Recovery from a Duna fly-by with limited dV

[Mast]

A few hours ago I launched a manned rocket towards Duna for a fly-by.

All went well, except I'm uncertain I'll ever be able to return to Kerbin.

I can't make a parking orbit around Duna (not enough fuel).

I can make it into the Kerbin SOI, but probably not into orbit.

The situation:

MET:

206d, 0h, 24m

LiquidFuel left:

292

Oxidizer left:

357

Engine:

LV-T45

Estimated dV left:

1750 - 2000 m/s

Leaving Duna SOI in:

1d, 1h, 43m

Acquired path to Kerbin SOI: (not necessarily optimized)

1736 m/s

Necessary dV to make Kerbin orbit after reaching SOI:

over 5km/s

Necessary dV to prevent leaving Duna SOI:

over 3km/s

Current distance to Duna:

46.000 km

Pe:

9.250 km

I'm flying without mods and the current craft has no docking ports (I was going to unlock those with the science retrieved from the fly-by).

The craft does have (probably) enough parachutes to survive landing on Kerbin without fuel.

Question:

How can I find out what's the best possible short-term scenario?

Preferably not just for this case, but also for similar cases. I'm not afraid of math, I just don't know what function to apply here (again, there is no parking orbit established).

By default it will return to a orbit around the sun which crosses the Kerbin orbit 2 times (ofcourse, Kerbin is not there when the craft is and vice versa). Retrieving it from there with a second craft would be fairly complex.

The long term goal is complete recovery.

Edited July 19, 2014 by Mast

[Mr Shifty]

You could wait a couple orbits around the sun. You'll probably get a Kerbin intercept at some point. If you wait until you pass out of Duna's SOI and are orbiting the sun again, put a maneuver node down at some point ahead of your vessel (a few hours in the future.) Make sure you select Kerbin as the target and you should be able to click the "+" button next to the maneuver node to advance the node one orbit each time. If you never see any intersections, you may need to alter your orbit so that it either nearly matches the plane of Kerbin's orbit or so that it intersects Kerbin's orbit at one of the ascending/descending nodes. You should be able to achieve intercept at some point within a couple orbits for a few tens of m/s.

Once you've got intercept, wait until you're closer to Kerbin's SOI, then tweak the orbit so that your periapsis is within Kerbin's atmosphere (make it ~20km). That way you can use Kerbin's atmosphere to aerobrake through capture and de-orbit.

[eempc]

I guess it is too late to aerobrake on Duna. If you flew into Duna's atmosphere with a periapsis of around 12 km, you can form an orbit easily with no fuel expended. Generally speaking, once you enter into a planet's SOI, you burn 90 or 270 degrees to alter your periapsis, this costs less than 10 m/s DV usually. After the aerobrake, a short burn at apoapsis will bring your periapsis above 41 km.

If you've already escaped Duna's orbit and can plot any course back to Kerbin, once you are in Kerbin's SOI, all you need is a small correction burn to bring your periapsis into Kerbin's atmosphere.

[Master Tao]

The easiest option may be to continue your Duna flyby, then set up a maneuver node near your solar AP and adjust it forward a few orbits for a cheap maneuver to a Kerbin intercept. (What MrShifty said)

Other options to consider:

You're just inside Duna's SOI. If you just entered Duna's SOI, you could fairly cheaply drop your PE well inside Duna's atmosphere to perform an aerocapture. You should have plenty of dV to do that and drop your parking orbit well inside Ike's orbit.

Try to find a maneuver near your Duna PE to give you an Eve encounter. Adjust that flyby to for Kerbin intercept. It helps to focus the view on Eve while adjusting that encounter.

Edited July 11, 2014 by Master Tao
switch solar AP/PE

[Pecan]

Unlike the others I think your Duna periapsis - 9.25km is probably so low that, unless you're really screaming past, you've about equal chance of crashing anyway. I'd pop the chutes over Duna and call that a landing.

For similar cases; avoid them ^^ - check the transfer windows and thrust required.

[davidpsummers]

If your orbit crosses Kerbin, then you can get an intercept (which I think you can do since you say you can get into the Kerbin SOI).

When you get the intercept, the periapsis will be listed. Do a burn (a small one as early as possible) to get it as low as possible. You will have to fiddle and try not to loose the intercept (at least to the point were you can't recover it) since the direction you need to burn will depend on where around Kerbin the periapsis lies. (If it is off to the north or south, you will need to do a burn that is at least somewhat normal or anti-normal). The best way is, IMO, as soon as you leave Duna's SOI, set up a maneuver node just ahead of you. Fiddle with the different directions and see what it does to the periapsis. (If it disappears completely, you might be going right into Kerbin, which is OK except for the G forces you astronauts will receive on reentry. But check to make sure you are really headed into Kerbin.). Using your RSC thrusters can be good for fine control.

