Microwave and Laser Thermal RocketsKevin Parkin2024-01-24T03:05:39-08:00
MICROWAVE THERMAL ROCKETS
LASER THERMAL ROCKETS
Traditional rockets are marginal at reaching orbit. Their chemical propellants do not have enough energy (in the form of chemical bonds) to carry their own weight all the way, let alone tanks and payload on top of that. Though rockets lighten as propellant is expelled, traditional rockets also need to jettison parts (stages) as they ascend. Most rockets can make it in 3 or 4 stages and a few of them in 2. The payload that is finally delivered into orbit is a tiny fraction of the rocket that lifts off from the ground.
Microwave and laser thermal rockets provide more payload for less rocket by bypassing the fundamental energy-carrying limit of traditional propellants: Energy is directed from the ground onto a heat-exchange layer covering the side of the rocket, which in turn transfers it into the propellant where it augments or replaces the energy released by combustion.
By swapping traditional propellants for inert monopropellants, and combustion chambers for heat exchangers, rockets can be made safer and cooler yet so energetic that they lift payloads to orbit in a single stage, avoiding the range constraints and engineering complexity of staging. Costs fall from $10,000 per kilogram delivered to low Earth orbit to less than $1,000/kg. For reusable rockets, payload costs fall from $3,000/kg to less than $300/kg. Even if future rockets are made for a dollar each by self-replicating robots, thermal rockets offer the next order of magnitude cheaper while being safer and more environmentally friendly (they can be propelled by ammonia alone, which is a possible way to decrease the environmental impact of rocket launches on the stratosphere).
The U.S. government alone spends$170M per week on launch-related activities (GAO-13-802R). Of this, 25% is spent on R&D and 75% on building rockets, launching them, and related non-R&D activities. Despite this remarkable and ongoing level of expenditure the U.S. government and everyone else launches payloads into orbit the same way as in 195865 years ago!; by chemical rockets.
Traditional chemical rockets achieve payload fractions of less than 4.2%. The rocket equation shows how this results from the relatively low specific impulse (Isp) of chemical propellants which long ago reached a practical limit of 460 seconds by reacting hydrogen with oxygenFor H2/O2 systems at high pressure. The highest chemical Isp ever achieved was 523 sec in vacuum using a Li/H2/F2 tripropellant combination (Arbit, Clapp et al. 1970), but this and other combinations have all proven to be highly impractical, volatile, and economically infeasible for mass production. The LOX/LH2 reaction releases 16 MJ per kilogram of propellant mixture, whereas the specific energykinetic energy plus gravitational potential energy for a 200 km circular orbit of low earth orbit (LEO) is 32 MJ/kg.
The price of payload delivery to low earth orbit (LEO) as a function of rocket payload capacity.
The price of launch remains above $10,000 per kilogram of payload delivered to low earth orbit for most models of rocket as shown in the interactive chart above (click ‘1’ or ‘0’ to show or hide layers). The data points are rockets from many countries spanning 60 years and representing hundreds of billions of dollars in evolutionary R&D investment. In contrast the payload’s energy would cost less than $1 per kilogram if propellant and structure did not need to be carried up as well. In practice, the energy cost of launch for a thermal rocket is below $100 per kilogram of payload, and potentially as low as $10 per kilogramEstimate based on 4¢/kWh electricity, 20% wall plug to jet efficiency averaged over the ascent trajectory, and methane propellant reaching a peak temperature of 2,500 K.
Forms of propulsion organized by the main metrics of T/W ratio and Isp. Data points are actual engines, grouped by type.
Thermal rockets bypass key limitations of traditional chemical rockets and in so doing are easily able to reach orbit using a single stage: A microwave or laser thermal propulsion system combines the specific impulse of a nuclear thermal rocket engine with the thrust to weight ratio of a conventional rocket engine, allowing the payload mass to be 3-12 times heavier for a given rocket. This direct effect can be shown using the rocket equation. Even more money is saved by using one propellant instead of two, and one stage instead of two to four stages. The combined effect is that the overall cost is 6-144 times cheaper than a conventional rocket, depending on the particular assumptions made.
