How to reduce the fuel amount needed?

[etse]

I am a person, and I am sure many people are like me, that usually put many large fueltanks on the ship alongside some huge engines for anything - and I do know that I in theory somehow should be able to get my payload into space with lesser fuel than what I actually use. But since I often make it to the places I want to go, with just a little fuel left, I am afraid to try and make smaller spacecrafts. Though I guess much of the fuel is actually just wasted on lifting all the extra fuel I added on the spacecraft. Just to give you an Image, by spacecraft usually end up with 1+6 large fueltanks in asparagus staging.

Something looking close to this:

Now say I jmy payload was a lander that I armed to get to minmus; I would guess I could not really need to much fuel for my 1st stage for getting out of the atmosphere? I mean, 7 mainsail rockets sounds verry inefficient. But I have trouble finding new designs that use smaller rockets and fueltanks that does the job well.

So in short, what I am asking is not for some complete spacecraft that does the job, I am looking for methods to figure out what engines I need (thrust to weight ratio maybe?) and how to know how much fuel I need to get my payload into orbit. (I guess those two are highly related).

[HoY]

I just threw this one together yesterday, easily gets to minmus with 2/3 of its fuel remaining on the top stage.

Doesnt carry a whole lot.. each of those balls is just a probe core, RTG, 2 nose cones, and 3 cubis struts.

it really depends on your payload..

[MBobrik]

from what I see too much thrust - you are wasting your fuel to air drag.

[etse]

Thats not really what I am asking for. That is kinda a completed spacecraft for me, does not really help. I want to be able to create my own designs what can get "anywhere", with as small amount of fuel as possible. Not just use something someone else have made.

For instance, how do you know when to use small rockets instead of big mainsails? and how do you know how much fuel you wanted to add? 'Cus I am in the situation where I add to much fuel I think - and this I need much more thrust to actually get this thing into orbit. I do also waste alot of extra fuel due to the extra mass. So I just want to learn how to get out of this bad trend.

from what I see too much thrust - you are wasting your fuel to air drag.

But I thought I can counter what with just reducing the power on the engines? I try to keep my velocity between 150 and 200 m/s 'til I do my gravity turn. That should limit the effect of air-resistance?

Edited May 13, 2013 by etse

[HoY]

its just a simple asparagus design that has a payload heavier than what your already carrying. its simply a matter of switching from the jumbo 64 tanks with mainsails to using 2 FLT800 tanks and a single LV30 engine. mine was setup to drop the empty tanks rather than stack them on top of each other, but that was just a quick fix because the first version didn't have enough fuel and didn't feel like removing the engines lol

[MBobrik]

But I thought I can counter what with just reducing the power on the engines?

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but then you are carrying useless weight. if you run the whole time at less than 1/3 throttle you could use only one third of the engines making your ship lighter and thus even less in need of big thrust, so at the end of the day you will end up with something much smaller, just as HoY has shown.

[etse]

its just a simple asparagus design that has a payload heavier than what your already carrying. its simply a matter of switching from the jumbo 64 tanks with mainsails to using 2 FLT800 tanks and a single LV30 engine. mine was setup to drop the empty tanks rather than stack them on top of each other, but that was just a quick fix because the first version didn't have enough fuel and didn't feel like removing the engines lol

But wiithout actually "trail and error", how do you know when to use what? How do I know when I need to stack fuel-tanks on top of each other, and how do I know when to use the Jumbo-64 tanks with mainsails instead of FLT800?

I mean, given I have a payload I want to send to minmus. This payload consists of a stage for getting to and from minmus with atomic-engines and a small 3 manned lander. How would I go to know the amount of fuel and thrust I need in my first tage to get this into orbit?

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but then you are carrying useless weight. if you run the whole time at less than 1/3 throttle you could use only one third of the engines making your ship lighter and thus even less in need of big thrust, so at the end of the day you will end up with something much smaller, just as HoY has shown.

okey, so as I run on about 40-50% thrust during the first stages with engines that has 1500 thrust I know I could swap them to engined with 750 thrust, but the smaller engines usually only give about 1/7 of the thrust the mainsail has. So I get the problem with that they won't even take off. I would guess this is due to the large amount of fuel I got, so I try to change to some smaller propultion tanks, but then I am afraid I will have to little fuel - which I in many case have.

edit: But I do get that having smaller tansk means you need less thrust, and with the lower mass in total you can get places with less amount of fuel. But I kinda have problem when understanding when to choose the small tanks and when to choose the big tanks with big engines.

