Nerdy math question! TWR calcs

[cwalrus]

I wasn't planning on doing this but i got sucked in - I want to calculate some of the math myself.

BUT

Starting with TWR - if is is (Force in KN) divided by ( (Mass) Times (Local gravity)

The mass on the chart* is listed in Tons, is it not??? So I'm dividing kgs by pounds... ! Furthermore it turns out you should just put in the number listed in the chart- e.g. for mass for the terrier engine, you put in .5 and get the correct TWR listed. You don't put in 1000lbs, or 453 kgs....

?WT∆?

Secondly, when it lists the TWR in a vacuum, why are we putting in Kerbin's gravity? won't there technically be no gravity (or almost none) when it actually is in space? OR is this number for a vacuum sitting on the planet's surface?

*Chart: http://wiki.kerbalspaceprogram.com/wiki/Parts#Liquid_Fuel_Engines

Edited June 17, 2015 by cwalrus

[Red Iron Crown]

You are measuring mass in tons, but you're also measuring force in kilonewtons, so the x1000 factor cancels out.

People often use Kerbin as a reference for calculating it, but you can calculate it for any celestial body using its gravitational acceleration. Properly speaking people should say "Kerbin-relative TWR".

There is always gravity, even in space. It does reduce with distance from the celestial body, though, so some calculating mods like KER list both current TWR and surface TWR. Some prefer to use TMW, or Thrust-to-Mass Ratio, for figuring out orbital stuff as it doesn't change (it leaves out the gravity term in the calculation).

[DeMatt]

I wasn't planning on doing this but i got sucked in - I want to calculate some of the math myself.

BUT

Starting with TWR - if is is (Force in KN) divided by ( (Mass) Times (Local gravity)

The mass on the chart* is listed in Tons, is it not??? So I'm dividing kgs by pounds... ! Furthermore it turns out you should just put in the number listed in the chart- e.g. for mass for the terrier engine, you put in .5 and get the correct TWR listed. You don't put in 1000lbs, or 453 kgs....

?WT∆?

Those would be METRIC tons (properly spelled "tonnes"), which are exactly 1000 kg each.

Secondly, when it lists the TWR in a vacuum, why are we putting in Kerbin's gravity? won't there technically be no gravity (or almost none) when it actually is in space? OR is this number for a vacuum sitting on the planet's surface?

*Chart: http://wiki.kerbalspaceprogram.com/wiki/Parts#Liquid_Fuel_Engines

Because it is a thrust-to-WEIGHT ratio. Weight requires a gravity source, so Kerbin's gravity is chosen as the most relevant.

The similar thrust-to-MASS ratio would, instead of giving you "how many times stronger than gravity can this engine accelerate itself?", give you "how many metres-per-second-per-second can this engine accelerate itself by?".

[cwalrus]

awesome answers - thanks

aha! Metric tons!

[Snark]

The whole "which gravity do I use for TWR" depends on why you care, i.e. what you would do with the number.

"I want to take off from Kerbin and am designing the rocket"

-> use Kerbin-relative TWR.

"I want to take off from <somewhere else> and am designing the rocket"

-> use <somewhere else>-relative TWR. (Same deal if you're landing there via suicide burn.)

"I'm out in space somewhere and just need to burn at a maneuver node"

-> you generally don't care about TWR per se. What you actually care about is "how many seconds will the maneuver node take", and that's where you care about thrust-to-mass ratio. So gravity doesn't enter into it at all.

"I'm trying to read the acceleration display on the navball"

-> this is showing your Kerbin-relative TWR for the amount of thrust you're doing right now, if you're in a vacuum. (It's actually showing based on total acceleration, i.e. what your ship is actually doing rather than just what your engine is putting out, so if you're experiencing significant atmospheric drag then the reading won't be the same as TWR.)

[Drew Kerman]

when in doubt, Scott Manley

[Arkalius]

The whole "which gravity do I use for TWR" depends on why you care, i.e. what you would do with the number.

"I'm out in space somewhere and just need to burn at a maneuver node"

-> you generally don't care about TWR per se. What you actually care about is "how many seconds will the maneuver node take", and that's where you care about thrust-to-mass ratio. So gravity doesn't enter into it at all.

If you just want to compare acceleration potential of various ships, sticking with a Kerbin-based TWR (or whatever body you desire really) gives you an apples-to-apples comparison. I only say this because KER gives you TWR and not thrust-to-mass ratio.

[ajburges]

KER can switch between surface gravity for TWR, Kerbin is just the default.

Common practice is to use Kerbin relative TWR to judge force to mass ratio of space craft (or gasp engineer acceleration!). TWR is already a common value to derive.

BTW anyone know of a way to display RCS acceleration in the hanger? I want to make sure I have more than .05 m/s² for all translations while full for easier docking. It is easy enough to calculate (kN/kg = m/s²) but it would be nice to automate. It seems like something RCS build aid should have.

I wish KER could display current max engine acceleration, it would save me a throw away burn node to calculate burn time to zero relative velocity. It might even get people to use the correct terms to define interplanetary stage capabilities! A burn time to zero target velocity would be even better!

[Red Iron Crown]

I wish KER could display current max engine acceleration, it would save me a throw away burn node to calculate burn time to zero relative velocity. It might even get people to use the correct terms to define interplanetary stage capabilities! A burn time to zero target velocity would be even better!

KER displays current maximum acceleration in m/s² in the Vessel tab, is that what you mean? Burn time to zero velocity is given by stock in the info tab in map view.

[Lynch]

Take a look at my Thread for further instructions on TWR, Liquid Fuel requirements and what engines to use for that TWR

[h=2]Kerbal Rocketeering 101 - By Professor Lynch [1.0.2][/h]