Ideal size for Jool moon landers

[AlpacaMall]

I'm heading to the Jool system for the second time soon, and just for fun, I want to see if it's possible to pull off a landing on all five moons in one go. I plan to bring two landers docked to a mothership, hopefully letting each kerbal in my five-person kerbonaut team claim their own moon. 

Without spoiling optimal designs, what kind of mass should I be looking at for my landers? I've currently got a parachute-descent Laythe lander down to 6.2t (4.2 dry), with a payload of just the command pod and smaller science instruments; I've also made a two-stage Tylo lander for 16.8t (5.4 dry), leaving a Science Jr. on Tylo's surface and bringing the command pod back up. I've added some landing struts on the Tylo lander's upper stage, and I'm planning on reusing it for Vall, Bop, and Pol (3.6t, 1.6 dry). Do these seem like reasonable masses for the respective landers?

(Of course, everything but the Laythe lander was designed purely through dV and TWR values, and it's more than likely that some of them will fail. But hey, that's part of the fun!)

[ManEatingApe]

For the Tylo lander, have you considered a command chair instead of a pod? The mass savings would be considerable.
A flags and footprints only Tylo lander can be in the 2 to 2.5 ton range (budget some more than this for the Science Jr)

Your plan to re-use the ascent stage of the Tylo lander for Val, Pol and Bop is a good idea and a great way to save some parts and mass.

Edited April 5, 2021 by ManEatingApe

[camacju]

Another option would be to make a Laythe lander that can do Laythe in a single stage, and reuse that for Tylo.

Alternatively if you want to use nuclear engines, then having a nuclear engine and a bunch of Mk0 fuel tanks can land on Tylo and return to orbit on one stage.

Edited April 5, 2021 by camacju

[king of nowhere]

8 hours ago, AlpacaMall said:

I'm heading to the Jool system for the second time soon, and just for fun, I want to see if it's possible to pull off a landing on all five moons in one go.

what you are going to try is called a Jool 5, and it's one of the most iconic challenges in the game. The link goes to the related forum thread, there are dozens of submissions from various people. you can look at all kinds of designs if you are facing a creative block.

[Empiro]

The main considerations for the mission are Tylo and Laythe. The other moons are all quite easy to land on and take off from. The typical design would be (at least) a 2-stage lander for Tylo, and then reusing the upper stage for the other moons, all while refueling from your main craft.

For Laythe, the most challenging aspect is landing properly. Laythe has so much ocean that you'll almost certainly be touching down in the water unless you use a space plane design. If you're using the same lander as for Tylo, you'll need to pack parachutes for Laythe, but want those to be detachable, as you don't want to waste mass carrying anything extra for Tylo.

[AlpacaMall]

Thanks for the responses!

On 4/5/2021 at 3:33 AM, ManEatingApe said:

For the Tylo lander, have you considered a command chair instead of a pod? The mass savings would be considerable.
A flags and footprints only Tylo lander can be in the 2 to 2.5 ton range (budget some more than this for the Science Jr)

I... kinda took this in the wrong direction. I'm using both a pod and a Science Jr on the landers now. 

On 4/5/2021 at 2:34 PM, Empiro said:

For Laythe, the most challenging aspect is landing properly. Laythe has so much ocean that you'll almost certainly be touching down in the water unless you use a space plane design.

I'm using the Trajectories mod, which is usually fairly accurate on Kerbin. Would I be right in assuming that this would help me find land?
(My Laythe lander is very top-heavy; testing it in Kerbin's oceans did not go well)

On 4/5/2021 at 7:58 AM, king of nowhere said:

what you are going to try is called a Jool 5, and it's one of the most iconic challenges in the game. The link goes to the related forum thread, there are dozens of submissions from various people. you can look at all kinds of designs if you are facing a creative block.

…this is going to be harder than it looks, isn't it
I see you're one of the people who's done it. Another quick question: how much dV would it take to bounce around Jool's moons? I'm trying to design my landers without too much reliance on established designs, and the traditional dV map isn't very helpful for this.

On 4/5/2021 at 7:43 AM, camacju said:

Another option would be to make a Laythe lander that can do Laythe in a single stage, and reuse that for Tylo.

I'm using a staged jet engine for the Laythe ascent, which I think should save some mass; this makes it pointless to try to reuse on Tylo, though. 

Edited April 7, 2021 by AlpacaMall
removed whitespace

[Empiro]

I haven't used the Trajectories mod, but I think it works pretty well. There is still a bit of risk that you land on a slope, which is also bad if you have a top-heavy rocket, but I don't think Laythe has too many of those. Using jet engines would work well on Laythe, but if you go that route, I'd recommend going for a space plane instead. Since you're bringing somethine separate just for Laythe, space planes are just way more fun, and will help you find a good landing spot. Anything that works well for Kerbin will work well on Laythe (and give you some extra room for error). Just make sure that you're able to land with a full tank!

The Jool 5 is both harder and easier than it looks. It's very easy to overplan and overestimate the delta-V requirements. It leads to a complicated launch and assembly since your craft is so large, but once you're at Jool, you'll find everything really easy since you have so much leeway.

