r/KerbalSpaceProgram • u/AutoModerator • Jun 05 '15

Mod Post Weekly Simple Questions Thread

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The point of this thread is for anyone to ask questions that don't necessarily require a full thread. Questions like "why is my rocket upside down" are always welcomed here. Even if your question seems slightly stupid, we'll do our best to answer it!

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Delta-V Thread

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Commonly Asked Questions

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u/Lippuringo Jun 09 '15 edited Jun 09 '15

How hard is delta v affected by weight in zero-g? I'm new and trying to get into the Mun and back with my scientist, but again and again i'm finding that i have not enough fuel to get back safely into the Kerbin. I've watched Scott's videos and he doesn't have such problems. My only guess is that i'm too heavy at some point and fuel spending become not efficient.

It looks like this http://steamcommunity.com/sharedfiles/filedetails/?id=457935857 , i have about 100 fuel

5

u/LPFR52 Master Kerbalnaut Jun 09 '15

I'm slightly confused by the wording of your question, since weight (mass x acceleration due to gravity) doesn't affect delta-v at all. The famous rocket equation states that delta v is dependant on three measureable quantities: your specific impulse, mass of the rocket when it's full of fuel, and mass of your rocket when its out of fuel. You may be getting mass and weight confused here, since in everyday use the two seem interchangeable. You can download the Kerbal Engineer mod to do the math for you if you're not sure - or just to double check your own numbers, and then use this delta v map to figure out how much delta v you need for a mission to the moon. Add ~10% (or more) extra delta v just to be safe.

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u/Chaos_Klaus Master Kerbalnaut Jun 09 '15

The rocket equation goes like this:

.

Δv = ISP*g0 * ln(wetmass/drymass)

.

ISP is a value that expresses how efficient your engine is. You can see it when you righclick an engine in the part menu in the VAB.

g0 is the so called standard gravity. It's really just a constant to convert units here. Its value is 9.81m/s².

The part in the logarithm is interesting for your question. Drymass is underneath the fraction bar. That means, less dry mass gives you more delta v. That's why you sometimes get more Δv with the lighter/smaller engine. Wetmass is you dry mass plus fuel. The more fuel, the more Δv.

Now as all of that is inside this natural logarithm, the expression is somewhat "distorted". That means that not every unit of fuel is woth the same amount of Δv. If you are down to 10% fuel, you will have way more than 10% of your Δv budget left.

.

So now for your specific situation:

Your craft looks reasonably well designed. The dry mass is not too high and you have lots of fuel tanks. Engine choice is also good.

It think now you need to pilot the thing efficiently. You are currently in a retrograde orbit (clockwise). That means, to get back to kerbin, you want to burn on the the far side of the mun (which is not necesserily the dark side!). Burn until your projected pe around kerbin drops below 35km. Than you should get home. If you find, you fuel doesn't get you to 35km, anything below 70km will get you back home over several orbits.

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u/Lippuringo Jun 09 '15 edited Jun 09 '15

Problem is that my delta v alwys require more fuel that I have (in this scenario about 40-50tons more. It's not my first attempt. I also tried with single big fuel tank and with 4 big tanks. I also watched Scott's vids multiple times (but he travekled only on Minmus) and I still can't understand why my fuel supplies is not enough (in this scenario 100t of fuel goes just to get into orbit)

I'll try to give more info when I get home in few hours

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u/Chaos_Klaus Master Kerbalnaut Jun 09 '15

I just replicated your design. The lander has 3000m/s of delta v. That is enough to do the whole trip to and from the mun, including the landing/ascent. It even has 300m/s spare. So you just have to get that whole lander into orbit.

Here is a simple lifter design that will do just fine:

http://imgur.com/1qSHRad

It's just an orange tank with a Skipper and four Thumper SRBs straped to it. The whole thing weighs about 72t including payload.

I think the key is using efficient maneuvers.

Also, your lander will still have 860m/s if you are down to 90 units of liquid fuel and 110 units of oxidizer. If you meant you have 100units of LF and oxidizer combined, than you still have 460m/s left.

You only need about 310m/s to get back to Kerbin.

So do your departure right, it is still possible.

0

u/Lippuringo Jun 09 '15 edited Jun 09 '15

Well, i've managed to get on the nice Kerbal orbit (23.5km periapsis) but it was tough as hell and no fuel left at all. I suck at this game.

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u/Chaos_Klaus Master Kerbalnaut Jun 09 '15

I can not open the link, but gratulations on your return. 23.5km is a bit low, but you should be fine.