Matching target - useful

[RW-1]

I had two craft in a parked orbit at the outermost region of Kerbin, just short of leaving SOI, they were awaiting a jool launch window.

I was playing with a maneuver node to get an intercept, and did so for my manned large lander, and off it went.

Came back to my other vehicle, a manned station, and just couldn't replicate the encounter, likely because of numerous factors, but, i found an elegant solution, and I had the DV to do it.

From the station, I selected the lander, and then burnt to it's retrograde marker, matching it up ....  Encounter! and then adjusted so that the station gets there first hehehe.

 

 

[Snark]

By the way... if you're going interplanetary, then being parked way out near the edge of Kerbin's SoI is about the worst thing you can do, in terms of efficiency-- it's horribly wasteful of fuel.  Hermann Oberth is spinning in his grave.

You'll save yourself scads of fuel and dV if you do your burn in low Kerbin orbit instead.

If you have some other reason to want to do it from high orbit-- e.g. roleplaying, or just-playing-around-with-oddball-mission-parameters-for-fun, or perhaps you're playing sandbox and everything is free and you don't care if you massively waste fuel-- then never mind. 

Just wanted to make sure you're doing it this way on purpose rather than unknowingly.  Not everyone realizes this aspect of orbital mechanics.

[Enceos]

@RW-1 That's very inventive

@Snark One the outer edges of the SOI the orbiting speed is so slow that one can send vessels in pack one by one matching the velocity of the leading vessel. It's gonna be very hard to send a synchronized fleet from the low orbit.

Edited May 23, 2016 by Enceos

[Brainlord Mesomorph]

4 minutes ago, Enceos said:

It's gonna be very hard to send a synchronized fleet from the low orbit.

Not if you know your departure angle weeks ahead of time using the technique in my sig.

 

19 minutes ago, Snark said:

You'll save yourself scads of fuel and dV if you do your burn in low Kerbin orbit instead.

Question: but what if I'm already in Mimus orbit. Is it worth the fuel to return to Kerbin, or plan a spoon out around Kerbin on my way out? or should I just leave from Minmus?

[Snark]

19 minutes ago, Enceos said:

On the outer edges of the SOI the orbiting speed is so slow that one can send vessels in pack one by one matching the velocity of the leading vessel.

Well, sure.  And if you're willing to completely waste a kilometer per second or more of dV because you can't be bothered to set up the navigation from LKO, it does make that convenient, yes.

But it's a huge waste.

If the player knows all about the orbital mechanics involved, doesn't care, and is fine with that, then great!  Please ignore what I said.    I certainly wouldn't presume to try to tell anyone how to play (or enjoy) KSP.  The fact that the game enables all sorts of different play styles is one of the great things about KSP.

Just wanted to make sure that the player's aware of just how incredibly wasteful the described mission profile is.  Some folks don't realize the degree to which proper use of Oberth effect can make a huge difference to dV requirements.

And if you want to depart in a fleet, there are ways to do that even from LKO that work pretty well and aren't too hard to set up.

11 minutes ago, Brainlord Mesomorph said:

Question: but what if I'm already in Mimus orbit. Is it worth the fuel to return to Kerbin, or plan a spoon out around Kerbin on my way out? or should I just leave from Minmus?

Depends on the mission profile, but probably worth dropping down to Kerbin for an Oberth maneuver, assuming that your burn requirement is more than just a couple of hundred m/s.

When you're boosting to go interplanetary, you want to add energy to your orbit.  Dropping down to Kerbin from Minmus orbit is reducing your energy, so that's an up-front "tax" (which you pay twice, because you have to regain it coming back out again, though the outbound tax is less than the inbound due to Oberth benefit).  But this is offset by the massive efficiency gain of your primary burn.

In effect:  by choosing to drop down to Kerbin for your main burn rather than ejecting directly from Minmus, you're flushing probably around 300 m/s of dV down the toilet right up front... but on the other hand, you're adding a pretty hefty multiplier to your main burn.  So as long as that multiplier (and the size of the burn) is enough to more-than-compensate for the 300ish m/s up-front cost, then it's worth your while.

Basically, the bigger the Kerbin-relative velocity that you need to eject from Kerbin's SoI with, the bigger the benefit you'll get from an Oberth maneuver.  I'd have to work some numbers to work out what the break-even point is, but I suspect it's pretty low.  Heading to LKO from Minmus orbit only takes 240ish m/s of dV, IIRC, so if your main burn is over 1000 m/s, I expect it's well worth it.

If you're curious, I'd suggest just pulling out a calculator and running the numbers.

