I understand TWR, but how do I know if I have enough fuel to get me to orbit?

[ultrarascal]

Sorry about bad title.

So I have been playing for quite a while now and I understand how TWR works. Each stage will have to have 1.0+ for my calculated TWR in or for me to get any lift. I have that much down.

My question is. How do I know if I have enough fuel to get me all the way to orbit. I could easily overcompensate and just put everything on a heavy launch system, but that is too easy and I would like to be "economical" about how I build things and not end up in space with 2k extra fuel. I'm sure you all understand.

Is there a calculation or anything that I need to know or something within mechjeb that I can use to tell me if I will just flat out have enough fuel to get me all the way to a 100k orbit around Kerbin?

Thanks for the help in advance everyone!

[Goldham]

You can calculate the Delta-v or use mechjeb or kerbal engineer my lifters generally have about 4k in the first stage

[stupid_chris]

Delta V, delta V, delta V! It is the key to success around here. Usually, a normal launch from Kerbin will use around 4500m/s of delta V, so you really want to plan your launch vehicle with at least that amount of fuel. Then you can sum up the delta V left in your ship and compare it to a delta V map and see if you have enough to go where you wanna go or if you need more fuel.

[ultrarascal]

So as long as I have 4,500ish delta V and all my stages are 1.0+ TWR my little guys should regularly be making it to orbit?

[capi3101]

Delta-V is the name of the game (or at least it would be if KSP didn't sound cooler or at least like the name of a fried chicken chain). You need on average 4,550 m/s of available delta-V just to reach a 100k orbit. How much fuel that equates to is going to depend on a) the mass of your payload and the specific impulse of your engines. And the equation you use - the Tsiokolvsky Rocket Equation - has got one of those nasty-looking natural logarithms in it. It goes like this:

delta-V = ln(M/Md) * 9.81 m/s^2 * Isp, where:

M = total mass of the rocket

Md = dry mass of the rocket (your rocket's mass after its fuel tanks are empty)

Isp = Specific Impulse.

When I plan missions, I usually assume the 1 atmosphere specific impulse of my rockets (i.e. when they are least efficient). Isp will increase as you enter space (i.e. your rockets will become more efficient), so it helps to use this planning value. On average, liquid fuel rockets in KSP have an Isp of about 290s on the ground (some more, some less, some a good deal more or less). Plug that figure into the equation and solve it for 4,550 m/s, and it turns out you want your mass ratio to be about 5:1. So figure out the mass of what you want to put into orbit (easiest way to do this stock is to just put the payload out on the launch pad without any booster elements, switch over to the map view and call up the info window) and then plan to put about five times as much mass into the booster. Bear in mind that only fuel tanks are going to affect that 5:1 ratio.

Kerbal Engineer Redux (KER) or Mechjeb will do all the math for you if you're into mods. Me, I still prefer to do the calculation by hand (even though I've installed KER and have found it to be a useful tool).

Edited July 16, 2013 by capi3101

[SRV Ron]

When designing for that 5 to 1 ratio, there is another factor to look at, rate of acceleration in the Kerbal lower atmosphere and the pull of gravity. Accelerate too slowly, and gravity will cause you to burn excess fuel to achieve orbit. Accelerate too quickly and atmospheric drag will cause excessive fuel usage for the first 20,000 or so meters. Engine efficiency as well as their power to weight ratio also affects orbital payload. Locking gimbals on outboard engines also increases efficiency as well as reduces shaking. Hit that right rate of acceleration and you will easily achieve that 5 to 1 payload ratio for orbit. 451 ton launch weight on my one Apollo design placed 51 tons into a low Mun orbit using onion staging, quad LV-30s on the onion stage, and quad LV45 for the center second stage in place of 7 Mainsails.

Edited July 16, 2013 by SRV Ron

[blizzy78]

Kerbal Engineer Redux does a pretty good job on showing you the exact Delta-v you have, for any body in the system.

[Eric S]

Kerbal Engineer Redux does a pretty good job on showing you the exact Delta-v you have, for any body in the system.

delta-v doesn't vary by body, except in how the atmosphere affects the ISP. What KER shows you for each body is your TWR. Very handy to know if your lander stage has enough TWR to lift off of Minmus without being so overpowered that it makes landing there harder.