The Importance of Delta-V

Delta-v. (Picture by Fred the Oyster)

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.

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2 Responses to The Importance of Delta-V

  1. Byzantine_Corporal says:

    Let’s distinguish between “fuel” as energy source and “fuel” as reaction mass.

    In a chemical rocket the fuel fills both roles. In a nuclear rocket, the roles are split. But all rockets must carry (and accelerate) reaction mass. This produces an exponential that dominates the rocket equation.

    An aircraft carrier doesn’t have to carry reaction mass because it can dump momentum into the ocean via its propellers, but it does need big honking nuclear reactors to turn them.

    The currently “hot” EM drive allegedly steals momentum from the [quantum virtual plasma]* in a way analogous to a propeller stealing momentum from the ocean. We’ll see. I remember the Dean Drive. It was good at sliding over carpets.

    But for fictive use a “space aircraft carrier” with an EM drive is solidly plausible, today. Unlike an aircraft carrier, all velocity is relative to arbitrary other objects, and slowing down is just as expensive as speeding up.

    Relativistic speeds are problematic if you have to carry even just your energy fuel. To accelerate 1 kg to 0.87c takes 1.02 kg of mass-energy. So total conversion of the entire mass of the vessel isn’t enough to go that fast. Only a staged stack of vessels can do better, and then the exponential comes back. Because you do have to accelerate your energy fuel.

    Bunkerage is going to be a problem. Better use a ramscoop or ZPE.

    I canz hard-sciffy, and so can you!

    *this collection of words may or may not have meaning

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