How much delta-V should I bring on my Jool probe?

[RealKerbal3x]

Pretty much as title. I'm designing a mission to Jool (my first time visiting that particular system despite having played for more than 2 years ) and I'm not exactly sure how much dV I should equip my probe with to be able to fly by each moon. I've consulted a dV map and have gathered that I'll need about 2km/s for the transfer, but it doesn't really tell me what I need for moving around the Jool system itself. Help would be appreciated

(also, side question: the dV map tells me that I need ~3.3km/s to land a probe on Tylo from a flyby trajectory. Is this accurate or will I need more?)

[bewing]

24 minutes ago, RealKerbal3x said:

(also, side question: the dV map tells me that I need ~3.3km/s to land a probe on Tylo from a flyby trajectory. Is this accurate or will I need more?)

Tylo has high gravity, so gravity losses are a huge factor. And gravity losses depend on how long it takes you to land, which is completely dependent on your TWR. So, how fast will your craft be able to go from 2000m/s to zero?

 

[RealKerbal3x]

Just now, bewing said:

Tylo has high gravity, so gravity losses are a huge factor. And gravity losses depend on how long it takes you to land, which is completely dependent on your TWR. So, how fast will your craft be able to go from 2000m/s to zero?

 

I don't know the exact numbers, but as far as I know my Tylo lander has a TWR above 2 fully fueled. And I have tested landing it from a hyperbolic trajectory - it seems to work, but that's coming from a mostly circular Jool orbit

[bewing]

That sounds like a pretty decent TWR. It'll still take 200 seconds to get down, though (2000 m/s at 10m/s^2). Which is an extra 2000m/s that you will have to counteract. So I'd be expecting something more like 4000m/s of dV to land. Then about 2500m/s to take off again, because your TWR should be higher on takeoff. Are you planning on just doing flybys of the other moons? Or orbiting them? Or landing?

 

[RealKerbal3x]

9 hours ago, bewing said:

That sounds like a pretty decent TWR. It'll still take 200 seconds to get down, though (2000 m/s at 10m/s^2). Which is an extra 2000m/s that you will have to counteract. So I'd be expecting something more like 4000m/s of dV to land. Then about 2500m/s to take off again, because your TWR should be higher on takeoff. Are you planning on just doing flybys of the other moons? Or orbiting them? Or landing?

 

It's only a probe, so I don't plan to take off again (in fact, even if my Tylo lander's engine and fuel tank is destroyed on landing, I'll call it a success, as all I care about is getting a probe core with science and power generation down to the surface)

My main probe is quite Cassini-esque - I intend for it to fly by each Joolian moon, detaching a small lander as I do and landing it.

[king of nowhere]

the deltaV map is very accurate for how much you need to land on the various moons. you should add a bit more for safety because using the minimum is very difficult, and only doable with high twr.

that said, you do indeed need 2 km/s to arrive on jool. you should be able to shave some of that with gravity assists, but it is very difficult; duna is never in the proper place, and eve requires multiple passes.

at jool insertion, you don't need a capture burn, you can use a laythe gravity assist to stay in the system for free: two examples

don't worry about setting an encounter when you launch from kerbin; you can intercept laythe later with a very small correction manuever (as shown in the second image)

 

afterwards, you should try to orbit one of the major moons with a low intercept deltaV. i tried for a long time to lower my apoapsis to get a cheap encounter with laythe, but to no avail. i'm sure it would be possible somehow, but hard. much easier to set up a cheap encounter for tylo instead, like in this example

60 m/s of correction manuever and a second laythe assist, and i could enter tylo orbit with only 170 m/s. if you have a classic lander/orbiter approach, then i suggest going to tylo first, because the tylo lander has to be very heavy, and getting rid of it will increase your fuel efficiency oitherwise

alternatively, you can get inside laythe's atmosphere on your flyby and aerobrake your way to laythe orbit directly. but you need a VERY good heat shield for it

from there on, you can't really cheat anymore, though.you need to circularize and land, and there's no way around that. circularizing around tylo requires some 800 m/s, which must be paid both arriving and leaving (add that to the 170 m/s injection burn and 2300 to land, and the 3300 figure is correct). for laythe it's about 600 m/s, but you can aerobrake on your way in.

vall is cheaper, around 250 m/s. going to bop from the inner moons requires about 800-1000 m/s, and from bop to pol a bit less.

so, to recap

2000 to get in jool intercept from kerbin, can potentially be reduced to about 1300 with lots of skill in multiple gravity assists

500 to enter an elliptic orbit around tylo (some extra because gravity assists are tricky and i was very lucky to find that one for 250 m/s). OR a good thermal shield and aerobraking to inject into laythe directly.

3300 from elliptic tylo orbit to land, and the same to go on other moons from the surface

free landing on laythe with aerobraking, but 2900 m/s to orbit and 600 more to go to other moons

2000 from vall intercept to land, and just as much to go back

about 900 to get to bop, plus 500 to land and orbit again

about 800 to get to pol, plus 300 to land and orbit again

about 1200 m/s to get from pol back to kerbin intercept, plus 1000 m/s to stay in elliptic kerbin orbit, from which you can aerobrake (again, you can skip the 1000 with a very good heat shield)

all numbers include a little bit of extra for safety

 

i'm not going to give you a total, because that will depend on your exact setup and mission profile. but that should give you a good idea of the costs involved.

the alexmoon planner can give you accurate measures of how much fuel you need from one moon to the other

 

 

Edited October 19, 2020 by king of nowhere