What is the ISP of an engine?

[numerobis]

Even rocket scientists screw up units sometimes. Intelligence is not a substitute for making complicated things easier on yourself.

[Specialist290]

Except that you've thrown in an arbitrary conversion factor of 9.8 m/s^2 in there. Not very universal, is it. The exhaust velocity makes much more sense if you're doing the actual thermodynamic nozzle calculations and talking about conservation of momentum.

Of course rocket scientists are skilled engineers and perfectly aware of the difference between mass and force. As I said a few pages ago, strictly from the definition, specific impulse is just impulse per unit [something]. The historical convention that we now have is that [something] is in units of propellant mass times g0. This is a pretty nonsensical choice, that came from dividing thrust in pounds by propellant flow rate in pounds per second, and calling the result specific impulse in seconds.

The factor of g0 is hidden when using imperial units, or any other gravitational unit system - we could use kiloponds and fold the conversion factor into the unit choice in the same way.

Let's set the record straight here: The reason "specific impulse" exists as a measurement of engine efficiency is because the United States military on the one hand did all of their measurements in Imperial units, while Wernher von Braun and his fellow German rocket scientists did all of theirs in metric. (Note also that the metric system was not nearly as universal in the 1950s as it is now; see also here.) Internally, both did measure their engines using raw exhaust velocity, like you're sugesting -- the Americans in feet per second, the Germans in meters per second. The "specific impulse" measure arose so that scientists from both sets could talk to one another without having to constantly resort to the tedium of converting units or confusing the other party.

Edited October 22, 2013 by Specialist290
Typo

[tavert]

Let's set the record straight here: The reason "specific impulse" exists as a measurement of engine efficiency is because the United States military on the one hand did all of their measurements in Imperial units, while Wernher von Braun and his fellow German rocket scientists did all of theirs in metric. (Note also that the metric system was not nearly as universal in the 1950s as it is now; see also here.) Internally, both did measure their engines using raw exhaust velocity, like you're sugesting -- the Americans in feet per second, the Germans in meters per second. The "specific impulse" measure arose so that scientists from both sets could talk to one another without having to constantly resort to the tedium of converting units or confusing the other party.

Actually the early German work used "specific propellant consumption," which was the inverse of specific impulse. Von Braun personally always used metric, but considering that his team moved to the US and worked with American engineers and fabricators, unit conversions were common.

I find it hard to believe that reluctance to convert between meters and feet was what led to the unfortunate convention that we're stuck with today... would be interesting to go looking for the original establishment of the convention though. I've read a number of old NASA publications that cite performance figures in lbf*s/lbm, which if we were being rigorous would be the most accurate - and can be converted to SI as kgf*s/kg. (Yes, kgf is a silly unit, but it gives you the same numerical value to satisfy a silly convention.)

Edited October 22, 2013 by tavert

[Specialist290]

I'll grant that's a fair point. I haven't actually had a chance to get "eyes-on" any of the old German stuff, myself, so my secondhand sources might be in error.

I'd also like to note that Imperial measurements do have a perfectly serviceable unit of mass that isn't a "pound-mass," so it's still possible to have consistent measures in an equivalent fashion to SI units. It's just that no one ever uses it, apparently. At this stage in the discussion, though, it's a bit of a moot point...

Edited October 22, 2013 by Specialist290

[Asimov]

n/m, answering question on first page that's been thoroughly covered already.

Edited October 22, 2013 by Asimov

[numerobis]

old NASA publications

kgf was still in use by NASA contractors in 2001. There was one number I saw that was quoted in something insane like kgf.m/sq in -- I've forgotten exactly the details, but it definitely had a mix of units. The young at the time (under 30, so under 40 now) were appalled at the imperial system; the old just couldn't get SI straight.

[Dunbaratu]

This is a pretty nonsensical choice, that came from dividing thrust in pounds by propellant flow rate in pounds per second, and calling the result specific impulse in seconds.

It is not nonsensical as it results in a value that is intuitive: "how long does fuel last".

[tavert]

It is not nonsensical as it results in a value that is intuitive: "how long does fuel last".

The nonsensical (or rather, arbitrary) part is that you're talking about a quantity used in rocketry and space travel, but basing its definition on the strength of gravity on the surface of earth.

