Does this count as a gravity assist?

[photogineer]

So, I (finally) put my satellite in orbit around Duna, it went pretty well actually! I used a little ship I built dubbed the ITV-1 (Interplanetary Transfer Vehicle-1) to put it there. Pleased with myself, and with the amount of propellant left in the ITV-1, I sent it home when the next Duna->Kerbin window opened up. Transfer went great, I came in inclined and hot, but it was no problem to insert in to orbit. I then used my high apoapsis to rotate the orbit to be equatorial, and burn my periapsis down to 120km to intersect my station orbit. Perfection! I started to really feel smug at this point, I played with the maneuver nodes and got it set up so I would burn most of the way down to a circular 120km orbit, and then one orbit later intercept the station. So chortling away like the genius I was, I did the burn and then warped around to closest encounter....

.....at which point my space station whipped by at 2x the orbital speed! Yep, all that clever interplanetary piloting and I was going the wrong way round Kerbin. Krap!

At this point I was low on fuel too, so I sent up another ITV ship (the ITV-2) in to retrograde orbit and docked with ITV-1. Together the ships burned in to a Mun encounter, at the Mun Pe I did a retro burn, but not enough to insert in to orbit, just enough to whip around and escape Mun going the other way. This dropped me in to a pretty huge Kerbin orbit, but in the right direction. This time I was able to rendezvous with and dock to my station, hooray!

So, I was wondering, that maneuver I pulled whipping around Mun to change my orbit direction, is that considered a gravity assist? I wonder only because that is a wicked amount of dV I got out of it, considering it totally reversed my orbit direction. Admittedly I was in a much higher orbit after, so it wasn't a 2x orbital velocity change, but still... not bad!

Just curious what I should call it when I describe it to my non-KSP playing friends (who really don't care... but I will tell them anyway! )

Matt

Edited June 26, 2014 by photogineer

[LethalDose]

Some (including myself) would say yes because you used an orbital body's gravity to modify your vessel's orbit.

Others would say/have said no, because you burned at periapsis. Since it was powered, it was by definition not a gravity assist.

[Taki117]

Some (including myself) would say yes because you used an orbital body's gravity to modify your vessel's orbit.

Others would say/have said no, because you burned at periapsis. Since it was powered, it was by definition not a gravity assist.

I beg to differ. Just because a burn is performed does not mean that a gravity assist didn't occur. http://en.wikipedia.org/wiki/Gravity_assist#Powered_slingshots

[allmhuran]

All encounters will change your velocity.

A gravity assist is simply an encounter that changes your velocity in some manner that is advantageous to you.

In this case you used the mun to assist with your change of direction, so yes, it was a gravity assist.

Additionally, even the burn itself was assisted by gravity in a slightly different way: By burning at your munar periapsis your burn efficiency was increased by the Oberth effect. So the mun's gravity assisted your burn efficiency - another kind of gravity assist.

[LethalDose]

I beg to differ. Just because a burn is performed does not mean that a gravity assist didn't occur. http://en.wikipedia.org/wiki/Gravity_assist#Powered_slingshots

Dude, read my post, I said I agree that it's a gravity assist.

I added the second part (which I don't agree with) because posters on this forum have gotten up my exhaust port for calling it a gravity assist. The instance that comes to mind was earlier this year, but I can't find the thread in my history.

You can "beg to differ" with my statement that people some people will not call it a gravity assist, but it's a fact they exist, even if they're wrong.

EDIT: Found it here.

Edited June 26, 2014 by LethalDose

[allmhuran]

The other thread isn't really discussing the same kind of situation.

A gravity assist occurs when you are orbiting one body and get an encounter with a different body. In that other thread, this doesn't happen.

It may seem like it happens since you start out on Minmus and then swing by Kerbin. But Minmus is itself orbiting Kerbin. When you escape from Minmus' orbit you are back in Kerbin orbit, and you can't get a gravity slingshot from a body you are orbiting. So I would say that the analysis in the other thread is correct: there is no gravity slingshot happening there. Rather, it is indeed just making use of the Oberth effect.

Not that you'd actually do this of course... the only reason for dropping back down to a low Kerbin periapsis in that thread is to make it easier not to miss the transfer window, since this significantly reduces your orbital period, meaning you are travelling in "the right direction for ejection" more frequently. Assuming Minmus was in even remotely the right spot at the time your interplanetary transfer window arrived, you'd certainly just go straight into solar orbit from Minmus.

As I said in my previous post, you could in some ways call exploitation of the Oberth effect a "gravity assist" in the sense that you are using the gravity of Kerbin to assist with your burn efficiency, but you aren't using Kerbin to change your orbital trajectory, only your engines are doing that work.

Edited June 26, 2014 by allmhuran

[LethalDose]

Fine, then ignore I tried to accommodate a different point of view or opinion that I don't even agree with, because even doing acknowledging it's existence apparently makes me wrong.

Edited June 26, 2014 by LethalDose

[allmhuran]

Sorry, I think you've misunderstood. What I'm saying is that "the other point of view as represented by the other thread" just doesn't apply to this situation, it applies to a different situation.

[tg626]

Yes.

http://en.m.wikipedia.org/wiki/Gravity_assist

[Xavven]

I'd call it a powered gravity assist. Even if you hadn't made a burn, if you entered and exited Mun's gravity well at the right angles, you would have adjusted your orbit in the right direction. You took advantage of your periapsis at Mun to further exploit the situation with a burn.

[photogineer]

Alright, that clears that up, thanks everyone! I'm learning new tricks all the time... I'm not sure that the burning required to actually do the slingshot around Mun was any cheaper than just burning way out and changing orbit directions at the Ap, but eh, this was more fun!

Matt

[Claw]

I'm not sure that the burning required to actually do the slingshot around Mun was any cheaper than just burning way out and changing orbit directions at the Ap, but eh, this was more fun!

Matt

If it wasn't cheaper, it probably save time since the craft didn't have to drift way out there.

And like you said, learning new tricks is always good.

[Red Iron Crown]

Dude, read my post, I said I agree that it's a gravity assist.

I added the second part (which I don't agree with) because posters on this forum have gotten up my exhaust port for calling it a gravity assist. The instance that comes to mind was earlier this year, but I can't find the thread in my history.

You can "beg to differ" with my statement that people some people will not call it a gravity assist, but it's a fact they exist, even if they're wrong.

EDIT: Found it here.

I wrote the post you're referring to and I would call the OP of this thread's maneuver a gravity assist. The critical factor for something being a gravity assist is using a body's orbital speed to change your orbit, by entering and leaving that body's SOI. So you can't do a gravity assist on your origin or destination body, only some third body that is not the parent of either of the first two bodies. In the OP's case, that third body is the Mun. In that other thread, Kerbin can't be used for a gravity assist because it is the parent of the Mun or Minmus; Mun and Minmus already have Kerbin's orbital speed in addition to their own. The best that can be done is to drop periapsis to maximize the Oberth effect, but that's not a slingshot because nothing is gained by the addition of Kerbin's orbital speed.

Burn or no burn is irrelevant, there are both powered and unpowered gravity assists.

Sorry if you feel I gave you a hard time about the difference (then or now), but the Oberth effect and gravity assists are confusing at first and often combined; conflating one with the other only makes things harder to understand. It is best to be clear and consistent in the use of the terms.