Solar System Exploration: The Missing Drive

posted by admin on February 10, 2013

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-exomoon

Nerva, courtesy Adrian Mann.

 

 

 


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31 Responses to Solar System Exploration: The Missing Drive

  1. lurscher says:

    This article is an inspiration for me, and i hope that it will be for many to remove the blindsights into this irrational fear of nuclear power.

    Really, really good writing

    • Glenn Thornton says:

      Lurscher, thanks for the kind words. You sound like a true believer in the new frontier. It’s great to know I’ve reached at least one fellow traveler.

      • lurscher says:

        Glenn, for some time i’ve been fascinated by the nuclear salt-water rocket proposed by Robert Zubrin on the 90’s ( http://path-2.narod.ru/design/base_e/nswr.pdf ), but i’ve never seen follow-up studies on it’s feasibility.

        I understand that the design is a bit ambicious, but what i don’t understand is the lack of interest of attempts to improve on something like that. In contrast, the nuclear-fragment rocket seem to be well understood.

        • Glenn Thornton says:

          Hi Lurscher, in a number of these replies that I’ve made, I seem to end up beating the same drum, more or less. I think there’s a method to my madness, but I’ll let you and the rest of the community, as well as casual visitors, make that call. I quite agree that nuclear research, in many areas, is lagging. From our point-of-view, it’s particularly lagging in terms of high energy rocket propulsion and surface and space nuclear power for manned missions to other planets; within the solar system and beyond. Any potentially “messy” nuclear research we might want to do is virtually impossible to do, owing to walls of environmental regulations and perceived – therefore true – threats to public safety and health. Even if you get all the necessary permission and the approved site for the work, what would have been a ten million dollar project ends up costing closer to a hundred million dollars. For this reason, in particular, I think it’s imperative that we get our backsides off the Earth as soon as possible and as far away as possible. To me that means Mars more than the Moon. I fear that the Moon will quickly become as hog tied as the Earth itself, in terms of treaties and international agreements, something like Antarctica, but orbiting a quarter of a million miles away. This is also why I’m inclined to go against the proverb and look very closely at any gift horse that comes to us though the government. I don’t see a robust manned space program coming by way of NASA, anytme soon. If it did, would we really want the baggage it would bring? If we can get out of Dodge, we can do all the messy nuclear research we want. I think we’ll do it with reason and consderation, but we’ll also do it efficiently and effectively. We’re going to have to move at least a small part of heaven and earth to get the NTR into service, but I think that’s possible. Once we set up housekeeping on Mars, many more things will be possible. Until then, we’ll be trapped within the walls of a website, imagining nuclear propulsion, but never realizing it.

  2. John Bensted says:

    Would good old H2O be a viable fuel instead of H for an NTR? I read once that H is difficult to store because it can easily escape due to its molecular size.

    • Glenn Thornton says:

      John, there are two paragraphs in the article that deal with this. Paragraph four talks about the best possible chemical rocket, the LOX/LH2 chemical rocket and paragraph seven compares the NTR, using just LH2, to that very same chemical rocket. The atomic weight of water is eighteen, while the hydrogen molecule, H2, is only 2. It’s the problem of the “heavy shot-put versus the “light” shot-put. This is the real edge that the NTR allows us. The LH2/LOX chemical rocket is the lightest combination of fuel and oxidizer possible that also produces very high energy. That’s as good as it gets for a chemical rocket; it can’t avoid the penalty of the higher mass to accelerate. The NTR, not requiring an oxidizer, can use the lightest reaction mass possible, LH2. You could run water/steam through the NTR and you’d get higher thrust, but much lower efficiency. In the end, you get much less bang for your buck. Does that help?

      • John Bensted says:

        Yes, thanks Glenn. My thoughts on this matter is long-term storage of fuel for long duration missions. You want to retain as much fuel as possible.

        • Glenn Thornton says:

          John, I agree that storage of fuel is an important issue, but LH2 storage has been done pretty successfully since Apollo. You may recall the fuel cells, requiring LH2 and LOX. I do think materials research may improve storage technology and that’s good, but it’s good enough at the present to make LH2 a clear winner.

