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struggling to get to any of the moons of jool


tacticalyeet
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so i have tried several times to get an orbit around any of Jools moons. i have tried Vall, Tylo, and Pol, but run out of fuel before slowing down enough to achieve an orbit. i built a 10K DV (vacuum) rocket that had a final stage with a Nerv and a bunch of fuel for it, for about 5k DV in a vacuum, and still failed to slow down to get an orbit around Tylo. i will say, i am NOT very efficient with fuel and DV. just wondering if it is a TWR issue, or another design flaw or if i am just executing it poorly.

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You have a great big old assistant right there nearby in the form of Jool itself. Try passing shallowly through its upper atmosphere to shed speed before going for a moon orbit. Caution: you will likely need to protect your ship against the heat while doing so. Or you can try to pass in front of the planet as it moves, since approaching from this direction will slow you down with Jool's gravity. The moon Laythe also has a thin atmosphere of its own which can help with braking. 

And welcome to the forum. :) 

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On 4/1/2022 at 5:22 PM, tacticalyeet said:

so i have tried several times to get an orbit around any of Jools moons. i have tried Vall, Tylo, and Pol, but run out of fuel before slowing down enough to achieve an orbit. i built a 10K DV (vacuum) rocket that had a final stage with a Nerv and a bunch of fuel for it, for about 5k DV in a vacuum, and still failed to slow down to get an orbit around Tylo. i will say, i am NOT very efficient with fuel and DV. just wondering if it is a TWR issue, or another design flaw or if i am just executing it poorly.

it is not a flaw with twr or design, only with your manuevers. you can reasonably get in orbit around a joolian moon with 3000 m/s from LKO. With a lot of optimization and gravity assists, it is possible to more than half that amount. you are not getting there with 5000 m/s, your manuevers must be very inefficient. Let's see

1) how are you reaching Jool? do you use a proper transfer window? Do you make the manuever in LKO, to make full use of oberth effect? here a low twr may screw you up, though there are ways to compensate. Should take roughly 2000 m/s, up to 2500 m/s if twr causes you to be inefficient. A common rookie mistake is to exit kerbin orbit and then make the apoapsis raising in solar orbit. it's easier to see what you're doing, but it's a lot more expensive. Another common rookie mistake is to forsake transfer windows and just use more fuel to brute force trajectories

2) jool insertion. It should be reasonably cheap, a few hundred m/s at most. However, a jool insertion is not an optimized manuever either; you can get captured by a gravity assist from tylo or laythe. if your target is one of the larger moons, you can also go for direct intercept on the moon. here there are so many common rookie mistakes i can't count them. actually, navigating the moons is quite hard and takes experience.

3) reach a moon. Again, not very expensive. You should lower jool apoapsis until you touch a moon at apoapsis, this will result in the lowest intercept speed. A common rookie mistake here would be to cross a moon orbit in a full frontal collision, resulting in a much higher intercept speed

From what you say, it seems intercept speed is your main problem. The most important thing to know here is that the more different are your trajectory and the moon's orbit, the higher the intercept speed will be. It's like a car crash, really; two cars going in the same direction and colliding laterally will only get scratched, while a full frontal collision will be devastating. This goes both for a planet intercept and for a moon intercept. So, first image is a good transfer. in this case it was with another planet, as I don't have readily available pictures from jool. But notice how the ship touches lightly the planet's orbit, and the two are moving in the same direction during the encounter. there is still an intercept speed because the planet is moving faster than the ship, but it's a lot smaller than it would be if the ship was moving slower AND in a different direction

rSFIFY3.png

and incidentally, here's how a gravity capture on jool looks like. there are rules for gravity assist, the most basic one is that you pass in front of the celestial body to slow down and you pass behind to speed up, but this is something you can do with trial and error. just try to pass in front of laythe or tylo and play with the manuever a bit, you'll see.

wKF99T0.png

Finally, about direct moon intercept. I said that you have to touch the moon's orbit with the same direction. again, example from another planet but works.

See that the ship is intercepting the moon at a time when they are moving in the same direction, and so the intercept speed is small (300 m/s)

bVEGDZ3.png

while here i changed things to show you how it canges if you don't get the moon at the right point; here it's a lateral collision, and the intercept speed increased to 450 m/s

BPAK1CS.png

and here a full frontal collision, for more than 800 m/s

zKEIAfU.png

And this is far more important the faster the moons move.

Wal orbits a smaller planet from afar, its orbital speed is only 400 m/s. The joolian inner moons have orbital speeds ranging from 2 to 3 km/s, and the difference in intercept speed for a wrong manuever will be proportionally greater.

Do notice that that's only the deltaV needed for capture. For circularization on a moon without atmosphere you need to rocket brake, and there is no trick whatsoever to avoid that, you have to pay the deltaV price.

Still, do try it on jool. Your ship with 5 km/s vacuum deltaV can make it.

P.S. To ensure that you hit the moon at the right time, do make a correction manuever where you burn a fraction of a m/s, or a few m/s at most, prograde/retrograde years before reaching jool. In a few years of travel, those few m/s going faster/slower will cause you to arrive a few days earlier/later, and you can use that to time your encounter to find the moons in the best position. Do perform this manuever in solar orbit once you've already left kerbin, it's a small manuever. Then use normal and radial corrections to get an intercept with the moon you want.

P.P.S. If the rocket has 10 km/s deltaV vacuum on the launchpad and you only have 5 left once in orbit, you are likely also doing the launch inefficiently. it should cost about 3.5 km/s on a normal rocket - unless one has horrible aerodinamic issues or somesuch.

P.P.S. 10 km/s on nuclear means a lot of fuel and few engines. twr is really a problem. it would be easier to have a higher thrust stage to reach jool. It would also be more efficient to split that 10 km/s with some staging mechanism or using drop tanks to shed some weight

P.P.P.S. I basically told you that you are doing everything wrong. don't take it bad; rocket science is difficult, optimizing trajectories takes a lot of knowledge and experience. you won't stumble over them randomly, unless you already know what you are looking for

 

Edited by king of nowhere
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