Advanced Vasimr plasma drives and SpaceX BFR trip times

The Vasimr rocket is on track to a 100-hour continuous firing this summer of the company’s VX200SSTM VASIMR® prototype at a power level of 100 kW. This would produce about 2 newtons of propulsion. It would be superior to current ion drives and would be 10-60 times more fuel efficient than chemical rockets.

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.

Using hydrogen propellent and a high quality nuclear reactor a Vasimr could reach 0.1% of lightspeed.

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.

A fast manned mission to Mars project would use several engines in the range of 20-to-40 MW per engine. 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.

The SpaceX BFR rocket could achieve 40-50 day trips to Mars by using faster Parabolic transfer orbits. A future 200MW powered Vasimr would only be able to have 10-20% faster trips to Mars than a SpaceX BFR.

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, high field 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.

A Vasimr with 40kW/Newton with an Isp of 5,000 second could produce 250,000 Newton with a 10 GW reactor.