Fuel efficiency question in regards to thrust.

[Moonfrog]

Hello again. I would just like to ask a question in regards to fuel efficiency and thrust. Just recently I tried building a craft to Laythe and began to find it was very difficult or very large for the return journey. It was built to do things the long way round, using solar orbit first then going to Laythe. I know using phase angles is cheap in Delta V, I was attempting to make it for someone else.

I learnt that may method of building the rocket was very inefficient, so did a bunch of experimenting and theorising. Came to the point of asking this question.

If I use a engine with say 215 thrust. Is it better to fill it up completely with fuel tanks before adding a new stage. In another wards is it best to only add an engine and decoupler when u need too? Of course based on what twr you want and such. but basically if I have 215 thrust that can lift 21.93 mass tons. Should I use 21.93 mass tons of fuel, minus the payload of course and other stuff?

Normally I wouldn't care, but places I'm traveling too now, and my friend has a parts limit of 150 because of their laptop.

Any help appreciated.

Edited August 22, 2014 by Moonfrog

[KerikBalm]

Have you looked into the asparagus staging tutorials?

[UmbralRaptor]

It depends on local gravity, but in general an on-pad TWR of ~1.2-1.8 seems to get the largest payloads. Low TWR engines and rocket SSTOs in particular do better at the lower end of that, while high TWR engines and asparagus staged craft can (and perhaps should) go for the lower gravity losses at the upper end. Upper stages (and obviously orbital ones) can get away with lower TWRs, below 1 in some cases.

[Xavven]

To answer your question directly, in general, yes. More powerful engines tend to be heavier, so you're usually wasting mass and getting less delta-v out of your rocket if you use an engine that gives you enough thrust to peg the g-meter above 2. Once in space, you can get away with even lower TWR and do fine. I personally prefer a minimum acceleration of 5 m/s (which is a Kerbin TWR of about 0.5).

If your stage has more TWR than you need, consider either using a lighter engine or adding more fuel tanks. Both will increase delta-v under most conditions, unless of course your lighter engine has worse ISP and its smaller mass can't make up for it.

[Poynting]

Hi Moon Frog

The reason we jettison stages is that as the mass of the empty tanks becomes significant compared to the remaining fuel our Dv is hurt be dragging the useless mass with us. However we dont want too many stages as we end up carrying alot of mass in the engines we arent using higher up. Its all a balance but a good rule of thumb is always to have about 10x as much fuel in the lower stage compared to the one above it.

For a Laythe return rocket I would suggest a ship with roughly 3 main stages.

The first: This stage will be your ascent stage to get you to orbit. Use the most powerful rockets, solid boosters and lots of fuel. Just keep the TWR above 2 and add as much fuel as you can via asparagus staging (side boosters that feed fuel into the remaining tanks to keep them topped up, so that you can ditch empty tanks as soon as possible). Once you have a Dv from this stage of about 5000m/s you are done. Jettison it once in orbit and the fuel is used up.

The second: This is your jool injection and manuvering stage. There is no longer a restriction to the TWR, you are in orbit and have all the time you like to complete the burn. You are just looking for the Dv you need to get to Jool which normally means looking for the rocket with the highest ISP (which is roughly speaking the efficiency of that rocket motor). I'd include plenty od SAS here so it doesn't take an age to turn the ship.

The third: This is your return stage. We've now dropped the stage that got us here as there is not point carrying the empty tanks back (Feel free to use us any remaining fuel though before ditching it). This stage should be as small and light as possible as adding any mass here will exponetially increase the required fuel mass for the first two stages.

Hope this helps, It's all pretty complicated so just keep trying. Try using kerbal engineer to get your delta v stats.

[Moonfrog]

Ok thanks for answering that guys. I have another question, with the parts I had, I built this to launch something into orbit. I was building it on the principle that using all my thrust was better than splitting it up into stages. Don't know if that is true. Here is a picture of it, I built it too twr of 1. Then used 3 solid booster rockets to help it get of the ground, fired all engines at once including boosters. It got into orbit. My question is, can it be done better? meaning my hypothesis is wrong. I don't have fuel lines yet. Click on image for it to get bigger. Also first time I ever used the radial engines, needed extra thrust for more fuel tanks.

