Fixing muffed xfer orbits?

[bewing]

A question for the orbit experts -- did I make the best choices to try to repair a mistake? (Stock KSP)

I have a contract to take four tourists to solar orbit = leave Kerbin SOI. They were all on a reusable ship docked to my fuel station in a 100km parking orbit over Kerbin. The ship had been partially refueled a month earlier with 4000 dV of fuel.

In order to save a little fuel, I set up a maneuver node for a Munar slingshot flyby. Forgot to do a quicksave -- but I timed the maneuver nicely, and lit the torch. Flipped to the map view to watch the orbit change. Three or four seconds later the engine gasped out its last newton of thrust and quit.

It took me another second or two to realize the engine had quit. Another second or two to realize that the sidesaddle tanks had been filled, but the core tanks had not -- and I needed to do a manual fuel transfer. Flipped screens, opened all the necessary context menus, clicked the "In" button -- and the engine roared back to life. Total time from engine shutdown was maybe 15 seconds -- but now my burn is late for my slingshot .... And it turned out that I'd missed my window.

So: onto desperate mission repair mode -- I burn by hand to try to get a Munar intercept with an escape trajectory. My Ap ends up a little above the Mun, makes this stupid little loop, goes into the Mun's SOI, goes through the Mun, turns hard as it passes by the Mun's core, and then proceeds to escape Kerbin's SOI. I sigh, and start planning my mid-course correction.

Desperate repair #2: I do the small mid-course correction, and now I have an actual 11km Munar Pe. But now the slingshot is borked and will only get me to a 16Mm orbit. I sigh some more.

Desperate repair #3: So, since a measly 5Mm above Munar orbit isn't worth a damn, I say "At least I'll have some damned Oberth at my Munar Pe, and I'll just burn there." So I start my burn at my Pe. It takes an incredible amount of fuel and dV! Something close to 400 dV, when I was already close to an escape trajectory in the first place! (I was burning and not expecting that I'd need to pay attention to those numbers.) My ejection orbit from the Mun straightens out completely, and my ejection orbit from Kerbin ends up being retrograde. I've never done a hyperbolic burn like that near the Mun before, but that didn't go at all the way I expected.

OK, so yeah, I'm on an escape trajectory. Whoopie. I bet I used twice as much fuel as I should have, somehow. And it's gonna get worse when I put myself back in Kerbin's SOI and have to fix my retrograde orbit -- you can't get a good braking slingshot when you are coming straight down .... And yes, it's only a thousand units of liquid fuel up in smoke, and the tourists are going to easily repay the costs. But still, I'm trying to save up some kash for my R&D upgrade, so this hurts a bit.

Was that truly the best I could have done, once the initial empty gas tank problem hit? Because it really felt like a total screwup by the end.

-- Or, once I saw that the Munar intercept was all messed up (as I left LKO at 3km per sec), should I have realized things were going to hell and just hit the gas and bypassed the Mun?

 

Edit: to partially answer my own question -- I realize after thinking about it that I threw away half my Munar slingshot by burning at my Munar Pe the way I did. I should have waited a bit longer. I would have lost a little Oberth, but gained a lot more from the slingshot.

Edited July 11, 2016 by bewing

[Streetwind]

It's a bit hard to give advice here... the situation is so specific to its initial conditions and how precisely it played out that the text description alone doesn't suffice, I'm afraid.

What I probably would have done is to not finish the initial burn fully, after realizing the error with the fuel. Instead, I would have tried to lift my AP up to shortly before getting a Mun intercept, and then stopped and deleted the maneuver. Then I would have made a new maneuver node, and tried to get the proper slingshot again. It should theoretically be possible via the use of the radial-out direction and the drag-and-drop of the node along the orbit.

But if I had discovered that the error was too large to get the planned slingshot, or too expensive to fix, I would have aborted, looped around to Kerbin, burned retrograde to lower my orbit again, and plotted a rendezvous to the fuel depot in order to get it right the next time the Mun looped around (takes only 6.4 Kerbin days). With properly filled fuel tanks this time.

