How much fuel I need

[funk3n]

So I have a mission to land on Mun and rescue some ♥♥♥♥♥♥ kerbal. Previously I have successfully landed on Mun and got back to Kerbin (w/ monopropellant and rcs lol). I have serious problem w/ fuel management. On top of my rocket I have Mk3 pod, service bay (the big one) w/ science tech, batteries and 2-4 monopropellant tanks, it weighs around 3.1t I think.

I tried to search some info about calculations on google, but it's too difficult for me to understand all this science terminology. If only somebody can explain basic calculations in easy language.

Also is TWR thrust to weight ratio? If yes, so it means I need to get TWR > 1 to get lift off, but how much fuel then I need?

[Azunai]

if the calculations are too difficult/tedious, you should consider installing the mod Kerbal Engineering Redux. it will give you a fairly accurate deltaV number for your rocket (deltaV is the amount of acceleration the rocket can do before it runs out of fuel, so basically it's the "effect" of the fuel).

there's also deltaV maps of the kerbin solar system that can give you an idea how much deltaV you'll need to get to various locations. for a mun landing & return, you'll need something like ~5500 m/s in total. ~3400 to kerbin orbit, ~1200 to low mun orbit, ~1300 for landing and launching back to orbit, 300 for transfer back to kerbin; you can do an aerocapture at kerbin, so you don't really need any more fuel once you have a return trajectory that gets you into the atmosphere of kerbin.

it's a good idea to have more deltaV than the bare minimum if you're new and don't have a good understanding of the game yet. you can easily use up an extra 500 or 1000 m/s for getting to kerbin orbit alone if you don't fly a good gravity turn, or use a few hundred m/s more for the mun landing if you don't know how to do it efficiently.

[jpcerutti]

You can also blow some deltaV by cutting transfers too tight and having the rock you're leaving pull you back (grumble). :) there is a good page for thumbnail guesswork though: http://forum.kerbalspaceprogram.com/index.php?/topic/87463-105-swashs-delta-v-map-continued-22-nov-17th-opm-update/

[Phoenix¹]

Don't do suicide (efficient) burn right at the start. If you just want to go to the Mun for now don't calculate (or minmus, or anything really). You should be able to do it (landing and return) with a poodle engine and a medium sized gray tank. Just pack another stage to set the Mun transfer burn, or one for circularizing AND transfer burn. You can use the delta-v that mkun told you and just compare to how much you got per stage. Remember that delta-v is the same no matter the size of the ship.

[Sapper Woody]

Originally posted by funk3n:

So I have a mission to land on Mun and rescue some ♥♥♥♥♥♥ kerbal. Previously I have successfully landed on Mun and got back to Kerbin (w/ monopropellant and rcs lol). I have serious problem w/ fuel management. On top of my rocket I have Mk3 pod, service bay (the big one) w/ science tech, batteries and 2-4 monopropellant tanks, it weighs around 3.1t I think.

I tried to search some info about calculations on google, but it's too difficult for me to understand all this science terminology. If only somebody can explain basic calculations in easy language.

Also is TWR thrust to weight ratio? If yes, so it means I need to get TWR > 1 to get lift off, but how much fuel then I need?

TWR is Thrust to Weight, yes. Greater than 1 means that you will lift off, but the higher the number, the faster your acceleration. I typically try to keep my TWR for my first stage at ~1.5. For me, any higher and find that I'm accelerating too quickly in atmosphere. After that, use whatever, but remember that higher ratios usually mean lesser fuel efficiency, which also usually means less Delta-V.

To calculate TWR, it's a fairly straightforward process. Simply, it is thrust divided by weight. But remember that to go up, you have to account for gravity. So, you want your acceleration to be higher than 9.8m/s/s. Simply put, if you divide your weight by your thrust, and then divide it by 9.8, and you come up with a number that is greater than 1, you will rise.

