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Tips for an interplanetory beginner


paul_c

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I've been playing KSP for a number of weeks and now Kerbin-orbital and Mun/Minmus trips are consistent, after a run of bad luck trying to land on/return from Mun. So I gathered a bunch of Duna/Ike contracts and the time has come to do the trip. I'd been twice before, but they were not-too-serious attempts with a big overpowered rocket and a bit of luck too. I transmitted a little bit of science data, not much, then got "lost in space".

So on my third trip I'd done a bunch of planning, and was taking an 8-pax station with a science lab and some Kerbals. I'd done a few tests on electrical demands (it seems 8x 1x6 solar panels is a good rule of thumb to keep up with a busy Science Lab) and I understand comms required transmitting power too. 

I knew the timing would be critical so I read up on the angles etc and made the trip, after waiting a number of days in Kerbin orbit (and was able to check out the science gear too). I got a burn which took me to an encounter with Duna but the Pe was 18Mm, and the orbital insertion burn was ~1000dV. The specs say it ought to be closer to 250. It was so finely balanced when & how much to do the outwards burn I was afraid to fiddle with it too much. I am guessing some kind of mid-course normal/antinormal or radial in/out would have lowered my Pe right down? 

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Yes, interplanetary transfers are always extremely finicky for the intercept. Really, all you can do during your initial transfer burn is to get any intercept at all. You always need a mid-course correction. That is always still fiddly, but it's doable to get a nice low Pe that minimizes your capture burn. For duna, it's fine to even lower your Pe all the way down into the atmosphere -- it's thin enough that there are no heat issues.

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Interplanetary travel is difficult in KSP because there's nothing to say when you should make your transfer burn and you can't easily create nodes far in the future. An 18Mm periapsis and 1km/s capture burn suggests that you made the burn at a time when the two planets weren't well aligned meaning you have a lot of excess velocity when you arrive at Duna, which to me suggests you burned too late. Generally speaking, when heading outwards from the sun you want the target planet to be about 45 degrees ahead of the one you're orbiting, and when heading inwards it's about 45-60 degrees behind the one you're orbiting.

It's often easier to make a pure pro/retrograde transfer burn towards the target and then match its inclination in solar orbit, which also gives you an opportunity to correct your trajectory and adjust your destination periapsis or to compensate for an inaccurate transfer burn- common for long burns with low TWR craft as many interplanetary vessels are.

While there are some helpful guides floating around the forums and the wider internet, the best tool for basic interplanetary travel is the Astrogator mod, which tells you the time until the next transfer window for every planet and the moons around the current planet, the delta-V required to get there and can even create the nodes for you to tweak as you like.

 

 

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Thanks, I was able to at least capture a broad orbit with the excess fuel I had. If I'd not had this, it would have been end of mission. It gave me a chance to do a bunch of science stuff at high orbit, its down now to about 80x200km (Ike is so huge, I was able to snag it on the retro burn and lower myself even more). I've parked it in a Duna orbit for now, there is a lot of science/data to process and the lads are working 24x7! (Or is it 18x4.3 since its Duna?) 

Thanks also for the mod suggestion - I am enjoying standard KSP and the challenges it gives at the moment, probably later on I'll get into mods.

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If you would like to practice transfer windows and alignment between celestial bodies, why not do so at home? If you think about it, planets around the Sun are no different from moons around a planet. ;)

So try making transfers from Mun to Minmus, or the other way around. Not only are both easy to reach from Kerbin and easy to return from, but their orbits take much less time than those of the proper planets. So you get transfer windows far more often. The dV requirements are also a lot lower.

 

Edited by Streetwind
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you want to go as close as possible to the planet for your capture burn. oberth effect, as well as being deeper into the gravity well, makes it cheaper. and yes, you need some course correction burn midway, it's more accurate than trying to do it in kerbin orbit. course corrections are rarely more than 10 m/s

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8 minutes ago, Streetwind said:

If you would like to practice transfer windows and alignment between celestial bodies, why not do so at home? If you think about it, planets around the Sun are no different from moons around a planet. ;)

So try making transfers from Mun to Minmus, or the other way around. Not only are both easy to reach from Kerbin and easy to return from, but their orbits take much less time than those of the proper planets. So you get transfer windows far more often. The dV requirements are also a lot lower.

 

I did think about this yesterday and its definitely something I want to try. I will need to do a little research and find out the angles and other numeric details; then find a good excuse to do it (for example, I think I have an M700 scanner which has done its work at Minmus, but need to check how much fuel is left in it).

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1 hour ago, paul_c said:

I did think about this yesterday and its definitely something I want to try. I will need to do a little research and find out the angles and other numeric details; then find a good excuse to do it (for example, I think I have an M700 scanner which has done its work at Minmus, but need to check how much fuel is left in it).

it's not particularly complicated. from mun, you want to move prograde compared to mun orbit. so, just manually find the position of your burn to exit mun's SoI parallel to its orbit.

and it needs very little fuel. if you do things right, most of the cost is some 200 m/s to get out of mun orbit. if i recall correctly, with 220 you can get a minmus intercept, and if you timed it right, you can spend 10 m/s on the capture burn. similarly from minmus to mun, you should be able to do the transfer with 300 m/s total

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9 hours ago, paul_c said:

I am enjoying standard KSP and the challenges it gives at the moment, probably later on I'll get into mods.

If you to use an out-of-game tool to help you find good times for interplanetary transfers, one of the more popular ones has been alexmoon's Launch Window Planner. I've also been working on a similar web app that plots your transfer trajectory and makes it easier to customize starting/ending orbits.

7 hours ago, Streetwind said:

So try making transfers from Mun to Minmus, or the other way around. Not only are both easy to reach from Kerbin and easy to return from, but their orbits take much less time than those of the proper planets. So you get transfer windows far more often. The dV requirements are also a lot lower.

I'll second this suggestion! Mun to Minmus or vice versa will be useful practice, especially since Minmus has a modest orbital inclination relative to the Mun. 

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