Making a long, multi-orbit burn

[Grogs]

Hi all,

I'm working on a Jool-5 mission. I've managed to get my rather large mother ship into orbit and dock all of my landers/tugs. I'm using Nerv engines, which gives me a rather low TWR. Even so, I'm not sure I can even use 100% thrust because I have several ships connected together via docking ports. As a result, a Jool ejection maneuver of ~ 2000 m/s will require at least 18 minutes, and possibly more. I put my mother ship into a 500-km orbit in anticipation of this, but even at this altitude the period is just over an hour. Given that the maneuver will require about 1/3 or more of an orbit, I think I need to split the burn over a couple of orbits, but I'm not sure the best way to go about it. A 500 m/s burn puts me in an orbit with an apoapsis of 4400 km and a period of 5 hours. 700 m/s gets me an Ap of 34000km with a period of over 10 days, and much more than that hits Kerbin escape. Given an orbital transfer window at time X, what's the best way to go about this as far as how to set maneuver nodes, when to start the first burn, etc. BTW, I'm running an unmodded 1.6.1 with MH if it matters.

 

Here's the assembled craft:

Spoiler

 

Thanks,

Grogs

Edited February 23, 2019 by Grogs
Fixed picture

[Gargamel]

Others will come along and post the best way to do multiple burns like you asked, but as for full thrust...

I would make a quick save, go to full thrust, see if anything wobbles in a bad way (I think this design is going to wobble regardless), reload the quicksave and autostrut to heaviest anything that is moving too much. 

Now..... the problem with this idea.    When you go to dock and undock, some/all of those autostruts will have to re-attach to another part.  That is known to cause Kraken attacks and rip the ship apart. 

But if you only autostrut one central part from each docked module to the heaviest part, then when you undock, you only have to turn off one autostrut.  The parts themselves should be attached solidly enough within each module, so one autostrut per module should suffice.  If, though, you are docking or undocking the heaviest part, you'll have to turn them all off.  Or quicksave, don't turn them off, and hope the Kraken doesn't notice you, if it does, load the QS, and turn them off manually. 

[4x4cheesecake]

Don't worry too much about the timing. On a long transfer, the timeing becomes less relevant and it is more about precision. On a transfer between Kerbin and Jool, a burn of ~1m/s dV can make the difference between "you got an encounter" and "you will miss the whole jool system". Even if you don't hit the perfect transfer window, you may need to spend ~10m/s dV more but that shouldn't be an issue.

Just launch 10-15 days ahead of the transfer window, split the burn 3 or 4 times so you will end up in an orbit with an Ap close to the orbit of Minmus. You can go further out but it will only take ~50m/s dV more to reach the edge of Kerbins SOI while the orbital preiode will increase much more. This is one of the things where you can spend hours of planning without getting a huge benifit from it, so if you have a little bit of dV to spare, I would suggest not to plan this maneuver too much. Just do it

Maybe someone here wants to do the maths but you will probably save more fuel by splitting the burn than by hitting the perfect transfer window

[bewing]

You're going to lose all the oberth benefits anyway, so I don't see any point of doing this inside Kerbin's SOI. Just gently get it into a Kerbol orbit a tiny bit ahead of Kerbin, and burn from there. Once you are out of Kerbin's SOI, setting up the maneuver node and burn will be trivial. (And therefore, I think the 500k orbit was a bit of a waste of fuel.)

Edited February 23, 2019 by bewing

[Loskene]

Everything you need to know was covered by Brad Whistance here:

https://youtu.be/wrIjWCEfBS4

[Aegolius13]

I usually start about a day before the ideal launch window, and do about 800 m/s of preliminary prograde burn (i.e., so my apoapsis is a little bit lower than the Mun's orbit).  If thrust is really low, this 800m/s can be split into two or more burns.  I find that's about the limit before (1) the Mun starts to potentially mess with your orbit, and (2) your orbit ends up taking too long to complete.  

After that, I do whatever's left in a single final burn.  This can be well over a thousand m/s, depending on the destination.  So not ideal in a low-thrust craft.  But as you say, you can't really split further than that without kicking yourself out of Kerbin's orbit or spending a bunch of time in slow orbits.

If you want to get really fancy, you can do the entire normal/antinormal portion of your burn while you're at your highest point before you do your final burn.  This should be the cheapest way to change your plane, even after accounting for the "pythagorean" savings of doing your normal and prograde burns at the same time.  It also reduces the total delta-v of your final burn a smidge, helping with the low TWR issue.

I agree with @bewing, it's not efficient to get into a high circular orbit before you start.  Keeping your periapsis as low as possible (potentially even in the upper atmosphere for the final burn, depending on your drag) will maximize the Oberth effect.  I've taken to leaving lowering my periapsis a smidge when I do the aforementioned plane change burn at Mun-ish altitude. 

[Laie]

6 hours ago, Grogs said:

A 500 m/s burn puts me in an orbit with an apoapsis of 4400 km and a period of 5 hours. 700 m/s gets me an Ap of 34000km with a period of over 10 days

OK, so you don't need periapsis-kicking explained, per se.

