Atomic Motors Not Worth It

[Awass]

Seems to me that atomic motors are not the best thing for last stage voyages through space. Despite having an 800 isp and 60 thrust, the weigh 2.25, as much as small diameter half fuel tank. In comparison, the LV-909 weighs .5 and has 50 thrust. I just performed a test with a small parachute on top of a small capsule on top of a small half fuel tank powered by an LV-909. After a vertical decoupler, the lower stage was a small diameter quarter tank on top of a full tank on top of an LVT-45. I launched completely vertically and made it to 1,275,000 meters. (The second stage ignited at 11k feet, almost in a vacuum, so the isp was already quite near optimal.) I performed the same test, except I swapped out the LV-909 for an atomic motor. It made it 525,000 meters.

Conclusion: Although the atomic motor is just over twice as efficient as the LV-909, it's thrust-weight ratio is worse due to its much larger mass. Therefore, it is actually more efficient to use an LV-909. If you have enough fuel, the atomic motor's efficiency may eventually outweigh its thrust disadvantage, but at that point, you'd probably be better off using a more powerful LVT-45, so your burns don't take forever.

Maybe I'm insane, but this makes sense to me. Thoughts, feedback, and your own testing results would be much appreciated.

[Baconluver25]

It's also good to note that the LV-N Atomic Rocket Motor also has a horrible 220 ISP at sea level within atmosphere. While the LV-909 Liquid Fuel Engine has a 300 ISP at sea level within atmosphere. Most of the atomic engine's efficiency is wasted in the early atmosphere meaning it can't have the same chances as the LV-909 in th early atmosphere.

[Grim187]

I have been thinking the same lately.

I haven't done any tests, yet.

i use the atomic rocket for just about everything, i don't think i will be swapping it out on my orange tank bus (4x fl-t100 + 4x atomic engines) but it is quite heavy and awkwardly big for landers and probes and such, which causes instability; the need for more struts; lagggggggg.

my inner nerd has (since i started playing the game) wanted to write up all the categories a rocket can preform in and test each one in at least 3 different situations....i just don't have the time.

if you would like the redo your tests or if you have raw data i would appreciate a spreadsheet/graph...maybe use telemachus.

[Brofessional]

TWR doesn't really matter for interplanetary trips. Delta-V is more important.

[Accelerando]

atomic motors aren't worth it for getting off the ground of a body with kerbin-like or greater atmosphere. for all vacuum operations they are actually quite good, especially on lower gravity bodies. the added mass is more than compensated for by the efficiency, although i do use chemical rockets for shorter burn times in lieu of a cost system that makes it less worthwhile

[RadHazard]

You are somewhat correct. The effectiveness of atomic motors vs other engines is dependent on the TWR of your craft. The higher the TWR of your craft, the worse the LV-N performs compared to the other rockets, due to it's relatively low TWR. The cutoff is roughly at 1 TWR. Below 1, it's more efficient to use LV-N. Above 1, it's more efficient to use the LV-30 or the LV-45. Efficient in this case means you get more dV for a given amount of fuel. There are several tools and charts available that can show you which engine is best for which situation.

[Awass]

For clarification, I used an LVT-45 for the firsts stage. The LV-909/atomic motor did not activate until 11k meters, at which point the atomic motor was already at just over 760/800 isp (so almost optimal.) I did not use them to get off the ground.

And the LV-909 does have more delta-v, at least with the second stage I was using. It had a much better T-W ratio, that outweighed its lower efficiency. So again, given enough fuel, the atomic motor will eventually surpass the LV-909 in delta-v. I will need to test to see at what point that is.

[spikeyhat09]

depends on the size of the craft. for small probes and vessels, it really isnt worth it. however, when your interplanetary cruise ship weighs in at 1200 tons, the extra isp helps a lot

ksp is always a tradeoff between patience and inefficiency

[Kerbart]

(...)I launched completely vertically and made it to 1,275,000 meters. (The second stage ignited at 11k feet, almost in a vacuum, so the isp was already quite near optimal.) I performed the same test, except I swapped out the LV-909 for an atomic motor. It made it 525,000 meters.

