So one Atomic Rocket Motor is better than four?

[Fenris]

So I'm messing around in the VAB and was looking at Delta V and noticed that with one Nuke I could have 13,039 m/s with a burn time of 1 hour 9 minutes and 45 seconds. If I swap out the one nuke and add four I go down to 9164 m/s with a burn time of seventeen minutes and 26 seconds.

I understand why the the burn time would go down, I'm burning four times as much fuel. What surprised me was that I lost almost 4k m/s ... I just figured with 4 engines I'd be pushing 4 times harder and keep the same Delta V, I would just get up to (or down to) the speed I needed for the maneuver quicker ..... guess not.

So if I'm understanding everything correctly then less is more and if I want a long range craft I'd go further with just one nuke? Is there something I'm overlooking or is that pretty much it?

Edited November 19, 2013 by Fenris

[SSSPutnik]

You're adding 4x the mass.

[Borogove]

The inputs to the delta-v equation are specific impulse and starting and ending mass. Final (dry) mass is higher with more engines.

The trade off is 4x lower TWR, so basically 4x longer burn times.

[Vanamonde]

less is more

This is one of the key concepts of spaceflight.

[DisarmingBaton5]

less is more

This is one of the key concepts of spaceflight.

What he said.

[mythbusters844]

Wait, so, if you had an imaginary engine with zero mass, and added one of them to a stage and compared the delta V with another stage with four engines, there would be no difference? I thought it did something much more complicated to the specific impulse or something.

[Fyzzi]

Wait, so, if you had an imaginary engine with zero mass, and added one of them to a stage and compared the delta V with another stage with four engines, there would be no difference? I thought it did something much more complicated to the specific impulse or something.

Yep, that would just increase the power of your burns, so you would not have to burn for that long...and yep, it's just about the mass (when stacking up engines of the same type) ofc it would be much more about specific impulse should you try to combine it with other types of engines

[SSSPutnik]

If it had zero mass it would shoot off at lightspeed

[Seret]

The LV-Ns are really heavy, they're the heaviest engine in their size range by a considerable margin. In general you want to use the minimum number of them that give you tolerable burn times. I find a pair is good for pretty much all operations in the vicinity of Kerbin, including pushing around big chunks of stations and larger ships. You'll probably only ever want want three or four for really big interplanetary ships where the mass is high enough that an extra 2.25t per engine doesn't hurt too badly.

[Kulebron]

Delta V is proportional to logarithm(M_start/M_end). Adding more engines you increase M_end and decrease Delta V.

You may drop 2 engines (if space convention allows you dropping nuclear stuff ) when needed and continue with the rest of them.

This is what I did once. Had 3 LN-Vs, dropped a tank and two of them. I couldn't just drop them, b/c thrust would be asymmetrical, so I had to use docking. (Very carefully, to not touch the engine, otherwise docking with a spinning object is very hard.)

The 2 other engines fell on Jool.

Edited November 18, 2013 by Kulebron

[SSSPutnik]

Very clever there, nice one.

[Seret]

I like that too, well done.

[Dispatcher]

You might also try a combination of one atomic and, say, 3 of another kind of engine. I've had success with the 48-7S.

[DrMonte]

When interplanetary travel time is accounted by days I don't think it does really matter if you will burn 5 minutes or 30.

[Seret]

When interplanetary travel time is accounted by days I don't think it does really matter if you will burn 5 minutes or 30.

Well, 25 minutes of staring at nothing happening on the screen matters to me.

[Moar Boosters]

Well, 25 minutes of staring at nothing happening on the screen matters to me.

You can install mechjeb and let that take care of the longer burns, whilst you go off and do something else.

[lajoswinkler]

When I have lots of mass onboard, I use 3 engines because I don't like waiting for half an hour at 4x time warp and burning on a large portion of orbital path.

I use a ship with 3 engines positioned radially, close to the top of the vehicle, and lots of detachable tanks (radially and axially positioned), so I start with lots of mass and end with less. That way it's easy to reach Eeloo and go back to Kerbin with fuel to spare. Actually, to this day I'm using the very same tug prototyped in v0.18. Granted, it requires a large lifter, but it gets the job done.

[Seret]

You can install mechjeb and let that take care of the longer burns, whilst you go off and do something else.

Which still takes the same amount of time to complete the burn. I don't have a huge amount of free time, I tend to play KSP in bursts of an hour or so about once a week. This is a pretty mellow game, but I like to get a reasonable amount done in that hour. I'm a bit cagey about time warp during burns too, I've had some twitchy results.

[capi3101]

For the OP: "better" is really a matter of your perspective - I think everyone else has already pretty well hammered home the point that the extra engines add deadmass, and that by adding them you're trading delta-V for thrust. You have to ask yourself which one is more important to you, and really I'm of the opinion that it depends on what job you're trying to do. Long range probe to another planet? You can probably get away with one (hell, you make it light enough and you're almost better going off sans nukes entirely - use a 48-7S instead; I put one of those on a five tonne probe, wound up with 4700 m/s and a Kerbin TWR of 1.15 and was able to shoot it into orbit). Massive Kerballed Duna lander around 60 tonnes? You want four. Not really any way to have your cake and eat it too.

And yes, delta-V is solely dependent on mass differential and the only parts that change mass over time are fuel tanks, so if you add more parts with unchanging mass, you decrease the ratio between full and dry mass and in the process lower delta-V. If you had a theoretical zero-mass engine, it would cause no change in delta-V. Of course, such a thing is impossible, but it's nice to dream.

[DrMonte]

Which still takes the same amount of time to complete the burn. I don't have a huge amount of free time, I tend to play KSP in bursts of an hour or so about once a week. This is a pretty mellow game, but I like to get a reasonable amount done in that hour. I'm a bit cagey about time warp during burns too, I've had some twitchy results.

In this case LV-N might be not the best choice at all. 48-7S or Aerospike might be more efficient. All usual space flight formulas assume that TWR in space is irrelevant, those ISP leads.

Edited November 18, 2013 by DrMonte

[Seret]

In this case LV-N might be not the best choice at all. 48-7S or Aerospike might be more efficient. All usual space flight formulas assume that TWR in space is irrelevant, those ISP leads.

I agree, the 48-7S and aerospike are great engines. I tend to reuse most of my propulsion modules for multiple missions though, and that 800s Isp starts to look better and better the more you refuel it.

[RadHazard]

It really is a matter of what you consider an acceptable level of thrust ratio vs how much dV you need.

<shamelessplug>

The design program in my sig can be pretty useful for figuring out the best design for your ships.

</shamelessplug>

[Vanamonde]

Wait, so, if you had an imaginary engine with zero mass, and added one of them to a stage and compared the delta V with another stage with four engines, there would be no difference?

As I like to point out, go look at the delta-V equation again. You'll find that engine thrust is not a term in the equation.

[SRV Ron]

Interplanetary asparagus with LV-909s or one LV-N with drop tanks. The latter has long 30 minute burns but nearly twice the delta V on equivalent weight in the 7 LV-909s and can easily reach Moho and Eeloo.

The advantage of dropping the three extra LV-N, the elimination of about 7 tons of mass in exchange for longer burn time but more Delta V from the fuel on board. Also, less launch vehicle or more payload into orbit will take place. That will give you even more Delta V to play with.

Edited November 18, 2013 by SRV Ron

[Franklin]

The idea that TWR isn't important in space is an opinion only batted around by those who let MechJeb play the game for them.