July 13, 2011

Zubrin Trashes the VASIMR

Robert Zubrin wants to go to Mars a couple of decades ago and he does not want to develop any new technology or wait for any new systems to get built. Robert Zubrin trashes the VASIMR plasma rocket in a recent article. VASIMR, or the Variable Specific Impulse Magnetoplasma Rocket has been claimed to be able to power future missions at faster speeds like going to Mars in 39 days if it gets scaled up to tens of megawatts.

I have reviewed detailed VASIMR proposals for the next 5-10 years.

The current one-way transit time is six months, exactly the same as a standard crew rotation on the space station. The six-month transit trajectory is actually the best one to use for a human crew because it provides for a free return orbit, an important safety feature which a faster trajectory would lack. Thus even if we had a truly superior and practical propulsion technology, such as nuclear thermal rockets (which the Obama administration is also not developing), we would use its capability to increase the mission payload, rather than shorten the transit.

The argument that we must go much faster to avoid cosmic rays is demonstrably false, as proven not only by standard radiation risk analysis — which estimates about a 1 percent cancer risk for the 50 rem dose that astronauts would receive on a Mars round trip — but by the fact that about a dozen astronauts and cosmonauts have already received such a cumulative cosmic ray dose during repeated flights on the international space station or Mir, and, as expected, none of them have evidenced any radiological health effects.

To achieve his much-repeated claim that VASIMR could enable a 39-day one-way transit to Mars, Chang Diaz posits a nuclear reactor system with a power of 200,000 kilowatts and a power-to-mass ratio of 1,000 watts per kilogram. In fact, the largest space nuclear reactor ever built, the Soviet Topaz, had a power of 10 kilowatts and a power-to-mass ratio of 10 watts per kilogram. There is thus no basis whatsoever for believing in the feasibility of Chang Diaz’s fantasy power system.

Space nuclear reactors with power in the range of 50 to 100 kilowatts, and power-to-mass ratios of 20 to 30 watts per kilogram, are feasible, and would be of considerable value in enabling ion-propelled high-data-rate probes to the outer solar system, as well as serving as a reliable source of surface power for a Mars base. However, rather than spend its research dollars on such an actually useful technology, the administration has chosen to fund VASIMR.

No electric propulsion system — neither the inferior VASIMR nor its superior ion-drive competitors — can achieve a quick transit to Mars, because the thrust-to-weight ratio of any realistic power system (even without a payload) is much too low. If generous but potentially realistic numbers are assumed (50 watts per kilogram), Chang Diaz’s hypothetical 200,000-kilowatt nuclear electric spaceship would have a launch mass of 7,700 metric tons, including 4,000 tons of very expensive and very radioactive high-technology reactor system hardware requiring maintenance support from a virtual parallel universe of futuristic orbital infrastructure.

VASIMR has demonstrated 60% propulsion efficiency

VASIMR VX-200 Performance and Near-term SEP Capability for Unmanned Mars Flight was presented at the Future In-Space Operations (FISO) Workshop

Ad Astra has demonstrated 60% propulsion system efficiency (DC to jet power) at an Isp of 4800 seconds and an input power of 220 kW DC

Solarosa (Stretched Optical Lens Architecture on Roll-Out Solar Array) has a technological readiness of about 3 or 4.

Deployable Space Systems, Inc. (DSS), in partnership with Entech Solar (ENTECH) and Carbon-Free Energy (CFE) will focus the proposed NASA Phase 1 effort on the development of our innovative SOLAROSA technology. SOLAROSA, named for Stretched Optical Lens Architecture on Roll-Out Solar Array, is a new lightweight, high performance space solar array that enables missions through low cost, lightweight, compact stowage volume, radiation hardness, high voltage capability, scalability to ultra-high power, and LILT/HIHT environment operability. SOLAROSA is a fusion of ENTECH's proven Stretched Lens Array (SLA) concentrator technology with DSS's innovative ultra-lightweight Roll-Out Solar Array deployable structural platform. The proposed SOLAROSA technology promises to provide NASA/industry a near-term and low-risk solar array system that provides revolutionary performance in terms of high specific power (over 400-500 W/kg BOL at wing level), affordability (over 50% projected cost savings at the array level), lightweight, high deployed stiffness, high deployed strength, compact stowage volume (over 80 kW/m3 BOL), reliability, high radiation tolerance, high voltage operation capability, scalability, and LILT and HIHT operation capability (LILT - Low Intensity Low Temperature, HIHT – High Intensity High Temperature).

Entech Solar is planning to commercialize the SolarVolt module in 2012. The technology is currently undergoing International Electrotechnical Commission (IEC 62108) certification testing and is protected by a number of issued and pending patents.

The SLA is a unique technology that uses ultra-thin arched Fresnel lens concentrators to focus sunlight onto highefficiency solar cells. Although the materials are different, the fundamental design is similar for space and ground PV concentrators. In space, the need is to minimize weight, while the focus on the ground is to reduce cost. The material efficiency of the SLA is critical to both. Space modules have used specially-qualified silicone lenses focusing sunlight onto multi-junction solar cells mounted to thin carbon-fiber composite radiators. The terrestrial SolarVolt module (40” x 65” x 6”) uses lower cost acrylic lenses focusing sunlight onto high-efficiency, low-cost crystalline silicon cells. Hundreds or thousands of SolarVolt modules can be installed in large arrays for utility-scale applications

Prior stretched lens array roadmap

Russians have begun funding a megawatt nuclear space reactor

The Russians are looking at nuclear power sources for space up to 1 megawatt for flying to Mars, moon base power and for orbital tugs.

About 15 tons for 1 megawatt.

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