Couple questions about planning interplanetary mission

[Khazar]

I have done many trips to mun and minmus, and have mining stations running on both and now I want to go for other planets.

If I want to go to Duna or Eve, would it be better to start from mun or minmus or from LKO and bring fuel there ?

Also, can I refuel SRBs or Monopropelant could be refined only for RCS ?

Is it a good Idea to start from eccentric orbit instead of circular ? I guess this could be good idea for LV-N engines ...

[Warzouz]

you have 3 questions

1- SRB refill : this is not possible since SRB don't use monoprop, but solid fuel which isn't an option for ISRU.

2- elipic orbit : elipitic orbit are quite hard to use. To go for interplanetary, you must do a (tangent) prograde burn so you have match the PE ou AP at the correct burn angle. With a random elipse, this is mostly unefficient. But you can "build an elipse" by doing multiple burns at PE, starting from a mostly circular orbit.

3- As for starting from LKO or minmus orbit, that's a hard question as it seems you would have to bring your fuel from minmus to LKO before departing. In that cas it maybe more efficient to start from minmu orbit. But in general, it's not.

[Khazar]

Thanks for the answers.

2. I meant eliptic orbit that would be pointed in "right" direction. I have low twr and I would like to get to Duna as fast as I can. I have 10k dv to spent and I'm aiming for 5k dv ejection bur. I will take some time do perform it, so I thought that if I would stretch my orbit first, I would be pointed in the right direction for longer part of the burn then if I would start from circular orbit. Is it true ? And if so would it be noticeable difference in efficiency ?

3. What are pros and cons ? If I'm supposed to eject from Kerbin orbit at certain angle, wouldn't I get same effect If I would eject from Minmus, when it is at that angle ? How long does it take for Minmus to circle Kerbin ?

[cybersol]

For you first interplanetary transfers I would start from a circular and equatorial low kerbin orbit. At first there is a plenty to learn about what day to leave (the launch window), where in your orbit to burn (angle to prograde/ejection angle). Later you can add more complications like below.

For example, starting from Mun for interplanetary is more straight forward because its at 0* inclination versus Minmus's 6* inclination. Assuming you are going interplanetary, and thus speeding a lot of dv, its most efficient to drop down from Mun/Minmus orbit to very low Kerbin orbit and perform your interplanetary burn there at periapsis without ever circularizing. By doing it that way, you would have an extra 800-900 m/s of speed at periapsis and thus gain more efficiency from the Oberth effect. However timing that so that you arrive at the right window and with the right ejection angle and inclination can be tricky. Plus if you mess it up, you can end up costing yourself much more than you would have saved.

EDIT: 2) A 5000 m/s ejection burn is a terrible idea and will result in a similar sized capture burn to Duna. You need to learn more about launch windows and ejection angles that I talk about above. Use something like http://alexmoon.github.io/ksp/ and wait to leave until the time it says, and make sure you are spending about what it says. Or better yet, save first and then try it your way and then the launch planner way to see the difference.

3) You have good intuition about waiting to the right time in Minmus orbit being the easiest way to leave Kerbin near the equatorial plane.

Edited June 8, 2015 by cybersol
OP Ninja

[Red Iron Crown]

The first link in my signature may be helpful for low-TWR ejections from low Kerbin orbit.

[Warzouz]

2. I meant eliptic orbit that would be pointed in "right" direction. I have low twr and I would like to get to Duna as fast as I can. I have 10k dv to spent and I'm aiming for 5k dv ejection bur. I will take some time do perform it, so I thought that if I would stretch my orbit first, I would be pointed in the right direction for longer part of the burn then if I would start from circular orbit. Is it true ? And if so would it be noticeable difference in efficiency ?

I think that would be even more efficient because your speed at PE would be higher each pass. So the Oberth effect should apply more and more. But the issue is more about timing. As you AP rises the time to do the full cycle will be higher and higher. To eject at the perfect time you have to know when to start your cycles. This is the tricky part. If you're not a the correct time, you'll meet the encounter with a less tangent angle which will cost you more fuel to capture.

