Tuesday, March 2, 2010

Moon Ice Water Rocket

NASA recently announced the discovery of vast amounts of water ice on the moon. If you are a lunar geologist, it is extremely exciting news - but most people don't see a way to take advantage of it in the short term. Cracking ice into rocket fuel is just too hard, and will take too much infrastructure.

But there is another way to use the ice - a simple water rocket!

From the moon's surface to lunar orbit takes an impulse of about 2 km/s. Going all the way back to the Earth takes about 3 km/s. Really good water rockets can get about 1000 m/s exhaust velocity, or an Isp of 100 seconds or so. Using the rocket equation:

delta-v=(exhaust velocity)*ln(mass ratio)

The required mass ratios are 7.5 to lunar orbit, or 20 to Earth return. That means that for every pound lifted off the moon, you would need 7.5-20 pounds of water melted from the ice. Gathering this is a lot more manageable than setting up a chemical plant on the moon! It could be as simple as smashing the ice using heated tools, and dumping it into a slightly heated hopper.

The engine is really just a water pump. There are many pump designs that can pump dirty water, so there is no need to even filter the "mined" ice. The pump's power is related to the flow rate and pressure required. For example, a 200 atmosphere pump that lifts 1 ton would need to pump 0.01 cubic meters of water a second. This would require only 200 kilowatts of power, while typical rocket engines generate megawatts or even gigawatts!

You can almost certainly do better than that, though. If you start with a normal rocket engine, and inject water near the throat you can get higher Isp at much lower pressures. Just using 5-10% normal rocket propellant in your mass flow provides enough energy to completely vaporize the water, greatly increasing Isp.

Each pound landed on the moon takes over 50 pounds of propellant to get it there. It costs about $10,000 or so per pound delivered to lunar orbit. So not having to bring your return propellant with you is a big deal!

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