[bewing]

Actually, I am slowly realizing that there is a generic lesson involved. But I'm still trying to explain it to myself. The Mun Pe burn was a 100% mistake.

The reason is: in a slow hyperbolic slingshot flyby, if you want to gain more energy, you burn retrograde, and I burned prograde.

I have seen this every time I'm doing a capture at the Mun, and it just never sank in. But why does retrograde work? Hmmmm. Is it just because it gives the Mun a longer time to accelerate you?

Edited July 11, 2016 by bewing

[Streetwind]

Instinctively, that looks wrong to me. Retrograde should always remove orbital energy. Even spending more time falling towards a gravity well will only consume the energy it gives you to restore what your burn removed... But as I said, that's my intuition speaking. I fancy myself pretty good at orbital maneuvering, but gravity assists are one thing I usually don't bother with.

One thing I can imagine happening is that the prograde burn made your final velocity vector point in the wrong direction. You had the energy, but it was not putting you where you wanted to go. Maybe a retrograde burn would have improved your vector, even at the cost of some energy?

[StarStryder]

On 7/10/2016 at 7:00 PM, Streetwind said:

What I probably would have done is to not finish the initial burn fully, after realizing the error with the fuel. Instead, I would have tried to lift my AP up to shortly before getting a Mun intercept, and then stopped and deleted the maneuver. Then I would have made a new maneuver node, and tried to get the proper slingshot again. It should theoretically be possible via the use of the radial-out direction and the drag-and-drop of the node along the orbit

No.  You're right that you might need to use a little radial thrust to re-gain the intercept but doing this after you've boosted your AP to nearly Mun altitude is wasteful.  As a general rule it's always better to combined burns.  Think about travel the long side of a triangle vs the two shorter sides.  Also, burning further away from your target gives bigger changes of position per m/s speed change than burning close to your target.

When this sort of thing happens, kill the original maneuver node and create a new one, as early as possible.

[Streetwind]

1 hour ago, StarStryder said:

When this sort of thing happens, kill the original maneuver node and create a new one, as early as possible.

Even if it happens 5 seconds into a 120 second burn, and you spend another 15 seconds noticing and fixing the problem?

The issue here is that you just missed a huge node completely, and you're in a tight orbit. You don't have time to spend a minute making a new node that's 1-2 minutes ahead of your ship. By that point, you went like 1/8th the way around Kerbin compared to your initial node. And remember: your target is not merely an encounter, but rather a precise gravity assist that is supposed to give you an encounter with a secondary target, and so you're reliant on coming in from just the right angle at just the right altitude while the Mun is in just the right position.

I'd rather get my AP lifted as soon as possible, so that my ship already travels along an approximation of its intended path while I plan my correction. The radial burn from that approximate path won't be very large at all.

And you know, we could just combine our approaches... continue the original burn and stop it short for a late correction like I would, but manually steer outwards to add a radial component into it like you would. You'd of course have to be careful not to overdo the radial, but if you eyeball it right, the correction should be minimized.

Edited July 13, 2016 by Streetwind

[StarStryder]

On 7/13/2016 at 7:16 PM, Streetwind said:

The issue here is that you just missed a huge node completely, and you're in a tight orbit. You don't have time to spend a minute making a new node that's 1-2 minutes ahead of your ship. By that point, you went like 1/8th the way around Kerbin compared to your initial node. And remember: your target is not merely an encounter, but rather a precise gravity assist that is supposed to give you an encounter with a secondary target, and so you're reliant on coming in from just the right angle at just the right altitude while the Mun is in just the right position.

Yes, because if you're in a tight orbit then waiting 1 more orbit is going to make likely to no difference on the launch window.  Interplanetary launch windows are measured in days, not minutes or hours.  There will be a small loss of efficiency in correcting for the small change in orbital position of the assisting body but since correcting early is vastly more efficient than correcting late, it's still much better.

Combine this with a course correction mid-way between the assist and the target (will be <10m/s) and you'll be fine.