# Creating gravity in space

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The force of gravity on Earth is roughly 9.81 m/s, yes? Okay, theoretically, we should be able to either accelerate at that 9.81 m/s for however long we want and it would feel like real gravity. Would this be correct? I understand we could use centrifugal force for the same thing but I'm just brainstorming for a science fair project.

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2 minutes ago, Willshikabob said:

The force of gravity on Earth is roughly 9.81 m/s, yes? Okay, theoretically, we should be able to either accelerate at that 9.81 m/s for however long we want and it would feel like real gravity. Would this be correct?

Absolutely. There is something called the equivalence principle that says that acceleration and gravity are indistinguishable. So (assuming you can't look out of the window!) you would no be able to tell if you were stationary in Earth's gravity or accelerating at 1g.

4 minutes ago, Willshikabob said:

I understand we could use centrifugal force for the same thing

Correct.

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3 hours ago, Strange said:

There is something called the equivalence principle that says that acceleration and gravity are indistinguishable.

Only if observed at a single point. Gravity usually has a gradient. Consequently, gravity at the top and bottom of an elevator car, will differ slightly. But the same two points in an idealized, accelerating frame of reference, will not exhibit that difference.

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7 hours ago, Rob McEachern said:

Only if observed at a single point. Gravity usually has a gradient. Consequently, gravity at the top and bottom of an elevator car, will differ slightly.

Good point. (I thought it might be more detail than the OP needed, though!)

Quote

But the same two points in an idealized, accelerating frame of reference, will not exhibit that difference.

Until you start to take relativity of simultaneity into account, maybe...

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On 1/5/2018 at 3:26 AM, Strange said:

Good point. (I thought it might be more detail than the OP needed, though!)

Until you start to take relativity of simultaneity into account, maybe...

How do we consider simultaneity in a non inertial frame?

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On 1/4/2018 at 4:55 PM, Willshikabob said:

The force of gravity on Earth is roughly 9.81 m/s, yes? Okay, theoretically, we should be able to either accelerate at that 9.81 m/s for however long we want and it would feel like real gravity. Would this be correct? I understand we could use centrifugal force for the same thing but I'm just brainstorming for a science fair project.

Actually, no. Strange forgot something.

You can only accelerate for so long before you reach the speed of light.

At which point, you simply can't accelerate anymore.

Additionally, you'd stop accelerating at 9.81 m/s within a hundred days, long before you reached even 1/3rd the speed of light because the exhaust from the engines would no longer be moving faster then you're moving away from it, resulting in less and less thrust the closer you got to exhaust velocity.

If my math is right, even if you could accelerate constantly at the same speed, you'd reach the speed of light in a little under a year, at which point you've hit the speed limit and would stop accelerating.

So, the use of acceleration as a gravity form is definitely a clever one and an interesting topic to discuss. However, you also have to note it's not a long-term solution to creating artificial gravity.

This is actually a large argument in my opinion against flat earth's theory that disk is accelerating at 9.81 m/s and that's why we experience gravity. But that's a whole other topic.

Best of luck!

Edited by Raider5678

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5 minutes ago, Raider5678 said:

If my math is right, even if you could accelerate constantly at the same speed, you'd reach the speed of light in a little under a year, at which point you've hit the speed limit and would stop accelerating.

I remain ready to be corrected, but As you get closer and closer to the speed of light, the energy required to keep accelerating increases. It’s not linear. Any object with any mass cannot get there because the energy required to do so exceeds all of the energy available in the entire universe.

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7 minutes ago, iNow said:

I remain ready to be corrected, but As you get closer and closer to the speed of light, the energy required to keep accelerating increases. It’s not linear. Any object with any mass cannot get there because the energy required to do so exceeds all of the energy available in the entire universe.

Anyways, this is true.

But I also pointed this out in my post.

"Additionally, you'd stop accelerating at 9.81 m/s within a hundred days, long before you reached even 1/3rd the speed of light because the exhaust from the engines would no longer be moving faster then you're moving away from it, resulting in less and less thrust the closer you got to exhaust velocity."

I haven't gotten rusty on my rocket physics quite yet  (this is technically my first time correcting someone on their physics that I'm aware of)

However, now I'm really curious.

Is there a formula for the curve generated by thrust deteriorating the closer you got to exhaust velocity?

Because like you said, it wouldn't be linear, but there should be a mathematical formula for it. I kinda want to know if NASA has to take that into account for rockets reaching escape velocity.

Edited by Raider5678

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39 minutes ago, Raider5678 said:

Actually, no. Strange forgot something.

You can only accelerate for so long before you reach the speed of light.

At which point, you simply can't accelerate anymore.

Additionally, you'd stop accelerating at 9.81 m/s within a hundred days, long before you reached even 1/3rd the speed of light because the exhaust from the engines would no longer be moving faster then you're moving away from it, resulting in less and less thrust the closer you got to exhaust velocity.

If my math is right, even if you could accelerate constantly at the same speed, you'd reach the speed of light in a little under a year, at which point you've hit the speed limit and would stop accelerating.

