Why specify the impulse?

[Human Person]

Simple question:

As we all know: 

exhaust velocity = specific impulse * g

 

What‘s the point of the specific impulse as a Unit? Why would anyone prefer knowing the specific impulse of an engine over the exhaust velocity? In other words: why do we need to specify this Unit to Earths Force of gravity?

Has anyone ever used something different than Earh’s or Kerbin‘s force of gravity to specify the impulse?

 

 

Edited January 18, 2018 by Physics Student

[IncongruousGoat]

We use specific impulse as a measurement because seconds are the only unit of measurement that's common to both Imperial and Metric. It's a workaround that was designed to allow U.S. rocket scientists to more easily communicate with German rocket scientists back in the late 40's and early 50's. g is only used as a coefficient because it's a constant with a known value in both systems. There's no strict physics or engineering reason to measure engine efficiency in seconds, or to use g specifically in the calculations, and at this point it's done for reasons of convention more than anything else.

Interestingly, for a while a similar convention was used in Germany where engine efficiency was expressed using specific propellant consumption, in units of s^-1.

[PB666]

20 minutes ago, Physics Student said:

Simple question:

As we all know: 

exhaust velocity = specific impulse * g

 

What‘s the point of the specific impulse as a Unit? Why would anyone prefer knowing the specific impulse of an engine over the exhaust velocity? In other words: why do we need to specify this Unit to Earths Force of gravity?

Has anyone ever used something different than Earh’s or Kerbin‘s force of gravity to specify the impulse?

 

 

Because when you launch you have to lift against the weight of the vehicle, and thus you can multiple by the weight fuel.

Specific impulse is useful when thrusting from non-inertial reference frames, it useless when thrusting from inertial reference frames (like space) Ve is a better choice. Thats why for ION drive systems a more frequent specification is Ve . . it just removes one term from the denominator.

Even one level higher is that F = power/C when you talk about photon drives or cannae drives.  ION drives, it is often convinient to consider Ev as a fraction of the speed of light, because this tells you basically what your relative efficiency is. For example if you are engaging in interstellar travel you want Ve ----> C (but not reaching C because that is very power inefficient,  and the only way to provide that kind of energy is a very efficient matter-antimatter electric generator).

 

Lets say rocket weight 1,000,000 pounds and the ISP is 200 how many pounds of fuel do you need to expel per unit tme to achieve an acceleration of 0.2 g at lift off that grows until half the weight is expelled.

 1,000,000 * 0.2 = (Thrust - Weight) such that Thrust is 1,200,000

1200000 / 200 = 6000 lbs of fuel per second.
Calculations are pretty simple,

 

Edited January 18, 2018 by PB666

[Pand5461]

After some googling:

Looks like it was introduced by the Germans during V-1 and V-2 development.

The original definition was probably "the ratio of thrust to fuel flow" and was applied to V-1's jet engine. So, at the time it wasn't even remotely close to the actual exhaust velocity.

The effective exhaust velocity might be a better option when they started developing rocket engine but the tradition was set, and the choice of seconds came in handy when Von Braun started working for the US.

And... It's rather convenient for aircraft because "we need X kgf of thrust for Y seconds, so have to load X×Y / Isp kg of fuel" makes total sense.

[mikegarrison]

1 hour ago, Pand5461 said:

And... It's rather convenient for aircraft because "we need X kgf of thrust for Y seconds, so have to load X×Y / Isp kg of fuel" makes total sense.

Nobody I know of uses Isp for airplanes.

We use SFC (aka tSFC, because it thrust-specific fuel consumption) to describe the engine efficiency, L/D to describe the aero efficiency, and typically "NAMS" to describe the overall efficiency. NAMS = "nautical air miles per unit of fuel". I'm pretty sure both Boeing and Airbus use NAMS, but Boeing uses nautical miles per pound of fuel and Airbus uses nautical miles per kilogram of fuel.

The new ICAO rule for CO2 efficiency uses a version of NAMS called SAR (specific air range) which is defined as km of flight per kg of fuel. (The rule actually uses 1/SAR, so that's kg of fuel per km of flight.)

I don't know what electric/solar airplane folks use.

-------------

In cruising flight, L=W, T=D.