[RW-1]

1 hour ago, Snark said:

By the way... if you're going interplanetary, then being parked way out near the edge of Kerbin's SoI is about the worst thing you can do, in terms of efficiency-- it's horribly wasteful of fuel.  Hermann Oberth is spinning in his grave.

You'll save yourself scads of fuel and dV if you do your burn in low Kerbin orbit instead.

If you have some other reason to want to do it from high orbit-- e.g. roleplaying, or just-playing-around-with-oddball-mission-parameters-for-fun, or perhaps you're playing sandbox and everything is free and you don't care if you massively waste fuel-- then never mind. 

Just wanted to make sure you're doing it this way on purpose rather than unknowingly.  Not everyone realizes this aspect of orbital mechanics.

Oh heavens no!

It wasn't planned, originally these were going to go elsewhere, but if staying in the Kerbin SOI I wanted to do an exit and back, to garner points for the crew.

But as far as using the one's departure and having the DV to spare on the 2nd's to match them up because I couldn't get the node to grab the intercept, well, I was quite pleased with myself for figuring that one out hehehe ...

I would make the assumption that I likely could have done the same thing in LKO, thing is if the maneuver target node would have had Kerbin in the way of a matching burn when getting the second vessel there, something you likely would not have out at the edge, but agreed totally better to head out from LKO in terms of total fuel, etc.

I wish I had more time to actually do those moon boost departures and arrivals, because it's fun! Just laziness on my part. The fuel you can save by using the planets and moons, well ... you know

Edited May 23, 2016 by RW-1

[Geschosskopf]

1 hour ago, Enceos said:

One the outer edges of the SOI the orbiting speed is so slow that one can send vessels in pack one by one matching the velocity of the leading vessel. It's gonna be very hard to send a synchronized fleet from the low orbit.

Huh?  I do it all the time.  Nothing to it.  KAC and MJ help a lot but aren't essential.  BetterBurnTime, OTOH, is pretty much required.

What you do is launch 1 ship into LKO.  Then select the target and use the porkchop plot in MJ's "Advanced Transfer to Another Planet" to create its transfer node.  Use KAC to create an alarm for that node.  Then launch the next ship and repeat the process, until you've got your whole flotilla in orbit with transfer nodes set.  As you create nodes for the other ships, you can pick departure times off the porkchop plot that don't conflict with those of other ships.  Spread them a few hours apart.  Then warp to the 1st departure time, burn that ship, warp to the next, repeat until all are on the road.

NOTE:  Also have KAC automatically create SOI alarms about 5 minutes prior.  You might have to alternate between burning ships and walking others across the SOI boundary.

[Pecan]

21 hours ago, Geschosskopf said:

Huh?  I do it all the time.  Nothing to it....

Geschosskopf's 'flotilla' tutorial is excellent for this, though he modestly doesn't point it out :-)

ETA: Link in his signature

Edited May 24, 2016 by Pecan

[pasukaru76]

On 23 May 2016 at 5:32 PM, Snark said:

If you have some other reason to want to do it from high orbit-- e.g. roleplaying, or just-playing-around-with-oddball-mission-parameters-for-fun, or perhaps you're playing sandbox and everything is free and you don't care if you massively waste fuel-- then never mind. 

I tend to launch my current interplanetary stuff from Minmus orbit, but that is only because all craft get fully refuelled by an ISRU outpost there. Usually that means more delta-v available at the target, since I basically leave Kerbin SOI with an almost full tank after a short escape burn from Minmus.

[Alshain]

On 5/23/2016 at 10:45 AM, Enceos said:

 

@Snark One the outer edges of the SOI the orbiting speed is so slow that one can send vessels in pack one by one matching the velocity of the leading vessel. It's gonna be very hard to send a synchronized fleet from the low orbit.

Actually, it's easier to do it that way.  Your craft orbit the planet so quickly that they get back to the burn location very fast.  So they don't all have to leave at the same time in the same orbit, but they still more or less end up as a fleet.  If you are using remote tech and you want constant communications without giant dishes on every single vessel, that is the best way.  You just need one long haul vessel (often the very sattelites you plan on putting in orbit of the destination) and the rest of your ships aim at that ship.  You don't want them all arriving at the same time anyway, you might miss an insertion burn.

On 5/23/2016 at 0:23 PM, Geschosskopf said:

Huh?  I do it all the time.  Nothing to it.  KAC and MJ help a lot but aren't essential.  BetterBurnTime, OTOH, is pretty much required.

