Question: The concept of laser propulsion has been around for decades. How has the concept changed since it was first proposed in the 1970s?
The idea originated in the early 1970s, when a pioneer named Arthur Kantrowitz proposed the idea of sending objects into orbit using laser beams. These early designs revolved around using ablative propellants and large single lasers. Since then, several things changed. In the late 1980’s, Arthur Kantrowitz and I came up with the "4 p" vehicle - payload, propellant, photons, period. The idea was to have payload on a block of propellant, with no tanks or nozzles.
Question: And the original plans called for using a single pulsed laser?
The original concepts, and the 4P concept, called for using a pulsed laser. But by 1991 the Strategic Defense Initiative Organization (SDIO) had cut back its program to use ground-based lasers to shoot down missiles, and we were no longer going to get funding for big pulsed lasers. So I came up with the idea of using a high-performance heat exchanger with liquid propellant. It is not as elegant as the 4-P approach, but it uses proven technology (except for the heat exchanger itself) and delivers the necessary performance to reach low Earth orbit.
Question: Why do you now advocate using an array of small solid-state lasers?
Building large lasers is both a technical and budgetary risk. Building multiple smaller lasers makes testing and debugging easier, and you get to mass produce them once you have the bugs worked out. They are also much closer to the kinds of lasers used in industry for cutting and welding, so there’s lots of work already being done to make them cheap and reliable. An array of smaller lasers is also inherently highly reliable, since if a few of them fail during a launch you don’t care.
Question: Have you ever considered using microwaves instead of lasers?
My colleague Kevin Parkin has been advocating using microwaves – actually millimeter waves, about 100 times higher frequency than a cellphone or microwave oven -- for the last few years. He and I have argued about this extensively. Microwave sources are currently less expensive than laser sources, but no one has ever tried to combine large numbers of millimeter wave tubes to obtain a coherent beam. Using microwaves requires using very large antennas, and the range you can get is still shorter than with lasers, which means you have to accelerate harder and you need more power. That cancels out some of the cost advantage. Moreover, millimeter wave systems are extremely sensitive to atmospheric moisture. So you would need to build such a transmitter on a tall mountain. So I believe that the laser approach is more technically viable, and assuming the cost of lasers continues to decrease as it has in the past, it will cost the same or less than the microwave approach.
Question: Doesn't the diffraction limit of laser beams place a practical limit on this technology?
There is a fundamental diffraction limit, which means that all laser beams spread out with distance, but how fast they spread out depends on how big the transmitting telescope (or antenna, for microwaves) is. So one just needs to build a large enough transmitter to deliver the beam. For launches, small telescopes, about half a meter (20 inches) in diameter, are big enough. With a high quality beam, a few meter sized mirror would be sufficient to deliver the beam to a spacecraft all the way out in geosynchronous orbit, where communications satellites are. So using laser power to power spacecraft anywhere near Earth may one day be feasible.
Question: How low could launch costs go using laser propulsion?
Laser propulsion has the potential to be extremely inexpensive. We envision using simple, throwaway vehicles that cost only tens of thousands of dollars to launch payloads weighing about 100 kg (220 lbs), and if that’s possible, we estimate the cost could be as low as $200 per pound) But if we designed a vehicle that was fully reusable, the cost could eventually go as low as 20 or 30 dollars per pound.
Question: How long do lasers last?
Modern lasers have lifetimes measured in decades. Diode lasers now have up to 100,000 hours of operation, and during a launch the lasers would only need to operate for perhaps ten minutes. Moreover the kind of lasers we want to use require negligible maintenance; they are being sold for 24/7 operation on industrial assembly lines. So the long lifetime of lasers is one of their selling points for the system.
Question: Could lasers be used to launch heavy multi-ton payloads?
Laser based propulsion systems are most economical with large numbers of launches. So it makes sense to break payloads up into pieces that weigh a couple of hundred pounds, and assemble them in orbit. For larger launches not amenable to orbital assembly, chemical rockets still make sense. But there’s no inherent limit to the size of a laser launch system. If you had enough space industry or space tourism to need thousands of tons launched every year, you could build a laser system to launch one-ton or ten-ton payloads.
Question: What sort of throughput would be feasible using laser propulsion?
We envision launching between 1000 and 10,000 times per year, which for a 200-pound payload is100-1000 tons per year. If we launch a few times per day, we get to 100 tons per year. For 1000 tons annually, the only difference is that you need a beefier power supply, and because of orbital dynamics you would need to be launching to several different orbits, not just to one place like the International Space Station.
