I thought it was some kind of rocket fuel but I’m pretty sure that’s wrong. Can someone explain it to me in a way I’ll understand? (imagine you’re trying to explain it to a 3-year-old, because that’s about my level of understanding :D)
Differential velocity. The speed difference between two things, in this case, spacecraft.
If you’re “carrying too much delta-V” during a docking maneuver, for example, you won’t have a successful docking procedure, but instead you’ll have a collision and break something.
Anything with differing speeds, even a LM and the moon surface, can be measured in delta-V. A safe moon landing could be done at, say, 5 feet per second “delta v”. More than that, and the LM breaks landing gear.
Delta V is the change in velocity of a spacecraft. Changing velocity changes your orbit. you’ll often hear the amount of fuel referred to as the amount of Delta V. that number takes into account the engines and their efficiency and the amount of fuel you have on board. With this Delta V number, you can calculate how much you can maneuver your satellite before running out of fuel.
It’s a way to measure how much energy a spacecraft needs to get somewhere.
This chart is at first confusing, but then it’s really simple, like a roadmap. Let’s say you have a spaceship at low earth orbit, roughly 350 miles above Earth. (See the point marked LEO.) It requires a modest amount of energy to go to a geosynchronous orbit, which is 23,000 miles above Earth. (See the point marked GEO.) Or, if you follow the couple hops on the chart to the Moon, you add up those numbers and see how much energy that requires. Here’s the really interesting part: look at how much energy you need to get to the Sun! It’s a lot, far more than going to Mars!
And just to be sure you recognize this, rocket engines of various types (whether on the big rocket that launches from Earth, or the very small ones on every kind of satellite) provide the propulsion to make these maneuvers. Sometimes the amount of fuel on a satellite is counted in delta v.
To clarify - Delta is the greek letter indicating differential in calculus. V is velocity. hence the engineering shorthand for velocity differential.
And to expand on this, it’s sometimes counterintuitive given our experience of earth-based travel. On or near the surface of the earth, energy requirements usually only need take into account what we need to do to start something moving, and to keep it moving against friction/drag. There is something to push against (the ground or the atmosphere) to turn it or to slow it down. An aircraft’s ailerons or speed brakes don’t require energy, they just push against the air.
But in space, without an atmosphere, an object will keep going indefinitely in the same direction and at the same speed unless pulled by gravity, or unless you apply a force (firing your rockets, or whatever) to change that. For example, if you want to go visit Alpha Centauri in a reasonable time, and you want to accelerate your spacecraft to 10% of the speed of light to do that, it will require just as much energy to slow it down as it does to speed it up.
In a way, it can be viewed as a measurement of how much fuel a rocket has or how much it needs to go somewhere or to stop when it gets there.
To explain it to a 3 year old, I would say something like this:
First, imagine you are on a huge frozen lake. So everything is slippery. Your brother is skating at 5mph. You are behind him, skating at 4 mph. If you just want to match his speed so that he doesn’t get any further away, you need to increase your speed by 1 mph. The Delta-V required to do that is 1mph. That’s the change in your velocity that is needed to match his speed. But of course, you will not catch him going the same speed. You need to go faster than him to catch him. The faster you go (the more delta-V you apply) the sooner you will catch him. Also consider that you are on ice, so if you don’t slow down once you get near him, you will continue to slide right past him! So, let’s say you increase your speed to 10mph in order to catch him. You’ve used 6mph of Delta-V to do that. If you were a rocket, you would have used 6mph worth of Delta-V. Then, once you’ve caught up to him, you need to slow down to his speed. You will need to apply 5mph of Delta-V (in the opposite direction) to slow down to his speed. No you can hold hands, since you are both traveling at 5mph on the ice. The total maneuver to do this used 11mph of Delta-V. Sometimes when we talk about rockets, we would say that we used 11mph of Delta-V to perform the maneuver. Of course, it is more common to use meters per second, but you’re 3 so I’m keeping it simple. If you started with 20mph worth of Delta-V, you’ve used 11 and you still have a Delta-V of 9mph worth of fuel remaining.
In other words, it’s a measure of why you want fuel. You could express the amount of fuel a spacecraft has in (say) kilograms, but that isn’t very useful: What can the spacecraft do with that fuel? For a large spacecraft, a few kilograms of fuel might not be enough to do anything meaningful, but for a small spacecraft, it might be enough for its entire operational lifespan. You want the fuel so you can change your spacecraft’s velocity, and so you measure it by how much you can change the spacecraft’s velocity.
Actually, it will take much less, because when you get to that point of your trip, you will have already burned a lot of your fuel (which will weigh much more than your payload), so what’s left will require much less energy to accelerate. Which is another illustration of why Delta-V is a useful measurement, because (unlike energy) you will have used up half of your Delta-V halfway through your trip.
It is also important to realize that the mass of fuel required to achieve a given delta v goes up exponentially with delta v. So if it requires a delta v of 10 to get to low earth orbit and 40 to get to the sun, it takes vastly more than 4 times as much fuel to get to the sun, than to low earth orbit.