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Interplanetary travel, accelerate where? Sun orbit first, or straight to other planet from Kerbin?


Magzimum
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I read several wikis and FAQs on how to get to Duna, such as this one. They recommend that you set throttle to max while in Kerbin orbit, and keep pushing it until you get a trajectory that meets up with the planet (e.g. Duna). I tried, and failed, a lot with this method.

Personally, I prefer the much easier method of getting out of the SOI of Kerbin first, where I depart from roughly the same place and keep the throttle open until I am on an escape trajectory. Then once I am in an orbit around the Sun, I plan and execute the Duna-intercept. Around the Sun, I can move the maneuver node along the Solar orbit, so if the planets weren't aligned correctly, I can correct this by just moving it ahead by a few days (or even a year if I launched at a rather bad moment). The ship will just follow along at roughly Kerbin's orbit, until it gets to the node, where I throttle up and get my Duna intercept. Maybe I am a newbie, but results with this method have been more successful, and certainly less frustrating.

I know of the Oberth effect, which says that I should accelerate when at the highest velocity (in this case that would probably be in low Kerbin orbit), so my strategy is a little less efficient, but in my opinion quite a lot easier. Does anyone know how much less efficient this is? Can it be quantified easily? Any other opinions?

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Performing a Hohmann transfer to Duna from a solar orbit near Kerbin requires a Δv of about 800 to 1000 m/s.  To this we must add the Δv that it takes to escape Kerbin in the first place, which is about 940 m/s.  This gives a total of about 1740-1940 m/s.  Performing the ejection burn from low Kerbin orbit requires a Δv of about 1050-1100 m/s.  Therefore, you are costing yourself about 690-840 m/s by executing the maneuver the way that you are doing it.

 

Edited by OhioBob
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2 hours ago, Magzimum said:

I prefer the much easier method of getting out of the SOI of Kerbin first

Easier in what way? You have to bring more fuel, so need a bigger lifter, which means more difficulty getting out of the atmosphere.

For me personally, once you get into space EVERYTHING is easy because you have as much time as you need to get it right with maneuver nodes before doing any burns, and you have hours, days, or sometimes even years to tweak things. That first few minutes from launch to orbit, though, you gotta get right all at once with no mistakes.

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Just wait until Duna is about 45 degrees ahead of Kerbin in it's orbit around the sun, then do your burn. You're pretty much guaranteed to get close enough. Set Duna as your target beforehand so that you know where your closest approach is. I think I made a professional MS Paint illustration on this for a different thread... *rummages through "Random pictures" folder* ...Here it is! This will save you literal metric tons of delta-V:

http://i.imgur.com/Ct8wrdD.png

You'll probably want to enlarge that image a bit to see the explanations.

Edited by cubinator
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It depends a bit on the TWR of your engines, but I'd guess about 2000 m/s of deltaV less efficient. Because you have about 2000 m/s more speed in LKO than you do in a solar orbit -- and one of the ways of thinking about the Oberth effect is that it's a trick to keep the extra deltaV that you already have.

Yes, it's easier to plan a maneuver when you are in the proper SOI (not Kerbin's) -- but there are ways around that.

And if you can find the proper moment when you should be leaving from Kerbin, you can combine the burn you already do to escape Kerbin's SOI with the burn you do to go to Duna -- all into one burn at LKO.

Edited by bewing
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for a trip to duna, the difference is almost "buy one, get one free". the burn to solar orbit costs about 950m/s and the burn that raises your solar orbit to duna's orbit is similarly expensive (haven't used that technique in a while, but i think it was something like 900 m/s)

doing the whole burn in low kerbin orbit reduces the total cost to about 1050 m/s. that's quite a difference. you need a much larger rocket to get a ship with so much extra fuel to kerbin orbit.

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The only way transferring from solar orbit is "easier" is that you don't have to put the node at the ejection angle in LKO.  But that's really a trivial thing to learn how to do and the fuel savings are huge.  It's very easy to eyeball both the transfer window and the ejection angle once you look up what they are, and this will get you close enough for government work.

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i found that the easiest method, when i didnt know what i was doing for IP transfers.  Once i understood how to do what i wanted and what would happen when i adjusted any 1 factor, doing a proper IP transfer was much simpler and easier.  Not to mention uses significantly less delta V, and actual time

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Oberth effect means you'll be getting much better bang for the buck in LKO than out of Kerbin SOI. But due to the curving orbit and poor precision you won't actually get the encounter.

What you do:

- Plan the maneuver in LKO, to encounter Duna.

- Disregard the "normal" factor of the burn (plane matching) and zero it after you got the encounter,

- Just perform the prograde burn.

This way you've spent great most of the fuel in the optimal way, the Oberth effect is on your side. You don't have the encounter, both due to imprecisions and wrong orbital plane, but don't worry, the first is cheaply fixed, and the second was actually more expensive "back on Kerbin".

- Plan a second maneuver about 90 degrees from the planned encounter location. This time aim for a very precise Duna fly-by, with a very low orbit. Thanks to the great distance, the delta-V required for the adjustment is small, and the norma/antinormal burn, while possibly still rather large, is still smaller than it would be otherwise.

It's still so far that getting the precise orbit you want may be difficult. Don't worry. Try to get the normal component to the dot (north-south orientation of the fly-by), don't worry too much about periapsis altitude.

- when nearing Duna's SOI perform a third burn to get the periapsis somewhere above 50km.

- perform circularization burn at Duna's periapsis. Again, Oberth effect is on your side: the lower you perform the maneuver, the more fuel you save.

Anyway:

- big, long-distance, strong-influence burns are performed in low orbit of the planet, to maximize benefits of the Oberth effect.

- precision burns that fine-tune the orbit are performed in deep space, far from the destination, to minimize disturbances of other effects.

You're not at liberty to follow this to the dot if you have a poor TWR, e.g. using nukes or ion engines on a big craft. The curvature of the departure orbit is too high to perform the burn at any reasonable accuracy. You first need to raise the orbit. Not beyond Kerbin SOI, but, say, 1000km altitude should suffice. That way you can perform the burn in a reasonably small fraction of the orbit circumference, instead of starting right towards the planet.

 

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