I mean, lets say I calculate my payload to be 10 tons. To get a successfull lift-off I would then need enough thrust in the early stages to be able to lift 10 tons, but I would also need enough fuel to run those engines til I get into orbit. But that means I would have higher mass, and I would need even more thrust to get up - which leads to requirement of more fuel. So the question is, how do I find the point where I have enough thrust and enough fuel?

Would you start by calculating the thrust required and choose engines accordingly? (and how do you actually calculate that to make room for the weight of the fuel required to reach orbit?), or would you start by calculating how much fuel is required and then choose engines powerfull enough to lift the while thing?

Edited May 13, 2013 by etse

[HoY]

There are maths to it, I admit I dont know them. I shamefully wasted most of my schooling playing video games and honestly regret it now.. SO for people like me there are Mods that show you your Delta V. It takes 4500~ to get a stable orbit around Kerbin, should always plan to have around 4800 or so delta V on your launch stage before your payload engine ever fires up. From orbit its simply a matter of knowing where you plan on going, landing or orbiting or any other planned maneuvers, and adding up the total ammount of delta V you PLAN on needing. General rule is to give yourself some wiggle room. I usually give myself about +10-15% for every single maneuver.

Knowing how much dV you need is collected from one of a few locations...

1: trial and error.

2: theres a forum thread here listing the dV requirements for getting to all the planets and moons from kerbin. Includes optional landing dV requirements and takeoff as well I believe

3: stick with what your doing, and send more fuel than you could need! over 3000m/s of dV inside the kerbin system is overkill for a single trip anywhere (3000 in orbit not on the pad)

[etse]

I think I am not good at formulating my actuall question, but I would love to know and understand the maths behind it, and not use mods. So I know I need 4800 delta-V to get into orbit. And I know that delta-V is related to thrust and mass. (calculated by the rocket equation). Anyone god any good tuturial that could link me to, where people explain (and I would love if they also have an example) how they actually figure out the amount of thrust and fuel needed through math?

[sgt_flyer]

Temstar gave some informations regarding how he created is Nova series in this thread.

http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29

note : you'll have to tweak around yourself to see what works or not, but that's a good start regarding the maths required (just go down a few posts to find Temstar's how to)

[karolus10]

Lighter (and often simpler) is always better, larger and heavier rocket needs more fuel to get into orbit and this fuel add weight too - it's an vicious cycle .

Also take care about proper acceleration in low atmosphere, if you go too slow you will burn longer and waste fuel... also if accelerate too fast in low altitude You will struggle with drag and not get any gain too.

When constructing rockets by trail and error method, G-meter (scale on right side of the nav-ball) is your best friend, after some test flights you will notice the pattern .

Also small rockets and probes are great tool for learning constructing larger rockets, so you don't start with overcomplicated designs.

Edited May 13, 2013 by karolus10

[etse]

Temstar gave some informations regarding how he created is Nova series in this thread.

http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29

note : you'll have to tweak around yourself to see what works or not, but that's a good start regarding the maths required (just go down a few posts to find Temstar's how to)

Thanks, post #7 in the thread seemed to explain much of the things I did not fully grasp yet. Thank you

[EndlessWaves]

The key thing is to start small with your fuel tanks - you want to get rid of the weight as fast as possible as the further you carry it the more fuel you have to use.

Using a fuel tank that's twice the weight of the vessel you attaching it too is very inefficient - as the fuel drains you end up wasting a lot of energy lumping around a great big fuel tank. I would start with fuel tanks that were something around 1/8 of the weight of the vessel you're attaching them to - if you want fuel efficiency then you shouldn't be using the big orange tanks until your vessel is already 500 tons and consists of dozens of smaller stages.

[etse]

The key thing is to start small with your fuel tanks - you want to get rid of the weight as fast as possible as the further you carry it the more fuel you have to use.

Using a fuel tank that's twice the weight of the vessel you attaching it too is very inefficient - as the fuel drains you end up wasting a lot of energy lumping around a great big fuel tank. I would start with fuel tanks that were something around 1/8 of the weight of the vessel you're attaching them to - if you want fuel efficiency then you shouldn't be using the big orange tanks until your vessel is already 500 tons and consists of dozens of smaller stages.

