Choosing the right engines

[dandrestor]

Hello!

After installing a few parts mods and RealFuels, I find myself more often than not building (what seems to me to be) over-sized rockets. A newbie's mistake, I believe, as I am completely puzzled by the large amount of engine choices, and I don't know exactly what I should take into account when I select an engine for any particular stage. I know to keep an eye on my TWR for the first stage, or my rocket won't get off the ground. I also have an idea of the dV I will need for any particular mission/stage, and I try to build a rocket that will top that requirement. But keeping only this in mind, I feel that my rocket building process resembles more of an arms-race between fuel weight and dV, rather than a guided, calculated process.

So my questions are: how do you select the proper engines for the stages of the rocket? Are there any guidelines? Am I correct in assuming that higher Isp and lower engine mass is better? How do you factor in thrust and type of fuel? What are the energy densities (if that's the correct term) of various fuels, and how does that affect the choice? Is there anything else I should be thinking about at design-time (concerning staging, engines and fuel) to ensure a fuel-efficient launch?

Thanks for your thoughts,

Dan

[DrMonte]

So why wouldn't you try the stock parts only first. (That's my thoughts)

[Tex]

There are no set guidelines for selecting the proper engines, but I'll tell you what I know! My only regret is your RealFuels mod, because I don't know how that affects the engines.

Lower mass and higher Isp is usually the way to go, yes, but you must also take into consideration the type of task the engines will be doing. If you're landing a vessel onto a planet/moon, shorter and lighter engines are the way to go. These are better for landing/lifting off and allow easier placement of landing gear. Radial engines always have pretty poor Isp, but can be placed in areas where no other engines are possible. Another (highly unorthodox) method to try is jet engines on rockets for their ridiculous efficiency. It's tough, but makes good sense.

If you have any other questions, let me know!

[Red Iron Crown]

I wrote a little primer on comparing engines for another poster, you might find it helpful. More aimed at setting stats for mod parts, but the same principles apply.

[StainX]

You need TWR > 1 for nice ascent to stable orbit ( i feel most comfy with TWR between 1,10 and 1,8).

If your in an orbit... TWR doesn't matter that much, ISP is more important, but also calculate in a good engine for descent

To the engines themself: ISP and Thrust are the two things that are most important for my choices for stage/altitude! You should also consider clustering them to get to an comfy looking TWR

EDIT: Oh, i also don't use the NERVA... cuz the risk of radiation holocaust is just to high

Edited April 28, 2014 by StainX

[Dispatcher]

I too would avoid using RealFuels until becoming proficient with getting to orbit and then to a target moon/ planet (and back). I don't see a problem with mod parts, as such (such as fuel tanks, etc.; see my sig line). I do recommend Kerbal Engineer or MechJeb for the dV, TWR and other data read outs; also a dV chart (you could Google for one of those).

Good luck!

[FREEFALL1984]

When building a munar lander (or a lander for any none atmospheric body) consider that higher thrust to weight will allow for a more accurate suicide burn meaning you can save fuel buy minimizing gravity drag. its often worth using a few aerospikes instead of the standard LV909 setup

[Red Iron Crown]

When building a munar lander (or a lander for any none atmospheric body) consider that higher thrust to weight will allow for a more accurate suicide burn meaning you can save fuel buy minimizing gravity drag. its often worth using a few aerospikes instead of the standard LV909 setup

I find a 48-7S cluster to be better still. Aerospikes are almost never best on bodies with no atmosphere, IME.

[Kryxal]

48-7S clusters are fairly good ISP and the best TWR you're going to see, Aerospikes just don't see a drop in ISP in-atmosphere.

[Heagar]

First:

When you really are a newbe, play with the stock parts first - as DrMonte suggested. That are enough parts to play with in the beginning.

Second:

• On an planet like kerbin with an atmosphere you should prefer the engines with the most thrust in the first stage - the mainsails, S3 KS e.g.. They have to lift the hole launcher up to (for kerbin) 20 to 25 km altitude.
  
• The second stage will be weaker engines with slightly more ISP with witch you can achieve an orbit - for example the Skipper, KR 2L; witch can also be used for the inital phase for an transit burn.
  
• For an interplanetary stage you use normally the engines with the highest ISP, witch means nuclear- or ion-engines, depending on the payload.
  
• For a lander on mun or a small moon, the weaker engines with high ISP's or with minimum weight are the best. Like the LV-909 or the 48-7s. Depending on the craft you want to build (probe, one/two man lander with one/two stages) even both could be installed. Of course you can use nuclear up to mainsails fort that, if you need to archieve a speacial goal (installing heavy parts of a base and so on).
  
• 
  

Last suggestion:

Especially for the first stage you should use the so called "asparagus-staging" witch means:

Around a center tank/engine-combination there are two to six tanks with radial decouplers (with/without engines) in pairs installed (use symmetry for that please). With fuellines the first pair is connected to the center, the second and third pairs are are connected to the next outer pair clock-/anticlockwise. As for the staging you decouple that pair first, that runs out of fuel. Than in the next stage the next pair and so on. For the outer tanks you can implement a toggeling of the gimbal of their engines in an action-group, witch prevents the launcher from wobbeling around. That means a slowed down, but more effective control of the rocket.

The result is a launcher with mutch more DV than a "normal" rocket (witch one is normal in KSP??). That's because all engines are running at liftoff (no dead weight) and only emptyed tanks are decoupled, while the rest of the tanks is fully loaded.

[theend3r]

Radial engines always have pretty poor Isp, but can be placed in areas where no other engines are possible.

Not true, you can always cubic strut them where you want them to be

[Pecan]

tavert has it all worked out for us.

Just select the TWR and vacuum/atmosphere chart you need and cross-reference the payload mass against the required deltaV.

[Tex]

Not true, you can always cubic strut them where you want them to be

What? You mean radial engines, right? It's impractical to put normal-sized engines were radials could go, even with struts and fuel lines.

[dandrestor]

tavert has it all worked out for us.

Just select the TWR and vacuum/atmosphere chart you need and cross-reference the payload mass against the required deltaV.

Beautiful! Thank you very much.

[dandrestor]

I wrote a little primer on comparing engines for another poster, you might find it helpful. More aimed at setting stats for mod parts, but the same principles apply.

Excellent write-up, thanks!

[dandrestor]

Thanks to everybody who answered. I understand that there are probably no clear rules, and having researched a little the notion of specific impulse, I realized that the choice of fuel is also irrelevant (Isp is fuel-independent, duh).

I disagree that a newbie should be playing with stock parts. First of all, even with stock parts, the same question would still apply. While playing with stock parts might allow somebody to get a 'feel' for what's going on, I believe it would offer no further understanding beyond simple intuition, which is what I am looking for here.

PS: can somebody tell me how I change the thread to "Answered"? I'm a forum newbie too.

[FREEFALL1984]

I find a 48-7S cluster to be better still. Aerospikes are almost never best on bodies with no atmosphere, IME.

the extra weight of the aerospike does make you right in some respects, but a lightweight lander with 4 aerospikes can burn off 300m/s in just a couple of seconds and a few times in the past I've killed all my vertical velocity just a few feet from the ground, sadly it doesnt always work out as planned

[Red Iron Crown]

PS: can somebody tell me how I change the thread to "Answered"? I'm a forum newbie too.

Edit the OP, go advanced, and there's a pulldown menu in the upper left where you set it.