Gravity Assist help

[W. Kerman]

I was playing KSP, and I saw other people on YouTube getting encounters and then focusing on the planet to do gravity assits for speeding up or slowing down. I looked at the wiki. But it was really confusing, what side of planet for speeding up, and which one for slowing down.

 

[TheDestroyer111]

If you don't understand it from the wiki, I'm afraid you will never do so.

Did you ever fly to the Mun? (if you think about gravity assists, you definitely did)

Well, look on which direction your post-flyby orbit goes and you'll figure out gravity assists. But if that very nice picture on the wiki doesn't help you, then the game itself won't help too.

[FullMetalMachinist]

It's a bit long, but this article is what made me really understand how gravity assists work, and how to use them in the game. 

The TL;DR of it is that if your orbit after the assist looks more similar to the assisting body than it was before, then that will speed you up. If your orbit after the assist looks less similar than it was before, then it will slow you down. 

[W. Kerman]

10 minutes ago, FullMetalMachinist said:

It's a bit long, but this article is what made me really understand how gravity assists work, and how to use them in the game. 

The TL;DR of it is that if your orbit after the assist looks more similar to the assisting body than it was before, then that will speed you up. If your orbit after the assist looks less similar than it was before, then it will slow you down. 

That was really long but it helped. To TD111, I have done gravity assist, I didn't know how to squeeze as delta v out of them as I could.

[Diche Bach]

@FullMetalMachinist Nice article. Very instructive.

I'm fairly "challenged" when it comes to physical sciences, so don't laugh but . . . this one point has me wondering

This is a general rule of thumb for gravity assists: if, after the encounter, the spacecraft is pointing more along the planet's direction than it was before the encounter, its speed will increase. But where does the energy come from to accelerate the spacecraft? In fact it comes from the planet's own energy of motion. In the Sun frame, there is a transfer of momentum and kinetic energy from the planet to the spacecraft. The planet slows down very slightly in its orbit, and the spacecraft speeds up. Newton's third law states, "To every action there is an equal and opposite reaction," and that's true in this case. Because the planet is so much more massive than the spacecraft, the transfer doesn't affect the planet to any measurable extent, but to the spacecraft it's a big deal. For example, we can calculate that during the Voyager encounters with Jupiter in 1979, Jupiter slowed down by roughly 10 to the -24th power kilometers per second -- a change much too small to measure. But each Voyager gained about 10 km/s, a pretty big number and enough to put them on a fast path to Saturn (and in the case of Voyager 2, to Uranus and Neptune as well) and eventual escape from the solar system.

So, humanity has effectively "extracted" some of the energy from Jupiter, Saturn, Uranus, etc., etc. Tiny fractions of the total energy of motion of these planets, but an appreciable quantity (at least from the standpoint of a spacecraft).

I wonder if using this effect, it might be possible (distant future of course) to "harvest" energy from celestial bodies?

[Superfluous J]

1 hour ago, Diche Bach said:

I wonder if using this effect, it might be possible (distant future of course) to "harvest" energy from celestial bodies?

Technically, we already do that. Most of our unmanned spacecraft harvest energy from the planets to get where they're going.

I can't think of a practical way to use that energy here, though, which is I presume what you mean

[Spricigo]

6 hours ago, Diche Bach said:

I wonder if using this effect, it might be possible (distant future of course) to "harvest" energy from celestial bodies?

Well...

Solar panels catch the energy radiated by the sun

Wind turbines catch the kinetic energy from atmosphere 

Geothermal powerplants catch the energy from tectonics 

... 

Harvesting energy from celestial bodies it's quite the standard in some sense.  Obviously harvesting it from earth (so close)  and the sun (astronomically huge output)  are particularly convenient. 

[Diche Bach]

On 8/16/2016 at 3:53 PM, 5thHorseman said:

Technically, we already do that. Most of our unmanned spacecraft harvest energy from the planets to get where they're going.

