What is ISP?

[Thomas988]

Hello everyone!

I've been thinking about my first Duna mission, and I know engine ISP is going to be important, or so I've been hearing. But what is it? I've looked it up on the Wiki, yet I'm still confused.

Could someone put ISP into simpler terms, and should I care about it when planning my missions?

[sal_vager]

It's basically rocket fuel economy, the higher your ISP, the less fuel you need for the same thrust over time

I think....

See wikipedia for more

[KvickFlygarn87]

Yes, a rocket with high I_sp is good. Here's an equation you wanna learn by heart:

Delta V = V_e * ln(m₀/m₁)

Tsiolkovsky's rocket equation.

V_e is exhaust velocity relative to the rocket. You calculate it with I_sp * 9.81.

m₀ and m₁ are full and dry mass, respectively

Now let's work it a bit. We take a rocket that ways 10t full and 5t empty, excluding the engine, and work out the equation with the efficient nuclear engine that weighs 2.5t.

V_e = 800 * 9.81 ~ 7848

m₀ = 12.5

m₁ = 7.5

Delta V = 7848 * ln(12.5/7.5) ~ 6682m/s

But if we take the less efficient and heavier mainsail, we get the values

V_e = 330 * 9.81 ~ 3237

m₀ = 16

m₁ = 11

Delta V = 3237 * ln(16/11) ~ 1213m/s

So, even while the Mainsail has far surperior thrust, the nuclear beats it at range.

[Grant_]

Isp = specific impulse = amount that the engine can change the craft's momentum per weight of fuel OR force applied per weight rate of fuel

[nine_iron]

Yes, a rocket with high I_sp is good. Here's an equation you wanna learn by heart

Is it bad I enjoyed that post? Not exactly what the OP was after but it made me giggle.

I understand ISP to be pretty much the thing that helps me rate engine efficiency for long burns. If I need to go somewhere in the vacuum of space I go high ISP.

If I need to go from the ground to space I need thrust.

[KvickFlygarn87]

Is it bad I enjoyed that post? Not exactly what the OP was after but it made me giggle.

Did I roast your brain too hard? I thought you wanted medium-black, not medium-rare!

[Grant_]

Is it bad I enjoyed that post? Not exactly what the OP was after but it made me giggle.

I understand ISP to be pretty much the thing that helps me rate engine efficiency for long burns. If I need to go somewhere in the vacuum of space I go high ISP.

If I need to go from the ground to space I need thrust.

Well if you're just making a very light probe you can get away with RAPIER engines which have very high ISP

[Smidge204]

Well if you're just making a very light probe you can get away with RAPIER engines which have very high ISP

The RAPIER engine is the worst rocket, Isp-in-vacuum wise, of it's radial size class and is arguably the worst jet engine in terms of overall efficiency. Its main (only?) advantage is it doubles as both a rocket and a jet engine, so you don't have to carry the mass of two different engine types.

=Smidge=

[Specialist290]

To put it still another way: Specific impulse is essentially a measure of how long it will take for a given engine to burn a given measure of fuel assuming a constant amount of thrust.

Purpletarget has a rather excellent video on the subject as part of his "Krash Test Kerbals" series, and I'd highly recommend you look at if you want a demonstration of what exactly that means.

[Grant_]

The RAPIER engine is the worst rocket, Isp-in-vacuum wise, of it's radial size class and is arguably the worst jet engine in terms of overall efficiency. Its main (only?) advantage is it doubles as both a rocket and a jet engine, so you don't have to carry the mass of two different engine types.

=Smidge=

It should be more than one stage.... have it go to about 8-10 km then switch to something else like a poodle.

[Thomas988]

Thanks guys!

[numerobis]

Tiny detail: in KSP, Ve = 9.82 Isp not 9.81 -- normally this is not enough of a difference to care about (you should have more than a part in a thousand of error buffer).

[DisarmingBaton5]

Simply put: Isp is a measure of fuel efficiency.

Higher number is better.

Note that Isp = specific impulse

[Semtex89]

ISP basically means fuel efficiency.

I always use the nuclear engines for interplanetary transfers to conserve fuel. They've got the highest ISP available. Although they don't pack much thrust, so you might want to go with at least two of them so you don't have to burn for fifteen minutes.