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Cheapest orbital capture.


Curveball Anders
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I've got a ship on it's way to Moho, but I've messed up my fuel management.

As in I have very little dV left to secure an orbit.

So my question is:
What the cheapest (dV) way to obtain an Ap within Moho's SOI?

Tweaking the entry is cheap (I'm still 30+ days out) but should I aim for a high or low Pe?
Prograde or retrograde?

Or should I just grab as much science I can with the flyby and let the probe return to solar orbit....

 

 

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You want the lowest possible Pe to maximize Oberth, but you'll probably need a bazillion m/s of dV to lower your Ap (unless you were really clever with your launch window), so you probably aren't going to get anything more than a flyby no matter how you do it. Check it with a maneuver node at your Pe to see how bad it will be.

Edited by bewing
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To answer the original question, it's actually cheaper to get a capture if you do so as far away from the planet as is reasonable within the UI.

Your orbit will be total garbage (as in, it will be circular way out at the edge of the SOI) but it will be stable and inside that SOI. So if that's the goal then that's how you'd do it.

Granted at Moho it's the difference between ridiculously fast and REALLY ridiculously fast, so if you're in that much trouble I doubt it'll help you. But I've done it at both Tylo and Mun.

Caveat: I'm not 100% sure my experience with it is good enough to be able to blanket statement say "it always works this way" but I'm confident enough that I'll believe it until either I have a chance to try it, or someone else does so)

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I remember my "normal" Moho missions, from entering the SOI to a nice, low Moho circular equatorial orbit. The initial breaking took something between 2500 and 3000 m/s (don't ask me for the exact numbers, it's been a while, my old 1.7.3 career) to get into an Orbit (something between a low Moho and an edge-of-SOI orbit, let's call it medium orbit), I made two burns due to the long burn time, first burn some 2000 m/s just after entering SOI, second burn as usual at Pe. Followed by a burn to get into equatorial orbit, final burn to circularize (with the idea to have a good orbit for a later landing). By no means my trajectory was optimized for low dv, I was already happy to have a proper encounter I could work further from.

Edited by VoidSquid
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Much more important than a high or low PE is the angle of your flight path in relation to Moho. If your encounter is not optimal, you will pay a much higher dV price. As the mass of Moho itself is rather small, savings due to oberth effect are not as plenty as with other planetary bodies or Tylo.

You can test the differences with a transfer window planner for yourself. Although a capture is cheaper than a circular orbit.
https://alexmoon.github.io/ksp/

 

Good luck!

 

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Tangential to Moho's orbit and prograde of course, Pe as low as you dare to maximise Oberth. It's expensive though in the best of cases so if you can't get one go with a flyby.

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22 minutes ago, Brikoleur said:

Tangential to Moho's orbit and prograde of course, Pe as low as you dare to maximise Oberth. It's expensive though in the best of cases so if you can't get one go with a flyby.

Absolutely: yes! My little expeditions though reached Moho at a much steeper angle, hence that big amount of dv I needed.

And: why not to go for a flyby? And send another rocket for orbiting and landing? Moho encounters are pretty frequently, compared to say Jool.

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3 hours ago, VoidSquid said:

Absolutely: yes! My little expeditions though reached Moho at a much steeper angle, hence that big amount of dv I needed.

And: why not to go for a flyby? And send another rocket for orbiting and landing? Moho encounters are pretty frequently, compared to say Jool.

I've concluded that it will be a flyby.

I'm currently aiming to grab as much science as possible before craft ends up in some kerbol orbit with next useful intercept in 65 years or so

 

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17 hours ago, VoidSquid said:

True, @jost. Old me though, I am quite too lazy for that way of optimizing. 

Just put a ridiculous amount of dv into my rocket (who cares for costs anyway), and: launch :D 

Me too :) But for Moho it's easier to pack a lot of delta-v AND do a bieleptocal transfer as explained in zhe secobd thread

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22 hours ago, Curveball Anders said:

So my question is:
What the cheapest (dV) way to obtain an Ap within Moho's SOI?

I see that you decided on a flyby, but I'll answer your question anyway.

The cheapest way (technically the only way, if you want to be pedantic about it) to reduce the delta-V you need to capture around Moho is to reduce the difference between your velocity and Moho's velocity.  Since you can't do it with fuel, your only option is gravity assists.  You'll be coming in faster than Moho, assuming that you're in a Kerbin-Moho transfer orbit, so that means braking assists.  Moho's small size and close proximity to the sun mean a lot of tiny assists, because Moho can't do much for you.  However, not much is better than nothing, so it can be done--it's a matter of patience at that point.

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Going out on a limb here, based on "next useful encounter in 65 years" -- rather than accept that or just do an impact, I'd be strongly tempted to search for an available gravity assist from Moho to get to an encounter with another body that can provide another gravity assist.  Eve is the obvious candidate here, but your odds of getting a return to Kerbin are actually somewhat better, and Duna or Dres might turn out to be better options (i.e. available for return assists with lower dV expenditure).  It's likely to still take years (but probably not 65) to get your craft back to Moho, and would likely be faster just to launch another mission.

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On 2/21/2020 at 2:03 PM, Superfluous J said:

To answer the original question, it's actually cheaper to get a capture if you do so as far away from the planet as is reasonable within the UI.

Your orbit will be total garbage (as in, it will be circular way out at the edge of the SOI) but it will be stable and inside that SOI. So if that's the goal then that's how you'd do it.

Granted at Moho it's the difference between ridiculously fast and REALLY ridiculously fast, so if you're in that much trouble I doubt it'll help you. But I've done it at both Tylo and Mun.

Caveat: I'm not 100% sure my experience with it is good enough to be able to blanket statement say "it always works this way" but I'm confident enough that I'll believe it until either I have a chance to try it, or someone else does so)

It isn't any cheaper to get high PE orbit capture than Low, if you make maneuver node to lets say mun periapsis when capturin, make it with both approaches, you will see that with same amount of dV you get circular orbit around mun, other is just higher than another. So to get cheapest you must aim low because you can then do high eccentricity orbit which costs less dV, and it is not possible to do when high orbit, with same amount of dV used.

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