Trying to get to mun or minmus, Burn time keeps increasing.

[John Kermin]

Hi,

i have a problem: I want to go from LKO to Minmus. So i plan the maneuver and start the burn. Everything *seems* fine until about 80-90% of the Burn then the m/s countdown slows down, stops (or even reverses slightly) and that keeps up until i have no more fuel. I tried using mechjeb in case it was some mistake i made but the same thing happens. The only explanation i can come up with is that Minmus is somehow "running away" from me, but 1. Minmus is in a stable orbit it is not getting faster and 2. even if that were the case, that should have been accounted for in the maneuver and not happen out of the blue. I can't imagine that this is some kind of bug. I think i would've heard something by now if it were impossible to get to minmus. Especially since i was there before and i didn't have that problem.

What i do is basically get into LKO (75x75) and then go to the map, set minmus as target, set a maneuver at the Ascending (or descending) node and pull the prograde vector until i get an encounter. Then i finetune that until i have a minmus periapsis of <100k and start the burn. According to the map i should arive at minmus and then start a circularization burn. Only i never get there....

Thx in advance for any Ideas.

Edited October 8, 2016 by John Kermin

[Superfluous J]

What does your map mode orbit look like as you're burning? Are you aiming at the maneuver node marker?

What it sounds like is you're straying from the maneuver node marker. The m/s left to burn indicator is m/s left to burn EXACTLY at the marker. If you stray from it, eventually it'll wander a bit and as you wander further, it'll go away faster. You likely passed it up and burned too MUCH, not too LITTLE. Another problem is, if you have a very low TWR, your time to burn will affect how accurately you can perform that maneuver.

That's why you should finish your burns in map mode, watching your projected trajectory. Where you're going is more important than how well you match the node.

[John Kermin]

Well, I use 4 NERVs, so my burn time is around 15 minutes. But shouldn't mechjeb account for all of that ? He seems to be spot on the marker the whole time. I tried it with or without mechjeb, with or without physical timewarp. Every time the same thing happens. Obviously i am not hitting the target but what exactly are the maneuvers for if they are not correct.

Edited October 8, 2016 by John Kermin

[Superfluous J]

Yeah 15 minutes is about 4x longer than I'd personally be willing to do, for exactly this reason. I personally prefer to keep LKO burns to under 2 minutes, really. 4 is just the absolute max. One option you have is to kick your apoapsis up slowly over multiple burns. Five 3-minute burns will be WAY more accurate and you won't lose anything but some time. Harder to plan your eventual encounter, of course, but hey that's just time too And watch out for Mun on the last burn or two.

I don't know much about MechJeb so can't really answer that part.

One important thing to note, this has more to do with mathematics and the laws of orbits than it does "the software can't handle it." The further you burn from your planned node, the less your burn does to help you and the more it does to hurt you. 7.5 minutes from the node is a LONG TIME.

[John Kermin]

Ok, i tried it again with a more powerful Engine. Now it worked, thx for the help. Still it is very baffling. What is the point of having very efficient engines if the resulting burn times make them completely useless and anywhere where i can use them and get <4min burn times i don't need them.

[DerekL1963]

7 minutes ago, John Kermin said:

Ok, i tried it again with a more powerful Engine. Now it worked, thx for the help. Still it is very baffling. What is the point of having very efficient engines if the resulting burn times make them completely useless and anywhere where i can use them and get <4min burn times i don't need them.

They're not completely useless - you just need to know how to use them.  In this case, your burn time is too large a fraction of your initial orbital period - and there are two main solutions.  Increase your t/w ratio by adding engines or lightening your vessel.  (And make sure you're using fuel tanks, not LFO tanks.  Otherwise you're hauling around a bunch of dead weight.)  Or increase the altitude of your initial orbit so that burn time is a smaller fraction.  (There is another - use a node splitter and make multiple burns,but I have little experience with them.) 

 

[steve_v]

11 minutes ago, John Kermin said:

What is the point of having very efficient engines if the resulting burn times make them completely useless

You just have to start from a higher orbit or split the burn into multiple nodes (and there's a mod for that too).

[John Kermin]

Ok, i will try this tomorrow. One more question. Is there a fast way to get from Minmus to Mun ? I tried returning to a Kerbin Orbit of 12k km (same as mun) and then rendezvous with mun. It works but it takes very long. I don't have a lot of supplies left (life support mod). Is there a way to start from minmus and directly intercept mun, maybe top it of with a suicide burn. That would be awesome.

[Superfluous J]

Also, in Sun or Jool orbits, 15-minute burn times are not a big deal. In Sun orbit, you can burn for a day and still have a pretty accurate node.

But going to Minmus? Naww I'd just bring more beef.

[Guest]

Besides which, high TTW is fun.

[GoSlash27]

I generally keep my t/w for orbital maneuvers at a minimum of .5. This allows for plenty- quick burns, yet still allows for efficient stage design.

Best,
-Slashy

[Fuzzra]

If you're starting that long a burn at 75k you're possibly ending up dunking yourself in wispy atmosphere for a while.

Not that I've ever done that.

