delta-v
Explanation
The Joke
The comic contrasts a "normal engineer" with a "rocket engineer" when asked "What's delta-v?" A normal engineer gives the textbook answer: "Change in velocity." A rocket engineer, however, responds enthusiastically with "Oh! I have some in this bucket!" -- as if delta-v is a physical substance one can collect and store.
The Humor
Delta-v (change in velocity) is a central concept in rocket science and orbital mechanics. In rocketry, delta-v is treated almost like a currency or resource -- spacecraft are designed around their "delta-v budget," and mission planning revolves around how much delta-v is available. Rocket engineers talk about delta-v so constantly in terms of having it, spending it, and needing more of it that the comic imagines them taking this to the absurd extreme of literally treating it as a tangible substance you could keep in a bucket.
The title "Rocket Engineers Are the Engineeriest Engineers" adds to the humor by suggesting rocket engineers are so deeply embedded in their specialized jargon that they've crossed over from understanding a concept abstractly to treating it as something concrete and collectible.
References
- Delta-v is the standard measure of the impulse needed for orbital maneuvers in astrodynamics. It determines what missions a spacecraft can perform given its fuel and engine specifications.