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longest burn time before doing it twice


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I'm planning to send a probe to Duna for the first time.  Never been past solar orbit before.  I found a window that says the probe will burn for over 25 minutes and the ordit's PE is 101k.  It is powered by 3 NERVs.  The longest I've burned before was over 6 minutes.  I've heard that  sometimes it is better to break up the burn into two or more burns. 

Where is the dividing line between one burn and two, and how are two burns planned?

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55 minutes ago, miklkit said:

how are two burns planned?

That is challenging, and I don’t think stock KSP has the tools to really do that accurately.   Ideally, you would want your final ejection burn to be pointed the correct direction, but there’s a lot of guesswork involved in making that happen- at least in stock KSP.

57 minutes ago, miklkit said:

Where is the dividing line between one burn and two

That is also hard to say exactly.  For me, from a 100 km Kerbin orbit, I draw the line at 3-4 minutes.  For anything longer I approach the ejection differently.

Your situation illustrates an important concept: sometimes using engines with a high ISP but low TWR can actually hurt your efficiency instead of improving it.  Specifically when you’re counting on Oberth effects for ejection burns.  With low TWRs you aren’t going to gain anything, and the sloppy alignment I get when trying to do multiple passes further hurts my efficiency.

Instead, when I have a vessel with a ver low TWR (Nukes or Ion engines..), I take one of the following approaches:

  • Raise my Kerbin orbit so that the long burn time keeps the entire burn to a smaller arc.  Maybe 250-350 km.  The burn time will be about the same, but I can do it in one pass, and keep the ejection path where I want it.
  • Or, eject just barely into Solar orbit, and do the transfer burn while in solar orbit.

The short answer is stock KSP does not really have the tools to do multiple periapsis kicks, and end up with a good ejection angle.

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it's tricky, but you can do it.

the easy way is to split your burn into smaller 4-5 minutes burn until you covered at least 500 m/s. orbital time will still be short, so you only lose a few days. which won't be enough to make a big change in the transfer window.

to perform the multiple burn, plan a normal manuever. then burn close to it, say from 2 minutes before the node to 2 minutes after the node. then delete it, and set up another manuever node in the same place; it will be cheaper, because you already burned some of the way. repeat as much as needed. you'll need some small correction manuever afterwards.

the hard - but better - way is to plan a mun gravity assist. of course, you won't be able to get to it. but you start raising your orbit just like in the easy way. finally, as your orbit is high enough, you time your last apoapsis raising so that it will send you back to periapsis just in time to cross mun orbit as mun is passing again. basically, you plan a gravity assist and you actually finalize it in the next mun orbit. the advantage is that having mun there will provide you a reference to keep your manuever straight; furthermore, you can use the gravity assist to get rid of small deviations instead of a correction manuever.

the straightforward way, if you can get away with only 1 periapsis raising, is to create a manuever node for the near future. say, you move your manuever node forward in time so it's 1 day away. Now you raise your apoapsis until your orbital time becomes exactly one day, then you use the manuever node normally.

 

by the way, how big is your "probe" that you need 3 nervs and still 26 minutes? either you're sending a 200-tons probe to duna, and i have no idea what could be in that probe to be so heavy, or you're burning for a lot more than the required 1100 m/s and you're taking an awfully inefficient trajectory

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8 hours ago, miklkit said:

I found a window that says the probe will burn for over 25 minutes

 

2 hours ago, king of nowhere said:

by the way, how big is your "probe" that you need 3 nervs and still 26 minutes?

My thoughts exactly.   I’m curious if the ‘window’ you found was via the stock maneuver planner, in which case it is probably not a very efficient transfer.  If the planned AP of the maneuver is significantly beyond Duna’s orbit, it’s not an efficient transfer, and you’ll use a lot more fuel at both the start of the transfer, as well as when you try to capture at Duna.

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Thanks for the answers.  Methinks I'll send it to solar orbit and go from there. 

The entire package in orbit now weighs 115 tons.  The probe is some antennas and some science along with power and batteries.  It has fuel and an engine to get it into a polar orbit once there.

I got that path by getting it into orbit and then trying to find an encounter.  Failed at that so used the planner and alarm clock.  The window is still 145 days away so there is plenty of time to make changes.

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6 hours ago, miklkit said:

Thanks for the answers.  Methinks I'll send it to solar orbit and go from there. 

The entire package in orbit now weighs 115 tons.  The probe is some antennas and some science along with power and batteries.  It has fuel and an engine to get it into a polar orbit once there.

