Using multiple nuclear engines

[r4pt0r]

Is there an advantage to using multiple LV-Ns for interplanetary pushers? Other than shortening your burn time you are adding a heck of a lot of extra weight to your craft.

[p331083]

Uhh yeah? Big craft + 1 LVN = 1 hour long burn.

[r4pt0r]

Uhh yeah? Big craft + 1 LVN = 1 hour long burn.

right but other than shortening your burn time? i can just burn when im near periapsis, and then orbit, and burn when im at periapsis, and orbit.........

[SpaceSphereOfDeath]

Less excess weight, more engines=more weight that isn't fuel (due to the rocket equation, more fuel weight = higher ratio of dry mass to wet mass.

[Renegrade]

right but other than shortening your burn time? i can just burn when im near periapsis, and then orbit, and burn when im at periapsis, and orbit.........

Well, that's only good if you're going to say Duna, where basically sneezing in the right direction will put you in a transfer orbit there... once you're leaving the SOI of Kerbin (or wherever), you can't make another pass at periapsis (not for a very long time at least, heh), and you'll have to do the rest of your burn that you couldn't pull off without any advantage of the Oberth effect.

That, and at your destination, you won't be able to leverage the Oberth effect at all if there's no atmosphere, since you have to be able to at least brake enough to enter orbit, which might mean far away from the planet if your acceleration is 0.02.

Also it takes up a lot of real world time to do that sort of looping launch etc... I don't know about you, but my job description doesn't involve playing KSP, so real world time does count for me.

[Vanamonde]

This is more of how-to and gameplay question, and so the thread has been moved to that subforum.

[Foxster]

You do have to watch out for the Oberth effect though. This is where the use of a rocket engine when travelling at high speed generates more useful energy than one at low speed.

[RexKramer]

In general, you will be more efficient by having as few engines as you can get away with. There are a couple of situations where the resulting low thrust to weight is a problem.

One is the fact that excessively long burn times means you are not burning at the ideal point, much of your burn is ahead of and/or behind the ideal point.

Also, there are situations where a low T/R ratio means you can't complete the burn in time at all. High speeds when trying to stop in a planets SOI are one. Trying to land on a surface when your T\R is less than 1 is another.

However, mainly (for me anyway) it's a matter of I don't have the patience to spend long periods of time watching painfully slow burns. Most of my ships would work just fine with half the LVNs I use, I just don't like waiting for slow burns.

Finally, I like to mount the LVNs radially, otherwise they make the ship wobbly because they are long and narrow. So for symmetry, I'm always going to end up with at least two LVNs.

[Gargamel]

Are supercars fuel efficient? Nope, but they sure accelerate fast though.

There's your answer.

It all depends on how efficient you want vs burn time.

I have built a semi heavy interplanetary mother ship with anywhere from 3 to 16 LVNs. There is about a 20-25% difference in potential EV with different cluster arrangements.

It all depends on how long you want to burn. My computer is so far below minimum spec requirements that my FPS is low enough that 10 real world seconds pass for every 1 in game second. That makes a 30 minute ingame burn into a 5 hour snooze epic. So shortening burn times while still maintaining acceptable efficiency is key to me.

[LethalDose]

Increasing the number of engines on a vessel increases TWR at the expense of efficiency. TWR helps improve burn precision, meaning you can hit your maneuvers closer to the planned node.

Because engines have mass, there's really no way around this problem, but there are design strategies to mitigate it.

However, IMO, saying "is there a reason to use more engines, other than to shorten burn times" is like saying "is there a reason to know CPR, other than to possibly save someone's life". No, there really aren't other good reasons, but the reasons given are really really good to start with.

Edited July 27, 2014 by LethalDose

[Red Iron Crown]

To maximize the amount of delta-V of a ship it is best to minimize engine mass, so more than one LV-N will mean less dV than just one. From a pure efficiency standpoint, the single LV-N with multiple burns at periapsis is best.

The tradeoff is lower TWR and thus acceleration in a single engine design. This means that burns are further away from the ideal of an instantaneous burn, though splitting up the burn into several smaller ones can mitigate that to a degree. It also makes burns a bit dull and more complex, as it is just minutes or hours of pointing at the maneuver node and you must split up the burn efficiently; whether this is important to you is more about what sort of player you are.

Personally, I dislike playing with ships with a TWR of less than 0.2 or so, lower than that and the lengthy dull burns start to be more of a factor to me than the increased efficiency. So my interplanetary ships usually employ more than one LV-N, often many more on bigger ships.

One other consideration is ship design flexibility. A single engine basically has to be mounted centerline at the rear of the ship, no other option makes much sense. With two or more you can mount them radially anywhere along the ship's length which can sometimes be advantageous (I'm partial to "puller" designs for ships held together with docking ports).

All this only applies to ships that remain in orbit. If a ship is intended to visit the surface of a body there will be a minimum TWR requirement that one LV-N might not meet.

Edited July 28, 2014 by Red Iron Crown
Typos

[technion]

Well, that's only good if you're going to say Duna, where basically sneezing in the right direction will put you in a transfer orbit there... once you're leaving the SOI of Kerbin (or wherever), you can't make another pass at periapsis (not for a very long time at least, heh), and you'll have to do the rest of your burn that you couldn't pull off without any advantage of the Oberth effect..

Yep, I tried hitting Dres today for the first time in a while. The SOI on that planet is tiny, and the six minute burn time I had was way too inaccurate to hit it. Sure, I could have broken that up into two, three minute burns. But after the first one, one full orbit will end up being a few days in length, and by that time, the ejection window and angle is no longer precise, and the correcting burn is significant.