Google Lunar X Prize

For a short while, I was on a team competing for the Google Lunar X Prize. Unfortunately circumstances conspired against us, but I'd like to share some of my thoughts on the difficulties involved in the prize.

The basics of the prize are that your team designs a lunar rover that can drive around on the moon and take video. Very possible - but you only win if your vehicle actually gets to the moon. The prize is about the rover - it doesn't matter how you get to the moon. Most teams were planning on tagging along on either a NASA or Russian flight.

To me, the primary issue is that the activity that a team undertakes to attempt to win the prize is a tiny percentage of the project effort, and the team controls almost none of the failure risk. Most of the challenge in driving a rover on the moon is in getting to the moon, not the rover! Getting to the moon costs billions in development for the transit vehicles - even the incremental costs are in the hundreds of millions.

It just seems a little suboptimal to have a prize that awards a team for getting a government space program to select your rover as additional payload on a government mission, rather than anything that an average team has any control over.

Poll results - How much would you spend to visit space?

Here is how readers have responded to "How much would you spend to visit space?"

33% Six month's pay
66% Only if I won the lottery

Nano-Satellite Launch Challenge

This is the most exciting news I've seen out of NASA in quite a while! This is right in line with what we have been planning.

Details:

Objective: to place a small satellite into Earth orbit, twice in one week.

PRIZE PURSE: $2 Million

Satellite mass - at least 1 kg
Satellite dimensions - at least 10 cm cube

Must complete at least one Earth orbit. Task must be accomplished twice in one week.

Purpose: To stimulate innovations in launch technology and to encourage creation of commercial nano-sat delivery services.

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!

Quick Update

Just a quick update, since we haven't said anything for a while...

We have been working on the business side of things for a bit: hiring people, seeking additional funding, that sort of thing. We have also simplified the engine design dramatically while keeping all the good features - hopefully we will be able to discuss that soon as well. We're hoping to have some engine tests in the coming months, making flames and shock diamonds!

More discussion on Selenian Boondocks

There is another good discussion going on at Selenian Boondocks.

Ribbon propellant discussion

There is a discussion of a few ribbon propellant usage options at Selenian Boondocks that may be of interest.

I've been meaning to say more about a few points raised by our contest entrants, but have not really had the time... so I will quickly touch a few points here. Both entrants discussed many good points about the properties of ribbon propellant systems that I will be skipping over.

Mr. Blake raised some good points about the difficulty of sealing the propellant feed system. This is definitely something that has to be addressed in your injector design. He slightly overstates the problem, however, when he says that any leakage is completely wasted. By aiming the leaks backwards, you can still get a significant proportion of the impulse even of the leaked gases. (Of course, good seals are even better.)

Mr. Cate's first concern is about throttling. He correctly states that deep throttling of liquid rockets takes some effort. Solids operate through a very different mechanism, however. Solid rockets can typically be throttled very easily, by customizing the burn profile. This is really a solved problem for solids, and we get to keep most of the advantages solids have in this regard.

Mr. Cate also took issue with the expectation of lift from the ribbon. To be fair, we were never really looking at using the lift generated by the ribbon for many up the reasons he states, but he does overstate the issue somewhat when he postulates that a ribbon provides no lift. In fact, it is relatively straightforward to experimentally prove this. Just get a thin ribbon, and dangle it in front of a fan. Instead of hanging down, it stretches out, blown by the wind. The key is that it is mostly still in a line - so the lift is manifestly spread out over the length of the ribbon. Ribbons produce lift in the same way that the Chines of the SR-71 do, by generating vortices.

I have previously addressed the drag issues in this blog, so I will skip over them here. Flutter is something that will be addressed - at this point, all I can really say is that it is a fairly well understood aerodynamic issue.

I have also addressed the injector pump mass and power earlier. The injector power and mass are actually lower than a comparable liquid rocket.

More to come later!