A Question About TWR

[RedDevilEA]

So, I know that T/WR on the pad uses 9.81 m/s2 as gravity. But if I want to calculate my T/WR at, let's say, a 100km orbit, should I use 7.21 m/s2?

I got 7.21 by multiplying 3.53E+12 by 700,000^2 since the radius of Kerbin is 600,000m + 100,000m orbit.

[Geschosskopf]

TWR is only really a factor when working against gravity, like during launch. What really matters from rockets (at least after initial launch) is TMR. Mass doesn't change with gravity and you're concerned about acceleration aka delta-V, so delta-V = Thrust / Mass. This ratio only changes as mass decreases with fuel consumption.

Problem is, things like MJ and KER actually show TWR and use the constant, surface gravity when doing so, depending on what SOI you're in. This is so you can build ships that can take off from where they've landed. AFAIK nothing really shows TMR because it's not a matter of life and death, just burn time.

I THINK. Could be totally wrong on this.

[Philoman]

Another way to find out is to put a probe into a 100 km orbit with the gravioli instrument. It will tell you

[Philoman]

Actually I have thought about something else, but related to TWR, so I think I'll just try and ask in this thread..

Am I totally wrong when I assume that as long as I'm below the terminal velocity and having a TWR > 1, then the rocket should be accelerating? Both numbers are from KER and it's the current TWR of course.

One thing is certain.. it's not what is happening in the game. From what I can tell, either the terminal velocity must be wrong or I have just no idea

[Kasuha]

So, I know that T/WR on the pad uses 9.81 m/s2 as gravity. But if I want to calculate my T/WR at, let's say, a 100km orbit, should I use 7.21 m/s2?

I got 7.21 by multiplying 3.53E+12 by 700,000^2 since the radius of Kerbin is 600,000m + 100,000m orbit.

If you got some orbital speed at that height, the centrifugal force subtracts from gravity, making the value even smaller. So the question is what you need the TWR for anywhere but on ground. In space, TWR usually does not matter very much.

Actually I have thought about something else, but related to TWR, so I think I'll just try and ask in this thread..

Am I totally wrong when I assume that as long as I'm below the terminal velocity and having a TWR > 1, then the rocket should be accelerating? Both numbers are from KER and it's the current TWR of course.

One thing is certain.. it's not what is happening in the game. From what I can tell, either the terminal velocity must be wrong or I have just no idea

Each TWR value has certain speed at which the ship stops accelerating. These speeds vary from zero at TWR <= 1 through terminal velocity at TWR = 2 to generally any higher speed if your TWR is even higher.

Terminal velocity is just the velocity of a freefalling object, i.e. under acceleration of 1 x gravity.

[Amaroq]

Not exactly. (Apologies, Kasuha, that was in response to Philo, not to you) You will accelerate if the force from thrust exceeds the force of drag plus the force of gravity. So, if your TWR is 1.1 but your force-from-drag is 0.2, you'll wind up decelerating due to drag. (Net upward force = 1.1, net downward force = 1.2)

The specific TWR you'll need to accelerate, obviously, will change depending on whether you are using the stock physics or the F.A.R. aerodynamics, the thickness of the atmosphere, your distance from the planet, and your current velocity. (Note that the wiki-listed "terminal velocity" is not applicable/correct if you are using F.A.R. instead of stock.)

For a F.A.R. launch, I tend to launch at maximum thrust to about 105m/s @ 1000ft, and then throttle back to about a 1.2 TWR, which is sufficient to continue accelerating right around enough to stay fairly below the F.A.R. terminal velocities without exceeding it ... but that formula would be way off for a stock launch.

Edited March 6, 2014 by Amaroq

[Geschosskopf]

If you got some orbital speed at that height, the centrifugal force subtracts from gravity, making the value even smaller. So the question is what you need the TWR for anywhere but on ground. In space, TWR usually does not matter very much.

I was raised in the belief that there is no such thing as centrifugal force. A ship is only in orbit because gravity is pulling it inwards. Nothing is pushing it outwards. If gravity stopped, the ship would go off in a straight line tangent to its orbit, not because a force made it do so, but because the force making it go in a circle was no longer acting. So how does this really work?

