Help: TWR formula in Visual basic?

[Brainlord Mesomorph]

I hate to show my ignorance esp. in things a lot of other people understand.

As a veteran VB coder I can grok some very heavy math as long as its typed in text. But when it comes to something as simple as this:

not so much.

I've got thrust in kilo-newtons (right?) KSP gives me mass in decimal tons (?) and g I assume is Kerbin (Earth) gravity (in m/s squared?) -- THAT's where physics always came apart for me. I get the concepts, but the unit-conversion in the math just drives me batty!

I'm trying to write a function to calc the TWR of an entire ship, Anyone want to show me what that should look like in VB?

Many TIA

Bra

Edited February 9, 2016 by Brainlord Mesomorph

[RizzoTheRat]

A Newton is a kg m/s²,hence how you get a ratio as the bottom of the equation is kg * m/s² 

So if:

• Fr = thrust in kN
• m = mass in tonnes

then

TWR = (1000*Fr) /(1000*m *9.81) 

The 1000's are obviously the conversions from kg to tonnes, and N to kN, and they cancel out so your original equation works fine

TWR = Fr/(m*9.81)

[Brainlord Mesomorph]

Oh, right metric tons. (I'm thinking 2000 lbs)  - and does that mean a ton is actually a megagram? (I've often wondered why ppl say thousands of kilometers, instead of single megameters)

well, that's too simple.

THANKS

[RizzoTheRat]

That would be a good source of confusion   KSP works in metric tonnes not imperial tons, so yes, a tonne is a megagram.  But to really confuse things the SI unit for mass is the kg not the g.

You're in good company though, NASA's Mars climate orbiter crashed because one bit of software was doing calculations in imperial, and another was acting on the data assuming it was in metric.

As a Brit we get used to working with both systems, I buy fuel in litres and beer in pints.  I weight myself if stone but my flight luggage in kg, and market traders now get fined if they sell in pounds.  Road signs are in miles and I calculate fuel economy in miles per gallon (a gallon being 1.2 US gallons).  I'm a farmers son and crop yields are in tonnes per acre, although most people from non farming backgrounds work in square meters and wouldn't know how long a chain is even if they play cricket (a wicket is 1 chain long).  Horse racing is in Miles and Furlongs, but running races tends to be 5km, 10km, 13 miles and 26 miles.

We're a strange country

 

 

 

[Brainlord Mesomorph]

WAIT: doesn't that mean the  ">1" part doesn't  belong there?

A ratio equals a fraction. that makes sense.  And your TWR must be greater than 1 *if* your going to move (off the ground in gravity G). But it isn't always true.  (some things don't move off the pad, but fly in space)

Why is that ">1" there?

Edited February 9, 2016 by Brainlord Mesomorph

[Snark]

44 minutes ago, Brainlord Mesomorph said:

Why is that ">1" there?

It's there in order to tell you "When making the calculation, you must arrive at a number greater than 1, or else you will not go to space today."

(i.e. you pretty much had it pegged)

Or, to express it in code (C# rather than VB here, but you get the idea):

public static readonly double kerbinGravity = 9.81 // m/s2

public static double CalculateTWR(double thrustKilonewtons, massTonnes)
{
    return thrustKilonewtons / (massTonnes * kerbinGravity);
}

public static bool CanIGoToSpaceToday(double thrustKilonewtons, massTonnes)
{
    return CalculateTWR(thrustKilonewtons, massTonnes) > 1.0;
}

A suggestion:  If you're dabbling in coding and you're in any way involved with KSP, it's worth learning C# rather than VB.  Not only is it a cooler language ... it's also what pretty much all KSP mods are written in, you'll find a lot more people in the KSP community who are willing & able to give advice in it.

(Since it's .NET, it's binary-compatible with VB.  For example, I believe it would be possible to write a KSP mod in VB.  But I've never come across anyone who actually does; they all use C#.)

Edited February 9, 2016 by Snark

[swjr-swis]

39 minutes ago, Brainlord Mesomorph said:

Why is that ">1" there?

To let you know that if your (vertically launched) rocket's TWR isn't over 1, you won't be going to space today.

Edit: I wish the forum would let us know before posting when someone has already answered a question. Mutter.

Edited February 9, 2016 by swjr-swis

[Brainlord Mesomorph]

32 minutes ago, swjr-swis said:

To let you know that if your (vertically launched) rocket's TWR isn't over 1, you won't be going to space today.

Edit: I wish the forum would let us know before posting when someone has already answered a question. Mutter.

I got that part. I'm saying its not part of the definition (or calculation of) TWR and it was confusing to me (and might be to others)

TWR will *not* always be >1

but it always will be = Fr/(m*9.81)

Edited February 9, 2016 by Brainlord Mesomorph

[Snark]

21 minutes ago, Brainlord Mesomorph said:

I got that part. I'm saying its not part of the definition (or calculation of) TWR and it was confusing to me (and might be to others)

TWR will *not* always be >1

but it always will be = Fr/(m*9.81)

Well, yeah.  So all one can say is either,

• "whoever gave you that formula was a lying sack of <something unpleasant>", or
• "whoever gave you that formula assumed you'd understand from context what the >1 meant", or
• "maybe it was clearer in the original context, but you presented the formula here without any reference to where you got it from, and therefore it's not really possible for anyone to comment on"

Joking aside, though... agreed that it can be confusing!

[RizzoTheRat]

1 hour ago, Snark said:

If you're dabbling in coding and you're in any way involved with KSP, it's worth learning C# rather than VB.  Not only is it a cooler language

9 lines to do what VB can do in 1? 

 

 

 

I'm an engineer/analyst not a programmer, I just hack stuff around in VBA instead since nobody lets me use Fortran any more.

Edited February 9, 2016 by RizzoTheRat

[Snark]

12 minutes ago, RizzoTheRat said:

9 lines to do what VB can do in 1?

Oh, hush.  You can code that in one line in C#, too. 

Not gonna get into a this-language-is-better-than-that-language debate here, just offering some friendly practical advice.

[regex]

 

But yes, C-<hash> is the preferred language of KSP.  If you've done any Java, or Microsoft languages, you can pretty much jump right in.

[JAFO]

15 hours ago, RizzoTheRat said:

As a Brit we get used to working with both systems, I buy fuel in litres and beer in pints.  I weight myself if stone but my flight luggage in kg, and market traders now get fined if they sell in pounds.  Road signs are in miles and I calculate fuel economy in miles per gallon (a gallon being 1.2 US gallons).  I'm a farmers son and crop yields are in tonnes per acre, although most people from non farming backgrounds work in square meters and wouldn't know how long a chain is even if they play cricket (a wicket is 1 chain long).  Horse racing is in Miles and Furlongs, but running races tends to be 5km, 10km, 13 miles and 26 miles.

We're a strange country

Ye gods that's a messed up arrangement.. at least in Australia, when we went metric, we just went all in.. no more imperial units in use at all. Now, there are several generations that only think in metric. Being 55, I lived through the switch, and can use either system comfortably.. anyone younger, not so much.

Edited February 10, 2016 by JAFO

[JAFO]

14 hours ago, RizzoTheRat said:

I just hack stuff around in VBA instead since nobody lets me use Fortran any more.

Hey.. even Ars Technica was forced to admit that Fortran still kicks butt..

 

http://arstechnica.com/science/2014/05/scientific-computings-future-can-any-coding-language-top-a-1950s-behemoth/

Edited February 10, 2016 by JAFO

[RizzoTheRat]

31 minutes ago, JAFO said:

Ye gods that's a messed up arrangement

Just Another F***ing Observer?

I worked for Rolls Royce some years ago. Due to high tolerances on components, international customers, and some parts commonality between engines, they had some drawings where a single component might have one bit measured in metric and other bits in imperial  And don't get me started on BTU/kg...

 

Sorry, wandering way off topic.

 

OP - Makes sure you use the sea level thrust not the vacuum one, I've had rockets sat on the launch pad at full thrust going nowhere because of that one .  Anyone know how the thrust changes with altitude?

Edited February 10, 2016 by RizzoTheRat

[JAFO]

6 minutes ago, RizzoTheRat said:

Just Another F***ing Observer?

Yup.

[jhousen]

21 hours ago, RizzoTheRat said:

OP - Makes sure you use the sea level thrust not the vacuum one, I've had rockets sat on the launch pad at full thrust going nowhere because of that one .  Anyone know how the thrust changes with altitude?

atmospheric pressure slows the exhaust velocity, effectively lowering the output thrust and isp, a simple case of needing to move the air out of the way

[kurja]

On 9.2.2016 at 5:56 PM, RizzoTheRat said:

 I calculate fuel economy in miles per gallon (a gallon being 1.2 US gallons). 

 

Why US gallons instead of UK/imperial gallons?

[RizzoTheRat]

19 minutes ago, kurja said:

Why US gallons instead of UK/imperial gallons?

Imperial, but pointing out to any that don't realise that a real gallon is 1.2 Us gallons

 

2 hours ago, jhousen said:

atmospheric pressure slows the exhaust velocity, effectively lowering the output thrust and isp, a simple case of needing to move the air out of the way

Sorry I meant what's the curve like.  Presumably pressure drops off as a function of altitude squared, but the thrust is related to the pressure differential, so if the exhaust pressure is significantly higher than atmospheric is it going to change more slowly?