Conclusion: Although the atomic motor is just over twice as efficient as the LV-909, it's thrust-weight ratio is worse due to its much larger mass. Therefore, it is actually more efficient to use an LV-909. If you have enough fuel, the atomic motor's efficiency may eventually outweigh its thrust disadvantage, but at that point, you'd probably be better off using a more powerful LVT-45, so your burns don't take forever

There's no such thing as one-size-fits-all in engineering. You could do a road test on a race track where you swap out the engines and come to the conclusion that there's no reason to use two-stroke low-rpm diesels. And yet the majority of the goods in the world is moved through those "worthless" engines because it's hard to beat their efficiency.

Your test seems to focus (unless I got it wrong) on one particular task: battling gravity (which is why I highlighted the "launch completely vertical" part. And yes, that's the one thing where the atomic motor completely sucks as it doesn't have a lot of thrust.

And indeed, long burns are the price to be paid for that. But: they save on the amount on fuel you need. The importance of that is hard to understate and usually well worth the longer burns. As an illustration: I did an experiment with seeing how small I could get a rocket to launch a probe (with an ion engine, which adds a lot of weight). The initial rocket had SRB's and consisted of two FL-T800 tanks... and some fuel left when getting into orbit. Sooo... could I replace one of the FL-T800's with an FL-T400 perhaps? Not enough fuel, but less weight as well... Why yes! That process was repeated until I was left with one friggin' FL-T200. Apparently nearly 90% of my rocket was used to bring that tiny bit of fuel that I originally had left over into orbit!

For launching the atomic motor is totally unfit. For an orbit-burn where time is of the essence you don't want to use one either. But even for a burn to the Mün or Minmus they start to look very attractive and once you want to leave Kerbin behind and go to other planets it's a lot easier to use the atomic motor as it keeps your fuel budget reasonable.

[Sovek]

The LV-N starts becoming useful when dealing with very large craft and lots of fuel, which is useful when putting ships/probes/landers into a inter-planetary trajectory. Its especially useful when you need 5 digit ÃŽâ€V figures, the LV-909 isnt going to get those numbers without massive amounts of fuel.

Right now you are still working when you want efficiency at the cost of weight, or lower weight at the cost of efficiency. When going to just Mun is the goal, weight, Mun+Minums, you may want that more efficient motor.

[swiftgates24]

You aren't really meant to use them inside an atmosphere. A better test would be to Hyperedit them to an exact 100,000 x 100,000 meter orbit around Kerbin, then see how far each can get from there.

[UmbralRaptor]

If the non-engine part of a craft masses more than 8.25 tonnes, you'll get higher TWR out of the LV-N, but at this point your acceleration is only ~5.7 m/s². If you need accelerations above 10 m/s² (eg: Kerbin ascent), both the LV-N and LV-909 are rather useless.

For vacuum use, which has the higher ÃŽâ€V depends on the amount of fuel and payload:[table=width: 600, class: grid]

[tr]

[td]Fuel + Oxidizer (L)[/td]

[td]100[/td]

[td]200[/td]

[td]300[/td]

[td]400[/td]

[td]500[/td]

[td]600[/td]

[/tr]

[tr]

[td]Payload (tonnes)[/td]

[td]0.88[/td]

[td]0.62[/td]

[td]0.39[/td]

[td]0.19[/td]

[td]0.02[/td]

[td]0.00[/td]

[/tr]

[tr]

[td]propellant + payload (tonnes)[/td]

[td]1.4425[/td]

[td]1.745[/td]

[td]2.0775[/td]

[td]2.44[/td]

[td]2.8325[/td]

[td]3.375[/td]

[/tr]

[/table]

(No testing required, just abuse of the rocket equation)

Edited September 29, 2013 by UmbralRaptor

[Awass]

There's no such thing as one-size-fits-all in engineering. You could do a road test on a race track where you swap out the engines and come to the conclusion that there's no reason to use two-stroke low-rpm diesels. And yet the majority of the goods in the world is moved through those "worthless" engines because it's hard to beat their efficiency.