[Plusck]

On 08/10/2016 at 8:32 AM, John Kermin said:

Ok, i will try this tomorrow. One more question. Is there a fast way to get from Minmus to Mun ? I tried returning to a Kerbin Orbit of 12k km (same as mun) and then rendezvous with mun. It works but it takes very long. I don't have a lot of supplies left (life support mod). Is there a way to start from minmus and directly intercept mun, maybe top it of with a suicide burn. That would be awesome.

Much the same as with interplanetary encounters - you can do anything if you have the fuel for it.

The quickest way to get to the Mun from Minmus, without spending ridiculous amounts of fuel, is to leave Minmus's SOI on a trajectory that heads retrograde and a bit towards Kerbin (so your resulting orbit around Kerbin has an Ap significantly higher than Minmus) and catch the Mun on the way down.
Since the Mun has an orbital period of about 6 days, while the cheapest return to the Mun's orbit takes about 11 days or so, that leaves a lot of leeway to speed up the return time and catch the Mun in just the right place. It only takes a couple of hundred ms dv to halve that time (or more), so if the Mun is in the right place when you start, you can get into orbit within about 4 or 5 days without too great an expense.

You should take a look at the Alex Moon porkchop planner to see what sort of timings are possible: https://alexmoon.github.io/ksp/#/Minmus/30/Mun/40/false/ballistic/false/1/1. If you click on advanced settings and reduce the minimum time to 3 days, you'll see that a 4-day travel time is possible without excessive cost once every eight days or so, there is an option for a reasonable 5-8 day transfer for about two-thirds of the time, and the rest of the time it is always possible to get a decent 9-10 day transfer.
What the planner is showing, in fact, is just the effect of accelerating more towards Kerbin to catch the Mun in the right place. The best 4-day transfers generally have you leaving Minmus heading directly towards Kerbin with a 400m/s escape burn.
And finally, since that shorter travel time leaves less time for Kerbin's gravity to accelerate you, you actually end up meeting the Mun at much the same speed as if you'd dropped slower, so the capture burn at the Mun varies surprisingly little.
 

Edited October 10, 2016 by Plusck

[Technical Ben]

21 hours ago, Fuzzra said:

If you're starting that long a burn at 75k you're possibly ending up dunking yourself in wispy atmosphere for a while.

Not that I've ever done that.

I did it yesterday... and just boosted back through it.

But as others have posted. As you are moving along the orbit while burning for the node/moon/planet, you run the risk of missing the target if your burn for too long.

Using multiple short passes, or one short burn with a more powerful rocket will help. Experienced players may use low thrust/small engines and multiple burns. Most new players use large rockets and get it done in a few seconds. I still prefer doing that as I don't have to wait too long while playing.

 

The example of this is Large Oil Tankers. It takes MILES to turn them around or to stop. The same for rockets. They look small, and "turn" fast. But actually moving your trajectory can take longer. Imagine Minmus as a little Tug Boat, and the rocket as an Oil Tanker... you just missed power slided the meetup.

Edited October 10, 2016 by Technical Ben

[John Kermin]

On 10.10.2016 at 2:27 PM, Plusck said:

Much the same as with interplanetary encounters - you can do anything if you have the fuel for it.

The quickest way to get to the Mun from Minmus, without spending ridiculous amounts of fuel, is to leave Minmus's SOI on a trajectory that heads retrograde and a bit towards Kerbin (so your resulting orbit around Kerbin has an Ap significantly higher than Minmus) and catch the Mun on the way down.
Since the Mun has an orbital period of about 6 days, while the cheapest return to the Mun's orbit takes about 11 days or so, that leaves a lot of leeway to speed up the return time and catch the Mun in just the right place. It only takes a couple of hundred ms dv to halve that time (or more), so if the Mun is in the right place when you start, you can get into orbit within about 4 or 5 days without too great an expense.

You should take a look at the Alex Moon porkchop planner to see what sort of timings are possible: https://alexmoon.github.io/ksp/#/Minmus/30/Mun/40/false/ballistic/false/1/1. If you click on advanced settings and reduce the minimum time to 3 days, you'll see that a 4-day travel time is possible without excessive cost once every eight days or so, there is an option for a reasonable 5-8 day transfer for about two-thirds of the time, and the rest of the time it is always possible to get a decent 9-10 day transfer.
What the planner is showing, in fact, is just the effect of accelerating more towards Kerbin to catch the Mun in the right place. The best 4-day transfers generally have you leaving Minmus heading directly towards Kerbin with a 400m/s escape burn.
And finally, since that shorter travel time leaves less time for Kerbin's gravity to accelerate you, you actually end up meeting the Mun at much the same speed as if you'd dropped slower, so the capture burn at the Mun varies surprisingly little.
 

Yeah, i am still a noob when it comes to Interplanetary Travel. In fact i was never farther than Minmus. I've been stuck in the Super-SSTA building cycle for far too long. Thx for the link. The diagram seems familiar i think i even have a launch window calculator mod. The problem is basically the Umbra Life Support mod. It depletes no matter if you fly or wait so if a wait for a perfect launch window that's just as bad as just flying. Of course it saves fuel, though.