How to make a round-trip, HEAVY reusable interplanetary rocket?

[Sudslv]

so i spent a bit of time calculating and experimenting.

i ended up building a massive interstellar ship which gives me 7.2k dV and probably someone like scott manley could make Eeloo land and return 100% reusable.

to assemble it in LKO it requires 7 launches (can be less). i would never suggest to do single mission for such a massive payload. split it up 3-4x.

P.S. there is a dV limit for all engines in ksp!

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The big 2 front fuel tanks weight 124T

Edited July 27, 2014 by Sudslv

[Claw]

There are a lot of good tips in here. I'm surprised it hasn't been hinted at until the last post.

You can use the rocket dV equation to calculate the maximum theoretical maximum dV that an engine (in particular the ISP) will provide. Because the fuel tanks max out at a mass ratio of 9:1 (liquid fuel, anyway), that's the highest mass ratio you can use in the dV equation.

dV = 9.81 * ISP * ln (9/1)

I only bring this up because if you are still aiming for 10,000 dV and a completely recoverable ship, the only stock engines capable of that are LV-Ns and IONs. You will either have to use those engines, accept lower dV requirements, or drop parts along the way. You can also send refueling tankers, but I don't know if that violates the "bring it all home."

Cheers,

-Claw

[Jodo42]

-nevermind-

Edited July 28, 2014 by Jodo42

[Red Iron Crown]

10,000m/s for a 120t payload is going to take a lot of fuel. Let's see how much (Warning! Math inbound!):

The theoretical maximum from a single stage chemical rocket in stock is 8,415m/s, So 10,000 m/s of dV in a single stage means nuclear or ion engines. Using ions will mean a near astronomical part count, as it would require a lot of tiny tanks to give enough propellant mass, and an abysmal TWR as well. So nukes are the only practical option.

Let's run the rocket equation on the problem, assuming a 120t payload, one LV-N for every 30t of mass (Kerbin TWR=~0.2, lowish but usable), and either 1.25m or 2.5m tanks for their better mass ratio:

dV= Isp * g * ln((Payload+EngineMass+WetTankMass)/(Payload+EngineMass+DryTankMass))

10000 = 800 * 9.82 * ln((120+0.075(120+n)+n)/(120+0.075(120+n)+n/9))

~808 = n (Thanks, Wolfram Alpha!)

So you would need 808t of fuel tanks (about 22.5 orange tanks) and 31 LV-Ns to achieve 10,000m/s of dV in a single stage with a TWR of about 0.2.

If you build this you must share pics.

[MerlinsMaster]

It's probably not what you want to hear, but I think the best advice is to reconsider your design strategy. It doesn't need to be totally reusable (rockets and be staged, and parts can be left on Eeloo), it doesn't need to be all sent together, and there are engines you're choosing not to use.

Well, total reuseability is my primary goal with this particular mission. I chose Eeloo because it's one of the harder places to get to. And if it's true that I can do it with 6000 delta v, then I think I can do it.

But I'm not ruling out any types of engines. Which ones were you thinking of?

Breaking up the payload could help, instead of sending 120t all at once. I usually send mission payloads for interplanetary missions in parts, e.g. for my last Duna mission, I sent the surface base & rover on one rocket, a station, return fuel, and lander on a second rocket, and then crew and science on a third, modular vessel (parts went up on separate rockets). Breaking the payload up into sections makes design and flying the mission much easier than a monolithic payload.

Actually, and I see how this will seem ridiculous, but I'm already sending stuff (larger rovers and hab modules) like that ahead of the manned ship. The landers and rovers on the ship are primarily in case that other stuff didn't make it. It's a big mission, but I 'm finding it to be a fun challenge. If it turns out I can't do it, that's fine. But I'm going to give it the old college try first.

Staging and drop tanks can also increase your dV for a similar payload fraction.

This is going to sound like a dumb question, but what are drop tanks?

[MerlinsMaster]

I'd definitely consider at least a few drop tanks if you're having that much trouble ... toss a controller on, a few solar panels, and a couple of separatrons, drop the first few tanks before you hit Kerbin SoI and just deorbit them. Enjoy the "free" fuel, that should get you around 800 extra delta-v (more if you're willing to clutter things up)

That's not a bad idea. I'll keep that in mind.

