≡ Menu

Sundiver: Hybrid Propulsion Emerges

New propulsion technologies are under study in the laboratory, even if finding the funding for such work is always a problem. James and Gregory Benford have demonstrated that a powerful microwave beam can push an ultra-light carbon sail even to the point of liftoff under lab conditions at 1 gravity. That’s useful information, for if we can leave the propellant at home, we can contemplate deep space missions driven by beamed microwaves, a technology that not only can pack a wallop, but is also less destructive to sail materials than a laser, meaning the sail can be brought to high temperatures more efficiently.

Unusual Acceleration

Yesterday we talked about a possible ‘Sundiver’ mission built around the microwave beaming idea. The Benfords’ version of this mission depends upon a second effect they observed in the lab. The photon pressure applied to the small sail they used could not account for the observed acceleration. Something was clearly coming out of the carbon lattice, but what was it? The sail material had been heated before the test to drive out any contaminants and the sail had been placed in a hard vacuum. James Benford told me about the feeling he got upon analyzing the sail’s performance under the beam:

I calculated and got two numbers for the total acceleration on our sail — somewhere between 9.8 and 13.5 g‘s. And I sat there and thought boy, what is this? How could we maintain such acceleration? The photons couldn’t do it, so something else was going on.

That something else turned out to be absorbed molecules — CO2, hydrocarbons and hydrogen — that become incorporated in the lattice when the material is made. Only at the highest temperatures do these residual molecules emerge. Mass ejected from the material under high temperature by this desorption process becomes another form of acceleration, observed in the laboratory as it forced the sail upward. Meanwhile, the original sail material remains unharmed by the desorption effect — the team’s final report is clear on the point, and I’ve seen Benford’s images of the intact sail.

The Uses of Desorption

Think, then, of a variety of compounds that can be ‘painted’ onto a sail, perhaps in multiple layers. Use the desorption process carefully and you have created a propulsive layer that can be triggered by microwave beam or the Sun itself. The Sundiver mission in this new guise now emerges: Launch the carbon sail via microwave so as to make layer #1 of this propulsive ‘paint’ desorb. Enhanced thrust through this combined beam/desorption method gives the sail 15 kilometers per second velocity, canceling most of its solar orbital velocity, and allowing a quick fall toward perihelion (using solar pressure alone would take years to spiral down).

At perihelion, the spacecraft rotates to face the Sun where, under intense sunlight, sail layer #2 desorbs, producing a 50 kilometer per second boost. The sail then moves away from the Sun, now acting as a reflective solar sail, its aluminum layer revealed. The Benfords have calculated that a Pluto mission could be accomplished in five years with these techniques, doubling the pace of our New Horizons mission, while paving the way for faster missions as the technology is tested and improved.


Image: The Benford Sundiver mission, aided by the propulsive effects of desorption.

What an intriguing concept desorption turns out to be, an effect discovered (but not anticipated) in laboratory work. Ponder what might have happened with the ill-fated Cosmos-1 mission if, upon successful deployment, the flight team had discovered the effects of desorption for the first time in space, with the sail showing propulsive effects no one had reckoned with. I asked James Benford about this in our conversation:

We had a design review at the Planetary Society with the Russian team on Cosmos and I raised the question, saying that we know that desorption takes place when you put any kind of a beam, any kind of heat source, on a material. And shouldn’t we take that into account in the experiment? The Russians said that effect was not important. Nevertheless, I got it onto the list of action items that it would be looked into.

The Russian team came back at the next design review and said actually it is important, but that it goes away in a couple of days. But Cosmos 1 as built had an asymmetric surface — the front and back were not the same. They were going to get more outgassing out the back surface than the front surface. So therefore when the sunlight hits it, the acceleration from desorption will in fact produce a negative acceleration. So the sail would actually be propelled backward for a little while.

Stabilizing the Beam Rider

We’re used to surprises on our space missions, but this one would have been a whopper. Now we can ponder the possible uses of this helpful effect in getting an extra boost to a mission that effectively combines a solar sail with beamed propulsion and, in a sense, rocket technology. But what about beamed propulsion over longer periods for moving beyond the Solar System? Could a sail riding a microwave beam for an extended period be stable? The answer is yes, not only through stable ‘beam-riding’ effects but because a microwave beam can communicate angular momentum to a sail to provide additional control. All of which is why microwave beaming is emerging as a credible deep space technology, and why we’ll be discussing it again in coming posts.

A good place to start digging on microwave propulsion is James Benford’s “Space Applications of High-Power Microwaves,” IEEE Transactions on Plasma Science, Vol. 36, NO. 3 (June, 2008). Zeroing in on desorption and its uses is the Benford’s “Acceleration of sails by thermal desorption of coatings,” Acta Astronautica 56 (2005), p. 593 ff.