          • Chris Schwehr says:

            While long term fuel tankage is a concern, I would think that long term power generation will be a primary goal also. Considering that any trip to a nearby star system will take generations with even the most advanced conventional propulsion systems, it appears to be obvious to me that nuclear power from a nuclear rocket system is the only practical solution. Fission being the only source currently, it appears that the course is obvious…develope a propulsion system that will be used as a long term power supply for the potential colonists during the voyage….

  3. Robert Lucas says:

    The only thing we actually have to get us at a fraction of light speed is at the moment ion drive or light sail. It looks like sheilding is vital as every particle that hits it hits it at that speed causing a reaction force which will slow it. The original mass needed may be prohibitive unless a hybrid of drives is used. It’s got to be big hasn’t it. A light sail would get holes in it, from atoms in space never mind anything else. What speed could you get from swinging around Jupitor and the sun, like Ulysses did? It could start something off.

    • Glenn Thornton says:

      Robert, while the optimistic and truly outward bound – I include myself – believe we will find a way to achieve a high fraction, even into the ninety percent range, of light speed, the problems facing us are anything but trivial. That’s one of the reasons I think we need to push ahead and start manned exploration of the solar system as soon as we can. It’s very likely that the advances and breakthroughs we need, in terms of propulsion and shielding, will happen in the “new world”, rather than the old. Still, even now, I think there are viable ideas that could get us to the nearest targets. While at Los Alamos, Dr. Howe, referenced in the article, suggested an alternate approach to the Orion concept. Instead of a pusher plate and atom bombs going off behind it at one per second, Dr. Howe believes we could eventually learn how to generate and safely capture/control enough antimatter to use it to make a more sophisticated version of Orion. The antimatter would be used to induce “micro-fusion” in hydrogen; this would replace the atom bombs from the original Orion concept. The pusher plate could be smaller and a bit less massive. The thrust could be more continuous and less discrete. The profile of the ship would be narrow and other power sources could provide the deflection of particles. I think these developments will come, or even better ones than anyone has dreamed, but they’ll come “out there”. We have to start pushing the envelope with what we’ve got right now.

      • Robert Lucas says:

        The Sun is the powerhouse for our Solar System. A maglev type of tube with electricity generated from that could launch vehicles from it presumably with onboard engines of their own into the Solar System. How far will a Laser travel in space?

        • Robert Gipson says:

          Voyager I very recently discovered as it left the solar system that cosmic rays and plasma are much denser outside of the solar system. This monumental discovery proffers this inexausitble energy as a means of propulsion, and the prospect of continuous acceleration (CA), with which relativistic speeds could be reached quickly. Humans can stand 2 G’s for a good while. But in front of any human flight could be a series of waves of micro-drones traveling at ever-increasing G until, approaching C, the mass of each drone slows them down; or (more likely) each drone hits the natural limit of speed of possible through using interstellar plasma & cosmic rays as propulsion. The waves of micro-drones, weeks and days ahead of the human craft, would have multiple sensors continously feeding back data to the human craft on collisions (e.g., major dust coulds) or other relevant environmental anomalies that lay ahead.

  4. Jake.-the dude man says:

    I think that a orion luanch would would be superior to NTR it could a lot more cargo and with modern tech fallout could be reduced sighnificantly.

    • Glenn Thornton says:

      Jake, in terms of greater capability and potential, there’s no doubt that Orion, or a similar propulsion system would be a step up. I think the way we get there is to take the first step into nuclear rocket territory by using the NTR for some interplanetary missions. We actually have experience with NTR technology, due to the ROVER and NERVA programs. It’s far more likely that the “nuclear nervous” public will accept the NTR as a first step, rather than the atom bomb propelled Orion. You’re absolutely right in terms of the scale of your vision, especially with respect to reaching the stars, but I believe it’s a matter of progression. Just as the V2 and many lesser chemical rockets preceded the Saturn V, I think we’ll need some intermediate steps before we reach for Orion and the stars.

