Reading the DV charts

[Some Kerbal]

Getting ready to start venturing out from the "safety" of Kerbol and looking to make sense of the dV charts.

Are the numbers on one of the charts it takes 1140 dV to get from Kerbol SOI to Eeloo intercept. Does it take an equal amount of dV to get from Eeloo back to Kerbol SOI. IE are the numbers the same for both directions?

[foamyesque]

1 hour ago, Some Kerbal said:

Getting ready to start venturing out from the "safety" of Kerbol and looking to make sense of the dV charts.

Are the numbers on one of the charts it takes 1140 dV to get from Kerbol SOI to Eeloo intercept. Does it take an equal amount of dV to get from Eeloo back to Kerbol SOI. IE are the numbers the same for both directions?

 

Yes, they're reversible, though some factors (atmospheres, particularly) can make it a lot easier to go one way than the other.

[Streetwind]

Before I get to the dV map stuff, I'd like to help you get your terminology right, because what you wrote is liable to confuse some people. The homeworld of the Kerbals is Kerbin. There is no place called "Kerbol" in the game, but some community members unofficially call the Sun by that name. You just referred to Kerbin as Kerbol, which may get people to think you're talking about solar orbit. As KSP is a fairly complex game, it is perhaps more important here than in other games to be very precise with your language, in order to avoid misunderstandings.

Now, as for dV maps: yes, they work both ways. Going from Kerbin to Eeloo takes about the same amount of dV as going from Eeloo to Kerbin - with caveats. For example, if you were to use things like gravity assists or aerobraking, then you'd pay less on one or the other leg of the trip than the map tells you. Also, the numbers assume a perfect Hohmann transfer. This is only valid during the "transfer window", a short moment in time when the planets are aligned just right. If you just fly to Eeloo, do your science and leave again, you're not going to get the same dV cost for the trip back as you had for the trip there, because you're flying out-of-window. You generally need to leave your spacecraft sitting in orbit of the target planet for several months until the "return window" comes up. But how long exactly that takes depends on each individual trip.

Also keep in mind that you must add up all numbers sequentially to figure out the total cost of your trip. In order to go to Eeloo from Kerbin, you must add up the costs of launching into orbit, escaping Kerbin, intercepting Eeloo, capturing at Eeloo, and finally lowering your orbit at Eeloo (or even landing).

[KevinW42]

I assume you're talking about this?

What the chart is saying is that from LKO, it takes 950 dV to escape Kerbin, and another 1140 dV to get an Eeloo intercept. Ideally these should both be done as one burn to take maximum advantage of the Oberth effect. If you just barely escape Kerbin and wait until you get to the Sun's SOI to get your Eeloo encounter, it's probably going to take a bit more dV. The 1330 on top of the previous number is there because Eeloo has an inclined orbit, so depending on what part of its orbit it will be in when you get there, you'll have to spend somewhere between 0 and 1330 dV on a plane change maneuver. Once you get to Eeloo, your relative velocity will be pretty high, so you'll be on a hyperbolic trajectory and you'll need another 1370 to enter a low circular orbit.

Heading back you just go in reverse. It's 1370 dV to escape Eeloo and get a Kerbin encounter (plus 0-1330 for plane changes). Then when you get to Kerbin it's another 1140+950 to get to LKO, but the arrows indicate that you can just let the atmosphere do all that work for you.

Hope this helps!

[GoSlash27]

3 hours ago, KevinW42 said:

If you just barely escape Kerbin and wait until you get to the Sun's SOI to get your Eeloo encounter, it's probably going to take a bit more dV.

It's definitely going to take a lot more DV. All of these charts assume the Oberth effect. Getting to the edge of Kerbin's SoI is 952 m/sec DV. Getting a Hohmann transfer from Kerbin's altitude to Eeloo's SMA is 2,955 m/sec, which varies widely due to Eeloo's eccentricity. Doing the burns seperately is 3,907 m/sec. Doing them both from LKO is 2,094 m/sec DV. Nearly twice the DV in this case.

