Lightcraft: A Laser Push to Orbit

by Paul Gilster on September 14, 2009

Not the least of the objections against using laser propulsion to boost a lightsail to the stars is the engineering required to build the system. But theorists like Robert Forward, who originated the laser lightsail idea, never thought we would simply create such a system from scratch. We might ask, then, in the area of laser propulsion, what ideas are being experimented with right now, and might be capable of development into more advanced designs?

Enter the Lightcraft

Laser lightcraft command the attention here. Extensive work has been done on them at the Air Force Research Laboratory (AFRL), building upon earlier work at the AFRL Propulsion Directorate at Edwards Air Force Base. These early designs aim not at the stars, of course, but at a much more accessible target: Low Earth Orbit. A ground-based laser transmits power to the spacecraft, which collects the incoming energy and uses it to power its propulsion system. The beauty of this is that ambient air becomes the working fluid, allowing designers to leave the energy source on the ground.

lightcraft1

Several lightcraft concepts have been considered, as Eric Davis (IASA) and Franklin Mead (Propulsion Directorate, AFRL, now retired — see addendum below) make clear in a recent paper. In the one just mentioned, what happens aboard the lightcraft is interesting indeed. The lower portion of the vehicle is a highly polished mirror (see illustration above — both images in this post are drawn from Davis and Mead’s paper). The craft itself looks something like a fat acorn. Kilojoule pulses from the ground installation are fed to the vehicle at the rate of 25 pulses per second. The system now turns ambient air into something more useful, as the paper explains:

The laser beam’s pulse interacts with the mirror, spreading out and focusing into an annular area inside the circumference of the craft. The intensity of the 18 microsecond pulsed laser is sufficiently high that atmospheric breakdown occurs in the annular area causing inlet air to momentarily burst into a highly luminous plasma (10,000 – 30,000 K), thereby producing a superheated plasma shock wave (with instantaneous pressures reaching tens of atmospheres) that generates thrust in the direction of the laser beam. A lip around the craft’s circumference, akin to a plug nozzle directs the expansion of the plasma, creating downward thrust expansion. Multiple laser pulses and an atmospheric refresh of breakdown air generate the flight.

Flight on a Beam of Light

A lightcraft, then, flies on a beam of laser light, turning its energy into thrust. Earlier designs examined the concept from various directions, including one that used a heat-exchanger aboard the rocket and transferred the beamed energy in such a way as to heat a working fluid like hydrogen or ammonia that would be carried onboard. That produces thrust through expansion through a nozzle, much like a chemical rocket.

Another possibility is to carry an onboard solid propellant. But the latest incarnation of the lightcraft operates in dual mode, using air as described above (turned into a plasma by the laser) and then switching to laser thermal rocket mode at higher altitudes (above about thirty kilometers).

The latter concept, of course, demands a small onboard fuel supply, but nothing like the massive fuel/payload ratios we see in today’s rockets. We’re talking about a spin-stabilized, single-stage transportation system to orbit. In its ‘airbreathing’ mode, the engine pulses at a variable rate to achieve what the authors call a ‘quasi-steady thrust,’ one that depends upon the Mach number and altitude along the craft’s flight trajectory.

Have a look at a diagram showing how the system would work. The lightcraft switches into laser thermal rocket mode as it climbs above the atmosphere, using the only fuel it needs to carry. And get this: The lightcraft system as envisioned by Davis and Mead is capable of mid-air hovering and powered descent and landing.

lightcraft2

Crunching the Numbers

Cost? The ground-based laser installation, the AFRL study found, is the major expense of this transportation system, comprising about eighty percent of the total lightcraft system life-cycle cost. Launch costs as low as $74,141 per flight emerge from the studies, with estimated payload costs (using a 10 MW N2/CO2/H2 laser design) that should raise some eyebrows:

…a final total cost of $2,793 to launch a 5.25 kg payload to LEO. This new final result represents a cost of $532 per kg of payload (or $241 per pound) launched to LEO, which is 41 times lower than the space launch industry cost of $10,000 per pound for conventional chemical propulsion rockets.

The theoretical and experimental work already conducted by the AFRL Propulsion Directorate has demonstrated the practicality of the lightcraft concept. Tomorrow I want to run through some of the lightcraft’s other advantages, discuss the background of the idea (aerospace engineer Leik Myrabo has been working on this concept for thirty years), and consider where we are today. The paper is Davis and Mead, “Review of Laser Lightcraft Propulsion System,” CP997, Beamed Energy Propulsion, Fifth International Symposium (AIP, 2008), pp. 283-294.

Addendum: A note from a reader points out that Franklin Mead is now retired from the Air Force and pursuing research at his own company, Mead Science & Technology.

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{ 25 comments }

Anthony Kendall September 14, 2009 at 11:23

This is a really interesting concept, and one that seems like we have the tools to start testing at the small scale.