So D=W/(L/D)=T

SFC= (fuel/sec)/thrust

TAS = nautical miles / hour

So with some constants converted like 3600 second per hour, you can relate NAMS to L/D and SFC based on the weight and true airspeed of the airplane.

NAMS = nmi/fuel => TAS/(fuel/sec) => TAS/(SFC*T) => TAS/(SFC*(W/(L/D))) => (TAS * (L/D)) / (SFC * W)

[I think I did that right. Feel free to check it.]

All that means, to increase your specific range, you either want higher speed, better L/D, lower SFC, or lower weight.

Edited January 18, 2018 by mikegarrison

[Human Person]

@IncongruousGoat @PB666 @Pand5461 @mikegarrison That‘s all very interesting, thank you!

[Bill Phil]

Specific impulse is impulse per unit mass. In some definitions, exhaust velocity is specific impulse. But, you can specify impulse per unit weight, which gives you a unit of seconds. Thus, we have to use the value for g because we used weight, not mass.

Edited January 18, 2018 by Bill Phil

[PB666]

32 minutes ago, Bill Phil said:

Specific impulse is impulse per unit mass. In some definitions, exhaust velocity is specific impulse. But, you can specify impulse per unit weight, which gives you a unit of seconds. Thus, we have to use the value for g because we used weight, not mass.

Yeah, that's right, there are two, one is ISP(sec) and the other is ISP(v). But if you don't specify its V_e/g  (m/s)/(m/s²) = ISP in seconds.

[GoSlash27]

Physics Student,

 The specific impulse is actually a pretty handy figure. If you take a mass of fuel and set the thrust so that it exerts the same force as that mass at sea level, your engine will take Isp many seconds to exhaust the fuel.

Aside from being easily convertible to exhaust velocity, it is also easily convertible to fuel flow rate. This allows you to calculate time to fuel exhaustion for a stage, the fuel mass consumed during a fixed time burn, or figuring the Isp of multiple engine types running in parallel.

In the same sense that DV takes 2 forms (the DV required to execute a maneuver or the DV a stage will produce), Isp also takes 2 forms (exhaust velocity or fuel flow rate). It makes it easy to chuck numbers over the wall from one realm to another.

Best,
-Slashy

[YNM]

I thought it has something to do with imperial measurements (where 1 lb.f = 1 lb * "g"), but apparently that's not the only reason.

[Nuke]

its really just a quirk of units conversion.

Edited January 19, 2018 by Nuke

[kerbiloid]

In the Russian literature the specific impulse is specified in m/s, as a synonym to the theoretical exhaust velocity.

I don't know, why does someone still specify it in seconds.

[mikegarrison]

1 hour ago, kerbiloid said:

In the Russian literature the specific impulse is specified in m/s, as a synonym to the theoretical exhaust velocity.

I don't know, why does someone still specify it in seconds.

Because no one outside Russia actually reads Russian literature. The novels are too long and nobody except Russians can pronounce the names.

[kerbiloid]

45 minutes ago, mikegarrison said:

Because no one outside Russia actually reads Russian literature. The novels are too long and nobody except Russians can pronounce the names.

And that's why you have to use s instead of m/s.

[Human Person]

1 hour ago, mikegarrison said:

Because no one outside Russia actually reads Russian literature. The novels are too long and nobody except Russians can pronounce the names.

Let me tell you, „everybody outside russia“ is missing out on that.

[GoSlash27]

1 hour ago, mikegarrison said:

Because no one outside Russia actually reads Russian literature. The novels are too long and nobody except Russians can pronounce the names.

And also excessively tragic and depressing

Best,
-Slashy

[PB666]

5 hours ago, kerbiloid said:

In the Russian literature the specific impulse is specified in m/s, as a synonym to the theoretical exhaust velocity.

I don't know, why does someone still specify it in seconds.

I only use ISP sec _here_ in discussions. If you read the literature on some of the drives they do specify Ve. But again I mainly focus on ION drives.

Don't forget we had a space craft crash into Mars because they forgot to convert between the two systems. If we all agreed to stop using ISP (sec) here and KSP switched also to Ve then we could get away from that ..

If wishes were horses beggars would ride.