Comments on this entry are closed.

  • John Gallagher April 1, 2009, 9:33

    Thermal desorption should give off material in all directions. How can this generate a net force?

  • Administrator April 1, 2009, 9:41

    John, the lab work shows evaporation of heated absorbed molecules from the hot side of the sail — the effect is dependent on the heat source. But it’s complicated and the effect does not seem to be well understood. This is from the final report:

    The magnitude of this effect can vastly exceed the microwave photon pressure if the temperature is high enough. The essential factor is that this force must be asymmetric, i. e., if the sail is isothermal there is no net thrust. A thermal analysis (see below) shows that the skin effect produces a substantial temperature difference between front and back. And that thermal conduction reduces it a lot, but differences still remain. However, analysis shows the sublimation rates at these temperatures don’t give thrusts sufficient to explain the observed acceleration. Therefore, we have speculated that there is a portion of the fibers which have locally high resistivity, perhaps at the joins, which increases dissipation of the microwave–driven currents flowing through them. If the mass of these resistive fiber joins is a fraction f of the total mass, they will be hotter on the front face than the back face and will sublimate faster. Comparing this model to our flight data shows that the fraction must be both small (<1% of fiber mass) and at very high temperature.

    So that’s one possibility. Further speculation from the report:

    There is another possible mechanism at work: Asymmetric Out-Gassing. This model is that acceleration of the sail comes from asymmetric gas emission. We observe that the RGA easily detects evaporation of gas from the sail during bakeout. This gas emission becomes undetectable after ~100 seconds of heating. Our standard bakeout is 250 Watts. With 10% absorption of the microwaves, the sail is heated to ~1000 K during this bakeout. This model must assume that there remains an asymmetric evolution of gas from the sail, always in the same vertical direction, sufficient to produce the observed acceleration but small enough to remain undetected by the RGA. However, we did see strong RGA signals on 4/10/00, though not the previous week. The signals were at mass 28, which could be N2 or CO. Further work must go into understanding the RGA signals.

    I’ll pass the question along to Jim Benford to get his latest thinking on the matter.

  • Mr. Schmee April 1, 2009, 14:39

    Although the mass is light, doesn’t infusing the sail with extra matter for dissipation defeat the purpose of ‘leaving your fuel at home’? I suppose the thrust/mass ratio is better because of the energy coming from the Sun, but wouldn’t be more efficient to capture energy and bleed off electrons instead of whole atoms? More finely tunable too.

    The other idea worth doing the math for is using a magnetic propulsion bottle / balloon to do the sundive. Perhaps some of the radiation energy encountered will diving in might be stored and used to strengthen the field on the way out. The big issue then is that the system is no longer ‘passive’ in the sense that the bottle will require near constant power.

  • Doowop April 1, 2009, 15:58

    This may be bit out of left field but can ground based microwave beamed energy make the airship-to-orbit concepte workable?

    The current concept calls for massive airship to float to the edge of the stratosphere where drag forces are minimal. Once in place, onboard ion engines will deliver continous thrust until escape velocity and orbit is achieved. The ion engines are to be powered by solar cells covering te airship’s surface.

    However, ion engines and the solar cells do not provide a sufficiently high ratio of thrust to weight to overcome anticipated low lift/drag ratios. But if the ion engines (and their weight) are completely removed and the surface of the airship is “pushed” with continuous microwave energy from ground stations, can the problem be solved?

    If so how much energy would be required and how long before the airship achieves escape velocity of 8 kps?

  • Enzo April 1, 2009, 17:11

    I find the possibility of fast aereocapture missions to the external planets very appealing (instead of a duplicate fly by to Pluto). Five years to Neptune, aerecapture and orbit sounds great.
    Similarly, a short (six months ?) trip to Jupiter instead of the current 6 years for the Europa orbiter would be a great improvement. I have to admit that I have no idea of how difficult it would be, once slowed down by Jupiter atmosphere, to then orbiting Europa.

  • Adam April 1, 2009, 21:44

    Hi Enzo

    A six month trip to Jupiter needs a heliocentric velocity of 60 km/s which wouldn’t be healthy for anything aerobraking at Jupiter – the survival of “Galileo” was iffy at best hitting the atmosphere at “just” 48 km/s. That transfer speed would mean the probe hits the atmosphere at a relative speed of 62.7 km/s and I’m not sure that’s survivable even for an aerocapture maneuver. Might be. Only have to knock off about 8 km/s or so.

