Question about different engines

[Sardonyx]

So I've been playing around with engines a bit and it seems some are more efficient at turning fuel into thrust. Is this actually the case or am I imagining it?

Other than the additional weight of the more powerful engines, which changes the thrust to weight ratio, is there any other difference in the amount of fuel used to generate a certain amount of thrust of the various engines? Right now I'm only thinking about the rocket engines.

[rryy]

Yes, the engines with a larger specific impulse are more efficient, and engines are more efficient in a vacuum than in an atmosphere.

[Phawks]

The ISP rating for each engine is a measure of its efficiency. Higher ISP = higher efficiency. You need to balance your outright thrust requirements with your desired fuel efficiency. For example, Mainsails are incredibly powerful. Efficient, not so much. Ion engines are the exact opposite; they take forever to move a fork across the table, but you can run them for an incredibly long time. Generally, you need more thrust for launch stages and more efficiency for extraplanetary stages. NERVAs are usually your best bet for the latter.

[ASnogarD]

Check the descriptions of the engine...

ISP - basically fuel efficiency, high ISP means the fuel will last longer

Power - More power but usually at the cost of fuel efficiency

Weight - A engine may have the perfect ISP and Power, but weigh too much.

In general , high ISP engines are usually heavy and have poor power in atmosphere and the engines with good power in atmosphere are fuel greedy.

[Ralathon]

What the guys above me said. The higher the specific impulse the more efficient the engine.

When picking an engine you have to consider 3 things:

- Weight: A mainsail will certainly lift your moonlander. But it's a tad heavy.

- ISP (specific impulse): The higher this is the better obviously.

- Thrust: You aren't getting off the ground if your rocket weighs more than your engines can push. It also makes your burns shorter.

Engines usually excel in one of these things but suck in all the others. The nuclear engine is very efficient, but also heavy and produces little thrust (same story but even worse for Ion engines). The mainsail will lift your rocket but it isn't exactly fuel efficient and it is heavy. The ant engine is very light but has horrible thrust and one of the worst efficiencies of all engines.

There are also a few jack of all trades like the red radial engine and the aerospike.

So you have to take into account what you want to do with your spaceship before picking your engine.

[Oddible]

Pick up the addon Kerbal Engineer and then go watch this Scott Manley

. It will help you a lot to understand the differences between the engines and what the strengths of each are. Each engine has a different listing for atmosphere and vacuum and Engineer will give you deltaV, ISP, and Thrust based on the weight of your entire craft and each stage.

[nyrath]

As a rule of thumb Isp and thrust are inversely related, that is, as one goes up the other goes down.

The classic example is comparing a mainsail to an ion drive. By comparision, the ion drive has a fantastic Isp, but the thrust is pathetically weak.

Isp similar to "miles per gallon" in an automobile.

Acceleration can be figured by dividing the thrust by the ship's mass. If you are trying to lift-off from Kerbin, and your ship's acceleration is less than 1 g, your ship is just going to vibrate on the launch pad while it burns all its fuel and goes nowhere. This is why you generally do not see any ion-drive booster rockets.

Thrust-to-weight ratio is a rule of thumb measure of how powerful an engine is. You divide the engine's thrust by the engine's weight (which is the engine mass times 1 g). If it is less than 1.0, you cannot use the engine to help a lift-off.

Generally the only time you care about the thrust is for purposes of lift-off or landing. But there are limits. Ion drive has a thrust so low that your burn time can be measured in hours.

Thrust power is the thrust times the exhaust velocity, then divided by 2. This tells you how many watts of energy the engine puts out. Since it takes into account both thrust and Isp, it is a good way to measure the relative power of all the engines. Exhaust velocity is Isp time 1 g.

If you are stuck with an engine with low Isp, your rocket will need huge amounts of fuel in order to create the required delta V. There is a limit to how much fuel you can add before it becomes counter-productive. The way to get around the fuel limit is to use staging.

[GusTurbo]

The ISP rating for each engine is a measure of its efficiency. Higher ISP = higher efficiency. You need to balance your outright thrust requirements with your desired fuel efficiency. For example, Mainsails are incredibly powerful. Efficient, not so much. Ion engines are the exact opposite; they take forever to move a fork across the table, but you can run them for an incredibly long time. Generally, you need more thrust for launch stages and more efficiency for extraplanetary stages. NERVAs are usually your best bet for the latter.

Don't take this the wrong way, but a player asking if certain engines are more efficient than others isn't likely to know that people often refer to the LV-N engine as "NERVA."