Your test seems to focus (unless I got it wrong) on one particular task: battling gravity (which is why I highlighted the "launch completely vertical" part. And yes, that's the one thing where the atomic motor completely sucks as it doesn't have a lot of thrust.

And indeed, long burns are the price to be paid for that. But: they save on the amount on fuel you need. The importance of that is hard to understate and usually well worth the longer burns. As an illustration: I did an experiment with seeing how small I could get a rocket to launch a probe (with an ion engine, which adds a lot of weight). The initial rocket had SRB's and consisted of two FL-T800 tanks... and some fuel left when getting into orbit. Sooo... could I replace one of the FL-T800's with an FL-T400 perhaps? Not enough fuel, but less weight as well... Why yes! That process was repeated until I was left with one friggin' FL-T200. Apparently nearly 90% of my rocket was used to bring that tiny bit of fuel that I originally had left over into orbit!

For launching the atomic motor is totally unfit. For an orbit-burn where time is of the essence you don't want to use one either. But even for a burn to the Mün or Minmus they start to look very attractive and once you want to leave Kerbin behind and go to other planets it's a lot easier to use the atomic motor as it keeps your fuel budget reasonable.

Well obviously the whole launching vertically scenario is not a practical one. I was merely using the test to show some general trends about the atomic motor. I had had a notion (I thought this was generally accepted) that the LV-N was the undisputed best for travel in a vacuum, so the results came as a surprise to me. The latter part of my conclusion was merely speculation, so you you may very well be correct, but for the situations I was considering (a Minmus mission, possibly interplanetary landing missions), the LV-N is not worth it. (I was considering trying to use it instead of a poodle for my main ship and on my lander instead of LV-909s). In fact I'll go edit my conclusion to be a little more specific.

[Awass]

You aren't really meant to use them inside an atmosphere. A better test would be to Hyperedit them to an exact 100,000 x 100,000 meter orbit around Kerbin, then see how far each can get from there.

The engines were lit at near optimal isp, so that wasn't much of an issue. However, burning from an orbit would allow for greater delta-v, so it probably would better highlight the LV-N's strengths.

[UmbralRaptor]

Silly question: What happens if you try the same test with an LV-T30, or an array of 2 or 3 rockomaxx 24-77s?

[Kerbart]

Well obviously the whole launching vertically scenario is not a practical one. I was merely using the test to show some general trends about the atomic motor. I had had a notion (I thought this was generally accepted) that the LV-N was the undisputed best for travel in a vacuum, so the results came as a surprise to me. The latter part of my conclusion was merely speculation, so you you may very well be correct, but for the situations I was considering (a Minmus mission, possibly interplanetary landing missions), the LV-N is not worth it. (I was considering trying to use it instead of a poodle for my main ship and on my lander instead of LV-909s). In fact I'll go edit my conclusion to be a little more specific.

You did a good job in thinking up a testing scenario and executing it. The issue is not that launching vertically is not a practical scenario--most testing scenarios are not practical and that is not important, because those scenarios allow us to test specific conditions.

There is no doubt that the atomic engine is the most efficient but you do not always need efficiency. To launch you need pure, raw, unregulated power. Efficiency is the least of your concerns on the launch pad -- get up to speed, and as quickly as possible -- is. And that is the problem with launching vertically. You're testing for efficiency in a scenario where efficiency is not the only factor.

Consider two engines. One is highly efficient, produces x amount of thrust, and uses only 10 kg fuel per second. However, because the TWR is low your vehicle accelerates slowly and it takes 25 minutes to reach orbit. In those 25 minutes you burned 25 × 60 × 10 = 15000 kg of fuel.

The second engine is horrible inefficient and uses 20 kg fuel per second. But because the engine is lighter and produces more thrust, you will only need 10 minutes to reach orbit. In those 10 minutes you burned 12000 kg of fuel.