The other way is to go with a smaller craft, but use complicated and efficient planning. It's quite cheap to transfer between Jool's moons if you're patient and wait for the right times.

There exist delta-V maps for Jool moon transfers (https://i.imgur.com/skAIz90.png). Though that's a bit outdated for Laythe, the other numbers are still basically correct. Note how cheap it is to go from a moon's escape to the transfer orbit of another moon.

You can also bring mining equipment, but I'm a bit of a purist, and like to do it without ISRU.

Some general advice for the planning:

• Use Laythe or Tylo to reverse-gravity assist to capture and lower your orbit at Jool.
• Avoid circularizing in low orbit around stuff as much as possible. Instead, leave your main ship in an eccentric orbit with a very high AP and very low PE. This way, if your burn at PE, you'll escape the moon with minimal delta-V. You will need to wait for the moon to be in the right position so that your escape trajectory will be pointing prograde or retrograde along the moon's orbit, however, which is why patience is important.
• Note that your lander will need more delta-V to land from an eccentric orbit. This may be a problem for somewhere like Tylo. However, even in this case, you could do something like this: have an extra tank of fuel attached to the Tylo lander when you detach from the mothership. Circularize in low orbit around Tylo. Top off your lander while still leaving a bit left in the tank you brought with you. Then, detach from the tank and land on Tylo, do your science, and take off. Rendevous with the fuel tank in low orbit, which has just enough to allow your lander to reach the mothership's eccentric orbit. Essentially your goal is to move as little mass as possible at all times leaving fuel in places you will backtrack to. It's kind of like folks who go on long hikes and drop full water bottles as they are going one way, and pick them back up to drink on their way back.
• When coming back, it's best to drop your PE down around Jool if you're currently around the tiny outer moons.
• Note that you will be entering Kerbin FAST. I'm not sure aerobraking is an option even if you have the inflatable heat shield (never tried it lately).

[king of nowhere]

9 hours ago, AlpacaMall said:

…this is going to be harder than it looks, isn't it

I see you're one of the people who's done it. Another quick question: how much dV would it take to bounce around Jool's moons? I'm trying to design my landers without too much reliance on established designs, and the traditional dV map isn't very helpful for this.

 

depends, really. if you are an experienced player, and you are mining your own fuel, then a jool 5 is nothing special. land on each moon, restock on fuel, move on. it becomes difficult if you add objectives.

if you want to do it without carrying any mining equipment, only with the starting fuel, then it becomes harder, because you have to carry a lot more.

as a rule of thumb for deltaV:

- LKO to jool, 2000 m/s. You can save some with gravity assists, but it always results in convoluted trajectories and very long travel time.

- jool intercept: free. you can get captured in the jool system by a gravity assist from either Laythe or Tylo, and it's easy and doesn't cost anything.

- getting captured by any joolian moon: roughly 200 m/s of intercept speed. I suggest you start with Tylo: you need a big heavy lander for it (unless you take the minimalistic approach, but i prefer to use actual spacecrafts), if you start at tylo youi don't have to drag the heavy lander through the rest of the system

- tylo: 800 m/s from capture to circularization. To leave tylo, you need the same 800 m/s, but after you leave tylo's SoI you can reach any other moon with no extra cost. So, 800 to get in, 800 to get out

- Laythe: if you perform correctly the burn from tylo, you can have a negligible insertion deltaV. Laythe's atmosphere is not good for aerobraking, but you can aerobrake gradually, so you can reach a circular laythe orbit almost for free. leaving laythe is 600 m/s, again once you're out of the SoI you can reach vall for no extra cost

- Vall: roughly 400 m/s for capture and circularization.

- Vall-Bop: roughly 1000 m/s

Bop-pol: roughly 1000 more m/s

 

so, to sum it up:

- 2000 from kerbin to jool

- 1000 from jool intercept to low tylo orbit (drop tylo lander)

- 800 from low tylo to low laythe (drop laythe lander)

- 1000 from low laythe to low vall (drop lander)

- 1000 from low vall to low bop (drop lander)

- 1000 from low bop to low pol (drop lander)

- 1000/1500 to return from pol to kerbin

of course, you don't have to orbit all of those bodies with the mothership.

Edited April 7, 2021 by king of nowhere

[ManEatingApe]

11 hours ago, AlpacaMall said:

I... kinda took this in the wrong direction. I'm using both a pod and a Science Jr on the landers now. 

The only wrong direction is if you capture into a retrograde orbit of Jool! (have done this by accident)

Seriously though, the great thing about KSP is that there are always many ways to accomplish things. Want a pod..use it! Want all the science..do it! Want to use nukes...go for it! You can optimize in any of several areas e.g low mass, low part count, low risk, low mission complexity, low requirements in real time. Then you can always go back and try the same mission in a different way just for kicks.

It's not strictly related to your question, but it's also fun to toss a probe into a one-way trip into Jool's atmosphere and let it get crushed. You can transmit some extra science and the added mass requirements are not too high, if you let aero-braking do most of the work.