Multiple in-line LV-Ns on Interplanetary Transfer Vehicle

[AlexinTokyo]

I'm building a general purpose ITV, and I'd like to add multiple (I'm thinking probably 3 or 4) LV-N nuclear engines.

I can do this (obviously) if I attach them under radially-attached stacks, but I'd like, if possible, to attach them in line.

If I go with in-line attachment, is there any way I can (reasonably) attach the lifter underneath the ITV? Would I have to go with entirely radial boosters to orbit?

If anyone could let me know any tips or tricks, it would be much appreciated.

N.B. Stock except for KER and KAC, so I'm not looking for mod-based solutions at this point.

Edited July 14, 2014 by AlexinTokyo

[helaeon]

You could use a short tank or one of the adapter plates with cubic octagonal struts to make mount points for multiple engines. Then you can use alt+click (alt copy just like in photoshop and illustrator) to make a copy of the adapter tank to use to get the octagonal struts in the same spot. The alt-copy will bring your engines so you want to copy it before you attach the engines, then put decouplers on each of the LV-Ns, then put your adapter flipped upside down, then build the rest of your rocket. May not be super reliable.

I know you said no mods, but for that sort of thing the procedural fairings in-line fairing piece can make this way easier and more reliable.

Another option is to launch the payload upside down with the engines facing skywards. Then use a probe core at the top of the lifter stack right below the payload so your nav ball is oriented correctly.

[Red Iron Crown]

I like LV-N clusters. Since 0.23.5 came out, I've been using 3.75m adapters or tanks for making those clusters, the 3x diameter means you can cluster up to six around one central one. I prefer using the BZ-52 Radial Attachment Point part rather than cubic octagonal struts, they make it easier to tell if the engines will clip.

If you want to mount your boosters below such a cluster it is best to either omit the center engine and attach it there or use the tweakable everything mod to turn of the fairings (otherwise when the fairing splits it will damage the cluster).

Here are some pics of an interplanetary ship that uses LV-Ns in clusters of six, with boosters mounted below (all stock parts):

With boosters attached:

Staging away the boosters, the 1.25m tanks have the decouplers on their tops:

Boosters gone, clusters visible:

Edited July 6, 2014 by Red Iron Crown

[Klingon Admiral]

You could launch the ITV upside down, with the LV-Ns facing skyward. You may only want to circularize before detaching the lifter, as turning the ITV may take some time. Works very well for small vehicles.

[Claw]

Depending on how you are mounting the engines, you might be able to use a modular girder as a core connector.

For the part that the engines are mounting on, leave the central node free and surface mount the engines to the free face like you are talking. If there is enough space between the engines for a modular girder to fit, then you can attach a decoupler and some modular girders to the node. You might have to remove the engines to get the decoupler/girder to snap on. Replace the engines, then you can build down from girder node.

After building the next layer, you'll almost certainly need to strut from the below the engines to the layer above the engines. If you strut that direction, then the strut mount points will go away from your ITV after decoupling (and won't add to part count).

It might work best if you are able to separate lower section with the modular girder attached so that it pulls cleanly out. But that'll depend a bit on your design.

(Sorry, I would have simply posted a picture but I'm not at a proper computer atm.)

Added:

Here are pictures of the general idea. Maybe you can imagine doing this with engine pods instead of lab modules (and pointed the correct direction). You can add a small decoupler to ditch the modular girder or keep it. I use this technique in a variety of ways and it is pretty versatile.

Javascript is disabled. View full album

Edited July 6, 2014 by Claw

[LethalDose]

I use Nuke "clusters" (so far, I've only used two in a cluster) for my IP vessels. I launch them upside down (as noted above) to be docked with the rest of the vessel in orbit. They typically go up like this:

It sounds like you don't want to do this, though. I've also launched vessels with paired nukes like this:

The 1.25m tanks are connected directly to girder radially, no cubic struts. The tank and pod on top of the girder aren't very wobbly, bu I don't think i'd want an entire IPV up there.

The nice is these can both go up under expanded 2.5m Fairings from KW, but it sounds like that's less of a concern for you since no mods. Anyway, hope that helps.

[Kasuha]

If you're ok with just four engines, it's possible to use a quadcoupler. The only thing is, it's tricky to place them so they don't get damaged by their own fairings - you need to install them rotated exactly 45 degrees with the "stitch" in their shrouds facing out. Also placing decouplers below shrouds need some additional struts because only one of them gets really attached to the quadcoupler below. And finally you need to first decouple, then activate engines - i.e. you need to leave engines in separate stage from decouplers. But with careful building it works.

I don't get it, I must be doing something wrong but Imgur always fails to make an album from uploaded images for me.

[Claw]

I don't get it, I must be doing something wrong but Imgur always fails to make an album from uploaded images for me.

Sidenote:

Drag and drop them onto the main imgur.com page. When the upload popup appears, there is a small selection bar near the center to select "Make it an album" with radio buttons to select the type. Select that and press upload. After the upload, it'll take you to an album page where you can do all the usual things (arrainge, add notes, etc). The five letter embedding works the same.