Your typical Jool transfer window has a lot of leeway, plus/minus several Kerbin days. Start at least two days ahead of time; push the AP as high as you dare (beware of Mun encounters!). If you can be back at PE at your nominal departure time +-six hours, that will be fine. Your eventual transfer burn will be imprecise enough that a few hours give-or-take on the timing barely matter. Don't overthink it.

[Grogs]

6 hours ago, Loskene said:

Everything you need to know was covered by Brad Whistance here:

https://youtu.be/wrIjWCEfBS4

Thanks, that's a great resource that explains a lot, especially the timing of the last couple of maneuvers. The one thing there that is quite a bit different is that he's launching to Eve. Starting from a 71-km orbit, the transfer is 1038 m/s, and from escaping the Kerbin SOI it only takes a small bit more burn. That lets him break it down into 8 pieces, and the last one that launches to Eve isn't all that much larger than the first seven. In contrast, when going to Jool if I tried to break it into 8 pieces, I would bring my apoapsis to the edge of the Kerbin SOI with #3 and escape with #4. Handling that last big chunk at the end seems like it might? require something different. I'm playing around with some tests to get an idea of how much this matters.

[Grogs]

3 hours ago, Laie said:

OK, so you don't need periapsis-kicking explained, per se.

Your typical Jool transfer window has a lot of leeway, plus/minus several Kerbin days. Start at least two days ahead of time; push the AP as high as you dare (beware of Mun encounters!). If you can be back at PE at your nominal departure time +-six hours, that will be fine. Your eventual transfer burn will be imprecise enough that a few hours give-or-take on the timing barely matter. Don't overthink it.

It seems pretty clear that the first step is to kick up the apoapsis over a few steps. Then after the penultimate burn arrive back at the periapsis at about the launch window time.

The parts that I'm not 100% sure on how to optimize are:

 

1. Which direction to point initially. I've just been setting a maneuver node that just gets me to Jool's orbit for the least amount of dV. The only problem is that Jool isn't in position for an encounter yet. 20-30 days later when it's time for the final kick, Kerbin has moved around the sun and the apoapsis is no longer aligned in the right direction. I noticed that the final maneuver node was several degrees past the periapsis when I gave it a try.

 

2. Does it matter whether I just go for it and make the final burn all at once, regardless of how far off periapsis I am? Or should I follow bewing's advice and burn just enough to escape Kerbin's SOI and then correct once I escape? I ran a couple of quick tests and they seem to suggest that just burning it all at Kerbin periapsis is better. I managed a close encounter with Jool for 2174 m/s (16% higher than the minimum value of 1872) with this versus 2534 m/s correcting after Kerbin escape. It seemed like handling the plane change might explain part of the difference. In any case, I know I included a good safety margin on fuel so the "don't overthink it" advice is probably sound. I'll just try to get a transfer that's acceptable for now and try to optimize it later for the absolute minimum if that's what I want.

[Starhawk]

There was a good thread about how to do this quite a while back:

HTH.

Happy landings!

[Grogs]

4 hours ago, Aegolius13 said:

I usually start about a day before the ideal launch window, and do about 800 m/s of preliminary prograde burn (i.e., so my apoapsis is a little bit lower than the Mun's orbit).  If thrust is really low, this 800m/s can be split into two or more burns.  I find that's about the limit before (1) the Mun starts to potentially mess with your orbit, and (2) your orbit ends up taking too long to complete.  

After that, I do whatever's left in a single final burn.  This can be well over a thousand m/s, depending on the destination.  So not ideal in a low-thrust craft.  But as you say, you can't really split further than that without kicking yourself out of Kerbin's orbit or spending a bunch of time in slow orbits.

If you want to get really fancy, you can do the entire normal/antinormal portion of your burn while you're at your highest point before you do your final burn.  This should be the cheapest way to change your plane, even after accounting for the "pythagorean" savings of doing your normal and prograde burns at the same time.  It also reduces the total delta-v of your final burn a smidge, helping with the low TWR issue.

I agree with @bewing, it's not efficient to get into a high circular orbit before you start.  Keeping your periapsis as low as possible (potentially even in the upper atmosphere for the final burn, depending on your drag) will maximize the Oberth effect.  I've taken to leaving lowering my periapsis a smidge when I do the aforementioned plane change burn at Mun-ish altitude. 

Yeah, I agree that moving to the 500km parking orbit wasn't the most efficient move. I just had extra fuel in my launch stage and moved it up as high as I could. The thing was so top-heavy it would snap-hook into the ground if I tried any sort of normal gravity turn so I included a lot of extra dV and kept it pointing straight up until it had a lot of speed.

[Superfluous J]

here's a trick, be in a similar orbit to another ship. Say you have a space station at 100km, and your ejection ship is also at 100km (or within a few km of that). Plot the projected ejection (one burn, to Jool) with the station,  then switch to your actual ship and target the station. You'll see the station's ejection burn in the orbit, hopefully at least a dozen or so days in the future.