This sounds verry much like "trial and error". Start small and add more 'til it works. It sounds verry expensive to du it that way. But I seem to have found some way to approach the problem:

1: Calculate the thrust needed to lift the payload. And find an engine that can lift the payload + some. Add the biggest fueltank that the thrust from this engine can lift. Just make sure the +some, is not to big.

2: Calculate what amount of delta-V you got. - this should be atleast X-amount delta-V

3: Create a "stage-part " that has as much fuel, or less, than the main stage and add a engine with just enough thrust to lift this section alone.

3.1: Add enough of those stages to reach 4800 delta-v. Keeping in mind that each new stage will add about half of the previous one.

So follow-up question. How much delta-V is good to aim for in the last launching stage? (mentioned in point 2). And is my assumption in 3.1 about right?

[RoboRay]

For instance, how do you know when to use small rockets instead of big mainsails?

The answer is to always use a small motor, unless you discover you actually need a big motor. And you will, for many of your launch vehicle lower stages.

The road to efficiency is to not build a big rocket that can do a mission, then try to shrink it down... but to build a small rocket that can't do the mission, then add to it until it just barely can.

and how do you know how much fuel you wanted to add? 'Cus I am in the situation where I add to much fuel I think - and this I need much more thrust to actually get this thing into orbit. I do also waste alot of extra fuel due to the extra mass. So I just want to learn how to get out of this bad trend.

To optimize your craft, you have to figure out delta-v. You need to determine how much change in velocity you need to accomplish to reach your goals, then design a ship that is as small as it can be while delivering as much as you need. Diminishing returns on added mass mean that a small ship is inherently more efficient than a large one. As you noted, you're adding fuel and engines to lift fuel and engines, not payload.

Kerbal Engineer or MechJeb can calculate your vehicle's delta-v for you if you don't want to do it yourself. And there are some good maps of the KSP solar system that lay out how much delta-v you need to reach various destinations, such as this one:

I would love to know and understand the maths behind it, and not use mods. So I know I need 4800 delta-V to get into orbit. And I know that delta-V is related to thrust and mass. (calculated by the rocket equation). Anyone god any good tuturial that could link me to, where people explain (and I would love if they also have an example) how they actually figure out the amount of thrust and fuel needed through math?

The requirement to reach orbit varies a bit with the efficiency of your ascent... I can usually reach orbit with 4200-4400m/sec expended. But 4800m/sec offers enough reserve that you can probably reach orbit even with some problems during the ascent.

If you want to do the math yourself, start with the Tsiolkovsky rocket equation. I can't point you at any specific tutorials because I try to limit my recreational math.

Edited May 13, 2013 by RoboRay

[EndlessWaves]

This sounds verry much like "trial and error". Start small and add more 'til it works.

Well, for the final amount I'd use Kerbal Engineer to give me DeltaV readings.

Engines are simple, you want the engine that gives you the most DeltaV when loaded down with as much fuel it can carry Maximum DeltaV is determined by efficiency (high ISP) and thrust - an engine with more thrust will carry more fuel while achieving the same acceleration so even if it's lower ISP it may be better.

In practice the LV-N is best for everything over a ton, except for Kerbin and Eve, where the LV-T30/Aerospike's much better lifting capability and equal or better ISP for some of the way tend to win out.

You want to keep the engine burning all the time if possible, better for it to be lifting it's own weight using fuel piped up from the lower stages than having another engine lifting it and the weight of the additional engine.

[Specialist290]

If you want a good explanation of why the factors in the rocket equation matter (including what, exactly, delta-v is), I'd highly recommend checking out this page, which explains the things you'll want to know in pretty simple terms. Wernher Von Kerman also has a fantastic video on delta-v (although he still apparently hasn't finished anything beyond the first part).

As for thrust: Once you're in a stable orbit, you can basically get away with as little as possible, since the only thing that really matters for orbital maneuvers is the total delta-v change, regardless of how long it takes to make that change happen. For taking off from Kerbin (and landing on / taking off from other worlds), however, you want to make sure at a bare minimum that your engines can provide enough force acting to push the rocket up ("thrust") to overcome the force that the gravity of the body in question exerts in trying to pull the rocket back down ("weight"). The number that measures this is your "thrust-to-weight ratio" (often abbreviated "TWR"), and can generally be figured out in a few easy steps:

1. Take the maximum power values of all the engines that will be firing on takeoff, and add them together. This will give you the total thrust for your first stage in kilonewtons.

