Planetary Intercepts - coming in too fast

[Fizbanger]

Hey guys, tried looking around but couldn't find a clear answer on this.  I'm starting to do more and more interplanetary trips and quite often I just seem to be coming in way too fast.  For example I just did a trip to Dres and once entering the SOI found that I would need to burn 3500ms to be able to circularize but looking at delta-v maps it shows something closer to 1200ms.

So my questions are:

1. Is this normal and I am just misreading delta-v maps and need to build to suit?

2. Is there a trick for most efficient intercepts (e.g. coming in prograde or retrograde)?

[Snark]

It matters how you arrive. This is what launch windows are all about.

The low velocity numbers you've read are assuming an ideal Hohmann transfer, in which your transfer orbit is an ellipse that just "kisses" the origin and destination planets' orbits at periapsis and apoapsis, without crossing them.

Here's an easy-to-spot warning sign that you have a "bad" approach: At the point of intercept, does your ship's orbit *cross* the target's, like an X? That's bad; it means a high relative velocity, since your ship and the planet are going in different directions, which is a big deal at interplanetary speeds. You want your approach to the planet to be on a path as nearly parallel as possible.

So what you need is to pick a good launch window. There are lots of good planners out there. My personal favorite is http://ksp.olex.biz.

[Mystique]

5 hours ago, Fizbanger said:

So my questions are:

1. Is this normal and I am just misreading delta-v maps and need to build to suit?

2. Is there a trick for most efficient intercepts (e.g. coming in prograde or retrograde)?

1. Most likely delta-v map uses "ideal" delta-v, based on assumpltion that you will wait till best transfer window, which means that your orbit will be quite similar to target's, which means you're heading almost same direction at almost same speed (in space scale, you know), so you won't need much "braking" to lose excess kinetic energy and remain in target's are of influence, i.e. in orbit. 

2. Like said in previous post, you can use only transfer planner if you feel like having enough time (well, it's worth doing several times on your own at least to get the idea of how it works, which will make things much easier in future) or you may use corresponding mods. MechJeb, for example, has interplanetary transfer planner with graphical interface, which will allow you to see when it's best transfer window, how much time you need to wait for it and how much time flight itself will take (it's important, because waiting time+transfer time is generally your mission's time, sometimes it's possible to spend 500-1000 more delta-v and greatly reduce this time). Even if you don't want to use this mod, it's useful to try at least in order to see how optimal and bad transfer orbit will look like and how much delta-v you'll need to spend to circularize in target's orbit in both cases. Then you'll be much closer to doing this stuff intuitively, even without using planning tools (not so precise and efficient, but c'mon, at least it's fast in real-time terms).

[Fizbanger]

The thing is I used Kerbal alarm clock to work out the right transfer window for Dres.

So I must have stuffed something up during the transfer and ended up coming in too fast.

Are there any best practice type of steps you guys follow to make sure the orbits are similar? 

[Snark]

20 hours ago, Fizbanger said:

The thing is I used Kerbal alarm clock to work out the right transfer window for Dres.

So I must have stuffed something up during the transfer and ended up coming in too fast.

Are there any best practice type of steps you guys follow to make sure the orbits are similar? 

Just eyeball the geometry.  A "wrong" orbit is immediately obvious to the eye if you know what to look for:  The right transfer orbit from Kerbin to Dres will be an ellipse whose periapsis is right at Kerbin's orbit and whose apoapsis is right at Dres' orbit.  It doesn't cross either orbit, it just "kisses" each one tangentially.

If it crosses either orbit-- that is, if the periapsis is significantly inside Kerbin's orbit, or the apoapsis is significantly outside Dres' orbit-- then it's wrong.

Note that doing a transfer burn is not simply a matter of launching at the right time; you also need to launch at the correct speed and in the correct direction.

I've never used Kerbal Alarm Clock, so I can't speak to that.  However, I can strongly recommend this tool:

http://ksp.olex.biz

Usage couldn't be easier.  You just tell it that you're starting from Kerbin, and going to Dres, and how high your parking orbit over Kerbin is.  It will tell you how many m/s to burn, in what direction, and when Kerbin/Dres are at what relative position.  Works like a charm.

