Designing a Jool Ship

[Halfdan]

Hello fellow kerbonauts

i am currently struggeling to design a craft that is capable of getting kerbals and equipment into the joolian system but i cant seem to get it right

i started by expandig on the ships i use to carry personell and equipment to duna but i have somwhat reached a dead end, i added more fuel and more nuclear engines to the base design of my craft (15 in total), and yes they are capable of reaching jool orbit using aerobreaking if possible (after a 15 minute transfer burn but that was expected) but lack the fuel for real exploration there let alone the return trip

so since i am somewhat out of moves here, i really need your help

what stock ship designs do you use?
nuclear engines or LF/OX?
what are basic design guidelines for this kind of craft?

thanks in advance

[Spricigo]

It depends.  What your mission objectives are? How many Kerbals do you want to bring? Planning to land or just orbit/flying high? 

Without further detail it's hard to say.  

 

 

[James M]

Remember that while Nuclear Engines have low Thrust to Weight ratios, they're super efficient (As I'm sure you're well aware) and it's best to bring a small amount of them with lots of fuel to maximize efficiency. When it comes to how many to bring, it really depends on how much time you have or want to spend on mission.

[juanml82]

We need pics to check.

Just in case, are you using LF only tanks or are you using LFO tanks and removing the oxidizer? Nervs should be used with LF only tanks (stock: the spaceplane parts like the MK3 line. Mods: Interstellar fuel switch lets you completely fill regular rocket tanks with LF. There are also plenty of part mods).

What's your payload? As in, what are you carrying above the nukes and their fuel tanks?

[bewing]

If your standard rocket won't make it, you need to downsize. Fewer kerbals. Fewer luxury items. Fewer parts. Fewer engines. Longer burn times.

[Starman4308]

There are also some tricks for how to do the transfer. Split up long transfer burns into a set of periapsis kicks, use a Tylo slingshot to capture into Jool orbit followed by some more gravity pingpong to adjust your Jool orbit, etc. Without a screenshot or craft file, it's hard to be certain, but my initial suspicion is too much payload and engine, not enough fuel (and remember that the nukes use only fuel, no oxidizer).

[TopGull]

As far as general design guidelines, I recommend you search for the "reverse rocket equation". It's useful for designing any sort of rocket-powered craft, not just interplanetary ones.  GoSlash27 has posted some great threads explaining what it is and how to use it. 

[Ripper2900]

I suggest you go look in the Jool-5 thread in the Challenges forum. It's a long thread, so be wary that some designs from the early posts might be outdated. 

Personally, I've done it twice, both with non-nuclear rockets, so it's not impossible. One advice I'll give, is to make a very large and powerful booster for the initial kick out of the Kerbin system, and then detaches. It's easy to get it into orbit at Kerbin, and it will leave a lot of dv for your interplanetary craft, so you don't have to arrive at Jool with a half-empty hulk that is expensive and difficult to maneuver. 

[Halfdan]

here is a screenshot of the ship i tested, the payload bay holds a mobile processing lab and some sciency stuff

i use MK3 parts for liquid fuel, and those that hold oxidizer have been empited

it was based on my duna transfer vehicle:

so how about i stick to the engine block of nine NERVs and ditch the side bits but add aditional fuel to the central fuselage?

[Snark]

1 hour ago, Halfdan said:

so how about i stick to the engine block of nine NERVs and ditch the side bits but add aditional fuel to the central fuselage?

Drop lots of the NERVs.  They're dead weight, dropping them will add dV.  Just how much it adds depends on the numbers.  What's your total ship mass prior to departure from LKO?

1 hour ago, Halfdan said:

those that hold oxidizer have been empited

So you're using LFO tanks, but without O in them?  In that case, you're being wasteful-- you're using fuel tanks that weigh too much for what you're getting out of them.  Ditch the LFO tanks, and use the Mk3 LF-only tanks instead.  You'll save a bunch of weight on empty tank mass.

 

[Spricigo]

1 hour ago, Halfdan said:

so how about i stick to the engine block of nine NERVs and ditch the side bits but add aditional fuel to the central fuselage?

For a given engine type there is only two ways to get more deltaV, add more propellant or reduce anything else. Adding more propellant will make your long burns even longer, reducing engines will have the same effect. If you are willing to take more time in maneuvers maybe you should consider to do the mission with multiple smaller vehicles.

Reducing something else, well you need to decide what you can afford to lose. Do you really need space for 20 kerbals? 8t of monopropellant? 6 docking ports?

 

 

 

 

[Halfdan]

okay i did some redesign on the craft:

i scaled the drive section down to eight engines, and it carries 35305 units of liquid fuel, and added a transfer stage to boost it out of kerbins gravity well bevore switching to the nukes for propulsion

mech jeb states that the main craft has DeltaV of 8309m/s 

does that look sufficient to you guys?

 

things i could do to save further weight could be:

scaling down the antennas (purely cosmetic in this case)

remove all docking ports except one (wich i would not really like because it reduces capacity for carrying landers for the moons)

cout down on the passenger section (perhaps swithcing it to a hitchhiker container in a cargo bay witch would reduce the size of the expedition)

any other things i could do to save weight?

to further explain the mission architecture:

i would like to do this in a sort of a mars semi direct style, with a dedicated crew vessel that carries kerbonauts and several unmanned cargo ships that carry equipment like habs, rovers, landing craft and SSTO planes for laythe, and a tug craft to get from moon to moon

[Reusables]

8km/s? That's a lot of dv!

I recommend packing just enough dv for a trip including safety margin, as tyranny of rocket equation will likely bloat out your ship. I think it is the first to shave off as much as one can, rather than the dry mass.

You should be able to reach Jool in 2km/s, and get into orbit with a gravity assist on either Laythe, Tylo or even Vall.

Also it won't take anything to return to Kerbin with gravity assists, and 500m/s~1km/s is enough to slow down the craft enough for aerobreaking. So at most 3~4km/s is enough for a mothership.

Or are you going for Pol/Bop landing with the craft?

[Spricigo]

 

48 minutes ago, Halfdan said:

to further explain the mission architecture:

i would like to do this in a sort of a mars semi direct style, with a dedicated crew vessel that carries kerbonauts and several unmanned cargo ships that carry equipment like habs, rovers, landing craft and SSTO planes for laythe, and a tug craft to get from moon to moon

 

Ok, that is your payload. You need to have it figured out before designing (or at least refining the design of) the mothership. DeltaV of 8km/s without all those loaded means nothing.

[juanml82]

You have too much monoprop. Worse, monoprop tanks are heavier than LF tanks. Just one of those MK3 monoprop tanks should be enough (more than enough, actually). OTOH, you lack reaction wheels. Unless you burn monoprop, that ship should be taking a lot of time to turn.

You don't need radiators.

The MRS mod has a few parts that can help you fine tune your ship. I'm thinking about 2.5m nervs, which equal to 4 stock nervs and 3.75m reaction wheels. You may also want to add a small cargo bay (or a 1.25m service bay at the nose) with rtgs, science experiments and extra batteries.