2. Find the total mass of your rocket in tonnes. (It's assumed that the mass values for all parts in KSP are given in tonnes -- or megagrams, which are exactly the same measure, if you're anal-retentive about making sure everything is in proper SI notation.)

3. Plugging these in to Newton's second law (Force = mass * acceleration, or F=ma) and solving for acceleration will give you the initial maximum acceleration your vessel has. (This value will go up in flight, because burning fuel reduces your mass.)

4. Once you have that value, divide it by the surface gravity value for the body in question. (Kerbin's is 9.8m/s^2, same as Earth). The final value will be your rocket's TWR.

5. If your TWR ends up being less than 1, then you can either reduce the weight, add more engines, or use more powerful engines, or any combination of the three. Then redo the equation until you're satisfied with the results.

Hope this helps

[Kegereneku]

Ok, Here is a few tips :

Regarding staging :

*- Don't be afraid of using several stages.

*- Each Stage have its own use, including boosting the next one.

- Useless Mass is the enemy. Get rid of it as soon as you can.

- Use engine at their maximum.

- ...but if you have too much thrust : you may have too much engine, get rid of them.

- Gain Horizontal Velocity as soon as you can (around 15km, out of the dense atmosphere)

Regarding how to choose engine :

- Always prefer Higher "ISP" to thrust, unless needed. (ISP = Specific Impulse)

- If you can take your time for a maneuver : use the lightest engine with the highest ISP.

- For Kerbin Launch : aim for 3G of acceleration

Regarding the quantity of fuel :

- the "range" of a rocket, depend of how much you can change your speed,

- The metric for this is the "Delta Velocity" (Dv in m/s)

- You cannot reduce the DeltaV needed for a maneuver, only the Dv your rocket have.

- DeltaV depend of : the mass of the "fuel", the mass of the "payload", and the efficiency of the engine.

LAST, if you want to do GOOD engineering : Use tool-mods

- You'll need mods to show you in real-time how much Dv your rocket have. (ex: you need 4400 to take of Kerbin)

- Each Stage have its own Dv

- Once you learn how to dock, leave behind mass you don't need (including fuel sometime)

Mechjeb can do that, it also include enough autopilot to save fuel during boring maneuver.

Edited May 14, 2013 by Kegereneku
forget important line in Staging

[MBobrik]

If you want to do the math yourself, start with the Tsiolkovsky rocket equation. I can't point you at any specific tutorials because I try to limit my recreational math.

actually, there is not much more to it than the equation, computed iteratively for each stage. That is basically what the engineer redux does for you.

It actually computes little more, like T/W but that is even simpler.

[MBobrik]

Maybe this example will help you, my newest and rather smallish ship ( with a similar goal in mind to HoY's craft ) and stage delta-vs as computed by the engineer redux.

Note that its upper stage is actually very overpowered so it can actually fly as far as to Gilly.

.

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I built it by building the upper stage, then adding the smallest tank and engine combination that gives me T/W around 2 and decent stage delta-v.

Of course it was not enough, so I added two asparagus side tanks with their own aerospikes. Still not enough, so I added 2 x side tank w/o engine because I had already enough thrust. Still not enough, so I made the throwaway tanks 1.25 x bigger and this was enough.

Edited May 13, 2013 by MBobrik

[etse]

It seems that the main issue I have had is that I have always had way to much thrust and that the mainsail engines are far heavier than what I imagined - and it seems I should never use that engine unless I have some very heavy payload. And instead I have started using the "LV-T30" for my first stages, which seems to do the job much better and saving me loads of mass. It seems like I have not yet really got a feeling on how much mass affects the delta-v.

actually, there is not much more to it than the equation, computed iteratively for each stage. That is basically what the engineer redux does for you.

It actually computes little more, like T/W but that is even simpler.

Yeah, I just got that part understood. So I manage to calculate the Delta-V manually. it is still a bit tidous and just try to add stuff and recalculate Delta-V all the time and check if it is enough.

Maybe this example will help you, my newest and rather smallish ship ( with a similar goal in mind to HoY's craft ) and stage delta-vs as computed by the engineer redux.

