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danielravennest t1_iv5ewq3 wrote

Reply to comment by stewartm0205 in Researchers Make Rocket Fuel Using Actual Regolith From the Moon by The_Weekend_Baker

That has pretty crappy performance.

The dark areas of the Moon (Maria) are made of basalt. Basalt fiber is only a little less strong than carbon fiber. Make an orbiting cable from the fiber and just pick up stuff from the Moon. The cable's orbit can be maintained using sunlight and electric propulsion. Oxygen extracted from lunar rock can be the fuel. Performance would be ten times higher than good rocket fuel and fifty times better than trying to burn oxygen with regolith slag.

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stewartm0205 t1_ivdazb0 wrote

A mass driver would be simpler and cheaper. You could fling cargo all the way to earth or Mars.

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danielravennest t1_iveia8n wrote

Electromagnetic catapults (mass drivers) are high peak power devices, since they have to do all the acceleration in a fraction of a second. Therefore they need a big power supply.

For example, getting to lunar orbit velocity over a 300 meter device takes 0.35 seconds. A 4 kg payload needs 5.78 MJ of kinetic energy, therefore 16.5 MW of power at perfect efficiency. If the mass driver is less than 100% efficient, input power needs to be higher.

A mechanical catapult with a rotating arm and electric motor can take as long as needed to come up to speed if it is operating in a vacuum. If you have an hour, the peak power is then 1.6 kW for the same payload. So it is better suited to low annual tonnage.

The original mass driver idea was for building space colonies, and launching half a million tons of material a year. At that kind of traffic rate you can justify the big power supply.

The orbiting cable approach doesn't put high g-forces on things like the other two. So you can carry people and complex equipment up and down. The catapults are suited for bulk materials. So which is better depends on what you are trying to do.

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stewartm0205 t1_ivvqxxz wrote

You can store the energy in capacitors or fly wheels. You biggest cost is the cost of shipping mass to the moon. You want to keep the mass of whatever you build to the minimum.

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danielravennest t1_ivz0io8 wrote

A rotating arm is a flywheel, just a straight rather than circular one.

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stewartm0205 t1_iwab8dd wrote

Have you calculate how heavy it would have to be?

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danielravennest t1_iwbr3xx wrote

For the Moon, the arm is about 6-8 times the payload mass. A complete system with drive motor and solar array is heavier. The figure of merit for these systems is the "Mass return ratio". That's how many tons of material delivered over their operating life, divided by tons of mining equipment, including catapults.

For a full discussion, see the Lunar Catapults section of my Space Systems Engineering book.

To verify authorship, click the View History tab on any page of the book and see the edits have the same user name as I have here.

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stewartm0205 t1_iwinued wrote

Total mass over lifetime. I would have liked to have seen a similar calculation for the linear system.

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danielravennest t1_iwlb1a7 wrote

The original space colony work by Gerard O'Neill and others assumed a "Mass Driver" (coilgun) which launched 4 kg lumps of lunar soil at 4 times a second, which amounts to 500,000 tons per year.

The gun would be 300 meters long and each payload takes 1/3 of a second to get up to speed. That means slightly more than one payload is in the gun at a time, and a near steady-state power supply.

Assuming perfect efficiency of the gun, it then requires 32 MW of constant electric power. With more realistic efficiency and running other stuff for mining or packaging the payloads, you are looking at 40 MW of power.

For comparison, this is in the range of a naval nuclear reactor, except you don't have the ocean to dump waste heat to. Current NASA work is towards 10-30 kW electric reactors for the Lunar suface. So factor of 1000 too small.

But such an electric catapult still needs 24 MW of peak power for launching single payloads at a lower rate. That's because all the acceleration happens over 1/3 of a second. So you need some kind of storage if your power source is smaller, and then release it in a burst.

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stewartm0205 t1_iwnntqj wrote

Solar panels and super capacitors should be enough.

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