Is ISP consistent between KSP and Real Life?

[cantab]

22 hours ago, weissel said:

The proper, correct, right, sane, ... measurement for the specific impulse is the exit velocity.[1]  Meters per second.  What comes out.  Very intuitive, very simple. 

Agreed, I think (effective) exhaust speed would be preferable. For rocket engines it's pretty intuitive - exhaust speed, minus a correction for any fuel that doesn't go out the nozzle - and it gets rid of that standard gravity factor. But as mentioned, specific impulse became popular because it's seconds whatever unit system you work in.

[Phineas Freak]

Remember that ISP is measured in SI seconds because it is more difficult to convert between imperial and metric than to use one single global measurement unit. Mass may be measured in kilograms or pounds, speed may be measured in meters per second or in feet per second but then you have to convert the units.

[weissel]

On 2.6.2016 at 1:42 AM, PB666 said:

Its probably a good idea not to use ISP once outside of LEO, unless you are going to express accelerations in g-forces. . 

So you'll have no efficiency at all outside LEO?  Great Idea(tm).

 

On 2.6.2016 at 3:34 AM, Bill Phil said:

Germany has pounds, but a different type ( 1 german pound is half a kilogram).

German has the "Pfund" (500g as per 1858 customs union definition), but it's only used in things like shopping (butter, minced meat, ...). It's not an allowed unit in any official or commercial communication (i.e. you'll never find anyone advertising a Pfund of butter for x.xx€).

Formerly, the Pfund varied widely from 467.41g (Lippe-Detmold) to 560g (Bavaria) and 560.012g (Austria) and even 1000g (Netherlands).
Frankfurt am Main, (i.e. the Frankfurt in Hesse) even had a light Pfund of 467.711g and a heavy Pfund of 505.128g ...

 

On 2.6.2016 at 2:55 PM, PB666 said:

Measurements or calculations, doesn't matter. [...] Ah yes and have to convert concentrations into LD50 values, ussually for other folks.

I can calculate the CO₂ use of aliens from Alpha Centauri, measuring their CO₂ use is somewhat more difficult ...

Also, what is the LD50 for oxygen in the pressure and concentration as found in the sea level atmosphere of Earth (as of 1950)?

 

On 2.6.2016 at 6:24 PM, Phineas Freak said:

Remember that ISP is measured in SI seconds because it is more difficult to convert between imperial and metric than to use one single global measurement unit. Mass may be measured in kilograms or pounds, speed may be measured in meters per second or in feet per second but then you have to convert the units.

Yes, that makes total sense.  Wouldn't do to have the USA have 9000 feet/second misunderstood by the USSR's 3000 meter/second during rocket scientist discussion in the cold war.  Translating between Russian and English --- or the fact that Russian uses a different alphabet --- is not as big as a problem ...
... and of course German scientists (-> Operation Paperclip) would obviously be used to using feet instead of meters.

[PB666]

54 minutes ago, weissel said:

So you'll have no efficiency at all outside LEO?  Great Idea(tm).

 

German has the "Pfund" (500g as per 1858 customs union definition), but it's only used in things like shopping (butter, minced meat, ...). It's not an allowed unit in any official or commercial communication (i.e. you'll never find anyone advertising a Pfund of butter for x.xx€).

Formerly, the Pfund varied widely from 467.41g (Lippe-Detmold) to 560g (Bavaria) and 560.012g (Austria) and even 1000g (Netherlands).
Frankfurt am Main, (i.e. the Frankfurt in Hesse) even had a light Pfund of 467.711g and a heavy Pfund of 505.128g ...

 

I can calculate the CO₂ use of aliens from Alpha Centauri, measuring their CO₂ use is somewhat more difficult ...

Also, what is the LD50 for oxygen in the pressure and concentration as found in the sea level atmosphere of Earth (as of 1950)?

 

Yes, that makes total sense.  Wouldn't do to have the USA have 9000 feet/second misunderstood by the USSR's 3000 meter/second during rocket scientist discussion in the cold war.  Translating between Russian and English --- or the fact that Russian uses a different alphabet --- is not as big as a problem ...
... and of course German scientists (-> Operation Paperclip) would obviously be used to using feet instead of meters.

There is ISP sec and ISP m/sec. I was suggesting that use of ISP sec being silly beyond leo, since it qualifies exhaust velocity by surface gravity, which is kind of silly far from earths surface. One is adding and removing an extra unnecesary term.. For instance using ISP with ion drive calculations is silly. 

[weissel]

6 hours ago, PB666 said:

There is ISP sec and ISP m/sec. I was suggesting that use of ISP sec being silly beyond leo, since it qualifies exhaust velocity by surface gravity, which is kind of silly far from earths surface. One is adding and removing an extra unnecesary term.. For instance using ISP with ion drive calculations is silly. 

m/s is much more sensible (in my eyes),that's why we are stuck with seconds.

I_sp is no more or less sensible with ion drives than with chemical rockets, the point is that you can (and want to) compare the efficiency.
IHowever, with air breathing turbines (especially high bypass ones) the effective m/s (as used in I_sp) and the actual exhaust speed are an order of magnitude away from each other ... so here seconds may be the better unit there.  Or not.  (Same would be true for rockets if you used some of the fury of chemical recombination to accelerate inert stuff with the spent fuel --- but why would you do that, when you can spend the same mass on stuff that actually carries energy?)

[PB666]

9 hours ago, weissel said:

m/s is much more sensible (in my eyes),that's why we are stuck with seconds.

I_sp is no more or less sensible with ion drives than with chemical rockets, the point is that you can (and want to) compare the efficiency.
IHowever, with air breathing turbines (especially high bypass ones) the effective m/s (as used in I_sp) and the actual exhaust speed are an order of magnitude away from each other ... so here seconds may be the better unit there.  Or not.  (Same would be true for rockets if you used some of the fury of chemical recombination to accelerate inert stuff with the spent fuel --- but why would you do that, when you can spend the same mass on stuff that actually carries energy?)

The equation using Isp m/s is

thrust = 2 power eff / Isp ..... where  Isp = avg. ion velocity

the equation using Isp sec is 

thrust = 2 power eff / ( Isp * g)  ... Where Isp = avg. ion velocity / g

So yeah an unneccesary term is entered and then has to be removed for no obvious reans.

 

It important to remember that ion drives produce thrust in the millinewton to micronewton range, so that leaving thrust in N/g units makes them even more difficult to deal with. 

Finally ion drives and nuclear thermal engines are strictly final stage engines, so that they are never used for lift-off. 

IOW, there is no sensible reason otger than tradition to use ISP sec with these engines in space. 

 

 

[weissel]

On 7.6.2016 at 1:35 PM, PB666 said:

It important to remember that ion drives produce thrust in the millinewton to micronewton range, so that leaving thrust in N/g units makes them even more difficult to deal with. 

Finally ion drives and nuclear thermal engines are strictly final stage engines, so that they are never used for lift-off. 

IOW, there is no sensible reason otger than tradition to use ISP sec with these engines in space. 

We are talking about 0.025 - 0.25N (says Wikipedia), so it's 0.00255 - 0.0255 N/g₀. Not exactly unhandy.

Even if an engine technology is "strictly final stage" stuff does not absolve it from being compared with other technologies that are first stage capable.  It's not exactly unheard of to use chemical fuel engines for the final stage.  Which has been the usual method.

So yes, you will need to have a good way to compare them old chemical engines with these new-fangled, up to no good, oh so magical amber-powered thingies.  I_sp is one of the ways that help comparing them.