  5. Think Big! says:

    Mr. Thornton, you are an inspiration!

    “I have seen the StarSeed future of mankind and it is not on planet Earth!” – Mellen-Thomas Benedict

  6. Robert Lucas says:

    I say the laser thing because if it’s big it may be able to power a craft over pretty big distances. Could always knock up a Dyson Sphere I suppose.

  7. John Bensted says:

    My recent comments and discourse with Glenn are missing. Why?

    • Glenn Thornton says:

      Hi John, I’ve seen the comments get “chopped” before; I don’t think it’s intentional. Whenever changes are made to the blog section, in terms of adding a new blog at least, it appears that some “disruptions” can occur. Maybe they’ll show back up again, but it’s all good. If you’re interested in some more direct exchanges, perhaps I can ask Richard to send my email address to you?

  8. Richard Obousy says:

    Our site just transferred hosting accounts so we lost some comments temporarily by the looks of things. I’ve had a look and there appears to be plenty of comments on this post so I believe we are back in business.

  9. Ron Stahl says:

    I’m sorry but I need to quibble with the seeming conclusion that “. . .it’s time to get behind the best technology available at the present time.”. NTR is not “available” at the present time. Even if we had a consensus, something like Pratt and Whitney’s TRITON would take years to design and build, and billions of dollars, all steps in the wrong direction, IMHO. People who are paying attention to the appropriate physics already know that J.F Woodward’s Mach Effect physics is a better answer with adequate lab results that it merits financial support and could easily pose a far better option than an NTR with less time and funding. If you think the NTR is our best available option, you need to catch up on the experimental work of Dr. Woodward. http://www.amazon.com/Making-Starships-Stargates-Interstellar-Exploration/dp/1461456223 I sincerely hope this audience isn’t going to participate in off hand judgements of work you’re not individually appraised of. If you’re not familiar with both the theory and the experimental evidence, your best course is to read the book, not make vacant comments.

    • Glenn Thornton says:

      I guess we agree to disagree. The NTR is a proven technology and a Mars mission capable NTR could be built, tested, and ready to fly in 3 years for a quarter of a billion dollars. Have a nice day!

      • Ron Stahl says:

        A quarter billion dollars? Em, I’m sorry but where do you find that figure? When the P&W team did the TRITON study a decade ago, they agreed to build such a thruster would cost at least a billion dollars and we all know what that means. It’s at least 3-4 billion dollars to build such a system. Pardon me to note it looks like you plucked your numbers from the air. Are you even familiar with the TRITON study? They wanted 15 years and you’re claiming you could accomplish this in 3? That hardly sounds credible.

        http://www.pwrengineering.com/dataresources/AIAA-2004-3863.pdf

  10. Ron Stahl says:

    As an aside Dr. Thornton, I should note to you that I supported NTR development in my advise to the Augustine Commission at the appropriate time with no results to show for it. I’m certainly an advocate for NTR. I am just noting to you, that there are other, more promising and cheaper options on the table.

    Back in 2004, I was hired to do an independent study of the various advanced propulsion options investors should consider. TRITON was one of them. However, my finding at that time was that both the peer reviewed theory of Dr. Woodward’s Mach Effect thrusters, and the experimental evidence at that time, both warranted a more careful investigation. After my study was complete, I found that Lock-Mart had done their own study as part of their “Millennial” program and come to the same conclusion, less than a year before me.

    We’re certainly agreed Dr. Thornton, that we need a next generation deep space propulsion system. We can’t go where we want to go around our planetary system using what we have now. I think though, before anyone proposes a new system that will inevitably cost billions of dollars, there needs to be far broader consensus about what this would entail. There need to be smallish investigations that fully vet our options.

    For every legitimate promise of a better system than we have at present–chemical–there are dozens of bogus, ridiculous promises posing as real answers. That’s why I was hired and that’s why Lock-Mart sent out their team of physicists to survey the landscape. We ignore these investigations at our own peril.

