Talk:Specific impulse

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Effective exhaust velocity

Farohki's "Aircraft Propulsion" https://archive.org/details/aircraftpropulsion2ed_201907 calculates values for effective exhaust velocities for rocket and air-breathing engines. However, his air-breathing example, similar to a CF-6 at cruise, has a value around 400 m/s which is nowhere near the 58,000 m/s shown in the Examples table in this article. Why is this? Thanks.Pieter1963 (talk) 16:44, 24 February 2021 (UTC)[reply]

For rocket engines, carrying the oxidizer, the result is related to the exhaust velocity. For air breathing engines, it is not. You should not try to compare the values between the two cases, just like you don't compare apples and oranges. Gah4 (talk) 08:26, 2 May 2022 (UTC)[reply]

"Specific impulse" of engines with no propellant

Nasa is working on an impulse engine based on the Mach effect (which has no Wikipedia page; the Mach effect page is about something else). This engine is driven by gravity, so it doesn't need to expel propellant. Yet it's still described as having a "specific impulse", which makes no sense if specific impulse is something that specifically applies to propellant, as the Wikipedia entry seems to imply. The article needs to explain specific impulse in a way that can be understood as it applies to engines like the one NASA is working on.

Seems to me that you have to see what WP:RS say, but I suspect you could do it based on energy instead of mass or weight. Gah4 (talk) 19:56, 22 December 2021 (UTC)[reply]

Definition of specific impulse in scientific terms

Impulse is, by definition, change in momentum. Anything specific in this type of context - such as specific heat capacity - refers to "amount per unit quantity" again by definition; in this case, it means mass (it can't possibly mean weight since - for example - propellant weights virtually nothing when floating around in free-fall).

Therefore a test of whether or not a source is reliable on the subject of specific impulse is to see if it uses that definition. This is applying editorial judgement, not doing original research: the point being that sources which are very obviously incorrect shouldn't be used.

The confused and confusing source at https://www.grc.nasa.gov/www/k-12/airplane/specimp.html fails this test - therefore, it is not a reliable source and should not be cited.

Think about it: that source states that the quantity of fuel for specific impulse calculations is specified in terms of the weight of propellant - calling it a force, which it is. Common sense will tell you that this varies with the acceleration of the rocket - does the amount of propellant being used by the rocket engine really increase hugely due to the acceleration of the rocket? Of course not, and the mathematical treatment on that page does not say that it does. The mathematical treatment actually uses what works out to be pounds-mass for the supposed "weight" of fuel. Editorial judgement, having spotted that obvious mismatch between the words and the maths, should treat this particular source as unreliable.

This source https://www.scienceabc.com/pure-sciences/specific-impulse-definition-formula-and-units.html contains a correct statement of specific impulse:

"it is the change in momentum per unit of mass for rocket fuels"

Very obviously, the old short description recently reverted to: "Change in velocity per amount of fuel" is completely wrong and must be replaced. Editorial judgement based on a quick reality check should make this obvious. Also, "fuel" tends to mean "not the oxidiser propellant" in the case of bipropellant engines, so it's clearer to replace the word "fuel" with "propellant". Again, editorial judgement rather than original research.

(Change in velocity of what, exactly? And how can "change in velocity" be a measure of rocket engine efficiency when the mass of a rocket is constantly changing? In any case, "velocity" is not "impulse" - they are entirely different parameters)

The fundamental error which leads to "seconds" being mistakenly used as the conventional unit for specific impulse is confusing pounds-mass (amount of fuel) with pounds-force (thrust of the engine): see here Pound_(mass) for more on the subject. This usage must be covered in this article because it's so common, but it's not the true definition of specific impulse in scientific terms.

The pound-mass has been defined in terms of the kilogram since 1959. Correspondingly, the Pound_(force) has long been defined in terms of the kilogram and a standardized value for acceleration due to gravity - or, to put it another way, in terms of the SI unit of force, the newton.

Both are necessary since the only current internationally recognised measurement standards are SI standards: the old US and imperial measurement references have long been abandoned.

Thus, specific impulse when calculated in US customary units works out as being defined, ultimately, as impulse measured in newton-seconds divided by mass measured in kilograms multiplied by a constant to convert the units from SI to US customary units.

The main point is that pounds-force and pounds-mass by definition cannot cancel each other out: they are dimensionally different, so common sense editorial judgement says any source which fails to address this important point must be treated as unreliable - although as I pointed out above, since the mistaken understanding which causes specific impulse to be cited in seconds is so widespread, this issue must be covered in this article. But it needs to be covered using reliable sources which seem to be hard to find.

I suggest that my changes stand until better sources to explain specific impulse are found. I've failed so far.

