The Oberth effect is simulated in KSP!

[Jouzu]

I have been thinking about this for a while, and this video by Scott Manley highligts the fact:

In short: The Oberth effect lets you trade potential energy (high altitude) for kinetic energy (higher speed). The result is that if you burn deep down in a gravity well you get more speed out of the manuver than if you burned at high altitude (again: watch Scott's video for a practical explanation).

Source: http://en.wikipedia.org/wiki/Oberth_effect

[Kosmo-not]

The Oberth effect just tells us that we add work energy to our orbital energy at a faster rate when traveling faster (work=force*distance). It's really nothing special. It is a basic principle of orbital mechanics.

[Iskierka]

Not even orbital mechanics, it's basic physics. If it didn't work, something would be majorly wrong.

[tsakali]

k not sure if this is the same thing, but do i get any benefits in adjusting orbit if I burn farther or closer to the muns gravity when all other things are equal? I'm kind of convinced that you don't...you shouldn't.

[Bluekommeh]

So pretty much the physics behind the slingshot maneuver in particular, am I right?

[Arkasai]

k not sure if this is the same thing, but do i get any benefits in adjusting orbit if I burn farther or closer to the muns gravity when all other things are equal? I'm kind of convinced that you don't...you shouldn't.

Apoapsis is your natural slowest point so it's logical to make course corrections from there as it uses the least fuel.

[nhnifong]

k not sure if this is the same thing, but do i get any benefits in adjusting orbit if I burn farther or closer to the muns gravity when all other things are equal? I'm kind of convinced that you don't...you shouldn't.

Only the body you orbit influences you. KSP does not simulate multi-body gravitation.

[Frederf]

No, slingshot is a conservation of momentum thing with three bodies that doesn't involve engine thrust at all. It's a lot like pushing off your friend in the pool. Your speed is at the expense of his.

Oberth effect is simply that rockets are more efficiently increasing craft energy at high speed. A given unit of fuel will increase the craft's speed by some amount. The amount of kinetic energy added to the craft by each unit speed is proportional to the speed. It's the "V-squared" term in the equation KE = 1/2 m v^2. The orbital technique is to apply thrust at high speed (deeper in the gravity well) to gain the most energy per fuel spent.

Notice that this logic holds true even for a constant mass rocket. There's a secondary benefit to burning deeper in the gravity well and that's not losing energy lifting fuel up high in the first place.

It's as surprising that the Oberth effect is noticeable in KSP as orbits are possible. Despite its name, it's just a noticeable feature of basic mechanics and not wizardry.

[Binky]

This is a slingshot of which I am inordinately proud

Using the Mun, I dropped my Pe by 5500km and Ap by 31000km using zero fuel.

Of course, I really lucked into it but I'm pretending it was all planned

[Ekku Zakku]

I can attest to this; with my old Munar lander, I had some SRBs attached to the service module as a launch escape system, but I also used them for a Munar injection burn (at the expense of having quite a bit of trash in Kerbin orbit). I did a test with them, and it was performing the Munar injection burns at different altitudes around Kerbin. I was wondering if the higher speed of a lower orbit would work better, or if a higher altitude that I already expended more energy to reach would be better. Turns out, the SRBs (which burn the same amount of time every time, thus making for a perfect test) brought my Ap higher when I was in a lower orbit. Thus, I now burn for the Mun and Minmus when I'm at a lower orbit around Kerbin.

A question though; using this effect, would it actually be better to get into a higher orbit around the Mun/Minmus, and then de-orbit for a landing? And then burn back towards Kerbin at a lower Mun orbit? I would think that it means that it's more efficient to de-orbit from a higher altitude, since there's less Delta V to get to 0 m/s, but then wouldn't you also use more Delta V to counteract the accelleration of gravity? So confusing x_X

Edit: Great slingshot there Binky! Would be nice to have NASA back us up, so we'd be able to time that kind of thing perfectly every time XD

Edited September 7, 2012 by Ekku Zakku

[bsalis]

Hey Binky.

You should set your CONIC_PATCH_LIMIT in the .cfg file to be 3 so you can see your next path.

Also, if you did a burn at Mun Pe you could have set your Kerbin Pe to 35km. Now THAT would be an efficient return!

edit: Oh yeah, on-topic...

It's really nothing special.

Yes, but it is certainly not intuitive. Same for slingshots.

[Frederf]

A question though; using this effect, would it actually be better to get into a higher orbit around the Mun/Minmus, and then de-orbit for a landing? And then burn back towards Kerbin at a lower Mun orbit? I would think that it means that it's more efficient to de-orbit from a higher altitude, since there's less Delta V to get to 0 m/s, but then wouldn't you also use more Delta V to counteract the accelleration of gravity? So confusing x_X

Assuming you start on the edge of the Mun's SOI with no speed you have escape energy. You want zero energy at the surface. To get rid of this energy you have to retro burn. You get rid of more energy per unit fuel at a high speed. Thus you want to do "just in time" landing burns (highest speed). Adjust technique for fault tolerance and safety.