ORIGINS OF THE 4 MAIN TYPES OF DIRECTED ENERGY PROPULSION
Konstantin Tsiolkovsky was the first to derive the rocket equation in 1897 and proposed directed energy rocket launch via a “parallel beam of shortwave electromagnetic raysShortwave radio is generally taken to be in the frequency range of 1.6-30 MHz” in 1924 (Tsiolkovsky 1924Tsiolkovsky, K. E. (1924). Spaceship, 1924, in Izbrannye Trudy, Compiled by Vorobev, B.N., Sokolskii, V.N., General Editor Acad. Blagonravov, Izdatelstvo Akademii Nauk SSSR, Moscow, Russia, 1962, 222 (in Russian). Edited Machine Translation prepared by Translation Division, Foreign Technology Division, WP-AFB, Ohio, on May 5th, 1966, 307). The two-pole magnetron had been invented only three years earlier and the time average power output at that time was so low that it would take the coherently combined output of millions of such devices to power a microsatellite-class rocket to orbit. In his 1924 book, Tsiolkovsky estimated that due to diffraction a ground-based beam director required an aperture diameter of 12.6 km to produce a beam concentrated enough. He wondered how any receiving material could withstand the intense heat generated by the beam and how it could be directed onto a rocket as it ascends to orbit, eventually concluding that “This method of imparting velocity raises quite a few difficult problems, the solution of which I shall leave to the future.”
In 1959 Willinski proposed for the first time a vehicle wherein “beamed power would be utilized to heat a propulsion fluid, such as hydrogen, ammonia, gasoline, or even water, which would then be expanded through a nozzle to produce thrust” (Willinski 1959Willinski, M. I. (1959). “Beamed electromagnetic power as a propulsion energy source.” Amer. Rocket Soc. J.29(8): 601-603). Noting the problems of large apertures and beam diffraction at longer wavelengths, and the opposing problem of atmospheric absorption at wavelengths shorter than 10 cm, Willinski dwelled predominantly upon a receiving antenna in the form of a large inflated balloon capable of focusing microwaves onto a central heat exchanger. But the vehicle was constrained to operate as an upper stage above the atmosphere because of the drag. Significantly, Willinski mentioned that “the skin of the vehicle itself could possibly be utilized as a surface antenna, thereby allowing operation in the atmosphere.” This line of reasoning is consistent with the later microwave lightcraft (Myrabo 1995).
In 1972 the idea of using a pulsed laser to propel rockets to orbit was proposed separately by Kantrowitz and Minovitch (Kantrowitz 1972Kantrowitz, A. (1972). “Propulsion to orbit by ground-based lasers.” Astronautics and Aeronautics10(74-76), Minovich 1972Minovich, M. A. (1972). The laser rocket – A rocket engine design concept for achieving a high exhaust thrust with high ISP. Jet Propulsion Laboratory, California Institute of Technology.). The laser had been invented only 12 years earlier yet it took another 20 years before laser thermal rockets, shown on the bottom left of the figure below, were proposed by Jordin Kare (Kare 1992Kare, J. T. (1992). Development of Laser-Driven Heat Exchanger Rocket for Ground-to-Orbit Launch. Washington, DC International Astronautical Federation Congress). The lower cost of microwaves relative to lasers motivated Kevin Parkin to invent the microwave thermal rocket in 2002 (Parkin and Culick 2003, Parkin, DiDomenico et al. 2003).
The four main approaches to directed energy propulsion form a grid. The bottom right approach was proposed in 1972 (Kantrowitz 1972Kantrowitz, A. (1972). “Propulsion to orbit by ground-based lasers.” Astronautics and Aeronautics10(74-76 ), Minovich 1972Minovich, M. A. (1972). The laser rocket – A rocket engine design concept for achieving a high exhaust thrust with high ISP. Jet Propulsion Laboratory, California Institute of Technology.), the bottom left in 1992 (Kare 1992Kare, J. T. (1992). Development of Laser-Driven Heat Exchanger Rocket for Groundto-Orbit Launch. Washington, DC International Astronautical Federation Congress), the top left in 2002 (Parkin, Culick et al. 2002Parkin, K. L. G., et al. (2002). A Microwave-Thermal Thruster for Ultra Low-Cost Launch of Microsatellites, Jet Propulsion Center, California Institute of Technology), and the top right in 2003 (Oda, Nakagawa et al. 2003Oda, Y., et al. (2003). An observation of plasma inside of microwave boosted thruster. Second International Symposium on Beamed Energy Propulsion, Sendai, Japan, Conf. Proc. AIP).
