Gravity Assist - Does Altitude matter?

[Jasel]

So when I aero-brake around Kerbin, to get the best results (slow me down the most), I dive down to 35K meters. Is there an equivalent affect for a gravity assist? I have a feeling I'm not understanding something correctly...

[rkman]

A gravity assist is more effective the closer to the moon/planet you get while avoiding atmosphere and mountains.

[cantab]

The effect will be stronger the closer you are to the planet. Of course the limit is when you'll hit the terrain or atmosphere. I don't know if an aerogravity assist - where you use lift to increase the deflection of your trajectory - is possible in KSP.

However, for gravity assists you don't simply WANT the biggest kick. You want the assist that gets you on to your desired location. If you're looking for a repeat assist from the same body, you want an orbital period that's a simple fraction of said body's, e.g. 3 orbits yourself for 4 Eve orbits for a second Eve assist.

[FREEFALL1984]

Aerobraking and gravity assists are two very different things, The altitude of a gravity assist only really matters if its a powered assist (ie where you burn at periapsis) in which case you want to be as low as possible in order to take maximum advantage of the oberth effect. With a passive gravity assist your angle of escape (or final orbit for gravity assisted braking) is determined by your altitude at periapsis, the lower the altitude the greater effect on the final vector. because the game doesnt use Nbody physics you wont gain experience a notable speed difference between a low peri and a high peri.

[LethalDose]

Aerobraking and gravity assists are two very different things, The altitude of a gravity assist only really matters if its a powered assist (ie where you burn at periapsis) in which case you want to be as low as possible in order to take maximum advantage of the oberth effect. With a passive gravity assist your angle of escape (or final orbit for gravity assisted braking) is determined by your altitude at periapsis, the lower the altitude the greater effect on the final vector. because the game doesnt use Nbody physics you wont gain experience a notable speed difference between a low peri and a high peri.

The altitude of a gravity assist does matter, because a lower altitude will lead to a greater degree of deflection from the original trajectory, as cantab pointed out above.

I would qualify that as an unpowered assist, but there are forum posters that get all prickly when you try to call anything that involves a burn a gravity assist.

[Red Iron Crown]

If doing a powered gravity assist, lower is better to maximize the Oberth effect. If doing an unpowered gravity assist, I don't think the amount you gain is dependent on altitude, it all depends on where you enter the SOI and where you leave it.

[FREEFALL1984]

The altitude of a gravity assist does matter, because a lower altitude will lead to a greater degree of deflection from the original trajectory, as cantab pointed out above.

I would qualify that as an unpowered assist, but there are forum posters that get all prickly when you try to call anything that involves a burn a gravity assist.

My original comment did indeed state that the altitude effects the escape vector...

[Starstrider42]

If doing an unpowered gravity assist, I don't think the amount you gain is dependent on altitude, it all depends on where you enter the SOI and where you leave it.

And the relationship between the two DOES depend on altitude.

To answer the OP, you'll need to experiment with the maneuver planner in map view. The new mode that lets you see the trajectory relative to the focused planet helps a lot with this. If, for example, you want the biggest outward boost, you want to adjust your flyby altitude until your post-flyby trajectory, relative to the planet, is parallel to the planet's orbit. Coming in higher or lower will both give you a lower apoapsis around the sun.

[Red Iron Crown]

And the relationship between the two DOES depend on altitude.

Agreed. I was meaning that low altitude is not advantageous in and of itself.

[linkxsc]

Kinda off topic. But what exactly is the technical term for a gravity assist that includes a burn?

On another note in response to cantab above. I do believe that you can do an aerodynamic... maybe skip would be the right word. I have before had craft (particularly one that had large wing area on it) go blasting through Jools upper atmosphere. And shifted its apoapsis ~10° clockwise rather than just lowering it a lot like a normal aerobrake maneuver. Havent experienced that on any craft since (or maybe just havent noticed it happen to such an extent)

[Kasuha]

If doing an unpowered gravity assist, I don't think the amount you gain is dependent on altitude, it all depends on where you enter the SOI and where you leave it.

Your "gain" depends just on which direction you are leaving. So there is no "best" altitude for unpowered assists, you need to set it up so you get sent in the direction you want to go.

Edit: "gain" is very fuzzy term in unpowered gravity assists. You don't have much freedom in what the outcome can be and your orbits before the slingshot and after slingshot are equal in the sense that your energy relative to the body off which you do the slingshot does not change.

Edit 2: more details regarding unpowered assists:

Regardless from which direction you come to the body you are going to use for the slingshot, your options are given by the speed at which you cross its SOI boundary. This speed defines your maximum orbit, minimum orbit, or perhaps maximum inclination difference you can achieve relative to the body you're using.

Maximum orbit means you need to exit the body's SOI exactly prograde relative to its orbital velocity. That will put your periapsis at the point where you did the assist, and your apoapsis will be at highest possible point given by your speed relative to the body.

Minimum orbit means you need to exit the body's SOI exactly retrograde relative to its orbital velocity. That will put your apoapsis at the point where you did the assist, and your periapsis will be at the lowest possible point given by your speed relative to the body.

