# Limits Of Rocketry.... Moving Entire Worlds

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The limits of the rocket equation really show itself when large mass payloads are involved.

Like this:

For example consider this: We want to move the moon, so we put big enough rocket nozzles for the needed mass flow on the moon, and then power the mass flow propulsion with antimatter.

So the problem here is the age old problem of high thrust vs high efficiency of the engines.

Either you get high thrust with high mass flow and run out of lunar propellant fast, or move at a snail's pace by doing the ion drive way... only with antimatter since we are pushing THE MOON

This whole concept is a bit ridiculous I admit, but if a civilization has the ability to put constant acceleration engines of 1g or higher on planets to push them... I think that easily puts them pretty high on the The Tsiolkovsky Civilization Scale (earlier post thread).

Anyways, what do you think?

Edited by Spacescifi
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A quasarocket.

Edited by kerbiloid
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Two thoughts. One - you've got a bit of a structural engineering problem here. Two - a quick back-of-the-envelope calculation can be helpful to assess quite how ridiculous an idea is or, conversely, what kind of completely awesome sci-fi would be needed to make it work.

For the current example.

According to NASA, the Moon has a mass of approximately 0.073 x10^24 kg.

The force required to accelerate the Moon at 1g is therefore approximately 0.73 x 10^24 N.

Rock compressive strengths are in the range of a few hundred megapascals. The highest value I found online was for nephritic jade which apparently has a compressive strength of 400 MPa. Lets go with that for the sake of argument.

So to avoid having your rocket engines completely pulverise the Moon when they try to move it, you need to apply that force over:  0.18 x 10^16 square metres.

Assuming that area is circular (again for the sake of argument), it would have a radius of approximately 24 million metres or 24,000 km.

However, the radius of the Moon is a mere 1,738 km.

Naturally, that's an extremely crude calculation with an awful lot of approximations there, not least in deciding that the compressive strength of the Moon was 400 MPa. However, the order of magnitude difference between the end result and the radius of the Moon allows a lot of wiggle room for accommodating those approximations and does illustrate quite how ridiculous the idea is - or what kind of completely over the top pusher plate you'd need to move the Moon at 1g.

Edited by KSK
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Another interesting complication would be the gravity of the pushed object. For rockets this can obviously be ignored, but on a planetary scale it becomes a factor. If an engine's exhaust velocity is below the body's escape velocity, the exhaust would eventually fall back onto the body and achieve nothing.

For the moon, chemical rockets would be sufficient (~4km/s vs. ~2.4km/s) but with a somewhat reduced effective Isp. (Not quite sure by how much though. I guess we would have to calculate the excess velocity for that.) For a body the mass of earth, we would need something with a bit more punch Isp-wise.

In the realm of antimatter-propulsion, all of this can probably be safely ignored though.

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First you put a planet's mass in the rocket equation under "massdry", include your Isp and then see how much fuel you'll need (and of course, make sure that Vexhaust>Vescape.  I suspect you have to subtract Vescape from Vexhaust, thus killing your Isp)...

Rocketry just isn't viable for moving planets, possibly even with anti-matter.  Consider bombarding it with photons or something (so you only have to deal with the planet's mass and any fuel).

From memory, there was a Star Trek book where this was the premise (the Enterprise had to pull a planet out of tidal lock).  When you finished it you discovered that the entire book was written to (correctly) include the line "then dawn broke on the planet".

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You would need a lot of antimatter.

If you want to move a rocky planet, your best bet is to build a fusion-based rocket candle on a lightweight gas giant and use its gravity to grab the rocky planet and shuttle it to where you want to go.

Spoiler

Edited by sevenperforce
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The regolith is full of 2713Al.

Just warm a piece of it up to the fusion temperature and get 5426Fe for free like in the stars.

The jet of iron plasma will push the Moon.

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A relevant video by Isaac Arthur on the logistics of moving planets

Hope this helps.

Edited by wafflemoder
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We already have a giant chunk of reaction mass orbiting once every month if we need to move the earth.

We could use hundreds of mass drivers on small asteroids to gravitationally nudge a large asteroid into an earth-crossing orbit, timing it such that it would slowly nudge the moon into a higher orbit over time.  Repeating the process with additional larger asteroids would accelerate the process. Once the moon escaped, the network of Earth-crossing asteroids could be used to nudge it into a Venus-crossing orbit that would suck orbital velocity away from Venus on one pass and add it to Earth on the next. The moon would slowly transfer energy away from Venus and to the Earth, raising Earth's orbit.

Of course you lose tides that way, but if the situation is so dire that you  need to fly the Earth out to Jupiter then tides are probably a minor problem.

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8 minutes ago, sevenperforce said:

We already have a giant chunk of reaction mass orbiting once every month if we need to move the earth.

We could use hundreds of mass drivers on small asteroids to gravitationally nudge a large asteroid into an earth-crossing orbit, timing it such that it would slowly nudge the moon into a higher orbit over time.  Repeating the process with additional larger asteroids would accelerate the process. Once the moon escaped, the network of Earth-crossing asteroids could be used to nudge it into a Venus-crossing orbit that would suck orbital velocity away from Venus on one pass and add it to Earth on the next. The moon would slowly transfer energy away from Venus and to the Earth, raising Earth's orbit.

