Interplanetary transfer from the Mun

[Lunar Sea]

I've got some ships in a 20km by 20km Mun orbit that I want to send to Jool, but I'm struggling to get the maneuver set up right. I've got the eject angles set up with Transfer Window Planner but ejecting along the Mun's prograde is not giving me any Dv savings at all. Can anyone suggest an effective method? Thanks. 

[HvP]

The trick with going interplanetary from a moon is that you have to wait until the MOON is at the point in its orbit that matches the ejection angle you need.

Basically, pretend that the moon is your ship. If you need to be at a 120 degree angle from Kerbin's line of orbit to launch interplanetary, then you need to wait until the Mun is at a 120 degree angle from Kerbin's line of orbit. When the moon has reached the right place then you create a burn that sends your ship in the moon's prograde direction.

Edited July 8, 2017 by HvP

[HebaruSan]

56 minutes ago, Lunar Sea said:

ejecting along the Mun's prograde is not giving me any Dv savings at all. Can anyone suggest an effective method? Thanks. 

How much savings are you getting? How much savings are you expecting? Checking some old screenshots, the difference between ejecting to Jool from LKO versus LMO is on the order of 100 m/s or less.

[Cpt Kerbalkrunch]

I'm not a physics guy, so I could certainly be wrong, but I don't think Mun can give you much from an orbit within it's SOI. It only has a fraction of the power of Tylo or Laythe. The only help I've ever gotten from it is when passing by the backside of it on a burn from LKO. And even then, I only do so if it happens to work out that way. I don't think it's worth going out of the way for. Plus, it can be tough to hit your target (except when going to Jool, like you are; who's huge SOI is very forgiving).

[HvP]

@HebaruSan @Cpt Kerbalkrunch I don't think that the original poster is looking to gain specifically by using the Mun for a transfer, but simply has ships already at the Mun for other reasons and can't get an efficient escape from the Mun to go interplanetary.

Another option is to drop down towards Kerbin when your periapsis near Kerbin will be at the angle you want to make your interplanetary ejection burn. The speed you build up by falling towards Kerbin uses the Oberth effect to make a more efficient transfer burn.

Edited July 8, 2017 by HvP

[Cpt Kerbalkrunch]

2 minutes ago, HvP said:

@HebaruSan @Cpt Kerbalkrunch I don't think that the original poster is looking to gain specifically by using the Mun for a transfer, but simply has ships already at the Mun for other reasons and can't get an efficient escape from the Mun to go interplanetary.

That makes more sense. I wonder if the dive back to LKO would do any good? It works great from Minmus (which is at a much higher altitude, of course), but I've never tried it from Mun. It's sort of in a no-man's land. Too high to get the full boost from Kerbin, but too low to make the dive worth it. I'm really just guessing, though. I've never really done any refueling or staging at Mun, so I can't remember the last time I tried a transfer from there.

[HebaruSan]

16 minutes ago, Cpt Kerbalkrunch said:

That makes more sense. I wonder if the dive back to LKO would do any good? It works great from Minmus (which is at a much higher altitude, of course), but I've never tried it from Mun. It's sort of in a no-man's land. Too high to get the full boost from Kerbin, but too low to make the dive worth it. I'm really just guessing, though. I've never really done any refueling or staging at Mun, so I can't remember the last time I tried a transfer from there.

Hmm, from LMO you pay ~280 m/s to get into an orbit that you'd have to pay ~850 m/s for from LKO. If the alignment is correct, I think it ends up being worth it.

[Cpt Kerbalkrunch]

4 hours ago, HebaruSan said:

Hmm, from LMO you pay ~280 m/s to get into an orbit that you'd have to pay ~850 m/s for from LKO. If the alignment is correct, I think it ends up being worth it.

So I checked it out and it looks like the dive is worth it. I tested a fuel shuttle I've had collecting dust in Munar orbit for about 60 years. The drop down to a 72km Pe cost just over 200 m/s. The transfer to Jool was then just over 1,200 m/s. A direct burn from Mun was about 1,900 m/s to Jool. These aren't the best numbers you can get, obviously, I just wanted to quickly check. My shuttle was a bit high at Mun, for starters (about 100 x 100), and I didn't try for the very best alignment I could get, but it was enough evidence for me. Not quite as good as a dive from Minmus, but still pretty good. So @Lunar Sea, if you're still looking for suggestions, maybe give it a shot and see if you agree.

