Is it more efficient to be captured at say, 50 km than at 1,000 km? (no atmosphere)

[Gus]

When landing on a planet/moon, is it more efficient (energy-wise) to tweak insertions to get closer to the planet/moon for your insertion burn?

[ngianoplus]

Changing your orbit way before you are captured or reach you periapsis (once you are captured) is extremely efficient. So go ahead and get as close as you can before you get captured, because it only takes a fraction of the fuel. I hope that answers your question

[stupid_chris]

Oberth effect, indeed. The faster a rocket is moving, the most efficient it is. So basically, the close you are, the lower your capture burn.

[Mr Shifty]

Oberth effect, indeed. The faster a rocket is moving, the most efficient it is. So basically, the close you are, the lower your capture burn.

Not always true. Lower orbits have higher energy, thus are harder to get into. The most efficient orbit depends on SOI entry angle and speed.

[TheDarkStar]

Not always true. Lower orbits have higher energy, thus are harder to get into. The most efficient orbit depends on SOI entry angle and speed.

If you make correction burns a while before the intercept, you lower the fuel cost quite a bit.

[DMagic]

It's harder to get into a circular low orbit. But if all you care about is actually getting into a stable orbit then it's easiest to do the injection burn very close to the planet/moon and stop once you get into a stable, highly eccentric orbit.

[Horn Brain]

Not always true. Lower orbits have higher energy, thus are harder to get into. The most efficient orbit depends on SOI entry angle and speed.

To get captured doesn't mean to end up in a circular orbit. To get captured is simply to bring the apoapsis of your orbit within the SoI. If you're just trying to get captured, lower is always better. You are of course right if you add the constraint that you want to get into a roughly circular orbit. However, if you're coming in very fast, you're probably best off coming in low, slowing down to get captured at periapsis, and then adjusting your inclination out at apoapsis before settling into your final orbit anyway. Unfortunately the forum monster ate a post I did that would have been helpful in showing all this, but unless you're coming in very slow, the savings from insertion directly into a higher orbit are usually trivial when they exist at all.

If your goal is to land, you should always go as low as possible when getting captured.

Also, lower orbits have lower energy, it's the difference in energy between that and your flyby trajectory that is large.

[Eric S]

Not always true. Lower orbits have higher energy, thus are harder to get into. The most efficient orbit depends on SOI entry angle and speed.

Lower orbits have higher velocity, but lower energy since they're farther into the gravity well. When you brake into an orbit, you're shedding velocity/energy. However, even at that, that's not what we're talking about here, since we're not shedding energy to have a low periapsis.

The way I have generally found that is the lowest delta-v method to get into any particular orbit:

1) Aim for a very low periapsis while still far away from the planet. I can often manage a sub-surface periapsis with less than 50 m/s delta-v spent fine tuning my transfer, and can always manage a sub-100km periapsis with that budget, once I have an actual intercept. Using conic mode 0 helps with the accuracy during this step, as you can focus on the target planet and see where your periapsis is in regards to the target planet.

2) When you cross over into the SoI of the target planet, use normal/antinormal/radial/antiradial thrust to put your periapsis right where you want it. Do not try to raise or lower your periapsis by burning prograde/retrograde. Normal/antinormal will affect your inclination, radial/antiradial will affect your periapsis. The more accurately you have placed your periapsis in step 1, the less effort it will take in this step.

3) Burn at periapsis to put your apoapsis where you want it. Note that if you have a really low TWR, this could be less than easy.

4) At your apoapsis, raise your periapsis to where you want it.

The only time I've found burning directly for your orbit to be more efficient than this is when I seriously messed up step #1 so that my initial periapsis was far away from the planet.

[Mr Shifty]

It's harder to get into a circular low orbit. But if all you care about is actually getting into a stable orbit then it's easiest to do the injection burn very close to the planet/moon and stop once you get into a stable, highly eccentric orbit.

Ah yes. A necessary qualification; I was only thinking of circular orbits.

Lower orbits have higher velocity, but lower energy since they're farther into the gravity well. When you brake into an orbit, you're shedding velocity/energy.

You're right, my nomenclature was incorrect. I should have said it takes more change in energy to get from a hyperbolic orbit to a lower eccentric orbit than than to a higher one.

[stupid_chris]

Not always true. Lower orbits have higher energy, thus are harder to get into. The most efficient orbit depends on SOI entry angle and speed.

Bah I was meaning with the general idea of getting in a low orbit or landing. The lower you set your periapsis originally (while staying reasonable now), the more fuel less fuel costly will be your approach to the planet. Of course if your closest approach to the planet while coming in is 1 000 000 000km, you can think over your fuel efficiency. I was speaking mostly for the best case scenarios, but there will always be specifics to ruin the idea.

[Gus]

I think that conservation of energy requires that regardless of how many higher orbits you enter as you cascade down to a specific base orbit, all paths are equivalent energy-wise, provided that all thrust is retrograde.

