Gravity assist fail

[THX1138]

I thought I could save fuel by going to Jool via Duna but all I seem to be able to achieve is to shift my orbit a little bit without raising my apoapsis. I found this thread which explains gravity assists but it's going to take a long time to read and figure out. In the mean time I'll go back to the crap way.

 

[Blaarkies]

I find low gravity worlds useless for anything other than easy landings. Take Dres/Moho for example, it uses over 1000m/s get capture yourself in orbit, while it only takes like 200m/s to capture at Eve(i know the relative speed difference between Kerbein/Eve is worse that Kerbin/Dres/Moho...but because of Oberth, it would still be far more expensive at Dres/Moho)

The Mun is great for a slingshot...to Minmus. Really, don't try anything more than that(maybe using a Mun assist to get into a higher orbit, resulting in cheaper injection burns into Mun orbit). It save a little more than a 100m/s dv getting to Minmus(since you get a Minmus transfer trajectory, but with Kerbin Pe way above 70km...it is like 5 000km at least).

A Scott Manley video about the JUNO mission has a piece of very educational visual gravity assist explanation. Basically, escape Kerbin going prograde(very slowly, less than 10% of the way to Duna). Then at Kerbol Ap, you should be behind Kerbin in the race...burn retrograde for about 200m/s, you will get an intercept with Kerbin somewhere past the "Starting line"(where you escaped at first). This sling should get you up to Duna orbit or even more...anyway, the trick is to start one year + a few(20?) days AHEAD of the departure date(so that you will swing by the same day that the transfer window opens up)

I am still no master at this, but I know it is useless for a trip to Duna/Eve...but for Jool this is perfect!  (but you really need to have your life in order to be able to achieve it...successfully)

[Speadge]

even better vid by scott (just a bit older):

 

Edited July 12, 2016 by Speadge

[Scotius]

26 minutes ago, Blaarkies said:

The Mun is great for a slingshot...to Minmus. Really, don't try anything more than that(maybe using a Mun assist to get into a higher orbit, resulting in cheaper injection burns into Mun orbit). It save a little more than a 100m/s dv getting to Minmus(since you get a Minmus transfer trajectory, but with Kerbin Pe way above 70km...it is like 5 000km at least).

Not true. Once i used a slingshot meneuver around the Mun to send a ship to Duna at the cost of whooping 200 m\s dV. I admit, it was a stroke of incredible luck - but it shows that a properly set slingshot can save you a lot of fuel.

[Jovus]

I haven't looked at the math, but if I were trying to slingshot to Jool, I'd use Eve or Kerbin instead of Duna.

Why? Because Eve and Kerbin have higher gravity wells, and the setup costs are lower.

[THX1138]

2 minutes ago, Jovus said:

I haven't looked at the math, but if I were trying to slingshot to Jool, I'd use Eve or Kerbin instead of Duna.

Why? Because Eve and Kerbin have higher gravity wells, and the setup costs are lower.

According to this thread (below) you can't use Eve to get to Jool if you're starting from Kerbin. The post says the source of the assist must be between your origin and your destination. I've no idea.

 

 

[Jovus]

Either the thread is wrong, or you're reading it wrong. Venus is a very popular assist option on the way to Jupiter.

[THX1138]

11 minutes ago, Jovus said:

Either the thread is wrong, or you're reading it wrong. Venus is a very popular assist option on the way to Jupiter.

It says "Thus, the primary use for a gravity assist is to use an encounter with an object intermediate between us and our target to help boost us to where we want to go. For instance, if we wanted to get to Minmus, we could plot a course for the Mun and use the Mun encounter to boost us into a higher orbit. Likewise, if we wanted to get to Moho, we could use an encounter with Eve to lower our orbit. However, we could not use an encounter with Duna to get to Moho if we were starting from Kerbin; the reason for this is that Duna marks the apoapsis of our Hohmann transfer orbit so we're already as low as we can reach from that encounter."

I think I must have read it wrong.

[Red Iron Crown]

You can use Eve to get to Jool. In fact you can get to Jool for about 1km/s using assists from Eve and Kerbin.

[Jovus]

OK, I think I understand the confusion.

The reason you can't use Duna to get to Moho is simpler than you think. In a Kerbin-Duna transfer orbit your apoapsis is at Duna. At apoapsis you're moving slowly. For a gravity assist, you want to be moving fast. In fact, a rule of thumb is, the faster, the better. So you couldn't use Duna for your Moho transfer - at least not profitably.

 

Aside, I think what the author means isn't that it's mathematically impossible, but rather that it's not worth the time and effort because your gains are either going to be miniscule or take an absurd amount of time to realize. However, again, I haven't actually run the numbers, so I don't know.