Please discuss the matter here and please do not revert the article to obviously incorrect information based on what common sense editorial judgement readily identifies as unreliable sources.

Michael F 1967 (talk) 19:46, 18 October 2021 (UTC)[reply]

- I have at least found a reliable source which specifies rocket motor specific impulse in SI units: "a specific impulse of 1780 N s kg -1 (sea level)" https://www2.le.ac.uk/departments/physics/research/xroa/astronomical-facilities-1/rocket-science-at-leicester/the-skylark-rocket

Michael F 1967 (talk) 20:26, 18 October 2021 (UTC)[reply]

"Anything specific in this type of context - such as specific heat capacity - refers to "amount per unit quantity" again by definition; in this case, it means mass (it can't possibly mean weight since - for example - propellant weights virtually nothing when floating around in free-fall)."

This bit is wrong and you've based your edits on it. That's a problem. GliderMaven (talk) 01:10, 19 October 2021 (UTC)[reply]

"The fundamental error which leads to "seconds" being mistakenly used as the conventional unit for specific impulse"

Not our problem, Wikipedia has to reflect the real world, and the real world does this. GliderMaven (talk) 01:15, 19 October 2021 (UTC)[reply]

Please stop using the edit button in an attempt to WP:RIGHTGREATWRONGS. GliderMaven (talk) 01:13, 19 October 2021 (UTC)[reply]

Yes, don't think of it in terms of weight. Think of it as a convenient unit system, and little g as the convenient unit normalizing factor. We could take the heights of people in the US and Europe, divide them by the appropriate little g, and end up with a value in square seconds. It would be convenient unit, independent of the local length unit. How convenient. Gah4 (talk) 08:38, 2 May 2022 (UTC)[reply]

General critique:

I really hate this page. 1. Using weight measures is archaic. Maybe some fields still do that, but this article ought to make sense to rational outsiders who know a little physics. The concept has nothing at all to do with g. The entire discussion should be in mass units, except with a section explaining how to convert to weight units. 2. I have tried to read it several times, and I still do not know an agreed perfectly general definition of specific impulse in mass units that is valid for jets, ordinary rockets, photon thrusters, and atomic rockets that heat up nonfuel to create exhaust. In fact, I'm left with the possibility that different fields might even use different fundamental definitions. If that is so, please label alternate definitions by fields. If not, please rewrite to make that clear. 3. Please rewrite to start out with a perfectly general definitions before mentioning any special cases. Burressd (talk) 06:13, 2 May 2022 (UTC)[reply]

Yes it is strange in weight units, but it seems to be the WP:COMMONNAME. It actually doesn't have anything to do with weight units or g, but is a convenient way to get a value independent of the system length unit. If you divide a velocity (in length units per second) by little g (in length units per second squared) you get a value in seconds, independent of length unit. In the case of rocket engines, that velocity has some physical meaning. In the case of jet engines, not much at all. (Only fuel, not oxidizer, goes into the calculation.) Since it is what people actually do, it is hard to complain about it. Gah4 (talk) 08:19, 2 May 2022 (UTC)[reply]

Move airbreathing engine tables

I think the 3 tabulations for jet engines would be more at home in a more aircraft-oriented article such as Airbreathing jet engine since the only numbers recognised by wikipedia, ie sourced, are aircraft specific, ie in the TSFC columns. Any comments? Thanks Pieter1963 (talk) 00:05, 5 June 2022 (UTC)[reply]

It looks like air breathing engine tables are much more than rocket tables. Seems to make more sense to move the non-air breathing tables. We could have two articles: Specific impulse of jet engines and Specific impulse of rocket engines. Seems to me, though, that one article is fine. It allows one to compare and contrast the two. Gah4 (talk) 02:13, 5 June 2022 (UTC)[reply]

Examples Tables and "SI" abbr.

In the head of the tables there is the "SI" column (s) als unit - this is extremely confusing, since the proper SI-unit is m/s which is stated als EEV in the next colum; both colums should get proper header cell (colspan 2) with I_sp, the left colum gets labeld with "weight (s)" and the right "mass (m/s)"

this should avoid confusion if sombody is searching for values and looking for the SI units and looking at the "SI" but not finding the proper SI unit there, since the specific impulse is the ratio between the impulse (Force * Time) and the ejected mass (N*s/kg which is effectively m/s)

i'll change that in a moment, since similar critique was added in Talk:Specific_impulse#General critique about the units Suit (talk) 17:49, 29 November 2022 (UTC)[reply]

Note that the article is Specific impulse, not effective exhaust velocity. It happens that, at least for rockets, they are related. Otherwise, it is a simple constant to get between them. As I said a few times, don't think of it as mass vs. weight, but just a unit conversion constant. Gah4 (talk) 21:11, 30 November 2022 (UTC)[reply]