What you do is launch 1 ship into LKO.  Then select the target and use the porkchop plot in MJ's "Advanced Transfer to Another Planet" to create its transfer node.  Use KAC to create an alarm for that node.  Then launch the next ship and repeat the process, until you've got your whole flotilla in orbit with transfer nodes set.  As you create nodes for the other ships, you can pick departure times off the porkchop plot that don't conflict with those of other ships.  Spread them a few hours apart.  Then warp to the 1st departure time, burn that ship, warp to the next, repeat until all are on the road.

NOTE:  Also have KAC automatically create SOI alarms about 5 minutes prior.  You might have to alternate between burning ships and walking others across the SOI boundary.

The more stock way to do it is to find the first ship's departure, burn it. and then switch crafts.  You will be able to see the first ship's trajectory as a grey line, so at it's apex around Kerbin, create a new node and drag it out until the lines overlap and you should be very close to an encounter, minimal tweaking depending on the planet.  Rinse, repeat.

[Snark]

30 minutes ago, pasukaru76 said:

I tend to launch my current interplanetary stuff from Minmus orbit, but that is only because all craft get fully refuelled by an ISRU outpost there. Usually that means more delta-v available at the target, since I basically leave Kerbin SOI with an almost full tank after a short escape burn from Minmus.

Sure, if you've got an ISRU station right there refueling it, it offsets the "waste" issue.  You're not "wasting" fuel to lift it up to Minmus if you can refill it once you're there.

However, the OP wasn't doing that (or at least, didn't mention doing that)-- he was just orbiting in free space way out on the fringes of Kerbin's SoI, didn't make any mention of Minmus refueling.

And in any case:  even if you do somehow have a fully-fueled set of ships out there... you'd still get more bang for your buck to drop your Pe down to LKO and do your main burn there, rather than ejecting directly from the high orbit.

As with Minmus.  If you have your ships fueled-up there, it's likely (depends on your destination and the size of the burn) that you can save dV by ejecting retrograde from Minmus to drop your Pe down to LKO, and then do your main burn there, rather than ejecting interplanetary directly from Minmus.

[pasukaru76]

16 minutes ago, Snark said:

And in any case:  even if you do somehow have a fully-fueled set of ships out there... you'd still get more bang for your buck to drop your Pe down to LKO and do your main burn there, rather than ejecting directly from the high orbit.

You're right, of course. I seem to recall a while back (must have been last year) someone did the math and the difference in delta-v between "eject directly from high orbit" and "eject from high orbit by lowering pe first and burn in low orbit" wasn't all that big for some situations, but always in favor of the second maneuver.

It seems installing an ISRU network has made me care less about efficiency. Interesting, I didn't expect that to have an impact on my style of playing.

[Norcalplanner]

59 minutes ago, pasukaru76 said:

You're right, of course. I seem to recall a while back (must have been last year) someone did the math and the difference in delta-v between "eject directly from high orbit" and "eject from high orbit by lowering pe first and burn in low orbit" wasn't all that big for some situations, but always in favor of the second maneuver.

It seems installing an ISRU network has made me care less about efficiency. Interesting, I didn't expect that to have an impact on my style of playing.

I think you mean this post from this thread.  Major kudos to OhioBob for putting this in an easy-to-understand format, using two common scenarios - transfer to Duna, and transfer to Jool.

 

[pasukaru76]

20 minutes ago, Norcalplanner said:

I think you mean this post from this thread.  Major kudos to OhioBob for putting this in an easy-to-understand format, using two common scenarios - transfer to Duna, and transfer to Jool.

Thanks for digging that up.  So it doesn't matter all that much for going to Duna, but for going to Jool it makes a surprisingly huge difference.

[RocketBlam]

On 5/23/2016 at 8:32 AM, Snark said:

By the way... if you're going interplanetary, then being parked way out near the edge of Kerbin's SoI is about the worst thing you can do, in terms of efficiency-- it's horribly wasteful of fuel.  Hermann Oberth is spinning in his grave.

You'll save yourself scads of fuel and dV if you do your burn in low Kerbin orbit instead.

I would do this, but I generally set up my transfer from orbit. Doing so, if you have to wait a year for the burn, you would like to accelerate time as fast as you can, and you have to be beyond 600km to get the max time warp.

[Snark]

1 hour ago, RocketBlam said:

I would do this, but I generally set up my transfer from orbit. Doing so, if you have to wait a year for the burn, you would like to accelerate time as fast as you can, and you have to be beyond 600km to get the max time warp.

Or you could just switch to the tracking station and warp as fast as you like.

[RocketBlam]

Just now, Snark said:

Or you could just switch to the tracking station and warp as fast as you like.

Haha, you know, after I typed that, I decided to do exactly that. Well, not exactly that, I just left the vehicle on the pad and warped from there, to almost where the phase angle was right. Maybe I've learned something new today!