Northlight

Only way to determine wheter to stage or not, (And how many stages you should have) is to do the math for each variant. Given a target delta-v, and payload. * How big (in tonnes) is the rocket if only using one stage? (easy calulation) * How big is the smallest (in tonnes) rocket with two stages? (still easy, but there is a lot of iterations) * How big is the smallest (in tonnes) rocket with three stages? (easy, but a LOT of work) Then you can see witch is the most efficient. (in terms of weight) But it would be cool if there was some "shortcut".

If you want a closer fit using regression, try a logarithmic regression. LnReg. http://forum.kerbalspaceprogram.com/threads/120256-ISP-Graphs There I tried the same as you, but with LnReg. Then NathanKell said that KSP uses the NASA standard atmospheric model, but at 80%. (As in 8km on Kerbin = 10km on Earth). The formulas for getting atmospheric pressure in the NASA standard atmospheric model are: (These are corrected for the Kerbin 80% atmosphere, so h is kerbin-altitude in meters) NASA model source: https://www.grc.nasa.gov/www/k-12/airplane/atmosmet.html

I had some brakes not working for me once, turned out i was out of electricity.

However, while fins can't control (steer) your rocket, they will help with stability.

Abolutely correct! And then add the text and arrows.

For comparison, this is what I used to rescue my stranded kerbal from Minmus. (It requires in addition to what tech you already have, the "Advanced Rocketry" for LV-909 and Mk-55, and "Fuel Systems" for the external fuel ducts to balance out the fuel-use on the first stage. Oh, that is 1x T400 in the L&R craft.

Don't have a good reason really, it just made sense at the time... I'll change the axis-label and update the graphs with NASA atmosphere. All graphs in post #1 and #5 updated to NASA Atmospheric Model (80%)

NathanKell: Cool! When I use the standard atmospheric model found on a NASA-website, i get a slightly different curve. Basically my model gave engines too high ISP in the lower atmosphere. And the bigger the difference in vac and asl-ISP, the bigger my error was.

I decided to trow in the LV-N "Nerv"

Shure I could make som graphs for the smaller engines too. Which ones would be good to group together?

I have long wondered what ISP the different engines have at different altitudes. That would make it easier to choose the best engine for a stage. Especially that 2nd stage burn halfway up the atmosphere. If you have done the same, this graphs may be useful. I have grouped the engines into 3 groups. the 1.25m, 2.5m and 3.75m. I have choosen the most common engines. UPDATE: Switched atmospheric model to NASA model (80%), for more accuracy.

Calculating the size of a rocket involves much more math than calculating a rockets ÃŽâ€V. The main equation for it looks like this: mprop = Mass of propellant (fuel) mpay = Mass of payload. Everything except fuel tanks. fi = How much of fuel tanks are not fuel. For tanks in KSP this is 0.11, execpt the T100 (0.13) and ROUND-8 (0.14) And that is just for one stage. With a singe engine, or multiple of the same engine.

There is a basic set of rocket equations that come in handy when playing KSP. (But you get a lot of the answers for free if you use Kerbal Engineer Redux add-on.) To find the ÃŽâ€V of a stage: ÃŽâ€V = ISP · g · ln(m0/m1) ISP is the ISP of the engine in that stage. (You find that info in KSP under the [more info] for the engine) Note: There is a differens ISP for sealevel and vacuum. This is because rocket engines become more efficient in vacuum. g is the standard gravity. For Kerbin (and Earth) this is 9.80665 (or 9.81) ln (LN) is the natural logaritm. You find it on more advanced calculators. On Windows calculator you find it if you switch view to scientific. m0 is the weight of the stage before firing. m1 is the weight of the stage after firing. (Basically the same as m0 minus the fuel spent) So for your 2 T400s pr. stage in the rocket: (I see now I made an error in my previous math) S3-m0 = Pod(0.84) + parashute(0.1) + decoupler(0.05) + 2xT400(4.5) + T45(1.5) = 6.99 ≈ 7 Each T400 has 2t of fuel+ox, so S3-m1 = S3-m0 - 4 = 3 S3 ÃŽâ€V = 320 · 9.80665 · ln(7/3) ≈ 2659 ÃŽâ€V S2-m0 = S3-m0 + decoupler(0.05) + 2xT400(4.5) + T30(1.25) = 12.8 Each T400 has 2t of fuel+ox, so S2-m1 = S2-m0 - 4 = 8.8 S2 ÃŽâ€V = 280 · 9.80665 · ln(12.8/8.8) ≈ 1028 ÃŽâ€V S1-m0 = S2-m0 + decoupler(0.05) + 2xT400(4.5) + T30(1.25) = 18.6 Each T400 has 2t of fuel+ox, so S1-m1 = S1-m0 - 4 = 14.6 S1 ÃŽâ€V = 280 · 9.80665 · ln(18.6/14.6) ≈ 665 ÃŽâ€V Total: 2659 + 1028 + 665 = 4352 ÃŽâ€V If we move 1 T400 from the 3rd stage to the 1st stage: S3-m0 = Pod(0.84) + parashute(0.1) + decoupler(0.05) + T400(2.25) + T45(1.5) = 4.74 Each T400 has 2t of fuel+ox, so S3-m1 = S3-m0 - 2 = 2.74 S3 ÃŽâ€V = 320 · 9.80665 · ln(4.74/2.74) ≈ 1720 ÃŽâ€V S2-m0 = S3-m0 + decoupler(0.05) + 2xT400(4.5) + T30(1.25) = 10.54 Each T400 has 2t of fuel+ox, so S2-m1 = S2-m0 - 4 = 6.54 S2 ÃŽâ€V = 280 · 9.80665 · ln(10.54/6.54) ≈ 1310 ÃŽâ€V S1-m0 = S2-m0 + decoupler(0.05) + 3xT400(6.75) + T30(1.25) = 18.59 Each T400 has 2t of fuel+ox, so S1-m1 = S1-m0 - 6 = 12.59 S1 ÃŽâ€V = 280 · 9.80665 · ln(18.59/12.59) ≈ 1070 ÃŽâ€V Total: 1720 + 1310 + 1070 = 4100 ÃŽâ€V So... Basically I screwed up my math, and your way was better. I think I was thrown off by the 3 stages to orbit. 2 is normally enough.

For a quick suggestion about the stages, move 1 T400 from the 3rd stage to the 1st stage, that will give you ~620 more ÃŽâ€V. Your Current rocket: 3rd stage: 2659 ÃŽâ€V 2nd stage: 466 ÃŽâ€V 1st stage: 312 ÃŽâ€V Total: 3437 ÃŽâ€V After move 1 T400 from 3rd to 1st: 3rd stage: 1691 ÃŽâ€V 2nd stage: 1300 ÃŽâ€V 1st stage: 1065 ÃŽâ€V Total: 4056 ÃŽâ€V

I find that you must have fins on the first stage. For your small rocket, the small rocket fins should do fine. This will keep it stable in atmosphere. If the 1st stage does not get you high enough, you might need fins on the 2nd stage too. One thing is that you have 2 T400 fuel tanks on all three stages, this is propably not a good idea. Stages tend to get smaller the further up the rocket you go. For eyeballing it, I make every stage about 2/3rds for the rocket. As in; 1st stage is 2/3 of everything. 2nd stage is 2/3 of what is left after 1st stage is dumped. And so on...