I got that path by getting it into orbit and then trying to find an encounter.  Failed at that so used the planner and alarm clock.  The window is still 145 days away so there is plenty of time to make changes.

ok, doing some math your ship accelerates at 1.5 m/s, with 26 minutes burn you have a deltaV of 2400 m/s. a duna intercept should cost 1000-1100 m/s from LKO, so there's definitely something VERY wrong with the planned manuever.

Doing the manuever from solar orbit can easily be twice as expensive. the good news, though, is that for duna it shouldn't matter too much. you'll spend perhaps an additional 500 m/s

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11 hours ago, king of nowhere said:

intercept should cost 1000-1100 m/s from LKO, so there's definitely something VERY wrong with the planned manuever.

Yes, I really suspect the in-game maneuver planner is doing the thinking here.  I've found it seems to prioritize a maneuver which occurs in the near future, over a maneuver which is fuel-efficient.

11 hours ago, king of nowhere said:

Doing the manuever from solar orbit can easily be twice as expensive.

With a reasonable TWR, that is accurate.  I disagree for extremely (ridiculously?) low TWRs though.  In that case, you:

  • Won't see any appreciable Oberth effects
  • Will likely eject on a less than ideal trajectory, which will need to be corrected later, using up any 'fuel savings' you might have had.

But yeah, from LKO 2400 m/s is probably enough to get you to Dres's orbit, maybe even Jool.  I think you hit the nail on the head, there is something amiss with the planned maneuver.

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1 hour ago, 18Watt said:

But yeah, from LKO 2400 m/s is probably enough to get you to Dres's orbit, maybe even Jool.

Dres for sure.  Jool I think is in the neighborhood of 3000 (if memory serves right).

...and now I have to go fire up KSP to test that.  Thanks, @18Watt, for giving me yet another reason to play this game!

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There’s a mod that can split long burns up to avoid this sort of issue- it’s called Maneuver Node Splitter and it does exactly what it says on the tin. As for when you’d want to do so, there’s no hard and fast rule but generally speaking you want to be burning as close to prograde as possible for as long as possible- if you’re more than 30 degrees off prograde at any point then you should probably split the burn; if you have to point further than 45 degrees of prograde, definitely split that burn.

Burning in a direction other than pro/retrograde will produce cosine losses, where you out in 1m/s of delta-V but your overall velocity changes by less than 1m/s, effectively meaning that some of your fuel gets wasted. Long burn times will produce a lot of cosine losses and so will waste a significant amount of fuel, so splitting your burn into several shorter ones can save you some delta-V.

There are other things to consider too, like planetary atmospheres (or even surfaces) that can get in the way if you do a long burn- the first part of the burn will be pointing “down” towards the surface and so will reduce your periapsis to a potentially hazardous level- and if you’re using mods that involve engine reliability or limited engine ignitions (e.g. Kerbalism or RO) then you need to balance the longer, less efficient burn against the risk of engine failure and/or a hard limit on how many burns you can do.

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On 12/24/2021 at 7:06 PM, miklkit said:

 Now I know to get into a solar orbit first. 

Don't do this (unless you really want to, I'm not going to stop you :blush:)

I know it seems simpler at first (that is exactly what I did on my first Duna mission) and it is possible, but needs lots of fuel. Instead, the efficient way to go to Duna is to first get into LKO (Low Kerbin Orbit) then keep thrusting at Pe until your orbit is elliptical. (you can plan this by playing with Maneuver nodes, instead of burning actual fuel)

The direction of the elliptical orbit should be so that it points in the same direction as Kerbin is moving around the Sun, this way when you add even more speed at Pe, you will escape Kerbin in the same direction as Kerbin's movement (those speeds kind of add up then).

You will notice that the escape direction and the elliptical orbit won't line up, it needs to point about 35° off to the side instead. Doing this seems complicated at first, but saves a lot of fuel. A youtube video as visual really helps get the point across.

Do you have some screenshots of what you have tried so far? :)

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Wow, lots of answers.  Thanks people. 

Yes the in game maneuver planner is doing all the thinking.  I tried to do it manually and got exactly nothing.  I have been playing around  trying to find something and this probe can easily make it to Jool orbit from solar orbit.   Can't find Duna tho.  Keep getting flickering and disappearing lines, but nothing I can work with.

Breaking the burn up is far beyond me now, so larger engines and more fuel is the answer.  Or more stages. 

