While gas first opening and Reaper opening are the mainstream builds in TvT, a gas first Reaper opening relative uncommon. The build I’m covering isn’t new, but I think it is worth blogging given the background behind it.

Let’s start with the build order.

Build order

10 – Supply Depot
11 – Refinery
13 – Barracks
17 – Orbital Command and Factory
@50 gas – Reaper
@100 mineral – Supply Depot
@75 mineral – Refinery
@100% Reaper – Reaper
@100% Factory – Hellion
@100 gas – Starport
@100 mineral – Supply Depot (Build Supply Depot according hereafter)
@100% Reaper – Tech Lab on Barracks
@100% Hellion – Hellion
@100% Starport – Viking
@100% Tech Lab – Reactor on Barracks
@100% Hellion – Hellion
@100% Viking – Raven (Swap Starport onto Tech Lab)
@100% Reactor – Swap Factory on it (Continue Hellion production)

Game plan

This is a defensive reaction build. More importantly, this is the refined version of the previous gas first Reaper opening. I want to talk about the similarities of these two builds, before I touch on the differences.

Both builds go for a gas first into Reaper, then Hellion and Viking opening. Unlike most gas first builds, there is no prefixed attack timing for this build. This build essentially prepares the necessary structures that allow you to react and defend any (known) TvT build if you have the information of what is coming. The early Reapers are mainly for scouting to see what build opponent is doing. There is a “poke the front timing” when you have two Reapers and two Hellions, and you can deduce what build opponent is doing based on what you see. Thus, this build requires good build order knowledge to execute.

I will not go too much into details regarding how to react, as I have covered it before in the earlier post. While this build can move towards a bio transition, it is smoother to go for mech given the structures set up.

Anyway, the highlight of this post is about the refined detail of this build.

Refined detail

Let’s discuss about the differences of this build and the previous version.

In contrast to other gas first opening, this build gets the Refinery at 11 instead of 12. This is a defining minor difference that improves the previous version. The 12 gas version gets a Marine and Factory when the Barracks completes, just like any other gas first opening. The reason you get a Marine and not a Reaper straight away is simply because you don’t have the gas to do so (unless you delay the Factory, which defeats the purpose of gas first). Thus, instead of waiting for 50 gas for the Reaper, it is better to just produce a Marine first then a Reaper right after. With a slightly earlier gas at 11, you can skip the Marine and wait for 50 gas for the Reaper.

So, what exactly is the difference between a 11 and a 12 gas first build?

Incidentally, I just had a discussion with someone on /r/allthingsterran on this topic. If you have done a gas first opening before, you will have noticed that you have enough mineral to put down a Refinery as early as 11 supply without pausing Scv production. However, as shown by many progamers, it is always built at 12 rather than 11 (except for this build to the best of my knowledge). The fundamental rationale is that every gas mined is an opportunity cost for mineral mined (vice versa), and this is extremely important in a build order at the beginning of the game when every bit counts. Therefore, you want to be as precise as possible in an opening to ensure you have the ideal mineral to gas ratio for what you want to do.

Going back to a standard gas first build with a refinery at 12, you want to have 150 mineral for an Orbital Command, 150 mineral and 100 gas for a Factory, and 50 mineral for a Marine when the Barracks completes. That is 350 mineral and 100 gas in total. While you can put down a Refinery as early as 11, you want to delay it to 12 in order to have 350/100. You can try putting it down at 11, and you will realise there is a bit of a hiccup with the mineral required. Conversely, for this 11 gas Reaper build, you need 300/100 first, then 50/50 for the Reaper. The earlier Refinery basically trades the tiny bit of mineral for a bit more gas, and this lines up better if you want to go straight into Reaper.

In terms of execution of putting down the Refinery in 11 versus 12 supply (I’m making it sound complicated), the difference lies between whether the Refinery is put down before or after queuing the 12th Scv. When you are at 11/11 with the 11th Scv building, you queue up the 12th Scv when you have 50 mineral for the 12 supply Refinery build, then build the Refinery when you have 75 mineral. If you do it with as little delay as possible, the timing the Refinery goes down is right when the number jumps from 11 to 12 (or close). In contrast, when you are at 11/11 with the 11th Scv building, you do not queue the 12th Scv first for the 11 supply Refinery build, and you put down the Refinery once you have 75 mineral. If it is unclear, just use the Scv that builds the Supply Depot to build the Refinery if you are doing a 12 supply gas first build.

Alright, so much for 11 versus 12 gas, and I have yet to discuss why the 11 is considered a refined version. The build is essentially the same except you trade a Marine for a Reaper. The Reaper can add more value to what you want to do in the early game than the Marine can. With two Reapers for the refined version, it is easier to scout what opponent is doing and this aligns better to the game plan. The extra Reaper gives you that seemingly marginal but important extra damage when you poke with the Hellions.

In sum, I think getting the Refinery at 11 instead of 12 is quite thought out, and is arguably better than the other version.