Would like some help with the Sciencey basics

[Caleb F.]

As per the way KSP works, in order to start making any voyages into the stars you have to have a grasp on payload capacity/weight versus how much fuel you are required to use which then comes onto the issue of adding too much weight.

Does anyone by chance have a good way to explain this to me? i always find myself with not enough fuel to either land on say the mun or can get there but not take off again. I think this is called the Thrust to Weight Ratio? Even after trying to watch videos from Scott Manley, I cant quite get it.

I guess this issue will also corolate to using the right pieces in the right context (best boosters for certain stages, etc)
Essentially im asking if there is a tutorial so in depth of the game that I can actually understand how this all works.

Any information would be great

 

- Caleb F.

[peteletroll]

Hello @Caleb F., welcome to the forum!

It looks like your question is about ∆v (Delta-V) and TWR (Thrust to Weight Ratio).

∆v: It's somehow related to how far can you go, so it's (mostly) about fuel: look around for "rocket equation" (if you're into math) and "delta-v maps" (to know how much delta v you need to go to Mun and back, for instance);

TWR: It's about how much your craft's engines can push: TWR < 1 means that your engines don't push enough to lift your craft. Very important on takeoff, of course, but also for powered landings (powered landing = no atmosphere, so no parachute, only engines).

There's much more to it, of course, but these are the key concepts. KSP provides you the tools to compute both values easily. You may try reading some KSPedia (the in-game help pages).

Fly safe, and come back if you have more questions!

[Vanamonde]

This is not a game which can be easily summarized, or encapsulated in a few equations. Even people who have been playing for a long time do a significant amount of trial-and-error. It sounds to me like you are going through the normal learning process.  

[rynther]

The key elements are weight, (how much stuff) Thrust, (how hard you can push on stuff) and fuel, (how long you can push that hard) 

The game does a lot of the heavy lifting for the math, but I tend to use the m/s as a shorthand, as an example, I like to have the first stage at about 16.4m/s of thrust (I use mechjeb to get the relevant info while building) I haven't done a bunch of mun landings, but it needs about 1400m/s worth of fuel to land, and re-launch to orbit, without being empty. (That does not include the fuel to get there, just the landing, take off, and a little bit left over)

If you are flying it manually, rather than using the autopilot, expect that you will likely use more fuel, unless you are very good at flying. (which requires a ton of practice)

BTW, the mun has very low gravity, so you don't need huge motors for the lander itself. (I use the small radial motors, or a terrier)

I hope this helps a little, but really, rocket science is complicated stuff, it takes time to learn about.

[king of nowhere]

14 hours ago, Caleb F. said:

As per the way KSP works, in order to start making any voyages into the stars you have to have a grasp on payload capacity/weight versus how much fuel you are required to use which then comes onto the issue of adding too much weight.

Does anyone by chance have a good way to explain this to me? i always find myself with not enough fuel to either land on say the mun or can get there but not take off again.

 

i hope you have a decent grasp of what is deltaV to start with (if not, ask further, but you can find a lot of good explanations there). That said, on the issue of how much you need to go places, refer to the deltaV map.

the deltaV map tells you how much deltaV (vacuum) you need. For example, to make a mun mission you need 3400 to orbit kerbin, + 860 to get a mun intercept, + 280 to get a circular mun orbit from there, + 580 to land, +580 to return to orbit after landing, + 280 to get a kerbin atmosphere intercept. from there you need another 860 + 3400 but you can get them free by aerobraking, as the arrow shows. so in total your rocket should have 6080 m/s of vacuum deltaV for a full mun mission with return.

A few things, though:

- those values represent optimized trajectories. if you are doing things wrong, you will need more, possibly much more. if you are very good, you can get away with a bit less. if you start using gravity assists, you can save some of those costs

- this map is in vacuum deltaV. be wary that vacuum deltaV and kerbin deltaV are different, because engines lose efficiency in atmosphere. make sure that your engines are adequate.

