Ion Propulsion Flies Again — When Will Sails?

by Paul Gilster on September 28, 2007

The launch of the Dawn mission to the asteroids makes me think about solar sails. I realize that Dawn uses ion propulsion, about which more in a moment, but watching ion methods as they mature makes an emphatic point: We need to bring solar sail technologies up to the same readiness level that ion propulsion currently enjoys. And we need to be shaking out sail ideas in space. The Russian Znamya attempts at a ‘space mirror’ were attached to a Progress supply ship, and interesting mostly in terms of their deployment problems, leaving the 2004 Japanese test of reflective sails in space as the only free-flying experiments I know about.

Which is not to say I’m a skeptic about ion propulsion. It will be fascinating to follow the performance of Dawn’s engines as the mission progresses. 54 feet of solar array produce the needed power to ionize their onboard xenon gas, which is four times heavier than air. The ions are then electrically acccelerated and emitted as exhaust from the spacecraft. The result is an engine of great utility over time, as JPL’s Keyur Patel notes:

“Each of our three ion engines weighs in at 20 pounds and is about the size of a basketball. From such a little engine you can get this blue beam of rocket exhaust that shoots out at 89,000 miles per hour. The fuel efficiency of an ion engine is an order of a magnitude higher than chemical rockets and can reduce the mass of fuel onboard a spacecraft up to 90 percent. It is a remarkable system.”

Remarkable indeed. Dawn’s engines will accumulate 2,000 days of operation during the course of its eight-year investigations, pushing the vehicle with about the same amount of thrust as the weight of a piece of paper in your hand. Days and months of thrusting add up, giving the vehicle an effective change in speed of about 37,000 kph by the end of its mission.

And ponder this. A chemical rocket’s exhaust speed is limited by thermal issues — the rocket nozzle has to be able to bear up to the temperatures involved. The ion thruster’s top exhaust speed is limited instead by applied voltage. Ion propulsion’s future seems bright — the greater the exhaust speed, the more efficient the engine. Dawn needed a boost from a Delta II Heavy to get off the Earth’s surface, but once in space the long push for the asteroids is remarkably stingy on fuel, delivering ten times as much thrust per kilogram as chemical rockets. No wonder JPL has taken to calling Dawn the Prius of space. Ion engines are ingenious and space-tested devices (Deep Space 1 was an early proof of concept, and the SMART-1 lunar mission again validated the basic ion design).

But if something about an ion engine’s gentle push rings a bell, you’re circling, like me, back to the similar situation with solar sails. The momentum imparted by photons falling on a solar sail delivers a vanishingly small push. A sail one-third of a mile square in Earth orbit would be pushed to no more than ten miles per hour in its first hour of operation. The solar flux at that distance is nine orders of magnitude weaker than the force of the wind on the Earth’s surface, so you can see the reason why.

Like ion engines, the speed of solar sails can increase bit by bit, day by day, and unlike them, sails carry no propellant whatsoever. As long as sunlight is plentiful (out to about 5 AU), sails should make an efficient alternative to ion engines, particularly in explorations of the asteroids. And while the continued development of ion propulsion is heartening, it shouldn’t be forgotten that sail strategies continue to be carefully studied. We need to find budgetary help for the work that will lead to the deployment of demonstrator missions. A sail in space will teach us much more about this potentially paradigm-changing form of propulsion.

{ 10 comments }

andy September 28, 2007 at 17:58

The annoying thing is that it seems there are numerous cheap ways to get around the solar system, but only once you’ve got out of the Earth’s gravity well.

So while all these technologies are definitely interesting, they don’t particularly grab me. It’s the getting into orbit part that’s the bottleneck and the real challenge. As I understand it, a space elevator on Earth is right at the limit of the theoretical maximum tensile strengths achievable, and as someone who’s a bit of a pessimist when it comes to futurism, my suspicion is that it is an impossible dream.

Which may well mean the overwhelming majority of us are stuck here.

Enzo September 28, 2007 at 22:34

Yes, the Dawn mission is exciting, but another adjective comes to mind too : excruciating. Excruciating is the wait for it to reach Ceres (which seems to be far more interesting than Vesta) . Yes, I know it’s either like this or just a quick fly-by but that’s exactly the point. By now we should be having probes going around the solar system with propulsion systems such as Mini Mag Orion. Things that can deliver large payloads quickly and even take them back. Instead these things are not even on the drawing board. A paper here and there and it all goes to sleep again for another 20 years. As far as I know there’s not even an half plan on when to build one. Even Prometeus (which is not really what I’m talking about here) but at least is some sort of start got canceled right away.
Sorry for the whine, but I see ionic propulsion only as scientists making the most of a bad situation, not as a real breakthrough.

Enzo

J. R. September 29, 2007 at 3:07

I see ion propulsion making huge breakthroughs in the somewhat distant future – 50+ years. Maybe in a few years they will have the thrust of ten sheets of paper. :)

I agree that propulsion systems like the Mini Mag Orion has the most potential for today’s technologies. It boggles the mind to think that if they took this seriously, there could be manned missions all over the solar system within 20 years.

By the way, whatever happened to all the hype about single stage to orbit technologies that they were talking about 10 – 15 years ago. Seems to me that they are still not using best available and lowest cost technology to even get out there. I guess that’s what you get when things are politically motivated.

