Need Much More Delta V For Interplanet

[ww1612]

Try to go to Jool.

 

The Community's Delta V map and the built-in travel planer both give similar estimation.

It takes 980~1250 m/s to intercept Jool

 

As I just escaped Kerbin and circled the the orbit, the orbit height of my vessel is pretty close to that of Kerbin and the angle is near 0. 

However, when I try to make a Hohmann transfer to Jool, it needs ~2500 m/s to raise its ap to Jool's

 

The similar siltation also occurs when going to other planets.

 

So I am wondering why it need much more delta v than estimated?

 

[18Watt]

Moved to KSP2 Gameplay Questions.

3 minutes ago, ww1612 said:

So I am wondering why it need much more delta v than estimated?

I am guessing those estimates assume you depart from LKO (Low Kerbin Orbit), and are dv values in addition to the dv required to just exit Kerbin's SOI.  

 

[Ezekiel]

I takes ~2000dV to intercept Jool from a 100x100km kerbin orbit. I just did it. Matches what the deltaV chart says here: https://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

[HebaruSan]

2 hours ago, ww1612 said:

As I just escaped Kerbin and circled the the orbit, the orbit height of my vessel is pretty close to that of Kerbin and the angle is near 0. 

However, when I try to make a Hohmann transfer to Jool, it needs ~2500 m/s to raise its ap to Jool's

This is the inefficient way of going interplanetary. The reason why is complicated but conventionally goes by the name "Oberth effect" if you want to read up on it.

[ww1612]

On 12/27/2023 at 2:46 PM, HebaruSan said:

This is the inefficient way of going interplanetary. The reason why is complicated but conventionally goes by the name "Oberth effect" if you want to read up on it.

Thank you for the explanation.

I guess this effect is stronger at lower orbit?

 

Besides, to be captured by the target planet, should I aim for lower fly-by orbit for delta-v efficiency?

 

[Superfluous J]

30 minutes ago, ww1612 said:

I guess this effect is stronger at lower orbit?

Specifically, the effect is stronger the faster you're going.

I like to think of it like this:

If you're in space orbiting at Kerbin's oribt, it takes ~2500 m/s dV to get to Jool.

When you're in LKO, you're traveling ~2100m/s so you get a "boost" and - in a perfect world - would only need to add 400m/s more dV to reach Jool. It's not a perfect world, though, and you need more (Among other things, once you stop burning Kerbin will pull backwards on you for a while, slowing you down). However, even though it's more than 400m/s, it's less than 2500m/s, which is a bonus.

It's even more of a bonus because you save the ~950m/s you spent to leave Kerbin's SOI, to get to the point you burned ~2500m/s to get to Jool. 2 burns is actually closer to 3500m/s, which is closing in on DOUBLE what it takes to do it from LKO.

Edited December 29, 2023 by Superfluous J

[HebaruSan]

9 hours ago, ww1612 said:

Thank you for the explanation.

I guess this effect is stronger at lower orbit?

Yes, that's correct.

9 hours ago, ww1612 said:

Besides, to be captured by the target planet, should I aim for lower fly-by orbit for delta-v efficiency?

Yes. A higher orbit requires you to be going slower, so you'd have to burn more to capture up there.

A higher orbit also doesn't add as much potential energy to your speed while you're falling inward, which by itself would decrease the magnitude of the capture burn relative to a lower one. But the two effects together still favor lower captures.