4 hours ago, mikegarrison said:

Because no one outside Russia actually reads Russian literature. The novels are too long and nobody except Russians can pronounce the names.

I don't know about that, seems to me that Russians have alot of good material to write about. I seen quite a few Russian movies in years gone by, they tended to handle some pretty heavy subjects.

[kerbiloid]

Spoiler

I'm confused a little...

Mentioning the Russian books with ISP values, I even couldn't suppose that physical references could contain drama, character names, long novels, tragedy, depression...
I'm really proud to have an honour of talking with people who can see all this behind the dry formulas and tables full of digits...

Also, I can remember maybe 3 novels mentioning ISP, and no movie about this. This is really sad...

Spoiler

6 hours ago, mikegarrison said:

The novels are too long and nobody except Russians can pronounce the names.

You just didn't look Polish ones.

 

Edited January 19, 2018 by kerbiloid

[PB666]

Here is why I don't like to use ISP [(sec)}. This is going to be a brain teaser.

Lets just suppose that we were fed up with the current administration and we elected Prince Harry as president of the US (of course only after he married an American and the law forbidding president to be someone of foreign birth). In an effort to solidify Americans into a single system of governance he made it illegal to use an metric unit in any government operation. So now we are completely adherent on using the Imperial System of measurements as adopted by Americans 120 years ago.

So we manage to build a Rocket and the Rocket has four stages and we have weight and thrust (in pounds, a force), and so during ascent we go through the four stages and we are delightfully in orbit.

But we are weightless.

Remember we are forbidden from using the kg m systems.

But we need to find- for a variety of reasons negotiating the earths fluctuating gravitational field, and numerous Oberth effects we intend to perform- the mass of our ship.

The ship does have a mass, it has a value in the English system of measurements, what is it?

Even 28 years after the units description we see this in the literature
 

Quote

No name has yet been given to the unit of mass and, in fact, as we have developed the theory of dynamics no name is necessary. Whenever the mass, m, appears in our formulae, we substitute the ratio of the convenient force-acceleration pair (w/g), and measure the mass in lbs. per ft./sec.² or in grams per cm./sec.². -wikipedia

Note: that if you find the Wikipedia page where this quote comes from you will find also find the name of the unit. I'm not going to tell anyone, I want everyone to specify in their mind what the unit is and search for and verify that unit.

The problem is that even if a 'King' decreed that we use the FPS system, no-one, unlikely 120 years of mass unit definition is going to use the FPS mass unit or be able to communicate with others using that system.
We know that mass is important because it specifies inertia. 

Quote

Physicists and mathematicians appear to be less inclined to use the popular concept of inertia as "a tendency to maintain momentum" and instead favor the mathematically useful definition of inertia as the measure of a body's resistance to changes in velocity or simply a body's inertial mass. - https://en.wikipedia.org/wiki/Inertia#Interpretations

and inertia is an important quality of momentum.

And these are two qualities you desperately need to know when you are in an . . . inertial reference frame (kah ching). So why use a unit that is specified for use in FPS, that does not have a 'coherant' mass unit, such that the unit always needs to be multiplied by µ_Earth / r²_{Earth,surface} when in fact your space ship is now orbiting the sun and about to make an oberth effect on Jupiter, or suppose you want to land a probe on Mars.  So where did NASA's spacecraft crash as it did not make the conversion from Imperial to metric units. . . . . . .Mars.

The moment that the stage 2 burn ends (i.e. you are now circularized), on the explicit condition that you are using that orbit to propel  the ship into space, the most important concern is the mechanical energy Hamiltonian. How much Energy do you need to put into your craft to get you to an exterior orbit from that you specify the dV you need dV =  V_e ln (m_o/m_f)     V_e = ISP (m/s) [= ISP * µ_Earth / r²Earth],surface  and M_x =  Pounds_x / 32 f/s². Fortunately in this equation the 32 f/s² in the numerator and denominator cancel, but this is not the case in all equations. Don't bother converting to mass, your final equation will be incoherent (except to an expert in trivia).

