ISP Comparison

[kahlzun]

Hi guys.

Been playing around with KSP for a while now, and am trying to figure out how the Specific Impulse (isp) of engines compares.

If I have 2 NERVA engines in vacuum, does that mean my equivalent thrust etc goes from

60kN

800 Isp

to

120kN

400 Isp?

If this is true, it compares unfavourably to the Toroidal aerospike which is

250 kN

390 Isp

or does Isp have a logarithmic effect? I checked wikipedia, and the article wasn't much help with explaining stuff, and my higher math isn't so great.

halp?

[phoenix_ca]

No, absolutely no. Isp is for each engine...I'm not sure where you get the idea that it'd be halved by two engines, but...no. Each engine will have an Isp of 800. They will each produce 60kN of thrust a piece.

Isp is a measure of efficiency, so the number of engines (assuming they're the same engines) you have doesn't change it. You'll still get just as much thrust-to-propellant ratio. The difference is that you'll be sucking-it-down twice as fast, producing the same amount of thrust in half the time.

You don't need higher math to understand this. Specific impulse is analogous to miles/gallon, or kilometers/liter for those who *ahem* know how to count. If you put two car engines together that had the same km/L rating, would they then start using even more fuel to get a km? No, dropping all the other messy things like friction, heat losses, drag, yadda yadda.

Edited October 4, 2012 by phoenix_ca

[maltesh]

Basically, all other things being eqal, if both craft have the same Full mass/dry mass ratio, delta-V (the amount of change in velocity the spacecraft can accomplish by burning all its fuel) scales linearly with specific impulse. ANd if the specific impulse of all the individual engines that are burning are the same, then the specific impulse of the entire engine system remains the same.

At the end of burning all its fuel from standsitll, in the ideal, no gravity, no atmosphere, no drag case, burning in a specific direction, using two ships that devoted the same fraction of their total mass to fuel, a spacecraft relying entirely on Nerva engines would wind up moving 2.05 times faster than one relying entirely on aerospikes.

Edited October 4, 2012 by maltesh
Deleted "thrust doesn't change" line. I don't know what I was trying to say with that, anyway.

[Person012345]

Fuel usage increases (volume/time) but at the same time thrust increases, so the specific impulse stays the same.

If you think of it in terms of an earth-bound object, say a car. Car A has 1 engine that does 6 miles to the gallon, and it carries 10 gallons of fuel. The one engine makes it go at 120 mph so the engines will burn fuel for 30 minutes, and in that time you will go 60 miles. Now you put another of the same engine in. It has 2 engines, so now it goes at 240 mph. Although fuel consumption has doubled, so it will only burn for 15 minutes, it goes twice as far in the same amount of time, so it will still travel 60 miles.

Of course, this doesn't factor in the extra weight of the engine and so on. In some cases it may be better to only have one engine because it reduces the excess weight that has to be hauled, it depends on exactly the type of operations you're expecting it to carry out. But two nerva will get you further than one aerospike I imagine.