Estimated And Actual Delta V's Don't Line Up

[Sauron]

So, I'm flying a mission to Jool (eight missions actually--I'm confining myself to some *very* small rockets without any Rockomax parts--but that's neither here nor there). While I am not flying the mission at the absolute best possible time, Protractor is giving me an estimate of about 1.6 km/s, but placing my nodes in the correct place, I've not managed to get an encounter with anything less than ~2 km/s (and my lander-carriers only have 2.25 km/s of delta-v, so there's really not a lot of margin for error). What's the problem here? Can I do anything better, or is it back to the drawing board?

[tavert]

What altitude are you starting from? I don't know what Protractor's getting 1.6 km/s to Jool from, since 2 km/s sounds closer (from LKO). Try http://alexmoon.github.io/ksp/ as it's more accurate than Protractor, taking eccentricity and inclination into account.

[Mr Shifty]

It's not possible to get a better burn than about 1900 m/s for a direct transfer. If you gravity sling off of Duna or Dres, you might be able to get there with less.

[Sauron]

Oh, right. Sorry. Forgot to mention that I'm orbiting at 140 km (which may account for a few hundred m/s). The second calculator is giving about 2.9 km/s for the insertion burn (which is really high!). Maybe I'm launching at a particularly bad time and protractor isn't factoring it in? Can't see why though. Jool's orbit isn't terribly inclined, and the SOI is large enough that it shouldn't matter that much.

In any case, those ships need to be redesigned by the looks of it (Bob and Bill are going to Duna again instead!). Bummer, that. I'm only allowing myself 10 ton payloads to LKO, so I'm probably going to have to fly three missions to get a Jool-capable ship that can actually move a lander.

[stupid_chris]

Protractor is highly not precise for the delta V needed, they'Re just a general indicator.

[Sirrobert]

Did you try making a node yourself and seeing if you get encounter?

[tavert]

Oh, right. Sorry. Forgot to mention that I'm orbiting at 140 km (which may account for a few hundred m/s). The second calculator is giving about 2.9 km/s for the insertion burn (which is really high!). Maybe I'm launching at a particularly bad time and protractor isn't factoring it in? Can't see why though. Jool's orbit isn't terribly inclined, and the SOI is large enough that it shouldn't matter that much.

The 2.9 km/s is probably including a capture burn. In order to assume aerobraking and only count the departure burn, leave the Final orbit altitude around Jool blank.

[Sauron]

The 2.9 km/s is probably including a ejection burn. In order to assume aerobraking and only count the departure burn, leave the Final orbit altitude around Jool blank.

No, that's just the capture burn! (I was as surprised as you ) The capture burn was another 1.2 km/s in addition.

My main issue is predicting this stuff accurately. Because I'm deliberately using the smallest possible rockets, I need to have a fairly accurate idea of my delta-v budget. The numbers I was getting with an encounter weren't lining up with protractor's, and the non-hohman calculator linked here is throwing out huge numbers. So I'm just trying to check my work here

Edited June 21, 2013 by Sauron