- just because there is an arrow, it does not mean aerobraking is practical. you can theoretically get aerocaptured by jool, but it takes a very special craft to survive the 9 km/s atmospheric speed. the one time i tried it, i got completely disintegrated before i was 3 km deep in the atmosphere. eve and laythe also have very unforgiving atmospheres, while duna is aerobrake-friendly. but you won't get there for a while, so this is a worry for the future.

all in all, you should take the deltaV map as a general guideline. but it is very useful to give an idea of how much deltaV you should pack.

 

adding weight decreases your deltaV. adding fuel increases you deltaV - but you may need bigger engines afterwards, and engines are weight, so you can only push this so far. also, some engines are better for use in vacuum, others are better for use in atmosphere.

Quote

 

I think this is called the Thrust to Weight Ratio? Even after trying to watch videos from Scott Manley, I cant quite get it.
 

 

if you say you lack fuel, then your problem is more likely to be deltaV. thrust to weight, or TWR, is how much your rockets can accelerate your rocket. 1 refers to the local gravity, so a rocket with twr 1 on kerbin will have 6 on mun, because on mun the gravity is 6 times smaller. it is very important for liftoff, because if it is below 1, then your rocket won't be able to lift - it won't be able to push against gravity. once in space, you can get away with lower TWR. as you spend fuel, your ship becomes lighter and your twr increases.

adding engines / using more powerful engines increases your TWR. adding anything else decreases it.

Quote

 

Essentially im asking if there is a tutorial so in depth of the game that I can actually understand how this all works.

Any information would be great

- Caleb F.

 

this one has got me laughing. no offence meant.

I mean, you can have a tutorial. or you can have an in-depth explanation. But you can't have an in-depth tutorial. this game is just too complex for that. in fact, i could spend hours writing, and still be missing stuff.  I tried to cover the basics. try to practice that.

try a mun mission with this new knowledge.

start with a lander. it must have enough deltaV (vacuum) to land and orbit mun; that's 1200 m/s. but it's not a lot; you would like to also be able to return to kerbin with it, and make some manuevers around mun. and landing with the exact amount of fuel is very difficult, better to keep some extra for safety. overall, 2000 m/s are a good target. and check your TWR. for a practical landing, you'll want at least 3 times the local gravity, so either 0.5 (kerbin, vacuum) or 3 (mun); you can set up the VAB to show you the TWR.

this lander covers everything from when you get in mun orbit. now you need to get there. so couple your lander on a rocket that must reach mun intercept. you'll want 3400 m/s for kerbin orbit, and 860 for mun intercept, plus some extra. 4500 is a good target. it's quite a large amount, so it's better to split it in two or three stages... check the deltaV and TWR on each one.

[jimmymcgoochie]

TWR only really matters if you’re trying to take off or land somewhere, in space it’s much less relevant- though a really low TWR will make everything take longer and you’ll lose a bit of fuel by firing the engines in a direction other than directly prograde/retrograde. What you really want in space is delta-V, which depends on two things:

• Engine efficiency, known as specific impulse or ISP. This is measured in seconds and represents how long a given engine can produce a given thrust with a given quantity of fuel; much like miles per gallon in a car, more is better as you’re sipping the fuel instead of guzzling it. Vacuum-optimised engines like the Terrier and Poodle have high ISP in space, but are usually pretty bad in atmosphere; in contrast, heavy lifting engines like the Mainsail and Vector have lower ISP overall but it doesn’t change much even at sea level on Kerbin.
• The rocket’s mass ratio. This is a key part of the rocket equation as it tells you how much of your rocket is fuel and how much is, well, rocket. You want the fuel fraction to be as high as possible, so using one FL-T800 will be better than 8x FL-T100 tanks as the dry weight of the bigger tank is comparatively less for the same mass of fuel. This is one key reason why rockets use stages- dropping the dead weight of empty fuel tanks and heavy booster engines makes you go further with what you have left. Cutting weight where possible e.g. using fewer, larger tanks or removing some parts you don’t absolutely have to have, will improve your delta-V under most circumstances, but the heavy stuff like science experiments are why you’re going out there in the first place so there’s always a balance to be struck.