Right now getting into solar orbit first seems o be the most efficient way to get way out there to other planets.  Most of the fuel goes to just getting away from Kerbin.  I have gas stations on the Mun and Minmus because of this.

vDjoEaS.jpg

Ha!  Imgur has failed me in the past, but this new account is working so far.  This is the Duna probe heading out to solar orbit.

Edited by miklkit
imgur test
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20 hours ago, Scarecrow71 said:

And that is why I use MechJeb.  

Eh?  I use mechjeb2 but only to hold a heading and altitude.  I just recently completed the tech tree for the first time ever, and it sounds like I need to take a good close look at mechjeb.

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23 minutes ago, miklkit said:

Eh?  I use mechjeb2 but only to hold a heading and altitude.  I just recently completed the tech tree for the first time ever, and it sounds like I need to take a good close look at mechjeb.

I highly recommend using MJ.  At a minimum you need the Tracking Station and Mission Control at level 2, and you'll need Advanced Flight Control unlocked (which it sounds like you do).  To create a maneuver to go to another planet, you simply:

  1. Switch to Map view
  2. Click on the planet/body you want to transfer to and select "Set Target"
  3. Open MechJeb and select the "Maneuver Planner" option
  4. In the window that opens, select "Advanced Transfer to Planet"; this should be in the top drop down.
  5. In the second drop down, select "Limited Time", and then change that to "Porkchop Selection".  I have found that unless you do this, the planner won't calculate appropriately.
  6. Click "Create Node".  Assuming you did everything right, the node will get created and it will tell you:
    1. How long to node execution
    2. How much dV is required
    3. How long the burn is
  7. Fast forward until you are within a comfortable-enough time for you (I generally fast-forward until I'm within 2 hours of node execution)
  8. Click Execute Node

And that's just maneuver planning.  I use MJ for all kinds of stuff, such as:

  1. Ascent to Orbit (with a nice gravity turn)
  2. Landing
  3. Return from a Moon
  4. Transfer to another planet

And that's not even barely the tip of the mod iceberg.

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On 12/23/2021 at 2:14 PM, miklkit said:

Where is the dividing line between one burn and two, and how are two burns planned?

5% cosine losses would be the dividing line that I know.  There's no physical threshold or change of state for this; it's a matter of convention, like the unwritten rule of thumb that says that launch stages should have a burn time of  about two minutes each, or that a launch thrust-to-weight ratio of 1.3-1.7 is ideal.

What are cosine losses, you ask?  I'm assuming that you know that the best time to thrust for an escape burn is at the periapsis.  There are a lot of reasons for this, but two of the factors involved in making an efficient escape burn are that it's better to adjust the apoapsis (because it's the part of the orbit that is closest to escaping in the first place), and that it's better to thrust prograde or retrograde to change the shape of your orbit (normal and radial do change the shape, but they primarily shift the orbit's orientation, which a prograde burn does not).  The periapsis is the one point on the orbit where both of these factors work together at maximum potential effect.

Cosine losses, then, are the inefficiencies that you get from burning anywhere but at the periapsis.  That's a generalisation, but it captures the idea.

For an easier-to-understand example, let's imagine that you have a rocket with two engines side-by-side.  Ideally, both engines point straight back:  you want the rocket to go forward, all of the available thrust is pushing you in that direction, and there is no loss.  Now, let's rotate the engines so that they both point outward somewhat in a V, like a lot of cartoon rocket ships do.  The combined thrust vector still points out the back, but it's smaller because some thrust goes out to the sides (and is counterbalanced by the side-thrust of the other engine).  That side-thrust is useless, the propellant wasted, and is an example of cosine losses.  In the extreme case, we rotate the engines by 90° to point directly sideways, which results in a full cancellation of thrust and the rocket going nowhere.  That would be 100% cosine losses.  This example has a caveat:  you don't need two opposed engines to have cosine losses, because any thrust that is not in the direction that you want to go is lost thrust.  One engine pointed sideways gives 100% cosine losses, and one rocket pointed sideways gives 100% cosine losses, too.  This means that thrusting normal when you want to thrust prograde is also an example of 100% cosine losses, even though in such a case, your rocket does go somewhere.  Thus, thrusting in a direction that is not prograde at the periapsis, even though it does something, and though that something may be mostly what you want it to do, is not 100% ideal.  Since it isn't possible to have a 100% ideal burn, cosine losses can also be said to represent the unavoidable difference between the ideal and reality.