Robin Goodfellow September 29, 2007 at 3:26

Electric propulsion has the substantial advantage that commercial spaceflight (orbital satellites) have developed and matured the technology over several decades. The transition to using ion engines for deep space missions was not that great of a technology or tooling shift and thus not that great a risk. Despite this it’s still taken quite a lot of effort to evangelize ion engine technology for deep space missions. Several deep space missions have used electric propulsion successfully already (Deep Space-1, SMART-1, Hayabusa), now Dawn adds yet another notch to that tick list. Given Dawn’s likelihood of being a high-return, trailblazing science mission it should do a lot to fully legitimize electric propulsion as a reliable means of getting around the inner Solar System.

Solar sails face a much more difficult battle for legitimacy and development. The technology itself is much less mature and thus the risk is much higher. Worse yet, because spaceflight outside of Earth orbit is, to date, entirely non-commercial there isn’t much motive other than sheer intellectual curiosity and engineering chutzpah to develop the technology. The real driver which could lead to a blooming of solar sailing development would be dramatically lower launch costs (which would enable entrepreneurs and adventurers to get their foot in the door).

Adam September 29, 2007 at 3:38

Hi All

Just to answer J.R.’s question about SSTO – basically it was at the bleeding edge of aerospace technology, and after blowing billions on trying to do it basically it got canned because we just don’t have the tech. Quite simply the only practical SSTO designs have TWO stages.

Which NASA should’ve realised after all the studies of the Shuttle concept in the early 1970s.

SCRAMjets might make SSTO viable, but they’re currently still in development – they proved a lot harder too. But at least we know they can work. And SSTO is relatively easy to do IF you don’t come back down. Thus One-Shot SSTO for bulk cargo is pretty straight forward. But there’s no bulk-cargo missions on offer at the moment.

Robin Goodfellow September 29, 2007 at 5:37

Adam, don’t fall in to NASA’s Wile E. Coyote engineering approach, just because NASA made an attempt at it and failed once does not mean that the whole idea is worthless in general. In fact, SSTO technology is almost certainly feasible in the very near-term without excessive R&D. As is fully reusable launch vehicle technology. However, the combination of these two is probably a ways off. NASA tried to jump over several generations of development, and tried to do so without sufficient funding, purpose, or organization, and failed tremendously. This doesn’t mean that dramatically lower orbital launch costs aren’t within relatively easy grasp. It just means that the establishment doesn’t have the smarts or the guts to reach in the right direction.

As for SCRAMjets, those are really a dead end for launch services. They have precisely the wrong characteristics for launch vehicles (they operate over too narrow a band of the atmosphere and too narrow a band of speed).

NASA would have you believe, as a salve for their many, many failures, that spaceflight is a fundamentally enormously difficult problem that requires wizbang futuristic technology to truly take in hand. The truth is that we have all the technology we need to get into space much more cheaply than we can today, it’s just that the blundering government agencies and the entrenched, bloated aerospace oligopoly has made it difficult for legitimate innovation and entrepreneurism to gain a foothold. That may be changing, ever so slowly, at the moment and I think we’ll see a flowering of development in launch services in the not too distant future. Look to SpaceShip-1 as an example. For a fraction of the cost and with a much, much simpler design that team was able to reproduce a lot of the relevant capability of the X-15, and it didn’t take space age carbon-carbon composites or state-of-the-art glass cockpits and fly-by-wire systems or SCRAMjets or aerospike engines, it took little more than a fiberglass hull, rubber and NO2, and a stick and rudder. These are the things the new space age will be made of. Solid no-nonsense design and guts, not buck rogers bells and whistles and fancy powerpoint slides.

Adam September 29, 2007 at 7:53

Hi Robin,
I’m well aware of the alternatives. But at the same time a 100 km sub-orbital flight isn’t an orbital return mission. Sure big NO2/rubber boosters might loft an orbital vehicle, but that vehicle had better be building on NASA/USAF experience or else it’ll burn-up.

I’d like to believe it’s easier than NASA’s “everything-and-the-toilet-sink” approach, but no one has demonstrated that it is yet either. When will an “amateur” effort launch a satellite? And don’t tell me SpaceX almost did – they’ve brain-picked NASA’s best and there’s nothing simple about their rockets either.

Once a few rocket-planes have spanned continents then we’ll see. But 100 km joy-rides aren’t space-flight. Not yet.

Darnell Clayton September 29, 2007 at 18:19

To get back on topic, solar sails seem to offer a lot of promise, however it seems that from descriptions of the sails (from weblogs that I’ve read) that they seem to be very fragile.

Has anyone figured out how to not only have them survive the rocket ride up but also open up without tearing?

Administrator September 29, 2007 at 21:37

Darnell, deployment is, as you correctly surmise, a major issue for solar sails. Some of the early work on the ground has developed deployment methods that have not yet been tried in space, so we’re very early in the game on that. I’m sure it’s a surmountable problem but tricky indeed at this level of development.

Frank September 30, 2007 at 13:59

I would have a design for a solar sail spacecraft which could deploy with the help of the launcher. This solar sail would carry a docking station with already docked in daughter units like asteroid landers.
Take a look at http://solar-thruster-sailor.info/figs/fig19-21d.html

or at http://www.solar-thruster-sailor.info/PosterSSS.pdf

Frank

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