Delta-V is simply how much can you change (delta) your velocity (v). Speed is distance (in metres, m) over time (in seconds, s) so delta-V is in metres per second (m/s); not to be confused with acceleration which is a change in velocity (m/s) over time (s) and is measured in m/s/s or m/s². Going anywhere in space requires changing your velocity so delta-V gives you your range- using a delta-V map gives you an approximate guide to where you can get to with the delta-V you have available and there are some in-game mods that are helpful for this too, such as Astrogator and Where Can I Go.

When designing a mission, start at the top with your mission payload. Decide where you’re going and if you’re coming back, then design your probe/lander/rover/whatever accordingly. Add the uppermost rocket stage to have enough delta-V to do what you need to do, whether that’s returning from the Mun or capturing into orbit of Eeloo, then add each stage below so that each has sufficient range to let the next one do its job with a small margin (I’d say 10-20%) on top. Keep the weight down by using a smaller number of bigger tanks e.g. one FL-T800 instead of 8 FL-T100s, and make sure to check the surface TWR and delta-V for the first stage to ensure it can get off the ground in the first place. One useful trick for launch rockets is to add fuel tanks on top of radial solid boosters and enable the crossfeed on their decouplers- the tanks on the boosters should drain first, powering the main engine(s) and then being dropped with the spent boosters. This can give you quite a lot more fuel without impacting your surfaceTWR too much as solid boosters are generally pretty powerful for their size and weight.

Trial and error is the best way to learn. Videos and tutorials can help, but they’ll only get you so far and you need to do it for yourself to really understand it. Don’t be afraid of making mistakes, by learning what doesn’t work you’ll also learn what does. Poke around with the stock crafts to get some ideas and if you don’t understand anything, ask! Everyone had to go through the same learning process one way or another and there are no stupid questions- it is rocket science after all!

 

Edited March 27, 2021 by jimmymcgoochie

[Snark]

On 3/26/2021 at 9:18 AM, Caleb F. said:

As per the way KSP works, in order to start making any voyages into the stars you have to have a grasp on payload capacity/weight versus how much fuel you are required to use which then comes onto the issue of adding too much weight.

Does anyone by chance have a good way to explain this to me?

As folks have said, "It's complicated". 

But it basically boils down to this:

1. Decide what mission you're going to run.  (Take a kerbal to LKO and back?  Land on the Mun?  Land on the Mun and return?  etc.)
2. Figure out how much dV you'll need to do that mission.
3. Design a rocket that has that much dV (plus a reasonable amount extra as safety margin).
4. For the parts of the mission that involve landing or taking off from the surface of a moon or planet, make sure your TWR is appropriate, given the local gravity.  (Until you start visiting other planets, taking off from the launchpad at KSC is the main place you need to be concerned with this.)

As to "how do you figure this stuff out":  each of #2, #3, and #4 is a whole complicated topic in its own right and one could spend a lot of time explaining it.  Since I don't know which bits you already know and where you need more understanding, I'll hold off on throwing great walls of text at you and let you ask more questions as you need. 

Broadly speaking, though:

• Figuring out #2 can get all mathy if you want, but there are dV maps such as what @king of nowhere posted above.  For example, getting from Kerbin surface to low orbit takes about 3400 m/s.  To go to the Mun from there takes about another 860 m/s, then 280 m/s to get into low Mun orbit, then 580 m/s to land on the surface.   Add 'em up.
• "Build a rocket with a certain amount of dV" is where you hone your design skills-- that's the tricky bit.    Basically you build it stage-by-stage and pick an appropriate engine and an appropriate mass of fuel for each stage.  You'll have a long learning curve to climb here.  One good way to learn is to just post a pic of your ship here, and we can offer suggestions about how to improve it (with explanations as to why).

[Caleb F.]

First off id like to say thank you everyone for taking the time to respond and help out any which way you can! its awesome.

