Engine Efficiency

[Stryde]

I've been looking around and I couldn't find an answer to why the 1.25 meter engines have an all around lower TWR and Isp compared to 2.5 meter engines(, with the exception of the Nerv). Are bigger engines more efficient than multiple smaller ones in real life or is it just for balance?

Edited July 26, 2015 by Stryde

[NathanKell]

Career mode.

[Glaran K'erman]

A) Smaller engines are naturally going to give you a lower TWR because they produce less thrust and this ratio is THRUST to WEIGHT, meaning smaller engines have less oompf the heavier the mass they are pushing.

The ISP levels are I believe meant to be balanced as you progress through the tree to simulate better technological advances. But these also have to do with what type of engine you are using because different propellants are consumed differently. An example where a smaller engine has a better ISP would be the Dawn propulsion system, one that would be considered an advanced technology by the games standards.

[Stryde]

Thanks for the replies, though the lower stats does make the Swivel and the Reliant engines pretty obsolete once the skipper is unlocked.

[Glaran K'erman]

Pretty much

[Ripper2900]

In ReaLife a single large engine *should* be better than several smaller ones. Usually, though, it's not so, because it's difficult and expensive to build very large engines. So the benefits of making the ultimate engine is very often outweighed by the savings of using less perfect, but cheaper hardware.

The ksp engines are mostly grouped with one "launch-class" and one "vacuum-class" engine in each diameter size. Then there's see specialized versions in each class.

The better TW of some of the larger engines doesn't mean that one is always better, because it's the TW of the ENTIRE rocket that's the important factor.

[Nich]

Yes because of volumetric efficiencys of larger engines will always have better performance. You could make a small engine almost as efficient as large engines but then they would cost just as much or more then there larger counterpart. Tolerances are much easier to hit on large parts because a .0001% error on a rhino is 1 mm where as the same tolerance on a terrior is 1 nm. Also larger engines get more complexity (cost) that is just not worth it to put in small engines. From real life fan blades from the 787 cost 60k apiece vs a DC9 at 2k. The blade on the 787 is 7.5% more efficient but you could never justify that extra cost on an engine that puts out 1/10th the power. The DC9 blades while 1/4th the size would cost about the same to produce (55k) because the materials are cheap the manufacturing is what is expensive.

[FancyMouse]

Don't underestimate engine mass itself. Remember Isp is not the only factor in the dV equation. There's the other hidden ln(m1/m0) term, which is also affected by your choice of engine.

1.25m engines are still good for appropriate payloads. If I use 2.5m engines for that payload, it will be just waste of fuel and extra mass.

[NathanKell]

That's not quite the case for rocket engines--combustion instability is a killer for large engines. That's why beyond about 2MN you see multi-chamber engines for staged combustion.

[X-SR71]

The swivel and reliant become obsolete in career, but I still use all other engines from time to time. The LV-1 Ant for example is perfect for very light orbital probes!

[tomf]

The swivel and reliant are great engines in career. If you want to lift a small probe into orbit the 2.5m engines are expensive and overkill and a tiny engine isn't going to be able to lift off.