[tsakali]

Hmm, isn't that going to run into a problem with the new planets and moons in 0.17?

and by 'run into problem', I mean feel unrealistic , therefore a lesser experience?

[Ekku Zakku]

Assuming you start on the edge of the Mun's SOI with no speed you have escape energy. You want zero energy at the surface. To get rid of this energy you have to retro burn. You get rid of more energy per unit fuel at a high speed. Thus you want to do "just in time" landing burns (highest speed). Adjust technique for fault tolerance and safety.

Yeah, I can understand that, since I have been able to land with much more fuel in my descent engine if I waited until almost the last second to kill off my descent speed. So I'm guessing this also then mean that a lower orbit would be more efficient to de-orbit from? (to answer my original question lol)

[tsakali]

I'm guessing having a lower orbit, might help saving fuel on the vertical, but a tighter orbit requires faster orbital speed yes? which you will need to bring to a complete standstill, so it's not that straight forward if lower orbits are more advantageous on fuel consumption .

[Thaniel]

I'm guessing having a lower orbit, might help saving fuel on the vertical, but a tighter orbit requires faster orbital speed yes? which you will need to bring to a complete standstill, so it's not that straight forward if lower orbits are more advantageous on fuel consumption .

Wich was the question to begin with

The difference here is that you are retroburning from either a high speed to a low speed, or from a lower speed to the same low speed needed for landing. Let´s try and modularize it a bit to make it clearer.

Prefered speed for preparing to land, let us say 50 m/s.

And origin orbits chould be: (Perimee values here, fantasy quality to point out the setting)

low, fast, meaning 10 km altitude, speed 600m/s

And the high slow orbit, 60 km altitude, 300 m/s.

So, the question here is, from wich orbit would you spend the least amount of fuel to deorbit from?

(PS, I just pulled the numbers out of my head, wich makes them wrong, but the question remains the same)

I have tried both aporaches, and so far, I have a feeling I use less fuel when starting from a high altitude orbit, and at apomee do a burn that brings perimee to or just above the surface, then I do a horizontal burn to bleed of the orbital speed. That usualy leaves me with only the local vertical speed to manage.

Edited September 7, 2012 by Thaniel

[PakledHostage]

This past spring, two challenges on this forum explored optimal descents to the Munar surface and optimal Munar orbit insertion from a given transfer orbit. And while these challenge threads pre-date v0.16, the results are still valid. Some of the experimental results were as expected but some were counterintuitive and go against the conventional wisdom.

Have a look at the threads here:

Optimal Descent to the Mun challenge

Optimal Munar Orbit Insertion challenge

[Kosmo-not]

I'm slowly making a video comparing a straight down vertical descent to a shallow (mostly horizontal) descent. The second one always wins.

[Ekku Zakku]

@PakledHostage Yeah I just came across those threads earlier today, and it turns out that it is true that killing off that horizontal speed quickly at a low altitude is more efficient (yet more dangerous) than killing it off at a higher altitude and coasting down. Well, the latter is actually MUCH more efficient if you abuse the fuel bug! XD I think that skewed the way I first interpreted Mun landings. Well, it's certainly going to make easy precision landings harder to pull off if efficiency is the goal =P

[tsakali]

In light of this, I would guess that the lower orbit descent is probably more efficient because of the human inefficiencies of having to gauge just how fast and at which point you should throttle a descent from a higher orbit. Seems that a lot of us aren't terribly efficient with our descent rates.

In any case, I bet it plays a role.

Edited September 7, 2012 by tsakali

[PakledHostage]

In light of this, I would guess that the lower orbit descent is probably more efficient because of the human inefficiencies of having to gauge just how fast and at which point you should throttle a descent from a higher orbit. Seems that a lot of us aren't terribly efficient with our descent rates.

In any case, I bet it plays a role.

I am certain that you are correct that human factors play a role in achieving the optimal descent, but they aren't the only reason that a low approach is more efficient. A lot of analysis was done by contributors to the Optimal Descent to the Mun challenge to verify the experimental results. Much of that analysis was idealised and didn't take human factors into account. There were also a couple of articles cited that support the low altitude transition from orbit to landing as the most efficient solution.

Check out:

The Apollo Lunar Descent and Ascent Trajectories article that Tarmenius posted

The Orbital Transfers textbook chapter that Closette posted

[tsakali]

interesting reads, thx