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gc1ceo

Looking for some advice on interplanetary flybys

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Hello,

 

I've played KSP for many years but for the most part have been confined to Kerbin, Mun, and Minmus so I don't have a lot of experience with interplanetary transfers but I'm trying to break through that and having some difficulties. I do use a number of add-ons such as MechJeb and the addon version of alex's interplanetary transfer calculator.

I want to start with manned flybys in the same vein as the proposed Apollo flybys and I've built my current vessel to give me about 1800 dV and about 200-300 kN of thrust (I've played around with a couple of different engines). I've watched a number of videos and read some tutorials on interplanetary transfers but they're either not very helpful or I've obviously missed something.

My closest success so far has been a flyby of about 11-20 million km from Duna using about 900-1200 dV however I feel completely clueless on how to get back to Kerbin from a flyby or how to get in for a closer flyby. My mission goal with the current vessel is for a Duna flyby of under a million km and then return in a reasonable time back to Kerbin although I want to do it without having to double my current craft.

Suggestions, pointers, etc would be helpful and as I said before I have read and watched a couple of tutorials to get the basics of interplanetary transfers and I've handled hops back and forth between Kerbal, Mun, and Minmus pretty successful for many years now.

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4 hours ago, gc1ceo said:

I've built my current vessel to give me about 1800 dV...

Why restrict yourself like this? Why not build a craft with, say, 6000dV and give yourself lots of room for trial and error?

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First of all: build as big a rocket as possible. If you aren't playing career, then go for it and overcompensate. Once you are a more refined player, you can build smaller and more efficient rockets.  

Second of all (to actually answer your question): If I'm completely honest, most of the skill I've gained has been from trial and error - just fiddle with maneuver nodes until you get it right. Sometimes I feel restricted to only use retrograde/prograde - but always try the others. Even use the circle ones (I forget what they're called :P) because they can actually be pretty useful sometimes, especially adjusting flybys. To get a closer flyby, first get an encounter with the planet you are heading to. Then make a maneuver node and focus on your destination. fiddle with the nodes and see how your flyby is changed - then do it with the actual engines. If it's too hard to memorize what you did with the node, then just use the actual engine for it. Tiny burns like the ones you will use barely take and fuel, just remember to quicksave so you don't accidentally un-encounter and can't get back to it. 

As for getting back to Kerbin from flybys - pretty much the same thing - trial and error. Find out the transfer angles and stuff and then just wing it. Just like I said with the rocket size, don't try to make things refined or super efficient - there will be time for that later. Just get your Kerbals and their precious science back to Kerbin. Even if that means getting Jeb out and making him RCS-push the capsule home. Just have fun.

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My builds are essentially based on the proposed Apollo flyby vehicle which was essentially the S-IV with a habitat module and some other modifications, one of them uses BDB while others are just using stock parts. This restricts the size and mass of the overall vehicle so I need to work within certain dimensions. I know from playing around with the plots that at least 1000 dV was reasonable to make the flight to Duna although it restricted me to certain launch windows which was ok. I was regularly able to make an intercept with Duna for about 1030-50 dV but decided I needed dV to make the return so was able to get my available dV up to about 1800 while keeping my builds' basic dimensions.

I've been trying to fine tune with additional nodes but it only helps so much and I'm really unsure where I should put these fine tune nodes, from what I've read I've seen people mention 2-3 different nodes to really fine tune the intercept but the details on when to make them seem sparse. The larger problem is that doesn't seem like any of the tutorials are based around manned flybys and the ones that mentioned manned flybys don't seem to give enough information.

 


 

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With Duna you could consider aerocapturing into orbit and then waiting there until the next Duna-kerbin transfer window. The aerocapture is almost free in terms of Delta-v and heating isn't much of an issue (as long as you can retract your antennas and solar panels to protect then from aero forces you should be good.)

Don't forget to click on Duna once you've got an encounter and select focus view to fine tune the manoeuvre.

