Jump to content

Atomic Motors Not Worth It

Awass

By Awass
in KSP1 Discussion

 Share

Recommended Posts

Meithan

Meithan

Posted
Posted
I often use them in my orbital tugs, or their KSPX cousin the LV-NB (balanced so it's exactly the same as sticking several LV-Ns on your craft, but lower part count). Those craft get launched once and stay in orbit pretty much forever, getting refueled in orbit as necessary.

That's exactly what I was planning for my manned mission to Duna. I already launched one LV-N powered tug taking a lander to Minmus and back, and I felt bad for deorbiting it upon Kerbin return -- such a waste of perfectly good hardware. Now I revised the design and it's meant to be launched once and re-fueled in orbit after each mission. I already got one prototype docked to my orbital fuel depot in low Kerbin orbit.

Link to comment
Share on other sites
JUDUFU

JUDUFU

Posted
Posted (edited)
To complement the single-engine comparison of the LV-N and LV-909, I've calculated the delta-v at which the LV-N becomes a better choice than the LV-909, as a function of payload:

The red curve is the delta-v (with its scale on the left) while the blue curve is the corresponding total ship mass (scale on the right).

SCIENCE!

I love it!

Nice work!

Edited by JUDUFU
Link to comment
Share on other sites
SDIR

SDIR

Posted
Posted

In my experience, the LV-Ns are definitely worth it if you can refuel them and reuse the vechicle. I have a tug system where basically a tug can make a round trip to Minmus without even having to use half of its fuel. But for planetary landings like Duna, it's best to use the 909's since you don't need too much power. A good combination would be a large mothership with LV-Ns for traveling to other planets while having a separate lander that uses 909's to land and take off.

Link to comment
Share on other sites
Awass

Awass

Posted
  • Author
Posted
As others have noted by now, starting the LV-N's in atmosphere isn't really where they should be used. At 11k, you've "barely" (well, in my mind anyway) started getting out of the atmosphere and are still fighting gravity and air friction. I don't use the LV-N's generally until after I'm in orbit. Also, from what I remember when Scott Manley was talking about them, much more than somewhere around 2-4 LVN's and you start loosing out due to the mass of the engine itself. I've used 2 LVN's when I was testing a kethane miner/lander. It needed very little fuel left in them to land, and I doubt I was using the best landing technique. Not like that is a conclusive test, as I haven't tried other engines.

I may have to try a test with one of my rockets I have with kOS on it. This way I can be sure to get relatively the same exact launch profile (other than straight up) and compare amounts of fuel left over once in orbit. Currently it uses 6 asparagus stages aerospikes, and one of the engines that has thrust vectoring. This is using all "normal" sized tanks...what ever they're called. :)

It's all about isp. Even at 11k meters, the LV-N was already at 760 isp out of a possible 800. That's up from 250 I believe at sea level, so the LV-N was already almost optimally efficient. However, for reasons stated by the others, the LV-N has its own strengths such as when it has a ton of fuel to work with.

Link to comment
Share on other sites
Awass

Awass

Posted
  • Author
Posted
Okay, I am absolutely FLOORED by the geniuses here. no sarcasm at all! I know this is a game using real world rocket science, but to see so many of you who have such a spectacular grasp of the math involved is awe inspiring!! I am truly a small fish among giants! Heck, I would not be surprised if Michio Kaku was wandering around here somewhere OR Professor Brian Cox, both super smart men! <bows down to his fellow players>

I could not agree more. Great community and a bunch of helpful replies to my post.

Link to comment
Share on other sites
JUDUFU

JUDUFU

Posted
Posted
To complement the single-engine comparison of the LV-N and LV-909, I've calculated the delta-v at which the LV-N becomes a better choice than the LV-909, as a function of payload:

The red curve is the delta-v (with its scale on the left) while the blue curve is the corresponding total ship mass (scale on the right).

Could you show us the math associated with this chart? I'm having a hard time wrapping my head around how you did this. Also what program are you using to produce the graphs?

Link to comment
Share on other sites
Specialist290

Specialist290

Posted
Posted
However for pure reusable landers they are usually an bad choice even then landing on low gravity world like Mun, again an miner who do the Minmus-LKO-> Minmus run will benefit from LV-N as most of its dV use is not takeoff, however you might want to add some chemical helper engines for takeoff, you just need to burn a 2-300 m/s then from low minmus orbit to LKO docking and return pretty dry.

In this situation, it seems to me that a modular configuration is ideal. Have a large nuclear tug and a much smaller lander docked together. The tug pushes the combination into orbit of the destination, while the lander undocks, lands, and returns. The tug then carries the combined vehicle back to the refueling point.

You can make a fairly small lander with reasonable delta-v, and it would take far less fuel in total than it would if you lugged all that fuel down to the surface and back with you.

Link to comment
Share on other sites
Meithan

Meithan

Posted
Posted (edited)
Could you show us the math associated with this chart? I'm having a hard time wrapping my head around how you did this. Also what program are you using to produce the graphs?

Sure. I started with the Rocket Equation and first solved for total mass as a function of desired delta-v:

gif.latex?m_{\text{total}}&space;=&space;\frac{8(m_{\text{payload}}&plus;m_{\text{engine}})}{9&space;\exp\left(\frac{-\Delta&space;v}{I_{\text{sp}}g_0}&space;\right)-1}

I can show you the detail if you want, but I think you already arrived at a similar expression.

I then set up the equation for two different engines, equated the total mass, and solved numerically for delta-v for several payload masses.

Edit: I forgot: I used gnuplot to make the graph. It's a free/open-source plotting software for Linux.