Edited October 22, 2013 by tavert

[numerobis]

I have never figured out why it's intuitive to scale my mass by g0 when I'm in orbit. And yet, I've heard this argument so many times.

I have also heard that it's totally intuitive that there are 12 inches in a foot and 16 ounces in a pint (or 20, depending), and much easier than keeping track of the conversion factor between kilometers and meters or millilitres and litres.

Science advances one funeral at a time.

[Kosmo-not]

I have also heard that it's totally intuitive that there are 12 inches in a foot and 16 ounces in a pint (or 20, depending), and much easier than keeping track of the conversion factor between kilometers and meters or millilitres and litres.

Who did you hear that? I would die of laughter if anyone told me that to my face.

[numerobis]

I'll admit that I am paraphrasing a bit, so in context and with the precise words it didn't sound so obviously ridiculous, but it was at NASA, talking to a contractor (from Boeing probably?).

[Dunbaratu]

The nonsensical (or rather, arbitrary) part is that you're talking about a quantity used in rocketry and space travel, but basing its definition on the strength of gravity on the surface of earth.

False. It's based on force and mass and the fact that the two happen to cancel out in Imperial units when at sea level on earth is NOT hardcoded in the defintion of what ISP means. You can always express it in terms of a unit of time, regardless of which system you are using. The fact that it happens to end up becoming exactly one second in Imperial units is NOT hardcoded into the defintion. But the fact that it CAN be expressed as an amount of time of some sort IS.

You are using your dislike of the Imperial system to try to justify the claim that it is incorrect to tell people to think of ISP as a measure of time.

The fact that it can be *accurately* explained in terms of a length of time is not in any way dependant on the fact that NASA used Imperial units. The fact that the unit of time happens to become exactly 1s is. The fact that it's a unit of time of SOME sort, however, is not. You could just as easily express it in MKS in a way so as to make it come out to be exactly 1 second as well if you like. The exact units used is arbitrary. The definition of ISP is not. You could just as easily mix and match units willy-nilly and express it as how many fortnights it takes to burn a gallon of fuel if pushing at a force of one nanonewton. That wouldn't change the fact that it can be defined as how long it takes to burn (some amount) of fuel when burning at (some force).

Edited October 23, 2013 by Steven Mading

[tavert]

My dislike of imperial units is certainly coloring my argument that the convention of including the acceleration of gravity on the surface of Earth in the conversion between effective exhaust velocity (which has a perfectly consistent physical definition and interpretation, with no arbitrary magic-number conversion constants thrown in whatsoever) and specific impulse as a time is a bad convention.

We can leave imperial units out of it for a second, if we're ignoring where the convention came from in the first place. If, as you had said earlier, specific impulse were defined as the amount of time 1 kilogram of propellant would last if burned at a thrust of 1 Newton, then specific impulse as a time might make sense - at least the conversion factor in there would be 1 m/s^2, which from F = m a is the acceleration that 1 kilogram would undergo if 1 Newton were applied to it. But this is not the case. There's a factor of g0 arbitrarily inserted into the definition. That factor has no business being there if we're trying to talk about universal physical behavior.

Or going even further down the rabbit hole, even kilograms, meters, and seconds have anthropocentrically chosen values, so 1 m/s^2 would also be a non-universal choice. If you want to call specific impulse a time, you have to at some point choose a ratio of thrust to mass (so an acceleration) to define as your reference value. There's no way to make that choice in a purely physical way that isn't biased by the values we've chosen to use, from our history and home planet. Unless we all used Planck units instead of SI, but those have crazy values and we don't know Newton's constant with enough precision for Planck units to be scientifically useful... So yeah, effective exhaust velocity is the only purely-from-physics unbiased convention that could rightfully be used here.

Edited October 23, 2013 by tavert

[iueras]

Way to take a question from a new guy and turn it into an unnecessary 4 page debate on how much you love/hate imperial units vs metric.

[AR3S_TGL]

On ‎17‎/‎10‎/‎2013 at 7:47 AM, Lheim said:

Don't worry bout the math.

It's about a rocket engine's efficiency. The higher the engine's Isp, the more delta-v you get per unit of fuel.

THANK YOU! A simple answer!  

[Streetwind]

Just as a heads-up to anyone who might want to come in here and continue the discussion...

--->  This thread is four years old. <---

 

 

Edited April 13, 2017 by Streetwind