running 4 nukes at 25%

[engraverwilliam]

I am building a mothership in orbit that will have roughly 3+ hours of Delta-V. I read here that if you run more than one nuke, that all you do is waste fuel. However, for balance purposes, I need to have FOUR nukes mounted on my ship... If I run them at max 25% on each engine, will it be the same as running ONE at 100% or will I only be doing 25% of the max thrust period?

I have been trying to wrap my mind around this one but cannot resolve the issue.

thanks

[Guest]

Running at 25% would just be 25% overall. This is because the engines have less TWR if you don't use all the thrust, and more weight to push. Running 1 nuke would have the same thrust but 4 times less weight.

You might as well run all of them at 100%. Not exactly sure about this stuff, just relying on logic.

[Monty_Droppings]

You are forcing yourself to carry the weight of 3 extra LV-N's, which are heavy, reducing your delta-V.

[Mr.Rocket]

Well, it will be the same amount of thrust as one if you do this, but i don't think it will be more efficient. The reason for the lost delta-v is because those nukes weigh alot for an engine, so moving them at all takes alot........ And overall, no. Running engines at lower power does not make them more efficient, it rather just makes the burns longer

[Greenspan]

With 4x the weight. If you have 4 engines you might as well run them at max thrust, they will use fuel 4x as fast, but you'll accelerate 4x faster, so you'll only have to burn for 1/4 the time to get the same amount of delta-v. So in the end the amount of fuel used will be the same.

The only difference is the extra mass of 3 more engines that is just dead weight in your rocket, which will require more fuel to get the same amount of delta-v.

[MockKnizzle]

It's never more efficient to run a rocket engine at anything less than maximum thrust, unless you're in an atmosphere (but only in Kerbal Space Program, not even in real life). If you have too much thrust, use a smaller, lighter engine.

[Whirligig Girl]

THe inefficiency comes from the dead weight that the engine has.

[Vanamonde]

Underutilized engines are a bad thing, as others have said, especially with the LV-Ns, because they're unusually heavy. You'd be better of re-designing your ship to use fewer engines. Post some pics of it, if you have questions about how to do that.

[Pecan]

(*whisper* build with symmetry-3. 3-legged stools (bases, landers) don't wobble, 3-engined tugs deliver them better, onion-3 is only marginally worse than asparagus for launchers)

[Vanamonde]

Pecan, 3x symmetry causes a bit of trouble in KSP. On one of the axes, there will be 2 units on one side of the center line but only one on the other. The unbalanced mass causes unwanted yaw while pitching, or unwanted pitch while yawing, depending on the axis. If the masses are not large, it's controllable, but it can be a nuisance while steering.

Edited February 13, 2014 by Vanamonde

[Pecan]

Ah thanks for that warning. I have been lifting and landing <40tonnes (payload) per vehicle so far and haven't noticed any trouble. Or, to put it another way, my manual flying is sufficiently erratic that if it has occurred I thought it was just me ^^. In the middle of re-designing my fleet for symmetry-3 because my one-piece -4 bases always seemed to end-up on a hillock of some sort where they wobbled like mad. Yesterday I downloaded KAS ... [in other words there are different solutions to the problem I had and I'll reverse and stop making a new one for myself]

[Red Iron Crown]

I am building a mothership in orbit that will have roughly 3+ hours of Delta-V. *snip*

I think this phrase reveals where you're having trouble wrapping your mind around this. You're equating a longer burn time with more delta-V, but that is not necessarily the case. Delta-V is not measured in units of time but in units of speed; it represents the potential change of velocity of a craft. It is calculated by the Tsiolkovsky rocket equation, which is regarded as the most important formula in all of rocketry:

dV= Isp * g * ln(m0/m1)

Where:

dV is the amount of delta-V in m/s

Isp is the Isp of the engine, a measure of its efficiency

g is the gravity constant, 9.82

ln is the natural logarithm function (available on any scientific calculator)

m0 is the mass of the ship fully fueled ("wet weight")

m1 is the mass of the ship empty ("dry weight")

From this equation you should be able to see three factors affect delta-V. You can increase delta-V by changing those factors:

Isp - Switching to a more efficient, higher Isp engine results in increased dV. You've already selected the best non-ion engine in this regard, the LV-N.

m0 - Adding more fuel will increase delta-V.

m1 - Decreasing the dry mass will increase delta-V.

Adding engines increases the dry mass (m1) without adding any propellant, so the fraction m0/m1 gets smaller, thus there is less dV. Burning the engines at a lower throttle setting will have no effect on dV, as none of those three factors changes.

Try to think about how this equation works, when you "get it" you will find designing rockets much easier.

There are some further subtleties regarding staging, which while adding mass in the form of decouplers and (sometimes) additional engines generally increases dV as it splits the rocket into smaller "subrockets", each with a smaller m1 but still governed by the equation.

[Red Iron Crown]

Pecan, 3x symmetry causes a bit of trouble in KSP. On one of the axes, there will be 2 units on one side of the center line but only one on the other. The unbalanced mass causes unwanted yaw while pitching, or unwanted pitch while yawing, depending on the axis.

I don't think this is so. Having two units on one side is exactly balanced out by the shorter moment arms on that side.

[Rocket Farmer]

Agreed Red Iron Crown

3x symmetry is fine and I have used it many times. I use 6x symmetry more often but the same theory applies.

On each axis of the symmetry lines there is one rocket with the symmetry line running through the middle of the 3rd rocket thus ensuring your thrust is always balanced.