[Xaiier]

So how does this really work?

http://en.wikipedia.org/wiki/Centrifugal_force#Fictitious_centrifugal_force

[RedDevilEA]

http://en.wikipedia.org/wiki/Centrifugal_force#Fictitious_centrifugal_force

I've always understood it to be a resultant force. Like one side of a triangle, I guess. Not the actual force, or the hypotenuse, but one part of it. But, as I've said before, I am not a clever man.

[Kasuha]

I was raised in the belief that there is no such thing as centrifugal force. A ship is only in orbit because gravity is pulling it inwards. Nothing is pushing it outwards. If gravity stopped, the ship would go off in a straight line tangent to its orbit, not because a force made it do so, but because the force making it go in a circle was no longer acting. So how does this really work?

Centrifugal force does not exist if you assume newtonian inertial frame of reference. In rotating frame of reference of an object moving horizontally in gravitational field, it is a very real force to be accounted for. It's because its frame of reference is not inertial.

[Kryxal]

It does exist, actually ... centripetal force on object a from object b means centrifugal force on object b from object a. It's that whirling-an-object-around example ... you have forces on both ends.

[SSSPutnik]

You could work it out by reading off current acceleration in MJ.

[Xavven]

Hey, everyone! As others have said, TWR is most important from takeoff from surface. Once you're in orbit, don't divide by the surface gravity. What you're left with is your maximum acceleration capability if you perform the calculation of Thrust / Mass. For example, for a transfer stage with a mass of 10 using one LV-N engine, you're getting 60 thrust / 10 tons = 6 m/s² of acceleration (every second your velocity goes up by 6 m/s when you're at full throttle). This can give you a good idea of how long it will take you to do a burn. For example, escaping Low Kerbin Orbit takes 950 delta-V, so it's going to take a little less than 950/6 = 158 seconds = about two and a half minutes to do the burn. Of course, as you burn fuel, your rocket gets lighter and your Thrust/Mass gets higher, making the burn shorter.

To answer the tangent about gravity, centrifugal force, and orbiting, the confusing thing about gravity is that it's not a real force.

Warning: wall of text incoming

Einstein has shown that gravity is space-time curvature. This means gravity is not a force, but a virtual force. This is not a trivial thing to explain as it is counter-intuitive to most people, but let me try here real quick. Let's start with an intuitive example about force. If a playground bully were to push you from behind, it's clear which direction the force is coming from and which way you accelerate (forward). The force comes from behind you and pushes you forward, and you end up falling forward. Okay, next example: If you get in your car and accelerate hard, you feel force pushing on your back. To be clear, there is no force pushing you backwards into your seat, rather there is a forces pushing you forward from your back. The car seat pushes you forward just like the bully was pushing you forward. If you left a pair of sunglasses on the dashboard while you floored the gas pedal, you'd see your sunglasses fly towards the back of the car, but did anything push the sunglasses back? Nope. The car moved forward while the glasses stayed still. Once hitting the car seat, it too gets pushed forward and appears to come to a rest (relative to you) but it is now feeling the same forward acceleration you are. STOP! If you don't quite get the above, then you won't get the next paragraph!

Now to tackle gravity. You get out of your car, find a nice grassy meadow to lie down in, and lie on your back. Hm, you feel the exact same force on your back that you felt when you were accelerating in your car. Gravity is not actually pushing you DOWN into the ground (if it were, you would feel pressure on your front, like if the playground bully pushed you downward into the ground). What's actually happening is space itself is flowing towards the center of the Earth. You don't actually feel space flowing (it's like floating in a river) but the ground is pushing you UP through that flowing space. Just like the car was pushing you forward, the ground is pushing you up, and that is the actual force you feel. If you throw your sunglasses into the air, it would appear that gravity is pulling it back down to the ground, but just as in the car example where it slid off the dash, it's not actually experiencing any forces! When it hits the ground, however, it starts accelerating upwards along with you and appears to come to a rest (relative to you) but it is now feeling the same upwards acceleration you are. Being on the ground on Earth is like being in a car that is perpetually accelerating UP!

