why are there people who stick more than one nuke on their ships?

[nholzric]

I've done some testing to determine the tradeoff between loss from added mass and gain from using the Oberth effect from Kerbin orbit.

...

This is what happened flying with 1, 3, and 5 engines:

1 engine: used 1185 dV from a total budget of 7085 dV.

3 engines: used 1121 dV from a total budget of 6236 dV. (+64, -849 vs base)

5 engines: used 1109 dV from a total budget of 5576 dV. (+76, -1509 vs base)

In short, while there is a saving to be made from the Oberth effect, it's eclipsed - for a ship of this type and for this run - by the loss of total dV budget entailed by the extra mass. You need less dV to get to the same place, but you have much less to begin with.

...

Thoughts?

Nice experiment!

One savings that could be important but that you didn't measure occurs when making a transfer burn from Kerbin to Duna for example. You need ~900m/s to escape Kerbin, and several hundred more to reach Duna. I believe the most efficient strategy is to burn all of that dV at Kerbin periapsis. If your ship is "underpowered" you will end up achieving the 900m/s after several orbits and then continue burning for a "significant" amount of time to in order to achieve the several hundred more to reach Duna. On the other hand, the more power your ship has the closer to periapsis you can achieve those final several hundred m/s. In the final orbit of an escape situation when you are not afforded another pass to burn at perigee power makes a more absolute difference than just convenience.

I don't know how to calculate the savings of the Oberth effect, does anyone have an idea? It would be neat to see if you could ever make up the loss of dV with more optimum burn locations. I would love to put math behind experiments like Silverchain's!

[nholzric]

...

Reason two: Nuclear engines fairings making everything explode. I hate finally getting a large ship into orbit only to have the fairing on a nuclear engine hit another part and make the thing explode. To have no fairings they have to not be attached and this means either off center docking or side mounted engines (see reason one for why you need at least 2).

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Engine fairing explosions are the worst!

Especially now that you can empty tanks before launch, I generally launch my drive sections upside down to allow 2-4 center-mounted LV-N's to launch fairings-free.

[SpacedCowboy]

That's just friggin' AWESOME !!! When I first glimpsed at that screen shot, I swear to God I heard the sound that thing must make !

[EtherDragon]

I've done some testing to determine the tradeoff between loss from added mass and gain from using the Oberth effect from Kerbin orbit.

Test craft Regenbogen Oberth in 150km x 150km LKO.

It is fitted with five LV-Ns, four of which are discarded here to carry out a burn using only a single engine.

(Strictly speaking, each LV-N has a docking port attached, so they're slightly heavier than normal; I could've discarded them but it's nice to have a stock in orbit!)

By save-scumming I started it from more or less the same point each run (the orbit is near circular anyhoo), and flew the best path I could to raise the orbit to a close approximation to 40Mm x 40Mm, slightly inside the orbit of Minmus.

I was flying manually, but it wasn't particularly complicated. It would be interesting to repeat with an autopilot. I made two runs for each config and chose the better.

((IMAGE SNIPED))

This is what happened flying with 1, 3, and 5 engines:

1 engine: used 1185 dV from a total budget of 7085 dV.

3 engines: used 1121 dV from a total budget of 6236 dV. (+64, -849 vs base)

5 engines: used 1109 dV from a total budget of 5576 dV. (+76, -1509 vs base)

In short, while there is a saving to be made from the Oberth effect, it's eclipsed - for a ship of this type and for this run - by the loss of total dV budget entailed by the extra mass. You need less dV to get to the same place, but you have much less to begin with.

I expect, though I won't try right away, that starting from a lower orbit (say 75km x 75km) would increase the gain from the Oberth effect more for the multi-engine rigs, and generally reduce the dV requirement to reach 40Mm x 40Mm for all configurations.

I expect that starting with a more massive ship (say if I stick another Big Orange on the front) would do the same thing.

I expect that starting from Eve or Jool orbit would do the same thing, and anywhere else would reduce the gain.

Thoughts?

I wonder how this experiment might change if you did part of the burn with some / all engines, and part without them

For instance, say it's a 5 minute burn with 3 engines, but a 15 minute burn with 1. What if you did the first minute with all 3 engines, and finished the last 12 minutes of burn with just one engine? In this way you might be able to capture the largest improvement from Oberth effect, while still gaining lots of Delta-V.

Also, try the same experiment with 1 nuke and 2 regular engines. There is definitely a point where the extra mass of extra nukes works more against your Total Dv budget than using a lighter, less efficient engine for things.