So, the use of acceleration as a gravity form is definitely a clever one and an interesting topic to discuss. However, you also have to note it's not a long-term solution to creating artificial gravity.

This is actually a large argument in my opinion against flat earth's theory that disk is accelerating at 9.81 m/s and that's why we experience gravity. But that's a whole other topic.

Best of luck!

As far as the occupants in the rocket are concerned they can continue accelerating and experience 1g of gravity forever (fuel allowing).  An outside observer will see their acceleration decreasing as they approach c, but they, inside the ship, will feel no slacking off of acceleration.

This does not mean that they could ever measure their speed relative to their starting point as ever exceeding c however.   This is due to the way velocities add in Relativity.    If you assume that they measure their velocity with respect to the Earth as 0.1c, they can add another 0.1c to their velocity, by their measure, but when they measure their speed relative to the Earth, they would measure it as being (0.1c+0.1c)/(1+ 0.1c(0.1c)/c^2 = 01.923 c and not 0.2c.

They can keep adding velocity onto velocity, but it will never sum up to a velocity greater than c with respect to the Earth.

The big problem is the ability to keep up that acceleration due to amount of fuel/reaction mass needed.

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Just now, Janus said:

As far as the occupants in the rocket are concerned they can continue accelerating and experience 1g of gravity forever (fuel allowing).  An outside observer will see their acceleration decreasing as they approach c, but they, inside the ship, will feel no slacking off of acceleration.

This does not mean that they could ever measure their speed relative to their starting point as ever exceeding c however.   This is due to the way velocities add in Relativity.    If you assume that they measure their velocity with respect to the Earth as 0.1c, they can add another 0.1c to their velocity, by their measure, but when they measure their speed relative to the Earth, they would measure it as being (0.1c+0.1c)/(1+ 0.1c(0.1c)/c^2 = 01.923 c and not 0.2c.

They can keep adding velocity onto velocity, but it will never sum up to a velocity greater than c with respect to the Earth.

The big problem is the ability to keep up that acceleration due to amount of fuel/reaction mass needed.

But wouldn't the occupants note that 400 days had passed, thereby allowing them to realize they're moving slower through time or..... I'm so confused.

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44 minutes ago, Raider5678 said:

"Additionally, you'd stop accelerating at 9.81 m/s within a hundred days, long before you reached even 1/3rd the speed of light because the exhaust from the engines would no longer be moving faster then you're moving away from it, resulting in less and less thrust the closer you got to exhaust velocity."

I haven't gotten rusty on my rocket physics quite yet  (this is technically my first time correcting someone on their physics that I'm aware of)

However, now I'm really curious.

Is there a formula for the curve generated by thrust deteriorating the closer you got to exhaust velocity?

Because like you said, it wouldn't be linear, but there should be a mathematical formula for it. I kinda want to know if NASA has to take that into account for rockets reaching escape velocity.

Exhaust velocity does not put any limit on the upper velocity of the rocket. (if this were the case, none of our rockets would ever get anything even into orbit as we are limited to exhaust velocities of ~4.5 km/sec and orbital velocities are ~7.9 km/sec.

The rocket equation illustrates this

Vfinal  = Vexhaust x ln(MR)

MR is the "mass ratio" of the rocket or the total mass of the rocket divided by the mass of the rocket after expending your fuel.

(ln means natural log)

As long as you have enough fuel/ reaction mass, you can reach any velocity you want.  The catch is that the lower the exhaust velocity, the larger the MR has to be to reach the same velocity.  So for example to reach 7.9 km/sec with an exhaust velocity of 4.5 km/sec you need a mass ratio of 5.787 ( you would need 4.787 kg of fuel for every 1kg of empty rocket.)

If you wanted to reach twice that velocity, the MR increases to 33.5 (32.5 kg of fuel per kg of rocket). Notice how this increases very quickly.   At this rate, to even reach 0.001c would take  roughly 1/20th the mass of the Sun in fuel.

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5 minutes ago, Janus said:

Exhaust velocity does not put any limit on the upper velocity of the rocket. (if this were the case, none of our rockets would ever get anything even into orbit as we are limited to exhaust velocities of ~4.5 km/sec and orbital velocities are ~7.9 km/sec.

The rocket equation illustrates this

Vfinal  = Vexhaust x ln(MR)

MR is the "mass ratio" of the rocket or the total mass of the rocket divided by the mass of the rocket after expending your fuel.

(ln means natural log)

As long as you have enough fuel/ reaction mass, you can reach any velocity you want.  The catch is that the lower the exhaust velocity, the larger the MR has to be to reach the same velocity.  So for example to reach 7.9 km/sec with an exhaust velocity of 4.5 km/sec you need a mass ratio of 5.787 ( you would need 4.787 kg of fuel for every 1kg of empty rocket.)

If you wanted to reach twice that velocity, the MR increases to 33.5 (32.5 kg of fuel per kg of rocket). Notice how this increases very quickly.   At this rate, to even reach 0.001c would take  roughly 1/20th the mass of the Sun in fuel.

Wait, doesn't fuel efficiency play a role in it?