Do you or the authors have a feel for the minimum size system? It seems like material durability on the lifting body, and the optics required for focusing are likely the limiting factors on this.

doowop September 14, 2009 at 12:19

Can this idea be modfied and adapted to the so called “air shipt to orbit” concept? ATO currently assumes an on board ion engine to accelerate an airship which had previously floated to the edge of space until it achieved escape velocity. However close examination of the calculations shows insufficient insufficient lift/drag and thrust/weigh ratios for hte plan to work.

Can these problems be overcome by using a ground based laser (or microwave emitter/maser), leaving the weight on the ground and improving the performance ratios?

Administrator September 14, 2009 at 13:35

Anthony Kendall writes:

Do you or the authors have a feel for the minimum size system? It seems like material durability on the lifting body, and the optics required for focusing are likely the limiting factors on this.

Let me run that past Drs. Davis and Mead to give you the more authoritative answer. Will be glad to do so.

doowop writes:

Can this idea be modfied and adapted to the so called “air shipt to orbit” concept?

I don’t see why laser beaming couldn’t be tried in the ATO concept, but I’ll pass this one along to the authors as well.

Tulse September 14, 2009 at 13:58

I’m curious as to the components of the launch cost, as I would imagine that one of the huge benefits of this approach is that most of the fixed capital costs don’t go up into space, but are on the ground. Because of that, one could presumably achieve much higher launch rates, and more importantly, the amortized cost of each launch using this approach would be much more dependent on launch rate than for approaches where the capital costs are tied directly to the hardware used for each separate launch. Put another way, this takes the economic advantages of reusability to a new level.

Do you have any idea what the assumed launch frequency was for the calculation of launch cost? And is my above analysis correct, that unlike many other systems the cost-per-launch gets significantly cheaper as the launch rate increases?

Gregory Benford September 14, 2009 at 14:17

An other constraint is stability. My brother & I explored this in detail for microwave driven sails, which are somewhat similar, and sails accelerated by blowing off substances “painted” on the sail. But any sail, light or microwave, must be spun to be stable as it rides a beam. The required rotation rate interacts with the sail geometry, especially moving in a fluid (air). Laser beams can’t rotate a lightcraft itself by angular momentum coupling, though microwave sails can, since the wavelengths are comparable to the “webbing” in a sail (struts etc). Lightcraft can spin themselves by heating an onboard fluid and ejecting it to rotate the craft. This adds an engineering complexity, but may not be a fatal problem.
I believe Myrabo’s lightcraft tumbled off their laser beams at heights of around 100 meters due to this problem. Maybe Leik should comment.

NS September 14, 2009 at 15:01

Could this concept be used in a general-purpose launch vehicle i.e. booster to low-earth orbit for a variety of payloads, including deep-space probes?

philw1776 September 14, 2009 at 15:01

Perhaps these unmanned Lightcraft could be initially spun up on the launch pad by electric motors or whatever. Would have to calculate the drag on the rotating object to see if the momentum would keep the sucka spinning long enough to get outside atmospheric drag.

P.S. Congrats to John Carmack & Armadillo for winning the Lunar Lander competition

kurt9 September 14, 2009 at 16:23

I’m reading Myrabo’s book, “Lightcraft”, right now. I’m only 50 pages into the book. Nonetheless, it is already apparent that this lightcraft system will require the concurrent development of lots of different technologies, including the fabrication of large SiC structural components and the wide area deposition of all kinds of thin-film materials as part of the lightcraft airframe. All of the technologies he discusses in his book (there are a lot of them) have to be integrated into the lightcraft vehicles and laser (both optical and microwave) power sources. As described in his book, it all looks like a rather complicated set of technologies, which makes me somewhat dubious of its feasibility. I tend to subscribe to the KISS (keep it simple and stupid) principle in engineering.

There is a competing technology, call ALP, which uses a propellant which is ablated off of the vehicle by the propulsion laser. The purported advantage of this system is that it uses lower powered diode lasers which are built into an array, thus keeping development costs low.

James M. Essig September 14, 2009 at 17:46

Hi Folks;

Perhaps some sort of electrodynamic-hydrodynamic-plasma drive system can be utilized to augment the basic atmospheric beam system.