Best,
-Slashy

[steuben]

I don't doubt the numbers. But do you have a non-heavy mathy reference for it... or a light mathy reference.

[Superfluous J]

41 minutes ago, steuben said:

I don't doubt the numbers. But do you have a non-heavy mathy reference for it... or a light mathy reference.

Yes, it's called KSP.

Get into LKO. create a maneuver node about 1/4 the way into the dark side of the planet, and drag it prograde until the resultant Sun orbit's apoapsis is about at Eeloo's orbit. Note how much it costs. It should be in the ballpark of 2000m/s.

Delete that node, and make a new one that just escapes Kerbin's SOI. It should be in the ballpark of 1000m/s.

Go out into the Sun Orbit of that 2nd maneuver node, and create another node and drag it prograde until the resultant orbit reaches Eeloo's orbit. It should be in the 3000m/s ballpark.

And there you go, a no (or as light as possible) math proof.

[Streetwind]

40 minutes ago, steuben said:

I don't doubt the numbers. But do you have a non-heavy mathy reference for it... or a light mathy reference.

Are you referring to calculating the Oberth Effect? If so, you don't have to worry about that. The dV maps already have it factored in, so you can follow those numbers relatively blindly.

For cases other than ideal Hohmann transfers from and to low orbits, I'm afraid that there is no such reference. There is an infinite number of possible variations of initial conditions, leading to an infinite number of results. Only when you define most of the initial conditions as fixed - such as what the dV maps are doing by calculating perfect Hohmann transfers from and to low orbits - you can receive singular answers.

[GoSlash27]

1 hour ago, steuben said:

I don't doubt the numbers. But do you have a non-heavy mathy reference for it... or a light mathy reference.

Steuben,

Escape velocity from any circular orbit is Vorb* sqrt(2)*. If you're going 2,296 m/sec in LKO, then Vesc is 3,247 m/sec and DVesc is 3,247-2296= 951 m/sec.

Calculating the DV to get from Kerbin's SMA to Eeloo's SMA is a bit more complicated, but @OhioBob has a good reference for it here:

http://www.braeunig.us/space/orbmech.htm

The important thing is that whatever this DV is, we treat it as "Vinf", or excess hyperbolic velocity.

To calculate the total DV, it's sqrt(Vesc²+Vinf²)-Vorb. That is, escape velocity and DV to transfer are vector added pythagorean style, then the original orbital velocity is subtracted to factor in Oberth Effect. This is how all the vacuum values are calculated for the DV maps.

* Escape velocity is actually less than this in KSP because it uses patched conics, but it's a safe value to use.

HTHs,
-Slashy

Edited December 13, 2016 by GoSlash27

[OhioBob]

I just recently made a Δv map for Galileo's Planet Pack.  It was actually quite a pain in the neck because their is so much variation from one transfer window to the next, it was hard deciding what numbers to put on the map.  The maker of the map has to assume something or they'd never be able to make the map.  The odds are, your actual conditions are going to be different then what was assumed.  Therefore the values on the map have to be used as with a lot of skepticism.
 

Edited December 14, 2016 by OhioBob

[Snark]

Here's another handy tool for checking dV:

http://ksp.olex.biz

It's simple and easy to use, for transfers from one planet to another.  Just tell it the height of your parking orbit around your origin, and it'll tell you where and how much to burn to eject to go to the target.

You can use it reversibly, too.  Let's say you're going from Kerbin to Dres.  You put in Kerbin as origin and Dres as destination, and it'll tell you how much dV you need to eject from Kerbin to go to Dres.  Then you switch them around, and use Dres as origin and Kerbin as your destination... and it'll tell you how much dV you need at Dres to capture to orbit.  Very handy. 

Completely unrelated-to-your-problem nit in spoiler section.

Spoiler

7 hours ago, Streetwind said:

There is no place called "Kerbol" in the game

Certainly this is correct.  Fire up KSP, and you won't find anything anywhere in the game that says "Kerbol".

