Payload to orbit mass

[Rainstorm]

Hello, I am currently trying to build my own rocket families. I found on a forum a formula to find the max payload weight your rocket can have. Using my java programming skills, I put it into a program. But then I figured out that the outcome of the formula was not correct. I tried moving stuff around, but it still wouldn't work. Whenever I tried putting a payload on my rocket that was under the payload capacity, it still didn't work, meaning their wasn't enough Delta V to get into orbit.

I was just wondering if their was any formula out their, that could help me and is simple, I'm too lazy to just test different payload weights and I want to be exact. Please help.

[zekes]

Hello, I am currently trying to build my own rocket families. I found on a forum a formula to find the max payload weight your rocket can have. Using my java programming skills, I put it into a program. But then I figured out that the outcome of the formula was not correct. I tried moving stuff around, but it still wouldn't work. Whenever I tried putting a payload on my rocket that was under the payload capacity, it still didn't work, meaning their wasn't enough Delta V to get into orbit.

I was just wondering if their was any formula out their, that could help me and is simple, I'm too lazy to just test different payload weights and I want to be exact. Please help.

The best way to do it is to get Kerbal Engineer Redux, which will give you stats of D/V In game, and just add payload to the rocket untill D/V goes down to about 5000-5500 m/s, that should be your max tonnage.

[Rainstorm]

I have kerbal engineer, I guess theirs no harm in trying that, but is their any formula I could use thats simpel and still gives the same outcome. Or even a calculator on the web? Thanks for the suggestion though.

[Starman4308]

Theoretically, payload fraction = e^(dV/g*Isp). In practice, you won't be able to get that much useful payload up, because some of that payload capacity gets swallowed up by stuff like staging equipment, aerodynamic control surfaces, command pods/drone cores, RCS, structural elements, etc. Also, the amount of delta-V to get to orbit is variable based on how well you handle the ascent: as a rough guideline, it takes 4400-4500 m/s to get to low Kerbin orbit if you don't mess up your ascent.

Good ascents in stock feature a straight vertical ascent followed by a gravity turn. For the vertical ascent, the most efficient speed to go is terminal velocity*: go faster, you lose fuel burning against atmospheric drag, go slower, you lose fuel burning against gravity drag. A stock gravity turn is to gently tip over eastwards at ~6-12 km until you are straight horizontal.

*Terminal velocity is how fast you would fall. There's a chart on the Wiki, else MechJeb and probably Kerbal Engineer tell you what terminal velocity is for your craft. As such, you want to design for a TWR of 1.6-2.0: while you need slightly > 2.0 TWR to hit terminal velocity, you burn fuel as you go up (increasing TWR), and engines are dead mass for the rocket equation.

The theory behind this is simple. Barring atmosphere (and lithosphere), the most efficient way to get to orbit would be to point dead east (in the direction of Kerbin's rotation), with just enough vertical pitch to keep yourself from hitting the ground, and just burn until you're in orbit. Horizontal thrust good, vertical thrust bad but necessary. However, Kerbin also has a thick souposphere which eats velocity: you need to go up to escape it, and gradually start turning over eastwards as you get to thinner and thinner atmosphere.

EDIT: Rainstorm, the rocket equation (dV = g * Isp * ln(start mass/end mass)) is the simple equation, although as mentioned, delta-V is not the entire story.

EDIT #2: The simple way to get to orbit is to have a launch TWR in the aforementioned range (1.6-2.0), about 5 km/s of delta-V (for safety), and tip over 45 degrees at 10 km, holding that until apoapsis is 80-100 km; you then wait until apoapsis, and burn prograde until you're circularized.

Edited November 28, 2014 by Starman4308

[UmbralRaptor]

If you assume SSTOs and ÃŽâ€V vs TWR follows a convenient path (as low as 4400 m/s for craft with >1.5 TWR, more like 4600 m/s for craft down around 1.2)...

Edited November 28, 2014 by UmbralRaptor

[GoSlash27]

The short version is there's not a "simple" equation. There's just the basic rocket equation and reorganized derivations of it. It can answer how much DV an assembly can generate, and deriving different forms of it can answer how much rocket you need to orbit a given payload.

Best,

-Slashy

[mhoram]

In my experience there is no simple formula to calculate the payload of a lifter. Factors like ascent path and TWR-distribution have a too big influence on it and they can not be calculated easily.

The payload capacities of my own lifter families I got by a lot of expermenting and testing.

If you take the reverse approach (building an asparagus-staged rocket for a certain payload mass), Temstar's rocket configuration guide might be helpful for you.

Edited November 29, 2014 by mhoram

[MockKnizzle]

It's very difficult to build a rocket first and then determine its payload capacity afterwards, because this depends greatly on the design details of the rocket in a number of complicated ways.

It's much easier to have a specific payload first and then stick fuel and engines under it until it can make orbit. To save yourself the work of building lifters from scratch every time you want to go to space, make some dummy payloads of convenient mass (quarter orange, half orange, full orange, and so on) and build standard lifters that can get them into orbit. Save the standard lifters as subassemblies, and you now have a family of rockets that all have a known payload capacity that you can plop underneath anything.

