Best nuclear engine setup for Kerbin - Mun/Minmus trips?

[OscarJade]

I've got orbital stations at Kerbin and its two moons, and a refueling infrastructure set up so that I can dock transports & landers, reusing them instead of always launching disposable rockets, which should save funds as I gather science, etc. My question: what is the best configuration of nuclear engine/fuel tank to move between Kerbin/Mun/Minmus? Or am I better off just sticking with the Poodle for this, and saving the nukes for interplanetary? Thanks

Edited June 5, 2015 by OscarJade

[kStrout]

Scott Manley made a video on the nuclear engine, put simply the nuke doesn't become superior to the LV-909 until around 5km/s of dv, don't know what it is for poodle, unless you're really patient and use 1 nuke per transfer ship the poodle is probably better.

[Warzouz]

I don't even use LVN for Duna/ike

[Randazzo]

I am making the following assumptions:

1. You are stating in LKO (~75-100km)
   
2. You want to complete a round trip in a single go
   

If you do want to do the whole circuit I'd suggest going to Minmus first, then transferring to the Mun, then back to Kerbin. The reason being that returning from Minmus has the potential to put you in a highly inclined orbit that could take a lot of dV to then match to your station. It is possible to intercept Minmus without changing your plane to match it, with a little bit of fiddling, and once there you can pull off pretty much any maneuver at a significantly reduced dV cost. At any rate, let's look at an approximate value for this round trip:

Kerbin - Minmus: ~920 (~1200 if you match planes first)

Minmus insertion: ~100

Rendezvous and docking: ~100-200 (depends on your ability, could be more or less)

Transfer to Mun: ~300

Munar insertion: ~250

Rendezvous and docking: ~100-200

Escape Munar SOI: ~250

Circularization in LKO: ~900

Rendezvous and docking: ~150-250

On the low end, that's a total of: 3070 m/s dV

That's just as much as an interplanetary trip. Of course these are estimates. It could be more, or less. Single destination trips would be somewhat cheaper. To the Mun and back would be around 2300 m/s dV with rendezvous (860 intercept+250 insertion+250 escape+860 circularization).

Really it comes down to what you want to take on the ship back and forth. Just crew? You can easily get away with a chemical engine. If you want to lug cargo back and forth I'd definitely shoot for a Nuclear tug, but that's just me.

Nukes aren't really necessary until the mass of your vessel hits a certain level.

Edited June 2, 2015 by Randazzo

[n0xiety]

I don't even use LVN for Duna/ike

That is so right. Actually you shouldn't even use them for anything but grand tour.

I used a LV-N craft for duna with a lander that used poodle and it was so overbuilt for the job. Ok heres what happened. Started with 5000dv for the main ship. I reach minmus get into orbit, pick up kerbals and get back up meet with the main ship. 16 kerbals total now. Go for duna aerobreak. After that connect the extra fuel and aerodynamic parachute pack to the lander. Get the main ship into orbit and switch to lander. After takeoff leave the extra fuel and parachute pack at Duna after the fuel gets used at takeoff. Meet with the Main ship and refuel lander. Go for Ike and land, takeoff meet again. Go for Kerbin. Get into high Kerbin orbit and change inclanation. At that point i had 5500dv..... More than i started can you believe that? Just because the lander used the fuel reserved for it.. The thing is i still had alot of fuel left reserved for the lander. Decided to go for minmus again and land all the kerbals for experience since some were never there before. After takeoff i have even more dV than before... Facepalming hard here... Go for Mun as well and land for the kerbals that have never been there... takeoff and meet with main ship. Lander still has %70 fuel left but the reserve atleast got used. The ship now has 5600dv... Ahhh... After that left the main ship at high orbit and landed at kerbin.

So i learned not to use LV-N for anything but Jool Moons Grand tour since it doesn't worth the burn times for the efficiency. That ship i told you about had 4 hours of burn time... If you are going for a single planet just use poddle or rhino if your ship is really big.

Edited June 2, 2015 by n0xiety

[Randazzo]

That is so right. Actually you shouldn't even use them for anything but grand tour.

I used a LV-N craft for duna with a lander that used poodle and it was so overbuilt for the job. Ok heres what happened. Started with 5000dv for the main ship. I reach minmus get into orbit, pick up kerbals and get back up meet with the main ship. 16 kerbals total now. Go for duna aerobreak. After that connect the extra fuel and aerodynamic parachute pack to the lander. Get the main ship into orbit and switch to lander. After takeoff leave the extra fuel and parachute pack at Duna after the fuel gets used at takeoff. Meet with the Main ship and refuel lander. Go for Ike and land, takeoff meet again. Go for Kerbin. Get into high Kerbin orbit and change inclanation. At that point i had 5500dv..... More than i started can you believe that? Just beacuse the lander used the fuel reserved for it.. The thing is i still had half the fuel i brought for the lander. Decided to go for minmus again and land all the kerbals for experience since some were never there before. After takeoff i have even more dV than before... Facepalming hard here... Go for Mun as well and land for the kerbals that have never been there... takeoff and meet with main ship. Lander still has %70 fuel left and the ship now shows 5600dv... Ahhh... After that left the main ship at high orbit and landed at kerbin.

