What does ISP stand for ?

[Vengeance4ME]

What does ISP stand for ?

[toinkertoy]

It denotes the "specific impulse" or how efficient the engine is at converting the propellent into energy or thrust.

It's not actually written "ISP", in equations it's denoted as an italicized "I" with the subscript "SP".

[Vengeance4ME]

Originally posted by toinkertoy:

It denotes the "specific impulse" or how efficient the engine is at converting the propellent into energy or thrust.

It's not actually written "ISP", in equations it's denoted as an italicized "I" with the subscript "SP".

So should I worry about it for my rockets when I'm in space ? I really never get that, should I worry about my thrust, delta-v or specific impulse. Sorry if this is off topic.

[toinkertoy]

I'd place more emphasis on the delta-V and thrust-to-weight ratios (abbreviated: TWR) when designing vehicles for a particular mission.

delta-V is pretty simple, you need what you need! For example: you need about 1200 dv (give or take 50-60 depending specifics) to have an encounter with Luna. There are no exceptions! you'll either have it, and make it. Or won't and don't.

Thrust-to weight ratio on the other hand is very important for initial lift off, landing on planet/moon surfaces and to lesser extent for big burns like a trans-lunar injection burn. For lift off of course you need a TWR greater then 1 or you will never get off the ground. For landing on moons/planets you need a TWR greater then 1 or you won't be able to slow down (slimier to lift off just going the other direction) For both I recommend a TWR around 1.5 to 1.6. For very large burns (say over 1000 dv) you want a decent TWR so that you're burn doesn't take long. If a burn takes over 3 minutes it starts to become less accurate and what you end up with can be quite different then what you set up with the burn node. So you want a fairly large engine that can conduct that massive burn in under 3 minutes.

A high ISP is nice! it means you can get more energy (or delta-v) with less propellant weight, but as I described above, if you don;t have the dv or an appropriate TWR then the efficiency doesn't really mean anything.

The Full-flow staged combustion engines are a good example of high ISP engines. They definitely out perform the Gas generator engine and even the staged combustion engines. They also are capable of high thrust to weight ratios. You can make your booster quite a bit less massive and get just as much dv from them then if you go with a gas generator engine. But it comes at the cost of a higher price (a much higher price).

The thermal nuclear engines are yet another example (I think they have the highest ISP/efficiency in the game). But they come at cost too and that is a much lower TWR! You won't be able to use them for lift off or landing on a planet/moon but they can work for large spacecraft in orbit looking to go to other planets! With some careful designing you can squeeze enough thrust out of them so that your burns don't take too long (and thus are still pretty accurate) and due to their high ISP, you can use much smaller propellant tanks then you would using chemical propellant engines (and that of course reduces mass, which increases dv, and further improves TWR etc.)

So I hope something in there was helpful, but if there are any other questions just feel free to ask!

[Vengeance4ME]

Originally posted by toinkertoy:

I'd place more emphasis on the delta-V and thrust-to-weight ratios (abbreviated: TWR) when designing vehicles for a particular mission.

delta-V is pretty simple, you need what you need! For example: you need about 1200 dv (give or take 50-60 depending specifics) to have an encounter with Luna. There are no exceptions! you'll either have it, and make it. Or won't and don't.

Thrust-to weight ratio on the other hand is very important for initial lift off, landing on planet/moon surfaces and to lesser extent for big burns like a trans-lunar injection burn. For lift off of course you need a TWR greater then 1 or you will never get off the ground. For landing on moons/planets you need a TWR greater then 1 or you won't be able to slow down (slimier to lift off just going the other direction) For both I recommend a TWR around 1.5 to 1.6. For very large burns (say over 1000 dv) you want a decent TWR so that you're burn doesn't take long. If a burn takes over 3 minutes it starts to become less accurate and what you end up with can be quite different then what you set up with the burn node. So you want a fairly large engine that can conduct that massive burn in under 3 minutes.

A high ISP is nice! it means you can get more energy (or delta-v) with less propellant weight, but as I described above, if you don;t have the dv or an appropriate TWR then the efficiency doesn't really mean anything.

The Full-flow staged combustion engines are a good example of high ISP engines. They definitely out perform the Gas generator engine and even the staged combustion engines. They also are capable of high thrust to weight ratios. You can make your booster quite a bit less massive and get just as much dv from them then if you go with a gas generator engine. But it comes at the cost of a higher price (a much higher price).

The thermal nuclear engines are yet another example (I think they have the highest ISP/efficiency in the game). But they come at cost too and that is a much lower TWR! You won't be able to use them for lift off or landing on a planet/moon but they can work for large spacecraft in orbit looking to go to other planets! With some careful designing you can squeeze enough thrust out of them so that your burns don't take too long (and thus are still pretty accurate) and due to their high ISP, you can use much smaller propellant tanks then you would using chemical propellant engines (and that of course reduces mass, which increases dv, and further improves TWR etc.)

So I hope something in there was helpful, but if there are any other questions just feel free to ask!

Thank you for your help, I play a lot of KSP more then Juno new Origins and I really want to get in this game.