Up to date Delta-V map

[haltux]

Hello,

I have just executed a mission to Jool and the Delta V requirements were way bigger than what they were supposed to be according to available delta-V maps. To get an intersect from Kerbin escape takes 2800 m/s, and not 980 as stated on the maps. Also orbiting Kerbin takes 4500 m/s, not 3500.

Did I misunderstand something or are these maps completely obsolete? Are there somewhere up-to-date maps?

Thanks

[ElWanderer]

4500m/s for Kerbin orbit is indeed out of date.

However, ejection from Kerbin to another body involves adding together two or three numbers: the amount to leave Kerbin's sphere of influence plus the amount to intercept the desired body, plus (potentially) the worst case inclination change value

https://m.imgur.com/yO0bQax

Personally I would use Alexmoon's transfer planner website to find out how much delta-v is required for the specific transfer you'll be doing. It can vary quite a bit depending on the relative positions of the bodies and whether you need a big inclination change or not.

Edited October 24, 2020 by ElWanderer

[HebaruSan]

 

26 minutes ago, haltux said:

Did I misunderstand something

Probably; the delta V maps haven't changed in a long time (if ever); the devs generally don't change the planets' orbits.

26 minutes ago, haltux said:

I have just executed a mission to Jool and the Delta V requirements were way bigger than what they were supposed to be according to available delta-V maps. To get an intersect from Kerbin escape takes 2800 m/s, and not 980 as stated on the maps.

Did you attempt this in a transfer window, or any old time you felt like it?

[Snark]

Moving to Gameplay Questions.

[jimmymcgoochie]

If you aren't using a rescaled solar system (which I doubt) then the only reason I can think of for your abnormally high delta-V requirements is that a) your launch was really inefficient and b) you did the transfer burn at the wrong time or with a really low TWR which does bad things to the accuracy of transfer burns.

To get the most efficient launches possible, try playing the tutorials about launching to orbit or use the GravityTurn mod to automate your launches and learn from seeing it succeed (or fail!);  there are a few different ways to do transfer burns, but Astrogator is perhaps the easiest to use as it shows you the time until the ideal transfer window, the delta-V required to get there and can even create the node(s) for you, though some fine-tuning is often required. It can also be a good idea to split your transfer burns when you have a low TWR- it will not only reduce the inaccuracy by having shorter burns, but also allows you to correct your nodes after each burn to stay on course and will maximise Oberth effect, which is a good thing. Maneuver Node Splitter can do this automatically but you can also do it manually by creating a series of burns to push your apoapsis higher each time until the final transfer burn pushes it out of Kerbin's SOI completely.

[Souptime]

I just use astrogator, makes transfers REALLY easy

[Superfluous J]

The m/s dV to get from Kerbin surface to Kerbin orbit is a function of a LOT of things, including TWR at launch, gravity turn efficiency, engines used throughout the flight, atmospheric drag, and who knows what else. 3300m/s is indeed a good estimate, and has been since KSP left beta and 1.0 was released. If you are taking 4500m/s your rocket and flight plan can very likely be optimized.

It does not take 980m/s to reach Jool from LKO. It also does not take 2800m/s. With a reasonable TWR it should take roughly 950+980=1930m/s. The first 950 will get you an orbit that just escapes Kerbin, and the remaining 980 will extend your Sun orbit all the way up to Jool. Note you can't just hit the gas you must do it at the right time both of the year (so Kerbin and Jool are at the right place relative to each other) and your orbit (so you're adding thrust in the correct direction).

I'm guessing you're burning 950 to leave Kerbin's SOI, and then expecting 980 to get you to Jool from there. That's not the way it works, sadly.

Also note that there are two basic measurements of dV, "atmo" and "vacuum". While it's useful to know your Atmo dV, we tend to always refer to Vacuum dV. The game shows Atmo dV by default, and you need to change this - every time you go into the VAB - or your numbers will not show you accurate information with regards to the dV map.

