planned manuever won't stay fixed

[king of nowhere]

I am trying to simulate a complex manuever involving a duna flyby for a gravity slingshot putting me in another flyby on the next orbit to hopefully get an assist towards jool. i know it won't unfold exactly as planned without some course correction, but i just want to study if the manuever is feasible, if it will net enough of a deltaV reduction to be worth the effort.

at first all is ok. i set up the first burn, the mid--course plane correction, and i slingshot past duna. i come out pretty close to be on intercept again, so i make another course correction. i get the second orbit to enter duna's SoI, i try to get closer... and suddenly the manuever skews and goes somewhere else. now i've been trying for a good ten minutes, every time i try to fine-tune the second flyby, at some point the orbits change. slightly, but enough to set me completely off.

here is one screenshot

notice the second duna periapsis, in T- 2 years

and notice how it's now no longer there. there is instead a closer approach for 110k km. i must stress, this is NOT me fiddling with the manuevers. i did absolutely nothing except reloading the game. you can see that the manuever has the same exact amount of deltaV, and it is made in the very same second. as i try fiddling my manuever, the game keep flickering between those two states, putting all my manuevers off.

 

Is there any way i can make a simulation with more than 3 or 4 manuevers without actually tying it directly? (this specific ship has pretty low TWR, i'd need to make a couple of apoapsis raising before the big burn, and i'd really rather not spend half an hour like that for just a simulation)

[Spricigo]

First thing first: Eve has the greatest  gravity of all the planet but jool, is right there in Kerbin's doorsteps and there is nothing but the spek that is Gilly around it. So  much more convenient than Duna. 

 

Anyways, you stumbled in a game mechanic limitation. The trajectory can precisely be calculate only to a certain point and if for any reason it need to be recalculated the values may be slightly different because of rounding errors.  That small deviation will propagate in a higher miss after 2y178d or similarly large (and larger)times.

Also, the actual burn will take more time in a pretty low TWR craft than the predicted instant  in the planner. It  don't help either.

On the other hand, it may take a trivial amount of deltaV to come from "missing the SoI entirely" to "within hundred meters"  when you do a correction burn later on.

[king of nowhere]

5 hours ago, Spricigo said:

First thing first: Eve has the greatest  gravity of all the planet but jool, is right there in Kerbin's doorsteps and there is nothing but the spek that is Gilly around it. So  much more convenient than Duna.

Yes, i realized that thing a few hours ago. i will look for eve slingshots too.

EDIT: i just tried, and i can raise my apoapsis as far as duna, so not worth it. i guess i could try to set up a second passage after that, but then there isn't much to gain over doing the same manuever with duna

 

Quote

Anyways, you stumbled in a game mechanic limitation. The trajectory can precisely be calculate only to a certain point and if for any reason it need to be recalculated the values may be slightly different because of rounding errors.  That small deviation will propagate in a higher miss after 2y178d or similarly large (and larger)times.

ouch. well, good to know at least.
 

Quote

 

Also, the actual burn will take more time in a pretty low TWR craft than the predicted instant  in the planner. It  don't help either.

On the other hand, it may take a trivial amount of deltaV to come from "missing the SoI entirely" to "within hundred meters"  when you do a correction burn later on

 

yes. that's why i don't worry about missing the trajectory even if i have a ship with long burn time.

the problem is, i would like to see where that second gravity assist would send me. Will it raise my apoapsis enough for a jool encounter? or will i be stuck halfway needing to burn more fuel than i would have with a direct trajectory? in that case, how much would it help if i burn at periapsis?

I was trying to simulate that stuff to get a good estimate of what would be the most convenient trajectory. but it seems the game mechanics won't let me.

Edited October 2, 2020 by king of nowhere

[Spricigo]

5 hours ago, king of nowhere said:

EDIT: i just tried, and i can raise my apoapsis as far as duna, so not worth it. i guess i could try to set up a second passage after that, but then there isn't much to gain over doing the same manuever with duna

 

Been a while since I messed with slingshots, but I'm under the impression that you may get more from Eve than that. (at the very last you can burn at Eve low orbit and enjoy the Oberth Effect)

Anyways, slingshots are not that simple...even the tools can be pretty daunting themselves.

 

[king of nowhere]

7 hours ago, Spricigo said:

 

Been a while since I messed with slingshots, but I'm under the impression that you may get more from Eve than that. (at the very last you can burn at Eve low orbit and enjoy the Oberth Effect)

 

I tried. i needed 1000 m/s to get up to jool's intercept, making it no more convenient than going straight there from kerbin.

i was hoping i could use the first trajectory i found, an assist from duna putting me directly on another intercept from duna the following orbit, but now i can't find it anymore. the first time i got it without even trying. On the other hand, i discovered i can get a similar effect by aerobraking on duna. in fact, i can perform an aerocapture directly without coming even close to damaging my ship.

i think i'm falling in love with duna's atmosphere.

 

ok, i was aiming for jool, why am i so excited about stopping at duna? i actually want to visit both, and i want to spend as little deltaV as possible before reaching laythe and tylo, because then i will drop the really heavy landers and i will be much more fuel efficient. but it would take me 2500 m/s to reach jool directly. yes, it's normally 2000, but with this low twr i needed a lot of course correction, even though i raised my kerbin apoapsis first. for jool i still need 1000 m/s after i went in kerbin escape trajectory, that's 15 minutes burn, i can't do it precisely, hence the extra cost. but for duna, i can reach it with a 900 m/s burn with a mun assist, and i can raise my periapsis first, reducing the manuever to a mangeable 5 minutes burn. i only needed, like, 30 m/s of correction. and from duna i can reach jool with 1300 m/s. which means that if i can aerobrake my duna insertion, stopping to duna first becomes cheaper than going straight to jool

Edited October 3, 2020 by king of nowhere