MICROWAVE VS. LASER
The case for microwave thermal rockets
Long wavelength microwaves sources led Tsiolkovsky and others to estimate aperture sizes exceeding a kilometerDue to the physics of light diffraction. After a century of microwave development high-power short-wavelength sources now enable aperture sizes on the order of 100 meters and increase the beam irradiance at which plasmadynamic breakdown occurs by several orders of magnitude. At this time, microwaves have the advantage of being one to four orders of magnitude cheaper than lasers on a $/Watt basis ($0.1-10/Watt vs. $100-1,000/Watt), and they are unaffected by atmospheric scintillation and more tolerant to weather, particularly at lower frequencies below 35 GHz.
The case for laser thermal rockets
In recent years the cost per Watt of lasers has fallen and there is no fundamental reason why they cannot be as cheap or cheaper than microwave sources. Commercially-available fiber lasers have high energy efficiency and do not require the high voltage power supplies that vacuum microwave sources do. Due to their shorter wavelength, lasers can employ a 1-meter primary aperture as opposed to 100 meters for microwaves. Not all the power is sent through a single aperture and many such beam modules are trained on the rocket. This has the benefit of reducing thermal blooming, and unlike microwaves the beam modules do not need to be in phase.
The case for both
Both approaches can work, there is a compelling need, so what is the reason not to pursue both laser and microwave approaches? It gets results soonest without prejudging what the market will ultimately choose. It is also possible to develop a flexbeam thermal rocket that can be powered by laser or microwave sources depending on what beam director is available at the time.
HOW IT WORKS
The current concept benefits from 20 years of feedback and design evolution. A single foil balloon tank holds a slush propellant. This propellant is then pumped through the heat exchanger, reaching close to the temperature of an incandescent light bulb filament just prior to being expanded through a plug nozzle to produce thrust. The beam tracks the heat exchanger, which faces the general direction of the beam throughout the ascent to orbit. There is only a single propellant, single tank, single turbopump, and single stage all the way from the ground to orbit.
Concept of microwave (or laser) thermal rocket operation
The concept of operations for a microwave (or laser) thermal rocket is shown above. In the simplest implementation, launch begins from the ground and begins vertically upwards. After a short time, a long-range beam director acquires the rocket. The rocket turns and accelerates horizontally until it reaches orbit, and then the payload is released.
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A beam heats prototype heat exchanger tubes to > 2,000 K for the first time in tests conducted on the 13 August 2013 at the General Atomics DIII-D Fusion Reactor in San Diego as part of the DARPA-NASA Millimeter-wave Thermal Launch System Program.
The video above shows a key milestone during a test campaign using a 110 GHz beam at the General Atomics DIII-D Fusion Reactor in San Diego California. For the first time, Mullite heat exchanger tubes were heated to very nearly their melting point by the beam thanks to an idea to coat them internally with a thin layer of graphite to act as an absorber. These tubes then formed the basis of a heat exchanger that was mounted to a small rocket and launched to just a few tens of meters altitude in early 2014. With refinement their performance would be sufficient for an orbital rocket. Such tubes can be modified to absorb laser light as well.
WHAT IS THE ECONOMIC/BUSINESS CASE?
The feasibility and performance of microwave and laser thermal rockets is affirmed by many analyses over the past 20 years sponsored by Caltech, NASA, DARPA and the U.S. Air Force. Over this time various methods have estimated that microwave and laser thermal rockets can reduce launch costs by one to two orders of magnitude. This being the case, focus has shifted to the infrastructure cost of an initial system: over a given payback period, how low must the initial infrastructure cost be for a directed energy launch to make economic sense? What is the minimum scale that can still launch a rocket to orbit?
Initial infrastructure cost is currently driven by beam director cost, which itself is comprised of the cost of the microwave/laser sources plus the cost of optics. Initial infrastructure cost can be expressed in $/WattThe Watts used here are Watts of ideal jet power produced by the rocket. One could also use Watts of transmitted power or Watts of ‘wallplug’ power, but these require additional efficiency factors to be included that vary with rocket size and add additional uncertainty. For a large rocket the beam spillage tends to <25%. and is its most important figure of merit. The $/Watt value at which directed energy thermal launch becomes economically better than present launch systems is calculated below under various assumptions and over a payback time of 20 years20 years is the time horizon used by both the NASA Office of the Chief Technologist and the Air Force Office of the Chief Scientist..
