By Dr. Robert Zubrin, SpaceNews, 07.11.11
“[C]ritical to deep space exploration will be the development of breakthrough propulsion systems.” — U.S. President Barack Obama, Kennedy Space Center, April 15, 2010
The Obama administration claims that it is developing a new breakthrough propulsion system, known as VASIMR, which uniquely will make it possible for astronauts to travel safely and quickly to Mars. We can’t go to Mars until we have the revolutionary VASIMR, they say, but just wait; it’s on the way, and once it arrives, all things will be possible.
Washington is a city known for its smoke and mirrors, but rarely has such total falsehood been touted as a basis for science policy.
VASIMR, or the Variable Specific Impulse Magnetoplasma Rocket, is not new. Rather, it has been researched at considerable government expense
Existing ion thrusters routinely achieve 70 percent efficiency and have operated successfully both on the test stand and in space for thousands of hours. In contrast, after 30 years of research, the VASIMR has only obtained about 50 percent efficiency in test stand burns of a few seconds’ duration, and that is only at high specific impulse. When the specific impulse is reduced, the efficiency drops in direct proportion. This means that the VASIMR’s much chanted (but always doubtful) claim that it could offer significant mission benefit by trading specific impulse for thrust is simply false. In contrast, this capability has been demonstrated by the ion-drive that propelled Dawn spacecraft on its way to an asteroid. Finally, if it is to be used in space, VASIMR will require practical high temperature superconducting magnets, which do not exist.
But wait, there’s more. 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. Yet it would still get to Mars no quicker than the 6-month transit executed by the Mars Odyssey spacecraft using chemical propulsion in 2001, and which could be readily accomplished by a human crew launched directly to Mars by a heavy-lift booster no more advanced than the (140-ton-to-orbit) Saturn 5 employed to send astronauts to the Moon in the 1960s.
That said, the fact that the administration is not making an effort to develop a space nuclear reactor of any kind, let alone the gigantic super-advanced one needed for the VASIMR hyper drive, demonstrates that the program is being conducted on false premises.
Far from enabling a human mission to Mars, VASIMR is primarily useful as a smokescreen for those who wish to avoid embracing such a program. Yet their entire case is disingenuous, because in reality, there is no need to develop any faster propulsion system before humans venture to the red planet. As noted, 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. (Cosmic ray dose rates on the space station are fully half of those in interplanetary space — half because the Earth blocks out half the sky. The Earth’s magnetic field does not shield effectively against cosmic rays. As a result, over the next 10 years, space station crews will receive the same number of person-rems of cosmic radiation as would have been received by five crews of equal size flying to Mars and back over the same period.) As for avoiding zero-gravity deconditioning, the practical answer is to simply prevent it entirely by rotating the spacecraft to provide artificial gravity rather than waste decades and vast sums in a futile effort to develop warp drive.
NASA has spent a lot on VASIMR, but its real cost is not the tens of millions spent on the thruster but the tens of billions that will be wasted as the human spaceflight program is kept mired in Earth orbit for the indefinite future, accomplishing nothing while waiting for the false vision to materialize. That is why, as unpleasant as it might be, this illusion needs to be exposed.
The Mars Society is holding its next international convention in Dallas, Aug. 4-7, 2011. Currently, we have a panel scheduled, titled: “VASIMR: Silver Bullet or Hoax.” I invite Chang Diaz and a colleague to come and take two of the four spots on it and defend the practical value of their concept in formal public debate.
Let the truth prevail.
Robert Zubrin is president of the Mars Society (www.marssociety.org) and author of The Case for Mars: The Plan to Settle the Red Planet and Why We Must, an updated edition of which has just been published by The Free Press.