Mortar fire

[Michele]

Originally Posted by Ulzgoroth

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.

There are no technical commonalities. There still are similarities: in the difference with the Gunner skill.

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.

The opposition exists in the very texts you mention.
It's in the word "area". Note it is absent in the second definition. With a mortar, a howitzer, an orbital weapon, an unguided torpedo, an ICBM and even a 406mm battleship battery firing from 15 miles away you are firing at a place. It changes little if you yourself can see it.

[Michele]

Originally Posted by 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?

With the exceptions already mentioned, determined by the fact that the mortar operator also is the FO, I would use the rules provided in High Tech.

[jason taylor]

I think there are a one or two quite odd mortars that can be used in direct fire mode. The one I remember though it's way back in the back of my mind was a Coast Guard piece of all things. How they managed that is odd as the Coast Guard would not want a weapon with a lot of authority as it's first priority. However it may have originally been a flare launcher.

I am speaking of something I read about a very long while back so sorry if I am vague.

[Anthony]

Originally Posted by jason taylor

I think there are a one or two quite odd mortars that can be used in direct fire mode.

I would think that any mortar could be used at a low angle if it had an appropriate stand, it's just that mortar stands aren't designed for low angles.

[Michele]

Originally Posted by jason taylor

I think there are a one or two quite odd mortars that can be used in direct fire mode. The one I remember though it's way back in the back of my mind was a Coast Guard piece of all things. How they managed that is odd as the Coast Guard would not want a weapon with a lot of authority as it's first priority. However it may have originally been a flare launcher.

I am speaking of something I read about a very long while back so sorry if I am vague.

Well yes, it equipped Coast Guard patrol boats deployed to Vietnam, and later USN boats too. It was a combo weapon, both in the sense that the mortar itself could be used for direct fire too, and in the sense that the mount also sported a .50 MG. Note the mortar had been initially intended mostly to fire illuminating rounds.

The point of this thing - which you understandably define "odd" - is that it was actually a gun-mortar, not a normal mortar. The tell is in that, if you wanted to engage a target with direct fire, you switched to "trigger mode". A true mortar's round is activated by dropping it down the tube.

[Michele]

Originally Posted by Anthony

I would think that any mortar could be used at a low angle if it had an appropriate stand, it's just that mortar stands aren't designed for low angles.

No. It would also need an appropriate round. A mortar round has to be dropped down the tube, and that is unsuitable for low-angled tubes.
You could push it violently down the tube, replacing gravity with muscle force - but that amounts to the risk of having an unfired round sitting at the bottom of your tube, something mortar crews intensely dislike, with reason.

There are the so-called gun-mortars, of course, like the one mentioned above. You can fire them with a trigger, and in some case you don't even muzzle-load them; they have an opening breech and you load the round like in a gun.
The Italian Brixia Modello 35 light mortar of WWII was designed that way, and the intention was exactly that it be able to fire direct, like an infantry support gun. In practice, it was seldom done.

So to amend my position as stated above, yes, if you have a gun-mortar, AND a proper mount, a proper activation system, and a proper round, you can fire a mortar as a gun - with Gunner. But it's an exception to the rule, and the conditions described here make it, really, a sort of short-barrelled, light-weight gun (regardless whether it's called a mortar), rather than a mortar proper.

[whswhs]

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.

On one hand, that's true. But on the other, it's not what is meant. Yes, if I throw a toy mouse across the room to pass just over my cat's head, it's following a curved trajectory. But the basic mental process is still "point and shoot [throw]." The arc followed by the mouse is close to the line from my eyes to the target. If I toss it up in the air, I can focus either on the toy or on my cat, but the one I don't focus on will be in my peripheral visual field, or even out of sight. Or I'll have to shift my gaze between them.

[Anthony]

Originally Posted by Ulzgoroth

But this is true of all projectile firearms.

For all shots at less than absolute max range, there are two solutions to the ballistics equations, and they are quite different; mortars use the higher angle solution, and are also very low velocity.

Consider a shot at 300 meters, which is a reasonable infantry engagement range. In all cases I am ignoring air resistance.

A rifle bullet will have a travel time of about 1/3 of a second, and must be aimed slightly over a meter over its target (assuming the sight is zeroed at 0m; if it's zeroed at 200m you only need to aim about 40cm above).

A pistol bullet will have a travel time of 2/3-1s, and thus must be aimed 5-10 meters above its target.

A 150m/s direct fire round (similar to a mortar) has a 2s travel time and must be aimed 40m above the target.

A minimum velocity round, fired at 45 degrees, has a velocity of 54m/s (38m/s v and h) and a travel time of 7.7s.

A 150m/s high angle round aimed 300m away has a travel time of 30s...

[geppo_68]

Originally Posted by Michele

With the exceptions already mentioned, determined by the fact that the mortar operator also is the FO, I would use the rules provided in High Tech.

This Is my initial problem in interpreting the rules of High Tech:what are the tables and mods involved in each of the 3 scenarios described in my first post? The thread Is derailing a bit...

[Fred Brackin]

Originally Posted by geppo_68

This Is my initial problem in interpreting the rules of High Tech:what are the tables and mods involved in each of the 3 scenarios described in my first post? The thread Is derailing a bit...

You always use the Speed/Range table _unless_ you have a Guided or Homing round. There is no other way around that. The Death Star firing at a planet uses the Speed/Range table.

This is one of the reasons that Artillery fire seldom achieves direct hits. Mostly you're going for "close enough" and after using the Scatter rules for rounds that don't get the direct hit you may well be close enough.

You always use Artillery Skill unless you have one of those odd gun/mortars and are using it in low trajectory mode. Regular mortars always use Artillery even when engaging in direct line of sight.

Take time to Aim and get Acc and the possible +2. You need all the help you can get.