Help with terms

[Dimetime35c]

Okay I am so confused on all the tech terms of KSP. What does ISP mean? How is thrust to weight measured? Is any positive number in TWR mean the rocket will lift off? A little clarification would be nice. I've tried looking online and on YouTube and there answers just confuse me even more.

[Rage097]

A positive number in TWR will mean that the craft will accelerate in vacuum. On Kerbin you need a TWR of 1 or higher to take off, I believe. I'm no expert and my data may or may not be correct. don't quote me.

[FITorion]

Don't know what ISP stands for but it is a measure of the fuel efficiency of the engine. The higher the number the longer your fuel will last.

Thrust to Weight Ratio. (TWR) Weight divided by thrust = the ratio. It is calculated based on the weight at Kerbin sea level. A TWR over 1 means you have enough thrust to overcome the weight of the craft and take off from Kerbin.

[Depth]

ISP is the effectiveness/fuel drain of an engine. Generally speaking a higher ISP gives lower thrust, so you make a long effective burn instead of a strong short fuel-hungry one.

Thrust to weight means how much thrust your engines gives vs the weight of the craft. If you have a 1 ton craft, you need more than 1 ton of thrust to lift off, 1:1. In space it's a bit more complicated, as the ratio between thrust and weight means how quickly you can accelerate.

Edit: Sniped

ISP stands for Specific Impulse. As far as I know, in real life it means the velocity of the propellant leaving the engine.

[Garek]

ISP is a measure of the efficiency of the engine. I don't know how exactly it works, but the higher the ISP the more acceleration it will generate from the same amount of fuel.

TWR is an indicator of how fast the rocket will accelerate. It's always positive (unless you trust backwards and count that as negative trust). It's also always relative to a certain gravity (as gravity turns mass into weight), if not specified, it's usually either Kerbin surface gravity or current gravity. A rocket can take off from a body if it's TWR for that body's surface gravity is greater than 1. E.g. a rocket with a Kerbin surface TWR of 0.2 can't take off from Kerbin but it could take off from the Mun, because the Mun has less than 20% of Kerbin's gravity, so there it's TWR would be greater than 1.

[LameLefty]

Isp is Specific Impulse. In short, it's a measure of rocket engine efficiency. The unit is seconds. Higher values are better. Here's a good description:

http://en.wikipedia.org/wiki/Specific_impulse

[RoboRay]

In a nutshell, Isp describes how long an engine can produce 1kg (or pound, or stone, or whatever unit you prefer) of thrust from 1kg (or whatever) of fuel. This is why it's measured in seconds, as the value is a period of time.

If an engine delivers one ton of thrust and has an Isp of 400, and you give it 10 tons of fuel, the engine could operate for 4000 seconds before the fuel runs out.

Edited October 8, 2013 by RoboRay

[Runboy]

You can visit the KSP wiki,It has all of the KSP terminology you will ever need.

http://wiki.kerbalspaceprogram.com/wiki/Terminology

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[Requiem762]

ISP is fuel efficiency

TWR needs to be greater than one in atmo/ on the surface but in space any positive number works

[Crown]

In a nutshell, Isp describes how long an engine can produce 1kg (or pound, or stone, or whatever unit you prefer) of thrust from 1kg (or whatever) of fuel. This is why it's measured in seconds, as the value is a period of time.[...]

Careful! Be aware that kg is a mass. Any engines produces Newton, which is a force.

The unit of the specific impulse is . But it is true what you say.

The reason why it is mentioned in seconds is that american (feet, pound) and german (meter, kilogram) rocket engineers had to work together in the 60s. And the only mutual unit is the second. When you eliminate the earth's acceleration and the mass the second is left.

As mentioned many times before, Wikipedia is a very good source for anything. It just takes a while to read it all.

ISP is fuel efficiency

TWR needs to be greater than one in atmo/ on the surface but in space any positive number works

The Thrust-to-Weight-Ratio is actually a Thrust-to-Mass-Ratio, because weight is a result of acceleration. And in space there is no gravitational acceleration.

Can there be a negative TWR?

For this either the thrust or the mass has to be negative. Mass can't and a negative thrust would simply mean to accelerate in the other direction.