What is important here, is that the inefficient engine have more surplus thrust than the efficient one. Since we're burning fuel per second, it is important to (within reason) limit the amount of time we're burning fuel by accelerating to higher speeds (thus spending less time) as quickly as possible.

Once you're in orbit things change. Now you simply need an x-amount of energy from your fuel, and time is no longer a factor in the energy calculations. As a result, the most efficient engine will take far less fuel. In your case, yes, you did test the atomic engine in a situation where it worked with the highest efficiency. But in a scenario where efficiency takes a back seat to the power-to-weight ratio.

The good news: you learned something!

[magnemoe]

depends on the size of the craft. for small probes and vessels, it really isnt worth it. however, when your interplanetary cruise ship weighs in at 1200 tons, the extra isp helps a lot

ksp is always a tradeoff between patience and inefficiency

Yes, for small probes the 7S is king, the LV-N don't win here until you pass 5000 m/s dV.

Moving any mass like an landing can, the LV-N wins over the 909 at 2000 m/s and below if the cargo weight increases.

You can land with LV-N on Mun and other places however if your TWR get to low it might be smart to add extra engines to use during liftoff

[csanders]

Don't consider the LV-N to be the best "in a vacuum," consider it to be the best "in orbit."

It usually is. Even then, with a small fuel tank, a lighter lower efficient engine can have higher Dv.

Edited September 29, 2013 by csanders

[arq]

The LV-N has very high ISP. However, it is also very heavy. Thanks to our dear friend the rocket equation, an engine is only 'efficient' if its mass is 'small' compared to the mass of the ship. If you use a single OSCAR-B tank, you will get the best mileage out of an Ant engine. If you use a Rockomax x32 tank (or really anything bigger than a T400, IIRC), no engine will get you further than a LV-N.

The issue is that the LV-N has such low TWR that it can't move enough fuel up to orbit (by itself) to make it efficient, hence its application as an interplanetary (or smaller-than-Kerbin body) engine. It works beautifully for Duna or especially Moho missions, for example, where you need a goodly bit of fuel even after you hit LKO.

Edited September 29, 2013 by arq

[TMS]

I've started to use these a lot: http://kerbalspaceprogram.com/radial-nuclear-engines/

Near-stock balanced, well-textured and far easier to attach to your designs.

The largest is 2.55 mass and 830 dV.

[NeoMorph]

I think that the LV-N is all about dV and fuel efficiency... but above all it rewards PATIENCE. The long burns are easily done with MJ and/or remote tech. I love setting a ship on a long burn and then doing what mission control does (go get a coffee?) while waiting to see the result of a burn.... though waiting to see what the LV-N has pushed you into takes time it does reward your patience by saving fuel.

[SpaceSphereOfDeath]

But haven't the LV-909s got less thrust then the atomic motors?

[UmbralRaptor]

But haven't the LV-909s got less thrust then the atomic motors?

Less thrust, but also lower mass, so the engine itself has higher TWR. The craft will also have a higher TWR if each engine (LV-N or LV-909) is pushing <8.25 tonnes.

[SpaceSphereOfDeath]

Less thrust, but also lower mass, so the engine itself has higher TWR. The craft will also have a higher TWR if each engine (LV-N or LV-909) is pushing <8.25 tonnes.

Ahhh, I see.

[Vlk]

hmm, nobody linked this page yet: http://wiki.kerbalspaceprogram.com/wiki/Tutorial:Advanced_Rocket_Design#Delta-V

for a little simplification you can remove the *gravity, because its the same always

according to this, if you have a jumbo fuel tank in space, with a lv-909, then you can make ln((36+0.5)/(4+0.5))*390=816 delta v

if you use atomic, its ln(38.25/6.25)*800=1449

even if you use 6 thrusters, wich gives you a very nice thrust (i usually use 6 atomic engines with 2-3 jumbo tanks): with lv909 is 700, with atomic: 831 - still better

if you use 6 engines, and add 10t payload: lv909: 412.854, atomic: 617