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

Fuel

[MyLongNickName]

It is very cost benefitial to not have to take fuel with you... or at least not as much.

My question is, what kind of payload is practical with this kind of thing? I've always read that to get any kind of larger payload, you cannot use solar sails. Do they get around this by using the microwave beam they talked about (ie higher energy per square meter)? I wish there were more numbers in the article...

Re:Fuel

[ThosLives]

Indeed. 60 kps is way past escape velocity for the solar system; from Earth mean orbit, solar escape velocity is only 42kps (give or take)!

Incidentally, I want to see their 60 kps calculation ... that's a huge momentum change over an hour! The average acceleration for 60000 m/s in 3600 seconds is: ~16.7 m/s2. I don't know about you, but methinks they got a decimal point wrong. Or their entire ship was made of aerogel and has very little mass...

Here's some fun math: 60 MW of (1mm/300GHz) microwaves will carry a momentum of 0.2 N; if the sail absorbs all of the photons, the force would be 0.2 N; if it reflects them completely the force would be 0.4 N. To get an acceleration of 16.7 m/s2 you need a force of 16.7 N per kilogram. All this says is that they're getting a lot of force from breaking the chemical bonds in the paint. Kind of like burning fuel.

Show my work: Energy per photon is h*f where h is plank's constant and f is the frequency. For 300 GHz microwaves, f = 3e11 Hz and h is always 6.626e-34 J.s; each photon has ~2e-22 J. 60 MW means you have ~3e29 photons per second. Momentum per photon is p = h/w, where w is the wavelength (1e-3 m), so each photon has a momentum of ~6.6e-31 N.s. 3e29 photons per second of 1 mm microwaves have a momentum flux of 0.2 N.

Then what?

[FroBugg]

So you're going faster than any interplanetary craft to date, and your only propulsion system requires you to be moving away from the sun (or the Earth, if they're using a laser to push you).

How do you slow down? Orbital insertion at that speed would be seriously difficult, if not impossible.

Re:Cast? What cast?

[paranode]

The idea is actually pretty good, but the problem is the technology is not there to support it yet. The most notable obstacle from the article:

"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.

Re:Why pipe microwaves from the surface?

[revscat]

Wouldn't it be far wiser to build solar panels in orbit, use them to power Microwaves, and avoid the attenuation in the atmosphere?

Well, from TFA:

The feat would require a 60-megawatt microwave beam with a similar diameter to the sail.

Now, I'm no EE, so somebody please correct me if I'm wrong, but I'm thinking that generating that much power from solar cells would be an undertaking in and of itself. You'd be hard pressed to generate that much energy in space.

Re:Why pipe microwaves from the surface?

[saigon_from_europe]

Now, I'm no EE, so somebody please correct me if I'm wrong, but I'm thinking that generating that much power from solar cells would be an undertaking in and of itself. You'd be hard pressed to generate that much energy in space.

It seems feasable.

I am EE, but I don't know what is the value of Sun's radiation. IIRC, it is about 400W/m^2 on Earth, so it is in worst case the same in orbit. With cell's efficiency of 25%, we get about 100W/m2, so for 60,000,000W we need only 600,000m2. It is sqare with side of 775m - not something too complicated to build even on our current technological level.

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.