I can't think of a practical way to use that energy here, though, which is I presume what you mean

What I was thinking of was something like this:

A. Some generator-like gizmo that converts "velocity" into electricity.

B. On a space ship(s)

C. Lots of them (maybe even like a "cloud" of them)

D. With very cheap/efficient propulsion.

E. Now send the swarm of "velocity soakers" out to do a gabillion gravity-assisted encounters with every possible celestial body in sight, packing their battery packs full of energy until they are low on fuel.

F. If you can "transmit" the energy back home, then do it that way, else send them back to an Earth Encounter to be retrieved full of more energy than when they launched

!?!

Like I said, my knowledge of physical sciences is fairly rudimentary, and I probably am way off the deep end with this idea.

Also, I know that something like a Dyson Sphere is already an idea, and perhaps this is just a more inherently efficient system.

But even Einstein had to start somewhere when he was moving from postal worker to history-changing physics genius!

[Spricigo]

1 hour ago, Diche Bach said:

What I was thinking of was something like this:

A. Some generator-like gizmo that converts "velocity" into electricity.

... 

 

I risk to say we probably have technology necessary to a gizmo like that but  the questions are 1) How efficiently it can be done?  2) Wich other better options we have? 

Gravity slingshots are convenient because we are taking kinetic energy of a planet/moon to give it to a craft. This is actually pretty easy* we just need to encounter  with the planet/moon in a convenient trajectory,  it's like going with the stream.

To generate electricity from this energy we  need to work harder, maybe  a very different approach.  I risk to say we probably have technology to figure out something** that works but probably it will be a complex and bulky device with low output.

 We have much simpler and efficient solutions if we go look elsewhere for electricity generation.   Maybe we will someday have solar farms in Mercury,  nuclear powerplants on the  Moon, wind turbines in Venus,  "geothermal"  powerplants in Enceladus or other fancy stuff. Until then we have solar panels,  and whatever we use to charge the battery before launching. 

 

 

*rocket science easy 

**my wild guess/bet is a device that use tidal forces to generate electricity.  But I'm no engineer,  mind you. 

[Diche Bach]

Yep, for now, digging up the remains of multi-million year old dead plants and critters and burning it is plenty cost-effective

[Diche Bach]

Question:

Is using Mun for a gravity assist to get to Minmus:

Good / Bad Idea in general?

Necessary to plan for "windows?"

or can the benefits of a gravity assist from Mun be gained in virtually any flight as long as one burns to extend the apo at the right spot relative to Mun?

I've got my first couple missions beyond Kerbin in my career play (impact an asteroid) so I've been fiddling with designing a "Hammer Time" probe with an upper stage impacter (including the "impact measuring thingamajig" and which has enough dV, battery, and radio power to be controlled from Kerbin) and a lower stage "monitor" (with the spectrometer thing that measures the explosion from the impact and a bunch of other science instruments). Starting out with dV in the 7000 to 8000 ballpark, it seems to have enough to get where it is going (tested in Kerbal Construction Time "simulation mode"), but in trying to use Mun for a gravity assist, I have to say, I wasn't immediately impressed.

My dV and "orbital speed" prior to the gravity turn were in the 5000 and 3400m/s ballpark, whereas after the burn(s) (it turned out the first one didn't achieve what the planner indicate it would achieve, so I had to do a couple more small burns to adjust) these were in the 3,300 and 700m/s ballpark. What! That gravity assist just slowed me down!?

I understand that the trick from a gravity assist (not a gravy "brake" but an acceleration) is to get the periapsis from the encounter BEHIND the body being "harvested" for velocity (relative to its orbital movement) and as close as possible and that if the orbit after the assist looks "more" like the assisting body than it did before, then it has "helped" in accelerating (to say nothing of changing the direction of the apoapsis relative to the original one). However, I'm not sure how to know what the actual "gain" is from such a maneuver, as I suspect the "orbital speed" indicator on the NavBall and that in Kerbal Engineer are telling me things relative to benchmarks that change during the course of a flight? (or at least as different targets are selected else as different SOI are passed through??).

The "Vel" for the asteroid is shown as "8,4000 m/s" (ballpark), and despite my NavBall suggesting that I had slowed down after this Mun gravity assist, it only took another 1000 or 1500 dV burn to achieve an encounter with the asteroid (though I ran out of time on the sim and didn't actually complete it).