You calculate cosine losses by taking the angle of interest (engine deflection in the previous example, and angle difference from periapsis in the orbital one) and taking the cosine of that angle.  The difference between that cosine and 1 is the fraction of the burn lost (so cos 90° = 0, and 1 - 0 = 1, therefore 100% of the burn is lost when you thrust sideways).

That said, some cosine losses are inevitable.  The only burn that is perfectly efficient is instantaneous, which of course does not happen in reality.  This is where burn time becomes important, especially for low-thrust rockets:  you want the burn times to be long enough to actually do something with the orbit, but not so long that you're wasting the thrust on something that you'll need to correct later.  5% cosine losses correspond to a window of a little under 13° on either side of the periapsis (and around two minutes in low Kerbin orbit)  Depending on what you're trying to do (and how tight your propellant reserve margin is), you can adjust that limit up or down.  I prefer something a lot tighter (I try not to go over 6° unless I can't avoid it), but I'm willing to split a long burn up into more parts to make that happen.  You may want a margin of 20° or more, but you should know that the loss goes up rapidly when you deviate from the periapsis.

 

As far as planning a split-burn, that's a little more complicated, but the key is that you usually want your last burn to do the work of setting up your interplanetary transfer.  This means that your next-to-last burn (and the others if you need more than two burns) works to set up your escape, and your very last burn must occur at the same time as you would have burned if you were doing it all in one big push.  It's tricky in KSP to start with a later burn and work backwards (KSP is set up for planning multiple burns in succession going forwards in time, instead), but it can be done.

Here's what to do:

  1. Note the transfer window time and location on the orbit.  Also get the delta-V for the burn, and note your orbital altitude (the altitude is important; don't forget it).
  2. Set up a burn that is somewhat less than an escape burn.  For a circular orbit, the delta-V needed to escape is a constant for a given orbital altitude.  Let's say that you're in a 100 km orbit of Kerbin:  to escape requires 3,176.5 m/s.  You're already going at 2,246.1 m/s just being in orbit, so the escape burn requires 930.4 m/s.  For the sake of easy numbers, let's go with a 900 m/s burn (to set up an orbit that almost escapes--and if that's too long of a burn, then yes, you can divide it into 3 burns of 300 m/s, or 9 burns of 100 m/s, or, if you're a dedicated masochist, 900 burns of 1 m/s).  Note the amount of time that it takes to complete one orbit (you can do this from the map view).
  3. Whatever the orbital period is for that orbit, you simply go back that amount of time from the transfer manoeuvre and set up the burn.  It likely won't be exact:  you need the final manoeuvre node to coincide with your periapsis, so to make that work, you'll need room for corrections and adjustments.  On the other hand, a close orbit of Kerbin takes about half an hour, so it's not like you're going to miss your window.  Remember to take the 900 m/s off of the final burn.

 

On 12/26/2021 at 5:05 PM, miklkit said:

Most of the fuel goes to just getting away from Kerbin.

Well, yes.  That's exactly how it works.  A typical transfer to Duna (typical defined as:  I used the alexmoon tool and took the first one it offered) costs 1,030 m/s of delta-V.  930 of that is spent just in escaping Kerbin's gravity.  That does not mean that only the last 100 m/s can be said as being for going to Duna.  That is exactly the wrong way to look at it:  the transfer burn needs to be considered as a whole rather than as a sum of independently-movable parts.  All of it is used in going to Duna, and all of it is spent in getting away from Kerbin to do so.  The truth of this is seen in the execution:  if you don't complete the burn, then you don't get the encounter.

You can split a burn in two and do it in two distinct steps, provided that you don't change location (that being, in this case, the periapsis of your ejection/transfer orbit).  You cannot split it into two pieces, do one in low Kerbin orbit and the second in solar orbit, and expect the burns to add the same way.  The reasons for this take a while to explain but the short version is that the energy of the orbit is distributed differently, which for you means that the energy available to exploit for the transfer is also different.   The same kind of thing applies to launching from the surface of Kerbin:  at or near the equator, you save a few hundred m/s by launching to the east because the surface of the planet is rotating in that direction.  If you launch from the north pole (putting aside for the moment that there is no east), it doesn't matter your direction because there, the surface of the planet is only rotating in place.  Kerbin's rotation didn't change; you just put your rocket somewhere where you couldn't take advantage of it.  Thus it is with your choice of interplanetary burn:  you end up wasting a lot of the second one by burning somewhere other than at the periapsis.  It's not cosine loss, per se, but it's a similar sort of problem.

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