Out of fuel.

[Fantorngen]

I don't know if this is only me, but i tend to always run out of fuel. In all the videos i watch, they never do, such as Matt Lowne's Minmus tutorial, but of course, i ran out of fuel waaay before he did. Anybody else getting this? and does anybody know any tips to not run out of fuel?

 

[Randazzo]

This is difficult to address without specifics, but how you fly can have a big effect on the amount of fuel you use to get to orbit. If you spend a lot of time going much too fast in the lower atmosphere you'll be losing a lot of fuel to drag.

[Axelord FTW]

Short of it, and the probable, is engine efficiency and TWR being mishandled.

Engines in KSP (and in real life to a very great extent) are more efficient in vacuum than in atmosphere. Always (mostly). The differential between those two level of efficiencies will vary greatly from one engine to another, however.
Fuel flow being a constant (throttle% being the same), an engine's thrust will increase as you rise up from Kerbin, to reach its maximum once you reach 70km. Two identical crafts, thus, will have different delta-v depending on where exactly they are.
Thing is, some engines are made to be as efficient as possible in different environments. Lower-stage lifter engines (the ones you want on your first and maybe-second stage) generally don't get that much more efficient in space, but the tradeoff being more powerful from the get-go. Upper stage engines, and vacuum-grade engines more particularly, can have bad to horrible efficiencies in-atmosphere, but do great when the air is rarefied or just absent altogether. This is the reason why Eve is such a deathtrap. The atmospheric pressure at sea-level there (and even at its peaks) is much higher than at Kerbin's sea level, thus making even strong, lifter engines there weak.

Ideally you want to make sure your dV budget when you're building in the VAB is enough to fulfil whatever mission your are building for, and that your TWR for any given stage is also high enough. Anything you want to land or take-off which has to have enough thrust to do their job efficiently, while you can get away with very low TWR when doing orbital/transfer maneuvers. For example, I usually hover around 1.4 TWR on the first stage on Kerbin, but that level of thrust is mostly unnecessary once in space, and chemical-rockets with 0.5 TWR are perfectly acceptable there. There are many tools available in the VAB to see exactly what you have, and you can also check your TWR in relation to different bodies. TWR of 0.5 is not nearly enough to land on Kerbin (without parachutes), but once you reach Minmus you could see your TWR ratio jump up to over 10 easily. You can right click on parts in the VAB to see more info, like their ISP (i.e. their 'atm' and 'vac' efficiencies). The higher the number, the better. Lifter engines tend to have those two values to be okay, but very similar, while vac engines will have horrible atm and great vac values.
You could also be loosing dV to inefficient maneuvers, as efficiency isn't all about engines either but what path you're taking to reach your destination. I always give myself more dV than I need, to account for inefficient moves, and/or mistakes in planning that I make. Not everyone is suited to pull a Stratzenblitz75 out of their bum and visit every single worlds in KSP sitting on top of one separatron or something, and giving yourself 25% extra dV is perfectly acceptable.

Edited January 1, 2022 by Axelord FTW

[king of nowhere]

Do you plan your missions in advance? I never run out of fuel because I plan the missions calculating how much deltaV I will need, and bring the fuel accordingly, with some extra. If I make a Tylo lander, I know it takes 2300 m/s to land, so I want at least 2500 m/s on the descent stage and as much in the ascent stage.

For more complex missions, I adapt the mission to the fuel budget. My jool explorer needs 1800 m/s to return back, but it only has 1400 m/s? My mothership must go from Ike surface to Moho and back, which requires normally at least 8000 m/s, and it has only 6000 m/s? I look for gravity assists to achieve the mission with less fuel.

My early game Minmus lander had an inefficient ascent, so that it reached Minmus with less fuel than it should have? I abort the landing and return to Kerbin, rather than getting stuck out of fuel.

You should never, ever run out of fuel. if you do, you weren't watching the fuel level.  You plan the mission, you know how much fuel you need, you bring enough. Something goes wrong, you spend more fuel than anticiapted, you change the mission accordingly to salvage what you can.