To figure up how much fuel you need, you have to figure out how much Delta-V you need. Delta-V literally means "change in velocity". Delta-V is basically a calculation of thrust to weight, with the fuel usage taken into account. If you don't have a background in (or an understanding of) at least college Algebra (and I'd recommend Calculus), then don't bother looking delta-v up on wikipedia. The easiest thing to do is, as others have said, install a mod that tells you the amount of Delt-V on the rocket. There are a couple that do this. Kerbal engineering will do this, and Mechjeb as well.

Edited for Spelling.

[r.zarth]

Originally posted by jpcerutti:

You can also blow some deltaV by cutting transfers too tight and having the rock you're leaving pull you back (grumble). :) there is a good page for thumbnail guesswork though: http://forum.kerbalspaceprogram.com/index.php?/topic/87463-105-swashs-delta-v-map-continued-22-nov-17th-opm-update/

the dV map is gold ! Thank you!

[SievertChaser]

Originally posted by Sapper Woody:

To figure up how much fuel you need, you have to figure out how much Delta-V you need. Delta-V literally means "change in velocity". Delta-V is basically a calculation of thrust to weight, with the fuel usage taken into account. If you don't have a background in (or an understanding of) at least college Algebra (and I'd recommend Calculus), then don't bother looking delta-v up on wikipedia. The easiest thing to do is, as others have said, install a mod that tells you the amount of Delt-V on the rocket. There are a couple that do this. Kerbal engineering will do this, and Mechjeb as well.

A handy term that hasn't been mentioned is the dV budget. Which means that dV is a hard cap on what missions your craft can undertake, its "range" if you wish.

Fuel to dV conversion is subject to diminishing returns. You can't squeeze more than 3500 m/s out of a chemical stage, no matter how much fuel you add. Therefore, a Mun mission will require 1-2 stage separations after Kerbin orbit. However, at that point you may attempt drop tanks rather than true staging.

If you have docking ports and RCS tech, now is when reenacting Apollo can save you money. Don't cart the return vehicle up and down Mun's gravity well (Direct Ascent). A Lunar Orbit Rendezvous is quite useful, although I haven't had luck in reducing launcher size using it, and ultimately am currently "cheating" by going nuclear:
http://steamcommunity.com/sharedfiles/filedetails/?id=644695071
http://steamcommunity.com/sharedfiles/filedetails/?id=644696642
http://steamcommunity.com/sharedfiles/filedetails/?id=644697406
http://steamcommunity.com/sharedfiles/filedetails/?id=644697696
http://steamcommunity.com/sharedfiles/filedetails/?id=644697982
http://steamcommunity.com/sharedfiles/filedetails/?id=644698335

[funk3n]

Thanks guys for help, gonna try it later today and then I will reply w/ my achievments. One more question, is it better to first establish an orbit around Kerbin and then burn to reach Mun, or w/o orbit straight to the Mun? Tried to calculate something and got that I need same delta-v w/o or w/ orbit around Kerbin

[SievertChaser]

Originally posted by funk3n:

Thanks guys for help, gonna try it later today and then I will reply w/ my achievments. One more question, is it better to first establish an orbit around Kerbin and then burn to reach Mun, or w/o orbit straight to the Mun? Tried to calculate something and got that I need same delta-v w/o or w/ orbit around Kerbin

Direct burns to the Mun are pretty damn tricky, not possible at certain times of month and generally ill-advised.

[funk3n]

Originally posted by dennis.danilov:

Originally posted by funk3n:

Thanks guys for help, gonna try it later today and then I will reply w/ my achievments. One more question, is it better to first establish an orbit around Kerbin and then burn to reach Mun, or w/o orbit straight to the Mun? Tried to calculate something and got that I need same delta-v w/o or w/ orbit around Kerbin

Direct burns to the Mun are pretty damn tricky, not possible at certain times of month and generally ill-advised.

still I managed somehow to do it, succesfully land on Mun and get back, but the going home part was a little bit tricky, because for orbit decreasing around Kerbin I used my RCS blocks..

[Empiro]

Direct burns are almost always less efficient than going into orbit first.

[t r a s h p a n d a]

Orbit is Best bro