Capital ship propulsion-- Isp or TMR more important?

[RSwordsman]

I'm getting ready to build my second large, orbit-assembled vessel and in the design of the thruster section, this conundrum occurred to me. In my first ship, I just used a big honkin' array of fuel tanks with a cluster of 5 LV-Ns. This one will be a bit bigger, and I want to keep a good balance of total dV against power, mostly for the sake of convenience. Those long burns get annoying XD.

But I have a sneaking suspicion that my original idea to throw more fuel and engines at it might not necessarily equate to more dV as the mass increased proportionally. Is there a tipping point in which it would be better to use engines with lower Isp, but higher thrust and lower mass? Or is Isp the dominant stat despite added mass and it's effective enough to cluster a ton of nukes together?

Edited January 22, 2014 by RSwordsman

[Tank Buddy]

You would probably get move DV from adding more LV-Ns just because of their Isp.

You can use this equation: Ve*ln((9x+Dry mass of ship w/o fuel tanks)/(x+Dry mass of ship w/o fuel tanks)) to find at what point one engine or engines provide more DV. (find the intersect with one equation having LV-Ns and another having your choice of engine)

X=dry mass of fuel tank and 9x is the mass of the tank(s) with fuel. When you have the mass of the tanks, you can do some ratios and figure out how much of that mass is liquid fuel, then use the 9:11 ratio to find the oxidizer. Using the mass of liquid fuel in the VAB, divide the mass of liquid fuel by the mass of the tank empty (on any normal tank, not Oscar to find the ratio. Cross multiply with your ratio (dry mass(x)/variable) You can experiment with different numbers of engines.

Solve with a graphing calculator or excel. I wouldn't try this by hand if I were you...

[RSwordsman]

^^That equation looks pretty good, thanks. But I am lost at where to put in the engine values. It looks like the only variable involves the dry vs. wet mass. **EDIT: Does Ve mean exhaust velocity? I just noticed that.

I also found a web app that allows you to plug in values: http://www.strout.net/info/science/delta-v/ It adds a spot for Isp, which makes it a little easier for the questionably-competent rocket scientist in me to understand. I haven't tried it yet, but do plan to test it with the equation you gave.

Edited January 22, 2014 by RSwordsman

[Taki117]

^^That equation looks pretty good, thanks. But I am lost at where to put in the engine values. It looks like the only variable involves the dry vs. wet mass. **EDIT: Does Ve mean exhaust velocity? I just noticed that.

I also found a web app that allows you to plug in values: http://www.strout.net/info/science/delta-v/ It adds a spot for Isp, which makes it a little easier for the questionably-competent rocket scientist in me to understand. I haven't tried it yet, but do plan to test it with the equation you gave.

Ve is in fact Exhaust Velocity, which is Isp * g where Isp is the specific impulse of your engine, and g is the standard gravitational parameter, in this case 9.81

[Specialist290]

As I always try to do when the subject of the rocket equation comes up, I'm going to recommend you do a little light reading. Specifically, I'm going to recommend reading the first four pages on the basics at MyKSPCareer.com and these three pages at Atomic Rockets. Both sources are written to discuss how things like delta-v, thrust-to-weight ratios, and other important variables for rocket engines and the craft they propel are important and what you can do with them to start optimizing your own designs. I especially found the Atomic Rockets pages to be very helpful when I was first getting into the game myself.

Hope this helps

[Tank Buddy]

As Taki117 said, Ve (Exhaust velocity) is a function of Isp*9.81. This is where the engine values go in. You graph the 2 equations, one for the LV-N (Exhaust velocity 7848) and another for your second ship, with a different engine. I used a TI-84, but you also can use excel. The purpose of the formula is, given 2 identical ships, with the only difference being the engines used, when do they produce the same amount of DV (Y value) and how much fuel has to be burned ((9)X value in tons)

I have also used that website, but I only used it for singe calculations for the DV of one stage. Feel free to send me the numbers on your ship if you want me to double-check. I used this equation for a project earlier this year and it was accurate, except I had to round a few places because the mass was something like 4.3156316545. The game did not like that when I make a custom part.

[tavert]

I've got a bunch of data here http://forum.kerbalspaceprogram.com/threads/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR that will tell you, for any given minimum TWR, payload mass (everything except engines, propellant, and tanks), and delta-V, which engine type you should use to minimize total craft mass. It's essentially running the rocket equation many times for each different type of engine, but also considering the number of engines you'll need to meet your TWR requirement, and how many fuel tanks to get the required delta-V.

[RSwordsman]

^^That chart is also something I was thinking of, but I couldn't remember the specifics and wasn't sure if it could be applied to multiple engines. It does suggest that the LV-N is the one to go to for large payloads.

But I'm also well acquainted with Atomic Rockets (one of the best things that ever happened to me for fiction writing) and I'll give those other pages a try. Thanks everybody! I know I'll be eager to brag with pics in the mothership thread soon when it's finished.

[TheDarkStar]

For any large ship, you'll be far past the point where smaller but less efficient engines are useful. Using the less efficient ones will probably increase the fuel required, leading to drastically increased burn times. Probably what you want to do is to design the strongest thing you can so that the ship can withstand physics warp. While it's burning, though, you might still want to find something to do like reading a book. Burn times for large craft are never short.