[cantab]

That does look like a lot of fuel on one stage, and a lot of engine compared to the stage above.

Sequential staging is always a tradeoff. The weight of an engine when you're not using it can be more than offset by the weight saving from ditching the previous engines and tanks - but then there are cases when it isn't. Parallel staging, with all engines firing from the start, gets round this especially with fuel crossfeed, but fuel crossfeed increases part count and complexity.

[juanml82]

Those are too many fuel tanks. Other people will explain it better, but here it goes:

Your engines down below have to push the weight of a lot of fuel at the launchpad. IOW, their thrust (which isn't increased by the amount of fuel they have available) has to cope with a lot of weight, lowering your thrust to weight ratio. Which, in turn, means you'll expend more time in the thickest parts of the atmosphere, where the air slows you down the most. So, at some point, you're achieving diminishing results.

You want to get above 10,000 meters as soon as possible, so you'll want a lot of thrust in your first stage. However, the atmosphere terminal velocity puts a cap in fuel efficiency. Go for a TWR of 5 and you'll waste a lot of fuel. Go for a TWR of 1.3 and you'll expend a lot of time fighting thick air.

Get Kerbal Engineer Redux. Monitor your TWR as you remove fuel tanks. IIRC, TWR at the surface with the stock game (ie, no FAR or NEAR) has to be close to 2. It's best to have less fuel tanks over more engines. Also those radial engines are convenient to place, but they suck. Once you have girders, you can make an "L" with them and mount non radial engines radially.

Edited August 21, 2014 by juanml82

[Moonfrog]

I appreciate people telling me the basics, but I already know the basics. I'm trying to fine tune what I already know. I explained I built that bottom part of the rocket to a TWR of 1, then added 3 solid boosters to add extra .2 of thrust to get it up. Fired all the engines at once. I know about using TWR, I know about delta V. I also understand principle of staging, what I can't figure out is relationship between between staging and adding engines.

I used as much thrust as I needed to get those fuel tanks, to get delta V I needed into space. On the principle that using your engines is better than having lots of them sitting doing nothing as dry mass. You mention theres a trade off, between staging and not staging. This is exactly what I'm trying to figure out. On small craft not such a problem for me, but on larger craft, everything becomes exponential almost, a single engine where u don't need it on a top stage, makes the bottom even bigger. I need to understand how to do it more efficiency. Personally I've seen people use asparagus and they do it wrong.

Perhaps I should just ask this. If you can add staging to that rocket to give it more delta V please tell me how u do it. Granted I have no fuel lines or bigger engines.

I do appreciated the advice, just the answer I need is more specific.

[Kasuha]

what I can't figure out is relationship between between staging and adding engines.

On liftoff, you need as much thrust as you can get. On bodies with atmosphere, ideal TWR is 2. On bodies without atmosphere, ideal TWR is as high as you can get because that allows you to reach orbit faster, losing less fuel "fighting gravity". As you get closer to circular orbit, you don't need that much TWR as you're close to optimum burn conditions - you're near the apoapsis raising your periapsis and if you have enough time, TWR is irrelevant.

That's why asparagus staging is so successful, it provides large thrust from all engines at the start and as you gradually drop stages, you also gradually circularize and don't need that much thrust anymore.

Perhaps I should just ask this. If you can add staging to that rocket to give it more delta V please tell me how u do it. Granted I have no fuel lines or bigger engines.

I do appreciated the advice, just the answer I need is more specific.

You can't have asparagus staging without fuel lines. That's the main idea of it - you use all engines and every time you stage, you have a "new" rocket full of fuel.

Without fuel lines you have to give up either on "use all engines" or on "full of fuel after staging".

If you don't use all engines, you can use classical staging, smaller rocket on top of a big booster that's powerful enough to send the rocket up.

If you use all engines, you need to give your central stages more fuel than your outer stages. Then after staging, you shed some mass of empty fuel tanks but remaining stages still have fuel.

