Talk:Main Page/archive/2013/02

Units of measurement for part masses and engine thrust

At the moment, the game doesn't specify units of measure for part masses or engine thrusts. We should agree on what to show in the wiki. There has been some discussion on this, but I figured it'd be useful to get some more feedback and hopefully a consensus.

A Wikipedia-style vote, with discussion at the end, seems appropriate. =) Please cast your vote by putting your signature (~~~~) in a list, and feel free to elaborate in comments at the ends of all the lists. =) — Elembis (talk) 04:32, 18 October 2012 (UTC)

sign with '~~~~' to vote

Killograms (kg), newtons (N)

1. Elembis (talk) 04:32, 18 October 2012 (UTC)
2. (your name here)

Metric tons (t), kilonewtons (kN)

1. Jlmr (talk) 04:50, 18 October 2012 (UTC)
2. UmbralRaptor (talk) 05:00, 18 October 2012 (UTC)
3. Kreuzung (talk) 13:08, 20 October 2012 (UTC)
4. Kahlzun (talk) 08:03, 23 October 2012 (UTC)
5. greenjinjo (talk) 22:11, 31 January 2013 (UTC)
6. (your name here)

Just like the game, no units

1. [[User:Thecoshman|Thecoshman] (talk) 07:58, 18 October 2012 (UTC)
2. [[User:Eli] 8:30 2/14/13
3. (your name here)

It's not really as much a matter of opinion, and as much as I enjoy the debate, it's a matter of simple physics! :) --Jlmr (talk) 04:52, 18 October 2012 (UTC)

I'm starting to think that Elembis's premise 3 was wrong, and rocket parts/kerbals are in different mass units from the star/planets/moons. Given that we can't push planets around, this seems workable.

Considering the game does not specify units, why are we even having this debate? If anything, there should just a page on units explaining that the game does not define units, but that they appear to be consistent with treating them in kg and N. And let's not forget, working in kg-N or Mg-N does not matter, it is just a different scale, though I would avoid using 'tonne' or 'ton' as it is a stupid term that has a bazillian interpretations. Never known of any cases where someone was wondering if you meant a UK Kg or a USA Kg. This also is some what relevant to my question above, are we aiming for UK English or US English. Thecoshman (talk) 07:58, 18 October 2012 (UTC)

There are multiple reasons to define the units. Several include: the wiki already defined them as kg and N (not physically possible), giving modders formulas to standardize the physical properties of new parts, converting realworld spacecraft into KSP, and above all, making the physics work on paper, the game already uses m/s L/s and meters as units of measure so it is not had to figure out that the units of measure for parts is in 1000's of kg and 1000's of N. I am not sure we should use kg and N, as it will involve a bit of work converting all the in-game part descriptions from tonnes to kg and from kN to N and probably confuse people who are looking at the in-game units and the wiki. Regardless, let's stick with SI as it is the unit already defined in-game for fuel consumption and acceleration. --Jlmr (talk) 19:54, 18 October 2012 (UTC)

Justification for t and kN (or 1000's kg and 1000's of N) This is a condensed version of all my arguments. --Jlmr (talk) 20:52, 18 October 2012 (UTC)

• An official NIST/SI publication: http://physics.nist.gov/Pubs/SP330/sp330.pdf states that kilonewton (kN) is a valid SI unit to represent 1000 N and ton (t), while historically a non-SI unit, has been accepted for use in the SI system by CIPM as a valid unit to represent 1000 kg for just over a century.

• We already know that the game uses SI units. These are specifically labeled: meters for units of distance, meters/sec for acceleration, liters for fuel volume, and liters/sec for fuel consumption.

• We know that the Mk1-2 Command Pod weighs 4 "units" from the in-game description. The command pod is roughly 2 m in height and 2 m in diameter. Calculating it's total volume gives us about 2.09 m3. Let's assume the capsule has 0.05 m (2 in) thick walls and subtract the interior void to get only the volume of the capsule walls. This gives us a volume of 1.80 m3.
  • Knowing the volume of the walls, we can calculate the density of the material making up the capsule, taking the 4 in-game units to mean kg, this comes out to be about 13.79 kg/m3. Unfortunately, is about 72.29 times less dense than water at room temperature and closer in density to most gasses! Perhaps Kerbals have discovered a super strong gas-like substance to make spaceships from? =)
  • However, if we take the 4 in-game units to mean metric tons, we get a density of around 13793.10 kg/m3, well within the range of the density of real building materials.

• The wikipage on Kerbin states it has a surface gravity of 9.81 m/s2, roughly that of Earth.
  • Given this we can calculate the following. If measured in kilograms(kg), a "9 kg" Rockomax X200-16 Fuel Tank on Kerbin, having a surface gravity of 9.81 m/s2, would be roughly the size of a 48 oz coffee tin and only hold about 1.5 L of fuel. Unfortunately for our intrepid Kerbals, 1.5 L of fuel would not provide enough energy to lift a 9 kg payload through 75,000 m of atmosphere, much less a few hundred meters.
  • If measured in metric tons, a "9 t" (9,000 kg) Rockomax X200-16 Fuel Tank under the same gravity, being about 0.67 m tall and 2 m in diameter, gives a volume of 2104.87 L, more than enough to hold the advertised 1600 L of fuel. Only a rocket engine with a thrust around 100 kN (100,000 N), could have any chance of lifting the 9 t (9,000 kg) fuel tank.
  • Similarly, a substance weighing only 8 kg with a volume of 1,600 L has a density of 5 kg/m3, only about three times greater than the density of gaseous oxygen, and would not make an efficient fuel. Whereas, a fuel weighing 8 t with a volume of 1,600 L has a density of 5000 kg/m3, closer to a realistic rocket fuel.

• On the wiki pages we could just as easily say 4,000 kg instead of 4 t. However, this takes up unnecessary space on the page.

• I know it's only a mod, but MechJeb already uses kN and tons for it's calculations of mass and thrust... =)