I am a big fan of electrodynamic-hydrodynamic-plasma drive interstellar manned space craft system concepts and perhaps the development of this atmospheric beam craft can help with the future refinement and eventual development of beamed powered interstellar electrodynamic-hydrodynamic-plasma drive manned space craft concepts , which might perhaps be capable of arbitrarilly high gamma factors commensurate with the ability to produce ever more extensive and intense electrodynamic drive fields.

djlactin September 15, 2009 at 2:06

This idea meshes with my favorite (amateur!) fantasy of a (petawatt) laser cannon on the moon to launch microprobes at a (fantasized) high fraction of c. My “calculations” have always assumed that photon momentum is the sole driving force, and have run into some practical problems (such as incineration of the probe…), but this idea of using gas/plasma transition (GPT) as the pusher never occurred to me (no surprise there). It might avoid the incineration problem. Of course, the complication for application to “my” idea is the moon’s lack of significant atmosphere. A launch tube temporarily amended with some quantity of gas would (in speculation) solve this problem, but I wonder how long the tube (i.e. the duration of the GPT would need to be, for the probe to attain significant velocity… Perhaps I will have an idea after doing some arithmetic.

As for Dr. Benford’s comment about the probe requiring spin to achieve stability, my low-tech solution is to “simply” spin-up the probe before launch. Of course, this would have to be done such that the axis of rotation is very precisely aligned with the trajectory, because any precession would be amplified.

tacitus September 15, 2009 at 4:01

I saw this clip when it was broadcast a few years ago which gives a nice overview of the system as envisioned (as well as some test flight footage):

http://www.youtube.com/watch?v=LAdj6vpYppA

Checking on Wikipedia, there doesn’t seem to have been much on an advance on the practical side in the last 10 years or so (the record altitude seems to be 233ft set in 2000).

Finally, give that the craft needs to be spinning to ensure stability, I assume that manned launches are out of the question… or would it be possible to only spin part of the craft — say the outer layer?

Adam September 15, 2009 at 5:43

Hi kurt9

Doesn’t the blown-off plasma get in the way of the incoming beam in ablative designs? I suppose a Lightcraft isn’t very different in basic design, just where the propellant is coming from low in the atmosphere.

Ron S September 15, 2009 at 13:11

tacitus: “…would it be possible to only spin part of the craft — say the outer layer?”

Doing it inertially might be difficult since even the slightest friction (including trace magnetic induction) would gradually bring the two sections back into synchronicity. The final, common spin rate would tend towards the unit with the highest relative angular momentum. However, I believe the spins could be maintained with a small, continuous impulse between the two.

Ron S September 15, 2009 at 13:12

djlactin: How about a mass driver instead.
http://en.wikipedia.org/wiki/Mass_driver

Gregory Benford September 15, 2009 at 13:26

djlactin said:
As for Dr. Benford’s comment about the probe requiring spin to achieve stability, my low-tech solution is to “simply” spin-up the probe before launch. Of course, this would have to be done such that the axis of rotation is very precisely aligned with the trajectory, because any precession would be amplified.

I believe Myrabo did spin his craft, and the rotation largely dissipated within 100 meters; hence they toppled. Right about aligning it with the velocity. So it must be an active system.

tacitus September 15, 2009 at 14:15

Getting a license to test a system like this will be a real pain. As the YouTube video I linked says, even during the smallest of test runs, they had to be sure to block the laser from beaming out into space to prevent it from blinding any satellites that happened to cross its path. Imagine what a full powered laser launch system could do!

One assumes that the full system would have to (a) actively adjust to the position of the craft above so that it always hits the target — e.g. sighting with low power laser pulses in between the powered pulses and (b) have the tracks of all satellites programmed into the system so that it can test only when there are no satellites within range. With secret spy satellites and all those foreign satellites, that might be tough to do.

tacitus September 15, 2009 at 14:30

From: http://www.lightcrafttechnologies.com/invest.html

LTI has established a set of concrete altitude and technical milestones for the venture. These will be incrementally achieved using small-scale LightCraft prototypes to minimize the technical risk involved as the systems are integrated and scaled for higher altitudes and larger payloads. Phase 1, which will culminate in acheiving 100 km altitude in near-vertical suborbital flights under full beam propulsion, will be completed within three years from program initiation at an overall cost of about $10M (US).

Sooo…

1. Three years from when? Now? Or when they get a $10 million investment?
2. Anyone got a spare $10 million seed money lying around?

Seriously, three years from initiation to a 100km sub-orbital flights (even if only for micro-satellites) would be extremely impressive, if true. But if it was really that easy, wouldn’t NASA be investing a few million into the technology? After all, if a private start-up company can really do that on that small a budget, then your talking about the possibility of achieving full LEO before the Shuttle’s successor is ready to go.

Even if the payloads are restricted to a few kilos, this is something NASA should not be ignoring *if* the promise of the technology is that great.

Darrell E September 15, 2009 at 15:32

I have serious doubts that this concept will prove to be viable. I have seen several ridiculously optimistic portrayals of this concept on documentaries or in popular science magazines for more than twenty years, but real world performance has barely improved in that time. Even if, after a long development, a system that can launch a 1 kilogram payload into low earth orbit is devised, that just is not very useful, unless you posit some fantastical leaps forward in nanotech as well. How much function can we fit into a 1 kilogram mass limit? I don’t think there will be much of a demand for micro payloads in this range, unless it turns out that the universe is so harsh that we just can never come up with a more efficient means of getting higher masses into orbit.

