Do g-forces significantly effect thrust-to-weight ratio?

[GibLit]

I checked everywhere for solid confirmation on this and found nothing. I'm just wondering if g-forces exerted on a spacecraft due to acceleration have any effect on the craft's thrust-to-weight ratio, even in a vacuum. So for example, if a craft is experiencing 10g, it would make it 10 times heavier, meaning it would require 10 times the thrust to propel it at the same rate of velocity as its original weight at 1g. Is there any truth to this?

I've noticed in KSP that it's more fuel efficient between takeoff and orbit to throttle down sometimes in order to keep a craft at 2g or under (within the green on the g meter). Is that strictly because of atmospheric drag, or does gravity have something to do with it?

[superm18]

No your craft dosent become 10 times heavier under 10g...it's just that the pressure on everything in and on the craft increeses 10 times under 10g...the total mass and thus the thrust needed to moves said mass remains the same.

[HoY]

In order to be more efficient while throttling down you most likely have a TWR of over 3. Anything less and you will probably not be getting anywheres near terminal velocity.

A good indicator (if you aren't using mechjeb) to know if you need to throttle down is if you are getting re-entry effect during your ascent. Below 10,000m if you have the white trails and above 10km the fire then yes, you can be more efficient by throttling down. I'm not sure if it was intentional but the speed at which those start IS terminal velocity.

[Nachtengel]

you have the logic a bit backwards, its actually the thrust to weight ratio that determines the G forces, change in acceleration causes the perceived effect of increased gravity, think of going up or down in an elevator. you do not get physically lighter or heavier, but for the moment the elevator starts, you feel it. just for a second. g and gravity are not the same thing, gravity is consistent (not constant, but its a function of distance nothing else) and your spacecraft will have the same acceleration exerted on it no matter what its doing. g is a designation for perceived weight, or the change in inertia. it is confusingly termed in amounts of gravities at earths surface. but nothing more. change in g force simply is a different method of saying you are accelerating faster.

in much the same way that we dont use meters for space distances (AU instead) we use g for accelerations, because it gets silly when talking about such large numbers, so a new system was invented to describe it.

[magnemoe]

One problem with high trust is that it might cause structural faults. main example is telescoping main stage during circulating orbit. as the stage is almost empty and have an huge mainsail it causes an high g-force, this might get the rocket to collapse. as with 5g the cargo is pushing 5 times as hard on the main stage than it did at the pad.

[Crown]

To be precise I should be thrust-to-mass-ratio. Because the mass stays the same even with different acceleration. Acceleration doesn't effect this. :-)

[superm18]

To be precise I should be thrust-to-mass-ratio. Because the mass stays the same even with different acceleration. Acceleration doesn't effect this. :-)

exactly

[Francesco]

A good indicator (if you aren't using mechjeb) to know if you need to throttle down is if you are getting re-entry effect during your ascent. Below 10,000m if you have the white trails and above 10km the fire then yes, you can be more efficient by throttling down. I'm not sure if it was intentional but the speed at which those start IS terminal velocity.

not true: you need to be travelling at almost two times the terminal velocity at that altitude to see mach effects appear.

[HoY]

I watch the terminal velocity indicator like a hawk during launches of my high TWR jet aircraft, and while I am not positive about the effect below 10km being present at terminal velocity I can confirm it as near fact that above 10km the re entry effect Does happen AT terminal velocity, because up there I ride that speed all the way up to 40km altitude and the effect comes and goes as my speed catches up with TV and then drops behind it slightly.

[korda]

To be precise I should be thrust-to-mass-ratio. Because the mass stays the same even with different acceleration. Acceleration doesn't effect this. :-)

I may be wrong here, but isn't it TWR when gravity comes to play? You can have excellent thrust-to-mass-ratio and still you can't get off planet with stronger g-force since you have bad thrust-to-weight-ratio.

[transcendentape]

I may be wrong here, but isn't it TWR when gravity comes to play? You can have excellent thrust-to-mass-ratio and still you can't get off planet with stronger g-force since you have bad thrust-to-weight-ratio.

Precisely. Rather than assuming the terminology in use for years is incorrect, it may be helpful to first consult basic reference resources like wikipedia. It's called thrust-to-weight ratio for a reason.

[transcendentape]

I checked everywhere for solid confirmation on this and found nothing. I'm just wondering if g-forces exerted on a spacecraft due to acceleration have any effect on the craft's thrust-to-weight ratio, even in a vacuum. So for example, if a craft is experiencing 10g, it would make it 10 times heavier, meaning it would require 10 times the thrust to propel it at the same rate of velocity as its original weight at 1g. Is there any truth to this?