Recheck this a couple of times on the way. As soon as you enter Kerbin SOI, make sure your obit intersects Kerbin, or has a periapsis that is low enough. (33 km will either send you to the surface or put you in a resonable obit which will decay the next time you pass the periapsis). You can be a bit above 33 km, but I'm not sure how much. You need to slow down enough to end up in orbit (or have to try and intercept Kerbin all over again), any obit because you will slow down more each time you pass through the atmosphere. Above 65 km you won't brake significantly.

[davidpsummers]

Note: aerobreaking at Duna will give you a chance to land or orbit Duna. I will also put you inside Duna's gravity well and you will need delta V to get out, and since return is you top priority, I'm not sure it meets your needs.

[LethalDose]

Given you're at 46 km above Duna, with a Pe of 9.25 km, you may be landing on Duna and there's not a whole lot you can do right now... If that's the case, just land, then send a rescue mission. I doubt you'd be able to to land safely and get back to orbit if you're asking this question (it may be possible, but if it were experienced enough to do it, you wouldn't be asking the question in the first place).

If you get captured by Duna but are able to stay in orbit, then you can probably stabilize your orbit (lower the better, but still above the atmo), and wait for your return window. In this case, you probably don't need a rescue.

If you aren't captured by Duna, and you have almost 2 km/s left of dV, I think the easiest way to get home is to burn at your solar orbit's apoapsis to bring your periapsis to the same level as Kerbin's altitude (13.6 million km). Then, at periapsis, burn until you get an encounter with Kerbin on your next orbit at your Pe (this may be prograde or retrograde, see which gets you a cheaper intercept).

Also, if you aerobrake at Kerbin (once or over several orbits), you won't need 5 km/s of dV to get back to the ground.

Best of luck, let us know how it turns out.

Edited July 10, 2014 by LethalDose

[Master Tao]

Some clarification would help: Is your PE 9250 meters or 9250000 meters? If it's 9250 m, LethalDose is right – you're about to have an unplanned crash landing on Duna.

[-root]

The long term goal is complete recovery.

I would say you're going to need patience for starters.

1.75km/s should be tons to get back to Kerbin.

As you go past Duna, ensure your orbit isn't being perturbed too much. I'd even suggest a burn in close to iron out any kinks that the fly-by might put into your orbit. That will gain you some oberth effect. The goal being, to get the kerbol periapsis as close to the same as Kerbin's orbit. If you can do that, you'll get an encounter sooner or later.

You may need another small correction to get the encounter. Just use the manuever node and advance orbits like others have said to find the encounter. Once you have it, work it around until you roughly close.

The next step would be to fine tune your approach to below 30km*. That will guarentee an aerocapture and zero fuel usage when you get in close.

*NB: provided you don't have deadly re-entry installed

[KerbMav]

I can't make a parking orbit around Duna (not enough fuel).

I can make it into the Kerbin SOI, but probably not into orbit.

You wanted to do a flyby anyway, also there is a different bonus for flyby, orbit and return from surface of X, and usually flybys and orbits get lost to missions being planned as landings, so take what you get!

If you can enter Kerbin SOI you can land, just change your flight path as soon as you leave Duna SOI and aim for a very low PE at Kerbin, keep checking as the path may shift due to calculation errors by KSP itself.

[cantab]

I'm unsure why the Duna capture is reporting 3000 m/s. Per the delta-V maps it should be more like 500 to capture and make low orbit, even less if you're happy with an elliptical orbit. Your ship should be more than up to the job of entering Duna orbit then leaving for Kerbin.

[Mast]

Perhaps it's noteworthy that a . in European notation is used as a thousand delimiter (compared to US style, comma and dot are role-reversed).

I'm not 46 km but 46000 km away from Duna and my Pe is 9250 km.

[Master Tao]

Good – you've got options and time.

One option that hadn't occurred to me earlier is to burn radial out to raise your Dunar PE even more. That would limit the effect of Duna's gravity on your solar PE, keeping it close to Kerbin's orbit. Then it's just a matter of waiting in orbit around the sun until Kerbin's close enough to your PE.

Edited July 11, 2014 by Master Tao
switch solar AP/PE

[LethalDose]

Perhaps it's noteworthy that a . in European notation is used as a thousand delimiter (compared to US style, comma and dot are role-reversed).

I'm not 46 km but 46000 km away from Duna and my Pe is 9250 km.

Oh. Thats way different then. I'd recommend that you burn radially to cheaply move move your Pe close enough to Duna to score "Space near" science (Probably around 50 km). This would also let you benefit from the Oberth effect and with some fiddling you may be able to efficiently get a return trajectory.

