propulsion 101

[KerbMav]

Can anyone push me in the right direction as to where I can find a good tutorial on the different engines regarding isp and thrust in particular?

I realized I did not understand which is what here - regarding payload/strength, acceleration, maximum speeds etc.

Edited September 6, 2013 by KerbMav

[stupid_chris]

Can anyone push me in the right direction as to where I can find a good tutorial on the different engines regarding isp and thrust in particular?

I realized I did not understand which is what here - regarding payload/strength, acceleration, maximum speeds etc.

Well the thing is, depending on your payload, you will get different specs. The only thing that will remain constant to an engine is it's thrust and ISP. The acceleration, will vary with the amount of fuel left and with the total payload, the maximum speed is virtually infinity and if intelligently made, almost any engine can lift any payload. I can't point you to a tutorial in particular, but maybe something like Kerbal Engineer or MechJeb's delta V and TWR stats could help you understand how different engines and different payloads affect your acceleration

[capi3101]

If what you're looking for is a comparative list of the capabilities of each engine, try the wiki. But yeah, what stupid_chris said is pretty much accurate: there isn't really such a thing as a "maximum velocity", and your acceleration is going to be sort of based on Newton's Second law (i.e. acceleration equals Force divided by mass, except your mass isn't constant because that's how a rocket works; your rate of acceleration increases as you burn fuel and your mass decreases).

[KerbMav]

My question was basically, what these figures "do".

More thrust moves more mass/payload - so far I think I understand.

But how does isp come into this?

My Science-English fails me today, sorry.

[UmbralRaptor]

Isp is a measure of propulsive efficiency. With the ingame unit choice (seconds), it measure how many pound(force) * seconds of impulse one pound(mass) of propellant provides. Higher is better, as it gives you greater ÃŽâ€V for the same mass ratio.

edit: I should probably mention how ÃŽâ€V and Isp are related...

Edited June 13, 2013 by UmbralRaptor
Tsiolkovsky

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

Thrust is the total force exerted by the engine when activated.

ISP is how efficiently it burns its fuel.

[Cranium]

Thrust is the total force exerted by the engine when activated.

ISP is how efficiently it burns its fuel.

That's actually a very good explanation. I was not sure about this either until that response. Thank you!

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

That's actually a very good explanation. I was not sure about this either until that response. Thank you!

No problem, happy to help

[KerbMav]

OK, lets try again if I got this right - feeling kinda dumb by now.

Thrust defines how well the engine handles/moves high mass (inc. lift off and orbiting) - isp defines how much fuel I would have to bring along?

So, would an atomic engine be of more use in space then as a lifter?

What about the "weak" ion drive - is it any good at moving bigger interplanetary ships? Or does it only take longer do speed up?

[AndersSchm]

Yes that is right, atomic engines are good as upper stages when you're in space, down low in the atmosphere you want your mainsails

[capi3101]

Thrust defines the amount of force the engine can output. Specific impulse represents the force with respect to the amount of propellant used per unit time (i.e. it measure's the rocket's "fuel efficiency").

An LV-N's Isp in space is just under four times what it is in atmosphere; it is thus just under four times more fuel efficient in space. For example, let's say you have a ship whose full mass is twice that of its dry mass (i.e. half its weight is fuel mass). In atmosphere at 220s of Isp, the Tsiokolosky Rocket Equation tells us that a rocket using an LV-N will have just under 1500 m/s of delta-V. Same rocket, same mass, but in space has 5,440 m/s of delta-V.

Ion drives...very high Isp but that low thrust output means you're going to have to keep the payload light. How light? Well...the thrust is only 500 Newtons, and your engine plus tank by themselves weigh 370 kilograms. Newton's Second Law therefore tells you that the maximum acceleration you can expect from just that amount is going to be 1.35 m/s^2. And that's not accounting for any other payload or for the electrical sources necessary to keep the engine running. Now, that said, if you can accept that you're not going to go anywhere quickly, ion engines may be a pretty good deal; I've got a geosynch satellite which I boosted with an ion engine.

[Jod]

Ugh. I've been calculating delta v for weeks now, still trying to get a small compact Eve lifter.

Thrust is basically the force that is applied to CoT. To get Thrust-to-Weight-Ratio you simply divide your thrust by 9,81*mass. So for example 1 aerospike can lift no more than 18 tons on Kerbin vertically. It is a good idea to have your TWR above 1 even if you use wings because current drag model gives absurd amounts of aerodynamic friction. I usually prefer TWR at about 1.05 for rocket engines or 1.4 if i use jet engines. Remember that as you consume fuel your weight will be reduced and TWR will increase.

Delta-v is technically an advanced analogy to mileage. The more delta v the further your craft can get. It depends on your fuel amount and on your Isp. I am currently working on a small craft with 1 aerospike and 2 LV-Ns that has 2/3 of its mass as fuel, in total it has around 7000 delta-v according to formula:

delta-v = Isp*9,81*LN(full mass/dry mass)

Where dry mass is mass without fuel.

The problem is actually getting a craft that has huge amounts of fuel relative to its total mass and efficient engines that can lift it all at the same time. Because efficient engines weigh a lot you have to combine them with other engines in a way that would give you a lot of TWR at take-off and an efficient burn later. This gets really hard, especially getting fuel ratio up when your Isp and TWR are capped.

Edited June 13, 2013 by Jod

[Eric S]

What about the "weak" ion drive - is it any good at moving bigger interplanetary ships? Or does it only take longer do speed up?

As much as I like the ion drive, it really isn't useful as a main drive for manned ships in KSP. It produces 0.5 thrust, which means that if you stick it on the lightest manned command pod and ignore fuel and electricity requirements, you'll get less than 1 m/s worth of acceleration.

Even unmanned probes can be too heavy to use an ion engine unless you've got a lot of patience.

It does make a nice engine for correcting satellite orbits, and an OK engine for lit probes, but for anything larger, you're either talking multi-hour burn times, using OP engines from mods, or strapping more of them onto a craft than you can realistically power.

And yes, the atomic engine is mostly for use in space. As long as you're patient enough, it doesn't matter if it takes 15 minutes for a transfer burn, since the actual transfer orbit is going to take months anyway. Technically this is also true of the ion engine, but at 1/120th of the thrust of the atomic engine, ion engines take saintly levels of patience if the ship you're moving has any real mass.

[stupid_chris]

Yep you got it right. The atomic is by far the best for interplanetary ships. The ion would require an incredible amount of them to have a decent acceleration, and a ridiculous amount of power supply, so it's really better for light ships or orbital adjustments.