I've noticed in KSP that it's more fuel efficient between takeoff and orbit to throttle down sometimes in order to keep a craft at 2g or under (within the green on the g meter). Is that strictly because of atmospheric drag, or does gravity have something to do with it?

I'm not sure why you're finding throttling down to be more efficient. It likely may be drag, or wobbly rocket construction, or too much gimbal. TWR does not decrease as acceleration increases, unless the acceleration is due to gravity. Thrust to weight ratio is simply what it states, and so is directly proportional to thrust and inversely proportional to weight (mass times acceleration due to gravity). The weight of the rocket does not change under acceleration. If you could imagine putting an accelerating rocket on some fantastic contraption that could match its acceleration and not burn up in order to weigh it, the rocket would not weigh any more than it does at rest.

Objects inside the rocket do weigh more, since the rocket is accelerating and pushing against them, applying a force which is indistinguishable from gravity. So twr does change for a rocket inside the accelerating rocket. For instance, say you constructed a situation where a kerbal had a jetpack with twr ratio of 1.5 inside of a rocket accelerating at 2g. Now, because the rocket is pushing against our jetpack kerbal at twice g, his effective twr is cut in half, and at a twr of 0.75, he won't be able to lift off of the floor.

[HoY]

And here's another theory of relativity debate! Did you know if your on a train traveling at the speed of light, and you turn the headlight on on the front, the light will not go farther ahead of you?

[transcendentape]

We are not even remotely dealing with relativistic speeds here

[HoY]

I know, it was somewhat of a joke.. But it brings up another debate I haven't had in years about objects disconnected from the vessel which thrust is being applied to, while still inside of it then applying directional force.. Say the kerbal in the rocket uses his jet pack with its 0.75 twr then jumps up off the floor which is now easy since 25% of his weight is carried by the pack. He's now inside the rocket disconnected from it, does his pack now have a higher twr relative to what it had while it was a part of the rocket?

[DMagic]

Assuming that there is air inside the rocket then he's not really disconnected from it.

[transcendentape]

Say the kerbal in the rocket uses his jet pack with its 0.75 twr then jumps up off the floor which is now easy since 25% of his weight is carried by the pack. He's now inside the rocket disconnected from it, does his pack now have a higher twr relative to what it had while it was a part of the rocket?

Assuming the rocket the kerbal is in does not change its acceleration, no. The presence or absence of air in the rocket is irrelevant. The kerbal's jetpack provides .75g of thrust as measured inside the rocket, and the kerbal makes a leap into the air similar to you jumping off of the ground. The effective gravitational force (my term, not sure what the correct term to use here is) which includes both the gravitational force felt by the rocket and the pseudo-gravitational force due to acceleration of the rocket has not changed any more than you jumping changes the gravitational force you feel on Earth.

Edited May 10, 2013 by transcendentape
changed rocket pack to jetpack to avoid confusion

[korda]

I know, it was somewhat of a joke.. But it brings up another debate I haven't had in years about objects disconnected from the vessel which thrust is being applied to, while still inside of it then applying directional force.. Say the kerbal in the rocket uses his jet pack with its 0.75 twr then jumps up off the floor which is now easy since 25% of his weight is carried by the pack. He's now inside the rocket disconnected from it, does his pack now have a higher twr relative to what it had while it was a part of the rocket?

No his TWR is same, at least the one measured relative to rocket. His acceleration can be measured relative to rocket - while he can accelerate very fast relative to Kerbin, he will still "fall" back to floor in rocket (since he has too low "local" TWR on rocket).

EDIT:

Btw, we are actually heading straight to Einstein theory. His thought process included something similar to what we are talking about - similarity of acceleration and gravity.

Edited May 10, 2013 by korda

[pebble_garden]

I've noticed in KSP that it's more fuel efficient between takeoff and orbit to throttle down sometimes in order to keep a craft at 2g or under (within the green on the g meter). Is that strictly because of atmospheric drag, or does gravity have something to do with it?

When launching in an atmosphere, drag is definitely an issue, though the problem decreases with altitude/atmospheric density. I don't know of a magic formula for finding the speed which keeps you from hitting the drag wall, but a little experimentation with different throttle settings will give you a clue. I did a video test of this problem, which you can see here.

On airless worlds, I believe there's no benefit to throttling back, though. The sooner you're clear of the high gravity regions, the better.

I don't know if there is any sort of efficiency loss with higher throttle settings in the engine simulation code, perhaps there is.

PS Results will vary greatly, depending upon the rocket and the world you're launching from, obviously.