/Sidenote

On topic: I love to see all the little tweaks people do.

(Makes me miss my computer. Hopefully soon!)

[Kasuha]

Sidenote:

Drag and drop them onto the main imgur.com page. When the upload popup appears, there is a small selection bar near the center to select "Make it an album" with radio buttons to select the type. Select that and press upload. After the upload, it'll take you to an album page where you can do all the usual things (arrainge, add notes, etc). The five letter embedding works the same.

/Sidenote

Okay, you got me here. I was experimenting with it when I started using Imgur and direct upload to album never worked reliably for me. So I settled with uploading the list, then converting it to album using a button on the screen. But for some reason it stopped working (at least for me) some time ago. Apparently the functionality of these two approaches was switched somehow.

Thanks for pointing it out, I'd probably never consider trying that rejected method again.

Javascript is disabled. View full album

[EtherDragon]

DON'T DO IT! It's a trap!!!!

If you are using more than TWO LV-N's you are doing it WRONG (in my opinion).

The whole point of using an LV-N is it's ridiculous efficiency in a vacuum. But you LOSE all that efficiency by strapping on a bunch of empty mass - which is exactly what a bunch of LV-Ns are. The ONLY time you will use more than 1 LV-N is when you are leaving a deep gravity well.

Remember your rocket equation - the more empty mass you bring with you, the less total Delta-V you end up with.

[Red Iron Crown]

DON'T DO IT! It's a trap!!!!

If you are using more than TWO LV-N's you are doing it WRONG (in my opinion).

The whole point of using an LV-N is it's ridiculous efficiency in a vacuum. But you LOSE all that efficiency by strapping on a bunch of empty mass - which is exactly what a bunch of LV-Ns are. The ONLY time you will use more than 1 LV-N is when you are leaving a deep gravity well.

Remember your rocket equation - the more empty mass you bring with you, the less total Delta-V you end up with.

I suppose that if your goal is to absolutely maximize your craft's delta V then that is good advice. However, it is also important to be able to use that delta V effectively, and that means a reasonable TWR. The further away you get from the efficiency ideal of an instantaneous burn, the more dV you will waste by burning at the non-optimal point. It is entirely possible to have a ship with a lower total dV but higher TWR to arrive at the same destination with more dV remaining.

Have a look at the ship I posted earlier in the thread, it uses 24 LV-Ns. Could I have built it with only 1 or 2? Definitely, but the TWR would have fallen from a tolerable 0.2 to a painful 0.02 or 0.01. And what would I have gained? About 300m/s of dV, on a ship which already has over 9500. For me, not having to wait through two hours of burning for an interplanetary transfer burn is worth not having the greatest amount of dV possible.

You certainly don't lose all the efficiency advantages of the LV-N by using more of them, though it is true you don't maximize the ship's dV. There are tradeoffs to be made in regards to vessel dV, vessel TWR, and how much fun the vessel is to play with.

[AlexinTokyo]

Thanks for the tips, everyone. I'll give them a try when I get home this evening and let you know how it goes.

[-Velocity-]

DON'T DO IT! It's a trap!!!!

If you are using more than TWO LV-N's you are doing it WRONG (in my opinion).

The whole point of using an LV-N is it's ridiculous efficiency in a vacuum. But you LOSE all that efficiency by strapping on a bunch of empty mass - which is exactly what a bunch of LV-Ns are. The ONLY time you will use more than 1 LV-N is when you are leaving a deep gravity well.

Remember your rocket equation - the more empty mass you bring with you, the less total Delta-V you end up with.

If you have infinite time, your advice makes sense. Otherwise, it does not.

I wanted my Eve ship to make it to Eve before I died from old age or went senile, so I used 10 LV-Ns.

If I had only used 2 LV-Ns, it would have meant my acceleration was only 0.1 m/s^2 (0.01g!!!). It would have taken hours just to do the transfer burn from Kerbin space to Eve space considering my computer is just barely chugging along at like 1/4X time when running this ship, and that's even considering if I had used time compression... I simply do not have this amount of free time.

[-Velocity-]

I suppose that if your goal is to absolutely maximize your craft's delta V then that is good advice. However, it is also important to be able to use that delta V effectively, and that means a reasonable TWR. The further away you get from the efficiency ideal of an instantaneous burn, the more dV you will waste by burning at the non-optimal point. It is entirely possible to have a ship with a lower total dV but higher TWR to arrive at the same destination with more dV remaining.

A lot of the time, you can break the maneuvers up into multiple parts (let's say, I want to go from a 100 km X 100 km orbit to a 2000 km X 100 km orbit- I can do this in a series of steps, burning each time at the optimal point). However, this again runs into the main reason I use enough LV-Ns to get me up to at least 0.5 m/s^2 of acceleration- TIME. Most of us DO NOT have an ETERNITY to sit there waiting for some maneuver to complete. Also, sitting there, watching the remaining delta-V on a maneuver SLOWLY tick its way down, 0.1 m/s at a time, is NOT FUN. It's like watching paint dry or trying to amuse yourself by starting at the second hand of a clock on the wall.

