Circularise or Periapsis first?

[THobson]

Having just entered the SOI of Moho (or any other body) at quite a high relative velocity which is the most fuel efficient:

1. Circularise at Periapsis (or anywhere else), then change Periapsis to the desired amount and then Circularise at Periapsis.

2. Change Periapsis to the desired amount and then Circularise at Periapsis

More importantly how can I work out which is the most fuel efficient?

Many thanks.

[Greenfire32]

my gut tells me to get your peri down to where you want it while still on escape velocity and then circularize when you hit peri.

[HebaruSan]

I believe #2 would be most efficient, since the Oberth effect says you should burn when moving as fast as possible (and therefore as low as possible).

11 minutes ago, THobson said:

More importantly how can I work out which is the most fuel efficient?

You can try setting up a sequence of multiple maneuvers to represent each option, and then if you have a mod like this one, you can then use it to see the total cost of all of the active maneuvers (see the "Trip Info" button).

 

[A_name]

52 minutes ago, Greenfire32 said:

my gut tells me to get your peri down to where you want it while still on escape velocity and then circularize when you hit peri.

Correct. This way you spend more time burning lower (faster), thus making the most of Oberth.

1 hour ago, THobson said:

More importantly how can I work out which is the most fuel efficient?

Many thanks.

I know you were asking about formulating this, but why not make a test? Save a quicksave right after entering SOI and have a go with each method, see how much fuel you end up with each time.

Edit: I think I misunderstood the question:

1 hour ago, THobson said:

which is the most fuel efficient:

1. Circularise at Periapsis (or anywhere else), then change Periapsis to the desired amount and then Circularise at Periapsis.

2. Change Periapsis to the desired amount and then Circularise at Periapsis

As long as with option 1 you make your first circularization at Pe (as opposed to "anywhere else"), these two methods should yield the same result, as you spend exactly the same amount of time burning at a given altitude (and thus orbital speed).

 

 

Edited June 15, 2016 by A_name

[Snark]

Agree with the prior comments about the value of Oberth effect and doing your burn down low.

Generally, you start your preparation when you're still very far away from the target planet (so that you can use a tiny burn to make big adjustments to you trajectory past the planet), i.e. at least several days before you even enter its SoI.  What you do is to adjust your trajectory so that it has the lowest Pe you can manage without actually scraping your toes.

Then you do your capture burn there, really low, practically at the surface.   Burn retrograde until you're captured, keep burning until you have lowered your Ap down to where you want your circular orbit to be.

Then you coast up to Ap and do a moderate prograde burn to circularize there.

[mk1980]

i agree with @Snark. the last time i sent something to moho, i adjusted the periaps during the plane change burn. afaik it gets more expensive the closer you already are, so if you can tweak it to where you want to have it while you are still very far away, it costs a lot less. i used a terrible transfer and the original capture burn would have cost >5000 m/s. with a 200m/s adjustment i lowered the PE to ~15 km (i think) and the capture burn went down to ~4000m/s.

same principle applies (in a much smaller scale) to something as simple as a transfer to mun. if you first capture into a high orbit and then lower it, it costs more than spending like 10 m/s on a correction burn early on to get the PE as low as you dare and do the capture burn at the lower PE:

[THobson]

Thanks.  I have just read about the Oberth effect on Wikipedia.  I now understand.

 

Thanks again for all the comments.

 

How do I close this thread?

Edited June 15, 2016 by THobson

[Snark]

3 hours ago, THobson said:

How do I close this thread?

You can't close it, per se-- anyone can still add posts to it, if they have something to add.

However, you can mark it as "answered", which is a good idea.  Just pick whichever post in the thread strikes you as the "best answer" (i.e. the one that really answered your question), and click the grey-circle "check mark" button next to it, on the left-hand side.

That'll move the answer up to the top of the thread (useful to anyone who may come along later looking for answers), and will also mark it as "answered" in the forum list view (so folks know it's taken care of).

[wumpus]

One reason not to close the thread is inclination adjustments.  Hopefully you had your inclination absolutely perfect as you aligned your flyby/periapsis.  If you want to adjust your inclination, you should stop your capture burn the moment the orbit closes.  Then fly out to either apposis or your ascending/descending nodes and burn there (this will minimize cost of inclination burns).  After this, adjust your periapsis at apposis (be *very* careful of adjusting ascending/descending nodes on the way down, you can easily hit a collusion course.  Just use your blue (up/down) nodes to move your periapsis inefficiently if you have to.

Edited June 15, 2016 by wumpus
arr. Some pirate stole my arr. Recovered.

[StahnAileron]

To add to @wumpus's comment: Inclination changes to a target orbit when approaching another celestial body should be done around the same time as the mid-course correction/adjustment. Ideally, it should at around the midpoint of the approach, well before SOI change. To oversimplify it, if you time a correction/adjustment burn around the midpoint of an approach, you have minimal lateral velocity (up/down, left right) and most forward velocity (i.e. straight at the target body). Your engines won't have to compensate for those velocity vectors, saving you DV. You can get a more precise inclination afterwards once in the body's SOI using wumpus's method as that will keep DV usage lower.

Personally, I use a maneuver node editor to help manage all that. (I just use MechJeb's node planner and editor due to convenience. There are alternatives like the one mentioned earlier.)