Quick specific impulse question

[Streetwind]

Sometimes I get a bit confused with maths and physics, and when trying to decide whether a thing works one way or the other, I make up compelling arguments for both sides and subsequently become unable to figure it out by myself. The best thing in this situation is to hear someone else spell it out.

In this case, I was trying to compare engines running on different propellants. Liquid fuel vs. monopropellant for instance. And I was wondering: is an Isp value given for an engine running on fuel type A directly comparable to one given for an engine running on fuel type B? Or is its meaning specific to one kind of fuel only, and you would need to translate the other number into that frame of reference before comparing?

I suppose this boils down to the result of the following hypothetical scenario - an engine with 500 Isp fed by fuel tank A and an engine also with 500 Isp fed by fuel tank B. Tank A contains 1000 units of fuel A, which weighs 1 ton in total. Tank B contains 5000 units of fuel B, which also weighs 1 ton in total. Do the two engines run out of fuel at the same time, or will one run for longer than the other?

Edited February 5, 2014 by Streetwind

[KvickFlygarn87]

They will run out at the same time, Tsiolkovsky's does not include any units other than tonnes, m/s and m/s²

[Streetwind]

...I could just have looked at the equation, couldn't I.

See, that's exactly my issue everytime this happens And all it takes is for someone to play Captain Obvious.

(I must be getting old...)

[radonek]

…an engine with 500 Isp fed by fuel tank A and an engine also with 500 Isp fed by fuel tank B. Tank A contains 1000 units of fuel A, which weighs 1 ton in total. Tank B contains 5000 units of fuel B, which also weighs 1 ton in total.…

You are looking for complications where there are not. Think of it this way: You have those two ships with equivalent engines and one ton of fuel, only difference is that one of them has fuel gauge in pounds and other in kilograms.

[Red Iron Crown]

It should be mentioned that Isp is not the same as fuel consumption, though fuel consumption is calculated using Isp and thrust. For example, the LV-T30 and the LV-T45 have the same Isp characteristics but the LV-T30 consumes fuel faster as it is generating more thrust.

[Soda Popinski]

Can someone explain what their Internet Service Provider has to do with fuel consumption?

[numerobis]

The Isp of your ISP is largely determined by the temperature of the air they blow in their advertising.

[radonek]

ROFL. You made my day numerobis.