Note that its upper stage is actually very overpowered so it can actually fly as far as to Gilly.

.

*large image*

.

I built it by building the upper stage, then adding the smallest tank and engine combination that gives me T/W around 2 and decent stage delta-v.

Of course it was not enough, so I added two asparagus side tanks with their own aerospikes. Still not enough, so I added 2 x side tank w/o engine because I had already enough thrust. Still not enough, so I made the throwaway tanks 1.25 x bigger and this was enough.

Not to be untankfull or anything, but that sounds like what I try to not do. Using addon to do everything for me, and just try adding stuff until it works.

[HoY]

Not to be untankfull or anything, but that sounds like what I try to not do. Using addon to do everything for me, and just try adding stuff until it works.

To be completely honest, if your going to be using a chart or calculator or website to do the math for you, you may as well install kerbal engineer. Yes its a mod, but All it does is do this math for you and present it to you. I'ts not an auto pilot, or game breaking overpowered engine or anything, its just a calculator with a spreadsheet that has the formula built in already.

But, if you want to stay as pure as you can, then open up excel and plug in the formulas, calculate your mass and thrust and specific impulse for each stage to get your delta V

barring that, trial and error, or "guestimating" is your only option

Edited May 13, 2013 by HoY

[etse]

To be completely honest, if your going to be using a chart or calculator or website to do the math for you, you may as well install kerbal engineer. Yes its a mod, but All it does is do this math for you and present it to you. I'ts not an auto pilot, or game breaking overpowered engine or anything, its just a calculator with a spreadsheet that has the formula built in already.

But, if you want to stay as pure as you can, then open up excel and plug in the formulas, calculate your mass and thrust and specific impulse for each stage to get your delta V

barring that, trial and error, or "guestimating" is your only option

a spreadsheet would be as much "cheating" in my opinion. I just don't like to hide the math - as I want to learn the physics and math behind it. It catches me as interesting. So if I just had a mod or a spreadsheep where I put in numbers and just got some result back it kinda removes much of the stuff I am looking for. In other words, I just want to learn I have nothing against mods, I use MechJeb for autopiloting - as I have done that so many times that it is not any interesting anymore; and I guess I will do the same for calculating delta-v and such later, I just need to do it a few times manually so I know I understand where the numbers are comming from, and why. It also helps be get a feeling on how to build my rockets properly.

So it is not about beeing "pure", it is about not hiding the maths and physics before I actually understand them to an acceptable level. And the problem here is clearly that even though I did know the formulas for how stuff worked i clearly do not understand them properly, and thus I end up with estimations that are far off, and rockets that are way to big. I could solve this by using a mod - but I am afraid if I do that I wont learn things properly.

[HoY]

I kinda understand,, its always great to learn how things work. I got the gist of it and thats good enough for me lol.

the old and the new versions of mechjeb all have that deltaV readout for your stages, so when you do decide to use it it is there.

[Specialist290]

A few tips: If you're calculating the delta-v for multistage rockets, you need to calculate it for each stage individually, including the combined weight of the other stages "on top" of your current stage as part of the payload. Then you add those together to get the combined delta-v total for the whole rocket. For asparagus staging or droptanks, each iteration of dropping tanks technically counts as a "stage" for the purpose of the math. Again, Atomic Rockets goes into a bit more detail for that.

Also, if you have multiple engines firing at once and some of them have different Isp values, you're going to want to find the mean Isp for all of them. I don't remember the equation for that offhand, but it's a bit more complex than just dividing the total Isp by the number of engines.

Those are the two things that tripped me up when I was learning the equations myself.

I can understand your reluctance to use mods, but I'd advise that you use Kerbal Engineer myself while you're learning, although if you're adamant about not using it then don't feel like you have to. Do the math out longhand yourself first, then compare your results with the totals that the Engineer gives you. They probably won't be perfect matches, but if they're pretty close, then you can satisfy yourself that you know what's going on. If they're way off, then either you probably didn't account for staging correctly (again, remember that for asparagus staging, each time you drop a set of tanks you need to start the math all over), or you're getting the order of operations wrong for the equation (which means you should have paid attention to your algebra classes ). That's what I did -- I worked out the math myself before I even put the Engineer chip on the rocket, until I was satisfied I understood the equations.