[SRV Ron]

Most certainly, use SRBs for launch. They can either be a stage of their own, or just add thrust, for the first 5000 to 10000 meters. That will make a big difference in the performance of the rest of the rocket.

Early Career Mun return vehicle.

A mod design using SRB assisted launch.

[Moonfrog]

I think I'm going to take from this that no one actually knows. Thx for trying to answer.

[Kasuha]

I think I'm going to take from this that no one actually knows. Thx for trying to answer.

My guess is it's either matter of taste or your question is unclear.

[waterlubber]

Personally, I start off with a lightweight upper stage, and continue adding fuel/reverting until my first stage brings me all the way to orbit, or extremely close. I always ditch the first stage once I'm in orbit anyway, since there is NO way I can manuever without RCS. Of course, I use crossfeed/asparagus, which I strongly recommend as you don't have to lug half filled cans all the way up.

[Simes]

I used as much thrust as I needed to get those fuel tanks, to get delta V I needed into space. On the principle that using your engines is better than having lots of them sitting doing nothing as dry mass. You mention theres a trade off, between staging and not staging. This is exactly what I'm trying to figure out.

I think what you may be missing is that empty fuel tanks are also useless dry mass and it's better to dump them than carry them all the way into orbit with you. And once you've dropped a bunch of dead weight, you no longer need those high-thrust low-efficiency engines to lift what's left, so you may as well drop those too instead of carrying them about. And now you're only on higher-efficiency engines, you don't need as much fuel to get the remaining dV you need.

Perhaps I should just ask this. If you can add staging to that rocket to give it more delta V please tell me how u do it. Granted I have no fuel lines or bigger engines.

I do appreciated the advice, just the answer I need is more specific.

How much deltaV does it have, and what is it meant to do? If you can supply that information, I'd bet a lot of people on here will be able to build something with stages which has better dV. It's hard to tell what the payload is meant to be when it's covered with a fairing.

Edited August 21, 2014 by Simes

[KerikBalm]

I think I'm going to take from this that no one actually knows. Thx for trying to answer.

I think your question needs to be worded better.

"If I use a engine with say 215 thrust. Is it better to fill it up completely with fuel tanks before adding a new stage."

Fill it up? what?

"In another wards is it best to only add an engine and decoupler when u need too? Of course based on what twr you want and such"

What defines when you need to? I'd say its better to add an engine and a decoupler when it improves your dV...

Adding a decoupler won't help your TWR at liftoff, and it TWR won't be a problem when its time to decouple stages... so...

"but basically if I have 215 thrust that can lift 21.93 mass tons. Should I use 21.93 mass tons of fuel, minus the payload of course and other stuff?"

No, that leaves no room for another stage, and also leaves you with a TWR of 1.

Go to the Tsiolkovsky rocket equation.

Basically:

dV = 9.8*ISP* ln (m_0 / m_1)

Essentially, every engine+decoupler you add, ads dead weight at the start, and removes deadweight at the end.

This breaks it up into essentially 2 stages.

Lets consider an example rocket, all monoprop drained, an aerospike engine, 13.5 tons of fuel+tanks, a mk1 pod, parachute, optionally staged with 1x decoupler and 1x Lv-909, so the ISP is almost always 390,

Total deadweight/empty weight as 1 stage = 0.8 (pod) + 13.5/9 (empty fuel tanks) + 1.5 (aerospike) + 0.1 (parachute) = 3.9

Loaded mass = 15.9

dV as per the equation= 5371.2

Now if we break it up into a 2nd stage with x tons of fuel tanks (and thus 9x tons of full fuel tanks), that 2nd stage has an empty mass of:

0.8(pod)+ x (tanks) +0.1 (parachute)+ 0.5 (lv-909) = 1.4+x

Its loaded mass is 1.4+9x

Its dv is thus 9.8*390* ln((1.4+9x)/(1.4+x))

Now lets find the dV of the first stage in terms of x.