OTOH, when you are launching you are playing out three different dynamic force vectors. There is the force of gravity, the force of drag, the force of thrust adding the dynamic acceleration in a rotational reference system called 'centrifugal acceleration' (a faux force). Because of the complexity of these the ideal dV required (Sqrt(2(u/2r₁ - (V_rot * cos Z)²/2 + (-u/r_o-u/r₁))) is only part of the equation. Since the ideal dV require can only be achieve by infinite thrust and no drag. 

The infinite thrust issue is also a problem with burns to escape from orbit, the reason is that δΘ/dT in an LEO = 0.00119 (0.068 degrees) per second, so that if an appreciable amount of the burn can occur in say 5' span it can occur over 73 seconds without much energy loss, and we can use kicks if it is longer. For example if your goal is mars,  you need say 3700 dV to get there from LEO, then an acceleration of 50 m/s² (5g) is required some loss of energy results by burning at 25 m/s² but the dV required is a magnitude easier to predict from ideal equations relative to launch from Earth. Since in any SME moment the optimal energy efficiency occurs when KE is max, any gain of radius during a burn increases PE at the expense of KE and if under thrust SME. So ideally we want the burn to occur at the lowest radius.

The point is that in a launch from Earth the Intergral of thrust  as a function of time is a very complex function in which mass is not a primary issue, but once you are in orbit, taking advantage of all the facilities space has to offer requires the definition of mass.

 

 

 

 

Edited January 19, 2018 by PB666

[Pand5461]

10 hours ago, kerbiloid said:

In the Russian literature the specific impulse is specified in m/s, as a synonym to the theoretical exhaust velocity.

I don't know, why does someone still specify it in seconds.

Not true.

Spoiler

Удельный импульс — основная характеристика ЖРД — у НК-15 на земле составлял 294 с, а на больших высотах — 331 с. Двигатели первой ступени «Сатурна-5» имели удельный импульс у земли 266 с и в космосе 304 с

http://militera.lib.ru/explo/chertok_be/25.html

The best explanation I managed to find so far (from Levantovsky):

Specific impulse is, by definition, the impulse produced by burning unit weight of fuel in an engine. Impulse is measured in Force × Time units, so the units for specific impulse should be Force × Time / Weight. Since weight is a kind of force, the unit of specific impulse is the same as the unit of time. With the introduction of SI in 1960, force is measured in Newtons, and someone clever also noted that it's more natural to specify impulse per unit mass of fuel instead of weight, so the specific impulse becomes the same as the effective exhaust velocity (for rockets, not jets) and should be measured in m/s.

[Human Person]

2 hours ago, PB666 said:

illegal to use an metric unit in any government operation.

Isn't the second a metric unit too, or does it exist in both systems with the exact same definition?

[kerbiloid]

20 minutes ago, Pand5461 said:

Not true.

Sure?

The International System of Units (SI) has been formally adopted in USSR in 1963 as the recommended standard.

https://ru.wikipedia.org/wiki/Удельный_импульс

Quote

Размерность удельного импульса есть размерность скорости, в системе единиц СИ это метр в секунду.

Quote

Quantity dimension of the specific impulse is quantity dimension of velocity, in SI it's meter per second.

(I.e. both are LT^-1.)

Chertok had graduated from the institute in 1930s, and the epoch described in his book is 1960s.
He has his own old habits. He can use tonne or tonne-force as a thrust unit, and grams as a unit of mass, or centimeters as a unit of length, because there were several obsolete unit systems, as you know.
ISP in seconds and thrust in tonnes are signs of "old school" and are not correct.

[PB666]

1 hour ago, Physics Student said:

Isn't the second a metric unit too, or does it exist in both systems with the exact same definition?

It is a shared unit, but in true metric use, you would avoid using hours, days, years (a year changes almost instanteously).

Here are two seconds:
SI = " The SI definition of second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom "
WikipediaBBC World

A system is defined by its last definition, and so a second is defined by its last definition. The Imperial system lacks certain definitions for instance (there is no defined mass in the Imperial system but there is a defined mass in the Customery US system of weights and measurement. The Yard and Pound where last verified in 1958 but that did not include the second. Prior to that the second was defined by the https://en.wikipedia.org/wiki/Mendenhall_Order referring to a set of agreed units in 1983. So that in FPS officially its the unit of a second in 1893, but since the agreement makes all units undefined by later acts equal to a metric unit times a scalar, you can unofficially take the time to be an SI second. 

