The �v Equation and Earth standard gravity.

[Whirligig Girl]

The Delta-V equation is a known and universal truth about how a rocket vehicle works. ÃŽâ€v=ln[m1/m0]*(Isp*9.81) But my problem is with the Effective Exhaust Velocity part of the equation (Isp*9.81). 9.81 is used only because it is Earth's gravity at sea level(9.81 m/s^2). This means that for a universal equation, it's arbitrary.

What if an alien civilization creates rocketry, their planet won't have a gravity of 9.81 m/s^2. Say hypothetically, for the sake of argument, if a civilization starts up on mars. To their physicists, is 3.71 m/s^2 instead! How do they create the delta-v equation? ÃŽâ€v=ln[m0/m1]*(Isp*3.71) will yield a different answer!

In trying to find an answer, I found one. Specific Impulse is equal to the Effective Exhaust Velocity divided by . If you create with the value 3.17 m/s^2, then you find that is just a medium with which you can transfer between a known Isp value or Ve value. So what "Isp" is to a martian is different than to a terran. If this is as such, why even bother having any number at all? Why not something like "2"? This is what becomes my question, and I have a feeling it all has to do with what is convenient, not what is a universal truth. Would this become an issue in the future if humans diverge into the various planets of our solar system?

[zekes]

The Delta-V equation is a known and universal truth about how a rocket vehicle works. ÃŽâ€v=ln[m1/m0]*(Isp*9.81) But my problem is with the Effective Exhaust Velocity part of the equation (Isp*9.81). 9.81 is used only because it is Earth's gravity at sea level(9.81 m/s^2). This means that for a universal equation, it's arbitrary.

What if an alien civilization creates rocketry, their planet won't have a gravity of 9.81 m/s^2. Say hypothetically, for the sake of argument, if a civilization starts up on mars. To their physicists, http://upload.wikimedia.org/math/9/b/0/9b0d20d96b222c9a2e2215029fa6e93b.png is 3.71 m/s^2 instead! How do they create the delta-v equation? ÃŽâ€v=ln[m0/m1]*(Isp*3.71) will yield a different answer!

In trying to find an answer, I found one. Specific Impulse is equal to the Effective Exhaust Velocity divided by http://upload.wikimedia.org/math/9/b/0/9b0d20d96b222c9a2e2215029fa6e93b.png. If you create http://upload.wikimedia.org/math/9/b/0/9b0d20d96b222c9a2e2215029fa6e93b.png with the value 3.17 m/s^2, then you find that http://upload.wikimedia.org/math/9/b/0/9b0d20d96b222c9a2e2215029fa6e93b.png is just a medium with which you can transfer between a known Isp value or Ve value. So what "Isp" is to a martian is different than to a terran. If this is as such, why even bother having any number at all? Why not something like "2"? This is what becomes my question, and I have a feeling it all has to do with what is convenient, not what is a universal truth. Would this become an issue in the future if humans diverge into the various planets of our solar system?

it is indeed arbitrary, but that's because ISP is arbitrary to gravity as well. It cancels out - ISP is arbitrary because it has to be, i guess.

It's like the Gravitational constant - it's strange because our measurement system is strange.

[maltesh]

Every unit in common use by humanity is arbitrary. There'd be no reason for a native Martian civilization use a second, a kilogram, a meter, or a Newton when constructing his own system of measurement..

If you take an engine, and set it to produce 1 [Force Unit] of thrust, and provide it with the amount of fuel that weighs 1 [Force Unit] under 1 Standard Gravity (which isn't the same thing as gravity at sea level on Earth, but is close enough), the Specific Impulse of the engine is the amount of time that fuel will last.

And you wind up with the same amount of time if you use pounds for force and feet/s^2 for the acceleration of 1 standard gravity, or if you use kilonewtons and m/s^2, or if you use dynes and cm/s^2, as long as you do your unit conversions properly. Ultimately it comes down to everyone who currently needs to compare the specific impulse of the rockets they want to use uses the same units for time understands what a second is, and what 1 Standard Gravity is.

[Bill Phil]

Isp is arbitrary....

Heck, 1g is arbitrary. It could be 0.5g somewhere else, and that'd be their 1g.

[K^2]

It's because US Engineers used pounds as a unit of both force and mass. When you do that, you end up with a factor of g in your equations. In USSR, they used metric system to define I_SP from the start, so they had it in units of m/s and I_SP is simply equal to effective exhaust velocity. No arbitrary factors!

[GoSlash27]

It is arbitrary, but immaterial AFA the math is concerned. It's like fretting over computing speed and distance in m/sec vs. miles per hour or furlongs per fortnight. The answer will be correct in the convention you assumed.

Best,

-Slashy

[cantab]

I think we would be better off if we used effective exhaust velocity more. It's more apparent what it actually means, at least for rocket engines.