Multipart ship engines

[ruiluth]

Let's say I have a 35 ton craft in orbit with no fuel or engines, but a big docking port on the back to attach engines to.

I want to take it to Jool and return home.

I have no refueling infrastructure in place, so I'm going to need at least 10 km/s for the trip.

...How does one even design a ship like that? Even with Kerbal Atomics' Liberator motor and USI's very dense hydrogen kontainer tanks, the engine modules I'm coming up with are enormous. I need a whole nuclear reactor just to cool the cryogenic tanks. I was planning on building larger ships in the future, maybe up to 120 tons, but if this is any indication, I won't be able to build engine modules big enough to support them.

How would you all solve this challenge? 10 km/s with a 35t payload. Any mods allowed, although everything needs to launch from the launchpad with mid-game tech without breaking the bank.

EDIT: I've managed to build a ship that meets the requirements, the KSS Dauntless:

Spoiler

I've increased the size of the payload to about 50t, mostly in fertilizer for the greenhouses and crew pods for habitation. The nuclear stage has side drop tanks and 13.5 km/s of dV. Her main design objective is to be able to reach Jool, Sarnus, or Urlum and be able to return home to report on the best sites to set up ISRU bases so that I don't need any more ships like this. The NTR on the bottom is the LV-N-GE "Liberator" from Kerbal Atomics, providing 820 kN of thrust at 1625s Isp. She ways 360t when fully fueled with LH2, giving a starting TWR of 0.23 which will rise to 0.63 when she's finished. Mission Control is still deciding her first mission profile but it's likely that she will make a few practice runs to the Mun and Minmus with a small lander and then Jeb, Bill, Bob, and Valentina will take her out to Jool or Sarnus on a voyage of discovery.

 

Edited July 31, 2016 by ruiluth

[SinBad]

If you have some of Roverdudes stuff, check out Karbonite and K+. A karborundum torch drive with a full tank will get your 35 tons there and back again with a few dozen km/s spare. Latest release makes solar harvesting of karborundum far easier than before, no more chromosphere surfing!

[ruiluth]

1 minute ago, SinBad said:

If you have some of Roverdudes stuff, check out Karbonite and K+. A karborundum torch drive with a full tank will get your 35 tons there and back again with a few dozen km/s spare. Latest release makes solar harvesting of karborundum far easier than before, no more chromosphere surfing!

I had forgotten about that mod... Unfortunately I don't have the whole tech tree unlocked yet and I don't have any ISRU infrastructure yet. I'll update the OP with a few caveats...

[lugge]

32 minutes ago, SinBad said:

If you have some of Roverdudes stuff, check out Karbonite and K+. A karborundum torch drive with a full tank will get your 35 tons there and back again with a few dozen km/s spare. Latest release makes solar harvesting of karborundum far easier than before, no more chromosphere surfing!

I never understood why one would use (obvisiously overpowered) engine mods to move things one cannot move in stock. Really, wheres the difference to just hyperedit your craft to Jool?

But, yeah, sure, it's singleplayer and sandbox, use whatever mod that fits your style ;-)

[evileye.x]

I would suggest nerva core + strap on side tanks. You can launch each tank separately.

Like this 

 

 

[SinBad]

In that case, check out near future propulsion for a few variants of high isp electric drives. bigger ions, resistojets, etc. some have decent thrust.

as far as op engine mods go, the k+ torch drive isn't an easy solution. its at the end of the tech tree, needs a rare resource that can only be gathered at the top and bottom of the sun's gravity well, as well as rock, as well as lots of ec. and its heavy. and its very expensive to build and fuel. but it is cool to do a duna transfer by burning at duna for half the trip, then flip over and burn retrograde to slow down again. 1g ftw!

[magnemoe]

5 minutes ago, lugge said:

I never understood why one would use (obvisiously overpowered) engine mods to move things one cannot move in stock. Really, wheres the difference to just hyperedit your craft to Jool?