Edited May 10, 2013 by pebble_garden

[Cal'Mihe]

I don't think the engines have an effiency loss based on throttle setting, the rocket motors for sure don't. The jet engines shouldn't either, they are instead affected by both altitude and velocity.

[Vanamonde]

"Weight" is a measure of the force exerted by a mass in the local gravity, so yes, if the local gravity is higher, the ship weighs more, and its T/W is reduced. However, the G meter on the navball is not measureing weight, and instead measures acceleration in earth-gravity-equivalents, and its reading sums acceleration due to both gravity and thrust. So during launch on Kerbin, the increase in G reading you're seeing has nothing to do with gravity (which is actually getting slowly weaker as you get farther from the mass of Kerbin), but is rather a measure of how hard the engines are pushing the ship. The fuel wastage you're seeing is due to air resistence rather than gravity, so you wouldn't need to throttle down like that around vacuum worlds, but it does save fuel in atmosphere to prevent your ship from exceeding the terminal velocity at that altitude.

And the throttle setting has no impact upon the ISP of KSP engines. It used to, but that was fixed a couple of versions ago.

[Specialist290]

To put it yet another way: Your thrust-to-weight ratio measures the ratio of the force your engines exert in pushing the rocket ("thrust") against the force that gravity exerts trying to pull your rocket back down to the surface ("weight"). If the resulting number is less than 1, then you are not going into space today. G-forces are just another way to measure the acceleration your rocket experiences relative to acceleration due to gravity at Kerbin's surface.

[Endoric]

here are a few questions on this topic.....

i heard TWR at launch is most efficent at 2.2. is that correct?

what should my TWR be once in space?

what should my TWR be at different altitudes?

also on terminal velocity: should i not exceed the speed given in the list i found on the wiki at each given altitude mark? i also notice mechjeb differs slightly when using an auto accent.

is mechjeb more efficent than a good pilot during accent?

is mechjeb more efficent during the gravity turn?

tanks!

[Eric S]

i heard TWR at launch is most efficent at 2.2. is that correct?

That's what I use, and it's at least approximately correct. You might have to throttle down a little bit as you're burning off fuel, however, since otherwise your thrust is constant but your weight is decreasing.

what should my TWR be once in space?

When you're in orbit, TWR doesn't matter much, as it all comes down to patience. I usually try to avoid going below 10 m/s accel for normal craft, and 2 m/s for big craft going a long way. For example, a transfer orbit to Duna takes about 1000 m/s of acceleration. With 10 m/s, that's 100 seconds, which is bearable. at 2 m/s, it takes 500 seconds, or a bit over 8 minutes, which can get annoying. At that rate, an Eeloo transfer orbit would take over 15 minutes.

Now, something to be aware of is that if you're using a low acceleration craft, it's quite possible to find yourself spending so little time in the SoI of a celestial object that you can't achieve an orbit before leaving the SoI. Two pieces of advice on that. First is to enter the SoI as precisely as possible. Not only do you have more time within the SoI to change your velocity, but you also benefit more from the Oberth effect. Second, sometimes even with that, you need to start matching velocity with the celestial object before you enter that object's SoI.

what should my TWR be at different altitudes?

I try to maintain a 2.2 TWR from launch until I hit the apoapsis. I don't mind circularizing with a TWR of 1.0 or so, but I prefer at least a 2.0 even at that point, as it makes getting a circular orbit easier since you don't have to burn as much before and after apoapsis.

also on terminal velocity: should i not exceed the speed given in the list i found on the wiki at each given altitude mark? i also notice mechjeb differs slightly when using an auto accent.

Generally a good idea to keep it in that general area.

is mechjeb more efficent than a good pilot during accent?

MJ is pretty good on some craft, and pretty bad on others, it really depends. A very good pilot will always match or beat MJ, however. Where MJ wins is in consistency. While I might occasionally have a very good or very bad launch with a specific vehicle, MJ will almost always have very little variation, which is kind of nice if you're testing small changes in a craft. Large changes don't give as accurate a result, as you can more easily be making the ship fit MJ's launch profile.

is mechjeb more efficent during the gravity turn?

Again, it's not bad, but I find I can usually do a bit better after some practice with the specific craft in question, since they all behave a little differently.

[Vanamonde]

In space, as long as you can complete a burn before the celestial motion window passes, T/W doesn't matter. That's why ion engines are sufficient to get small ships to distant places, and why the efficiency of LV-Ns is more important than their pitiful thrust. I'll leave your other questions to the numbers experts, but that takeoff T/W and terminal velocity numbers are at least reasonable.