see the table below. The cover DR for given thicknesses of other common materials are also shown on the table below. If a material is not shown on the table, figure cover DR is that of water times the material's density, rounded to something convenient.
Material Cover DR/cm   Cover DR/m   Notes
Vacuum or Trace Atmosphere neg. neg. possibly up to 1/3000, depending on pressure
Very Thin Atmosphere neg. 1/120 between 1/3000 and 1/60, depending on pressure
Thin Atmosphere neg. 1/45 between 1/60 and 1/40, depending on pressure
Standard Atmosphere neg. 1/30 between 1/40 and 1/25, depending on pressure
Dense Atmosphere neg. 1/25 between 1/25 and 1/20, depending on pressure
Very Dense Atmosphere neg. 1/5 between 1/20 and 1/3, depending on pressure
Superdense Atmosphere neg. 1 can be anything over 1/3, depending on pressure
Water 2/5 40
Wood 1/3 30
Nano-carbon 1/2 50 Commonly used in advanced armors and high-strength materials
Rock, ceramic 1 100
Metal (light) 1 100 Aluminum, beryllium
Metal (common) 3 300 Iron, copper, titanium, manganese, cobalt, nickel, chromium, zinc, silver, molybdenum, or tin
Metal (heavy) 6 600 Lead, gold, platinum-group metals, uranium, and trans-uranics*
* If you ever find yourself needing to shoot through a meter of plutonium, you have bigger things to worry about than hitting what is on the other side.
DR from materials whose strength is not dependent on chemical bonds, such as superscience collapsed matter or superscience force screens, may be much more resistant to highly penetrating beams than matter whose DR comes from strength and toughness due to its chemical bonds. Against these materials, the armor divisor never gets more penetrating than (10).

For armor or shields, the large area and flat geometry give odd results if you try to use HP to find cover DR. On the other hand, figuring out thickness can be a pain. The following method will give reasonable and consistent results:
  1. Find the armor's mass in kg. Call this M.
  2. Add up a factor proportional to the surface area protected by the armor for all hit locations the armor protects. Call this value A. Values of A for typical humans are listed in the table to the right. For odd geometries, multiply surface area in square meters by 25 to get A (M measured in kg) or 55 to get A (M measured in pounds).
  3. Divide M by A. Round down. This is the cover DR provided by the armor.
    DR = M/A
  4. If this is more than the armor's HP/4, use HP/4 instead.
"A" value of body hit locations for a typical adult human
Location Surface Area "A"
(M measured in kg)
Surface Area "A"
(M measured in pounds)
Head 5 10
Torso and Vitals 15 33
Arms 8, or 4 each 18, or 9 each
Hands 2, or 1 each 4, or 2 each
Groin* 5 11
Legs 16, or 8 each 36, or 18 each
Feet 4, or 2 each 8, or 4 each
Whole Body 55 120
Small Shield 3 7
Medium Shield 5 11
Large Shield 10 22
* Assumed to also include the pelvis, to reflect the frequency of hits.
For example, none of the non-powered armor listed in GURPS Ultra-Tech provides any cover DR. A TL 10 heavy battlesuit has a mass of 220 kg, so it would provide cover DR 5 to those inside of it, and cover DR 10 to those on the other side of the armor from the beam (since the beam has to penetrate the front and the back of the armor).

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