Secondly ill respond to most of the responses where necessary. Most of them are repeats of each other and will only respond to some that I feel need to be Responded to. (Doesn't mean I haven't read yours, I still appreciate every piece of advice and thought in every comment)
 

@rynther  Thanks for taking the time to answer. I've tried using the terrier engine but it never seems to have enough thrust to slow me down descending to the Mun. Am I too heavy or am I using it incorrectly?

 

@king of nowhere This helped a lot in explaining most of what i was struggling with, (this game really is complicated! XD (Not sure what i expected, as what everyone is saying, it is literally rocket science)). Most of what you explained I did already have a realistic grasp on (say the fact that the more you put on your rocket the heavier it will be, the more thrust required, thus requiring more fuel, thus making the rocket heavier). As stupid as it sounds, for some reason I hadn't thought to actually take the time and make each section piece by piece making sure they each had enough fuel and the correct boosters for their designated spot on the trip. I'll get back to this later but I have done multiple attempts at a Mun mission each time failing. I made it there but always never had enough fuel to take off again, essentially being stranded. I also found the terrier engine never had enough thrust for the fuel it used to slow me down enough to land either so stopped using it. Could I be using it wrong?
Anyway, I will definitely take what you have contributed into account and It has genuinely helped me in understanding DeltaV and TWR a little more. Thank you

 

@jimmymcgoochie  I wont lie, kind of got lost there a bit. however like I said to King, I understand most of the realistic ideals in the game, eg heavier you are, more thrust is required, or that one big fuel tank is better than multiple small ones. Once again what I said to King, I also hadn't thought to lay out my entire mission as the stages, essentially going in reverse not from Kerbin to Mun but rather Mun to Kerbin. Starting with the lander and making sure it can land and take off and then proceed to enter Kerbins Orbit close enough to hit the atmosphere for an airbrake. Then looking at what it takes to go from Kerbins Orbit to the Mun, then the stages to take off etc. You talked about using side boosters but essentially running a main engine plus the side boosters off of their fuel. Im assuming this is with the fuel lines (which at my point in the game dont have researched yet but will keep that in mind as I can see how you can still get the starting push up with the side engines then once in the air they are gone but still full on fuel in the main fuel tanks.
Anyway, I appreciate your comment and will take note of the things you have pointed out and appreciate your time.

 

@Snark Noting down your System I will try it. As per your suggestion I will post a picture of my Mun Lander Rocket (I called it the Mooner). It could get me to the Mun and Land but never had enough fuel to take off again (Could get up a couple thousand meters but always came tumbling back down instead of finding a stable orbit). Tried different engines on the lander stage as well as extra fuel but always fell short as then my lower stages couldnt get me there in the first place! However, I for some reason can no longer find the save file for this specific rocket (whether i tried to upgrade it and forgot to rename so it overwrite the craft or something I dont know. I will try to make one that I see working and ill try it. Any Feedback on the craft would be greatly appreciated when I get it going.

 

[king of nowhere]

7 hours ago, Caleb F. said:

 I also found the terrier engine never had enough thrust for the fuel it used to slow me down enough to land either so stopped using it. Could I be using it wrong?

either you are using it wrong, or you are too heavy.

ok, technically, what counts as "too heavy" depends entirely on you. if you want to land a 1000-ton thing on mun, you can, and of course in that case a terrier is inappropriate. but for a simple early mun lander, up to 10 tons of mass on the lander, a terrier is perfect. so, if your lander weights less than 10 tons (and i doubt you alredy started adding features like i do), then you are using the terrier wrong. probably you started burning too late.

if you failed your mun missions because you didn't have enough fuel to return, you either have insufficient fuel in the rocket, or you are using too much in the landing. easy to find, look how much deltaV you have when you start descent. if you want to be able to land, return to orbit, and return to kerbin, you need 1500 total, though you'll probably need 2000 because i won't try to coach you on a tight landing yet. anyway, if after you land you have 700 m/s less than when you started, then all is fine. if you have 1000 m/s less than when you started, you are making a bad landing and wasting fuel.

or perhaps you spend more because you are making bad ascents? do you know to gravity turn for efficient orbiting? if you go straight up and then straight laterally, you end up spending some 20% extra.