[Lunar Sea]

I set up the nodes and it looks like they'll work. the transit time is somewhat longer than I wanted but I just want to get it done and move on to my next missions! I've been sweating this Jool thing for months. We're talking like 10 separate vessels - I know, kind of got out hand. 

Thanks for the tips!

Edited July 9, 2017 by Vanamonde
Mind the language, please.

[Hesp]

On 09/07/2017 at 0:38 AM, Cpt Kerbalkrunch said:

I'm not a physics guy, so I could certainly be wrong, but I don't think Mun can give you much from an orbit within it's SOI. It only has a fraction of the power of Tylo or Laythe. The only help I've ever gotten from it is when passing by the backside of it on a burn from LKO. And even then, I only do so if it happens to work out that way. I don't think it's worth going out of the way for. Plus, it can be tough to hit your target (except when going to Jool, like you are; who's huge SOI is very forgiving).

Actually, it helps quite a lot in reducing the size of the transfer stage.

All my interplanetary missions are leaving directly from a refuel station in Mun orbit (fuel is refined and shipped from Minmus).

Of course if no refuel is planned, starting from LKO is way cheaper.

 

[Spricigo]

I'd expect higher cosine losses in LMO than in LKO.  And for the same reason more trouble getting the ejection angle right.

Unfortunately the transfer planner don't take that in consideration 

[Kryxal]

If you plan to do much of this sort of thing, I'd put a sat in a nice equatorial orbit of Kerbin so you can use it to pre-plan burns.  Target the sat, plan Mun ejection to drop your PE on top of the node, maybe do an orbit or two for time, and match your outbound trajectory to what the node would give.

[Hesp]

1 hour ago, Spricigo said:

I'd expect higher cosine losses in LMO than in LKO.  And for the same reason more trouble getting the ejection angle right.

Unfortunately the transfer planner don't take that in consideration 

It's exactly the opposite... I haven't got numbers at hand right now, but orbital period in LMO (50km) is higher than in LKO (150km), so cosine losses are lower (and the burn is way shorter too!).

Regarding the ejection angle, just zoom out the map and focus on the ejection trajectory. If the timeframe (mun position and node position in munar orbit) is right, the trajectory will follow.

[Spricigo]

16 minutes ago, Hesp said:

It's exactly the opposite... I haven't got numbers at hand right now, but orbital period in LMO (50km) is higher than in LKO (150km), so cosine losses are lower (and the burn is way shorter too!).

well, my comment was based in my intuition (a.k.a. poor substitute for experience). I'm not surprised if I was wrong. However I'd try to look for LMO=20km since that is what the Op is using to be sure (let alone all the detail we can include  in the analyses).

In any case, what he attempted is something I never considered in my own game (if I'll use refuel I’ll  do at Minmus followed by an Oberth
Dive to LKO)  so, while I hope my guesswork to be somewhat helpful, take it with a grain of salt.

[Zhetaan]

@Cpt Kerbalkrunch, @HebaruSan, @Lunar Sea:

@OhioBob did a rather exhaustive analysis of savings for various starting orbit altitudes.  I know it's not the original question, but since the discussion is turning that way, I thought I'd supply the reference.  It's something to consider if any of you want to make a serious effort at understanding the physics behind orbital refuelling stations, advanced interplanetary transfers, and the like.

This also talks about the subject, if you're interested, and it has some very interesting mathematical plots (including ones that choose the best of either a direct prograde transfer or an LKO dive).

Edited July 10, 2017 by Zhetaan

[Spricigo]

 

 

42 minutes ago, Zhetaan said:

@OhioBob did a rather exhaustive analysis of savings for various starting orbit altitudes.

While, for obvious reason,I don't contest the analysis made in this thread, it is often misinterpreted. (irony at its finest)

Quote

I've always thought the term "Oberth effect" was a bit overused around here and somewhat misunderstood. It is not the hard and fast rule that everyone seems to thinks it is.

I suppose OhioBob didn’t intended to replace the old "rule" everyone used without thinking for a new one equally mindlessly used. He pointed that the "rule" was not always true, food for thought. So he said:

Quote

Of course, V_orb and V_esc are both functions of the orbital altitude.

