What is Delta/v?

[Reventage]

I've heard the term lots, but I don't know exactly what it is, I hear lots of youtubers like Scott Manley saying ''We're going to need a lot of delta/v for this burn'

So what exactly is it?

[rryy]

Delta-v means "change in velocity." If you are travelling at 2200 m/s and need to accelerate to 3100 m/s, you will change your velocity by 900 m/s, which means the burn will take 900 m/s of delta-v.

[Lheim]

Very useful measure for space flight, that.

How much you can change your velocity depends on the efficiency of your engines and the mass of your ship. It's actually not all that obviously related to how much gas you got in the tank. You can have a very light probe with a highly efficient engine that'll have a delta-v of 10 klicks a second, while a gas-guzzling monstrosity might only manage 4 k/s. In real turns, that means the probe can go to Eeloo and back, and the monstrosity'll barely make a return trip to duna.

Sending the two ships out to Duna, both would eat up about the same amount of their delta v to get there. But way different amounts of fuel.

[Jackissimus]

In physics the word "delta" means change. If you have 7 eggs in your fridge and you increase the number to 10, then you just increase the amount by deltaEggCount = 3. V is for velocity, so if you are driving on the highway at 80kph and increase your speed to 100kph, you increase your speed by deltaV = 20kph. That's all there is to it, you just need a lot of deltaV to go from 0 to escape velocity.

[capi3101]

Delta-V is, as has already been said, "change in velocity", velocity being a vector consisting of speed (a scalar value) and a directional heading. You change velocity either by changing the speed or direction; either counts as delta-V. It's important in spaceflight because you need to know how much a spacecraft can change its own velocity (without accounting for gravitational forces at work) before it runs out of fuel.

[Specialist290]

To put it yet another way: Imagine you have a rocket standing still in a perfect vacuum, with no other forces acting on it. If your rocket has 1000 m/s of delta-v, then if you fire the engines in one continuous burn without changing your heading, your rocket will be moving at 1000 m/s in the direction you're pointed once you empty the tank. If you only burn half of your fuel, you'll be moving at 500 m/s; if you flip the rocket around and burn the rest of your fuel against your direction of travel, you'll come to a complete stop again.

Wernher von Kerman has

, and I'd also highly recommend reading these three pages over at Atomic Rockets if you want to know more about how to calculate the delta-v capability of your rockets yourself.

[dlmarti]

In physics the word "delta" means change. If you have 7 eggs in your fridge and you increase the number to 10, then you just increase the amount by deltaEggCount = 3. V is for velocity, so if you are driving on the highway at 80kph and increase your speed to 100kph, you increase your speed by deltaV = 20kph. That's all there is to it, you just need a lot of deltaV to go from 0 to escape velocity.

lol, delta-egg!

From waking up, to me finishing my coffee today, took 2 delta-egg.

[Imaginer1]

Keep in mind that delta-v doesn't account for gravity on Kerbin or atmospheric drag almost 100% of the time, so if you're doing the math shave off a little bit.

[Anizer]

Delta-v means "change in velocity." If you are travelling at 2200 m/s and need to accelerate to 3100 m/s, you will change your velocity by 900 m/s, which means the burn will take 900 m/s of delta-v.

Why don't we just use acceleration?

[carazvan]

Why don't we just use acceleration?

acceleration represents the instanteneous change in velocity (aka the result of the current forces applied to the craft). For KSP this is usually measured in the TWR (Thrust to Weight Ratio) since you need a possible accelaration over 9.8 m/s^2 (TWR above 1) to lift off Kerbin

deltaV represents to the total change in velocity over the life of the craft. (so you could have a craft with a 1m/s^2 accelaration burning for 400 seconds getting the same deltav as a craft with a 400m/s^2 acceleration burning for 1 second) <- (note that this refers to vacuum as on a planet you have gravity and an atmosphere to worry about)

Edited July 31, 2013 by carazvan

[maccollo]

Why don't we just use acceleration?

Isn't acceleration "change in velocity per unit of time" or something like that.

Edited July 31, 2013 by maccollo

[dlmarti]

Isn't acceleration is "change in velocity per unit of time" or something like that.