Question: How much human labor would be required to launch 1000 tons into space annually? How much would all this cost to maintain?
I don't really know how much human labor would be required to support that kind of infrastructure. I estimate that total costs for operating such a system, would be in the range of $200 million per year, but it might be much less. We wouldn't need large ground crews, but we probably would require multiple small teams of people to prepare the vehicles for launch. The laser system should take very little labor to maintain, except maybe to clean the telescopes occasionally.
Question: Is laser propulsion appropriate for putting humans into orbit?
Yes, it could provide a smooth ride for astronauts; the acceleration would be similar to the Space Shuttle. One of the advantages of this system is that it has the highest reliability of any proposed launch system. If one laser out of an array of 1000 fails, you don't even notice. The laser system would have backup power generators for emergencies if main power goes offline. Moreover, this system can be tested a thousand times before putting humans in it. So it could easily become the preferred method for putting humans into orbit.
Question: In the nearer term, could laser propulsion be used to power unmanned aerial vehicles (UAVs)?
Yes, the company that I co-founded, LaserMotive (http://www.lasermotive.com/), is working on that. UAVs can be remotely powered using “laser cells” (similar to solar cells)_to receive the power. This technology has immediate practical applications. In theory, even a commercial airliner could be laser powered, given a powerful enough series of lasers. The main advantage of laser powered UAVs is that they can stay up indefinitely, given sufficient laser power. We think laser power beaming is ready to be a commercial technology, so we’re starting with some down-to-earth applications even as we keep looking toward space.
The thing that makes beaming power to UAVs, and other applications LaserMotive is working on, practical is that we can use arrays of diode lasers, basically similar to the ones in DVD recorders. Diode lasers are up to 70% efficient, are simple to operate, and are low maintenance. Diode lasers can be pooled together in large arrays. They are already the cheapest kind of laser, and the price is falling steadily. Diode lasers don't yet have the desired range to use for launching things to orbit, but we hope to eventually be able to use diode lasers even for that.
Question: Didn’t LaserMotive recently prove the viability of this concept?
Yes, we recently demonstrated powering a miniature quadrotor device for over twelve hours, at 30 feet up. We could have kept it at an altitude of several hundred feet up, with minimal modifications. It only required 190 watts to keep the device aloft. The device only weighed a few kilograms but it showed the feasibility of keeping aircraft aloft indefinitely. There are no intractable engineering issues to scaling up this technology, so it is primarily a question of funding at this point.
Question: What about weather? Presumably all of these craft would need to carry backup power supplies.
Lasers cannot travel through clouds, so beam-powered UAVs would be limited to flying below clouds, and couldn’t fly in heavy fog, for example. But we are also looking at, for example, high-altitude UAVs that would fly on laser power most of the time, and on backup power when the laser transmitter was “clouded out”. We’re also considering UAVs that would be able to fly away from the laser beam on battery power for a mission, then come back and recharge without ever landing.
All of these craft would also need batteries to briefly supply power if the beam were interrupted for safety reasons; the laser would have sensors to turn the beam off if a bird or an airplane (or a person) got in the way. The batteries would recharge themselves once the laser reacquired the aircraft. The batteries would only need to provide power for seconds or minutes, so they wouldn't need to be large.
Question: How does the safety of laser propulsion compare with that of conventional chemical propulsion?
Laser propulsion is clearly safer than chemical propulsion. The spectacular explosions that plague chemical rocket launches simply would not happen with laser propulsion. UAVs would have backup power systems, and would be able to land safely if there were an extended power interruption. So a laser propulsion system would be simpler and more reliable than chemical propulsion, for either rockets or UAVs.
Question: Assuming adequate funding, when could the first laser-based propulsion system come online?
LaserMotive expects to start delivering laser power beaming systems for UAVs and other industrial and military applications within the next year.
Perfecting the technology for a launch system to send payloads into orbit would take perhaps five years, given adequate funding, and deploying and testing the system would take about another five. It would be a challenge to start offering launches to orbit within a decade, but it is definitely doable.
Question: Could laser propulsion become the preferred method to deliver payloads to orbit?
It is silly to insist that there is only one way to deliver payloads to orbit, so I foresee multiple ways of going into space. But I see laser propulsion as having the potential to become the high volume, low-cost method of putting things into orbit.
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