In fact... in a microgravity environment, for example when you're in orbit, being in a rocket that is accelerating at 9.8 m/s² gives you the same behavior as standing on the ground on Earth. If the rocket were wide enough, you could play tennis-- the players on the court, the ball, the net, everything would behave the same as on Earth as long as the rocket continues accelerating.

Oookay, next lesson. Centrifugal force. It is not a real force. Centripetal force, however, is a real force. Back to the car example: You are going 60mph and you make a right turn around a bend in the highway. The sunglasses on the dash "get flung to the left" side of the car, and your body gets pressed against the left door. In which direction is the force accelerating everything? Hint: It's actually NOT to the left. It's to the right! The car moved to the right while the stuff inside of it wanted to go straight ahead. The left side of the car catches up to the inside contents and exerts a force to the RIGHT to keep everything inside the vehicle. Centripetal force is that force of the driver side door (assuming not in the UK) pushing everything to the right. It is a true force and always points towards the center point of the rotation. Even though it looks like stuff is getting thrown to the left, there is no actual force pushing it to the left -- instead the car is going to the right and the stuff is initially going straight!

Now let's talk about orbit. When you're in orbit, you do not feel any acceleration (a fancy way of saying you are "weightless"). What's truly happening is you are traveling through space-time curvature. You are traveling in a "straight line" as far as forces are concerned, but that line just happens to be warped around the planet because of the space curvature. There is no centripetal force, either, because you aren't technically turning.

Hope that helps

[Xavven]

Oops, I forgot to talk about virtual forces and tie it back to TWR. Hehehe. Here we go!

Remember back in the meadow when you threw your sunglasses into the air, and I said, "gravity isn't pulling the sunglasses down"? Well, it isn't - there is no force pulling the sunglasses down (other than air drag which we will ignore right now). But it sure LOOKS like the sunglasses are being pulled down, at least relative to where you are. Enter "virtual forces". These are things that make it look like stuff is being moved around you even though it is you moving around it. Virtual forces "act" in the opposite direction of the actual force that something ELSE is experiencing in relation to it. In other words, you are accelerating up towards the sunglasses, and you are the one experiencing force. The sunglasses are not experiencing forces, but it looks like gravity is "pulling it" towards you, so we call that "downwards force" a virtual force.

Back in the car around a right-hand highway bend example, when the sunglasses appear to be flung to the left, that is the virtual force (centrifugal force) "acting" on the sunglasses to the left. But again, what's really happening is the car is accelerating to the right towards the sunglasses.

Now to tie this to thrust to weight ratio. When your rocket has not yet ignited its engines and it's sitting on the launchpad, it is in fact being accelerated upwards at 9.8 m/s² (remember because gravity = space flowing downwards but the ground pushes you up). If you want to get further away from the ground, you're going to have to accelerate at more than 9.8 m/s². So let's say you go exactly 1.5 times that, or 14.7 m/s². You are now actually experiencing 14.7 m/s² of acceleration (or 1.5g). How fast are you accelerating relative to the ground? Well, the ground is accelerating upwards too, but at only 9.8 m/s². Relative to the ground, you are accelerating at 14.7 - 9.8 = 4.9 m/s². This is "gravity drag" but the name is so misleading. It much more analogous to two drag cars racing each other. They are both accelerating, but one car is accelerating faster and starts to pull ahead. The rate at which the faster car pulls ahead is the difference between their acceleration rates, and if you want to express that in terms of percentage better acceleration, you get "TWR"!

[Kasuha]

Whether gravity is or isn't a real force is actually irrelevant. Imagine you have a paper covered in iron particles and you draw something in these particles by removing some of them. Now, you put the paper beside a large electromagnet and switch on the electricity. You may observe that all particles start accelerating towards the magnet but if the field is uniform enough, your drawing in them will stay unchanged all the time until particles crash into the magnet. The "being" you have drawn does not feel any forces affecting it because all particles are accelerated by the same force.

Similarly your body does not feel gravity pulling on it because every atom of your body is accelerated by the same force in the same direction. What you feel is the force exerted by ground on your feet because there the force is applied only to a part of the body. But still, there is an action (gravity pulling on you) and equal and opposite reaction (ground pushing against your feet).

