Help with calculating delta v and twr

[Screeno]

Hi fellow kerbonauts! I've been designing an ssto, that when refueled in LKO is meant to go to the mun, I'd like to know how much delta V is have when I'm refueled, so I don't have to make the trip and then realise I can't get back... I much rather know if its possible or impossible before hand XD I'm very crap with numbers, so if someone could explain how a delta V calculation goes that would be great. Oh and twr too lol thanks.

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When its refueeled it has exactly 3000 units of liquid fuel, and it weighs 46.1 tons at that point. Also it is using 2 nuclear rockets with 60kn each and an isp 800

[Invader Jim]

I use http://www.curse.com/ksp-mods/kerbal/222685-kerbal-engineer-redux

[Screeno]

but it seems that mod involved putting an extra part on your ship, i thought it was just a ui mod... you see i want my ships to all be stock parts.

[Randazzo]

but it seems that mod involved putting an extra part on your ship, i thought it was just a ui mod... you see i want my ships to all be stock parts.

Then you'll need to read up.

Here's the most basic stuff: Tsiolkovsky Rocket Equation

[adinfinitum]

For deltaV, the equation is g*Isp*ln(m1/m2). g is 9.81m/s, aka gravity at Kerbin sea level, Isp is the given value for the engine, you want to use vacuum Isp if you're in space, m1 is the mass with fuel, m2 is mass with fuel drained.

For thrust to weight ratio, you take the total thrust, divide it by the gravity of the body you are landing/taking off from, and divide that new number by the mass of your craft.

Both calculations are easy to do by hand on a scientific calculator, and I think most phones now even have ln buttons.

[Screeno]

what does "In" mean? is that dividing? multiplying? what to i divide and multiply by what ? ive now got a load of numbers, but i donrt know what toi do with them.

edit> omg i did it lol so i now.. think that my craft has 3089 Delta v in space. is that enough to land on the mun and back? the thing is i will end up wasting delta v a lot on landing because the craft would be heavy and ive only got 2 nukes.. argh im just gonna try it. if this doesnt work i think ssto's to the mun are dead.

Edited May 13, 2015 by Screeno

[Goomblah]

The "In" is ln (lowercase L, lowercase N), which stands for the natural logarithm, the inverse function of e^x. It has its own button on pretty much any given scientific calculator.

[adinfinitum]

The ln stands for natural logorithm, it's just a certain type of function that comes up a lot in physics.

That should be plenty of dV to land on the Mun and come back, but if your thrust to weight ratio is low make sure you start your landing burn early, there's not much worse than getting down to a hundred meters and finding out you've still got 50m/s to kill

[kurja]

what does "In" mean? is that dividing? multiplying? what to i divide and multiply by what ? ive now got a load of numbers, but i donrt know what toi do with them.

edit> omg i did it lol so i now.. think that my craft has 3089 Delta v in space. is that enough to land on the mun and back? the thing is i will end up wasting delta v a lot on landing because the craft would be heavy and ive only got 2 nukes.. argh im just gonna try it. if this doesnt work i think ssto's to the mun are dead.

ln = natural logarithm. logarithm of fuel mass fraction times constant g times Isp.

-well, ninja'd. As usual.

[cybersol]

but it seems that mod involved putting an extra part on your ship, i thought it was just a ui mod... you see i want my ships to all be stock parts.

You can switch KER to partless mode in its settings menu. Then all your craft will be stock. I still thinks its a great idea to learn to calculate deltaV and TWR yourself initially.

[Screeno]

Cool thanks I didn't know you could do that, looks like I will get KER then!

[amorymeltzer]

You can also just use KER in the VAB/SPH to get the initial numbers right, then fly stock. I don't use KER (or MechJeb) in flight until I've "earned it", but I figure it's safe beforehand to figure out numbers, especially the stage-by-stage stuff.

[SoberSquid]

Breaking this down a bit more for any kerbalnauts that want to do this by hand for fun and profit. Hopefully my math isn't off here, if it is, feel free to correct me!

For anyone who doesn't know what a logarithm is, it calculates out "How many times (x) do we multiply y to get number z" (log ^ y (z) = x) , for example, 5 * 5 * 5 = 125, so log^5(125)=3. For the rocket equation, we are looking at a natural logarithm, which uses Euler's Number (2.71828) as Y. So ln is the same as saying log^2.71828, or log^e.

So lets break down the rocket equation based on the information in the OP

When its refueeled it has exactly 3000 units of liquid fuel, and it weighs 46.1 tons at that point. Also it is using 2 nuclear rockets with 60kn each and an isp 800

The rocket equation when using Isp is DeltaV=Isp*g0*ln(M0/M1), where g0 = Gravity (9.81m/s^2), M0 is mass when full of fuel, and M1 is empty weight. So pulling down from above, we have an approximate empty weight of 12.8 (Fuel units weigh about 0.0111 each, so 46.1-(0.0111*3000))

DeltaV=800*9.81*ln(46.1/12.8)

DeltaV=7848*ln(3.6015625)

DeltaV=7848*1.325

DeltaV=10398.6

So you should have plenty of DeltaV for things, assuming you have a way to get all that into space directly. The harder part of those engines is they are pretty bad in Atmosphere by comparison (Having an Isp of 280 instead of 800 in atmosphere, plugging that into the equation you come out to 3639.51, on those engines alone, you won't even get into orbit from the surface!

