Noob Question on Calculating Specific Impulse

[inigma]

Just need a math whiz to show me how to calculate the specific impulse of the following two examples:

In a vacuum:

Example 1. two LV-T45 engines with Isp of 370 each

Example 2. one LV-T45 engine with Isp of 370 and one 909 engine with Isp of 390

I just want to understand the math and where to get and plug in the numbers. I probably will update the wiki with the detailed answer.

[CaptainKipard]

This is a good question. All i know is that in example 1 the Isp will still be 370 if both engines have 370. Example two is trickier. It's not exactly an average of the two but it's close IIRC.

[regex]

http://forum.kerbalspaceprogram.com/showthread.php/34315-How-Do-I-calculate-Delta-V-on-more-than-one-engine

[inigma]

Thanks reg. So essentially adding engines does not increase delta V. Adding fuel obviously does. So for a noob the two factors in spaceflight rocket science are:

1. delta V corresponds to how far your vehicle can travel which roughly translates into how much fuel you've got.

2. TWR corresponds to how much you can lift at once and corresponds to how many engines you have.

So to fixup a rocket:

More Distance = More Delta V = More Fuel/Efficiency

More Lifting Power = Thrust:Weight Ratio = More/Stronger Engines

[regex]

1. delta V corresponds to how far your vehicle can travel which roughly translates into how much fuel you've got.

Well, it's not that simple. Delta-V, as I would explain it, is a measure of how much you can change your vector. Also, adding more fuel isn't the only solution to more delta-V since you're adding mass that your engines have to shift (change vector). Adding another engine along with fuel can often result in more delta-V since you're able to change your vector faster with stronger engines (and more fuel).

2. TWR corresponds to how much you can lift at once and corresponds to how many engines you have.

Not exactly, since higher thrust from a single engine (Mainsail vs. Skipper vs. Poodle, for instance) can be better than multiple engines (which mass more and thus mean more force needed to change your vector).

When designing a rocket TWR, delta-V, and mass are all important concerns. You need the thrust to shift that mass and the fuel to make use of that thrust, but adding more fuel means more mass, which needs more thrust, etc...

After that, it's all struts.

(side note, I searched out the link because I was interested in the topic, not to be snarky, happy you didn't take it that way).

[inigma]

Thanks for the link, I couldn't find it when I searched for a clear math example.

[purpletarget]

1. delta V corresponds to how far your vehicle can travel which roughly translates into how much fuel you've got.

2. TWR corresponds to how much you can lift at once and corresponds to how many engines you have.

So to fixup a rocket:

More Distance = More Delta V = More Fuel/Efficiency

More Lifting Power = Thrust:Weight Ratio = More/Stronger Engines

It's not entirely unlike a car, but one level up...

Delta-V is like the Range of a car, how far it can go on a tank of gas. But instead of a car measuring KM's, you're measuring a change in speed or velocity, hence the V.

ISP is like the MPG, or L/100km in a car. It's a measure of fuel efficiency of the motor. The higher the ISP, the more Delta-V that motor can squeeze out of a L of fuel.

And where you might look at how fast a car can go in km/h to get from A to B in x time, the Thrust of a rocket motor plays into how fast it can accelerate to the Velocity you need to get somewhere.

You can also check here for math, and explanations, including how to solve ISP for a mix of engines. (I recommend against the Thrust Weighted formula from the Wiki tutorial.)

[tavert]

http://forum.kerbalspaceprogram.com/showthread.php/34315-How-Do-I-calculate-Delta-V-on-more-than-one-engine

Glad you found that link, saved me the trouble of looking up where we had that discussion. Looks like the equation on the wiki (http://wiki.kerbalspaceprogram.com/wiki/Tutorial:Advanced_Rocket_Design) hasn't been corrected yet, can someone who's good at wiki-markup have a shot at that?