    Case in point, the M-E based thruster investigations could be replicated and enhanced for high thrust iterations for just a couple hundred thousand dollars. Current materials science has yielded cutting edge ceramics to be considered. Proprietary power systems are easy to engineer given almost insignificant funds. However, we’re not going to see the proper replications and appropriate scientific investigations, so long as we are constrained by tunnel vision and can only look at what was accomplished by NERVA and ROVER so many decades ago.

    We need a fresh look at our options and I heartily recommend to you a few hours inquiry into Woodward’s work before you press forward on seriously old technology that only forms a partial answer to our space-faring needs.

  11. Ron Stahl says:

    As an aside Dr. Thornton, I should note to you that I supported NTR development in my advise to the Augustine Commission at the appropriate time with no results to show for it. I’m certainly an advocate for NTR. I am just noting to you, that there are other, more promising and cheaper options on the table.

    Back in 2004, I was hired to do an independent study of the various advanced propulsion options investors should consider. TRITON was one of them. However, my finding at that time was that both the peer reviewed theory of Dr. Woodward’s Mach Effect thrusters, and the experimental evidence at that time, both warranted a more careful investigation. After my study was complete, I found that Lock-Mart had done their own study as part of their “Millennial” program and come to the same conclusion, less than a year before me, that ONLY Dr. Woodward’s work was worthy of further investigation.

    We’re certainly agreed Dr. Thornton, that we need a next generation deep space propulsion system. We can’t go where we want to go around our planetary system using what we have now. I think though, before anyone proposes a new system that will inevitably cost billions of dollars, there needs to be far broader consensus about what this would entail. There need to be smallish investigations that fully vet our options.

    For every legitimate promise of a better system than we have at present–chemical–there are dozens of bogus, ridiculous promises posing as real answers. That’s why I was hired and that’s why Lock-Mart sent out their team of physicists to survey the landscape. We ignore these investigations at our own peril.

    Case in point, the M-E based thruster investigations could be replicated and enhanced for high thrust iterations for just a couple hundred thousand dollars. Current materials science has yielded cutting edge ceramics to be considered. Proprietary power systems are easy to engineer given almost insignificant funds. However, we’re not going to see the proper replications and appropriate scientific investigations, so long as we are constrained by tunnel vision and can only look at what was accomplished by NERVA and ROVER so many decades ago.

    We need a fresh look at our options and I heartily recommend to you a few hours inquiry into Woodward’s work before you press forward on seriously old technology that only forms a partial answer to our space-faring needs.

    • Glenn Thornton says:

      It’s not “Dr.”, I’m a digital design guy. I have a good mentor on the Phd. physics side of the equation. His name is Dr. Steven D. Howe. He’s the Director of the Center for Space Nuclear Research. He paid about twenty years of dues at Los Alamos National Laboratory, before taking the helm of the CSNR. Google it and take it up with Steve, but you might want to be a little humble. I think he knows a little more about the NTR he’s been designing and developing than you do.

    • Glenn Thornton says:

      As an aside, Dr. Stahl, from what I can see of the Mach effect/Woodward effect, you might as well power it with cold fusion while you’re at it.

  12. Ron Stahl says:

    Are you saying you have it from Dr. Howe that it’s possible to build an NTR for $1/4B in 3 years? Still wondering where you got those numbers. I have no doubts that Dr. Howe is a legitimate authority on the NTR. I am just having a very hard time believing you are accurately representing his views.

    My point is that whatever we choose as a next generation deep space transport system, it is going to be expensive both in money and in time to develop it. Given this, it behooves us all to understand what are the legitimate options.

    If you tell anyone, that a fully operational NTR can be had for $1/4B, you should have a legitimate source for that number.

    • Glenn Thornton says:

      Dr. Stahl, my blog article and my comments to you are all fact based. I recognized early on that we would have to agree to disagree. You can champion and support what you think is most promising and I shall do the same. Have a good day; time will tell, regarding the NTR and your preferred technology.

  13. Ron Stahl says:

    I’m not a doctor either. Please feel free to address me as “Ron”. And yes certainly, lets agree to disagree. This doesn’t make us adversaries but rather folks with a diversity of opinions all looking for the same sorts of outcomes!

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