[MerlinsMaster]

What you're looking for is delta-v. The amount of delta-v to Eeloo remains constant for your given Hohmann transfer; you need as much to get a 20 ton payload to Eeloo as you do for a 120 ton one. It's just much harder to get that delta-v with the 120 ton payload, because you're pushing a lot more. But there's actually a way to reduce the overall amount of delta-v needed to get places. You've stated you basically want to do a simple Hohmann transfer with a mid-course correction, but for something that size, you may want to look into gravity assists. That's a big-bad phrase around here, even for some of the more experienced users, but they're honestly not all that difficult. Allmhuran has a

explaining how they work, but getting them to line up in space and time can be tricky. There's a few tools out there to help you get timing right, like this one or this one that's specifically oriented towards finding flyby series. Again, these programs look daunting, but they're relatively easy to read and use once you've looked them over. On the topic of cutting delta-v, as others have mentioned, aerocapture at Kerbin on the return trip is going to save you a lot and won't take hardly any delta-v from Eeloo's altitude.

I'll definitely check those out. Even on the off chance I don't end up needing them for this, they will really come in handy when I try to do a grand tour mission.

You say that 120 tons is set in stone. Is it? You'd likely be able to cut a significant amount of delta-v off the return trip if you ditch those landers and rovers. Why would you want to bring the rovers back anyways? Besides, Eeloo landers need to be little more than Mün landers, which should be cheaply and easily replaceable once you've returned to Kerbin's SOI. You might also consider ditching the command section once you're ready to return, if you know you have enough delta-v (I'll get to how to know that now).

Those landers and rovers are going to be left there, but they're not a huge portion of the payload. As for the command section, that would be basically breaking the ship apart. I might as well have stages if I do that.

Getting 120 tons to Eeloo, and back, would be a significant challenge even for some of the best players. Take a look at the staging article on the KSP wiki if you're not sure how serial/parallel/asparagus staging works. Cut your ship design down as much as you can. Something that might be very helpful for you would be Kerbal Engineering Redux, a mod that tells you exactly how much delta-v you have to start out with in the VAB, and gives you a bunch more information in flight, including your sea-level and ground-reference altitude to a body, remaining delta-v in your rocket, remaining burn time, and much more. Mechjeb does something similar, but also has some nice autopilot features for your rovers and the transfer burns to and from Eeloo. If you decide to go with a basic Hohmann transfer, Olexander Savchuk has a wonderful site that gives you a graphical representation of burn positions, http://ksp.olex.biz/. Alternatively, Alex Moon has a more number-y site that'll tell you an exact time and date that you should leave on to get the best possible transfer, as well as letting you pick earlier or later launch dates and slower or faster transfers.

Oh, trust me, I don't go anywhere without KER and Mechjeb. I don't know what I'd do without them. As for olex.biz, I've had it bookmarked for a while now, but haven't had a chance to really get into it yet.

I wish you the best of luck in this very Kerbal venture. Consider posting your progress in a thread Mission Reports in an Imgur album, or make some videos and post them in the Live From Mission Control subforum. I'm sure there's many users, including myself, that would like to see this pulled off.

Welcome to the forums.

Thanks! The pointers and advice that everyone here has given me has got me pretty energized. I can't wait to start implementing them and posting the results afterwards.

[MerlinsMaster]

so i spent a bit of time calculating and experimenting.

i ended up building a massive interstellar ship which gives me 7.2k dV and probably someone like scott manley could make Eeloo land and return 100% reusable.

to assemble it in LKO it requires 7 launches (can be less). i would never suggest to do single mission for such a massive payload. split it up 3-4x.

P.S. there is a dV limit for all engines in ksp!

Wow! That's impressive! I'm amazed you got the delta v that high with so little fuel. I can't wait to try this out. Thanks!

[cantab]

This is going to sound like a dumb question, but what are drop tanks?

Fuel tanks that you ditch when they're empty. As opposed to staging proper when you ditch the engines as well. Obviously you don't benefit as much as if you drop the engines as well, but in career or for roleplaying keeping expensive engines and just ditching cheap fuel tanks might make sense. In real rocketry the most prominent example is the Space Shuttle's external tank.

[Laie]

Slightly off-topic. I think you can still shave a few tons off of your vessel.

(EDIT: damn, that's not your ship, but sudslv's -- I'm commenting on the wrong ship. Sorry.)

That's a lot of solar arrays you got there... On average, you'll need almost no electricity at all. I suggest you bring a lot of battery capacity (on the order of 4000 units) and only very few solar panels. If the trickle recharges the battery within one hour or three, that will be enough. Personally, I'd skip the arrays and take nuclear generators instead. Two will be plenty.

Also, what's the deal with those radially attached docking ports? If you ever want to re-attach the outer tanks and engines, getting the alignment just right will be a PITA, and I'm not 100% confident that the gimbals can handle it. I suggest double-docking with two standard ports and no girders in-between (the standard ports can easily take the load from a poodle). Also, girders tend to bend and wobble a lot, while tanks or the structural fuselage won't.

Next up, the choice of tanks. Rockomax-sized equipment has a slightly better payload fraction (88,8% of the mass are fuel, compared to Kerbodynes' 87,5%). Over your entire vessel, that amounts to ~10 tons.

