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scourge of the airways

Starcraft has 3 suicide units: spider mines, scourge, and infested terrans. They need to be handled much differently than other units. All of them do high damage but are easy to destroy, so they are units with a lot of leverage (meaning that using them well brings a lot of benefit). Spider mines are difficult because they don’t move after they’re placed; you have to foresee the future to place them well. Scourge is difficult because it does move. Infested terrans are difficult because they are rarely any use unless they coordinate with other units.

Steamhammer is one of the best bots at scourge usage, but objectively it is distressingly weak. It often hits a target with more scourge than are needed to destroy it, is overcautious about attacking targets defended by ground units, suicides scourge against mass air units that can defend themselves, retreats scourge that have no immediate target too far from the fight, and other blunders. But it’s daunting to think of all the work needed to play well with just this one unit type.

How many scourge per target? Well, each hits for 110 damage, so divide. If a mutalisk target (120 HP max) is hurt more than slightly, you can kill it with 1 scourge, and you don’t want to waste any. But if there’s a long chase to catch it, it might heal during the chase and you’ll need 1 extra. Theoretically you could account for that beforehand. Or a target might recharge at a shield battery, or suddenly be mass-repaired, or d-matrixed. That’s hard to predict, and if it happens you might want to turn the scourge away—unless extras are already handy.

Scourge can be shot down on the way to the target. A battlecruiser has 500 HP, so it dies to 5 scourge. It can fire one shot before the scourge strike, and the shot can kill one scourge, so you need to send 6. Well, actually whether it kills the scourge depends on upgrades. 2 scourge kill a corsair. 4 scourge kill 2 corsairs. But 12 scourge do not kill 6 corsairs; the corsair damage splashes and protoss may defeat most or all of the scourge. In general, any enemies nearby may cooperate to kill your scourge. I don’t have a better idea than to simulate the attack and see how many scourge may be shot down, to decide how many are needed and whether it is worth it.

Usually you want to send enough scourge to kill the target. Don’t sent 2 at a time against that lone battlecruiser (as many bots do, including Steamhammer and PurpleSwarm), because only 1 will hit and you’ll need twice as many. Usually, but not always. A mutalisk has 120 HP, so a scourge hit leaves it with 10. A muta with 10 HP is nearly useless in an air fight, so if you follow up the scourge with your own mutas then the enemy muta will flee or die, or both. Neutralizing an enemy mutalisk with 1 scourge hit is a crazy-efficient use of gas.

What tactics? Scourge see corsair and chase; corsair sees scourge and flees toward safety (like a cannon or a group of dragoons). The corsair can try dodging micro moves to throw the scourge off; I doubt that works well against a bot. Pro players like to predict the path of the fleeing corsair and intercept it with another pair of scourge sent from another direction. I don’t know any bot that tries that.

Later, there may be many scourge and many corsairs. To minimize the splash damage to the scourge, try converging in a wide arc. That may make it worth while.

With lurkers versus protoss, you want to snipe observers. Undetected lurkers can delay protoss a long time, so it may easily be worth it to spend several scourge to kill a single observer that tries to stay safe among dragoons. But don’t fly in headlong, try to analyze the dragoon locations and choose the best angle.

In general, good scourge planning and pathing is complicated and depends on the tactical situation. And how are you supposed to do the planning, which has to happen before you can do an accurate combat simulation, without combat simulations first to figure out what works? I guess expensive methods may be needed.

Where to go when there are no immediate targets? Scourge are fast, so if the enemy has little anti-air, which is common versus protoss early in the game, a good idea may be to swoop around the map looking at stuff. But scourge have short sight range, so that’s dangerous once the enemy is good at shooting up. Scourge want to stay near where enemy air units may appear: Over or near the front-line army (which will protect them), above cliffs or unwalkable areas near the fight, along paths where dropships or corsairs may fly, or in your own base to defend against air raids. Making the best choices means predicting the enemy, not a simple skill.

The bottom line: Scourge use is awesomely complex. I don’t want to code all the details, I think I have to aim for more general methods of search and machine learning.

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Jay Scott on :

I could have also written about coordinating scourge with other units. Steamhammer loses a lot of value in ZvZ by moving its scourge and mutalisks independently.

MarcoDBAA on :

In my opinion, scourge are practically useless vs all bot air units, that are as fast as them, with mutalisks as the only exception, because of limited muta weapon range and also (normally) limited ZvZ ground anti-air.
Iron bot for example often obliterates scourge without taking any damage. Just retreat into turrets or other anti-air, including even other air units...

You can buy some time maybe vs the first harassing corsair, that needs to flee from both scourge at first, but corsairs, valks and wraiths aren´t really that vulnerable in the bot world vs them.

Bot mutas however are clearly better vs corsairs and valkyries, because they can spread out better than human controlled ones and avoid splash damage. And spreading out on the other hand gets more difficult for scourge, because they probably need to close in on the same path to allow them to suicide, if the enemy unit is fleeing, all while getting fired on by another corsair.

Scourge might be more useful vs slower air units however here, because you can still close in vs them on a non-direct spread out path. In that case corsairs cannot protect their carriers as well as normally. Although carriers vs zerg is problematic anyway, bot or not.

Also think, that retreating your scourge is seldom a good choice, if you already committed them (if they are in enemy range already). They just don´t have enough hitpoints to retreat.

Jay Scott on :

Even if scourge never hit, they still serve a purpose in keeping air units at bay and holding drop attempts at risk. A vessel that has to keep retreating toward marines will not get as many good irradiates.

MarcoDBAA on :

Drop ships (except protoss upgraded shuttle) are slower than scourge anyway, and therefore weren´t included as being bad targets. Yes, I actually think, that they can be very effective here indeed, because the damage is made instantly ("drop ship" might not unload or is forced to unload at a bad spot). You may patrol your bases, if you suspect a drop. Bots see the danger immediately.

Not sure if it will be (cost) effective to try to just keep air units away. BUT it should be useful in a lurker strat vs protoss (and maybe terran, but they have comsat), where the advantage is really huge, if the observer has to retreat or gets killed. But observer (and science vessels) are also slower than scourge anyway (not contradicting my first comment), therefore a good target.

Irradiate should be a lesser treat in bot play. You can (and should :P, is it in SH again?) suicide the irradiated unit into your opponent immediately. And bot mutas might be spread out already anyway. On the other hand, bots can probably micro spellcaster units better. krasi0 vessels are annoying for example.

Dan on :

I've experienced the frustration of trying to answer Iron's air units with Scourge. I do agree, as long as you avoid suiciding them into ground units, that they usually still pay for themselves by keeping more expensive units from fighting.

Vessels are a tough case because the range of Irradiate is so long that as long as you keep your Vessels and Marines together -- easier for a bot than a human -- that Scourge will cease to be an effective answer.

jtolmar on :

> I don’t have a better idea than to simulate the attack and see how many scourge may be shot down, to decide how many are needed and whether it is worth it.

One improvement would be to send enough scourge to kill the low-end estimate in one wave (ideally hitting within a couple frames of each other), then a second wave with the additional scourge for the high-end estimate, far enough behind for Steamhammer to react and pull some back. This leaves you with the excess scourge somewhere in the enemy formation, where they can be used on the next target.

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