[Karretch]

While I use 3x as well, I think I can probably explain better what that person meant. Imagine a triangle. Everyone initially imagines the axises landing dead through all of the points on the triangle. However that's not always the case. Depending on how the rocket is assembled and oriented, assuming the usual 90 heading it is possible to have 1:2 ratio falling on one side of the rocket. First we have to imagine that first triangle with a cross super-imposed over it, the center of the cross on the center of the triangle. For simplicity sake we'll only focus on the vertical line which for this example will always represent gravity. Rotate the triangle 90 degrees either way and you see the problem, you have two points on one side of the vertical line. I've experienced the problem since I'm relatively new and don't think about it when building but when performing the gravity turn in at least atmosphere the craft rotates and makes it harder to manually correct heading. Careful planning, however, won't leave you with the problem tho if you align the triangle to have one point with the gravity axis during the turn.

Edited February 13, 2014 by Karretch

[Superfluous J]

I would be surprised if 4 engines was necessary. If your ship is symmetrical 4 ways, it's probably also symmetrical 2 ways. I bet you could toss 2 of the engines.

That said, if your craft is HONKING HUGE 4 LV-Ns isn't as bad as a lot of people say. A lot of times, adding an extra LV-N to a craft will take 5% of your dV but cut your burn time in half. That's a trade-off you should be willing to consider if not embrace.

[Red Iron Crown]

While I use 3x as well, I think I can probably explain better what that person meant. Imagine a triangle. Everyone initially imagines the axises landing dead through all of the points on the triangle. However that's not always the case. Depending on how the rocket is assembled and oriented, assuming the usual 90 heading it is possible to have 1:2 ratio falling on one side of the rocket. First we have to imagine that first triangle with a cross super-imposed over it, the center of the cross on the center of the triangle. For simplicity sake we'll only focus on the vertical line which for this example will always represent gravity. Rotate the triangle 90 degrees either way and you see the problem, you have two points on one side of the vertical line. I've experienced the problem since I'm relatively new and don't think about it when building but when performing the gravity turn in at least atmosphere the craft rotates and makes it harder to manually correct heading. Careful planning, however, won't leave you with the problem tho if you align the triangle to have one point with the gravity axis during the turn.

No. The side with two engines on it has those engines working through half the moment arm so it exactly cancels.

Imagine a perfectly 3x symmetric rocket with three identical 100kn engines 1m from the centerline. These engines can gimbal to a full 90 degrees. When viewed from below, the engines can be seen as an equilateral triangle with the center of thrust for each engine at each point and the center of mass in the exact center of the triangle. If we then imagine the worst case scenario you describe, the line runs through the center of mass, with one point of the triangle at 90 degrees to that line on one side and two engines at 30 degrees from that line on the other side. The single engine is 1m from the line, the other two engines are 0.5m from the line. Torque is force*distance. The torque delivered by the single engine is 100kn*1m=100knm. The torque delivered by the two engines is 2*100kn*0.5m=100knm. So they apply no net torque in the roll axis.

Edited to add awesome MSPaint:

Edited February 13, 2014 by Red Iron Crown
Added image.

[Karretch]

~Snip~

Are we both talking about pure rotation, as in the q and e key type? Because I'm not sure I said that properly. If so, I'll accept that my limited understanding was wrong.

[Merinsan]

That said, if your craft is HONKING HUGE 4 LV-Ns isn't as bad as a lot of people say. A lot of times, adding an extra LV-N to a craft will take 5% of your dV but cut your burn time in half. That's a trade-off you should be willing to consider if not embrace.

I completely agree with this. A recent ship I built had 4 LV-Ns. The burn time was 90 minutes. If I had 1, it would be more efficient, but it would have been a 6hr burn. I'm lucky when I get to play KSP for 6hrs, I don't want to be doing a single burn for that time!

[arq]

The 4 engines are 'worse' than 1 because they are heavier and for no other reason. So run them at full. If your ship is heavy (IE hours of burntime) then chances are that your extra engines only slightly reducing your dV. Also, there is a point where you have so little thrust that you cannot efficiently use your dV. If people are obsessed with maximizing efficiency, they should use ion engines exclusively. At some point it just isn't worth it.

Remember: if your burn time is 3hrs, you will need to sit there at your computer for 3hrs waiting for it to burn.

I would never rely on a single LV-N to move a 200t ship. I could add an extra engine, reducing dV by a tiny amount (<100m/s) but halving burntimes.

[magnemoe]

Reducing trust has just two purposes as I see it, one is to restrict trust on some engines like another type or the central one in a stack who tend to overheat, because of this you can run the other engines on 100% giving larger overall trust.

The other is to deal with unbalanced ships.

[Merinsan]

Reducing trust has just two purposes as I see it, one is to restrict trust on some engines like another type or the central one in a stack who tend to overheat, because of this you can run the other engines on 100% giving larger overall trust.

The other is to deal with unbalanced ships.

Another reason is if the ship is not stable. Admittedly not a real issue with the nukes, but I have had problems with mainsails being too powerful, destroying a ship.

[engraverwilliam]

Ive reduced it down to two engines. the drive section alone is 152 tons fueled. I am putting it up nearly empty at 52 tons and will send up tankers to fuel it up for its journey. running the engines at full will give me just about 1h 45 mins of burn time. I will also have a living space for 10 kerbals and four full sized landers....i will post the sucker in the mothership thread when I am finished building it.

[engraverwilliam]

I just had the greatest idea ever! (prolly not original) i put two more engines back on facing retro and made an action group... now I dont have to spin the whole mothership to slow down! I love this game.

[Jouni]

I usually try to use one nuclear engine per 30-35 tonnes of ship mass. Going too much below TWR 0.2 (at Kerbin) is a sure way to achieve incredibly boring and wildly inaccurate interplanetary launches.