Mortar fire

[geppo_68]

Dear all,
could you someone explain to me in detail the use of mortars with a line of sight to the target?

They obviously use always indirect fire, but:
a) do you use the Artillery skill?
b) does the range/size/speed chart apply?
c) could you Aim and then use the Acc bonus?

conversely, I reckon that without line of sight as in High Tech p 139
"Observed lndirect fire" they
a) use Artillery skill coupled with the FO skill
b) do not use the Range/size/speed chart
c) do not use the Aim or Acc bonus
d) they can use the bonus given by the FO rolls only to offset the intial -10 "shooting blind" malus

And "Predicted Indirect fire" they
a) use Artillery skill
b) do not use any bonus/ malus for range or other reason, just the flat Artillery skill

Is it the correct interpretation?

Thanks and best regards

[Ulzgoroth]

I'm pretty sure we've had this discussion before. Try searching? Not sure what the results were if any.

[geppo_68]

Originally Posted by Ulzgoroth

I'm pretty sure we've had this discussion before. Try searching? Not sure what the results were if any.

Yes thanks, I tried but then after 10 or so pages the discussion derailed about house rules etc..
I am trying to understand the rules as written, and they are not clear to me

[Ulzgoroth]

Well, I think there's a decent case that by the rules as written you should be using Gunner for laying a mortar on a visible target... In which case you could certainly Aim.

Range/speed always applies to ranged attacks, unless they're Guided. Size always applies, though artillery commonly attacks a ground hex rather than any particular individual target.

[Anaraxes]

I think that was the "Skill of the Week" thread for Artillery / Forward Observer.

[Michele]

I wouldn't use Gunner with a mortar, regardless of who is correcting the fire (a separate spotter or the mortar layer himself). Because in GURPS, Artillery covers all ranged attacks that don't rely on the straight-shooting alternative, Gunner. A mortar round will still be fired on a curved trajectory, and doing the math for that (or judging it by mere eyeballing) is really another skill.

Note the Mortars Table, GURPS High Tech p. 146. As all weapon tables, it begins by telling us which skill to use, which is Artillery (Cannon).

So, the artilleryman will serve as his own FO. He will skip the navigation roll. Locating the target will require a Forward Observer roll anyway, but the GM can reduce the time needed because that includes the time for transmitting the information.
For firing the mortar, the margin of success or failure on the Forward Observer roll will add directly to the Artillery roll; no -10 penalty for not seeing the target; and add any Acc bonus.

[Ulzgoroth]

Originally Posted by Michele

I wouldn't use Gunner with a mortar, regardless of who is correcting the fire (a separate spotter or the mortar layer himself). Because in GURPS, Artillery covers all ranged attacks that don't rely on the straight-shooting alternative, Gunner. A mortar round will still be fired on a curved trajectory, and doing the math for that (or judging it by mere eyeballing) is really another skill.

But this is true of all projectile firearms. Rifles, cannon, grenade launchers, machine guns - all of them fire on curved trajectories that must be compensated for if you want to hit your target beyond literal point-blank range. A mortar is a bit more sensitive to accurate ranging than most guns that are fired at lower elevations, but a tank gun, a destroyer's main guns, a sniper rifle, or a panzerfaust all require you to handle the same basic problem.

[Michele]

Originally Posted by Ulzgoroth

But this is true of all projectile firearms. Rifles, cannon, grenade launchers, machine guns - all of them fire on curved trajectories that must be compensated for if you want to hit your target beyond literal point-blank range. A mortar is a bit more sensitive to accurate ranging than most guns that are fired at lower elevations, but a tank gun, a destroyer's main guns, a sniper rifle, or a panzerfaust all require you to handle the same basic problem.

OK, call it a high curved trajectory. A mortar, a howitzer, and similar guns are designed for that - it's a feature, not a problem you have to correct for, as in the case of weapons that work best with the straightest trajectory possible.
Note that we're talking GURPS here, not ballistics, and the fact that torpedoes, bombs, orbital bombardment etc. also use the Artillery skill tells us what similarities to look for.

By the way: naval artillery uses Artillery when fired at a target area. 406mm guns are a specific example in the skill description.

[geppo_68]

Michele, your suggestions seem spot on. Speaking of direct line of sight Fire, Do you suggest to use the range table malus and the +4 area Fire bonus as well?

[Ulzgoroth]

Originally Posted by Michele

OK, call it a high curved trajectory. A mortar, a howitzer, and similar guns are designed for that - it's a feature, not a problem you have to correct for, as in the case of weapons that work best with the straightest trajectory possible.
Note that we're talking GURPS here, not ballistics, and the fact that torpedoes, bombs, orbital bombardment etc. also use the Artillery skill tells us what similarities to look for.

By the way: naval artillery uses Artillery when fired at a target area. 406mm guns are a specific example in the skill description.

A mortar does specifically aim on a high-angle trajectory - that is, on the higher of the two firing angles that would allow it to hit the intended target, above 45 degrees elevation. A howitzer usually doesn't, unless extreme plunging fire is specifically needed for the mission.

The Basic Set tells us that there are no commonalities to look for across Artillery except for the shared rules text. Torpedoes and air-dropped bombs aren't like mortars in any substantive way, but GURPS doesn't pretend that they are either.


Part of the problem is that the Artillery skill text and Gunner skill text give contradictory definitions of indirect versus direct fire. According to Artillery, indirect fire is "to put fire onto a target area via a high ballistic arc or similar path". According to Gunner, direct fire is "to aim and fire at a target to which you have a line of sight". Both strongly imply that these are in opposition, but this is patently untrue. We're currently talking about fire that meets both of those definitions. Fire that meets neither also exists - low-elevation shooting into blind areas is entirely possible, probably most commonly done with infantry machine guns.