Subject: P-Beams
From: "David L. Pulver" <[email-masked]>
Date: Wed, 17 Dec 1997 21:53:44 -0500 (EST)

A correspondant pointed out GURPS particle beams are total
garbage in space compared to lasers -- they don't have the range
to do anything. This contradicts SPACE (and may mess up GURPS
TRAVELLER). It's also not what I intended. To test this, I
designed a series of weapons with similar weights and power
outputs, and I agree there is a significant problem with space
ranges.

Suggested Errata to GURPS MECHA AND GURPS VEHICLES
     P. VE126, p. VE187, p. ME 51. 
     Neutral particle beam and antiparticle beam get 200 times
their range in vacuum, not 10 times their range.
     Disruptor, flamer, fusion gun get 100 times range, not 10
times range.
 
    {Note: These weapons become proportionately more effective
in vacuum, although their base ranges are rather low. I forgot,
when doing the rules, to factor in the base "R" value. For
reference, this gives a neutral particle beam exactly the same
range per kJ as a space-optimized laser (UV laser) but under half
the range per kJ than the higher TL x-ray or rainbow lasers even with TL 
adjustments to range, which is what was intended.}

========================================
From: "Dr Kromm" <[email-masked]>
Date: Thu, 18 Dec 1997 11:35:50 -0500
Subject: Re: P-Beams

On 17 Dec 97 at 21:53, David L. Pulver wrote:
> A correspondant pointed out GURPS particle beams are total garbage
> in space compared to lasers

Unfortunately for game balance, this is quite realistic. Charged-
particle beams are easily collimated with electromagnetic "lenses,"
but fly apart in a vacuum due to internal repulsion. Neutral-particle
beams don't fly apart, but beams of massive, uncharged particles are
extremely hard to collimate at the outset (at least without serious
energy loss). In either case, focusing a PB to lethal intensity at any
significant range is difficult. Contrast this with lasers, which don't
experience internal repulsion and which are almost trivially easy to
collimate. Lasers are also subject to fewer things that will bend them 
off course in space.

This is why I didn't change these things when we were putting Vehicles
together -- I assumed that the lower efficiency of PBs vis-a-vis lasers
was intentional, a bow to realism.

> It's also not what I intended. To test this, I designed a series of
> weapons with similar weights and power outputs, and I agree there is
> a significant problem with space ranges.

Before you start changing things or calling for errata, consider that
at a given power output, the laser *should* get better range.

> [...] this gives a neutral particle beam exactly the same range per
> kJ as a space-optimized laser (UV laser)

Which is actually very unrealistic. A UV laser in vacuum should have
essentially infinite range next to any kind of particle beam.

SP.

========================================
From: Anthony Jackson <[email-masked]>
Date: Fri, 19 Dec 1997 13:16:00 -0800 (PST)
Subject: Re: P-Beams 

Part of the problem with why p-beams have such short ranges is that the ranges
for lasers are actually unrealistically long.

1)  For a megajoule laser to be able to penetrate 2 cm of steel (56 pts), it
needs to be focused into an area not more than ~2 cm across.
2)  While not theoretically disallowed, focusing a larger lens down to 2 cm at
extreme ranges is a horrifying engineering concept.  Thus, we can assume that a
megajoule laser has a 2 cm aperture.
3)  The '1/2D range' of a laser is presumably where energy density is halved,
which is at 3 cm.
4)  The minimum beam divergence angle for a beam is equal to
(wavelength/aperture) radians, based on diffraction.  If we assume 4000A light,
this works out to (4e-7m/2e-2m) or 2e-5 radians.
5)  The beam will thus gain 2 cm width per kilometer.  Therefore, the 1/2D
range of a 1 megajoule laser is 500 meters (550 yards)
6)  According to VE2, the 1/2D range of an extreme range megajoule laser, in
space, is (sqrt(1000) * 200 * 8 * 10)) yards, or about 500,000 yards.  This is
roughly 900* the theoretical limit for a 2 cm beam; a 60cm (2 foot) lens would
have that much range, but would have no penetration to speak of.

In order to improve range, without reducing penetration, your only real choice
is reducing wavelength, and getting a UV or X-ray laser.

Now, the theoretical limit for a particle beam is the same as for a laser, but
(a) particle beams will have much higher individual particle energies, and thus
much shorter wavelength (typically by several orders of magnitude), and (b)
focal area is unlikely to be the primary issue for particle beams; the problem
is more with beam temperature.  A 500 kilovolt particle beam accelerating
protons will generate beam velocities of roughly 10,000 kps; cool the beam down
to room temperature and cross-beam velocities will average about 1 kps, for
1e-4 radians -- effective range 100 meters, assuming that a 2cm beam is still
fair.  Note that this has very little to do with atmospheric range -- range in
atmosphere is mostly a matter of generating a current in the beam, creating a
magnetic field which keeps the beam coherent.

In order to increase range, our choices are (a) reduce temperature, and (b)
increase per-particle energy.  Neither one of these is terribly useful in an
atmosphere -- (a) will only last until it hits atmosphere, and (b) has the
effect of _reducing_ the current relative to the beam energy, which will tend
to cut atmospheric range.  For vacuum purposes, however, both can be done, and
probably would be.

Of course, this winds up with another issue -- if you start throwing 500 MeV
neutral particles at a target, you'll sprays of X-rays and other particles,
some of them extremely penetrating -- you can get through several inches of
steel without actually making a hole.  This significantly improves the useful
range of high-energy particle beams, since you no longer really need to focus
to a small point to penetrate.  If we assume 1 MeV gamma rays, spread a
megajoule particle beam across a 1 meter radius and an unshielded target inside
that area will still take somewhere around 100,000 rads, and will be lethal (if
not immediate) through 3" of steel.