    Things improve a little as you aim further out. At Neptune the relative speed is down to roughly the same as “Galileo”, but you have to shave off a bit more speed than at Jupiter. Won’t be as bad as “Galileo” so it just might work. A slower transfer speed might allow an aerobraking ballute to be used to drop a probe onto Pluto, which might be worth the longer wait – let’s see what “New Horizons” finds.

  • Chan April 4, 2009, 0:51

    The other idea worth doing the math for is using a magnetic propulsion bottle / balloon to do the sundive. Perhaps some of the radiation energy encountered will diving in might be stored and used to strengthen the field on the way out. The big issue then is that the system is no longer ‘passive’ in the sense that the bottle will require near constant power.

  • Adam April 4, 2009, 2:31

    Hi Again

    I looked a bit deeper at the speeds and braking required for those aerocapture scenarios. For a 5 year flight-time to Neptune a speed of 135.4 km/s is needed at 0.1 AU – which is just 2.2 km/s over the escape velocity at that radius. The encounter speed at start of aerocapture is 46 km/s and some 25.46 km/s needs to be shed, minimum, for capture. Call it 26 km/s. In energy terms that’s about 73% of what “Galileo” had to dissipate during its entry maneuver, but peak acceleration can probably be lower – the “Galileo” entry probe peaked at 250 gees (!) So, roughly, a high-speed aerocapture at Neptune might be feasible.

    The 6 month trip to Jupiter might not be as problematic as I’d assumed either. Jupiter’s gravity well is so deep that the capture maneuver only has to shave off 15.7 km/s – the total energy shed is roughly the same as the 5-year transit-to-Neptune aerocapture. However the entry speed is 62.6 km/s and that’s much higher than the “Galileo” baseline of 48.5 km/s. However the sail has to launch off from 0.1 AU and to get there typically requires a slingshot past Jupiter anyway. I guess a microwave beam could send it inbound towards the Sun, but furling it automatically then redeploying could be tricky. Perhaps a two-sail system, with one optimised for the desorption boost, would be feasible.

  • thomas schluender April 7, 2009, 8:52

    When is that comanion book to “Frontiers of Propulsion Science” Going to become available?

  • Administrator April 7, 2009, 9:13

    thomas, I have no word yet on the schedule for the companion book to Frontiers of Propulsion Science, but I’ll keep readers advised. Right now everyone’s still exhausted from getting FPS out the door, all 739 pages of it!

  • george scaglione April 7, 2009, 14:31

    yes paul please do keep us advised as to the availability of above mentioned book! there is no debate that that 739 pager must have been some herculian task for all involved! speaking only for myself i think the companion book would be better suited to me – the BIG one might be just a tad too technical- lol better suited to the likes of marc millis and company! but lol again,what am i talking about!? it was THEY who wrote it! but i do find the above discussion valuable also…i was never all that much of a fan of solar,(space) sails,but after reading the above i begin to have my doubts.anyone who can further enlighten me please post below or feel free to write me directly at udt109@aol.com as usual thank you one and all very much your friend george

  • James M. Essig April 9, 2009, 10:58

    It is interesting to consider the possible high gamma factor achievable with a dive and fry sail manuever around a high end O class star. The maximum temperature of the surface of these stars is on the order of 100,000 K, someone correct me if I am wrong, and so by the Black Body approximation, the total integrated spectral radiancy on a uniit area sail around such a star wherein one would approach the star inorder to have the star subtend a 45 degree angle would be (100,000/5,800) EXP 4 times the total integrated spectal radiancy of the sun as such on the same unit area of sail. This works out to 88,370 fold increase of pressure on the sail utilizing a high end O-type star.

    The absolute luminosity of both Blue Supergiants and Red supergiants is as great as 4 million solar luminosities thus providing a much longer effective pathway for accellerating outbound space craft. No doubt, efficiencies would drop off rapidly as relativistic gamma factors mount due to relativisitc redshift of radiation impinging on the sail, but my feeling is that if suitablly refractive and reflective sails can be built, perhaps sails of some form of exotic material that are 1 nanometer thick and have a net like configuration that comprises nanometer threads seperated in a weave by distances of 100 nanometer, potentially fantastically high accelerations can be reached.

  • george scaglione April 9, 2009, 13:58

    “fantastically high accelerations can be reached” i hate that hip new phrase “thats what i’m talking about” but in this instance,yes,i am forced to say – THAT IS what i’m talking about! more and more i begin to suspect that those space sails are not such a bad thing! what a shame that experiment by the planetary society did not work out.what a fine first step that would have been…hopefully still can be. thank you and as always i hope i will soon hear alot more through our participants here.all the best your friend george

  • ljk October 27, 2009, 23:44

    Orion’s Arm’s take on the Beamrider concept: