Some comments on Vasimr by Paul March on the Nasa Forum
Vasimr has different quoted exhaust velocity ranges for different propellants –
* argon has a limit of around 50,000 m/s
* 300,000 m/s is possible with hydrogen.
The 39 day mission to Mars using VASIMR propulsion takes 200 Megawatts total input power driving 4-to-8 yet to be designed VASIMR engines that will sink 25-to-50 MW each. VASIMR can change its ISP from 1,000 seconds to over 30,000 seconds so its thrust generation efficiency can be adjusted from ~10,000 Watts/Newton at 1,000 seconds all the way up to ~300,000 Watts/Newton at 30,000 second Isp, so it really needs a big set of nuclear reactors to drive them. Dr. Chang Diaz needs to have a set of three, 75MW or larger reactors to drive his proposed 39 mission to Mars. If we could double that power level then we could deliver even shorter Earth to Mars trips times on the order of 4 weeks (28 days) or less.
Franklin Chang Diaz gave a UHCL lecture Friday night here in Houston and he disclosed that by the time the VF-200 VASIMR flight engines flies to the ISS, the development cost for Ad Astra Rocket Company will be over $150 million.
Paul March asked Franklin during the Q&A session what the VASIMR engine power level was for planning his proposed 200 MW manned mission to Mars project, and he stated that it was something in the range of 20-to-40 MW per engine. That implies 5-to-10 engines would be used for this 39 day mission senario tied to three nuclear reactors. Each engine would be consuming something on the order of 40 kW/Newton, so each of the 40 MW VASIMR engines would be producing ~1,000 Newtons.
Franklin Chang Diaz has proposed several deep space manned Mars missions using nuclear powered VASIMR engines over the last decade that take anywhere from ~120 days to ~40 days for a one-way trip time to Mars per Ad Astra’s mission calculations. The total required propulsion power runs the gamut from 12MW-e constant power for the 120 day one-way trip time up to 200MW-e for the 40 day trip time. I have been told that each of these manned Mars mission scenarios and several in between utilize three to four VASIMR engines tied to three semi-redundant fission or fusion power reactors/power converters that when summed yield the noted power levels.
These VASIMR mission proposals are being taken seriously by NASA/JSC management, so [Paul March] been tasked to perform a survey study that will look into how we could generate that kind of power levels, looking at solar, nuclear fission and fusion power generators. All of these missions require a power plant with a specific mass of less than 10kg/kW-e. It appears that the solar power option is viable for the lunar tug option in cis-lunar space, but that it becomes problematic for solar array sizes any larger than 5-to-10MW-e in size due to deployment and dynamic stability issues. You also have the issue with losing approximately half of your solar constant at Mar’s orbit, so nuclear power for Mars missions and beyond make more sense to pursue.
Vapor Core Reactors: Light and Powerful and Helped with Stronger Superconducting Magnets
A gas core reactor coupled to a disk MHD unit with superconducting magnets is the basis for a high performance topping cycle in a proposed MHD-GT (Brayton)-ST (Rankine) heat recovery combined cycle for a future Generation IV nuclear power plant Optimized studies show that such a power plant could reach nearly 70% energy efficiency. [Pulsed Magnetic Induction Gas Core Reactor, or PMI-GCR, Vapor-Gas Core Nuclear Power Systems with Superconducting Magnets]
This design would be a high fuel burn-up system with online extraction of fission products and most importantly for environmental and long-term economic viability this proposed concept would enable a completely closed fuel cycle (the only unspent nuclear fuel would result from the minimal amount of fuel in the reactor loop at shut-down when the plant would be decommissioned, this poses no long term storage problem whatsoever). These systems require two key advanced technologies, (i) materials capable of withstanding greater than 2000K temperatures and chemically compatible with uranium tetrafluoride vapor, and (ii) light-weight, highfield superconducting magnets with good radiation hardness properties.