M_1 of the first stage will be the weight of the 2nd stage + aerospike + decoupler weight + empty 1st stage tanks

The empty weight of the tank is (1.5-x)

M_1 is thus: (1.4+9x) + (1.5) + (0.05) + (1.5-x) = 4.45+8x

M_0 is the same as in the first case + 0.55 (for the Lv-909 and decoupler) = 16.45

The dV of the 1st stage is thus:

9.8*390 * ln (16.45/(4.45+8x))

total dV of both stages is thus:

9.8* 390* ( ln((1.4+9x)/(1.4+x)) + ln (16.45/(4.45+8x)) )

I'm not going to do the calculus to find the maximum, I'm jsut going to use my computer to plot it.

x reaches a maximum at 0.55066, with a dV of 6881.6 m/s

X is the empty weight of the tanks, so that would mean the 2nd stage basically gets a FL-T800 tank

Compared to the original 5371.2... we gained a lot of extra dV (1510 m/s) by staging it.

Note we'd get even more if we used a 48-7s, but that would change the optimal x as well.

Granted, our rocket was .55 tons heavier, if you want to throw another .55 tons on to the non-staged rocket, we can....

*assuming a 9:1 empty full ratio as most of the tanks have:

9.8*390*ln (16.45/3.961111) = 5441.77

So 2 rockets of equal weight, 6881.6 vs 5441.77 m/s of dV.

26.% more dV / 1440 m/s more dV by adding 1 staging event.

Use the same math, but tweak the numbers to correspond to your engines and pods and payloads and such.

You can do similar math for 3 stages and more. Or you can do it iteratively... Like suppose you want to build a 100 ton rocket, find what x gets you the most dV, and use that to determine the amount of fuel you use for your first stage, then you've defined the size of your 2nd stage, and you can just repeat the math to optimize your 2nd and 3rd stage (as if they were 1 and 2)

[cantab]

Perhaps some pictures would help. Here's a simple single-stage rocket, with an LV-T45 engine:

That's respectable enough, it'll get you into orbit. But once you've burnt up most of the fuel and are down to say your last can or two, you're still dragging around the empty fuel tanks. Empty tanks in KSP are kind of heavy. You're also using an engine that's more powerful and heavier than you need.

Here's a simple two-stage rocket, with the same amount of fuel, and an LV-909 engine for the second stage:

Even though the first stage has to lift a bit more weight, once you're down to your last two cans of fuel you can ditch the empty tanks and engine from the first stage. The upshot is that we get 25% more delta-V from the same amount of fuel. This rocket could land on Minmus (but not come back), or fly past the Mun or even Duna or Eve.

This is a rocket with silly staging:

Don't build rockets like this unless it's as an example (like here) or for a laugh. The extra weight meant I had to switch to bigger engines (Skippers) for the lower stages. The whole thing weighs over 2 1/2 times what the single stage rocket did, and I shudder to think what the cost would be like in .24. Yet despite that, even this gets more delta-V from the same fuel than the single stage!

[Moonfrog]

Maybe your right the question is unclear, but I also feel everyone just tried to explain stuff as well regardless of what I asked. I've done some maths of my own and experimentation, I have come to the answer that, for Serial staging like Apollo you should not fill up an engines thrust with fuel tanks before staging.

My maths isn't very clear either, could still be wrong. But what I think I discovered was, you should stage slightly above the max mass ratio for your engine and fuel tank mass divided by two(as far as I know all basic fuel tanks have same ratio, u can use any of basic ones). This should insure that your two stages will always have more mass ratio than a single stage, since a single stage has a max mass ratio based on engine and fuel tank, of same engine or engines. At the same time, u are not over building by adding too many fuel tanks, which although would give you more delta V in the stage, per mass the fuel becomes less efficient in comparison to staging for some reason.

I used some of the peoples maths above and there advice, so thx for the help I have managed to figure out the answer, even if it wasn't made clear. Anyway Thank you. Your maths and stuff did help.

Using this basic idea I built this. with the same payload as that earlier rocket I made.

Now to figure out how to eliminate wobbliness.

Edited August 22, 2014 by Moonfrog

[juanml82]

Moar struts, of course.

Seriously, put some struts between your first and upper stages. That will fix it.