For TWR you are looking at TWR=Thrust/(m0*g), so in this case, TWR on Kerbin would be 120/(46.1*9.81), or 0.265, whereas in space it would be 2.603 (120/46.1).

As such, I would expect your ship to work quite well in space with those engines, but on Kerbin, It wouldn't lift off by itself. Taking that ship, strapping it to a rocket and getting it to space first is your best bet. Once in space, the ship should easily be able to make it to the mun and back.

Edited May 13, 2015 by SoberSquid

[iDisOrder]

Breaking this down a bit more for any kerbalnauts...

I'm certain that you did a wonderful job simplifying this for most here,

but for those of us that your response causes migraines, is there a mod that allows you to enter a destination and calculate it for you using the ship you've built?

(yes, I'm a total n00b at this in case your wondering)

[Red Iron Crown]

I'm certain that you did a wonderful job simplifying this for most here,

but for those of us that your response causes migraines, is there a mod that allows you to enter a destination and calculate it for you using the ship you've built?

(yes, I'm a total n00b at this in case your wondering)

Conveniently, the delta-V to a destination doesn't change with ship design, it's the same for all ships to a given destination. You can calculate the delta-V cost for body to body transfers with an online calculator like these two: simpler and more complex.

For a mod that does the same you can look at Transfer Window Planner, or the Maneuver Planner in MechJeb.

[SoberSquid]

Conveniently, the delta-V to a destination doesn't change with ship design, it's the same for all ships to a given destination. You can calculate the delta-V cost for body to body transfers with an online calculator like these two: simpler and more complex.

For a mod that does the same you can look at Transfer Window Planner, or the Maneuver Planner in MechJeb.

Excellent resources. Yeah, the math definitely isn't for everyone. I don't often do the math by hand, other than workplace doodles. Mostly I use Kerbal Engineer to figure out Delta-V levels when building, and rough estimate charts like this guy when playing. I just wanted to outline how the math works out for people who wanted to know

[iDisOrder]

Bookmarked all the suggestions given.

Thanks for all your help gentlemen!

[GoSlash27]

Excellent resources. Yeah, the math definitely isn't for everyone. I don't often do the math by hand, other than workplace doodles. Mostly I use Kerbal Engineer to figure out Delta-V levels when building, and rough estimate charts like this guy when playing. I just wanted to outline how the math works out for people who wanted to know

A big +1 to the sentiment, but I'm really big on embracing the math. As much fun and satisfaction there is in this game, it's even better when you're able to work out concepts on the back of the envelope and vindicate it because the math said it would work.

Best,

-Slashy

[ArmchairGravy]

My advice is to get KER and use the tape deck. It's too awesome not to use.

[Falkenherz]

Sorry to chime in a bit late, but something is unclear to me: KER gives me a reduced dV output, which does not match to the deltaV formula, as soon as TWR (in relation to Kerbin) goes below 0.5. This is not expressed in the deltaV forumula, and I wonder what variable I need to add? It seems to make sense to me that a lower TWR leads to longer flight times and KER is definitely correct, as far as I can tell from my tests.

EDIT: Disregard. I might have gotten confused when I added mass, will test again tonight. My goal is a high dV with TWR > 0.5 and that´s where LV-Ns definitely do not shine and Poodles might be a better choice.

Edited June 26, 2015 by Falkenherz

[Warzouz]

but it seems that mod involved putting an extra part on your ship, i thought it was just a ui mod... you see i want my ships to all be stock parts.

Not if you have an engineer on board or an advance probe core, or if you play in "science" or "sandbox" game mode

[tutike2000]

but it seems that mod involved putting an extra part on your ship, i thought it was just a ui mod... you see i want my ships to all be stock parts.

You don't need to add any parts with KER.

In the VAB you'll see your TWR, mass, Delta-V, etc. There's also a setting to allow you to continue using KER even without adding any parts to your ship.

You can probably delete the part files from your mod install if you don't want to see them in your part list.

[Nefrums]

never mind...

Edited June 26, 2015 by Nefrums

[GoSlash27]

A discussion that's relevant here:

http://forum.kerbalspaceprogram.com/threads/125893-Kerbal-Rocketeering-101-By-Professor-Lynch-1-0-2?p=2038270&viewfull=1#post2038270

In this post I'm explaining how to set up a spreadsheet to mathematically design stages based on DV requirement, payload, and minimum acceleration. Starting from these criteria it is possible to derive the exact number of engines, mass of fuel and oxidizer, and mass of tanks required.

From that you can calculate total stage mass including payload, payload fraction, fuel fraction, fuel cost, stage cost, etc.

If this is done simultaneously for every engine type, it allows you to find the ideal design for a stage immediately.

This is an incredibly powerful tool and I find it indispensable. KER doesn't give you this capability.

Best,

-Slashy

[Red Iron Crown]

A spreadsheet, Slashy? I thought you were a pencil and sliderule sort of person.

Kidding aside, I have to agree that the dV calculating mods don't answer all the interesting questions. They answer one question really well, "What is the delta V of this design?", and that is enough for many players (and often enough for me, tbh). But if you want to answer questions like "How much fuel and engine mass do I need to give this design x delta V and y TWR?" or "How much payload can this tug push to body x?", then they become a bit tedious to use and a spreadsheet or calculating by hand becomes better.