Air Force Research Laboratory has a 2008 report that provides an overview of advanced concepts for space access.
* attempt to understand whether chemical systems can ever be replaced
* assess the technology’s potential of providing a cost-effective means of placing objects in orbit around the Earth within the next 15 to 50 year timeframe.
* Two Air Force relevant missions have been used in this study to assess potential launch concepts. The first mission involves placing a large communications satellite in geosynchronous Earth orbit (GEO) which is complicated by the requirement of a relatively large Delta V. For the GEO mission, an analysis based on the availability of a notional space tug, a LEO to GEO transfer vehicle, has been performed to assess the usefulness of the space tug concept from the perspective of launch vehicle design. The second mission involves placing a micro-satellite, with a mass of about 100 kg, in a low-Earth orbit (LEO) which is complicated by the requirements of low-cost and rapid response.
The 2008 Space Access Situation
Photonic Laser Thruster, Lasers and Mirrors
Recent work by Bae has brought the possibility of launching small payloads to orbit using photon pressure. In the Photonic Laser Thruster (PLT) concept, a thrust amplification of up to 3,000 times has been demonstrated by forming an optical cavity between two planar surfaces. Assuming no losses, approximately 5 MW of laser power would be required for a thrust-to-weight (F/W) ratio of one for a 10 kg payload. Although unlikely to provide enough thrust at reasonable power levels for launch with current laser systems, the trend in high power laser development could make this a viable system for launching small payloads in the future.
Photonic propulsion and fusion work of Young Bae (nextbigfuture)
Photonic laser propulsion (nextbigfuture)
Laser ablation propulsion is capable of providing much higher thrust levels than the PLT concept; however, this benefit comes at the price of carrying propellant (laser ablatant) on the vehicle. In two notional designs by Phipps, et al. the payload mass fraction to low Earth orbit ranged between 4% and 27.5% depending on the size of the vehicle and the number and configuration of laser launch stations utilized.
In order to achieve a relatively high thrust laser propulsion system, high power lasers producing high temperature gas flows are necessary. Plasma formation in a nozzle can create temperatures as high as 10^4 - 10^5 K. However, sustaining a plasma in a high mass flow environment requires power levels of 100 to 1000 MW for a typical launch system. Pulsed laser systems have been proposed to ionize the propellant inside a nozzle increasing the thrust generated by creating a high temperature plasma jet. The power density required to ionize a typical working gas is in the range of 5 x10^14 and 10^15 W/m2 once again emphasizing the need for high power laser systems. These concepts generally suffer from the requirement for highly accurate focusing optics on the launch vehicle.
The laser lightcraft concept is envisioned to be a multi-stage system with the first stage driven by an air-breathing aerospike, utilizing a beamed, ground-based laser to form air detonations that propel the vehicle. Two types of lightcraft engines have been examined using either simple laser-thermal or more complex magnetohydrodynamic (MHD) concepts. The initial lightcraft design was a reconnaissance or telecommunications vehicle weighing 100 kg and envisioned to be boosted by a 100 MW-class, ground-based laser. (Smaller arrays of thousands of lasers can be used)
Microwave Propulsion Concepts
Microwave propulsion concepts can be put into two categories that include heat exchanger or propellant heating and plasma formation options. Since the 1930’s, microwave source development has seen exponential increases in Pf (power x frequency). Current estimates suggest that an array of 300 gyrotron sources operating at 140 GHz and 1MW power levels is sufficient to place a 1000 kg satellite into orbit. Gyrotrons appear to be one of the most versatile vacuum electronic devices capable of producing high average power in the 30-300 GHz range. The maximum average power range for gyrotrons is approximately 2 MW; however, some types of gyrotrons can produce upwards of 30 MW peak power. Conversion efficiencies of approximately 50% are expected from current gyrotron sources. In a notional design, Parkin and Culick suggest a payload mass fraction approaching 5-15% after system optimization.
Comparing Beam Propulsion
The efficiency of both laser and microwave transmitters is expected to be in the 35% to 60% range with laser system efficiency generally higher. Using the notional laser and microwave heat exchanger concepts, Kare and Parkin estimated the cost of a beam source capable of launching 100 kg. Their conclusion was that both systems would cost in excess of $2 billion USD with the microwave system costing slightly less (but probably within the error estimates of the comparison). Even though the microwave system exhibits about 30% less cost per Watt of power generated, it will require almost 2.5 times the power to be generated at the source.
Propellantless Launch Concepts
The cost per unit mass could be as low as $600/kg compared to $20,000/kg of the space shuttle if the required launch rate can be achieved. Laboratory rail systems have achieved very high exit velocities, accelerating a small 7g launch packages to 7 m/s, showing that there is no fundamental barrier of achieve the require muzzle velocity. The amount of energy needed to be stored for an EM rail launch system is tremendous. To launch a modest 1,250kg package would require muzzle energy or 35 GJ. Assuming an 80% energy conversation, an input energy of 44GJ would be required. It has been suggested that these energy levels could be supported using high-speed rotating electrical generators. Even a very light payload of 10 kg would require 250MJ which is comparable to one of the largest energy storage facilities at Sandia National Laboratory. Launch velocities for a railgun launch to LEO require velocities of approximately 10.6 km/s including losses.
There is more discussion of space elevators, space towers, launch assist and breakthrough ideas. This will be covered in a follow up to this article. Pretty much all of the ideas in the overview have been covered here at Nextbigfuture. I will add links to the relevant Nextbigfuture articles (you can also follow the tags below). The beamed propulsion section of the overview had some details that Nextbigfuture has not covered.
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