What is fine for the analysis he was doing but don't tell the whole story. Take a periapsis of 100km around kerbin, what is the orbital velocity(V_orb)? Answer: take the vis-viva equation V²=GM(2/r -1/a) ...and notice the missing data: semi-major axis(or apoapsis if you prefer). He just considered circular orbits and continued his analysis, and for what he proposed it was the simple end effective. Its just when people use it for something else without considering how the situation is different that problems arise.

Moral of the story: It is not the hard and fast rule that everyone seems to thinks it is. (and tanks for @OhioBob for sharing a bit of his expertise with us)

 

[Cpt Kerbalkrunch]

1 hour ago, Spricigo said:

...and notice the missing data: semi-major axis(or apoapsis if you prefer). He just considered circular orbits and continued his analysis, and for what he proposed it was the simple end effective. Its just when people use it for something else without considering how the situation is different that problems arise.

Moral of the story: It is not the hard and fast rule that everyone seems to thinks it is. (and tanks for @OhioBob for sharing a bit of his expertise with us)

 

Exactly. We've seen the OhioBob thread referenced several times before. I've read through it a couple times (and I assume everything in it is true and don't dispute a word of it), but it usually doesn't quite apply to the conversation in which it's referenced. I'm not a physics or mathematics guy (a far cry from an OhioBob, sadly), but I know the game enough to know truth when I see it. There are 2 things that the referenced thread doesn't take into account: the first is that when leaving from Mun or Minmus, you most likely went there to refuel (the 2nd thread referenced does touch on this). This adds a dynamic that changes the entire conversation. Now it becomes; are you better off leaving Mun or Minmus fully fueled, or leaving from LKO with what you have?

The 2nd point would be, when leaving Mun or Minmus, did you drop down into the gravity well to build up speed? Now with a highly elliptical orbit of an Ap around Mun or Minus, and a Pe just outside Kerbin's atmosphere, you have built up a tremendous amount of energy and can make a relatively easy transfer. The savings from one to the other is pronounced and will quickly become obvious during testing. The toughest part is usually lining up for the proper ejection angle.

You could also add a 3rd question and ask, did you do both 1 and 2? If you went to Mun or Minmus, refueled, then did the dive before your burn, it's going to be superior to leaving from any stable orbit, IMO. This is provided you didn't stage away your engines in order to get to the moon in question in the first place. Also, @Red Iron Crown had a great refueling, drop down and burn method that sounds like about the best you can get for a single ship (using a reusable refueler/booster), but for a multi-ship convoy like the OP is talking about, I think the dive and burn is the way to go.

All this is, of course, assuming that you don't mind the real-time spent traveling to whichever moon (Minmus being the best bet, IMO) and doing all the refueling in the first place. That's something everyone would have to weigh for themselves.

[OhioBob]

4 hours ago, Spricigo said:

I suppose OhioBob didn’t intended to replace the old "rule" everyone used ...

The "rule" to which I refer is the often stated incorrect claim that it is always best to eject from the lowest possible orbit to take advantage of the Oberth effect.  The only reason for the thread in question was to show that that's not necessarily true.   For me the thread was nothing more than reporting an observation, and then spending the next several pages trying to find a mathematical explanation for it, and exploring the implications.  This lead to the discovery of something none of us had ever heard of before, a gate orbit.  I suggested in the thread that it is up to each individual to use the information as they see fit and to determine whether or not they can find a practical application for it.  No attempt was made to replace an old rule (or fallacy) with a new rule.  We simply went were the math took us.  What I enjoyed about that thread was that those of us involved were making some real time discoveries (new to us at least).
 

4 hours ago, Spricigo said:

He just considered circular orbits and continued his analysis ...

That's not true.  A considerable amount of the thread discussed elliptical orbits with ejection at periapsis.  One of the implications of a gate orbit is that it provides a dividing line between when it is best to eject directly from the initial orbit, versus lowering the periapsis and ejecting when passing through periapsis.  Much mathematical analysis was devoted to this issue (did you not read it?).  The math shows that if you start out above the gate orbit, it is most efficient to lower the periapsis to just above the atmosphere and eject from there.  On the other hand, if your initial orbit is below the gate orbit, it is most efficient to eject directly from the initial parking orbit without lowering the periapsis.

(That's not a rule, it's a mathematical fact.  Use the information as you like to make your own rule.)
 

Edited July 10, 2017 by OhioBob

[OhioBob]

1 hour ago, Cpt Kerbalkrunch said:

The toughest part is usually lining up for the proper ejection angle.