Fissioning plasmas, such as are proposed in the aforementioned concepts, are much less dense and much lower temperature than fusion plasmas, therefore gaseous fuel systems (GCRs or VCRs) employing fission power have an immediate technological advantage over fusion power systems for stringent space exploration requirements. This is true of all fission reactors at present, but vapor-fueled reactors are the most advanced fission power sources for at least two reasons. First, they allow direct energy conversion of the heat energy released into the fuel at the highest possible quality. This is possible for example by using magnetohydrodynamic (MHD) generators through which the activated ionized fission plasma can flow. For this effect to generate hundreds of kilowatts up to many megawatts of power in a compact low mass system requires high field magnets of up to 4 to 10 Tesla or more. Even higher fields would be advantageous due to the Hall effect mode that the disk generator operates. Secondly, vapor core reactors can be constructed at almost half the mass and scale of conventional solid fuel reactors, this is because many subcomponents of conventional nuclear reactors are simplified or entirely removed from gas or vapor core reactors.
There is also a need for a new generation of navy propulsion systems. Seawater is highly conducting and a suitable fluid for MHD power or propulsion effect. Both in space power systems and on-board nuclear power for navy applications, the power conditioning sub-system delivering power to thrusters can comprise a considerable extra mass and complexity. If a G/VCR reactor was used to provide the gas flow for an MHD power generator, then this could be fairly simply and naturally coupled to a reversed MHD unit that injects energy (directly by ion acceleration) into seawater for navy vehicle propulsion. This is a potentially very efficient and compact way to provide ship propulsion. Both for space and navy propulsion a solid fuel reactor could also be used if the coolant could be activated enough by fission products and radiation to become electrically conducting, then MHD generators could again be employed for direct power conversion
Gaseous fission reactor at wikipedia
The vapor core reactor (VCR), also called a gas core reactor (GCR), has been studied for some time. It would have a gas or vapor core composed of UF4 with some 4He and/or 3He added to increase the electrical conductivity, the vapor core may also have tiny UF4 droplets in it. It has both terrestrial and space based applications. Since the space concept doesn’t necessarily have to be economical in the traditional sense, it allows the enrichment to exceed that which would be acceptable for a terrestrial system. It also allows for a higher ratio of UF4 to helium, which in the terrestrial version would be kept just high enough to ensure criticality in order to increase the efficiency of direct conversion. The terrestrial version is designed for a vapor core inlet temperature of about 1500 K and exit temperature of 2500 K and a UF4 to helium ratio of around 20% to 60%. It is thought that the outlet temperature could be raised to that of the 8000 K to 15000 K range where the exhaust would be a fission-generated non-equilibrium electron gas, which would be of much more importance for a rocket design.
Polywell and Vasimr Scaling Speculation
From Paul March from memory:
The 0.30 meter radius WB-7 Polywell reactor ran at power levels around 1,000 Watts. Scale this 0.30 m radius up by a factor of 10 or 3.0 m, that would indicated that the power output would go up by a factor of 10^7 = 10,000,000 times. A 3.0 meter reactor that theoretically could produce 10^10 Watts or 10 Gigawatts.
The WB8 project proposal discussed 100 milliwatts.
The report shall address the conceptual requirements for a polywell fusion reactor capable of generating approximately 100 milliwatts.
From M Simon:
At 100 milliwatts for a follow on reactor they are starting to get into the power range. If they can get that kind of power with .3 m diameter. coils and .8 T fields, then a reactor with 3 m coils and 10 T fields should produce about 2.5 Mega Watts if the scaling laws hold.
Adjusting the off the cuff commen:
If one were to scale up to a radius of 15 meters (30 meter cube), then that would indicated a scaling factor of (15/0.3)^7 = 781,250,000,000 * 0.1 = 7.8125×10^10 Watts or ~78.1 Gigawatts, or 78 one-GW reactors.
A Vasimr with 40kW/Newton with an Isp of 5,000 second could produce 250,000 Newton with a 10 GW reactor.
If there was a successful Mach Effect propulsion with 1.0 kW/Newton propulsion system that would imply a total thrust of 10,000,000 Newtons from a 10 GW reactor.
Superconducting Wire Will Become High Volume and Affordable and Are Making Powerful Magnets
Superpower inc is a leader in superconducting wire and magnets. They describe their roadmap to achieving mass market success with superconductors. Incremental progress increasing production throughput and improving current capacity of large volume wire will achieve market success and societal impact in about 2017.
There is 32 tesla superconducting magnet project and there are detailed designs and projects for 60 tesla superconducting magnets
Powerful and relatively cheap magnets in high volume will enable better VASIMR and could also enable the vapor core reactor and help with various nuclear fusion projects.
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Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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