But, yeah, sure, it's singleplayer and sandbox, use whatever mod that fits your style ;-)

Why not add isru on an 35 ton base, now you need 3.5 km/s to get to Pol or Bop and can refuel. 

[ruiluth]

8 minutes ago, lugge said:

I never understood why one would use (obvisiously overpowered) engine mods to move things one cannot move in stock. Really, wheres the difference to just hyperedit your craft to Jool?

The difference between hyperedit and advanced engines is that hyperedit has no explanation. Fancy engines at least have some sort of justification behind them, and usually are only available after a lot of effort. They're a reward for all the science you collect. For example, the only reason I'm able to consider nuclear power at all is because of the fleet of probes I've already sent throughout the solar system. Hyperedit is just cheating.

2 minutes ago, evileye.x said:

I would suggest nerva core + strap on side tanks. You can launch each tank separately.

That may be what I have to do, although the goal with this ship originally was to make it single stage and completely reusable. Launching new fuel tanks for each mission almost defeats the point

1 minute ago, SinBad said:

In that case, check out near future propulsion for a few variants of high isp electric drives. bigger ions, resistojets, etc. some have decent thrust.

as far as op engine mods go, the k+ torch drive isn't an easy solution. its at the end of the tech tree, needs a rare resource that can only be gathered at the top and bottom of the sun's gravity well, as well as rock, as well as lots of ec. and its heavy. and its very expensive to build and fuel. but it is cool to do a duna transfer by burning at duna for half the trip, then flip over and burn retrograde to slow down again. 1g ftw!

NFP is on my radar, but unfortunately the electric thrusters are prohibitively small and power-hungry. I had considered them and even built a few ships to test them, but since my fleet of interplanetary probes with TWR <0.2 I've decided that all my ships henceforth with have more thrust. And it seems that NTRs scale up better than plasma jets, the biggest MPDT requires a very very large reactor to run it, so much that the added mass totally offsets the better thrust.

1 minute ago, magnemoe said:

Why not add isru on an 35 ton base, now you need 3.5 km/s to get to Pol or Bop and can refuel. 

I could do that, although it would completely change the mission profile and require a ground-up redesign. The ISRU equipment would likely double the mass of the payload, as well.

[purpleivan]

I use a pretty standard nuke powered tug to haul my vehicles around the system. My first step was to use these to put in place mining and orbital storage of fuel at Minmus and Ike.

Station component in flight to Duna

 

Tug at Ike

 

Mining base that was delivered to Ike using the tug

 

Fuel storage section headed to Minmus

 

I even used a couple of modified versions of these to push my Eve return vehicle there and used one of them as the return vehicle.

 

 

 

[Streetwind]

1 hour ago, ruiluth said:

I want to take it to Jool and return home.

I have no refueling infrastructure in place, so I'm going to need at least 10 km/s for the trip.

Why do you need 10 km/s for a Jool trip and return? You're looking at maybe 3000 m/s for each leg of the trip.

And why are you hauling a 35 ton payload? I mean normally I'd say "oh sure, he has a Tylo compactible lander on there, among other things", but you specified no fuel and no engines. And since you want to return the whole thing, it's not a space station or asteroid either. How mysterious!

 

As for the actual question though - if you want that much dV out of a non-electric spacecraft, then you either need to stage at least once, which you say you don't want, or slowboat with nuclear rockets. Because even those are going to scratch at their limits.

The absolute edge of what I personally consider sane for a rocket stage in KSP is a mass fraction of 4, since by the time you're at that point, you're getting serious diminishing returns that only ramp up even harder from there on out. Let's add a senior docking port and four LV-N's, for 12.2 tons, to your existing 35 ton dry mass. A mass fraction of 4 would require you to bring 4 times as much mass in fuel as the entire dry mass of the spacecraft (now at 47.2 tons) plus the dry mass of the tanks you're going to add. According to the wiki - I don't know if it's up to date - the mk3 liquid fuel fuselage long carries 50 tons of fuel in a 7.14 ton tank mass envelope.