Edited by Meithan
Link to comment
Share on other sites
Crown

Crown

Posted
Posted
[...]

gif.latex?m_{\text{total}}&space;=&space;\frac{8(m_{\text{payload}}+m_{\text{engine}})}{9&space;\exp\left(\frac{-\Delta&space;v}{I_{\text{sp}}g_0}&space;\right)-1}[...]

Sadly the graph doesn't show up :(

But why is there a 8/9 in the equation? Shouldn't this be just

gif.latex?m_{\text{total}}&space;=&space;\frac{(m_{\text{payload}}+m_{\text{engine}})}{&space;\exp\left(\frac{-\Delta&space;v}{I_{\text{sp}}g_0}&space;\right)-1}?

Or is this a factor I didn't know about before?

Oh, and thanks for the link to this awesome formula editor. :cool:

Link to comment
Share on other sites
rkman

rkman

Posted
Posted
Its more likely mission dependent. The Atomic engines are great if you wish to get to Mun or Minmus, but you are right about the weight and atmosphere performance is not great.

It is easy to get to Mun or Minmus with normal engines, but for missions to Jool or Eeloo the atomic engine makes a really big difference.

Link to comment
Share on other sites
Silverchain

Silverchain

Posted
Posted
Protip thrust does not matter on interplanetary missions what matters is deltaV

Thrust does not usually matter as much on interplanetary missions.

You need thrust for:

Carrying out any manoeuvre in a reasonable amount of time. This matters in KSP much more than it does in real life because watching a two hour engine burn as your single LV-N boosts a giant stack of Jumbo tanks to Eeloo is boring!

Carrying out any manoeuvre near planets that is time critical....

...an important subset of which is making best use of the Oberth effect by carrying out burns at the right time. (Ideally you want an instantaneous burn at the point in your orbit where your speed is highest.)

Once you start flying missions to Jool having a reasonably nimble ship is very useful.

Link to comment
Share on other sites
Meithan

Meithan

Posted
Posted
Sadly the graph doesn't show up :(

Here's the direct link to the last graph: http://i.imgur.com/4HVffEp.png

But why is there a 8/9 in the equation? Shouldn't this be just

gif.latex?m_{\text{total}}&space;=&space;\frac{(m_{\text{payload}}+m_{\text{engine}})}{&space;\exp\left(\frac{-\Delta&space;v}{I_{\text{sp}}g_0}&space;\right)-1}?

Or is this a factor I didn't know about before?

It comes from the fact that I'm including the mass of empty fuel tanks in the calculation. Adding more fuel also adds more tank mass which you have to carry. And in KSP most fuel tanks have an empty mass equal to 1/8th the fuel they carry (or 1/9th their mass when full).

Oh, and thanks for the link to this awesome formula editor. :cool:

Yep, it's great. And has a lot of options to "share" the produced graphic.

Link to comment
Share on other sites
Technical Ben

Technical Ben

Posted
Posted
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.

Thanks for this. I'll keep it in mind when planning my missions. Now to get that 8 orange tank monster to move (LV-Ns OR 30s)...

(I have up to a 12 LV-N engine base I can swap for in orbit, oh and everything in between. Though I found the Rocomaxs to be far too big and usually shake the craft apart. But I might revisit that. Use some 909s occasionally for orbital tugs too.)

Link to comment
Share on other sites
WafflesToo

WafflesToo

Posted
Posted

(I have up to a 12 LV-N engine base I can swap for in orbit, oh and everything in between. Though I found the Rocomaxs to be far too big and usually shake the craft apart. But I might revisit that. Use some 909s occasionally for orbital tugs too.)

I ran into this recently as well. The problem is the engines gimballing around. Lock the gimbals on all of your engines except for the center one and it should clear up.

Link to comment
Share on other sites
Meithan

Meithan

Posted
Posted
But why is there a 8/9 in the equation? Shouldn't this be just

gif.latex?m_{\text{total}}&space;=&space;\frac{(m_{\text{payload}}+m_{\text{engine}})}{&space;\exp\left(\frac{-\Delta&space;v}{I_{\text{sp}}g_0}&space;\right)-1}?

Or is this a factor I didn't know about before?

By the way, the fact that fuel tanks do have mass also means there is a maximum delta-v for any engine:

gif.latex?\Delta&space;v_{\text{max}}&space;=&space;I_{\text{sp}}&space;\,&space;g_0&space;\ln&space;(9)

The 9 here has the same origin as in my previous formula: it's because fuel tanks have a full mass that is 9 times their empty mass. As you keep adding fuel mass to the rocket, the mass of the engines and payload (which is fixed) becomes negligible, but the tank mass keeps increasing (proportionally to the fuel mass). So in the high-mass limit the rocket is just fuel and tanks to contain it, and the total-to-dry mass ratio becomes constant, thus resulting in a maximum delta-v value.

For the LV-909, the maximum value is 8406.4 m/s. For the LV-N, it is 17244 m/s.

Link to comment
Share on other sites
Technical Ben

Technical Ben

Posted
Posted
I ran into this recently as well. The problem is the engines gimballing around. Lock the gimbals on all of your engines except for the center one and it should clear up.

Probably. I put it down to a stupid TWR and a the "rolling rocket" problem KSP occasionally has. I scaled down the weight/size of my pulling/pushing tugs and it works a lot better. Still hard to get a controllable design using docking ports only and big fuel tanks. Currently waiting for a Duna, Eve and Jool window to test subsequent designs for a long burn. The most stable might be a 3 row of ports. It's a scaled up version I made way back. Which reminds me, I still have to launch the fuel for that one!

PS, notes to himself not to try and hit 18k DV... must remember (especially as I've had 10k DV and 12k DV craft easily!).

Link to comment
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
 Share
Go to topic listing
×
×
  • Create New...