If it were me, I would try to find some combination of increasing my solar Ap and then burning at that Ap to get the Pe to Line up with Kerbin when Kerbin is at that location. Alternatively, you can just try to get a Pe around Kerbin's orbit, and eventually, you should get an encounter (it may take a long time though).

[cantab]

If you don't want to enter orbit, set Kerbin as the target then try playing with a manoeuvre node set fairly soon (an hour away, say) and see if you can get a Kerbin encounter or at least a fairly close approach lined up. Duna's not as massive as Kerbin or Eve but it's still capable of giving you a reasonable gravity assist.

[Mast]

With some fiddling entering Kerbin SOI isn't the problem anymore.

However, I still can't get myself close enough to Kerbin to make a reasonable Kerbin PE.

It feels like poking around, trying what works to make it more efficient. Is there a way to make the guessing more targeted instead of random trial and error?

So far I've managed to get a Kerbin PE of 55000 km for less than 200 dV. However, I can't close a gap that big with the remaining dV.

Is there a standard approach to solve these kind of problems? Circular orbits can be solved with a Hohmann transfer, is there something similar (but probably more complex) for non-circular transfers?

[Kryxal]

The earlier the better, and just play around with it ... do you have a screenshot in map view or something? Closing a 55000 km gap a few hours before you get there is different from a few days, after all, and how much delta-v do you have left?

Further note, if this is trying for the direct intercept into atmosphere from Duna, I wouldn't worry too much if you're passing above/below Kerbin, drop a note halfway there and you can probably fix it for a few m/s. Just quicksave before your burn in case things really mess up.

Edited July 12, 2014 by Kryxal

[cantab]

It feels like poking around, trying what works to make it more efficient. Is there a way to make the guessing more targeted instead of random trial and error?

So far I've managed to get a Kerbin PE of 55000 km for less than 200 dV. However, I can't close a gap that big with the remaining dV.

Don't worry. Make a correction later on, for example right after you enter Kerbin's SOI to deflect the periapsis down.

The further out you make a correction, the smaller it needs to be, but TOO small and you can't be precise enough. In that case you just wait until later.

[Kryxal]

55 thousand km isn't small on the scale of intercepts, though it's not exactly huge either. It's just hard to know what to advise with just a number to work with...

[LethalDose]

The earlier the better, and just play around with it ... do you have a screenshot in map view or something? Closing a 55000 km gap a few hours before you get there is different from a few days, after all, and how much delta-v do you have left?

Further note, if this is trying for the direct intercept into atmosphere from Duna, I wouldn't worry too much if you're passing above/below Kerbin, drop a note halfway there and you can probably fix it for a few m/s. Just quicksave before your burn in case things really mess up.

If this is your situation, you're probably 90% of the way to a successful mission. You should probably be able to adjust your Pe to below 25 km with only a few hundred dV if you burn just inside of Kerbin's SoI in the "radial in" and "retrograde" directions (see wiki).

Let Kerbin's atmosphere slow you down on a pass or two, and you'll be landing in no time.

[Mast]

After a lot of poking around, finally found a cheap burn for a Pe of under 3000km.

However, turns out the orbital speed is way too high...

Got it saved at pre-burn, so not all is lost yet.

Now I just need to find out how to get it at Kerbin in one piece.

Edited July 14, 2014 by Mast

[cantab]

Radial burn on the outskirts of Kerbin's SOI to lower periapsis and then aerocapture (or just land).

And you really are hooning it around with an entry speed like that.

[LethalDose]

well, based on the amount of resources on the vessel, I'm guessing it's pretty small, which is good because you can slow down quickly and you should have a lot of dV. As Cantab suggested, burn radially to put your Pe into the atmosphere (like, immeadiately), and then burn retro at Pe to get more out of the aerobrake. It may take a pass or two to kill all the orbital velocity, but /shrug you got time.

[Kryxal]

I beg to differ about WHERE you make that burn to catch the atmosphere ... you want to make a burn to put you fairly close (like within 50-100 km of your final trajectory) well out from Kerbin SoI, possibly even a quarter-orbit from your intersection point (that's where you can normal-burn to best effect if need be). You then do your fine-tuning once you cross the SoI, but you want to do MOST of the work where you can get a greater effect for less force.

Still, from the position at that last screenshot, I'm estimating a 150 m/s burn to aerobrake, and you can use http://alterbaron.github.io/ksp_aerocalc/ to get actual figures (and a target height if you don't want to go straight for a landing).

As for why you're carrying so much velocity, it has to do with the way you dropped your orbit inside of Eve's to get the intercept, your speed may not be hugely different but the vectors are more so...