Solar System Exploration: The Missing Drive

This guest blog is a contribution from Glenn Thornton. Glenn is recently retired from a career at Los Alamos National Laboratory. He worked on underground nuclear testing at the Nevada Test Site for four years and then joined the Lab’s satellite design group. He made important contributions to several satellite projects, some NASA funded projects among them. He designed the data acquisition systems for the Lab’s neutron spectrometers that flew on Lunar Prospector and a similar instrument for Mars Odyssey. Lunar Prospector was the first orbiting lunar probe to use remote sensing (neutron spectrometer) to definitively identify water ice deposits in the permanently shadowed craters at the lunar poles. Mars Odyssey is still orbiting and returning data from Mars. It has produced detailed global maps of subsurface ice deposits on Mars. The neutron spectrometer can see about a meter deep and identify the percentage of water ice within a “footprint” that the instrument sees as it travels over a region. Glenn currently lives with his wife Pam and their mischievous cat, Tilly, in Santa Fe, New Mexico.

A year before I was born and a full decade before Sputnik, one of the pioneers of modern science fiction had his first novel published – the year was 1947. In a world that still struggled to recover from the horrors of the Second World War, the author depicted an optimistic future. Ironically, the backdrop for his vision was the V2 rocket; Nazi Germany’s most destructive Vengeance Weapon. He was able to look past its dark origin and see the V2 as a promising technology. Previous developments, military or otherwise, were nothing more than upgraded fireworks in comparison to the V2. The V2 was the first rocket to reach suborbital space, but to Robert A. Heinlein, it did much more; it pointed to the Moon and beyond…

Perhaps a bit giddy about the promise of the V2, Heinlein envisioned a world where the chemical rocket was in everyday use. He saw rockets zooming around the planet, delivering mail, freight, and passengers. The rocket was simply the next step in high speed transportation and he expected it to be adopted as quickly as the train, automobile, propeller aircraft, and the jet. After jet comes rocket; what could be more logical? Yet, as inspired and optimistic as he was, he still couldn’t see chemical rockets taking us beyond the Earth, not even to our nearest neighbor.

Heinlein understood the energy density and efficiency problems that confronted the chemical rocket. He realized that even a trip to the Moon would require a monstrous tower of fuel and oxidizer. It was too extreme for serious consideration, but he couldn’t foresee Sputnik; the Soviet first strike that put a priority on advanced rocket technology and launched the Space Race. Spurred on by the heat of the Cold War, NASA and Wernher von Braun constructed that tower of chemical propellants and engines, stretching three feet higher than the entire length of a football field, including end zones. The Saturn V was both an amazing achievement and a definitive statement about the limitations of chemical rocket technology.

The best possible chemical rocket uses liquid oxygen, LOX, and liquid hydrogen, LH2. Their combustion produces plenty of heat and pressure, and consequently very high levels of thrust. Yet, the efficiency, or specific impulse (Isp) is low, about 450 seconds at best. The thrust is high, primarily due to the high atomic weight of the water molecule – eighteen – but that’s also the reason for the low Isp. Heavy atoms and molecules are harder to accelerate than lighter ones and efficiency is all about the exhaust velocity. For a given energy input, lower mass atoms or molecules can reach a higher exhaust velocity. Just imagine yourself choosing between an eight pound shot and the Olympic sixteen pound shot; which could you accelerate more effectively and get the most distance from?

One alternative, and perennial “new girl” on the block, is electric-propulsion, or EP. There are numerous flavors of EP, but they all involve ionizing a gas in some fashion so it can be accelerated electrically, hence electric-propulsion. EP drives are efficient, in terms of the exhaust velocity of the reaction mass, but the key word is “electric”. EP drives don’t produce high thrust levels and they need a source of electrical power – that means mass. If you choose nuclear power – necessary if you expect to travel past the orbit of Mars – or solar, you’ll be christened NEP or SEP accordingly. In either case, there’s a very delicate balance between thrust and the mass of the electrical power plant needed to produce it. In turn, this puts more severe restrictions on the actual payload that can be transported. Realistically, you have to include the mass of the power source, along with the mass of the chosen EP drive, to compute the T/W ratio, but most EP proponents choose to draw their electrical power from the future. In the future, a low mass, high output power source – probably nuclear –  will be available and that’s the one they’ll use; when that happens, EP, or NEP, will be much more attractive.

