Solar Super-Sail Could Reach Mars in a Month

ti-coune sent us a story running on newscientist describing solar super sails and how they could one day get us to Mars in a month. The key is a special new paint. The cast of Trading Spaces is unavailable for comment.

Comments

Re:Cast? What cast?

[the_mad_poster]

Yea, I imagine carbon monoxide poisoning is probably the biggest issue facing unprotected free-floating humans in space.

Re:In related news

[Frymaster]

The invention of a warp drive... could get us there immediately

man, if you have a time machine, words like "immediately" don't really mean anything.

Re:Cast? What cast?

[Jonathan_S]

"The feat would require a 60-megawatt microwave beam with a similar diameter to the sail. It would also have to be capable of tracking the craft as it accelerated away. But this power level could not be delivered by any existing microwave transmission system. The deep-space communications network that NASA uses to communicate with Mars rovers and the Cassini probe now orbiting Saturn can only manage half a megawatt. The Benfords say the power could be ramped up in future and hope to persuade NASA to consider doing this as part of a future upgrade to the network.

So basically NASA's currently-used equipment is 1/120th of the power needed to get this sail to Mars. I would say this idea is not in our near future for sure.

Um, NASA's current communications system puts out 1/120th of the power needed for this sail.

That's like saying since your cell phone can only put out 1/2 a watt it's impossible to heat things in your microwave.

A couple of points of reference, the radar mounted on US Aegis cruisers can put out 4 MWs and the stationary Cobra Dane early warning radar that went online in 1977 puts out 15.4 MW.

I don't think we are that far away from building a 60 MW transmitter now that we have a reason to.

Why this is not helpful; other useful technologies

[Dr. Zowie]

The sail is being used as a rocket engine. That is not too helpful, as the exhaust speed of the paint is still limited by chemical bonds (it's hard to make individual paint molecules leave with more kinetic energy than is contained in a chemical bond). That means you would need just as much propellant (in the form of paint) for this scheme as you would need for a normal chemical rocket engine.

These guys are definitely on an interesting track, though. The problem with rocket engines in general is that they have a tradeoff between mass efficiency (you want to put as much momentum on each piece your propellant as possible, so that you get as much push from it as possible) and energy efficiency (it costs energy to push propellant, and you have to supply the energy).

Chemical rockets can't get much more efficient than the Space Shuttle Main Engines, because the amount of energy available for each molecule of exhaust gas is whatever you can get by chemically reacting your fuel to make the propellant molecule. The SSMEs use one of the most energetic-per-unit-mass chemical reactions around: hydrogen and oxygen (fuels) combining to make water (propellant).

Electric ion rockets do better because each molecule of propellant gets much more energy than would be available from chemical reactions. The problem there is that you still have to produce the energy. Nuclear electric propulsion uses plutonium to generate heat, which is converted to electricity and then used to run the ion rocket. Solar electric propulsion uses solar panels to generate electricity that runs the ion rocket. The problem is that both of those schemes are limited by the power available: it's hard to make energy rapidly with either a conventional radiothermal (noncritical) generator or solar panels, so while the rocket is extremely fuel efficient it is also quite slow.

Pure solar sails use the best/worst propellant in the Universe: photons. Best, because photons are disposable -- "use all you want, we'll make more!". Worst, because photons use the most energy per unit delivered momentum of any propellant in the universe. So a sail transduces huge amounts of power (at least in the inner solar system) but uses a very inefficient process to convert that energy to momentum.

Making the sail into a hybrid rocket is a Good thing, but using this paint scheme doesn't help, because the ejected molecules don't ever get much more energy than their own chemical binding energy into the paint -- that means they're being more or less wasted as propellant, because you want to put as much kinetic energy on the propellant as possible.

A better scheme is to use a curved solar sail as a concentrator to heat up a high power electrical generator, and then use the electricity to drive an ion rocket. In 2000 or 2001 I and a colleague worked up the numbers for such a scheme (there are technical problems with making high-power ion rockets; but we considered just energy flow). A smallish curved solar sail (say, 120m in diameter) can concentrate 10 megawatts of heat onto a heat collector. At 10% conversion efficiency to propellant power (15% for conversion to electricity, times 67% efficiency in the rocket engine) that would still be a megawatt of power, enough to provide hundreds or thousands of Newtons of thrust. In several scenarios we considered, the acceleration of the whole craft is higher than the unloaded self-acceleration of the sail, so it would be necessary to repel the sail electrostatically or something like that to keep its shape correct.

Ion rockets can be 100 to 1000 times more propellant-efficient than chemical rockets, provided that there is enough energy available.