When do I use which engine?

[Talkingstreet]

I am relatively new to KSP, but have done some manned an unmaned missions to duna, the mun and minmus and build a small space station (Sandbox). The problem that I keep having is the trouble of choosing the right (rocket)engine. Eg: In the beginning I literally just used the Mammoth engine for the first stage, no matter the space craft.

Although I have learned a bit, some engines still seam very similar to me, or I don´t really know what engine to use for the job.

What are the radial mounted engines used for? Like Spider and Twitch

What first stage setup would you use for a) a crewed close distance mission to LKO or the moons of Kerbin b) a crewed mid distance mission eg to Duna c) a crewd long distance mission eg Eeloo?

The same a) b) and c):

What setup would you use for second and third stages?

And as a last question, which engines would you say have the same usecase, but one is just better than the other?

[Echo__3]

Engines have a value called ISP that is the specific impulse of the engine. Basically it is how efficient the engine is. The game will give you the engines ISP at sea level and in a vacuum when you right click on an engine in the build menu. The engines with a high vacuum ISP are best used in space. The once with have 280 or more sea level ISP are your booster stage engines.

Engines have a Thrust value. If you are launching somethings small, you don't need as much thrust.

Engines come in different sizes and weights. For a small probe or a little command seat with a small fuel tank, an Ant engine may work best.

You will try to use the most efficient engine for the job. 

[Talkingstreet]

Thank you!

But for what TWR would you aim on a first stage, second stage and third stage?

[Regor]

5 hours ago, Talkingstreet said:

Thank you!

But for what TWR would you aim on a first stage, second stage and third stage?

First stage > 1.5 at sea level. Should get you some of the way to orbit. You want engines with high thrust and usually high ATM ISP.

Second Stage > 1 at vacuum. Should get you completely to orbit, and probably a bit beyond. Here you want engines with the most VACUUM ISP. 

Third stage doesn't really mater, as long as it is higher than about a half. Otherwise, burns start to get efficient. Here you can use weak but extremely efficient vacuum engines.

[Snark]

19 hours ago, Talkingstreet said:

(Sandbox)

As a side note to your actual questions-- you might want to consider trying a career play-through, at least for a bit.  There's nothing wrong with playing in sandbox, but it throws all the options at you, so it can be a bit bewildering for someone who's new to the game, sometimes.    One of the things that can be handy about career mode is that it takes the confusion away by drastically limiting your options to just a tiny handful of parts to begin with.  It can be a useful way to learn your way around.

You don't have to do that, of course-- just mentioning it as something you might want to consider.

19 hours ago, Talkingstreet said:

What are the radial mounted engines used for? Like Spider and Twitch

Those are useful in certain ship designs where, for whatever reason, it's either not possible or impractical to have an engine mounted below a stage.  There are a lot of potential reasons around that, I won't go into them here-- they boil down to "fancy design options".

Typically, radial engines tend to be somewhat less "efficient" than equivalent stack-mounted ones (e.g. lower Isp or something), so you may want to stick with the stack-mounted ones for now, particularly if you're new to the game.  Basically:  if you come to a point where you're trying to build a rocket and something about the design is awkward when you try to fit the engine in there... then that's when you might want a radial engine.

9 hours ago, Talkingstreet said:

But for what TWR would you aim on a first stage, second stage and third stage

Pretty much what @mabdi36 said, above-- that's good advice. 

A good way of thinking about "what's the right TWR" in general is, you want the lowest TWR that you need for the job at hand.

Why do you want low TWR?  Because the biggest limiting factor for rocket designs, for the most part, is available dV.  You want lots of dV, and designing for that is hard.  So anything that gets you more dV is usually a good thing.  And the enemy of dV is dead weight-- because the heavier the rocket is, the less dV you have.  You don't want to "waste" mass.  And engines are heavy.  And bigger engines are heavier.  So if you have "too much TWR", then that means you're carrying more engine power than you need, which in turn means that your engines are probably a lot heavier than they need to be, which means you're wasting dV.  (Also, the really high-thrust engines tend to have lower Isp, and you really want higher Isp when you can.)

"Okay then," I hear you cry, "why don't I just make it really really low for everything?"  The answer is that you do need to have "enough"-- for example, if you're launching off the pad, then you need to have TWR significantly higher than 1 because you're fighting gravity.  Gravity loss is a thing.  It's a huge source of wasted dV for the first stage, so that's where a higher TWR helps you.  It becomes a balancing act.

First stage:  You need high TWR.  1.5, as mabdi36 suggests, is a reasonable "happy medium".  In practice, anything from 1.3 to 2.0 can work, though you need to design the rest of the rocket accordingly.

Second stage:  Around 1.0 is not bad, though it can be higher or lower depending on how much dV your first stage has.  Here's how to think of it:  At the time that your first stage is jettisoned, what angle are you flying at-- i.e. how far along your gravity curve have you gotten?  If you're around 45 degrees, then you probably want a TWR of around 1.0.  If you're steeper (more vertical) than 45 degrees, then you probably want something higher than 1.0.  If you're shallower than 45 degrees (i.e. mostly horizontal), then you probably want something lower than 1.0; even 0.5 or so would probably be fine.

Third stage:  You want a low TWR.  The lower it is, the less mass you've wasted lugging around engines you don't need.  It should definitely be no higher than 0.5ish, but just how much less will depend on various things such as your mission profile and so forth.  Play around with it for a bit and find out.  (I'm assuming that this is the stage you're using to circularize orbit to LKO.)

Fourth stage and beyond:  Let's say you've gotten to LKO, but now you're going on somewhere further, such as the Mun or Minmus or other planets.  You're already in orbit.  In this case... you generally want your TWR to be really low, so that you're saving mass and getting good dV.  Pick an engine that has the highest Isp you can get, and it's fine if it has low thrust.

[Zhetaan]

 

50 minutes ago, Snark said:

[What @Snark said]

 

@Talkingstreet:

I mostly go along with what has been said already, but I will add one caveat to this:

1 hour ago, Snark said:

Fourth stage and beyond:  Let's say you've gotten to LKO, but now you're going on somewhere further, such as the Mun or Minmus or other planets.  You're already in orbit.  In this case... you generally want your TWR to be really low, so that you're saving mass and getting good dV.  Pick an engine that has the highest Isp you can get, and it's fine if it has low thrust.

Don't forget that if any mission you plan involves a landing, then you need to include engines on that lander such that its thrust-to-weight ratio is greater than one for the body it is landing on.  Remember that you can turn engines off using the part action window (meaning right-click the engine--and you can also assign action groups to do that with a button press, as well) so that you don't use your lander engines when they are not wanted.