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"about 1800 dV"
Mun it is then (860m/s each way), or Duna (1,130 m/s each way) without drawing breath and hoping to get good aerobraking.
"11-20 million km from Duna"
See, the thing is you can do some stuff with a weak rocket and you can fly a good rocket with weak skills.
Both weak together - doesn't really work.

You could try MJ getting your Duna intercept but I wouldn't even trust that with so little fuel.  You're just going to have to practice with the maneuver nodes to get a better intercept.

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Two things to add on to the useful advice above:

-Sounds like you're familiar with the concept of launch windows (i.e., when the phase angle of the two planets puts them in the right place for a low-delta-v transfer).  If you're planning a flyby-and return, then you're probably NOT departing Duna at a favorable launch window for the return to Kerbin.  In other words, even if you adjust your orbit to take you back out to Kerbin's, Kerbin is probably not going to be in the right place when you get there.  This is not really an easy problem to solve.  Either you need to be willing to wait lots and lots and lots of orbits for a better alignment, or you need to expend delta-v to make an intercept happen sooner. So this is another reason your return margin is probably just too tight.  

-On a flyby, you should also take note of how your orbit around the sun changes before vs. after being in Duna's SOI.  It will most likely get altered in some fashion by the interaction with Duna's gravity.  This is a gravity assist, and can be very useful in the right circumstances.  But if you're not planning for it, it's equally likely to screw with your return path a bit.  So if you're not particularly interested in getting as close to Duna as possible, you may want to fine-tune your approach to Duna's SOI so that you end up with a favorable assist for your return mission.  Duna has relatively weak gravity so none of this is nearly as big a deal as at Eve or Jool, but just something to think about if you're on really right delta-v margins.  

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To get closer, you just do a small correction burn mid course.

It is actually easier to insert into orbit and then wait for a transfer window to go home.

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9 hours ago, Reactordrone said:

With Duna you could consider aerocapturing into orbit and then waiting there until the next Duna-kerbin transfer window. The aerocapture is almost free in terms of Delta-v and heating isn't much of an issue (as long as you can retract your antennas and solar panels to protect then from aero forces you should be good.)

Don't forget to click on Duna once you've got an encounter and select focus view to fine tune the manoeuvre.

 Possibly, although I have to get alot closer to Duna to do an aerocapture which is the first part of my problem.

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46 minutes ago, TheFlyingKerman said:

To get closer, you just do a small correction burn mid course.

It is actually easier to insert into orbit and then wait for a transfer window to go home.

Oh I definitely agree I need to do some correction burns, the real question is where there's the best place to do that in my orbit of Kerbol (The Sun).

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4 hours ago, gc1ceo said:

Oh I definitely agree I need to do some correction burns, the real question is where there's the best place to do that in my orbit of Kerbol (The Sun).

About halfway to your destination.

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Heading to Duna, I'd say even earlier than that's likely to be better.  As a rule, plane-change maneuvers are best done either as high as possible, at the AN/DN, or midway, while prograde/retrograde and radial in/out (which is handy for adjusting arrival times) are best done as early as possible.

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Ok I'll play around with it, I'm familar with the AN/DN guideline for changing planes and that you make a bigger effect with other changes the earlier you do them in the trip but my experience is largely just Kerbin/Mun/Minmus.

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Adding to the other great advice above. Why not start with a simple probe so you can practice technique. Even if you're going for "realistic" it's worth remembering that NASA wouldn't throw people at another planet until they'd done a few "practice runs" with unmanned probes.

Edited by Tyko
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I decided to add some more fuel giving me 2395 dV while staying near to my desired dimensions and have successfully made some flybys using a mixture of launch windows and small correction burns along with returns to Kerbin. I'm still experimenting but I found myself making very small correction burns around the time I left the Kerbin SOL to be the most helpful and I got myself to within a hair of Duna's atmosphere so I think I'm on the right track.

Edited by gc1ceo
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