Of course you lose tides that way, but if the situation is so dire that you  need to fly the Earth out to Jupiter then tides are probably a minor problem.

You can get the same effect by using the moon itself as a gravity tractor for earth. This simplifies the design, and saves both the tides and venus. A large solar laser could be used to push the moon, or even the earth itself, further simplifying things and allowing for much faster timescales.

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We could mine metal oxides on the Moon, produce sitall glass out of them, make a huge orbital lens, and vaporize regolith with sunlight to make a jet.

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9 minutes ago, wafflemoder said:

You can get the same effect by using the moon itself as a gravity tractor for earth. This simplifies the design, and saves both the tides and venus. A large solar laser could be used to push the moon, or even the earth itself, further simplifying things and allowing for much faster timescales.

I think momentum transfer is much faster.

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20 hours ago, sevenperforce said:

We already have a giant chunk of reaction mass orbiting once every month if we need to move the earth.

We could use hundreds of mass drivers on small asteroids to gravitationally nudge a large asteroid into an earth-crossing orbit, timing it such that it would slowly nudge the moon into a higher orbit over time.  Repeating the process with additional larger asteroids would accelerate the process. Once the moon escaped, the network of Earth-crossing asteroids could be used to nudge it into a Venus-crossing orbit that would suck orbital velocity away from Venus on one pass and add it to Earth on the next. The moon would slowly transfer energy away from Venus and to the Earth, raising Earth's orbit.

Of course you lose tides that way, but if the situation is so dire that you  need to fly the Earth out to Jupiter then tides are probably a minor problem.

20 hours ago, wafflemoder said:

You can get the same effect by using the moon itself as a gravity tractor for earth. This simplifies the design, and saves both the tides and venus. A large solar laser could be used to push the moon, or even the earth itself, further simplifying things and allowing for much faster timescales.

Well the other option is a giant pusher plate and AM bombs, but that is only marginally better.

I think laser ablation boosting stations from the moon are an optimal way for space ships to fly fastest.

Since if it can move a planet....

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25 minutes ago, Spacescifi said:

Well the other option is a giant pusher plate and AM bombs, but that is only marginally better.

If you are pushing the whole planet you don't need a pusher plate.

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1 hour ago, sevenperforce said:

If you are pushing the whole planet you don't need a pusher plate.

+1

As the Earth is flat, it's a pusher plate itself.

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3 minutes ago, kerbiloid said:

As the Earth is flat [...]

Alternative plan: Produce 300 megatons of space turtle bait and hitch a ride.

On 5/4/2020 at 3:52 PM, wumpus said:

I suspect you have to subtract Vescape from Vexhaust, thus killing your Isp

I'm not a 100% sure, but I think instead of a simple substraction, you would need to calculate the hyperbolic excess velocity, which would come out somewhat more favourable. The faster your exhaust leaves the gravity field, the less affected it would be by it.

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1 hour ago, Piscator said:

I'm not a 100% sure, but I think instead of a simple substraction, you would need to calculate the hyperbolic excess velocity, which would come out somewhat more favourable. The faster your exhaust leaves the gravity field, the less affected it would be by it.

You are correct -- very astute.

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5 hours ago, sevenperforce said:

If you are pushing the whole planet you don't need a pusher plate.

No matter how much you may have liked the show, there was no way Space 1999 could have worked like that (even had it been a nuclear ammunition dump instead of a waste disposal site).

This may help.

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16 hours ago, sevenperforce said:

If you are pushing the whole planet you don't need a pusher plate.

Yes, you certaily need a pusher plate. As KAK already told, planets are very much like liquid droplets. Their structural strength is negligible compared to their mass and internal forces. You can no bolt technomagical superengine to bedrock and give thrust.

Energy is another issue. If you want to make some major rearrangement of planets in Solar system you need tens of km/s dv. Kinetic energy is comparable to gravitational binding energy of planetary sized bodies. For example binding energy of Earth is about 2E32 J. It corresponds to velocity of 8.2 km/s. If you think for example saving earth when the Sun becomes a red giant, you need more. It is known that energy efficiency of rockets are very poor, and the higher ISP the poorer efficiency. Even we can assume unlimited energy source (which is not trivial in such enormous energy levels),  we must also have technology which radiate loss energy to space instead of vaporizing the transported planet.

For example bomb type solutions use very large part of energy to heat pusher plate. They would certainly not work even pusher would be larger than Earth. I think you have to assume violation of conservation laws in any case or accept that operation takes hundreds of millions of years.

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9 hours ago, Hannu2 said:

Yes, you certaily need a pusher plate. As KAK already told, planets are very much like liquid droplets. Their structural strength is negligible compared to their mass and internal forces. You can no bolt technomagical superengine to bedrock and give thrust.

Adding a pusher plate does nothing. Materials on that scale wouldn't even work. It's precisely because the planet is a liquid droplet that you can use an entire continent as your pusher plate and the mantle as your shock absorber, if that's your aim.

9 hours ago, Hannu2 said:

For example bomb type solutions use very large part of energy to heat pusher plate. They would certainly not work even pusher would be larger than Earth. I think you have to assume violation of conservation laws in any case or accept that operation takes hundreds of millions of years.

That's why you use the ablative properties of your pusher plate: https://what-if.xkcd.com/13/

Seriously, the only way to actuall do anything like this is to use successive gravitational flybys. Gravity works well for that.

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