Oberth effect Vs Gravity assists

[FREEFALL1984]

I've been perusing the forum and other sites trying to find a simple way to decide what would be the most efficient method of interplanetary travel. Many people, including Scott Manley, state that the escape burn should be performed in a low Kerbin orbit. Logic on the other hand states that it would be far more efficient to burn towards a normal or antinormally positioned moon (normal/antinormal relative to kerbin trajectory)

If you hit the munar periapsis at the same time as you hit your interplanetary transfer window, you should be able to gain some of the relative speed of the mun. obviously you would need to be quite precise in your manuver nodes but the benefits would be huge. On one occasion I set up manuver nodes to enter mun SOI and had MJ execute them while I was AKF, I was AFK longer than I thought and when I returned I was on an escape trajectory a long way past the mun and obviously I spent no more fuel than I would normally of spent cleanly getting into a munar SOI

So can someone explain why people think the oberth effect is more efficient than a munar gravity assist, in laymans terms

[Padishar]

I don't think they do as such. Setting up a slingshot with the Mun and ending up on an accurate course to your intended destination is difficult and you will almost certainly need to make a correction burn. Adding this correction burn to the burn that took you to the Mun will probably end up bigger than doing a direct shot from LKO...

[wasmic]

The most efficient way to do it would be to get a gravity assist from Mun, with a burn at Mun periapsis if needed. Very hard to pull off, though, so most people burn from LKO.

Funny thing is, if you use Extraplanetary Launchpads and launch from Mun, it's actually more efficient (except for Duna and Eve) to escape Mun with your Kerbin periapsis lowered to LKO altitudes, and then do an Oberth boost there.

[SSSPutnik]

You aren't really getting an Oberth boost lifting straight from LKO. One of most efficient is to raise apoapsis, then boost at next periapsis. That uses Oberth as your speed is greater at periapsis which is where Oberth kicks in.

There is a huge thread on it here:

http://forum.kerbalspaceprogram.com/threads/70685-oberth-effect

[samiamthelaw]

Oberth effect and gravity assist aren't really comparable in terms of "efficiency." Oberth tells you when you should burn to maximize the effect of your craft's delta-v; gravity assists give you free delta-v. There's no reason you can't use both.

A gravity assist from the Mun will give you a few hundred m/s delta-v for free, but that's only helpful if it has you pointing in the right direction after the maneuver. Making that happen requires more serious planning (i.e. launch timing) than KSP players tend to get up to.

[JumpsterG]

I think why you see gravity assists happen less is that you need the assisting body to be in the right place at the right time, which requires some time warping, luck, and maneuver node tweaking. Kicking straight out of low orbit can be done pretty much anytime and fairly easily. Gravity assists, when aligned well, are always going to give you more bang for your buck, since you're getting added velocity essentially for free.

I don't think anyone says that utilizing the oberth effect is better than using a gravity assist. It just means it's more efficient to increase your orbit when you're velocity is greatest (when close to an orbited body). In fact, you can utilize both at the same time, burning for a close moon encounter, then burning at the moon periapse to help kick yourself out to solar orbit (I have not run any numbers, maybe this could be more efficient than burning entirely at Kerbin and having the moon arc you out?). Again, this takes some timing and maneuver node tweaking, but there's no reason you couldn't set up something like this with a little patience.

[Edit: Ninja'd]

[Claw]

Oh boy, that's a big question that will probably get a lot of response.

First off, if MJ was executing nodes that you set up, it may have warped past the SOI transition at a high rate, which sometimes leads to wacky game errors. Or if you set up more than one node, then it probably didn't execute correctly.

Oberth and gravity sling shots are different but overlapping concepts. Without getting bogged down in math and details...

Gravity slingshots are based on your approacing and departing trajectories from a body. The craft basically steals some of the planet's (or moon's) momentum to gain speed (or leaves momentum to lose speed). As such, slingshots only require dV to set up, but need none to execute.

Oberth is a byproduct of expending fuel. A craft always steals kinetic energy from engine exhaust. However, because of how the physics works, the higher the craft speed, the more kinetic energy the craft can steal. Your orbital speed is faster when you are closer to the planet/moon than when further away.

So efficiency here isn't necessarily directly comparable because they are using different effects. However, there is generally only so much dV you can steal from slingshotting. So if you need more than that, you have to burn an engine. If you're going to burn, taking advantage of Oberth gets the most bang for the buck. Ideally you would combine those gravity assist and Oberth when possible.

Edit: Lots of ninjas here. Probably because I'm such a slow typer, ha.

[Geschosskopf]

So can someone explain why people think the oberth effect is more efficient than a munar gravity assist, in laymans terms

It costs about 850-900m/s to get from LKO to Mun. It costs about 1000-1100m/s to get from LKO to Duna. IOW, going to Mun costs you about 80-90% of what it costs to go to Duna without touching Mun's SOI. Therefore, if you can derive any benefit at all from passing by Mun on the way to Duna, it be no more than 10-20% of what you'd spend otherwise.

