Weight measurement?

[ItchyDani3l]

I'm trying to do some calculations for a rocket here, and I'm really struggling with this.

My craft is 2.8 "t", which is tons of course, but there are many different standard measurements for 1 "ton".

In the UK one ton was originally 2,240 pounds (1,016 kg), until they adopted the metric system.
The "metric ton" or "tonne" is 1000 kg.

AND in the United States, a ton is defined as 2000 pounds. Which is 907 kg.

These make big differences when dealing with rockets.
Can someone help?

Note: I couldn't find any information on this anywhere for the life of me on the KSP wiki.

[ItchyDani3l]

http://wiki.kerbalspaceprogram.com/wiki/Parts

I'll leave this here for anyone who has a similar problem to me.

The answer is Metric Tons. 1 "t" = 1000kg

I just found it, but it was hidden in my defense.

[Dark MAvL]

IIRC, the ton is the only weight mesure of stock KSP (some mods display small weight in kg though), so the unit only matters when you want to figure out what will happen with a given thrust. And thrust being given in Kilo Newton, you really want it to be metric tons.

More generally, scientists, engineer and science related stuff uses (or really should use) a standardized unit system, the System of International Unit (derived from the metric system). The international unit of mass is kg, hence the metric pound.

[ItchyDani3l]

Yeah I was doing calculations for Velocity that a given amount of fuel will get me.

Which is the thrust in kN * time in seconds, divided by mass in tonnes, assuming a vacuum.

[Langkard]

This is why Kerbal Engineer was invented, for which I am eternally grateful. I can do the math. I just hate it. Even with a calculator. I am taken back to my high school days and having to use circular slide rules for physics and math classes, because the new-fangled HP and TI hand-held calculators weren't allowed in class until my senior year when we could finally bring a TI SR-50a to class. Luckily, college wasn't so strict (except for not allowing calculators with memory functions in tests).

And to think the USA and the Soviets did orbital mechanics in the 60's with slide rules - masochists!

[Rene]

Originally posted by ItchyDani3l:

Yeah I was doing calculations for Velocity that a given amount of fuel will get me.

Which is the thrust in kN * time in seconds, divided by mass in tonnes, assuming a vacuum.

For very short burns that is more or less correct. For longer burns, your mass changes significantly, so you should use the Tsiolkovsky rocket equation[en.wikipedia.org] (or a mod that does it for you).

[ItchyDani3l]

Originally posted by Rene:

Originally posted by ItchyDani3l:

Yeah I was doing calculations for Velocity that a given amount of fuel will get me.

Which is the thrust in kN * time in seconds, divided by mass in tonnes, assuming a vacuum.

For very short burns that is more or less correct. For longer burns, your mass changes significantly, so you should use the Tsiolkovsky rocket equation[en.wikipedia.org] (or a mod that does it for you).

The reason I was using the equation was to determine how much fuel I need. So if I assume the full mass for the entire duration of the burn, it will overestimate how much fuel I need. That won't be a problem I think, because it will be sure to make it, and I can simply remove fuel as I see fit for later designs.

[Wolfe]

Originally posted by Langkard:

This is why Kerbal Engineer was invented, for which I am eternally grateful. I can do the math. I just hate it. Even with a calculator. I am taken back to my high school days and having to use circular slide rules for physics and math classes, because the new-fangled HP and TI hand-held calculators weren't allowed in class until my senior year when we could finally bring a TI SR-50a to class. Luckily, college wasn't so strict (except for not allowing calculators with memory functions in tests).

And to think the USA and the Soviets did orbital mechanics in the 60's with slide rules - masochists!

I have to agree with this, Kerbal engineer is the only mod I use and I think it's essential to avoid wasting time on complex equations.

There's a good reason why the most brilliant of the Nasa and Soviet mathmeticians and programmers stayed on the earth while fly boy test pilots actually flew the first space missions - it's a very tough call to do it all.

It also is utterly incredible that Tsiolkovsky somehow had the foresight to create the delta V equation, I can barely fathom how he managed to do it all that time ago and for his principles to be sound in practice, way before the technology itself was possible.

[J-Curwen]

Originally posted by ItchyDani3l:

The reason I was using the equation was to determine how much fuel I need. So if I assume the full mass for the entire duration of the burn, it will overestimate how much fuel I need. That won't be a problem I think, because it will be sure to make it, and I can simply remove fuel as I see fit for later designs.

It will overestimate a lot, like 50% and more most of the time. Eg commandpod+fl-t400+lv-909 would give you 2.1 km/s instead of 3.1 km/s.

The ideal rocket equation is not harder to do than the calculation you do.

Its just dV = V_exhaust * ln (M_full/M_empty)

with V_exhaust = I_sp * 9,81 m/s^2

If you want error margins just add 10 % on that. But this equation works both for rockets containing lot of fuel and those with a lot payload and little fuel. And you dont need to calculate the burntime. Because if you include that, the rocket equation should be faster as well, from the typing numbers into a calculator point.

[AlexMBrennan]

tbh I am somewhat surprised that the formula is using tonnes instead of kg...

[J-Curwen]

Originally posted by AlexMBrennan:

tbh I am somewhat surprised that the formula is using tonnes instead of kg...

Which formula? If you use the ideal rocket equation the units cancel out. If you do it like ItchyDani3l wanted to do it, its necessary because he uses kN for thrust. The factor 10^3 cancels out there as well kN/t = 1000 N/1000 kg. kg and Newton are SI units[en.wikipedia.org], Newton is kg m /s^2

[AlexMBrennan]

If you do it like ItchyDani3l wanted to do it, its necessary because he uses kN for thrus

That's the one I meant. Just use standard SI units (N, kg, s, m, etc) and aborb the rest into a constant.

[IronSides]

Rocket scientists are worse than supermodels.

So much worry about being heavy.

Just embrace it!!!!!

As the wise kerbaler once said....

MORE BOOSTERS!!!!!


On a side note wait until you have KSP and 2 different info mods telling you 3 different weights for your vessel.

[J-Curwen]

Originally posted by AlexMBrennan:

If you do it like ItchyDani3l wanted to do it, its necessary because he uses kN for thrus

That's the one I meant. Just use standard SI units (N, kg, s, m, etc) and aborb the rest into a constant.

I feel you, but it is commen to use t instead of Mg (and kt for Gg, Mt for Tg). I never saw someone using the term Mega-gram, even if it is the right name.
We are also using litre or bar, they arent SI units either, but can quickly be changed to SI units by a multiple of 10.
For daily things those units are helpful, but i agree. In science or engineering we should stick to SI. Im a chemist and ill always feel a little hate coming up, if the pressure is stated in bar. Its a great source for errors if you dont be careful.