Edited October 25, 2020 by Superfluous J

[king of nowhere]

sign me up on "your trajectories were pretty bad" train.

the deltaV maps are accurate, but they show the very minimum amount you need if you optimize everything well. most often, you will need a bit more. nominally, it takes 580 m/s to land on mun, but i use about 650-700 in a normal situation. once in a challenge i had to optimize things really hard, and by using a very dangerous trajectory that came extremely close to crashing on the surface, i managed even to save 10 m/s on what's nominally required. For orbiting tylo, instead, i never could do it without spending at least 50 m/s more than the map says. and you could get to jool with 1930 in theory, but 2000 is a more likely scenario.

On the other hand, you can save fuel on interplanetary transfers with gravity assists.

on the other hand, 2800 for a jool intercept or 4500 for a kerbin orbit are NOT normal. the kerbin orbit value makes me think you burned straight upwards until you were out of the atmosphere, and then circularized; back when i was new to the game i was doing it, and i had similar expences. you may also have horrible aerodinamics increasing your costs. not sure what you could be doing wrong for jool intercept, but there are many possibilities.

anyway, you can show us some screeenshots of your manuevers and we can ty to give advice on how to improve efficiency.

[haltux]

First, thanks for all your answers and sorry for not having reacted faster.

My issue is the cost of the trip to Jool. I said it takes 4500 m/s to orbit, that was a rough estimation, I know it depends of plenty of parameters, so let's say I just said somethin wrong, that was not the main post of my question, sorry for those who took time to anwser to me on this.

But for the Jool transfer I am still not understanding what is wrong.

Here is a screenshot, not from Kerbin but from an orbit around Kerbol slightly closer to Kerbol. Still you get the idea:

3109 m/s just to reach Jool orbit from Eve/Kerbin circular orbit.

 

On 10/25/2020 at 4:44 PM, Superfluous J said:

It does not take 980m/s to reach Jool from LKO. It also does not take 2800m/s. With a reasonable TWR it should take roughly 950+980=1930m/s. The first 950 will get you an orbit that just escapes Kerbin, and the remaining 980 will extend your Sun orbit all the way up to Jool. Note you can't just hit the gas you must do it at the right time both of the year (so Kerbin and Jool are at the right place relative to each other) and your orbit (so you're adding thrust in the correct direction).

As you can see on the screenshot in my case "extend [my] Sun orbit all the way up to Jool" does not cost 980 but 3109 m/s. It would be a bit less just after a Kerbin escape because I would be a bit further from Kerbol. It would be 2800 m/s.

This is driving me mad because I know I did that long time ago, I would have noticed it if the delta-v map would have been that off. So I assumed it was a recent change, but you tell me it's not. 

I am even wondering whether I could have installed a mod at some point that never properly uninstalled. But I checked, nothing suspicious in my gamedata.

On 10/25/2020 at 12:09 AM, HebaruSan said:

Did you attempt this in a transfer window, or any old time you felt like it?

My issue is not to get an intercept but just to extend my apogee to Jool orbit. So it does not matter when I do it. But to answer to your question yes, I attempt that in a transfer window.

 

Edited October 28, 2020 by haltux

[FleshJeb]

@haltux The problem is that you're planning that transfer from interplanetary space, and not from Kerbin orbit. It's missing out on the considerable savings of the Oberth effect. If you do the planning when Kerbin is in the position where you've got your node, you'll see a lower number.

Is your ship currently outside of Kerbin's sphere of influence? It seems so from the orbital velocity shown on the navball.

[HebaruSan]

41 minutes ago, haltux said:

not from Kerbin but from an orbit around Kerbol slightly closer to Kerbol

from Eve/Kerbin circular orbit

It would be a bit less just after a Kerbin escape because I would be a bit further from Kerbol

It's probably worth eliminating those confounding factors; here's a 1924 m/s burn from Kerbin orbit, with Jool encounter:

[Superfluous J]

5 hours ago, haltux said:

As you can see on the screenshot in my case "extend [my] Sun orbit all the way up to Jool" does not cost 980 but 3109 m/s. It would be a bit less just after a Kerbin escape because I would be a bit further from Kerbol. It would be 2800 m/s.