So i learned not to use LV-N for anything but Jool Moons Grand tour since it doesn't worth the burn times for the efficiency. That ship i told you about had 4 hours of burn time... If you are going for a single planet just use poddle or rhino if your ship is really big.

So.... I ran some numbers. For that lander to have expended only 30% of it's fuel and landed on all those bodies, it would have required a wet/dry mass ratio of over 60 to 1, which while impossible enough on it's own, would have given a TWR so incredibly low that "landing" would not be the appropriate word. I'm not even going to bother with the main ship.

Though I get the point you are making, the story you're using to prove it is sort of backwards. The LV-N is in fact -the- engine for moving redonkulously large vessels. You can easily go to Duna and back on conventional engines, and maybe push it to Eve, but beyond that chemical engines are one-way trips for probes.

Edited June 2, 2015 by Randazzo

[n0xiety]

That's an interesting story. How big was this lander you brought? How much dV did it have? I find it extraordinarily hard to believe you returned while still carrying the lander, with more dV than you started with. You would have literally had to have dumped half the starting mass of your vessel (more!) without losing any fuel beyond what you were burning to have returned with that amount.

I did dump alot actually since i had fuel tanks that i can dump and the aerodynamic fuel extention with parachutes for the Duna landing and takeoff + all the liquid fuel tanks were mk3 parts. Doing the retrograde burns for getting into orbit while the lander is decaupled might have helped too. I had bad twr as you can imagine since the craft had more than 4 hours of burn time but that made it all the more efficient i guess.

If i remember it right Lander had 1800dV that is why i used an extra parachute pack with more fuel for Duna landing and takeoff.

Oh and using mechjeb for everything must have helped too.

Edited June 2, 2015 by n0xiety

[Red Iron Crown]

Reusability changes the equation a bit. For a single trip from LKO to the Mun or Minmus chemicals are better, but if you are making 10 or more such trips? Then the savings become much greater.

I would suggest mounting three or four LV-Ns to short Mk2 LF tanks, with those attached radially to another central 1.25m LF tank that mounts your utility equipment (probe core, battery, docking port, etc). Add some small wing parts to the Mk2 tanks as radiators and you should be good to go.

[EtherDragon]

I get the sense that the LV-N is going to get a bit of a buff in the 1.0.3 balance pass.

[OscarJade]

Thanks all! What I would like to use is a 3-man command pod plus the 4-person hitchhiker container...I'm sure that means larger fuel tanks for four LVNs. I built this with 2 LVNs and a 2880-gal red tank, and it used up most of the fuel just getting to the Mun My chemical option would be a red tank with a Poodle. Either way I can refuel them at any of my three orbital stations, the main expense of course being station refueling runs (I'm not drilling/making my own fuel yet).

[Red Iron Crown]

I get the sense that the LV-N is going to get a bit of a buff in the 1.0.3 balance pass.

Maybe. I'm not convinced it needs it, the orbital vehicle design game is more interesting now that the LV-N isn't the solution to almost all problems.

(Unless you mean having a sensible 2.5m LF tank, which would buff the LV-N in a way and it much needed.)

Edit:

Thanks all! What I would like to use is a 3-man command pod plus the 4-person hitchhiker container...I'm sure that means larger fuel tanks for four LVNs. I built this with 2 LVNs and a 2880-gal red tank, and it used up most of the fuel just getting to the Mun My chemical option would be a red tank with a Poodle. Either way I can refuel them at any of my

three orbital stations, the main expense of course being station refueling runs (I'm not drilling/making my own fuel yet).

Don't use the red tank with the LV-N, the dry mass is murderous for the amount of LF it holds. Better to use a Mk2 or Mk3 LF-only tank. Probably the shortest Mk3 tank for the amount of payload you're pushing.

Edited June 2, 2015 by Red Iron Crown

[n0xiety]

Don't use the 3 man command pod + 4 man hitchiker mate. Their total mass would be close to the mk3 16man compartment. And you can use the mk3 compartment with mk3 LF fuel tanks as Red Iron stated.

[OscarJade]

Don't use the red tank with the LV-N, the dry mass is murderous for the amount of LF it holds. Better to use a Mk2 or Mk3 LF-only tank. Probably the shortest Mk3 tank for the amount of payload you're pushing.

Thank you! Certainly makes sense; I'll look into it. Not exactly familiar with the LF-only tanks, but I'm sure I have them somewhere, LOL.

[Vorg]

I see a lot of talk about nukes, but not much about xeon

[OscarJade]

Don't use the 3 man command pod + 4 man hitchiker mate. Their total mass would be close to the mk3 16man compartment. And you can use the mk3 compartment with mk3 LF fuel tanks as Red Iron stated.

Thanks for the tip - am just wanting to transfer crew, but don't think I have the Mk3 16-man yet...guess I'd best focus on unlocking that!

[MarvinKitFox]

what is the best configuration of nuclear engine/fuel tank to move between Kerbin/Mun/Minmus?