Edited January 22, 2014 by EtherDragon

[Mobjack]

@Silverchain

That orange fuel tank isn't pushing any payload, which means towards the of the burn, it will weigh much less. This will skew the results in favor of less engines.

In that scenario I will agree that one LV-N is the best.

But if you have a larger ship where there are many stages, the advantage of one LV-N will be much smaller.

[SuperBigD60]

If I understand the purpose of ISP in the math properly, (and there's no garantees there), for a big ship where adding the weight of a few nukes won't appreciably increase the weight, Adding more nukes is not going to decrease your dV by too much.

Unless I'm incorrect, ISP effectively helps determine your dV per unit thrust, so as long as you use nukes, it shouldn't decrease efficiency (except by adding more weight).

So I guess it comes down to how much weight you can add to your ship before you don't have enough dV to get where you're going.

Unless my logic's faulty

[Randox]

I usually aim for two on any given module that uses them. It's a standard that I find easy to work with and combine. I try not to be overly wasteful, but I'm not here to build fuel sipping econo rockets either. If that means my 100T or less interplanetary package has 4 or 6 LV-N's, that just means my burns are that much more awesome. If I ever need more than one pass to complete a burn, that is entirely unacceptable to me. I want all my burns to be shorter than my average Kerbin takeoff, which puts the ceiling someplace around 12 minutes. Without time compression.

[RSwordsman]

The loss of efficiency by adding more engines is what I'd consider a convenience charge. I want my ships to be powerful and get to places in a timely manner, so I add a butt-load of engines. Call me crazy but I have not ragequit the game yet because of dV inadequacy.

Quite the contrary, actually. I'm still building up to the point where I have a Kethane infrastructure set up so I can fly my inefficient muscle car spaceship all around the solar system and refuel when necessary. And that goal is incredibly motivating.

[Lohan2008]

Answer: Because you can ! Besides you need three for symetry. One for main drive, and two expendible feed tanks for the Eve/ Duna voyages.

Edited January 23, 2014 by Lohan2008

[WafflesToo]

LOL! Why does this have 9 pages worth of replies when it became obvious on page two that OP is just being a troll anyway?

At Isp 800 the engines would have to become a large fraction of your mass before other engines become more attractive. Go take a look at Zisteau's "Dres-tacular", even with all those atomic powerplants it still beats out LV 909s for dV

[thunderstar]

because they want shorter burns, but yeah, and orange tank with a 3-5 ton payload and a nuke can have up to 12,000 m/s of Dv, but with two nukes it just gets merely 9800 m/s

[Dooz]

Usually on small half tank craft i use one nuke but if Its a rockomax large grey fuel i use 2 nukes

But its for deltav or thrust

[Tw1]

I once built a rocket that only used LV-Ns. Took an awful lot of them to take of on Kerbin.

[zxczxczbfg]

As someone said, it's to reduce burn times, but there's another thing: I have a tug that's basically a pod and fuel tanks, with a normal docking port on the front and a Senior docking port on the back. With just one engine and correct CoM and CoT alignment (which I always require), there's not enough space for the Senior. Having two side-mounted nacelles allows me to just put the Senior on the bottom of the tank without compromising the CoM-CoT relationship.

[Mitchz95]

I usually use two nukes per ship, slung alongside the hull. While it's not quite as efficient, it 1) reduces burn times, 2) makes it unnecessary to rest all the launch weight on a fragile engine, and 3) prevents having to worry about those fairings flying off and destroying nearby spacecraft.

[DJEN]

Yup. See the "suggestions" and "addons" forum for far more people trying to dictate how others enjoy their game (mostly claiming that any way except their way is wrong and nobody should be allowed to play different from them).

Them ideo-fanatics will force us to do their ways!

They must stop such things!

[DJEN]

I once built a rocket that only used LV-Ns. Took an awful lot of them to take of on Kerbin.

That should've been an absurd piece of engineering.

[Woolywoo]

Challenge accepted.

[maccollo]

Oh dear XD

What is the payload fraction to low orbit?

[Tw1]

That should've been an absurd piece of engineering.

Found it. Forgot that I used some solid boosters to add to the the initial acceleration, so turns out to was only mostly Lv-Ns. Here it is:

Edited January 25, 2014 by Tw1

[Superfluous J]

Today I sent a three-LV-N ship to Moho. 3 LV-Ns were a better TWR than any number of other engines for the same weight and dV. I would have done 2 but the amount of stuff I was landing in one trip was enough that my TWR would have been less than 1, even on Moho.

Also, the insertion burn will be 10 minutes. With only 1 engine it'd have been 30 minutes and nobody wants that.

[Sargent890]

I do it to mount them radially