Correct algorithm for calculating the delta v that your rocket have.

[kUSer]

I am currently using this algorithm to calculate the total delta v my rocket has: dV = ln(Mstart / Mend) x ISp x 9.81.

I have a couple of questions though.

Do I need to multiply every time with 9.81 (Kerbin's gravity) or should I multiply with the gravity of the area that I'm in, e.g. on Minmus it would be: dV = ln(Mstart / Mend) x ISp x 0.481, but this would cause a problem if I am in space, as multiplying with 0 (for the gravity) always = 0

Is there a better algorithm for this or am I just confused?

Thanks.

[K^2]

This is correct algorithm. The only time you won't get correct result is if I_SP changes during flight.

Gravity in that equation is always 9.81. It's actually Earth's gravity, and comes from the fact that US Engineers were using pounds for both the weight and mass of the fuel. Because pounds of mass are always defined with respect to Earth's gravity, you always get that 9.81 factor in the I_SP formula, rather than anything relating to local gravity. It's purely a units conversion factor and nothing more.

Soviet engineers defined their specific impulse per mass of the fuel, so they don't have that 9.81 hanging around. Instead, if you read translations of any Soviet rocketry texts, you'll see I_SP defined in m/s instead of s, and it is equal to effective exhaust velocity. It's a much less confusing formula, but Squad chose to go with US convention.

[Rune]

I am currently using this algorithm to calculate the total delta v my rocket has: dV = ln(Mstart / Mend) x ISp x 9.81.

I have a couple of questions though.

Do I need to multiply every time with 9.81 (Kerbin's gravity) or should I multiply with the gravity of the area that I'm in, e.g. on Minmus it would be: dV = ln(Mstart / Mend) x ISp x 0.481, but this would cause a problem if I am in space, as multiplying with 0 (for the gravity) always = 0

Is there a better algorithm for this or am I just confused?

Thanks.

I fully understand you issue, let me explain where that 9.81 comes from. First, the correct way to write the rocket equation:

dV = ln(Mstart / Mend) x V_ex.

Where V_ex is the rocket exhaust's velocity. If you throw rocks, actual velocity, if not the average of the gases that make up the exhaust. The thing is I_sp, which is arbitrarily defined in seconds from the impulse equation... don't sweat it, just understand that it is a bogus unit kludged together for engineering purposes, and would be perfectly fine defined as I_sp=V_ex/9,81.

Which means that equation will work always, everywhere, and that has still nothing to do with Earth's surface gravity.

Rune. I_sp, confusing people since it was invented.

[kUSer]

Thanks guys.

[GoSlash27]

Actually, you should use 9.82 instead of 9.81 in KSP.

Otherwise, ^what they said.

Best,

-Slashy

[UmbralRaptor]

Actually, you should use 9.82 instead of 9.81 in KSP.

It got changed to 9.80665 (I think) in 1.0:

[Xannari Ferrows]

The formula is ÃŽâ€V = [isp • G] • Ln[M1/M0]

I'm not really adding anything new, but always remember that G is a constant. For the instance of TWR calculation, however, G is 9.81m/s multiplied by the result of the target body's gravitational acceleration divided by 9.81.

[Bill Phil]

The formula is ÃŽâ€V = [isp • G] • Ln[M1/M0]

I'm not really adding anything new, but always remember that G is a constant. For the instance of TWR calculation, however, G is 9.81m/s multiplied by the result of the target body's gravitational acceleration divided by 9.81.

I think you mean little g. Big G is the gravitational constant of te universe...

To OP:

Always use 9.81 for Dev in meters per second. If you want it in feet per second, use 32 and some change. Basically, the acceleration at Earth's sea level on the units you want.

IsP in seconds is more generic, allowing for thevequation to be used internationally by anybody who can.

[Xannari Ferrows]

I think you mean little g. Big G is the gravitational constant of the universe...

I keep getting that... My translator can't define them differently. I told it to capitalize any character by itself, unless the context of the sentence around it demands otherwise.

I really need to fix that...

[GoSlash27]

It got changed to 9.80665 (I think) in 1.0:

Oh, this is big news if true! I see a lot of recalibration of math models in my future...

[Agate]

Isp, confusing people since it was invented.

I'm a physics professor, and when I teach this stuff, I teach the exhaust velocity form Rune listed. I figure if I don't tell students about "specific impulse", maybe eventually it'll go away.