[stupid_chris]

I think that conservation of energy requires that regardless of how many higher orbits you enter as you cascade down to a specific base orbit, all paths are equivalent energy-wise, provided that all thrust is retrograde.

Not necessarily. If your goal is to land on the said planet, managing to have a peripapsis of 50km straight away when you enter the SOI will prove to be much more efficient fuel wise than if you're coming in to a periapsis of 1000km.

[KerbMav]

What about using a gravity assist to slow down the vessel - what was it ... flying past infront of a body to slow down, behind it to accelerate? Could this be factored into this?

[Mr Shifty]

I think that conservation of energy requires that regardless of how many higher orbits you enter as you cascade down to a specific base orbit, all paths are equivalent energy-wise, provided that all thrust is retrograde.

Most of the rocket's energy is stored in the form of chemical propellant, which is expelled into space. Oberth effect, for instance, seems to violate conservation of energy until you realize that it works because it represents maximum transfer of energy from the propellant to the rocket.

[Eric S]

I think that conservation of energy requires that regardless of how many higher orbits you enter as you cascade down to a specific base orbit, all paths are equivalent energy-wise, provided that all thrust is retrograde.

As stupid_chris pointed out, that isn't quite correct.

As to why, the oberth effect is not to be underestimated. Rather than get into the math of it, think of it this way. As the craft dives into the gravity well, it will pick up energy from gravity, based on how far into the well it is and how much time it spends there. Then the craft climbs out of the gravity well, and it loses as much energy as it gained. Simple enough, and just what you'd expect if you're thinking in terms of the conservation of energy.

Now, instead of passively going in and out, let's burn some delta-v at the periapsis. Now, instead of losing as much energy as it leaves the well as it gained while dropping into the well, it spends more time leaving the well, which means it spends more time affected by that gravity, which means it loses more energy than it gained. EDIT: since we mass less because we burned fuel, replace energy with velocity.

We haven't actually violated the conservation of energy, though it's not easy to see where the extra energy went. It went into the rocket's exhaust basically, since it accelerated rather than decelerated at periapsis. Which means that it left the gravity well faster, so it slowed down less, so it will leave the SoI with more energy than it entered it with.

Does that make sense? I'm not sure I've made this clear enough.

EDIT: Ninja'ed by Mr Shifty, how appropriate *snicker*

Edited July 16, 2013 by Eric S

[Gus]

Thanks guys on the Oberth effect. I'm glad others found this difficult as well.

My first objection to it was, "So who's to say how fast this rocket is going?", but the velocity has to be it's orbital velocity around the body at the center of the potential energy well to be consistent.

Edited July 16, 2013 by Gus

[Tarmenius]

I hosted this challenge a year ago in an attempt to answer just this question. Even though much about the game has changed in the year since this topic was created, the methods contained within are still very much applicable. I hope this helps, and remember not to necro such an old topic

[Gus]

Exploiting The Oberth Effect (again) in Kerbal Space Program: A video by Scott Manley

[Lexif]

As to why, the oberth effect is not to be underestimated. Rather than get into the math of it, think of it this way. As the craft dives into the gravity well, it will pick up energy from gravity, based on how far into the well it is and how much time it spends there. Then the craft climbs out of the gravity well, and it loses as much energy as it gained. Simple enough, and just what you'd expect if you're thinking in terms of the conservation of energy.

I'm feeling like nitpicking: In the above paragraph, you are mixing "energy" (which is conserved), "potential energy" and "kinetic energy" and just use the term "energy" for all of them. When you just pass through the SOI of a body, your total energy is conserved, but potential energy is converted into kinetic energy as you descend into the gravity well of a planet, and back as you ascend out of it.

[Frederf]

A: Yes.

Why: When encountering a massive body from beyond the SOI you have net positive orbital energy. Orbital energy is a combination of gravitational potential energy (which is conventionally defined as zero far away and some negative number on the surface) and kinetic energy due to motion (always positive, proportional to speed squared). Transgressing the SOI of a body without doing anything results in leaving SOI at the same speed as you entered. This ignores any slingshot effects and assumes you don't actually hit the body at some point.

Anyway, to achieve capture you must have net negative orbital energy. At any given position in the gravity well your speed must be less than "escape speed" or rather you cannot have escape kinetic energy. The way to reduce your orbital energy is to slow down. To that end the most efficient kinetic energy lost per unit fuel happens at the highest speed. The highest speeds happen deepest in the gravity well. And so it's seen that a retrograde burn at the lowest possible periapsis sheds the most kinetic energy per unit fuel.

[Eric S]

I'm feeling like nitpicking: In the above paragraph, you are mixing "energy" (which is conserved), "potential energy" and "kinetic energy" and just use the term "energy" for all of them.

I don't consider it nitpicking. Really, I should have used velocity rather than energy, or talked about the conversion between potential and kinetic forms of energy. Not sure why I didn't.