If you're still having problems:  can you post a screenshot of your map view, showing the path from Kerbin to Dres?  (Ideally, taken right at the point of departure from Kerbin.)  Would make it easier to point out "here's what's wrong with that trajectory, and how you can tell from looking at it."

Edited December 27, 2015 by Snark

[Hannu2]

On 26.12.2015 at 7:46 AM, Snark said:

http://ksp.olex.biz

Usage couldn't be easier.  You just tell it that you're starting from Kerbin, and going to Dres, and how high your parking orbit over Kerbin is.  It will tell you how many m/s to burn, in what direction, and when Kerbin/Dres are at what relative position.  Works like a charm.

 

This assumes that orbits are circular and inclinations are zero. It works for most planets but those in inclined or eccentric orbits may be problematic. Optimal trajectories may differ significantly from Hohmann orbits. Moho is an extreme example but also Dres can take 1000 m/s more if Window is far from optimal. DV requirements of windows from Kerbin to Dres vary between about 2900 m/s and 3600 m/s (from 300 km circular orbit with near optimal inclination to 100 km circular orbit).

I recommend Alexmoon's porkchop plotter. There is also a mod based on that, Transfer Window Planner, which is very practical. They are somewhat more complicated to use but they take into account true orbits and give much more accurate predicions. I need typically 10-30 m/s for corrections from Kerbin to Dres. Worst cases are about 100 m/s.

[Snark]

3 hours ago, Hannu2 said:

This assumes that orbits are circular and inclinations are zero. It works for most planets but those in inclined or eccentric orbits may be problematic. Optimal trajectories may differ significantly from Hohmann orbits. Moho is an extreme example but also Dres can take 1000 m/s more if Window is far from optimal. DV requirements of windows from Kerbin to Dres vary between about 2900 m/s and 3600 m/s (from 300 km circular orbit with near optimal inclination to 100 km circular orbit).

I recommend Alexmoon's porkchop plotter. There is also a mod based on that, Transfer Window Planner, which is very practical. They are somewhat more complicated to use but they take into account true orbits and give much more accurate predicions. I need typically 10-30 m/s for corrections from Kerbin to Dres. Worst cases are about 100 m/s.

An excellent point.  However, the http://ksp.olex.biz tool works pretty well for all the stock planets, in my experience-- even for Dres or Moho.  You're right, it won't be perfect, and there will often be a 2nd burn needed (e.g. at the ascending/descending node to correct for inclination), so it's best to keep some "safety margin" dV to allow for that.  However, the stock planets are close enough to circular & coplanar that it "basically works"; it doesn't generate trajectories that are egregiously wrong.

The reason I point this out is that I think it will suffice for the OP's needs and is very simple to use.  The OP's problem is that he has a really wrong trajectory coming in-- he shouldn't need anything like 3200 m/s to circularize at Dres.  Therefore, the problem is not one of optimization, but of the essentials of "what does a Hohmann transfer look like".  I think http://ksp.olex.biz does a good, workmanlike job at that, and is also a useful tool for understanding what's going on.

[ElWanderer]

On Saturday, December 26, 2015 at 5:17 AM, Fizbanger said:

The thing is I used Kerbal alarm clock to work out the right transfer window for Dres. 

I'm not convinced Kerbal Alarm Clock's transfer window alerts are fuel-optimal. In the past I've had it tell me there's a window to Eve, only to find I needed a lot more delta-v than I'd expected. I went back to using an online planner after that.

I second Alexmoon's website. It'll tell you when the cheapest window is, but will also tell you what to expect to spend for any given transfer date. So you can double check KAC's window alerts and plan accordingly.

[FancyMouse]

Just in case - even if you transfer optimally, you might be doing something wrong elsewhere - for example I've seen people trying to burn retrograde at boundary of SoI trying to slow down. Better let us know what exact steps you did and we can debug for you which step you're doing wrong.

Edited December 29, 2015 by FancyMouse