That' the deal about interplanetary burn : if you don't do it correctly, you pay the bill on capture, not at the start (Sometimes, you can partially fix thing mid course)

3. What are pros and cons ? If I'm supposed to eject from Kerbin orbit at certain angle, wouldn't I get same effect If I would eject from Minmus, when it is at that angle ? How long does it take for Minmus to circle Kerbin ?

It's always more dV efficient to burn prograde or retrograde at lowest orbit you can (that's not valid with normal and radial burns). That's the Oberth effect. Elipic orbit might even be more efficient because you have higher speed at PE.

If you're on high orbit you go slower, so your burn is less efficient. In general you don't want to go there. A rule of thumb would be to consider loosing the circularize burn on AP.

But if your fuel is already on high orbit you would loose more to bring it to LKO.

[mrclucks]

If you're starting from a lko from say 100x100km then a very efficient way to achieve escape would be to burn retrograde opposite of where your ejection burn will be to get your periapsis down to 80-70 km so you can start getting your oberth effect. Then burn a small amount each time you get to periapsis( ill usually do 100 m/s dv just to make it an even number) until your apoapsis is around 8.5 Mm( mun will never capture) and set up your burn.

If you can time it correctly, then increase your apoapsis just outside of minmus for even more oberth savings ( dv price for raising it from 8.5Mm to 70 wish is not much more). This should also be done a few orbits below your transfer window so you're not wasting time( or do it, set up your maneuver with mechjeb, and add an alarm with kerbal alarm clock if you wanna be cheaty like me).

I had just set up an orbit like this in prepperations for a duna intercept for about 500 dv and my exit burn only cost me 200 and change dv, but that was with my apoapsis at 8.5 so you can imagine what it may of been if I raised it to around 60Mm.

[Khazar]

I am using calculator from this link http://alexmoon.github.io/ksp/ but I am willing to send small probe that would start scanning and mapping Duna before I will send larger crewed craft. I am aware that 34 day isn't best day to launch for Duna. I designed simple craft that has various scientific tools and scanners and using single LV-N engine and set of fuel tanks, I have 10k dv after getting to LKO. In this case I want to save time not fuel.

I was able to set a node that would allow me to get to Duna for jusr 2k, but it would take 190 days. I was hoping for shorter, more expensive trip.

I will try both ways, dropping periapsis after launching from mun, and raising apoapsis in regular steps. If something will go wrong, well it's only a probe...

[Warzouz]

But if you're dropping from Mun, you might not be in the correct ejection angle. Burning at the PE could send you far from optimal direction.

[Snark]

If you're launching from the Mun, the most efficient way to conserve dV in ejecting interplanetary is to make one (fairly lightweight) burn to drop from Mun to a very low Kerbin periapsis, then make your major burn at Kerbin periapsis to take optimum advantage of the Oberth effect.

Doing it this way can make planning your ejection angle a little tricky, since you have to time your departure from the Mun correctly so that your Kerbin periapsis happens at the right angle. One little trick to help with this:

1. Keep a satellite "dummy" that's permanently parked in circular LKO.

2. When you're about ready for your main ship to depart Mun for Duna or wherever, set a maneuver node for the dummy satellite (as if you were going to send it, not the main ship) to eject to your target. This will show you where the periapsis burn needs to happen.

3. Switch back to your main ship, and set the dummy as your target. Now you can see where the dummy's maneuver node is.

4. On your main ship, set up your maneuver node so that you'll hit Kerbin periapsis right where the dummy's maneuver node is. (You may need to wait a bit until the Mun is in a good position.)

5. Once you've done your burn to send your main ship towards Kerbin, you can go ahead and delete the dummy ship's node, then set up a node on your main ship at periapsis to do the burn to take you to your destination.

[Khazar]

Nice idea. And its perfect for my plans. I have established mining base exactly at mun's equator. I wonder how would it work with minmus as it has such a nice low gravity, flat surfaces and its farther from the Kerbin, so the effect would be greater. I wonder if it would outweigh the inclination issue.