Art project involving the moon

[davebreak]

Hey rocket folks -

In light of those artists that deal with massive art, like covering an outdooors area with cloth and the like I thought it would be interesting to propose a world shaking art project. Yes, I know this would never fly with the world and it might cause problems with the tide and the moon itself but I wonder - could it be done with current technology, what would be the best approach and what are the forces involved?

Background: The moon rotates about once per 28 days, which is why it always shows one side to the earth. It used to rotate at a higher rate but "frictional effects associated with tidal deformations caused by the Earth [??!]" has slowed it so one side points toward the earth.

My idea is to get the moon rotating again with respect to the earth. It would be quite the sight to see it up there spinning. Fast enough that you could see it move slowly around.

Proposed solution is to strap rockets of some sort to the surface, turning them on and off to start a slow rocking process until finally one of the blasts starts a complete rotation. This would probably require nuclear powered blasts. Assume a homogeneous spherical mass - 3500 km in diameter and 7.3 x 10e22 kg.

1. What are these "fricitional effects" and how large are they?

2. Could we cause the moon to begin to rotate again using current technology (and infinite supplies of money)?

3. Is the proposed solution (nuclear powered rockets, on and off) the best approach?

4. What are the forces involved? Could this be done with just a few rockets? In theory a few pop bottle rockets shold do the job but that doesn't seem reasonable.

 

[DaveC426913]

I was about to suggest that rocking was not necessary but it's possible the Moon is held in place by not being truly spherical. I'm not sure what force is required to overcome that. It could be great (when you compare it to the man-made propulsion you'll be using).

You will want to ensure you pair your rockets on both sides of the Moon, pointing in opposite directions. You don't want to impart any translational motion upon the Moon or you will affect the altitude of its orbit.

Also, don't use rockets. All you need is the reaction and a way to direct it. Figure out way of using the existent minerals of the Moon to make a reaction in a subsurface cavern (worst case, you'll have to import the fuel). Direct that reaction tangential to the surface via an appropriately-facing tunnel.

After you get one working, just start replicating the engine everywhere (near the equator, anyway). Make them in antipodal pairs and fire each pair up as it is completed.

 

[davebreak]

Ah so. I hadn't thought of translational forces.

 

[AlephZero]

Also, don't use rockets. All you need is the reaction and a way to direct it. Figure out way of using the existent minerals of the Moon to make a reaction in a subsurface cavern (worst case, you'll have to import the fuel). Direct that reaction tangential to the surface via an appropriately-facing tunnel.

This won't work unless
(1) you vaporize the material before you eject it, or
(2) you eject it faster than the moon's escape velocity.

(Hint: what happens to the angular momentum if solid material lands back on the lunar surface?)

 

[DaveC426913]

This won't work unless
(1) you vaporize the material before you eject it, or
(2) you eject it faster than the moon's escape velocity.

(Hint: what happens to the angular momentum if solid material lands back on the lunar surface?)

Not outside our ability.

Some typical values of the exhaust gas velocity Ve for rocket engines burning various propellants are:

1.7 to 2.9 km/s (3800 to 6500 mi/h) for liquid monopropellants
2.9 to 4.5 km/s (6500 to 10100 mi/h) for liquid bipropellants
2.1 to 3.2 km/s (4700 to 7200 mi/h) for solid propellants

http://en.wikipedia.org/wiki/Rocket_engine_nozzle

Moon's escape velocity is only 2.38km/s.

 

[K^2]

So what, nobody's bothered by the fact that the moon has moment of inertia something like 10^32 kg*m² and that it'd take something like 5x10^23 J to get it to spin at 1 revolution per day? That's at 100% efficiency. For comparison, the largest hydrogen bomb ever detonated released only 2x10^17 J of energy. Or roughly, 2.5 million times too weak.

 

[DaveC426913]

So what, nobody's bothered by the fact that the moon has moment of inertia something like 10^32 kg*m² and that it'd take something like 5x10^23 J to get it to spin at 1 revolution per day? That's at 100% efficiency. For comparison, the largest hydrogen bomb ever detonated released only 2x10^17 J of energy. Or roughly, 2.5 million times too weak.

It's an engineering problem. How many rockets burning for how long?

Budget is stated as unlimited.

 

[K^2]

You can have an unlimited budget, but the energy supply is limited. The above figure exceeds all known fuel reserves on this planet by orders of magnitude. Suggestions?

 

[DaveC426913]

You can have an unlimited budget, but the energy supply is limited. The above figure exceeds all known fuel reserves on this planet by orders of magnitude. Suggestions?

Extract the fuel from the Moon itself. You don't have to worry about making a self-contained engine (such as a vehicle), or transporting anything anywhere. You don't have to worry about pollution, ecology or hazardous materials.

If you can figure out a way to combine elements of the Moon into an exothermic reaction, you can scale it up as-needed right in-place. It's not at all like a typical powerplant - it is vastly simplified. Even what we use for fuel does not have to be typical. Again, no tanks, containment or cooling needed.

At its simplest:
- dig a tunnel into the regolith at a shallow angle
- feed reactants into it (if we choose the location well, the rubble might contain the reactants)
- ignite it

Ideally, if we could find veins of a useful reactant, we'd just make our cavern there. The walls of the cavern would act as both combustion chamber and fuel source - like the solid fuel boosters for the shuttle do.

The key question is: what abundant materials on/in the Moon could be combined to create an exothermic reaction? And do they possibly exist in concentrations?

 

[AlephZero]

The key question is: what abundant materials on/in the Moon could be combined to create an exothermic reaction? And do they possibly exist in concentrations?

I can't find "green cheese" in my materials database

But seriously, if there were any easy-to-start exothermic reactions, I guess the impacts that formed the huge craters would have already used up most of the reactants. If the moon was made of rocket fuel, it wouldn't still be there.

 

[DaveC426913]

I can't find "green cheese" in my materials database

But seriously, if there were any easy-to-start exothermic reactions, I guess the impacts that formed the huge craters would have already used up most of the reactants. If the moon was made of rocket fuel, it wouldn't still be there.

Yeah. I'm realizing it is not a "simple" task to simply combine some abundant materials on the Moon. The Moon has been dead for a long time, likely most of the interesting elements are not near the surface like with volcanically active Earth.

 

[davebreak]

Ok, so I think I have the answer to one of the questions -What are these "fricitional effects"

The reason the moon settled facing us is that when it rotates w/r/t the Earth, it compresses a little and gets out of round due to the gravity exerted on one side by it's proximity to the earth. The Earth does this too but it shows up primarily as tides. This compression and decompression of the moon's mass causes frictional heat, much like when a ball is bounced.

The energy to do this comes from the rotatational energy of the moon. The "lowest energy" state is when one side of the moon faces the Earth all the time, since the compression and decompression stops completely at that point.

Therefore, in order to get the moon rotating again we have to overcome this plus add whatever additional forces we need to increase the rotational velocity.

 

[DaveC426913]

The reason the moon settled facing us is that when it rotates w/r/t the Earth, it compresses a little and gets out of round due to the gravity exerted on one side by it's proximity to the earth. The Earth does this too but it shows up primarily as tides. This compression and decompression of the moon's mass causes frictional heat, much like when a ball is bounced.

Yes. This is what I was alluding to in post 2 but I didn't bother going into details. I don't know how distorted the Moon is or what magnitude of force keeps it one-face.