I first read about delta-v as it relates to space travel on Winchell Chung’s Atomic Rockets website. At first, I dismissed it as just another boring limitation on fun space opera adventure, a bit like his descriptions of why gigantic starships can’t work or why space fighters are useless.
But as I thought about how I’d write my own space opera, I considered supply needs in deep space, even in a Star Trek or Star Wars-style story: food and water would need to be carefully rationed, energy usage would have to be monitored closely, and one would have to pay close attention to how fuel is used.
That’s when it hit me. That’s when I realized what “delta-v” really signified.
Delta-v is related to fuel level. In space, you don’t need to burn fuel to keep moving, but you do need it to speed up, slow down, or change direction; by contrast, on Earth you don’t need fuel to slow down because friction and air resistance take care of that for you. This means that you can only change velocity a limited number of times since all such changes require fuel to be spent — and based on your fuel levels, you can estimate the total amount of velocity changes you can make.
Let’s say your Cool Starship (because habitat modules can go eat feces) has two million gallons of fuel. If you were to expend all of this fuel at once when the starship is at rest, the final speed is, let’s say, 447,040 meters per second, or one million miles per hour. That 447,040 m/s figure is your delta-v; this means that you can make changes in velocity that total 447,040 m/s, whether speeding up, slowing down, changing direction, or any combination thereof. If you run out of this before reaching your objective, you will either:
– Drift through space endlessly, or
– Crash into the nearest astronomical body or sentient-made structure.
Another thing to consider is that every gram counts. The more mass aboard the ship, the more fuel required to accelerate the ship to the same speed. Therefore, more mass means less delta-v for the same amount of fuel.
I cannot emphasize enough how important this is for verisimilitude. Limitless delta-v is like limitless supplies, or limitless food and water, or something equally silly and Mary Sue-ish. It is the same as limitless fuel. Just think through the logic of space and its frictionless environment — if you can’t change your velocity, you’re toast. It boggles the mind how alien outer space really is.