As others have already said, you can't burn from Sun orbit. Your only burns from Sun orbit should be to make little corrections and tweaks. You don't burn 980 in LKO, and then 950 in Sun orbit. You burn 1930 in LKO. All at once.

Do this, with your ship in LKO.

• Make a maneuver node about 1/3 of the way around the dark side of Kerbin, so you're doing the burn in the dark but not yet opposite the Sun.
• Make the maneuver node about 1930 m/s.
• Zoom out to see what your resultant Sun orbit will be.
• Be amazed

 

Edited October 29, 2020 by Superfluous J

[haltux]

On 10/29/2020 at 4:13 AM, Superfluous J said:

As others have already said, you can't burn from Sun orbit. Your only burns from Sun orbit should be to make little corrections and tweaks. You don't burn 980 in LKO, and then 950 in Sun orbit. You burn 1930 in LKO. All at once.

Do this, with your ship in LKO.

• Make a maneuver node about 1/3 of the way around the dark side of Kerbin, so you're doing the burn in the dark but not yet opposite the Sun.
• Make the maneuver node about 1930 m/s.
• Zoom out to see what your resultant Sun orbit will be.
• Be amazed

 

I am amazed! Thank you all, that was the problem. I was starting from Minmus and making the main burn once escaped. Now I tried going back to Kerbin just for a fly by, then going straight to Jool, and it worked great. It made it a little more difficult to time everything for an optimal trip Minus-Kerbin-Jool but it was worthing it.

Now I realize how awfully difficult it became to aerobrake at Laythe since last time I tried (which was many years ago). But that's another problem...

[Echo__3]

You need to make sure you are using a good gravity turn to get into orbit. Also make sure you are using a good transfer window when you eject from Kerbin's sphere of influence. These videos might explain why you are using so much ∆v.

 

 

 

[king of nowhere]

On 11/2/2020 at 9:40 PM, haltux said:

I am amazed! Thank you all, that was the problem. I was starting from Minmus and making the main burn once escaped. Now I tried going back to Kerbin just for a fly by, then going straight to Jool, and it worked great. It made it a little more difficult to time everything for an optimal trip Minus-Kerbin-Jool but it was worthing it.

Now I realize how awfully difficult it became to aerobrake at Laythe since last time I tried (which was many years ago). But that's another problem...

to expand, oberth effect has quite a complex explanation, but the overall result is that a prograde/retrograde burn works better the deeper you are into a gravity well. you can experiment this in kerbin orbit; start in a low circular orbit, then make a big periapsis raise, say by 500 m/s. then on apoapsis circularize, and keep track of the expence. it will be more than 500 m/s, because you'll be farther from kerbin and your burn will be less efficient.

as you already discovered, when you are on minmus the most optimal trajectory to go far is to fall back on kerbin, and make the big burn at periapsis, to get advantage of oberth effect.

 

as for aerobraking on laythe, you are better off using a gravity assist from it to get into jool orbit.

[Spricigo]

1 hour ago, king of nowhere said:

oberth effect has quite a complex explanation

I disagree. In layman terms it's just "the same  change of velocity produces a greater change in kinetic energy if going faster"

What is a lot more complex to figure out is how much of an effect it will have in the actual maneuvers, specially when we also consider several other factors that will affect it.

[king of nowhere]

7 hours ago, Spricigo said:

I disagree. In layman terms it's just "the same  change of velocity produces a greater change in kinetic energy if going faster"

for "explanation" I mean "why that happens". and it has to do with kinetic energy being distributed differently between the ship and exhaust. And that's rather complex, especially if we want to dig into the calculations. Just stating the principle is pretty straightforward, though, i'll give you that