 

anyway, i tried troubleshooting the most likely reasons, but if you want specific advice to fix your mun mission, you need to give us more information on it. with pictures, possibly

[rynther]

For my mun minor project, a 30t drilling and conversion station, I'm using 6 cub vernier engines (32Kn each), using the landing autopilot uses about 1000m/s of fuel (which is far better than I can do by hand) with a fuel payload of 1500m/s, this was not meant to return on the remaining gas, rather it needs to mine the difference, or just stay put.

The best I've managed flying by hand (mind you in like 0.91 or earlier, because I haven't played with manual mun descents in years) was about 3000m/s

The moral of this story is simple, manual landings that are good on gas take a excrementsload of practice.

[rynther]

One thing I did catch, was the fuel priority, you don't need fuel lines to crossfeed fuel,  radial decouplers can crossfeed fuel as well, so you can use that even in relatively early game designs, so I thought I would mention it.

[rynther]

At the risk of post flooding, I'll mention some stuff I use a lot, first, SRB's are soooo your friend, learning how to tune them will give you lots of cheap thrust, in fact SRB's are the cheapest thrust you can buy.

This comes with the price that you must set the throttle in vehicle assembly, and once lit, they burn until they are empty. Most of my first stage rockets are pure SRB, designed to burn to 70km altitude. (yeah, the first liquid fuel engine to light will already be in vacuum) Now look, this is not the most efficient way to burn, requiring delayed turning, over thrusting, and a pretty strange looking steering curve. However, it's MUCH cheaper than conventional first stage rockets, by about half, to as low as a quarter, with the right payload.

To put this into perspective, you can launch a mk-1 with 2 hammers (trashcans full of boom) and a flea, and get a vertical apoapsis of about 300km.

The recipe is a mk-1 capsule, 1 mk-16 parachute, (2 radial droge chutes if you have them, just to smooth out the ride), 3 td-12 decouplers, 2 rt-10 hammers, 1 rt-5 flea, and 18 basic fins. Capsule first, chute(s) on top, (throw a couple barometers and goo on it for the points if you like) td-12, flea (set to 40% thrust), 6 fins on the flea, td-12, rt-10 (set to 50% thrust), 6 fins at the bottom of the rt-10, td-12, rt-10 (set to 100%) 6 fins on the bottom. (yeah, that will actually fly straight, don't forget the fins, and I really don't recommend turns)

This is a big point to consider, because  the cost of your projects really matter, and saving money can be as important as conserving TWR, or more so. (as if there weren't enough points to consider)

I tend to focus on "lift per dollar" rather than "lift efficient" it's a different perspective to look at the question from. Not sure how helpful this might be to your specific goals or playstyle, but it is food for thought.

Second, if your rocket "wobbles" adding control rings will likely make it worse, add struts instead, add control rings when steering is too sluggish. (if I'm using a very long rocket, I'll use a girder segment  as a high point to attach the struts vertically, but if using struts between stages, keep it to 3 or 4 struts, too many, and you'll need a stack separator, or it won't let go.)

Third, always check your electric draw, and make sure you have enough batteries (and/or panels), no amount of fuel will save an out of control rocket with dead batteries. ( I've done this too many times not to mention it)

[paul_c]

There's a lot to take in in previous replies, but to make things simple, the only two things you need to worry about are:

deltaV
TWR

Forget about aiming for a specific weight, or thrust, or amount of fuel, or using a particular engine etc (they're wrapped up in the above anyway).

For Mun landings, when I did them I'd always budget on: 3000m/s left in LKO, to be able to 1) get to the Mun 2) be captured in its orbit (retrograde burn) 3) adjust to a low Mun orbit for landing 4) land 5) take off.

If you've never landed on the Mun, make the first mission a "land only", ie don't worry about the extra fuel needed for a subsequent take off and orbit. This will simplify things a lot - no heatshield/decoupler/parachute(s) needed.

Edited March 29, 2021 by paul_c