How to figure how many tons a launcher stage can carry into Orbit?

[SpaceSmith]

So yeah, title is self explanatory. How do I figure this out? I have MJ and Engineer and can see how much it weighs, and it's twr, and delta v, but how do I figure out that launcher x can carry 55 tons of weight to an orbit I want?

[FacialJack]

i build my rocket wiz 123 rules: 15/30/45% for 3/2/1 stage, + 10% of total mass of utile payload.

I know, that dont answers to ure question, but that maybe can give u an idea

[Ignamious]

http://exploration.grc.nasa.gov/education/rocket/rktwtp.html

http://en.wikipedia.org/wiki/Payload_fraction

Just add weight to it. you have MJ so you have a readout of ∆v you can add fuel tanks and see what the ∆v is after each one. 4500 is average to LKO i like 5000 for wiggle room.

or the Kerbal way, and just push launch.

[Seret]

how do I figure out that launcher x can carry 55 tons of weight to an orbit I want?

Use the rocket equation? Plug the ÃŽâ€v into it for the orbit you want (eg: 4500ms^-1) and see what kind of mass you can go up to. Try this tool:

http://www.quantumg.net/rocketeq.html

[Palec]

Add dummy weight to launcher (for example those 55t) and check deltaV in Mechjeb / Enginner. If it shows 4500deltav or more, it will get into stable orbit

Edit: If you are using full fuel tanks as dummy payload, be sure to right click on it and disable fuel flow from it, otherwise it will count into deltaV calculations. Or you can use this mod, which adds nice dummy payloads http://forum.kerbalspaceprogram.com/threads/76231-0-24-x-Kerbal-NRAP-adjustable-test-weight-for-rocket-building%21-v1-5-30-07-14

Edited July 31, 2014 by Palec

[blizzy78]

I'd rather start with the payload mass, then work my way to build a launcher that can transport that.

http://forum.kerbalspaceprogram.com/threads/46569-WEB-Online-engine-cluster-calculator

[Jouni]

It's pretty much trial and error. While there are general guidelines, it really depends on the particular design, how much delta-v you need and how high the TWR must be for each stage.

You may start from the assumption that 12-15% of launch mass is payload, which corresponds to a rocket with a few boosters, two vertical stages, and no fuel lines. Adjust that down, if you use inefficient staging (purely vertical staging or even a single-stage lifter), or up, if you use particularly efficient staging (onion or asparagus). SRBs and other fuel-inefficient engines decrease the payload fraction further, while some particularly efficient engines may increase it a bit.

Consider also which orbit you are targeting. The usual numbers assume a low orbit at around 100 km. Higher orbits obviously reduce the payload fraction, especially if the upper stage must be able to deorbit itself.

[GoSlash27]

It's just simple math, really.

e^(DV/9.81Isp) gives you your required ratio of wet to dry mass in order to make orbit.

Knowing how much your launcher weighs wet and dry, you can rearrange the rocket equation to read

Mp=(RwdMld-Mlw)/(1-Rwd) where

Mp is the mass of your payload

Rwd is your required wet to dry ratio

Mld is the mass of your launch stage dry

and

Mlw is the mass of your launch stage wet.

This will tell you exactly how much payload you can lift with your launcher.

Best,

-Slashy

[DocMoriarty]

I play it the Kerbal way:

Since i play without any nifty engineering mods i do it very simple. I use the rocket i want to test, put a probe core on it and launch it. The remaining fuel (LF+LOX) x 5 = roughly the payload it can get to LKO.

[rkman]

As said, it takes about 4500m/s delta-v to get to LKO.

Also the stages of the lifter each need a twr of about 1.8 (although a final orbital insertion stage can get away with a twr of about 1).

[Jouni]

As said, it takes about 4500m/s delta-v to get to LKO.

Also the stages of the lifter each need a twr of about 1.8 (although a final orbital insertion stage can get away with a twr of about 1).