If you stack five of those together - yes, five of the longest mk3 LF fuselages - you will end up with 250 tons of liquid fuel, compared to a combined dry mass of 82.9 tons. That results in a mass fraction of 4.0157. With the Isp of your nuclear engines, that gives you 10,907 m/s worth of dV. Let's round to 10.9 km/s since you'll need four tiny octagonal struts to tape the engines to the tanks anyway.

However, you're now trying to push a 333-ton vessel with 240 kN of thrust. A TWR of 0.073 is ultimately doable, but is it pleasant for you to fly? More engines will require a sixth fuel fuselage to maintain the mass fraction, and launching (and docking!) five of them at once is a herculean task already. You probably don't want to make it any more difficult than that. Without adding more fuel, adding four more LV-N's will reduce your dV to 10.12 km/s, and up your TWR to 0.142. Another four (for a total of 12), and you'll be at 9.45 km/s, 0.206 TWR.

The only way to get around needing that high of a mass fraction would be electric propulsion, but stock KSP doesn't really supply you with the tools to do this effectively for 35-ton payloads in the outer solar system.

Edited July 28, 2016 by Streetwind

[Foxster]

This stock craft will do it...

The top section above the docking port weighs a little over the required 35t payload and with the engines and fuel attached it has a bit over 10kdV. Initial TWR is 0.1 so the burns will be long but are do-able. 

This isn't really the answer though, the answer is not to ship a single 35t payload to Jool and back. 

Just to show it is is stock-do-able, here's this craft after Laythe gravity-braking into a Jool orbit...

 

...and back to Kerbin with loads of dV spare...

Edited July 28, 2016 by Foxster

[ruiluth]

12 minutes ago, Streetwind said:

Why do you need 10 km/s for a Jool trip and return? You're looking at maybe 3000 m/s for each leg of the trip.

Good point, I checked a delta-v map and realized that was more than I needed. Although more never hurts

13 minutes ago, Streetwind said:

And why are you hauling a 35 ton payload? I mean normally I'd say "oh sure, he has a Tylo compactible lander on there, among other things", but you specified no fuel and no engines. And since you want to return the whole thing, it's not a space station or asteroid either. How mysterious!

Good question-- the answer is that I'm playing with USI-LS, and I wanted to have a ship that would house and feed 4 kerbals for the duration of the trip, to explore good areas to build a more self-sufficient base. So a most of the mass is in crew cabins and fertilizer to run the greenhouses.

(Also just because I built a ship and I think it's awesome and I want to take it everywhere )

The ship:

Spoiler

Supplies will actually last much longer, since the window does not factor in the fertilizer to be used in the greenhouse.

I actually managed to build a drive section that seems to be up to the task using USI kontainers, which hold a lot more LH2 than Nertea's cryo tanks and come in larger sizes. The TWR is bit low and it's still pretty big but it does what I asked it to do. The other main reason I wanted to overbuild like this is so that in the future I can haul large station or base modules without having to launch more tugs. I think I'm going to have to set up a fueling station on Minmus though, because this is a LOT of LH2...

Spoiler

Thank you all for your advice, I think I've realized that this is more than most people ever try to do, and it's pushing the limits of the physics of KSP, even with mods. I probably won't do this ever again... although that depends on how much fun this ship proves to be once I get it to orbit!

[SinBad]

Foxster has a point, drop the tanks you dont need, twr increases as you burn fuel and drop tank mass. Simplest way to arrange it is as a pendelum rocket as you can drop one tank at a time and shed the weight sooner than the pairs that radial tanks would require. Maybe add a single seperatron to each tank to ensure its vector diverges from the main ship though. You dont want the last one you dropped to catch up the first time you burn retrograde.