[Red Iron Crown]

A lot of the time, you can break the maneuvers up into multiple parts (let's say, I want to go from a 100 km X 100 km orbit to a 2000 km X 100 km orbit- I can do this in a series of steps, burning each time at the optimal point). However, this again runs into the main reason I use enough LV-Ns to get me up to at least 0.5 m/s^2 of acceleration- TIME. Most of us DO NOT have an ETERNITY to sit there waiting for some maneuver to complete. Also, sitting there, watching the remaining delta-V on a maneuver SLOWLY tick its way down, 0.1 m/s at a time, is NOT FUN. It's like watching paint dry or trying to amuse yourself by starting at the second hand of a clock on the wall.

As a rule, I try not to break up burns into more than two parts. More than that and the second and subsequent interim orbits start taking a lot of game time to complete, making it difficult to catch a transfer window.

Your point about slow burns being dull is well taken, I agree that it is often worth sacrificing maximum efficiency for better gameplay.

[Dispatcher]

I've had good success using 3 radially mounted LV-Ns on my transfer stages. I mount small fuel tanks radially, and mount the LV-Ns to those. I feed fuel from tank(s) in the central stack. If need be, I've used larger tanks radially. I have also used onion and asparagus staging of other radial tanks (which of course do not have engines attached to them).

[NecroBones]

Your point about slow burns being dull is well taken, I agree that it is often worth sacrificing maximum efficiency for better gameplay.

KR-2L engines to the rescue.

[magnemoe]

DON'T DO IT! It's a trap!!!!

If you are using more than TWO LV-N's you are doing it WRONG (in my opinion).

The whole point of using an LV-N is it's ridiculous efficiency in a vacuum. But you LOSE all that efficiency by strapping on a bunch of empty mass - which is exactly what a bunch of LV-Ns are. The ONLY time you will use more than 1 LV-N is when you are leaving a deep gravity well.

Remember your rocket equation - the more empty mass you bring with you, the less total Delta-V you end up with.

Most players want a TWR higher than 0.1 for long burns, LV-N is so efficient that the engine mass don't matter so much in itself.

But yes nuclear powered boosters with cross feed might help here. Another option is to use an tug to push your Ap out to minmus, it can then return to Kerbin or perhaps go to Minmus to refuel from a kethane miner. For a trip to Eve or Duna you don't need much extra trust.

[Red Iron Crown]

KR-2L engines to the rescue.

I've tried them on a couple of interplanetary designs, and while the high TWR with reasonable efficiency is gratifying, they don't work all that well for me. I like to build IP ships with the engines in a "pull" configuration, which means I need at least two and that's a whole lot of extra thrust/engine mass that I don't really need.

It is pretty cool to do a transfer burn in under a minute, though. Jeb definitely approves.

[NecroBones]

I've tried them on a couple of interplanetary designs, and while the high TWR with reasonable efficiency is gratifying, they don't work all that well for me. I like to build IP ships with the engines in a "pull" configuration, which means I need at least two and that's a whole lot of extra thrust/engine mass that I don't really need.

It is pretty cool to do a transfer burn in under a minute, though. Jeb definitely approves.

Yeah, I've had varying results with these myself. In the push-configuration for my Jool-5, they were a mistake, since I could only run them at 1/3 thrust to avoid folding the ship in half (but then again, there were four of them, total overkill). Using a single KR-2L to push my Eve lander away from Kerbin? That worked wonderfully.

I go back and forth between wanting to fly efficiently, versus just wanting very short burns. I'm usually not afraid to build big, to compensate with lots of fuel.

[EtherDragon]

Try this instead:

Bring a single high powered engine, and use it to pull out of gravity wells. Your deep space and orbital adjustment maneuvers can all be on the LV-Ns. You'll find you have tons of Delta-V, and still a good TMR. When you are at a point in your mission where you no longer need the big engine - decouple it.

[Kasuha]

Try this instead:

Bring a single high powered engine, and use it to pull out of gravity wells. Your deep space and orbital adjustment maneuvers can all be on the LV-Ns. You'll find you have tons of Delta-V, and still a good TMR. When you are at a point in your mission where you no longer need the big engine - decouple it.

That's IMO the least efficient method of all mentioned here.

[-root]

That's IMO the least efficient method of all mentioned here.

I think we're talking about transfer burn speed now... Trading off efficiency for TWR and getting a compromise between the two. Thats a good solution if thats the aim, but as you say, fairly inefficient. If I'm in an impatient mood, I build my ships the same as above. Get the the ejection and a ballpark encounter with a skipper and then jettison. Then I use a LV-N to make the correction at the AN or DN.

[Kasuha]

Mounting more LV-Ns is much better compromise in trading off efficiency for TWR. For large transfers you still end up with lighter ship than if you mount a powerful engine and fuel to get the same dv.

Of course if you're impatient you need a lot of TWR. But that's not a compromise. That's brute force.

[Red Iron Crown]

I wish there was an engine in the 5-600s range for Isp with better TWR than the LV-N. Not necessarily realistic, but would make for more variety in IP ships. LV-N is the best choice in almost all IP scenarios, it would be good to have more variety of efficient choices.