There's also the issue of having to wait for the proper day when Mun is in just the right position to sling you straight into Duna. I have no idea when this day would happen, and there's nothing says it has to be on the same day that the Duna transfer window opens. The transfer window is when it costs the least to get to Duna, so if Mun isn't in the right place that day, waiting on Mun would mean the base trip to Duna would be more expensive than it needs to be, so any savings from Mun would be even less than on an optimal day.

There are folks who do things the other way around, though. Use Kerbin instead of Mun for the slingshot by starting at Minmus. They say the dive back down from Minmus to Kerbin gives them enough extra starting speed at LKO altitude that they save about 500-600m/s on the transfer burn. HOWEVER, first you have to get to Minmus, which is ~900-1000m/s from Kerbin, so you actually end up spending about 1500m/s in total to get to Duna this way, which is more than if you just left from LKO. The reason for doing this, however, is that it allows for smaller interplanetary ships and/or faster interplanetary burns. Instead of having a big, slow-burning transfer stage, you have big boosters that get you to Minmus. There, you refuel if necessary from a kethane establishment, and your ship effectively has already done 500m/'s of the transfer burn.

[Claw]

The most efficient way to do it would be to get a gravity assist from Mun, with a burn at Mun periapsis if needed. Very hard to pull off, though, so most people burn from LKO.

Well, that's not entirely true if you're referencing Oberth. When you get to the Mun, your speed is a lot slower than in Kerbin LKO. The large difference in velocity greatly reduces the Oberth effect.

That's why people reduce altitude back to LKO before interplanetery burns.

[Claw]

You aren't really getting an Oberth boost lifting straight from LKO. One of most efficient is to raise apoapsis, then boost at next periapsis. That uses Oberth as your speed is greater at periapsis which is where Oberth kicks in.

There is a huge thread on it here:

http://forum.kerbalspaceprogram.com/threads/70685-oberth-effect

Oberth effect is always present, whether in circular LKO or not. Because engines cannot instantaneously apply dV, you can improve the ability to harness Oberth by doing two burns. One to establishing the ellipse, and a second to do the rest of the burn.

[cantab]

Scott Manley mentioned it in his video on gravity assists. There's just not enough benefit from a Munar slingshot. I believe the Mun just isn't moving fast enough and isn't massive enough. You can use it to escape Kerbin, but not really to get anywhere else. Though you can set up to get back into Kerbin's SOI for a gravity assist off it, but even that's of questionable benefit for the player time setting things up and game time in transit.

[maccollo]

The most efficient way to do it would be to get a gravity assist from Mun, with a burn at Mun periapsis if needed. Very hard to pull off, though, so most people burn from LKO.

If you just want to escape Kerbin, sure, but if you actually want to go anywhere you wont gain anything from a Mun gravity assist. It's gravity just isn't strong enough.

At best you might be able to save a couple of meters per second going to Duna. Anything beyond that you're going to get massive losses.

[MarvinKitFox]

So can someone explain why people think the oberth effect is more efficient than a munar gravity assist, in laymans terms

In SIMPLEST terms:

A gravity assist *adds* to your delta-v: V = v + sling With sling being at most 2 * orbital velocity of your slingshot partner, upper bound to escape velocity of it.. For mun, that is about.... 800m/s expect more like 250 to be achievable.

Oberth effect *multiplies* your delta-v. V = v * x , with x depending on your initial velocity. (usually achieved by orbiting close to something heavy, but this is not a prerequisite.) For Kerbol 100km parabolic flyby, relative to being in low kerbin orbit (as in the example below), Oberth effect x is about 3.2 That is a 3.2 *multiplier* to your effective delta-v.

Taken to the extreme, If I boost from Low Kerbin orbit with 25000 delta-v, and head directly out, I will cross Jool orbit on Kerbol escape at about 8000m/s

If I instead "waste" 10000 delta-v by first going out, then *way* in , and then burn the remaining 15000 delta-v while skimming Kerbol's surface, Oberth allows me to have an exit velocity at Jool orbit of 52500m/s.

NO WAYS will any combo of slingshots give you that level of boost

When planning to do something with a hefty delta-v requirement, (such as interplanetary boost), a multiplier is very likely to outscore an add.

[Vanamonde]

Isn't it also the case that Kerbin's moons just aren't going that fast? Mun orbits at, what, 542.5m/s? And you won't get a geometry that would apply 100% of that to your ship's velocity. So if interplanetary transits require outgoing speeds on the order of several thousand m/s, it's just not going to end up being much of a boost. Or so it seems to me.