In all cases, you will be exiting the body's SOI at exactly the same speed at which you entered it - just in different direction.

Edited May 9, 2014 by Kasuha

[meyst]

The effect will be stronger the closer you are to the planet. Of course the limit is when you'll hit the terrain or atmosphere. I don't know if an aerogravity assist - where you use lift to increase the deflection of your trajectory - is possible in KSP.

I was once curious about this too.

KSP Aerogravity assist is possible

I think a real one would melt and send debris, everywhere.

[Northstar1989]

A passive gravity assist *IS* affected by your periapsis relative to the gravitational body, first of all. The closer your approach the body, the greater the average effect of the body's gravity on your craft during the time you are in its SOI...

Think of it this way- the closer your periapsis, the greater the possible effect on your trajectory heading relative to the gravitational body. And certain exit trajectories represent a greater increase in final energy relative to the body the moon (or planet, for a gravity assist relative to Kerbol) is orbiting, whereas others represent lesser increases in energy or even a decrease in energy.

In the most extreme case, exiting a moon's SOI moving precisely retrograde relative to its velocity would decrease your kinetic energy by the most relative to the body the moon was orbiting (assuming you came in at a different angle) whereas exiting moving precisely prograde would increase your kinetic energy by the most relative to the planet...

Exiting moving precisely prograde gives you the most KINETIC energy, but doesn't give you the most TOTAL energy. This is because TOTAL Energy = Kinetic + Gravitational Energy.

This is most obvious for a moon with a large SOI and strong gravitational well relative to the body it is orbiting- such as Ike relative to Duna.

You *could* exit Ike's SOI moving precisely prograde, at the same altitude as Ike's orbit around Duna, for instance.

Or you *could* exit Ike's SOI moving at a 30 degree altitude to prograde, radially outward, at a HIGHER altitude relative to Duna.

In the 2nd case, you exit with the EXACT same velocity relative to Ike as in the 1st case, and thus the same Kinetic energy, but with greater Gravitational potential energy relative to Duna- as you exit at a higher altitude.

I *HIGHLY* recommend seeing the tutorials on the KSP Wiki about gravity assists (http://wiki.kerbalspaceprogram.com/wiki/Gravity_assist).

A gravity assist that will provide you with the MOST increase in energy relative to the body you are orbiting (because unpowered gravity assists *CAN* increase your total energy- by increasing your gravitaitonal potential energy relative to the planet without hurting your kinetic energy relative to the moon) is one where an imaginary line drawn perpendicular to your periapsis with the moon passes precisely along the prograde vector of the moon's orbit relative to the planet, and your periapsis is as close to the "retrograde" side of the moon as possible without impacting its surface...

Please note that gravity assist CANNOT provide you with kinetic energy relative to the MOON they are around in KPS's current physics model, but they CAN provide you with kinetic energy relative to the PLANET the moon is orbiting around. This is because any degree you are moving SLOWER than the moon relative to the planet in the direction of the moon's orbital trajectory actually counts as moving FASTER relative to the moon within the frame of reference of the moon. Therefore, you can exit the moon's SOI with both more Kinetic AND Gravitational Energy relative to the planet than you entered the moon's SOI with...

Regards,

Northstar

P.S. Where does this extra energy from an unpowered gravity assist come from, you might ask? In real life, it comes from the moon itself- which either gains or loses an equal amount of energy to that lost or gained by the craft passing nearby to it. A moon is usually much more massive than a spacecraft, however, so the change in the velocity of the much more massive moon is very small compared to the change in velocity of the much less massive spacecraft...

Edited May 10, 2014 by Northstar1989

[Northstar1989]

Also, for those of you curious, since the potential energy you can gain or lose from a gravity assist is determined by the orbital velocity of the moon around the planet it is orbiting itself (or planet relative to the sun it is orbiting), you can actually gain slightly more energy from a gravity assist around a body with an elliptical orbit when it is at periapsis than when it is at apoapsis...

This makes little difference for the moons and planets in the stock game, which all have relatively circular orbits, but can make a HUGE difference if you are playing with Krag's Planet Factory- which adds a "planet" called Ascension which is in a comet-like trajectory around Kerbol...

You can gain a LOT more energy relative to Kerbol if performing a gravity assist around Ascension at its periapsis with the sun than at its apoapsis- which helps if you are trying to reach Kerbol escape velocity...

Regards,

Northstar

Edited May 10, 2014 by Northstar1989

[LethalDose]

Kinda off topic. But what exactly is the technical term for a gravity assist that includes a burn?

I vote to refer to it as an "Oberth Maneuver".

[Jasel]

I've read through everybody's comments (thank you all very much), and will try and summarize to my specific question: Does altitude matter when performing a gravity assist.

Altitude does seem to matter, however it is not needed as long as you enter and exit at the correct angles. If your going for skipping off the atmosphere, you'll need a pretty good wingspan. From Meysts post, the altitude seems to be the same as aero-braking.

I saw the Wiki page NorthStar but it's blocked from my current location...I will try and give it a read when I get home.