[Wjolcz]

10km/s is a slight overkill. You can go to Jool and back having about 4km/s. Anyway, here's how I did it a long time ago:

http://imgur.com/a/romH6#0

http://imgur.com/a/utisk#0

http://imgur.com/a/0POu5#0

http://imgur.com/a/XpWjG#0

Keep'em modular and easy to rearrange.

I can't seem to include imgur albums into the post for some reason. Why is that?

Edited July 28, 2016 by Veeltch

[Blaarkies]

1 hour ago, ruiluth said:

Hyperedit is just cheating.

"How dare you call that cheating?! It is a playstyle, in a single-player sandbox game"... I am glad you didn't received any of those comments, but I just had to do it now 

[ruiluth]

21 minutes ago, Blaarkies said:

"How dare you call that cheating?! It is a playstyle, in a single-player sandbox game"... I am glad you didn't received any of those comments, but I just had to do it now 

Hey, I'm not against cheating my any means. I cheat regularly in almost every game I play

Although, in KSP i only use hyperedit for testing. I find this game so well balanced that there's nothing I feel compelled to "fix" through cheats

[Blaarkies]

6 minutes ago, ruiluth said:

Hey, I'm not against cheating my any means. I cheat regularly in almost every game I play

Although, in KSP i only use hyperedit for testing. I find this game so well balanced that there's nothing I feel compelled to "fix" through cheats

Except the wheels...Yes i don't hyperedit, I invoke affirmative action as the victim of a bug!

Spent multiple missions getting a special lander to Moho -> Go in for perfect smooth landing..."cannot deploy while stowed".
Switch back to the massive mining rig on Minmus -> Rig believes it is a Kangaroo, tumbles down the slopes, ripping into() 5 million pieces.
Come in for landing with a spaceplane after a perfect cargo mission -> Front wheels gets a hickup and knock you on-target towards the SPH fuel depot!

Stuff like that happens, and HE is a beautiful tool(need some quicksave periodically as well). I get what @lugge was saying, where do you draw the line(is it wrong if I mod the "Spark" to have mammoth thrust and ion Isp?)...i guess, since Karbonite, NFP and those stuff are big mods  then "5 million fans can't be wrong" comes to mind. Even if it is the same HE results, it was fun doing it...with microwaves and stuff(sorry, I am old traditional...I will never let go of my Reliant engines!) 

[monophonic]

2 hours ago, ruiluth said:

That may be what I have to do, although the goal with this ship originally was to make it single stage and completely reusable. Launching new fuel tanks for each mission almost defeats the point

Since you don't have ISRU yet, you have to launch all the fuel you refuel with. Using single stage tug means you have launch, dock, transfer fuel and undock. Using drop tanks means you launch, dock and you're done! Remember, you dropped the tanks as they dried up during the journey.

When you do get ISRU in place, you can still refuel the drop tanks through any free docking port. Of course only if you don't drop them in interplanetary space, which means you lose dV hauling about those empty tanks. Then again when you have permanent bases and can refuel at Jool you don't need as much dV either. Or you can keep dropping those tanks and bring much bigger loads to build your bases with.

[GoSlash27]

1 hour ago, ruiluth said:

Good point, I checked a delta-v map and realized that was more than I needed. Although more never hurts

As you can see by your difficulties here, more actually does hurt. Unneeded DV is dead weight, and dead weight is the difference between a straightforward problem and an impossible one.

 Had you designed this mission with the bare minimum of equipment and the exact required DV (plus a safety margin), it would've been a lot easier to accomplish.

Best,
-Slashy

[Streetwind]

2 hours ago, ruiluth said:

I actually managed to build a drive section that seems to be up to the task using USI kontainers, which hold a lot more LH2 than Nertea's cryo tanks and come in larger sizes.

Woops, looks like I completely missed the line where you said you used LH2 for nuclear engines, didn't I? That of course changes things.

