Delta-V

[Bigun]

To start, I know what it is (the capability of a vessel in a current state, to achieve a certain number of meters / second, at least that's my understanding). But I don't understand a few things:

- What does "delta" actually mean?

- How is it calculated?

Edited April 29, 2014 by Vanamonde

[rkman]

- What does "delta" actually mean?

"delta" means difference.

For instance when launching from Kerbin's surface to low Kerbin orbit, you need to increase your speed from equal to the rotational speed of Kerbin's surface to some 2270m/s.

How is it calculated?

Depends on the situation. In the calculation, losses from moving against the direction of gravity (and drag losses in case of launch on a planet with an atmosphere) have to be taken into account.

Others have figured out how much delta-v is needed to get to the planets in ksp, and made a map: http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

To know how much delta-v your rocket has you either do fancy calculations, or rely on ksp plugins such as KER, mechjeb or VOID:

http://forum.kerbalspaceprogram.com/threads/12384-PART-0-23-5-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-2

http://forum.kerbalspaceprogram.com/threads/18230-0-23-Kerbal-Engineer-Redux-v0-6-2-3

http://forum.kerbalspaceprogram.com/threads/54533-0-23-5-VOID-Vessel-Orbital-Informational-Display

Edited April 29, 2014 by rkman

[Red Iron Crown]

"Delta" means change. So delta-V is shorthand for change in velocity.

Delta-V of a craft or stage is calculated with the Tsiolkovsky rocket equation. In short:

dV = Isp * 9.81 * ln(m₀/m₁)

Where:

Isp is the specific impulse of the engine(s), in seconds (s)

m₀ is the fully fueled mass of the craft/stage ("wet" mass)

m₁ is the mass of the craft/stage without fuel ("dry" mass)

ln is the natural logarithm function, available on any scientific calculator or spreadsheet

From this, it can be seen that to increase dV, you can:

- Use engines with higher Isp

- Increase wet mass by adding fuel

- Decrease dry mass by reducing payload or removing extraneous parts

Less obviously, you can increase total dV by separating your ship into stages, decreasing its dry mass as it goes.

[Baythan]

Well, the non-smarta** response is that (oh, post removed while I was writing, and others beat me to it as well)

"Delta" or "ÃŽâ€" means "Change." It's a mathematics/science term. As you know, ÃŽâ€v means "Change in Velocity." How it's calculated is much more complex.

On the launchpad, your rocket has a specific mass based on the weight of all of its parts and fuel. Each engine has a specific thrust and ISP (Specific Impulse, or amount of fuel used per unit of velocity gained). Some engines work better in atmosphere and some in a vacuum.

As your rocket burns fuel, it gets lighter, and therefore easier to push. This leads to Asparagus staging being the most efficient method of rocket construction in KSP (not in real life, because fuel can't be pumped that fast between tanks) because you drop excess empty tanks to shed weight. Total ÃŽâ€v is advanced calculations based on how much fuel you have and how much thrust you can generate with that fuel.

Sadly, without mods, it's not easy to calculate total ÃŽâ€v in KSP, and knowing how much ÃŽâ€v you have left mid-mission is even harder. Adding maneuver nodes helpfully tells you the amount of ÃŽâ€v needed to accomplish the burn, but nothing tells you how much ÃŽâ€v you CAN use. MechJeb and Kerbal Engineer Redeux are the two most common mods(that I know of) that calculate this for you.

There are two major factors in knowing 'Whether or Not You'll Be Reaching Space Today'; ÃŽâ€v and Thrust-to-Weight Ratio(TWR). You could have 7,000m/s of ÃŽâ€v but if your TWR is less than 1, you won't move. When building a rocket, be sure that each stage has a TWR greater than 1. The higher the TWR, the faster you'll be able to change your velocity.

Edited April 29, 2014 by Baythan

[Bigun]

"delta" means difference.

For instance when launching from Kerbin's surface to low Kerbin orbit, you need to increase your speed from equal to the rotational speed of Kerbin's surface to some 2270m/s.

Depends on the situation. In the calculation, losses from moving against the direction of gravity (and drag losses in case of launch on a planet with an atmosphere) have to be taken into account.

Others have figured out how much delta-v is needed to get to the planets in ksp, and made a map: http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

To know how much delta-v your rocket has you either do fancy calculations, or rely on ksp plugins such as KER, mechjeb or VOID:

http://forum.kerbalspaceprogram.com/threads/12384-PART-0-23-5-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-2

http://forum.kerbalspaceprogram.com/threads/18230-0-23-Kerbal-Engineer-Redux-v0-6-2-3

http://forum.kerbalspaceprogram.com/threads/54533-0-23-5-VOID-Vessel-Orbital-Informational-Display

Oh! So you could say difference velocity and be somewhat correct, also helps me understand the meaning of delta.