In the baseline case above there continues to be no directed energy thermal launch system and 100% of the market is served by families of traditional chemical rockets, the largest of which is capable of launching a 20 metric ton satellite to LEO. The cost of launch to continues to be $125M/week for the U.S. Government, excluding R&D. This equates to a $130Bn expenditure over the 20-year period.
In the pessimistic case a directed energy thermal rocket with a 200 kg payload capacity to LEO captures 1% of the total revenue for satellite launch. It has a relatively poor jet power per unit payload mass (though not the worst) and a relatively poor cost reduction factor. Consequently its initial infrastructure needs to cost less than $2/Watt in order to save money over the period. Current estimates of microwave beam director cost are $1-5/Watt at this scale and further research is expected to lower this value (and its associated uncertainty). Laser beam director costs are also falling.
In the optimistic case a family of directed energy thermal rockets capable of launching up to 20 tons to LEO captures 100% of the revenue for satellite launch, replacing traditional rockets altogether. They have a relatively good jet power per unit payload mass (though not the best) and a relatively good cost reduction factor. Consequently, the initial infrastructure needs to cost less than $22/Watt in order to save money over the period. Current estimates of beam director cost are one to two orders of magnitude lower than this depending in part on the choice of frequency.
The interpretation of these results is that a business case does not yet exist for a 200 kg directed energy thermal launch system that fits within the cost, risk and schedule of private capital. More work is needed to lower technical risks and in particular to define the beam director and constrain its associated costs. That being said, the economic case to replace traditional rockets with directed energy thermal rockets is compelling, and there is every reason for governments to make substantial R&D investments toward the initial goal of a small directed energy thermal launch system. This would be a technology push without regard to the cost impact on launching small satellites, since the overwhelming economic benefit is gained from the rest of the market.
THE BOTTOM LINE
The conventional rocket industry reached maturity many decades ago. Hundreds of billions of dollars have been spent on developing families of rockets and launching them. Yet directed energy launch was proposed almost 100 years ago by Tsiolkovsky himself, and slowly over time the conditions needed for it to be realized have fallen into place: Lasers were invented. Vacuum and solid-state microwave sources were invented. The $/Watt for beam sources has fallen by orders of magnitude. Detailed concepts have been invented for pulsed laser rockets (1972), laser thermal rockets (1992), microwave thermal rockets (2002), and pulsed millimeter-wave rockets (2003). Of these, the laser and microwave thermal rocket engines outperform chemical engines in T/W and Isp. Assuming that the U.S. Government is the only customer, simple cost-benefit analysis shows that the $/Watt is already low enough to make microwave thermal rockets cheaper than chemical ones, and laser thermal rockets are not far behind. At $60-125M per week the price of inaction is great indeed ($26B-$55B from when these words were first written in 2015 until 2024).
FREQUENTLY ASKED QUESTIONS
WHAT IS THE MAXIMUM PAYLOAD?
There is no maximum payload mass for the microwave or laser thermal rockets. The launch systems, which include the beam directors, can be scaled up to launch all payload classes, through 1 metric ton payloads and even above 100 ton payloads, if needed. Once a system is built that can launch small satellites, there is every reason to scale it up.
WHAT IS THE MAXIMUM BEAM DIRECTOR SIZE/POWER?
Due the physics of beam diffraction, beam directors become smaller as the rocket size (and payload capacity) increases. For these, the limiting factor is power density, or, depending on the way in which microwave sources are combined to form a single beam, it can be spectral characteristics that limit the number of microwave or laser sources per beam director. However, these limits are for a single beam director. Once one beam director has been built, there is no reason a second cannot be built and reuse the same spectrum. Both can then be trained on a single rocket designed for double the power, and due to efficiencies of scale, the rocket payload mass will more than double. Any number of beam directors can be trained on a single rocket in the same way that overlapping spots from flashlights do.
WHAT IS THE MINIMUM BEAM DIRECTOR SIZE/POWER?
Due to the physics of beam diffraction, beam directors become larger as the rocket size (and payload capacity) decreases. Above a certain aperture size, it minimizes the system cost to build more microwave sources and spill more energy than to further increase the beam director size. Curves for cost-minimized beam directors show that lighter rockets always result in a cheaper beam director.
