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aceman67

Capturing to a celestial body with out abusing Orberth Effect

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I'm planning on doing a full Beyond Home playthrough and I've never really gone farther than Minmus in the base game, and a Reddit user challenged me to get to a deep system moon (around a gas giant no less) chemically and without abusing the Oberth effect. So basically what he's asking is to just brute force my way there, MOAR BOOSTERS basically. Am I right in my assumption?

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Yeah, "abusing the Oberth Effect" makes as much sense as a phrase as "abusing burning retrograde."

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Posted (edited)
4 hours ago, aceman67 said:

I'm planning on doing a full Beyond Home playthrough and I've never really gone farther than Minmus in the base game, and a Reddit user challenged me to get to a deep system moon (around a gas giant no less) chemically and without abusing the Oberth effect. So basically what he's asking is to just brute force my way there, MOAR BOOSTERS basically. Am I right in my assumption?

That all depends.  What constitutes abuse?

Otherwise, yes.  Oddly enough, you're probably in a position to be better at this sort of thing than a more experienced player.  The longer you play, the more comfortable you become with difficult gravity-assist transfers and razor-thin fuel/mass margins.  Early players who are first going to Duna tend to either under-fuel and never get there, or else they overload their vessels with so much fuel that they use less than half of it over the whole trip.  That is a waste of fuel, but it's also a successful mission--since your challenge essentially is to waste fuel, your lack of interplanetary experience puts you in a great place to complete it.

Edited by Zhetaan

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4 hours ago, aceman67 said:

without abusing the Oberth effect

I'm also curious what this would mean. "The Oberth effect" is just a name for an intrinsic property of orbital dynamics, that (prograde/retrograde) burns are generally more efficient when done near a lower periapsis. I can think of two times when it's relevant to such a mission, and one way to avoid Oberth for each of them:

  • Departing from Kerbin - circularize at a very high orbit before executing the departure burn
  • Arriving at the destination - don't fine tune your encounter to have a low periapsis, i.e. capture way the heck out in the outer part of the destination SOI

But neither really represents "abuse." It sounds like you're being asked to deliberately sabotage the mission by maneuvering inefficiently.

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Gravity assists I could see people (ab)using, but Oberth?  It just sort of happens that way, avoiding it is going out of your way to plan badly...

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I've actually been fretting about this in an unhealthy - albeit lighthearted - way.

I am imagining an entire contingent of players who consider doing things efficiently to be "cheating."

"Oh of course you were able to get that into orbit. You cheated by doing a gravity turn and having a nominal TWR."

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On 3/22/2019 at 8:21 AM, aceman67 said:

…and without abusing the Oberth effect…

I am also interested in what that means. Does he mean to say no periapsis kicking (that is, all escapes and captures are performed in a single maneuver instead of multiple burns over multiple orbits)? Otherwise, there is getting the job done, or failing the mission.

You said this came from Reddit, so surely there is a post you can link us to?

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On 3/22/2019 at 9:21 AM, aceman67 said:

So basically what he's asking is to just brute force my way there, MOAR BOOSTERS basically. Am I right in my assumption?

I've thought some more about this, and the Oberth Effect is exactly that; it's an effect that naturally arises from the use of propulsive thrust.  Saying not to abuse the Oberth Effect with a rocket is akin to saying not to abuse lift with a spaceplane.  I could do that--namely, I could take off all of the wings and fly strictly prograde to orbit from a near-vertical launch (to eliminate body lift)--but then it's arguably no longer a spaceplane.

That could make an interesting paper:  Has anyone tried to examine the rocket as a degenerate-case or zero-lift spaceplane?  But I digress.

To make this more understandable, there is a way to completely avoid any use at all of the Oberth Effect--look at it and then decide whether it is what the challenge requires.  Namely, to completely eliminate the Oberth Effect from a transfer manoeuvre, you need to begin at the lowest possible periapsis (presumably, 70 km) and then increase the apoapsis by a metre.  Then circularise.  Raise the apoapsis by another metre and circularise.  Treat the manoeuvre as a series of individual transfers to higher orbits and be certain to circularise all intermediate orbits.  Continue doing this until you reach the limit of Kerbin's sphere of influence and finally escape to solar orbit.  Then begin the same process again, only now with the sun as the primary.  Continue changing your orbit until you reach the deep-system gas giant's orbit.  Once you enter its sphere of influence, begin lowering your orbit a metre at a time until you reach the moon.  On the way out, there are no retrograde burns, and on the way in, there are no prograde burns--all engine power goes directly to adjusting the orbit in the desired fashion, so technically, there is no wasted fuel.

The Oberth Effect is not a special offer from physics to buy one orbit and get another at half price.  The Oberth Effect is an observation that changes to a rocket's kinetic energy are partly dependent on the amount of kinetic energy it already has. Since you cannot actually go anywhere without the use of kinetic energy, there is no way to avoid it--even my wildly impractical exercise above is only an approximation that links kinetic energy changes to minuscule potential energy changes.

Thus, I can no longer say with surety that you are right in your assumption--all I can say for certain is that your challenge is plagued by an ambiguous understanding of what the Oberth Effect is.

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Posted (edited)
On 3/25/2019 at 7:18 PM, Zhetaan said:

I've thought some more about this, and the Oberth Effect is exactly that; it's an effect that naturally arises from the use of propulsive thrust

It's really cool how you can also achieve the same thing in a car driving towards an uphill :wink:


* Note: People in said car did not understand why I got so excited at that same hilltop every time :blush:

Edited by Blaarkies

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Regarding Oberth effect in automobiles, I know it wasn't meant seriously,
but it made me think about the differences between automobiles and rockets.

Rockets are limited by the fact they have their own exhaust to push against. Best efficiency means making the most of the exhaust, which means getting the highest exhaust velocity possible from the combustion.  Then thrust = (fuel-mass per second × exhaust velocity).  To boost your orbit to the Moon, you need a given amount of energy = thrust × distance.  Firing the engine when moving quickly gets the needed thrust×distance in less time, so less fuel.

Automobiles take full advantage of having the entire Earth as reaction mass.  The engine has some maximum torque, but with a gearbox you can convert that to whatever propulsive force you need to accelerate.  If we need more thrust to get up a hill, we downshift.  Without the constraint on thrust per fuel-burn-rate, automobiles reach efficiencies limited by the energy in the fuel -- power per fuel-burn-rate.  To get the energy to climb a hill, we need to run the engine for a certain time at its speed for best power per fuel-burn; we can do that in 5th gear when approaching the hill, or in 3rd gear while climbing it.

So when driving through hills, the Oberth effect would let me avoid downshifting, by accelerating sufficiently before each up-hill section, but that doesn't save me any fuel.

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