[Bill Phil]

I'm a physics professor, and when I teach this stuff, I teach the exhaust velocity form Rune listed. I figure if I don't tell students about "specific impulse", maybe eventually it'll go away.

But Isp is more general. It's only unit is seconds, and then conversion into a velocity for calculation can be done for any unit. Even cubits... if you wanted to use those for some reason.

It allows people who don't want to convert "ft/s" to "m/s" to just convert seconds into meters per second, which is much easier.

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Plus, the reason Isp is so confusing to people is that they aren't really given that much of an explanation. If people bothered to explain it...

[morat242]

But Isp is more general. It's only unit is seconds, and then conversion into a velocity for calculation can be done for any unit. Even cubits... if you wanted to use those for some reason.

It allows people who don't want to convert "ft/s" to "m/s" to just convert seconds into meters per second, which is much easier.

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Plus, the reason Isp is so confusing to people is that they aren't really given that much of an explanation. If people bothered to explain it...

To quote Ignition!, once again:

Probably the best way of thinking of specific impulse is as a velocity expressed, not in meters or feet per second, but in units of 9.8 meters (or 32.2 feet) per second. That way you retain the concept of mass flow, which is relevant everywhere, and doesn't depend upon the local properties of one particular planet, and at the same time lets European and American engineers understand each other. When he hears Isp = 250, the European multiplies by 9.8 to get the exhaust velocity in meters per second, while the American does the same with 32.2 and comes out with feet per second. (When will the U.S. ever change over to MKS?!)

Probably if the US had metricized, we'd be using exhaust velocity (thankfully, we abandoned the WW2 German convention of "specific propellant consumption", the reciprocal of specific impulse). But that hasn't happened, and now we're probably stuck with Isp for historical reasons even if we do go metric.

[Agate]

The fact that "seconds" is the only unit metric and American units have in common is not a good reason to use Isp. I mean, I could tell you that my own mass is 2 seconds, if by that I mean the period I would bob up and down at if tied to a certain bungee cord. But that would be ridiculous, and Isp is exactly that same ridiculous.

But we're stuck with it, so sigh.

[Jolbucley]

I think Isp is a little bit less ridiculous than your example, but I haven’t got any good reasons why, and it’s only a very little bit anyhow, so it’s probably not statistically significant at any reasonable α.

[Rune]

But Isp is more general. It's only unit is seconds, and then conversion into a velocity for calculation can be done for any unit. Even cubits... if you wanted to use those for some reason.

It allows people who don't want to convert "ft/s" to "m/s" to just convert seconds into meters per second, which is much easier.

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Plus, the reason Isp is so confusing to people is that they aren't really given that much of an explanation. If people bothered to explain it...

As an American using weird and confusing units in general, I get how you might think that way. But the truth is (IMO and with a generous amount of irony), you have a bigger problem using feet as a unit of measure. I use them to walk. XD

Rune. Specific propellant consumption, BTW, is really useful in aeronautical engineering to work out ranges and endurances and such, shows up in a lot of equations and makes sense. I_sp gets converted to V_ex every time.

[TheGamingNoobster]

The GRAVMAX reports a 9.81ms^-1 at sea level not a 9.82ms^-1! A change! (Now for sarcasm) Such a change! think of the fuel savings! It totally makes up for the ISP cuts of rockets which means reduced efficiency! (Now out of sarcasm) The small changes count. The ISP's were changed due to the new aero system. Take no offence to the sarcasm, it is there for effect. If it offends you please notify me.

[Reactordrone]

The GRAVMAX reports a 9.81ms^-1 at sea level not a 9.82ms^-1! A change!

The gravmax has always recorded 9.81m/s^-1 at sea level but the delta v calculation always used 9.82 for some reason.

[K^2]

I'm a physics professor, and when I teach this stuff, I teach the exhaust velocity form Rune listed. I figure if I don't tell students about "specific impulse", maybe eventually it'll go away.

It is still, technically, specific impulse. Amount of impulse per unit of mass of propelant, dp/dm = <v_e>. It's only when your m is in pounds and p is in pound-seconds that you end up with silly units and a conversion factor. In metric, effective exhaust velocity is the specific impulse. N s/kg = m/s.

But I suppose, simply talking about exhaust velocity and not even mentioning specific impulse might be less confusing at introductory level, so you probably have the right idea there.