That's just one way to do it. Depending on the staging you use, the initial TWR with the highest possible payload may be below 1.1, and the upper stage may start with TWR around 0.5.

[Seret]

I don't think I've ever bothered having a TWR as high as 1.8 for lifters. If the TWR was above about 1.3 and the engines couldn't be changed I'd consider adding more fuel.

[NecroBones]

My rule of thumb is a minimum of 1.8 TWR at the pad, and for at least the first few km of altitude. Preferably a TWR of around 2 until starting the gravity turn. If you can do that, 4500 m/s of dV to get to orbit works, and below that you may need more. But I like to aim for 5000 m/s, for some wiggle room, and to have fuel left over for de-orbiting the portion of the lifter that makes it to orbit. Now that recovery matters in career mode, that helps a lot. Prior to that, I used to just use the remaining fuel to help start the escape burn (with a small reserve tank ready to de-orbit the stage). I've started with 1.5 to 1.6 TWR on the pad, and maintained that most of the way up, but I think the dV cost to get to LKO is then something like 4800 to 5000 m/s.

With these things in mind, and using Engineer or MechJeb to see the stats in the VAB, you can just add mass to the payload until you get into the right ballpark, and you'll get your answer.

Edited July 31, 2014 by NecroBones

[Sirrobert]

If you already have the lifter, it's easy.

Just add mass ontop of it until Mechjeb sais you have 4500 deltaV left.

I usually stack Cupola command pods as mass

[K77LostNSpace]

I play it the Kerbal way:

Since i play without any nifty engineering mods i do it very simple. I use the rocket i want to test, put a probe core on it and launch it. The remaining fuel (LF+LOX) x 5 = roughly the payload it can get to LKO.

I try to play KSP without any mods, and knowing the payload a lifter can put into a stable orbit is useful. This idea seems very reasonable. Launch one test craft and use it as the base line for future information. question i have is.

LF = liquid fuel?

LOX = what is this

once i add the numbers and multiply by 5, this should be the total weight of the payload?

[NecroBones]

LF = liquid fuel?

LOX = what is this

Yes, and Liquid Oxygen.

[Red Iron Crown]

Ideal TWR is a subject of some debate. Minimizing dV consumed involves getting to and staying near terminal velocity without exceeding it; that means a TWR of >2 for most of the ascent. Maximizing payload fraction and, generally speaking, cost seems to do better with an initial TWR of 1.3-1.4; more dV is expended but less fuel is consumed and fewer engines are required. It was surprising to me that minimizing dV expenditure and minimizing fuel consumed are not the same thing (was likely less surprising to the rocket scientist types ).

I'm in blizzy's school of thought that starts with the payload and builds the lifter for it (I've come to realize that I like building the rockets so I don't use standardized lifters anymore). My payload fraction for multistage rockets is generally about 1/6, so looking at one of my lifters alone without payload I can guess that it can throw a fifth of its mass to orbit.

[UmbralRaptor]

Like Seret and GoSlash27 said, invert the rocket equation. For simpler craft, it's possible to generate an optimal (maximum payload for a given ÃŽâ€V and TWR requirement) design simply from knowing the engine(s) used.

[vexx32]

Moved to Gameplay Questions.

[GoSlash27]

I try to play KSP without any mods, and knowing the payload a lifter can put into a stable orbit is useful. This idea seems very reasonable. Launch one test craft and use it as the base line for future information. question i have is.

LF = liquid fuel?

LOX = what is this

once i add the numbers and multiply by 5, this should be the total weight of the payload?

This would seem to be a useful rule of thumb, but as several people have pointed out (myself included), you don't actually have to launch anything in order to answer this question. The inverted rocket equation I gave you will spit out the answer immediately.

I also don't use any mods for KSP, so it's just that much more important to understand and use the math in your design process. For certain, you'll never be able to design optimized vehicles without going through the math... and there are many things you simply can't accomplish without optimized vehicles.