That's about like asking which brand of bulldozer to use in a cycle race.

Nuke is good, but only for big ships, and bigger delta-v' requirements.

[OscarJade]

That's about like asking which brand of bulldozer to use in a cycle race.

Nuke is good, but only for big ships, and bigger delta-v' requirements.

Well, I do plan on hauling personnel (more than 3 at a time), and am considering long-term fuel efficiency with reusable spacecraft.

[Laie]

I'd say, take the biggest Mk3 tank and (if part count allows) slap on a good number of nukes, like four to eight.

The many engines to give you a half-decent TWR, the big tank because it can go back and forth a few times before you need to top it off.

For a convenient Mun/Minmus shuttle, you want an acceleration of something like 4m/s² -- still three to four minutes burn time, but can be done in a single pass without extraordinary losses. Getting over 6m/s² isn't really worthwhile, getting below 3m/s² means that you need to plan ahead and split your burn. That will be getting old pretty quickly.

[OscarJade]

I do have the Taurus capsule mod & have unlocked it; that's a 7-man pod...I just don't have the widest tanks (Mk3?) & super heavy lifters unlocked...

Edited June 2, 2015 by OscarJade

[ajburges]

For reusable space craft fuel economy is key. NEVAs take a mass hit, but that crazy ISP allows for a lot of payload flexibility.

I am also designing a planetary transport. Though mine is more multi-mission mother ship. It has generous chemical stores (for landers), crew seating, and docking adaptors. Fully loaded it should gave silly dV and enough juice for a lot of lander outings. Switch fuel payload to nuke fuel and it should be good to visit any interplanetary target. (perfect for a crew ferry)

I charted out the tanks in a Google sheet. The Mk2 long jet segment has the best dry mass ratio for pure liquid fuel of any part. The short one is actually a slightly worse ratio and all Mk3 are less mass efficient than the long Mk2. Only resort to them if you need the greater capacity. The dry mass ratios of tanks are weirdly balanced. The Mk1 fuel fuselage and NCS cones have horrid dry mass ratios. Seriously stay away from them unless you need them for aerodynamic purposes.

A Mk2 nuke design will also want small delta wings for heat radiators and to move the CoL back for aero braking. As long as you use cargo bays, you can descend to 30 km at orbital speeds without worrying about burn up, but aero brakes could still be useful for greater brake control.

Careful using a two nuke design for a Mk2 pure space ship. Making it massive enough for a TWR of .3 full makes for a rather wobbly craft and space sausages don't naturally have need of structures that would be good strut points. My first design is a usable failure because the wobbles prohibit accurate Klaw grapples.

Edited June 3, 2015 by ajburges

[Requia]

A Mk2 nuke design will also want small delta wings for heat radiators and to move the CoL back for aero braking. As long as you use cargo bays, you can descend to 30 km at orbital speeds without worrying about burn up, but aero brakes could still be useful for greater brake control.

I'm not getting these numbers from the mk3 standard and long, (the mk3short is bad though). It's 87.50% for the mk3 and mk3 long vs 87.52% for the mk2 long. It comes to an extra .01 tons for an entire mk3 long, you'll win on needing fewer struts (assuming you need that much fuel, and it doesn't add too many fuel lines)

Edited June 3, 2015 by Requia

[ajburges]

I'm not getting these numbers from the mk3 standard and long, (the mk3short is bad though). It's 87.50% for the mk3 and mk3 long vs 87.52% for the mk2 long. It comes to an extra .01 tons for an entire mk3 long, you'll win on needing fewer struts (assuming you need that much fuel, and it doesn't add too many fuel lines)

I misspoke, the short Mk2 tanks are less mass efficient than most Mk3. But the long Mk2 is still the most efficient. The part count savings is worth it for heavy transfer tugs, but for a single planet ferry, Mk3 is a little overkill. Less tends to be more and Mk2 is plenty big for 1-2 nuke craft. I'm a fan of nuclear craft that don't need struts by design, but the loss of the gimbal makes pullers more attractive and those need struts to make properly ridged arms.

MK3 is also not a lifting body, so you don't need as much consideration for aerodynamic forces. The radiator effect is more for justifying the wings than real concern. Your tug should not burn long enough for heat to be an issue if you stay in the same planet's SoI.

Edited June 3, 2015 by ajburges

[OscarJade]

The best, most immediate payoff for me has been docking landers above the Mun & Minmus...I've built/launched my

last landers for Kerbin's neighborhood

[Ferdoni]

Also bear in mind that the LV-N doesn't use oxidizer!!! Only liquid fuel. I'm ashamed of the fact that I found it just a week ago, also in the turorial of Scott Manley.

[tomf]

I just built a ship designed to take four untrained kerbals from my LKO station and land them on both the Mun and Minmus for the XP

It is built around the 4 man mk2 crew cabin, some of the MK2 liquid fuel tanks and a single LV-N with about 5000m/s delta-v

It is right on the cusp where a chemical engine might have been cheaper and easier over a single flight, but by the time it has done several trips the cheaper refueling costs will have easily made up for it.