The equation looks simple enough, I just hate relying on plugins to calculate. But it looks like just collecting the source information (weight at beginning of thrust, weight at end of thrust, specific impulse, etc) is 90% the battle and is a major pain to collect. I'll wait on KSP to put it in vanilla. Thanks for the equations/links guys!

[Red Iron Crown]

ISP (Specific Impulse, or amount of fuel used per unit of velocity gained).

That's not quite right, Isp is proportional to the mass of fuel used per unit of thrust. Similar, but not the same.

[Red Iron Crown]

Bigun, that's some extreme lurking you've got going on. Four posts in almost two years!

[Deadweasel]

"Delta" can mean change, or more accurately in this context, "deviation".

Delta-V indicates a vessel's total potential velocity adjustment capability. In other words, if you're moving at 1000m/s with a D-V of 1000, you have the potential capability to double your velocity or cancel it out completely (assuming you're burning precisely pro- or retro-grade). If you work with maneuver nodes, you'll see the estimated Delta-V value changing as you set up your maneuver. This is the amount of change to your current velocity that will be required to achieve the currently-set maneuver.

Another delta reference we use in my line of work is "delta-E", which gives a numerical value to how far a printed color has deviated from the intended color.

[Bigun]

Bigun, that's some extreme lurking you've got going on. Four posts in almost two years!

Long time forum member/KSP user, just now getting social with it. I felt as if everyone who played this was better than me, and until I could handle SSTO's, landing modules without assistance, and docking without assistance, I would be wasting people's time even talking about it.

My latest project (indulge a little off-topic here), was putting up a "tow-vehicle" and attaching a couple of SSTO's to land on planets with atmosphere.... with Eve being my first target... heh, lesson learned. My next endevour before I head to Duna will be putting some kind of landing-gear parking brake for vehicles I attach to this beast so the wiggling will be kept to a minimum.

Image

Thanks again for the info!

[Baythan]

That's not quite right, Isp is proportional to the mass of fuel used per unit of thrust. Similar, but not the same.

I'll pretend I understand the difference.

I was just trying to explain it as best I understood it in layman's terms. I'm no mathematician or scientist, just a relatively intelligent gamer that's learned a lot from this game and I have begun trying to share my knowledge from that perspective. I figure if some people explain it scientifically and others from a gamer perspective, people will find the information they were asking for somewhere in the middle.

Long time forum member/KSP user, just now getting social with it. I felt as if everyone who played this was better than me, and until I could handle SSTO's, landing modules without assistance, and docking without assistance, I would be wasting people's time even talking about it.

My latest project (indulge a little off-topic here), was putting up a "tow-vehicle" and attaching a couple of SSTO's to land on planets with atmosphere.... with Eve being my first target... heh, lesson learned. My next endevour before I head to Duna will be putting some kind of landing-gear parking brake for vehicles I attach to this beast so the wiggling will be kept to a minimum.

Image

Thanks again for the info!

Same here, except for some stuff in the modding forum, I hadn't felt I had much to contribute until recently. My post count has more than doubled(if not tripled) in the last two weeks. Of course, some of my posts were lost back when the forums server got corrupted.

Those SSTO's look awesome, but I don't know if they'll work on Eve due to the different atmosphere(I assume that was the lesson learned). As far as making them wiggle less, you have most of their mass in front of their docking port connection to the 'tow vehicle,' so it's PUSHING them through space, which I would guess makes them flex all over the place.

Edited April 29, 2014 by Baythan

[Bigun]

...As far as making them wiggle less, you have most of their mass in front of their docking port connection to the 'tow vehicle,' so it's PUSHING them through space, which I would guess makes them flex all over the place.

It only wiggles when I turn to burn, most of the weight is being dragged behind the nuclear engines (the SSTO's engines are disabled). But if I need to make a small correction during a burn, that's when things start to get hairy.

Thanks for the compliment on the SSTO design. I was honestly worried what people would think. I used Scott Manley's tutorials to get the basic flight path of an SSTO down, then made my own model SSTO using some of the other basics in his other tutorials. After I did, it couldn't get enough Delta-V to get orbit. I actually had to make a small adjustment to his tutorial to get orbit, and managed to save even more fuel in the process.

But for the love of all that's holy, do not try to roll a vehicle with two or more ASAS modules not directly centered on the main vehicle's ASAS module. Nearly ripped the ships right off of the docking ports.

[Capt. Hunt]

Oh! So you could say difference velocity and be somewhat correct, also helps me understand the meaning of delta.

The equation looks simple enough, I just hate relying on plugins to calculate. But it looks like just collecting the source information (weight at beginning of thrust, weight at end of thrust, specific impulse, etc) is 90% the battle and is a major pain to collect. I'll wait on KSP to put it in vanilla. Thanks for the equations/links guys!