Yes its meters per second per second.

Its not something the Human mind is comfortable with, when making micro decisions.

[capi3101]

The reason why delta-V is used and not acceleration is because the kinematic equations governing how acceleration is defined make one key assumption: that the mass of an object is a constant. With rocketry, this assumption is violated; in order to make a rocket move, you have to dump mass. End of story. Your acceleration isn't going to be a constant value because your mass changes with time, so you'd have to go down one more level of integrated gooiness and come up with a concept called "jerk", which has units of meters per second per second per second. And then if your rate of mass change with time isn't constant (such as what happens when you've got a throttle setting)......well, then your jerk isn't constant. I don't know what's after jerk...

In the long run, the Tsiokolvsky Rocket equation (which is used to calculate delta-V) involves a lot less math; just a natural logarithm and some basic multiplication.

[longhornchris]

The OP's question has been mostly answered, but I'd like to add a few more details that I think are germane to the discussion.

Delta-V can be thought of as a normalized, or linearized, measurement of a rocket's available fuel. Unlike cars, airplanes, etc, a spacecraft has 'unlimited' range but changing vector velocity either in speed or direction requires fuel OR an outside force (gravity & drag being the two most common). Delta-V is the measurement of how much change in vector velocity is available. As capi3101 noted, the Tsiolkovsky Rocket equation is logarithmic. The equation also accounts for the engine efficiency, or ISP (Ve / g) and provides a measurement that can be treated as linear.

What Delta-V allows you to do is compare rockets and probes in a normalized manner. As an example, after some practice (or mechjeb) you can find out how much Delta-V is needed to reach a specific orbit. As an example, lets say you place your orbit at an altitude that requires 3000m/s (if I remember that's between 80km and 120km but its been a while since I flew stock, I've got FAR installed). Your other rockets intended to reach that orbit should have that much delta-V (not counting propellant for use on orbit). There will be some variability for drag and gravity losses, but this gives you a baseline.

Also, the amount of delta-v needed to change orbits will (with few exceptions) be the same regardless of spacecraft. As a simple example, it will take the same amount of Delta-V to go from 80km circular to 100km circular. Larger rockets will require more fuel by mass (assuming same engines), a craft with more efficient (higher ISP) engines will require less fuel, and one with a higher thrust-to-weight ratio will take less time for each burn, but the delta-V should be the same or very close.

[tavert]

To put it yet another way: Imagine you have a rocket standing still in a perfect vacuum, with no other forces acting on it. If your rocket has 1000 m/s of delta-v, then if you fire the engines in one continuous burn without changing your heading, your rocket will be moving at 1000 m/s in the direction you're pointed once you empty the tank. If you only burn half of your fuel, you'll be moving at 500 m/s; if you flip the rocket around and burn the rest of your fuel against your direction of travel, you'll come to a complete stop again.

Careful, not exactly half the fuel. It'll be slightly more than half the fuel on a linear scale (you're heavier during the first burn), half the mass ratio on a log scale.

The reason why delta-V is used and not acceleration is because the kinematic equations governing how acceleration is defined make one key assumption: that the mass of an object is a constant. With rocketry, this assumption is violated; in order to make a rocket move, you have to dump mass. End of story. Your acceleration isn't going to be a constant value because your mass changes with time, so you'd have to go down one more level of integrated gooiness and come up with a concept called "jerk", which has units of meters per second per second per second. And then if your rate of mass change with time isn't constant (such as what happens when you've got a throttle setting)......well, then your jerk isn't constant. I don't know what's after jerk...

Sometimes called jounce. Or if you want to be cute, the 4th, 5th, and 6th derivatives of position with respect to time are semi-jokingly called snap, crackle, and pop.

As others have mentioned or alluded to, delta-V is the integral of acceleration over time.

[Specialist290]

Careful, not exactly half the fuel. It'll be slightly more than half the fuel on a linear scale (you're heavier during the first burn), half the mass ratio on a log scale.

Fair enough. Like all analogies, it isn't meant to be perfect when examined more closely than it needs to be.