Quote

In 1930 the British Standards Institution adopted an inch of exactly 25.4 mm based on the 1927 light-wave definition of the meter. The American Standards Association followed suit in 1933. By 1935 industry in 16 countries had adopted the "industrial inch" as it came to be known.^[8][9] In 1946, the British Commonwealth Scientific Conference recommended that members of the British Commonwealth adopt the inch as exactly 25.4mm, and the 36-inch yard as exactly 0.9144 meters.^[10] The British Commonwealth Scientific Conference's recommendations were accepted by the Canadian Standards Association in 1951.^[11

Since we know how so in essence it is the if C =  δd/δt of light,  then δt = δd/C  then its in essence the speed of light in 1927 in inches x 2.54 / 1000 divided by the speed of light (δ) as it was known in 1927 divide by the current definition of the speed of light. This is the nitpicky answer. The exact value of C is299,792,458 SI meters/SI second ( the meter is defined officially as the length that light travels in 1/299,792,458 second), In 1927 the speed of light was 299,710,000 so that sec = 0.999725 second if one takes the 1927 measure. 

So here is how you should think of the FPS in reality, these are all based secondarily and tertiary on the SI system.
Second = 9 192 631 770 periods of radiation . . . . . . . of cesium 133 atom
Meter = C * Sec/299,792,458
Inch = .0254 Meters.
Liter = The volume of a cubic meter divided by 1000.
Kilogram = (mass of 1 liter of water @ 3.984'C)/ 0.999975 wear the atomic composition of the water reflects the isotopic of H and O averaged between all the worlds oceans. Or the mass of a Kg stored in France.
From the Kg we get pount
Pound =  0.45359237 kg  x 9.80665 g = 4.4482216152605 N.
Foot = 0.3048 meters
 the
American slug (there is no imperial slug, thus the slug is really not a coherant measure of mass) = 14.5939 kg =   0.45359237 * 32.17405 ft/sec²

IOW all units, weather Metric or FPS are current if they are derived from SI system. So the answer is yes, but that also to realize that a second in the FPS second that goes undefined may not be exactly as second.

If you are looking at the table above you are probably wondering why anyone would use the FPS system, since all FPS units are secondary to the Metric system, some units (i.e. the Slug) are not defined internationally and would not be coherent if they were, and the system does not have clear definitions. . . . .and you would be correct to wonder that.

 

Edited January 19, 2018 by PB666

[mikegarrison]

You are right about the Polish literature, I've never tried it.

6 hours ago, PB666 said:

The ship does have a mass, it has a value in the English system of measurements, what is it?

The problem is not a lack of units of mass. The problem is there are too many units of mass. One choice is the lb-mass, which is simply the mass that weighs 1 lb-force under 1 standard g. The other is the slug, which is the mass that is accelerated 1 ft/sec^2 when under a force of 1 lb-force. 1 slug is about 32x as large as 1 lb-mass.

[PB666]

21 minutes ago, mikegarrison said:

You are right about the Polish literature, I've never tried it.

The problem is not a lack of units of mass. The problem is there are too many units of mass. One choice is the lb-mass, which is simply the mass that weighs 1 lb-force under 1 standard g. The other is the slug, which is the mass that is accelerated 1 ft/sec^2 when under a force of 1 lb-force. 1 slug is about 32x as large as 1 lb-mass.

That's true for just about every FPS unit, no matter which unit you choose I can find an implimentation in which that unit is different.  For example what is a gallon. So unless the person specifies the convention the unit they name is ambiguous.

EDIT: BTW the pound(mass) has the same name as the pound(force) which means if you use that unit, then every time you use pounds of force you have to specific that it is a force, and every time you use a pound of mass you have to specific that it is a mass. So lets say you go to the moon and tell earth you found a special rock of 100 lbs on the moon (which unbeknownst to them you snuck along a fisherman's scale). Well if you are talking about mass, that's fine you have 45.4 kg of rock, but if you are talking about force you have 270 kg of rock, and of course if they don't know that then good luck getting your rock and yourself back to Earth.

The fps systems loses all utility past any stable orbit, which means it basically looses utility above the edge of the atmosphere.

 

Edited January 19, 2018 by PB666