In all the videos you watch, they planned the mission, and tried it, and rehearsed it, until they verified that all worked correctly. You don't think they'd just sit down and put together a complex mission from scrap and it would work flawlessy from the first try?

[N_Danger]

If  you don't run out of fuel occasionally, you may be over building.  Just my thoughts from learning the hard way.

The most important thing IMO is to get a handle on transfer windows and Delta V maps/the transfer window planner.

Most Delta V maps give a delta V cost assuming the most efficient transfer window. You can use the in game planner or a mod (KER, Mech Jeb etc..) to find what time that window opens up in game. 

Check out the Gameplay questions and tutorials for some really good tips if you are a beginner. I learned a lot there when I first started out.

https://forum.kerbalspaceprogram.com/index.php?/forum/16-gameplay-questions-and-tutorials/

 

[MechBFP]

On 1/1/2022 at 9:03 AM, Fantorngen said:

I don't know if this is only me, but i tend to always run out of fuel. In all the videos i watch, they never do, such as Matt Lowne's Minmus tutorial, but of course, i ran out of fuel waaay before he did. Anybody else getting this? and does anybody know any tips to not run out of fuel?

 

What you are running into is called the "tyranny of the rocket equation".
This equation to be exact:

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

A very rough summary is that a linear increase in mass of the rocket will cause an exponentiation decreases in the amount of Delta V you will have to use.

Which means that a little increase in mass can potentially cause a MASSIVE increase in the amount of fuel you need to compensate to get the same Delta V. However adding more fuel means your engines may no longer be strong enough, which means you need to add more powerful engines, which are heavier, which means you need to add more fuel to compensate.... you see where this is going I hope.

So when people say you are "overbuilding" what they really mean is that your rocket is too heavy. I.E. using engines that are way more powerful (and therefore heavier) than they need to be, and/or you are wasting fuel between stages (which again makes the ship heavier than it needs to be).

The best thing to do is to make your stages are light as possible. Try to aim for a TWR of about 1.3 to 1.5 for launch and then once in orbit your TWR doesn't really matter as it just means burns will take longer, but I usually aim for around 0.7 to 1.0 as that lets my ship get places without extremely long burn times. Too low of a TWR can complicate things, like needing to do multiple burns, or getting into a higher initial starting orbit, so I would avoid that until you have a better understanding of the complications a low TWR causes.

On landing your TWR just needs to be above 1 (once again I aim for 1.3 to 1.5). On Minmus however the planet has almost no gravity so your ship is virtually guaranteed to have a TWR much higher than that anyway no matter how you design the landing stage.

The game now has built in tools that show you how much Delta V each stage will have as well (make sure to switch between sea level and vacuum as appropriate for the stage). The same area will also show you your stages TWR, but make sure to switch to the appropriate celestial body when looking at each stage.

For example, a lander with a TWR of 1 with respect to Kerbal will be something like 20 or 30 with respect to Minmus because it has way less gravity.

 

I usually design rockets top-to-bottom. So I will first make a landing stage, using those tools to make it have an appropriate Delta V and TWR for that purpose (i.e. landing on Minmus, then taking off and getting to orbit, and then getting back to Kerbal all in one stage), then I will design the 2nd stage to be an "in orbit" stage for getting from Kerbin to Minmus, then I will design a 3rd, 4th, (or more as needed) that is used from going from launch to orbit.

 

Edited January 2, 2022 by MechBFP

[Mahnarch]

If worst comes to worst, you can always get out and push.

 

I've run out of fuel more times than I'd like to admit coming back from Minmus.

With a Pe at Kerbin of 500,000m or so, I have to go EVA. Get behind the craft and wedge myself, pushing with the jet pack to get the craft Pe into, at least, the low 40s.

 

[Warning! Don't use ALL of your EVA juice whilst pushing. Make sure you can get back in when low to refill. Lather, rinse, repeat.]

Edited January 7, 2022 by Mahnarch