[Zylark]

There are too many variables to keep track of to make a good dV calculation on your own - Kerbal Engineer Redux or MechJeb does a good enough job of it and takes a lot of the guesstimates out of the equation. Especially when multiple stages are involved. My own approach is pretty much what MockKnizzle described. I got a payload, MJ tells me it weighs say 15 Tons, and then I build a rocket with at least 5000dV to get it to orbit. If successful, I then save that rocket, sans the payload, as a Subassembly and name it something along the line of '015T Lifter'.

[kerbiloid]

I have a primitive but enough efficient way to calculate this.

It is simple, but enough long to tell, so I've created a post about how I solve this problem  to easily find it in future.

http://forum.kerbalspaceprogram.com/threads/101697-A-silly-method-of-payload-mass-estimation-and-launcher-composition?p=1570959#post1570959

[KerikBalm]

It's very difficult to build a rocket first and then determine its payload capacity afterwards, because this depends greatly on the design details of the rocket in a number of complicated ways. It's much easier to have a specific payload first and then stick fuel and engines under it until it can make orbit.

I disagree. Especially with stock aero, payload mass is payload mass.

To save yourself the work of building lifters from scratch every time you want to go to space, make some dummy payloads of convenient mass (quarter orange, half orange, full orange, and so on) and build standard lifters that can get them into orbit. Save the standard lifters as subassemblies, and you now have a family of rockets that all have a known payload capacity that you can plop underneath anything.

Well, thats sort of the same thing... Even using a dummy payload, and sticking on rockets and fuel until you have the required TWR and dV... there will still be some uncertainty.

It may get an orange tank to orbit... but can it get an orange tank + a FL-T400?

What I like to do, is send up a rocket with a test payload, and when it gets to orbit afterwards, I see how much fuel it has remaining. Its pretty easy to calculate the weight of fuel remaining. This fuel might as well be payload, and if you use a tank combinationto reduce your rocket's fuel capacity to just over the amount you took, you can just add that mass to your rocket's payload.

Or don't reduce the fuel of your lifter rocket, and still add that mass to its payload, for a little extra margin of error.

[capi3101]

Myself, I'm apt to build a custom booster for every payload. Nothing wrong with the notion of building a rocket family, though. For purposes of figuring out the maximum payload capacity after the rocket's done, I'd suggest NRAP - that way you can adjust the payload mass without having to actually add or subtract parts. It's a pretty handy tool for this sort of thing (and despite what KSP says, it works perfectly fine in 0.25 - though I do wish the mod creator would get around to doing something about that damn incompatibility warning). KER is also quite useful for this kind of work. Stock solution probably would be to add small fuel tanks (FL-T100 or FL-T200 tanks) until the booster doesn't make orbit any more based on how you fly it. It's your rocket family, after all...

Myself, I utilize Temstar's guidelines for asparagus boosters:

1 - Determine the mass of your payload (either by shoving it out on the pad and going to map view before it collapses into a heap or by just getting the information from KER)

2 - Assume a 15% payload fraction and a launch TWR of 1.65 (divide the mass of the payload by 0.15, multiply the result by 9.8 and multiply that result by 1.65).

3 - Assume 22% of the required thrust will be in the core engine. Assume 13% of the required thrust will be in each booster if you're using 3 booster pairs or 9.75% if you're using 4 booster pairs. Find a set of engines capable of providing the required levels of thrust (and be willing to use the thrust limiters - Temstar wrote his guidelines in an age before thrust limiters were in the game, where an engine was all or nothing).

4 - From the assumed launch mass (i.e. the payload mass divided by 0.15), subtract the mass of the payload, the mass of the selected engines and an additional two tonnes (this should cover the mass of additional parts such as RCS, probe cores, decouplers and extra nick-nacks for most cases). What's left is the mass of fuel tanks. Divide that amount by 7 if you're going with three booster pairs or 9 if you're going with four. Find a set of fuel tanks that gives you roughly that amount of fuel and go on the high side.

For single-stage boosters (i.e booster-payload assemblies), I generally assume a 4% payload fraction and a launch TWR of 1.2. Onion and serial assemblies I assume 9% and TWR 1.5. The whole process is generally the same, though.

I suppose if you wanted to build a rocket family, what you could do is start with your engine clusters (i.e. what engines you're going to use). Leave them at full thrust, determine what that full thrust level is then determine what the maximum launch weight is for the desired TWR. From there it's just step 4 of the asparagus setup - take out the assumed payload fraction, the mass of the engines and an additional tonne, and get your fuel tank configuration from there. Start with the stack decoupler/seperator as the root part if you want to save the booster as a subassembly, using one that's the same radial size as the core engine.

Edited December 1, 2014 by capi3101

[justidutch]

I built my own Delta-V calculator, perhaps you might find it useful.

http://jscript.ca/ksp.aspx

Cheers!