There’s always room to dream, to question, to look outside the box to find an edge, but developments tend to follow a progression. The chemical rocket will continue to have its uses and EP drives will continue to evolve, but we need something better to do the heavy lifting for interplanetary manned missions – now. At the present time, the only effective way to apply nuclear power to propulsion is to make a fission powered rocket. That fission powered rocket is the nuclear-thermal-rocket, or NTR. The NTR is a true nuclear rocket; the heat of fission is used to directly heat hydrogen to produce thrust. It’s simple, direct, and reliable. The acronym NTP is also used, referring to nuclear-thermal-propulsion.

The NTR can be effectively compared to the LOX/LH2 chemical rocket. The NTR uses uranium and the heat of fission to replace the LOX. In the bargain, a great many tons of LOX are carved out of the mass of the rocket and the LH2 becomes the total reaction mass. Narrow channels in the nuclear fuel rods transfer the heat of fission to the hydrogen, producing a higher exhaust velocity and a rocket that’s at least twice as efficient as the LOX/LH2 chemical rocket. If we want to climb higher – and avoid falling flat on our faces – we ought to use all the rungs on the ladder. They each have something to teach us.

If we take the EP approach and plug our NTR into the future, we have the possibility of even more advanced drives. These far off visions would employ compact fusion power plants, or even stored antimatter that can be controlled safely, to superheat hydrogen in a direct application of nuclear power to generate high thrust at very high efficiency. These implementations would also be true nuclear rockets, but we don’t have fusion power plants, much less low-mass, portable fusion power, and the ability to generate, bottle up – and control – serious quantities of antimatter, is nowhere near the horizon.

The NTR has its detractors, but many are the same folks that are against nuclear power in general, while others are looking at old data. One of the concerns about the NTR has always been thrust to weight ratio, with respect to the engine, or reactor. Early on, the T/W was just a little more than one to a high of about three. Current technology, as reported by the Center for Space Nuclear Research, CSNR, in Idaho Falls, Dr. Steven D. Howe – Director, supports a T/W of twelve for their tungsten cermet fuel elements. The NTR, with a T/W of twelve, could potentially lay claim to one of the holy grails of rocket propulsion; the SSTO, or single-stage-to-orbit. These figures refer to the solid core NTR. Almost no development and very little study has been done on the liquid core and gaseous core versions of the NTR.

Utilizing all the tools at our disposal, the liquid fueled chemical rocket, SRBs, and the NTR, we have the means to return to the Moon and mount the first manned missions to Mars. If we get to work now, we’ll be a multi-planet species much sooner than we expected. If we take a pass on the NTR, it would be as sensible as our stone age ancestors passing up the stone club or ax, preferring to stick with the good old hand held rock until the machine gun comes along. Apollo happened and succeeded because there were forces that pushed us in one direction and forced us to make the most of the best technology available, the chemical rocket. NASA and von Braun pushed that technology to its absolute limits. We know the results. Now, if we want to take the next step into that new frontier, it’s time to get behind the best technology available at the present time.

I’m concerned about the present time, because our only path to the future is through the present. Each day, it seems, Kepler reveals a new, more Earth-like planet than the day before. As Kepler’s search compiles more data, the statistics favor ever greater numbers of possible new “Earths” and point to the probability that at least one will be in our neighborhood. Current predictions, fueled by data from Kepler, indicate that an Earth-like planet may well be found within a dozen light years of our home world. Still, a light year is long haul, much less a dozen of them. Some form of nuclear power will be an absolute necessity to future interstellar travelers. They’ll need a formidable propulsion system and the power to maintain themselves through what we hope will be a “relatively” short trip, from their perspective. The path to an advanced nuclear propulsion system needs a beginning. I think the fission-based NTR is that beginning.

Like the chemical rocket that preceded it, the NTR is no panacea; it simply has twice the efficiency of its predecessor with very good T/W potential. Future development and use will undoubtedly widen that gap. We can stick with the chemical rocket and continue to explore LEO, while we wait for fusion power plants that fit in a briefcase, or we can pick up where Apollo left off and start exploring the solar system now. The NTR, is the next rung on the ladder. We should step up and use it.


Nerva, courtesy Adrian Mann.