Thoob! BOOM!

You'd be right to think that a grenade launcher can't blow a door open, at least not in the way that's seen here. This is NOT an error on my part - it's intentional. The logic behind the behavior will be hinted at over the next few strips, and either explained or remarked on in further detail later in the story.

If you just can't wait : The grenades have proximity fuses. The ship has proximity sensors. While the ship isn't keen on Yang and Aleph leaving with shiny bits, it's even less keen on sustaining further damage. Hence the ship automatically opening the hatches a split second before the grenades explode, and hence Yang successfully using his boomstick to force his way out.

It also makes for a nice spot of fantastically stupid violence.

Not shown : the lead-up to this scene, in which Yang and Aleph are trapped, Yang admits it's a dumb idea and tries it anyway.