I see no reaction wheels. How long does it take to turn that craft around when the engines aren't running?

Edited July 28, 2014 by Laie

[Foxster]

Here's a stock craft that will lift a 120T payload from Kerbin to a 100Km orbit: https://www.dropbox.com/s/jgww6graghcuf1l/Far%20lifter%204.craft

If you are careful with the lift or use MechJeb, there will be enough fuel left in the last lifter stage to get you started on the way to the outer planets, then throw away the last lifter stage.

It will have about 10K dV left by Kerbin orbit, you will "recover" dV when you throw away each of the lower tanks as you empty them during your burns.

[Skorpychan]

I second the drop tank idea; attach your engines radially, and stick fuel tanks onto the back with docking ports, that you can leave in orbit around Eeloo.

It's still technically reusable, should you go back.

[Sudslv]

it was a very fast and simple build and i think i will redo it and create an actual working 120+ t eeloo orbit and return however i dont plan to go 100% reusable on that, thats overkill for me. it was a 100% vanilla build so i wont use these "nuclear generators", the solar panels was just for show and a bit for making it 125~ tons.

As well the thread seems to be [Answered], the OP splitted his ship or went with something like i have displayed. i think i could have increased dV by swapping engines for aerospike, but that would increase burn time.

[Laie]

it was a 100% vanilla build so i wont use these "nuclear generators",

They are stock parts: http://wiki.kerbalspaceprogram.com/wiki/PB-NUK_Radioisotope_Thermoelectric_Generator

Usually I just call them "nuclear batteries" but tried to stick with their proper name in my post. I've grown rather fond of these things, one or two of them plus a large battery are all it takes to power even a huge mission. Low part count, and you can't forget to deploy or retract them.

[MerlinsMaster]

it was a very fast and simple build and i think i will redo it and create an actual working 120+ t eeloo orbit and return however i dont plan to go 100% reusable on that, thats overkill for me. it was a 100% vanilla build so i wont use these "nuclear generators", the solar panels was just for show and a bit for making it 125~ tons.

As well the thread seems to be [Answered], the OP splitted his ship or went with something like i have displayed. i think i could have increased dV by swapping engines for aerospike, but that would increase burn time.

Hey everybody,

I know it's been a few days, but I just wanted to let you know how it's going. Basically, what I did is I used Sudslv's approach as a starting point and modified it. It's true that connecting the tanks together radially with docking ports is not a good way to go, because if they're docked off angle by even a degree, the engines will be pointing in different directions. So instead, I docked all the engines to a docking assembly that makes up the front of the drive section. It turned out that I could also get ample delta with just five engines instead of nine.

While it didn't take me very long to build the entire thing in the VAB, I've spent all week launching the individual components and docking them together in orbit. Here is a pic just to give you a general idea, and I'll be posting an imgur album later on when I'm finished.

Right now there's a fuel tank where the command section is going to be. I'm in the middle of filling up all of those massive tanks. Once I do that, I will timewarp to the next available launch window for Eeloo, and fly the command section up with the crew. Once it's docked, they will set off for Eeloo.

[DasValdez]

Looks like you're already well on your way, but in the future consider a Kerbin system ski lift approach. I have a fleet of tugs can be used to boost ships up to Kerbin escape velocity, then detach and swing around for refueling and orbital parking. The tugs can be docked together to increase deltav for the payload... a quick test showed that three tugs could provide 120t with between 2400 dv @ 1.7 TWR and 4700 dv @ .7 TWR.

Good luck on your mission!

[MoonMan22]

Check out this vid... its exactly what your looking for.

[Vanamonde]

Oh my, I really need to get around to updating these designs, but they should still work in the current sandbox version of the game.

[MerlinsMaster]

Looks like you're already well on your way, but in the future consider a Kerbin system ski lift approach. I have a fleet of tugs can be used to boost ships up to Kerbin escape velocity, then detach and swing around for refueling and orbital parking. The tugs can be docked together to increase deltav for the payload... a quick test showed that three tugs could provide 120t with between 2400 dv @ 1.7 TWR and 4700 dv @ .7 TWR.

http://i.imgur.com/Nd8PxYnl.png

Good luck on your mission!

That's a pretty cool idea. I'm definitely going to try to implement that somewhere.

[MerlinsMaster]

Oh my, I really need to get around to updating these designs, but they should still work in the current sandbox version of the game.

You know, it was one of your other threads regarding this subject that made me want to do a reusable interplanetary rocket to begin with. I tried to reverse engineer one of them by examining the pictures, but didn't have a lot of luck. Now that I know that you've posted craft files, I will look at them and see how I can incorporate some of their aspects into my design.