How seriously should we take the claim that within three years LTI can go from 75 meters to 100 kilometers? I would really like to be proven wrong sometime in the next 5 to 10 years, I would be delightfully shocked, but the evidence I’ve seen leaves me very skeptical.

Ross Harper September 15, 2009 at 17:02

While this is a fantastic idea, surely the infrastructure required for any mass use of the technology is prohibitive. The recently published Myrabo book talks about the presence of huge laser beaming stations in orbit in order for this to work. And is there any hazard with the idea of having many MW lasers criss-crossing around, ionizing the atmosphere??

Tom Duffey September 15, 2009 at 19:41

It should be noted in the article Dr. Mead retired from the Air Force last year and started his own business, Mead Science & Technology, to pursue his research.

Mark September 15, 2009 at 20:50

Interesting concept in reverse… Land a robotic laser facility on Mars (or other desired target world) and you can use it to bring a craft in for a soft landing. “Beam in” thousands of small payloads and have them assemble themselves into something bigger and more useful like a base or a factory or a city or a robotic land rover.

Beam up millions of small payloads into an orbital trajectory and have them assemble themselves into a massive kinetic energy weapon to strike fear into the hearts of our enemies, both real and imagined.

Ocean based launches could be done if you could strap these lasers to the heads of trained sharks.

Administrator September 17, 2009 at 18:08

I asked Eric Davis about the stability questions that have surfaced on this and the other lightcraft thread. Eric sent this response from Frank Mead:

The AFRL had a program with a small business in Huntsville, AL to control both lateral and tipping stability of the laser Lightcraft. Because of the proprietary nature of the concept, I have never been allowed to discuss their approach. We were on the verge of testing the hardware in the lab and on outside flights when the AFRL cancelled the program, leaving the small business phase II contract in limbo. The only thing they (Eric’s note: the small business) could do was write a final report which is restricted for I think 5 years (that 5 years might be up, I’d have to check). The hardware produced under contract was shipped to HELSTF where I assume it sits in storage to this day. I have a lot of confidence that their approach would have worked because we conducted a lot of tests in the lab to collect appropriate data.

forrest noble September 19, 2009 at 1:44

I saw a video of a small craft like the above, maybe 3 feet wide, lifted about 1000 ft by a pulsing laser. This was maybe 5 years ago. I like the idea of superheated air as plasma drive for a lightcraft. If this works as planed then it would seem the possibilities could be many as far as practical applications.

A supplemental idea might be that a small column of air could be lifted hundreds of miles high to augment the lightcraft ascent. A set of sequentially pulsing lasers in a circle maybe ten feet wide might enable a column of air to be heated on the interior causing a rising cylindrical column of air to rise vertically. If pulsing lasers could somewhat contain this rapidly rising heated column of air then the saucer shaped craft could be propelled by a central pulsing laser push, also turning the air into a plasma by the laser heat for the lightcraft propulsion to orbit as indicated above.

Leik Myrabo December 9, 2009 at 17:10

Contrary to what Gregory indicated (no offense intended Greg), the 72 m world altitude record on Oct. 2, 2000 — for vertically laser boosted flight — was accomplished in less than 4.5 seconds, but the lightcraft hovered on the 10 kW PLVTS laser beam longer than 10 seconds — with perfect stabiity. The reason it didn’t go any higher was because the PLVTS laser beam had expanded to the point that the rear parabolic optic could only capture enough power to just maintain hover; if the transmitter could have been adaptive with a variable focus, to maintain the desired spot size on t he vehicle, PLVTS could have been boosted it very much higher — perhaps a kilometer or more. At launch, the Lightcraft was spun up to 13,000 RPM, and it was still spinning when it returned to earth, retrieved into a large salmon net. Yes this lightcraft was spin-stabilized, and had autonomous beam-riding capability — requiring no on-board control system.

Leik Myrabo

tom May 22, 2012 at 11:47

The technical demonstrations in this field are fantastic. I could see why the Air Force wanted to develop a ground based laser array to pinpoint and power an object to go into orbit. Even configured into a completely different architecture made funding this very feasible. An ATO would be an ideal craft to integrate with this kind of system. To be a bit more ambitious, after the LEO objective, deploy a lasersail array to go to GEO/Cislunar/Lagrange or even a MOI? But unlike Goddard’s first liquid fuel prototype to the diversity of missiles & rockets we have now…. lightcrafts could be developed and integrated vary quickly and possible at cost relative to what is being estimated for some of the subsystems in the Constellation program(now canceled). Even if it isn’t developed into a spaceflight technology….there is a couple of other overlapping fields that would benefit from this work. Small projects with vigorous funding and good management are what is need in these times.

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