seeking clarification on Effective Thrust calculation for TWR from Wiki

[Drew Kerman]

referencing this KSP Wiki entry, how exactly does effective thrust affect TWR? Should I actually be using fEff in place of fT in the TWR equation? I've been unable to find a practical example. Currently I'm doing:

fEff = fT * cos(a)

TWR = fEff/(m * g)

I'm actually using the current gravity as well.

Edited November 2, 2015 by Gaiiden

[FancyMouse]

Depending on what you use the number for. Effective is mostly used when you want to calculate in flight whether you get upwards or downwards acceleration.

[Snark]

The particular equation you're using simply refers to the vertical component of thrust. Not sure why you particularly care about that.

TWR is just the ship thrust divided by its weight.

Can you explain a bit more about what you're trying to figure out? Would be helpful in providing a useful answer to questions.

[Drew Kerman]

I guess a better way of putting it is I'm looking for an accurate measurement of TWR while in flight of a rocket during ascent, and I'm wondering if factoring effective thrust into the equation is worthwhile towards that end. TWR gives a good indication of both the rocket's power and acceleration.

[SchweinAero]

The particular equation you're using simply refers to the vertical component of thrust. Not sure why you particularly care about that.

Isn't this equation more about engines that are angled relative to the rocket's body? Because that certainly affects TWR regardless of maneuver.

[Yasmy]

I guess a better way of putting it is I'm looking for an accurate measurement of TWR while in flight of a rocket during ascent, and I'm wondering if factoring effective thrust into the equation is worthwhile towards that end. TWR gives a good indication of both the rocket's power and acceleration.

If you want a measure of the rocket's acceleration, use TWR.

If you want a measure of the rocket's vertical acceleration, use the effective TWR. This depends on your current flight path, rather than just on your rocket mass and engines.

The effective TWR (as defined by the wiki) is just the vertical portion of your acceleration in units of g. If your effective TWR is greater than 1, your vertical velocity will increase. If your effective TWR is less than 1, your vertical velocity is will decrease.

(Technically, instead of g, you would need to use GM/r^2, but for most purposes, g is sufficiently close to GM/r^2 in atmosphere or low orbit.)

- - - Updated - - -

Isn't this equation more about engines that are angled relative to the rocket's body? Because that certainly affects TWR regardless of maneuver.

Nope. While you are correct that if your rocket engines are all angled by angle 'a' relative to their symmetry axis, your TWR will be reduced by cos(a), that's is not apparently the effective TWR as defined in the wiki. Personally this is what I would have called effective TWR, but the wiki disagrees with us.

Edited November 2, 2015 by Yasmy

[Snark]

I guess a better way of putting it is I'm looking for an accurate measurement of TWR while in flight of a rocket during ascent, and I'm wondering if factoring effective thrust into the equation is worthwhile towards that end. TWR gives a good indication of both the rocket's power and acceleration.

Unless you have some particular, special reason for wanting to know the vertical component of your acceleration at a particular time (and if so, I'd love to hear it, because I have trouble thinking of one), I would contend that this "effective TWR" isn't relevant and there's no point in trying to track it.

It's true, as other posters have indicated, that an effective TWR greater than 1 indicates an increasing vertical velocity, whereas less than 1 indicates decreasing. But why do you want to know that? What use is it? Do you have some reason why you would want it to be one or the other? I ask because I sure don't... and I'm a physics-major math geek who does lots of calculations at design time and never uses KER because figuring things out on a calculator is fun. In a year and a half of intensive KSP playing, I've never once had a reason to want to calculate this "effective TWR".

At launch, you're pointed straight up, so your simple TWR (no fancy trigonometry) is all that matters.

As you start to tip into your gravity turn, the cosine will be so close to 1 that it doesn't matter... and unless you're tipping way too much too soon, it will be more than offset by your increasing TWR as you burn fuel and your mass goes down.

By the time you're tipped over far enough for the cosine term in effective TWR to make much mathematical difference, you no longer care about the vertical component of your TWR, because at that point you're mainly concerned with building up your horizontal velocity to achieve orbit, and the vertical component can take care of itself, as long as your ascent stage isn't grossly underpowered.

[Drew Kerman]

Thanks Snark, that's some informed reasoning I was looking for.