In inertial frames of reference, inertia is not considered a force because it does not cause any changes in position or motion of objects.

In non-inertial frames of reference (such as inside of a car turning to the left), though, inertia is not so inert and it makes objects move. From the point of view of such frame of reference, inertia acts as a force and gets various names according to what it is causing. Such as centrifugal force. Yes, it is not a real force for someone who only considers inertial reference frames valid. But for many purposes, non-inertial reference frames are not only valid, they may be even beneficial. And treating inertia as a force in such reference frames is essential.

[Xavven]

Whether gravity is or isn't a real force is actually irrelevant. Imagine you have a paper covered in iron particles and you draw something in these particles by removing some of them. Now, you put the paper beside a large electromagnet and switch on the electricity. You may observe that all particles start accelerating towards the magnet but if the field is uniform enough, your drawing in them will stay unchanged all the time until particles crash into the magnet. The "being" you have drawn does not feel any forces affecting it because all particles are accelerated by the same force.

Similarly your body does not feel gravity pulling on it because every atom of your body is accelerated by the same force in the same direction. What you feel is the force exerted by ground on your feet because there the force is applied only to a part of the body. But still, there is an action (gravity pulling on you) and equal and opposite reaction (ground pushing against your feet).

In inertial frames of reference, inertia is not considered a force because it does not cause any changes in position or motion of objects.

In non-inertial frames of reference (such as inside of a car turning to the left), though, inertia is not so inert and it makes objects move. From the point of view of such frame of reference, inertia acts as a force and gets various names according to what it is causing. Such as centrifugal force. Yes, it is not a real force for someone who only considers inertial reference frames valid. But for many purposes, non-inertial reference frames are not only valid, they may be even beneficial. And treating inertia as a force in such reference frames is essential.

This is an excellent description of gravity in Newtonian terms and I'm not saying it's incorrect. In Newtonian physics, objects can't accelerate towards one another without a force acting on them. In this case, you are describing gravity as accelerating all parts of the object at once such that a mass spring wouldn't measure any force being exerted.

However, in the theory of gravitation in general relativity, as described by Einstein, the force you describe becomes fictitious. I learned general relativity in college, not from Wikipedia, but I do think this Wikipedia article has a fairly good explanation: http://en.wikipedia.org/wiki/Introduction_to_general_relativity. I'd write more but I need to get to work. I'd love to continue this discussion when I get home!

[Kasuha]

However, in the theory of gravitation in general relativity, as described by Einstein, the force you describe becomes fictitious. I learned general relativity in college, not from Wikipedia, but I do think this Wikipedia article has a fairly good explanation: http://en.wikipedia.org/wiki/Introduction_to_general_relativity. I'd write more but I need to get to work. I'd love to continue this discussion when I get home!

I wrote it is irrelevant, not wrong. We don't need anything more than newtonian physics for KSP. You don't need to think in curved spacetime if the mechanism is for all practical purposes equal to forces between charged particles.

In fact Einstein's gravitation theory is very useful but it is very likely to be found being a consequence of quantum mechanics, just like electrostatic force or inertia. We're just waiting for someone to find a way how to put them together.

[Xavven]

I wrote it is irrelevant, not wrong. We don't need anything more than newtonian physics for KSP. You don't need to think in curved spacetime if the mechanism is for all practical purposes equal to forces between charged particles.

In fact Einstein's gravitation theory is very useful but it is very likely to be found being a consequence of quantum mechanics, just like electrostatic force or inertia. We're just waiting for someone to find a way how to put them together.

Okay, fair enough. I was just trying to answer Geschosskopf's question about centrifugal force and the force of gravity, and how it works when in orbit. If you want to truly understand it, you have to realize they are not real forces counteracting each other - they are virtual. Nothing is actually pushing or pulling the craft towards or away from the barycenter.

For the purposes of KSP, I'm perfectly happy with someone saying that they accelerate towards periapsis and decelerate towards apoapsis with respect to the central body due to the force of gravity. I had simply detected that the conversation had switched from "how does this game work" to "what's the theory behind this concept of centrifugal force?". So there you go - free physics knowledge. Take it or leave it.