A Giant Sword, A Shield That Blows Up, and Reactive Armor
 

In the universe of Muv-Luv Alternative, giant robots (called TSFs) sometimes use shields. These shields have reactive armor plates on them, which are frequently used to punch aliens in the face with explosions. How would one model such a shield, and that attack? I've not been able to find a plain reactive armor system-just EMA from ultratech and Laminates from High-tech, one of which is too high-tech for the setting and the other of which is not quite what's being used. I'm tempted to just have it provide a bonus to crushing damage when shield bashing, but I'm not real sure what a good benchmark for that bonus damage would be.

Along similar lines I've had some trouble modeling the swords used by the giant robots, as they're SM +6 with dST 30, and the low-tech companion 2 table only covers ST up to 100. Is there some further extrapolation of it I could use? The mecha are being done up under Spaceships rules.

Further explanations of the exploding shields are here. The world of the games is TL 8 with bits of TL9 appearing as cutting-edge equipment and the tech required to get mecha running.

Re: A Giant Sword, A Shield That Blows Up, and Reactive Armor
 

Based on more advanced rules for reactive armor from 3rd edition and assuming late TL 8/ early TL 9 plate technology I can see each charge doing only about 8d+2 cr ex damage per charge but given how they're being used I'd count it as a contact explosion. It should also give about DR 1,000 vs shape charges, half that against piercing damage and 1/5th as well against all other types of damage and weight 110lbs and cost about $200 per square foot of coverage.

Re: A Giant Sword, A Shield That Blows Up, and Reactive Armor
 

Just make up stats given the constraints of

1. It shouldn't deal enough damage to reduce hit points of the shield or TSF.
2. It should be able to penetrate non-armour-plate BETA DR.
3. It should be reasonable given plausible REFs for the TL and eyeballed dimensions of reactive armour patches.

Ultra-Tech's Reactive Armour Paste is TL 10 but its ratio of 1d damage to user to provide DR 20 (200 vs shaped charge) and progression of x1.5 DR at TL11 and x2 at TL12 might be a useful starting point.

I'd probably have it do an amount of damage dice that can't penetrate the shield and then multiply that by three and use the resulting number as either DR (x10 if against a shaped charge) against an attack or as an incendiary crushing damage with double knockback explosion if used as an attack that's capable of penetrating normal BETA DR.

You can use the Under the Hood: Weapon and Armor Scaling Table box to scale past SM +6. If the subject is SM +6 (though note that some TSF's such as the Type-04 Shiranui Second just barely squeak into SM +7) that just means that they have a lot more ST than needed for proportionally sized swords. 10 times normal min ST would mean a lot of lost damage for dST 30 due the the max ST cap. Fortunately its quite justifiable to increase the min ST. Look at the Type-74 PB Blade, its very thick and made with TL8-9ish materials. Low-Tech Companion 2 doesn't handle making weapons heavier to efficiently capture the user's ST without making them larger but Reverend Pee Kitty's MyGURPS does ( http://www.mygurps.com/r_minst.html?p=irha&v=0 )

Re: A Giant Sword, A Shield That Blows Up, and Reactive Armor
 

And note that this is going to be a contact explosion on the shield itself, for maximum damage. I'd probably limit it to (Shield's DR)/2 in maximum damage, just in case it gets detonated in an enclosed environment (which doubles damage, see HT181), such as might result from punching a particularly squishy foe.

Note if the above damage is insufficient for this, you can state the explosive plates - which appear to be designed more for offense than defense - are actually shaped charges with metal liners, which can justify anywhere up to a (10) Armor Divisor against the target (no divisor against the shield it's mounted to).

I can't imagine casing/etc taking up more than 10% of the patch's weight (although I could be mistaken), so going with that we can fill it up with a 2:1 mix of CL20 and HMX (this mix being the highest power TL 8 explosive that's safe to transport that I can readily find online). The end result is going to be that the explosive patches have overall REF of around 2.0.

A typical shield scaled up to SM+6 is going to have DR 40 or so, but ST 300 is more appropriate for an SM+9 character, so scale shield weight up to that. An SM+9 shield would have DR 80, but with only 1/3 the surface area this gets boosted to DR 160. So, your shield is safe for a 50d or so explosion. That's around 8.7 lb of armor patches being detonated at a time. If 50d cr ex (arguably maximized for contact) is enough to punch through and appropriately injure typical BETA's, you're set. If they have too high of DR for this, give the patches sufficient armor divisor to get through. If they have too high of HP, consider giving BETA's a Vulnerability to explosions. If this is too much damage, either have fewer patches explode per hit or use a lower-yield explosive. This explosion will occur concurrently with the punch/shield bash, so each should have to deal with armor separately (it may be possible to delay the explosion so that it functions as a follow-up instead, in which case a punch that gets through DR can result in the explosion not having to deal with such). Note that, with ST 300 giving 31d thr and 33d sw, this explosion is probably the mecha's best melee attack - which I suspect was indeed the intent.

EDIT: As for the swords, you can scale them up to SM+9 (or SM+8 if it normally has MinST above 10) for weight and damage, SM+6 for Reach. So a thrusting broadsword would do thr+60 imp, sw+30 cut, which works out to around 48d imp, 41d+2 cut (yes, the scale is a bit broken here).