Agreed.  It is easy to say that one should drop into an elliptical orbit and eject at periapsis, but executing it is an entirely different matter.  You can't just place the periapsis at any old spot.  The maneuver has to be executed in such a way the spacecraft passes through periapsis at the right place (proper ejection angle) and at the right time.

The analysis made in the referenced thread just tells us that if we start out at an altitude higher than the gate orbit, then we should be able to eject using less Δv if we first drop the periapsis to just above the atmosphere, and then eject when passing through periapsis.  That's what the math tells us.  However, if the execution is off, we could end up with a suboptimal burn because of incorrect ejection angle or some other misalignment or mistiming.  We could waste enough Δv that we squander any potential savings.  What mathematically sounds like an advantage could actually end up costing us more.

That's why I don't intend to propose any particular set of rules for what a person should do or when.  It is up to the each individual to decide for themselves what they fell comfortable doing and what they think they can properly execute.  However, knowing what the math says is an important first step.
 

[Spricigo]

3 minutes ago, OhioBob said:

  No attempt was made to replace an old rule (or fallacy) with a new rule.  We simply went were the math took us.  What I enjoyed about that thread was that those of us involved were making some real time discoveries (new to us at least)

Exactly what I understood. My point is that, ironically, people now skim over the first post and get some of the values as the new ideal height for the transfer. (e.g. so 303km is the ideal height to transfer to Jool.)

8 minutes ago, OhioBob said:

That's not true.  A considerable amount of the thread discussed elliptical orbits with ejection at periapsis. 

Sorry, what I meant was that a first approach considered circular orbit. Since I was criticizing* people that don't read it past the first post I didn't mentioned the later discussion, my fault.

Also my fault that I did the same mistake for awhile, after I take the time to read and try to comprehend the entire discussion. Only when I noticed that I was wasting a lot of fuel/time/effort to reach a ideal orbit that was not ideal at all in my circumstances (I read the thread again, said D'oh! , read a few times more to make sure I really understood and promised to myself: not be so lazy and to pay attention when studding something. )

Hope I made myself clear now. And thanks again.

 

*In my pedantry I rather prefer to say alerting, but criticizing is the correct word.

[OhioBob]

5 minutes ago, Spricigo said:

My point is that, ironically, people now skim over the first post and get some of the values as the new ideal height for the transfer. (e.g. so 303km is the ideal height to transfer to Jool.)

OK, that's a valid point.  If one just reads the OP and not the entire thread, they'd definitely miss out on a lot of important follow up discussion.  I can see how somebody could possibly misinterpret to intent of the OP and read as a recommendation to eject from the altitudes given.  However, even in the OP I qualified my statements with the following:
 

On ‎10‎/‎6‎/‎2015 at 1:04 PM, OhioBob said:

What these results don't take into account is that, although it may require less Δv to eject from a higher orbit, it takes more Δv to reach the higher orbit in the first place. When we add together both the launch Δv and the ejection Δv, the lower orbit results in less total Δv.

 

[Spricigo]

1 minute ago, OhioBob said:

However, even in the OP I qualified my statements

That says much about the quality of your analysis. Unfortunately we can't guarantee people will not ignore that because... "is just a game" let alone that people are just people.

I really suggest people read, with attention, the thread, and some other obviously good stuff about rocket science and orbital mechanics. Just remember that IS rocket science, pay attention.

 

[Lunar Sea]

good stuff folks, thanks to all who expounded on this technique. In my case it worked and vessels are on their way. 

[Freshmeat]

I have done some practical work inspired by a similar thread around Christmas or so. The outcome was that refueling at Minmus had substantial savings due to lighter payloads sent to orbit, and the drop to a periapsis did indeed save some dV, the savings where rather small due to steering losses, wrong inclination of Minmus (this could get very bad), and the fact that it is quite a challenge to find the exact escape orbit from Minmus to get the correct argument of periapsis.

That being said, figuring out how to calculate ephemerides where quite interesting, and as a bonus I know the launch time for heading to Minmus with the correct inclination.

PS: Writing this I just realised that there must be a way to launch from Minmus to get a low inclination  on the Kerbin orbit, by tilting the orbit from Minmus. Investigation pending.

[Kryxal]

Near Minmus AN and DN, getting near-equatorial is easy enough, but doing it at whatever point you need to eject is the real trick.