On the other hand,

7 hours ago, Streetwind said:

some community members unofficially call the Sun by that name

...I think the unofficial "Kerbol" name is a little more widely-recognized than that.  Cases in point:  there's the official KSP wiki, whose primary designation for the sun is Kerbol, though it does acknowledge the name's unofficial nature.  (If you try searching the wiki for "Sun", it redirects you to "Kerbol").

Then there's the first paragraph of Squad's official announcement of 1.2,

On 10/11/2016 at 0:40 PM, UomoCapra said:

Kerbals hit a milestone, they are now interconnected and they can hear you Loud and Clear! Let your imagination fly with new possibilities, build communication networks, control vehicles remotely and explore every inch of the Kerbol System in ways that weren’t possible before with this new update!

...or if you prefer something more recent,

On 12/1/2016 at 0:36 PM, UomoCapra said:

Kerbin’s position and tilt in relation to Kerbol makes it very similar with planet Earth

I hasten to add that I'm not particularly advocating for one terminology or the other, and certainly there's no "official" name... but I think it's a little more broadly accepted than "some community members unofficially call."  Certainly it appears to be what Squad folks tend to refer to it as, informally at least.

 

[paul23]

3 hours ago, Streetwind said:

Are you referring to calculating the Oberth Effect? If so, you don't have to worry about that. The dV maps already have it factored in, so you can follow those numbers relatively blindly.

For cases other than ideal Hohmann transfers from and to low orbits, I'm afraid that there is no such reference. There is an infinite number of possible variations of initial conditions, leading to an infinite number of results. Only when you define most of the initial conditions as fixed - such as what the dV maps are doing by calculating perfect Hohmann transfers from and to low orbits - you can receive singular answers.

Anything other than a perfect hohmann transfer is always "required" if you don't have the perfect launch opportunity - or have other time constaints (ie with a life support system). I always compare hohmann transfers with throwing balls: if you want to reach the basket when you throw a ball the least amount of energy/force you need to apply is if you "just" reach it. You just reach the height when throwing. However this is also the slowest throw: you can throw faster if you throw harder and then block/stop once you reach the height of the basket.

Now there are of course an infinite number of speeds you can throw: when throwing a ball this doesn't matter. However consider the basket is moving, then to reach the basket you have to throw the ball at exactly the right moment with the right speed. - If the basket is closer you throw faster (instead of the minimal speed).  This is also how intercepts work (Ok it would also mean your platform from which you throw is moving): there is an ideal speed & time to launch, and there are an infinite number of other speeds/direction at which you can move towards your target.

To analyse these in space engineering we use something called a "porkchop plot": 

The horizontal axis in such a plot is the date of departure, and the vertical the date of arrival. The red lines are the trip time. But most importantly are the blue contours: they describe a line of "constant C3 energy" - translating to a "constant delta-V". Thus the ideal departure dates would be at the center of these "porkchops". Typically there are about 2 dates per slightly less time than a year.

In above plot the least energy would mean an escape energy (C3) of roughly 15 km^/s^2 - launching early September. - Though the mission would take over 400 days. Maybe due to mission requirements/life support limitations a faster launch is more benificial: for just under 16 m/s we can launch at the 10th of June: taking less than 200 days to reach our destination. - So 1 m/s increase (but greatly different launch dates)  means 200 days less of life support/mission maintenance. This is a tradeoff the mission designer has to make.

Maybe even that time (200 days) is not fast enough and mission requirements (retrieval of an object) state we HAVE TO be there before 2006: then our only option is to launch early august and go for a C3 energy of 25-30 km^/s^2. These porkchop plots tell us exactly the mechanical requirements of a transfer.

 

Now these porkchops are basically numerically decided: by just testing a lots of arrival/departure conditions (and they can hence take slingshots etc into account); this can be done in kerbal space program either manually (very, very time consuming) or just using mechjeb. (Sadly only for already flying spacecraft.)

 

Delta-V maps are ideal for comparing the delta-V between planets. Once you decided a target, a porkchop plot will give you the exact delta-V needed and is optimized for that trajectory: Without porkchop plots you'll always be shooting in the dark.

Edited December 13, 2016 by paul23