I'm going to try to put together a comprehensive tutorial on all the different forms of the rocket equation and how it can be used to answer all sorts of questions along these lines; how much payload a given rocket can accelerate to a given DV, or how much fuel a given rocket needs in order to accelerate a given payload to a required DV, etc.

Best,

-Slashy

Edited July 31, 2014 by GoSlash27

[NecroBones]

Ideal TWR is a subject of some debate. Minimizing dV consumed involves getting to and staying near terminal velocity without exceeding it; that means a TWR of >2 for most of the ascent. Maximizing payload fraction and, generally speaking, cost seems to do better with an initial TWR of 1.3-1.4; more dV is expended but less fuel is consumed and fewer engines are required. It was surprising to me that minimizing dV expenditure and minimizing fuel consumed are not the same thing (was likely less surprising to the rocket scientist types ).

These are very good points/observations. Personally, I often neglect the overall fuel usage, in favor of dV, so this was a good reminder for me to see. I may have to do a few experiments.

I'm in blizzy's school of thought that starts with the payload and builds the lifter for it (I've come to realize that I like building the rockets so I don't use standardized lifters anymore). My payload fraction for multistage rockets is generally about 1/6, so looking at one of my lifters alone without payload I can guess that it can throw a fifth of its mass to orbit.

This is the direction I've gone lately as well. I had standardized lifters for a while, but when every payload is different, building the lifter to the payload is often a good way to go.

[K77LostNSpace]

This would seem to be a useful rule of thumb, but as several people have pointed out (myself included), you don't actually have to launch anything in order to answer this question. The inverted rocket equation I gave you will spit out the answer immediately.

I also don't use any mods for KSP, so it's just that much more important to understand and use the math in your design process. For certain, you'll never be able to design optimized vehicles without going through the math... and there are many things you simply can't accomplish without optimized vehicles.

I'm going to try to put together a comprehensive tutorial on all the different forms of the rocket equation and how it can be used to answer all sorts of questions along these lines; how much payload a given rocket can accelerate to a given DV, or how much fuel a given rocket needs in order to accelerate a given payload to a required DV, etc.

Best,

-Slashy

I tried to read your prior post regarding Wet and Dry mass. Information You posted "e^(DV/9.81Isp) gives you your required ratio of wet to dry mass in order to make orbit." not sure what "e" stands for. or how to get DV without mods. and why you are using 9.81 isp.

very much looking forward to your tutorial.

[Sirrobert]

I tried to read your prior post regarding Wet and Dry mass. Information You posted "e^(DV/9.81Isp) gives you your required ratio of wet to dry mass in order to make orbit." not sure what "e" stands for. or how to get DV without mods. and why you are using 9.81 isp.

very much looking forward to your tutorial.

http://en.wikipedia.org/wiki/E_%28mathematical_constant%29

Except poket calculators with only +-*/, all off them have a button for E already

[K77LostNSpace]

http://en.wikipedia.org/wiki/E_%28mathematical_constant%29

Except poket calculators with only +-*/, all off them have a button for E already

How hard can rocket science be anyway?

thats the slogan right?

well your link answers that question.

after I pick up my gray matter from the brain explosion ill try to go over that wiki site slowly and understand what i'm reading.

[UmbralRaptor]

"e^(DV/9.81Isp) gives you your required ratio of wet to dry mass in order to make orbit." not sure what "e" stands for. or how to get DV without mods. and why you are using 9.81 isp. .

e^x is an exponential equation, or an inverse logarithm using the constant e (approximately 2.71828). Aside from simple cases like an integer raised to an integer (eg: 2^3 = 2*2*2 = 8), no one does exponents purely by hand -- using a calculator is much less hassle.

The 9.81*Isp is a consequence of the rocket equation, and how we measure fuel efficiency. Isp is a figure of merit, and 9.81 is a unit conversion factor.

(For additional !!Fun!!, KSP uses 9.82 rather than 9.81. But the difference is small enough to rarely matter)