Still, the math with the mass fractions remains fundamentally the same. In particular, keep in mind that "holds a lot more LH2" is to be always taken relative to the tank's dry mass. If it holds twice as much but the dry mass is twice as high, you're not gaining anything except a slightly smaller part count.

[ruiluth]

12 hours ago, Blaarkies said:

Except the wheels...Yes i don't hyperedit, I invoke affirmative action as the victim of a bug!

Yes exactly ^^

12 hours ago, monophonic said:

Since you don't have ISRU yet, you have to launch all the fuel you refuel with. Using single stage tug means you have launch, dock, transfer fuel and undock. Using drop tanks means you launch, dock and you're done! Remember, you dropped the tanks as they dried up during the journey.

When you do get ISRU in place, you can still refuel the drop tanks through any free docking port. Of course only if you don't drop them in interplanetary space, which means you lose dV hauling about those empty tanks. Then again when you have permanent bases and can refuel at Jool you don't need as much dV either. Or you can keep dropping those tanks and bring much bigger loads to build your bases with.

That is true, and I had actually constructed a few designs that used staging to achieve more dV. My main problem with them is that they were too big and ugly

12 hours ago, GoSlash27 said:

As you can see by your difficulties here, more actually does hurt. Unneeded DV is dead weight, and dead weight is the difference between a straightforward problem and an impossible one.

 Had you designed this mission with the bare minimum of equipment and the exact required DV (plus a safety margin), it would've been a lot easier to accomplish.

Best,
-Slashy

That would be accurate, if the mission were a simple one. However, the design goal for this ship was to create a powerful craft that would be able to complete multiple missions without needing to refuel while I get all my base infrastructure in place. The plan is to have it refuel only at Kerbin and carry enough fuel to be able to function anywhere in the solar system and still return home to refuel. Additionally, extra fuel means extra maximum payload in the future when I get my bases operational.

12 hours ago, Streetwind said:

Woops, looks like I completely missed the line where you said you used LH2 for nuclear engines, didn't I? That of course changes things.

Still, the math with the mass fractions remains fundamentally the same. In particular, keep in mind that "holds a lot more LH2" is to be always taken relative to the tank's dry mass. If it holds twice as much but the dry mass is twice as high, you're not gaining anything except a slightly smaller part count.

True, but the main problem I was having with the LH2 tanks was mainly that they were too bulky and not big enough. I was going to need something like 10 3.75m tanks to be able to carry all the hydrogen, and it was frustrating to not be able to just have one 6.25m tank or something of the like. USI's 5m tank was exactly what I needed to compress all those tanks into a smaller, more manageable space.

Edited July 28, 2016 by ruiluth

[SelectHalfling0]

Rather than one giant ship, I send a bunch of smaller vessels, each with a different part of the mission. It tends to be less expensive, and I often refuel and re-use the engines.

[ruiluth]

1 hour ago, SelectHalfling0 said:

Rather than one giant ship, I send a bunch of smaller vessels, each with a different part of the mission. It tends to be less expensive, and I often refuel and re-use the engines.

That would work, if the life support equipment wasn't so heavy. The 35 tons is a long range ship with 5 years of life support.

[SelectHalfling0]

11 minutes ago, ruiluth said:

That would work, if the life support equipment wasn't so heavy. The 35 tons is a long range ship with 5 years of life support.

Does it all have to be on the ship? Could you possibly send some with little cargo ships that would re-supply mid-mission. If you have a base, send little cargo delivery units to the base.

[ruiluth]

4 hours ago, SelectHalfling0 said:

Does it all have to be on the ship? Could you possibly send some with little cargo ships that would re-supply mid-mission. If you have a base, send little cargo delivery units to the base.

Unfortunately yes, it does. This mission is the scouting mission to send the first kerbals to explore the Jool system for future colonization. If I sent other ships it'd only add complexity by requiring docking all the way out there. Also, a lot of the weight is in crew space which can't really be detached. In the future though, I absolutely will be setting up resupply stations