[Claw]

Isn't it also the case that Kerbin's moons just aren't going that fast? Mun orbits at, what, 542.5m/s? And you won't get a geometry that would apply 100% of that to your ship's velocity. So if interplanetary transits require outgoing speeds on the order of several thousand m/s, it's just not going to end up being much of a boost. Or so it seems to me.

Pretty much, yes.

[Claw]

Also, I can't stress enough that Oberth is not predicated on an elliptical orbit. It isn't like Oberth ceases to function if the orbit is circular. Oberth is Oberth. It turns out that the best place to capitalize on it is where your speed is the fastest. That doesn't mean that the orbit must be elliptical.

[adinfinitum]

There are folks who do things the other way around, though. Use Kerbin instead of Mun for the slingshot by starting at Minmus. They say the dive back down from Minmus to Kerbin gives them enough extra starting speed at LKO altitude that they save about 500-600m/s on the transfer burn.

Just to help avoid possible confusion to people that are reading the thread, diving from Minmus to Kerbin takes advantage of the Oberth effect, and is not a gravity slingshot.

[Geschosskopf]

Just to help avoid possible confusion to people that are reading the thread, diving from Minmus to Kerbin takes advantage of the Oberth effect, and is not a gravity slingshot.

Actually, it's both. It's a powered slingshot.

[SSSPutnik]

Oberth effect is always present, whether in circular LKO or not. Because engines cannot instantaneously apply dV, you can improve the ability to harness Oberth by doing two burns. One to establishing the ellipse, and a second to do the rest of the burn.

Yes agreed.

[Claw]

Just to help avoid possible confusion to people that are reading the thread, diving from Minmus to Kerbin takes advantage of the Oberth effect, and is not a gravity slingshot.

Actually, it's both. It's a powered slingshot.

And it's really only advantageous for Oberth if you have a low TWR. Otherwise you're just killing time.

Besides, if the goal is to ease using Oberth, using the Mun for such activities is way more simplistic since the delta V is pretty close to Minmus, but you don't have to wait nearly as long for the orbits to line up or have to deal with orbital inclination.

[Geschosskopf]

And it's really only advantageous for Oberth if you have a low TWR. Otherwise you're just killing time.

Besides, if the goal is to ease using Oberth, using the Mun for such activities is way more simplistic since the delta V is pretty close to Minmus, but you don't have to wait nearly as long for the orbits to line up or have to deal with orbital inclination.

I agree. I don't do such things myself. The OP was asking about slingshotting away from the Kerbin system using Mun. I was merely pointing out that if he really wanted to slingshot out from Kerbin, he should use Kerbin, not Mun. And that even then it wasn't worth the trouble, at least for a short trip to Duna or Eve. Might be worth doing for a longer trip but why bother when it's pretty simple just to go direct.

[adinfinitum]

Actually, it's both. It's a powered slingshot.

If I read your earlier post correctly, and you are in orbit around Minmus, then lower your periapsis so that it is near Kerbin to perform your burn at a higher velocity than from an LKO orbit, then it isn't a slingshot.

A powered slingshot would be using the Mun for a slingshot and then burning as you pass by, that way after you escape Kerbin your apoapsis is actually high enough to get to the planet you want to travek to.

[Rakaydos]

Hmm... a Mun sligshot can resonably give you an elipse almost to Kerbin SoI. From there, it should be fairly easy to "correct" into a minmus-style drop and slingshot.

[wasmic]

All right, so from Minmus and Mun, you should drop down towards Kerbin and burn at periapse, IF you are going further than Duna outwards or Eve inwards. This isn't a gravity assist, though, as that happens when you encounter a body and uses its gravity to accelerate/decelerate. Mun gravity assits aren't really useful for anything further than Kerbin Escape. Powered slingshots from Mun may be useful, but that would require someone to do the maths, as I don't know them.

[Claw]

Again, dropping from an elliptical orbit and doing an interplanetary transfer burn at an LKO PE only helps harness Oberth if you have low TWR.

Gravity does play a part in this, but it's inherent to the orbit so it's not apparent when intraplanetery. There's a reason why you get the highest solar AP when burning on the midnight side of Kerbin. (Multiple reasons really.)

Anyway, the advantage of descending from Minmus or the Mun is that you could have a refueling station there and still have some control over the start of the elliptical orbit.

So if you have a high TWR and aren't planning on refueling, the gain you get from creating an elliptical orbit to "maximize Oberth" will be minimal and is probably not worth the complications of setting it up.

At the risk of throwing out numbers, I just completed a challenge where my design forced me to make an elliptical orbit to maximize Oberth. It used a nuke and took over an eight minute burn for the entire fuel load. Setting up the elliptical orbit netted a gain of around 200 dV for a roughly 3000dV burn (the entire fuel load). So actually not a bad gain. But, if I had 4x the thrust, the burn would have been 2 minutes, I wouldn't have to do the ellipse, and I would have had fuel left over.

Edited March 14, 2014 by Claw