[Blaarkies]

17 hours ago, Scotius said:

Not true. Once i used a slingshot meneuver around the Mun to send a ship to Duna at the cost of whooping 200 m\s dV. I admit, it was a stroke of incredible luck - but it shows that a properly set slingshot can save you a lot of fuel.

Did you seriously send a ship to Duna from LKO via Mun for only 880m/s dv? For comparison, you can "touch" the Mun SOI for about 840m/s from LKO. Most gravity assist Gurus say the Mun gives a maximum of about 150m/s dv, and less than that the faster you go through the SOI(the change in exit angle decreases)

[specialopsdave]

Personally, I've just avoided gravity assists altogether by sticking to the tried-and-true "moar boosters" method. In other words, insane amounts of delta V.

[storm_soldier2377]

@Red Iron Crown

In my experience, gravity assists only work if the assisting planet is between my destination. Like getting an assist from Eve to Moho or Jool to Eeloo. I can't seem to get assists from Eve to Jool for example because my ship will be moving faster than Eve at the encounter, ensuring that I will encounter Eve from behind which will give me the opposite effect of what I desire.

Edited July 13, 2016 by storm_soldier2377

[Draco T stand-up guy]

Gravity Assist

Quote

A gravity assist around a planet changes a spacecraft's velocity (relative to the Sun) by entering and leaving the gravitational field of a planet. The spacecraft's speed increases as it approaches the planet and decreases while escaping its gravitational pull (which is approximately the same). Because the planet orbits the sun, the spacecraft is affected by this motion during the maneuver. To increase speed, the spacecraft flies with the movement of the planet (taking a small amount of the planet's orbital energy); to decrease speed, the spacecraft flies against the movement of the planet. The sum of the kinetic energies of both bodies remains constant (see elastic collision). A slingshot maneuver can therefore be used to change the spaceship's trajectory and speed relative to the Sun.

Just to help people clarify what a gravity assist is and how it works.

[Blaarkies]

2 hours ago, storm_soldier2377 said:

In my experience, gravity assists only work if the assisting planet is between my destination. Like getting an assist from Eve to Moho or Jool to Eeloo. I can't seem to get assists from Eve to Jool for example because my ship will be moving faster than Eve at the encounter, ensuring that I will encounter Eve from behind which will give me the opposite effect of what I desire.

If you only tangentially touch the orbit, then you will have that problem.
What i mean by this is, imagine you try to intercept Eve from LKO, with very little dv so that your Kerbol Pe is barely touching Eve's orbit. When you arrive at Eve, you will be moving faster than Eve(since you have a greater Kerbol Ap than Eve). Now since you are moving forward, there is now way to bend you inner Eve SOI trajectory to point more "forward"...(there is an inclination trick to still achive this with Eve, ignore that for now)
However, think of it this way: Since you are moving faster, it is easier to get "in-front" of Eve in Eve SOI, since you will be hitting Eve from behind...but if you are going slower, then Eve's SOI will catch up to you and you will swing "behind" Eve.

The trick to skip all this, is by dropping your Kerbol Pe to slightly below Eve's orbit(about 5% of the way to Moho), it seems wasteful but bear with me: When you intersect Eve's orbit now, you will have a greater difference in speed(Eve is racing past you, since you aren't really even movin in the same direction as Eve). This extra speed is relative to Eve, so if you can bend this trajectory as to exit Eve SOI in the same direction as Eve's orbit, then you suddenly got that extra speed when you exit into the Kerbol SOI.

*you can easily play/test this out on Kerbin alone. Escape Kerbin, deep space burn retro grade to drop Pe slightly below Kerbin, ???, profit!

[Red Iron Crown]

2 hours ago, storm_soldier2377 said:

@Red Iron Crown

In my experience, gravity assists only work if the assisting planet is between my destination. Like getting an assist from Eve to Moho or Jool to Eeloo. I can't seem to get assists from Eve to Jool for example because my ship will be moving faster than Eve at the encounter, ensuring that I will encounter Eve from behind which will give me the opposite effect of what I desire.

The trick is to not set it up as a Hohmann transfer to Eve, you want to arrive when your trajectory is not tangential to Eve's. Have a look at this mission done by a master of gravity assists, @PLAD:

 

[PLAD]

Duna is small and a flyby of it can only save about 100m/s tops over a direct flight to Jool. On the other hand if you want to do just one flyby on the way to Jool then Duna is the best one to do. If you can do two flybys then Eve-Kerbin saves a lot though, and as shown above E-K-K is best of all.