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5 minutes ago, Raider5678 said:

Wait, doesn't fuel efficiency play a role in it?

Only in the same way that you pissing in the ocean changes its ph

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21 minutes ago, Raider5678 said:

But wouldn't the occupants note that 400 days had passed, thereby allowing them to realize they're moving slower through time or..... I'm so confused.

For them 343.568 days will have past (assuming 400 days have passed on the Earth) and they will have a velocity of ~0.75c relative to the Earth. The Earth will measure their distance as being ~0.493 ly,  while they will measure the distance as 0.326 ly (length contraction)

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9 minutes ago, iNow said:

Only in the same way that you pissing in the ocean changes its ph

Huh.

But now I'm confused.

The mass to fuel ratio and the exhaust velocity is the only thing that matters for the speed of a rocket?

What about deltaV? Where does that come in play?

If a rocket with 10,000m/s deltaV is next to a rocket with 12,000m/s deltaV, wouldn't the first rockets top speed, starting at 0m/s be 10,000m/s and the second rocket's 12,000m/s?

Regardless of mass to fuel ratio?

Edited by Raider5678

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12 minutes ago, Raider5678 said:

Wait, doesn't fuel efficiency play a role in it?

In rocketry Specific Impulse of an engine can be used as a proxy for "fuel efficiency", but it directly depends on the exhaust velocity. The higher the velocity of stuff that comes out of the back end, the less of it you need to use to achieve the same delta-v. And exhaust velocity is already there in the rocket equation.

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Just now, pavelcherepan said:

In rocketry Specific Impulse of an engine can be used as a proxy for "fuel efficiency", but it directly depends on the exhaust velocity. The higher the velocity of stuff that comes out of the back end, the less of it you need to use to achieve the same delta-v. And exhaust velocity is already there in the rocket equation.

So different rocket fuels only affect exhaust velocity, which then, in turn, affects the ISP of the rocket?

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1 minute ago, Raider5678 said:

Huh.

But now I'm confused.

The mass to fuel ratio and the exhaust velocity is the only thing that matters for the speed of a rocket?

What about deltaV? Where does that come in play?

Delta V = Vfinal  if you assume your initial velocity as being zero.  Otherwise it is the change of velocity caused by burning your fuel.

As far as fuel efficiency goes, that where exhaust velocity comes from. A more efficient fuel releases more energy per kg and thus generates higher a higher exhaust velocity. ( with a chemical rocket, the by-products from the combustion of the fuel becomes the reaction mass.)

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2 minutes ago, Janus said:

Delta V = Vfinal  if you assume your initial velocity as being zero.  Otherwise it is the change of velocity caused by burning your fuel.

As far as fuel efficiency goes, that where exhaust velocity comes from. A more efficient fuel releases more energy per kg and thus generates higher a higher exhaust velocity. ( with a chemical rocket, the by-products from the combustion of the fuel becomes the reaction mass.)

Ah, okay.

All makes sense now.

Delta V, as I thought, is the change of velocity.

However, delta V is related to ISP, which in turn is related to exhaust velocity, which in turn is directly related to how much energy per kg is released per kg.

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6 minutes ago, Raider5678 said:

So different rocket fuels only affect exhaust velocity, which then, in turn, affects the ISP of the rocket?

In a nutshell, yes. There are a lot of other considerations in play, such as TWR that's possible to reasonably achieve, resulting engine mass or how easy it is to store fuel/oxidant or whether or not fuel is extremely poisonous and/or radioactive.

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Just now, pavelcherepan said:

In a nutshell, yes. There are a lot of other considerations in play, such as TWR that's possible to reasonably achieve, resulting engine mass or how easy it is to store fuel/oxidant or whether or not fuel is extremely poisonous and/or radioactive.

Well, yeah. It's rocket science after all.

I'm one of those people who doesn't think rocket science will ever become routine.

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2 minutes ago, Raider5678 said:

I'm one of those people who doesn't think rocket science will ever become routine.

I certainly hope it does

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Just now, pavelcherepan said:

I certainly hope it does

Rocket travel, yes.

The act of designing rockets, making them safe, etc?

I just kinda doubt it.

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The obvious way to create artificial gravity in space is using a rotating space station, creating artificial gravity. Once it's rotating at the correct speed it needs no further fuel. And you don't end up millions of miles from where you started. You just spin it up, till you weigh the same as you would on Earth. Going outwards from the centre of rotation, the magnitude of the artificial gravity will increase, so you will probably put the main floor just inside the outer casing.

The only problem with it, is that the rotation causes a sickly feeling in humans because of it's effect on the inner ear. The bigger the diameter of the spinning floor, the less noticeable this is, and it's reckoned to be undetectable at diameters of more than 200 metres, and forces of 1g.

So the space station needs to be big, but you can live as normal in space, just by spinning. In some ways, it will be better than living on Earth, because if you are old or sick, you can choose to live closer to the centre, and feel lighter. But for most people, 1g would be the best environment, for normal health functions.

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6 minutes ago, mistermack said:

And you don't end up millions of miles from where you started.

Unless you want to end up millions of miles away!