HOW MUCH POWER DOES IT TAKE?
10 MW of ‘wall plug’ power per kg of payload for the lightest rockets, falling to below 1 MW/kg for the heaviest rockets. This is because the payload fraction and transmission efficiency both improve as rockets become heavier.
HOW MUCH OF THE PAYLOAD COST IS FOR POWER?
$10-100 per kilogram of payload, depending on electricity cost, the choice of propellant, the peak propellant temperature, and the efficiency of energy transfer to the rocket, which increases with payload size.
HOW IS THAT MUCH POWER GENERATED?
The power needed by the thermal rocket engines is comparable to the power developed in conventional rocket engines of the same thrust, though of course this power is generated on the ground for thermal rockets. 2 GW can be obtained directly from the electrical grid for beam directors that are located near to high capacity transmission lines, for example in California, and this is enough to launch a rocket with a payload of about 100 kg. This power would cost on the order of 10¢/kWh. For heavier payloads, on-site pulsed power is used. Several of the technologies used or being developed for grid energy storage, such as batteries and flywheels, are suitable for the beam director site. For these technologies the amortized cost adds < 3¢/kWh to the electricity cost (Viswanathan, Kintner-Meyer et al. 2013Viswanathan, V., et al. (2013). “National Assessment of Energy Storage for Grid Balancing and Arbitrage. Phase II. Volume 2: Cost and Performance Characterization.” US Department of Energy). Capital cost is on the order of $1/Watt. In comparison, the capital cost of a solid-state microwave source is $0.5-10/Watt, and the capital cost of a millimeter-wave source (i.e. a gyrotron) including its power supply and supporting equipment is around $5/Watt. Costs will be reduced by further R&D and/or as production quantities increase.
IS PLASMADYNAMIC BREAKDOWN A PROBLEM?
At very high beam intensities, the atmosphere breaks down into a plasma, and this limit was shown to be orders of magnitude greater than that needed for small satellite launchers (Parkin 2006). It is possible to approach this limit with very heavy payload rockets, but it is always possible to flatten the rocket and increase the receiving area to keep the beam intensity well below the breakdown threshold.
HOW IS THE ROCKET AND PAYLOAD SHIELDED FROM THE BEAM?
By metal foil. Aluminum kitchen foil is easily thick enough, and there are standard techniques to ensure that microwaves do not propagate through holes or along any external cables or tubes through to the payload.
WHAT IF A BIRD/PLANE FLIES INTO THE BEAM?
Beam directors are located at arid, high-altitude sites in controlled airspace where there are few birds. If a bird flew just above the beam director during operation, the power density it would experience is 1,000 times lower than at the rocket, which is well above the altitudes of birds and planes. The bird would initially experience a growing sensation of heat on its underside, somewhat like opening an oven door, and would flee, it is hoped, in a different direction. The vicinity of the beam is monitored by radar, particularly objects that are capable of intercepting it within the duration of an ascent trajectory (3-6 minutes). If something is about to enter the beam, it can dodge, dim, or douse Dickinson 1978R.M. Dickinson, “The Beamed Power Microwave Transmitting Antenna,” IEEE Trans. Microwave Theory and Techniques, Vol. MTT-26, No. 5, pp. 335-340, May 1978 until the obstacle is clear, or it could hand over to a nearby unobstructed beam director. Migrating birds can achieve remarkable altitudes, and the North American flyways are quite well documented. Research on bird radar systems (King 2013King, R. E. (2013). Research on Bird-Detecting Radar) has resulted in commercially available solutions that are capable of the kind of short-range tracking needed, such as the Merlin Avian Radar System offered by DeTect Inc.
WHAT IS THE BEST PROPELLANT?
Ammonia and methane are best and exceed the performance of LH2. Methane is of interest because it is nontoxic and soot formed by its thermal decomposition can potentially be used as a microwave absorber to post-heat the propellant to the melting point of carbon. Ammonia is of interest because it has potentially less of an environmental impact on the stratosphere. Water does not perform nearly as well but is still a viable propellant.
HOW HOT DOES THE HEAT EXCHANGER NEED TO BE?
Hotter than 1,500 K for slush ammonia or methane. Hotter than 2,000 K for slush hydrogen. Hotter than 2,500 K for liquid water.