I'm assuming you're doing 2 burns, one to kick the apoapsis up to some high value near or beyond Mun, and another to eject. So plot your first burn X days before that station's ejection burn, at the same place, where X is the period of your first elliptical orbit.

Now you know your first burn, do it, and then your second burn will be in the same place, but enough to actually eject. You can actually just make its trajectory match the station's projected trajectory, and you'll very likely hit Jool.

[Kryxal]

One thing to note, if you push your apoapsis past the Mun, you can manage to get yourself into a bit of a mess ... hit it just wrong and you'll end up with a surprise Mun encounter when you plot your ejection burn.  Always plot your ejection burn while still low on the way up, so if you need to you can adjust orbital period to avoid such an issue.

[Grogs]

11 hours ago, Kryxal said:

One thing to note, if you push your apoapsis past the Mun, you can manage to get yourself into a bit of a mess ... hit it just wrong and you'll end up with a surprise Mun encounter when you plot your ejection burn.  Always plot your ejection burn while still low on the way up, so if you need to you can adjust orbital period to avoid such an issue.

Good point, thanks.

13 hours ago, Starhawk said:

There was a good thread about how to do this quite a while back:

HTH.

Happy landings!

Thanks. I tried searching for something like this, but "long burn" and "multi-orbit" didn't find it.

12 hours ago, 5thHorseman said:

here's a trick, be in a similar orbit to another ship. Say you have a space station at 100km, and your ejection ship is also at 100km (or within a few km of that). Plot the projected ejection (one burn, to Jool) with the station,  then switch to your actual ship and target the station. You'll see the station's ejection burn in the orbit, hopefully at least a dozen or so days in the future.

I'm assuming you're doing 2 burns, one to kick the apoapsis up to some high value near or beyond Mun, and another to eject. So plot your first burn X days before that station's ejection burn, at the same place, where X is the period of your first elliptical orbit.

Now you know your first burn, do it, and then your second burn will be in the same place, but enough to actually eject. You can actually just make its trajectory match the station's projected trajectory, and you'll very likely hit Jool.

Thanks, cool trick. Playing with this, I guess I answered my first question which is how to get pointed in the right direction initially. I started my maneuver at T-30 days, moved it to the launch window using the "+1 orbit" button repeatedly, then set up the minimum dV direct transfer to Jool. Once I had it, I moved it back to the current orbit using the "-1 orbit" button. Now I can start pointed in the right direction, which will (hopefully) get rid of a lot of the 16% extra dV I had when I tested this before. Playing with all these maneuver nodes has also reminded me of just what a pain setting them up is with the stock controls. I'm going to try and complete the Jool-5 without them just as a matter of pride, but once that's done I'm adding a mod that lets me create/modify them more easily ASAP.

[Grogs]

Hi all,

I flew the Kerbin escape to Jool this evening and wanted to share the results. Alex Moon's launch window planner showed an ejection dV of 1942.6 m/s for this launch window. I executed the burn as follows:

1. Set up a generic maneuver node along the orbit, then hit '+1 orbit' until it reached the launch window. I set the node to 1942.6 prograde and moved it in the orbit until I got a Jool encounter.

2. Moved the maneuver back to the current orbit using the '-1 orbit' button. I was about 30 days out at that point.

3. Performed the burn in 5 steps. After each of the first three burns, I hit the '+1 orbit' to move the node ahead to the next orbit. The burns were:

3a. 117 m/s This was how much dV was left in the final launch stage.

3b. 295 m/s. I ran the Nervs for 3 minutes

3c. 171 m/s. This got me to just inside the Mun's orbit. Any higher and I would have to start worrying about encounters

3d. 135 m/s. I had to wait one orbit to avoid a Mun encounter, then burned long enough to return to periapsis just at the launch window. The orbit ended up outside the orbit of Minimus with a 25-day period. Periapsis/final maneuver ended up within 20 mins of optimum launch window.

3e. 1225.4 m/s Final burn for Jool. This one took 10:48 and my Nerv engines started to overheat at the end.

According to the dV display on the stages, I used 1994 m/s of fuel on the escape. That's only 2.65% over the optimum value, so losses for burning off periapsis are pretty small. I would honestly be quite happy to get within 2.65% on a single burn with a higher TWR craft.

I made a small correction burn to fix my inclination about a year out and managed to get a Jool capture from a Tylo assist for only 70 m/s. Now I just have to set up a Laythe encounter/capture once I reach the system and my Kerbals can start their landings.

Thanks for all the help,

Grogs

Edited February 25, 2019 by Grogs

[FleshJeb]

On 2/23/2019 at 8:56 PM, 5thHorseman said:

here's a trick, be in a similar orbit to another ship. Say you have a space station at 100km, and your ejection ship is also at 100km (or within a few km of that). Plot the projected ejection (one burn, to Jool) with the station,  then switch to your actual ship and target the station. You'll see the station's ejection burn in the orbit, hopefully at least a dozen or so days in the future.

I had NO idea you could do that. How long has that been a feature?

[Superfluous J]

2 hours ago, FleshJeb said:

I had NO idea you could do that. How long has that been a feature?

Hm. I recall them adding it, but not when. A few years probably. 1.1?