More accurately, Delta-V is another term for acceleration. In this context, it's how much your ship can either accelerate or decelerate before it runs out of fuel.

[Baythan]

Oh! I failed to notice exactly where the tug engines were! Thats my fault. I see the design now.

I had some problems with SAS ripping one of my vessels apart when trying to change direction for orbital burns and discovered it wasn't because of the 25+ large SAS I had(4 RCS claw vehicles with 6 SAS each attached radially via normal docking ports). It turns out I was trying to control the vessel from the middle instead of one end(extra fuel docked to the back of the RCS Claw carrier, again via normal docking ports), so the SAS was having trouble getting everything to orient correctly. I'd show a picture but I've since re-designed after unlocking larger docking ports, and now I'm getting Null-Refs on the new design.

Edit:

I think the new vessel is corrupted.

[KerikBalm]

Acceleration is the rate of DeltaV.

Acceleration is change in velocity per unit time.

DeltaV is change in velocity

DeltaV is not acceleration... it requires acceleration though

[Red Iron Crown]

Acceleration is the rate of DeltaV.

Acceleration is change in velocity per unit time.

DeltaV is change in velocity

DeltaV is not acceleration... it requires acceleration though

Yep. I think of it this way:

Delta-V is the amount of velocity change.

Acceleration is the rate of velocity change.

[Bigun]

Yep. I think of it this way:

Delta-V is the amount of velocity change.

Acceleration is the rate of velocity change.

You just can't get this kind of wisdom from a wiki!

[Baythan]

Yep. I think of it this way:

Delta-V is the amount of velocity change.

Acceleration is the rate of velocity change.

Exactly. ÃŽâ€v is change in velocity, or the amount of extra velocity you can give yourself in any direction, so burning against current trajectory adds velocity in the opposite direction, decreasing current velocity. So, in other words, ÃŽâ€v is POTENTIAL energy. Nothing about your vessels current ÃŽâ€v will tell you when to start doing a burn.

Acceleration, caused by thrust, is how quickly you can make that change. As lajoswinkler showed with his math, the heavier you are, the harder it is to accelerate. If you have better engines with higher thrust, you can accelerate faster because you increase your TWR. As you burn off weight your TWR increases. KSP can actually calculate how long a burn will take depending on current thrust, as you increase throttle the time for a burn decreases. What KSP doesn't do is calculate that information for you BEFORE you activate engines, so switching vessels erases it's previous calculations for current burn times.

[Vanamonde]

Okay, removed 10 posts arguing for and against "poop" jokes and comments, and we're back to the original subject of the thread. Please keep it there.

[Cannon]

I don't know much about it in ksp but we studied it in physics many times, delta v =v2 - v1

V1 is start velocity (if object was at rest it's 0 ofc)

V2 is final velocity,

And when you calculate that you get change in object's velocity.

[Red Iron Crown]

I don't know much about it in ksp but we studied it in physics many times, delta v =v2 - v1

V1 is start velocity (if object was at rest it's 0 ofc)

V2 is final velocity,

And when you calculate that you get change in object's velocity.

It's important to note that we're talking velocity here, not speed. So the subtraction operation in your equation is vector subtraction, not subtraction of the speed components.

It is possible to spend delta-V without changing speed, by making an eccentricity or inclination change in the orbit.

[rkman]

ISP (Specific Impulse, or amount of fuel used per unit of velocity gained).

That's not quite right, Isp is proportional to the mass of fuel used per unit of thrust.

I'll pretend I understand the difference.

I think you overlook that acceleration (velocity gained) per unit of fuel spent (or per unit of thrust) depends very much on vehicle mass. It also depends on gravity and/or drag loss, and Oberth effect.

[Red Iron Crown]

I think you overlook that acceleration (velocity gained) per unit of fuel spent (or per unit of thrust) depends very much on vehicle mass. It also depends on gravity and/or drag loss, and Oberth effect.

I don't think Oberth effect is relevant here, but your other points are spot on.

[rkman]

I don't think Oberth effect is relevant here

Are you sure? Don't we get more acceleration per unit of thrust, the faster we go?

[Aphobius]

Don't we get more acceleration per unit of thrust, the faster we go?

No, we don't.

[Baythan]

Well, I know my definition of ISP was not entirely correct as I didn't do much technical reading on it, I just checked the KSP wiki which linked me to the wiki and I gave a rough definition based on what that said.

What it boils down to, if I understand correctly, is that higher ISP means the engine is more efficient. This doesn't mean it has higher thrust, it just gives you more ÃŽâ€v for amount of fuel used. A different engine might have more thrust and therefore accelerate faster, but uses more fuel to do so. This is the important part when mission planning, because if you can do longer burns, you want the more efficient engine, but for shorter burns you might want more thrust instead.

Of course vehicle weight matters, that's where TWR comes in.

And now I've lost my train of thought.