Saturday, January 5, 2013

Using Nuclear Rockets Safely for Manned Space Travel

I am not a specialist in this topic (nuclear rocketry), but I am an older, well-experienced aerospace/mechanical engineer, and I am well-read about these things. I have no hard numbers here, but these concepts really do look feasible and fruitful for manned Mars missions (and more).

Using solid-core nuclear rockets as propulsion in anything resembling a safe manner is not a trivial issue, to be sure. I do think the applications of orbit-to-orbit transport, and planetary landing, end up addressing this risk entirely differently.

For the orbit-to-orbit transport, most interestingly a Mars mission, issues of artificial gravity should interact very constructively with the need to provide radiation shielding from your reactor. There is a need to stage-off emptied propellant tanks after every “burn”, but if the design comprises a set of docked modules, it is easy to reconfigure into the same length “slender baton” shape at each stage-off. Spin the “baton” end-over-end for gravity: 56 m radius at 4 rpm is 1 full gee at a tolerable spin rate. (This is true even for chemically-powered designs.) See Figure 1.

Figure 1 – Reconfigurable Docked Module Design Provides Both Shielding and Artificial Gravity

This reconfigurable “slender baton” shape not only maintains radius for artificial gravity at low rpm, it also maintains the much longer distance that is so very necessary for getting shielding benefits out of your remaining propellant tanks. Somewhere around 40 meters of propellant tank fluids and structures should be quite effective at shielding the crew from nuclear radiation, during or between “burns”.

The lander is a vastly different proposition. Compact as it has to be for landing stability, shielding “steady state” by distance with tanks and fluids is impossible. The alternative is tons of lead or concrete, etc, also very undesirable. But since the descent and ascent “burns” are brief (minutes only), there is no need to shield “steady state”. Using what little tankage-and-structures shielding benefit that there is, the crew need only endure brief intense exposures, integrating to a very modest accumulated dose, actually. But this does require that the crew evacuate to a surface shelter remote from the lander during the surface stay. And you keep your distance from these landers in orbit, too, except when in use. See Figure 2. However, the numbers show such landers could fly both descent and ascent on a single fueling, and in a very practical design with significant cargo capability, at Mars.

Figure 2 – Nuclear Mars Lander Safety Depends Upon Short Exposure Times

The intensity of the exposures might be mitigated slightly by a shift to thorium reactors instead of uranium. This gets the worst-offender plutonium-239 out of the picture. But, fission leaks neutrons, no matter what, so it remains a very serious risk. I like the thorium approach better than uranium, in part because of the slightly-reduced danger from shut-down cores, but mostly because it is a more plentiful fuel. But, I recognize that we have to start with what we know: uranium.

Dangers of radiation from a contained core after engine shutdown (the worst risk of all) could be mitigated greatly by the open-cycle gas core concept, which is essentially an “empty steel can” between “burns” (see Figure 3). The light-bulb concepts feature a retained core, and suffer the same risks as solid core after engine shutdown. Only induced radioactivity in the engine shell is still a problem, and that is far less intense, and it decays far quicker. That’s why I’m such a fan of developing the open-cycle gas core technology as soon as possible, although we still have to start with what we know, that being a highly-enriched uranium solid core. No one has ever actually yet built and tested an open-cycle gas core engine, however.

Figure 3 – Radiation Danger Sources With Nuclear Rockets

I am also a very big fan of revising the nuclear rocket engine to use water instead of liquid hydrogen as the propellant fluid. I think the better heat-absorbing characteristics of the water may allow higher reactor power levels, thus offsetting in part the loss of specific impulse due to the higher molecular weight. This has never been implemented and tested. The water provides a better radiation shield, and logistically is far easier to handle and store in space. Further, it seems to be very widely available as native ice, at many interesting destinations in the solar system, including Mars.

There are several related articles on this site where I have posted design concepts and estimated performance numbers for “typical” designs, for both the orbit-to-orbit transport, and the landers. The best and most realistic of these articles are listed below, by date, title, and a content summary:

9-6-11 Mars Mission Second Thoughts Illustrated - looks at a revision of the original modular design in my original Mars Society convention paper, one with solid core nuclear/min energy transfer propulsion, instead of gas core/fast trip. Otherwise, the mission scenario and hardware designs are the same as that paper. This is revisited,  and the discussion extended,  in 4-23-12 Update to Mars Mission Design.

7-25-11 Going to Mars (or anywhere else nearby) the posting version – is the posted-here version of my original Mars mission paper at the Mars Society convention in Dallas, Texas, August 2011. The transit vehicle baseline propulsion in that paper was gas core nuclear/fast trip. The reusable solid core nuclear single stage lander performance is outlined very well in that paper. This lander design is very over-conservative, as no credit was given to aerodynamic drag during descent: the descent delta-vee was assumed to be the same as the ascent delta-vee.

7-19-12 Rough-Out Mars Mission with Artificial Gravity – this is the first analysis I ran that deliberately explores the integration of spin-generated artificial gravity into a slow-boat mission by using reconfigurable docked modules to build the orbital transport vehicle. This one assumes the same 60-ton reusable nuclear landers as the original paper, plus solid core nuclear/slowboat propulsion. The landers go with the transport in this analysis. In the original paper, they went separately. But the lander propellant supply still goes separately to Mars in this article. This design uses the 56 m radius figure at 4 rpm to provide 1 full gee of artificial gravity.

6-30-12 Atmosphere Models for Earth, Mars, and Titan – this provides realistic and traceable data for the “typical” Mars entry environment for lander design purposes. (Typical atmospheres for Earth and Titan are also defined.)  I got these data from a posted NASA paper,  cited within. 

7-14-12 “Back of the Envelope” Entry Model – this provides a traceable and realistic means of quickly estimating how much velocity reduction can be achieved on descent to Mars (or any other location with an atmosphere), given a ballistic coefficient, a velocity at entry interface, and a descent angle at entry.  I corrected the heat transfer model from a 1956-vintage warhead re-entry calculation that was discussed in the same posted NASA paper.  The dynamics were fine. 

8-10-12 Big Mars Lander Entry Sensitivity Study – sensitivity study of end-of-entry altitude to ballistic coefficient,   primarilyfor grazing entry from low Mars orbit.  The inherent grazing entry angle from low Mars orbit is important,  and this shows up in the sensitivity study.  Run with the corrected 1956-vintage model.

8-12-12 Direct-Entry Addition to Mars Entry Sensitivity Study – grazing entry for a typical interplanetary direct transfer speed. As expected,  higher entry speeds do put end-of-hypersonics at a much lower altitude.  Run with the corrected 1956-vintage model.

8-28-12 Manned Chemical Lander Revisit - A two-stage non-reusable chemical Mars lander design and performance estimate, using the entry results and direct rocket braking for the descent propellant requirement. Ascent is “standard” rocket equation with experiential “jigger factors” for gravity and drag losses. This is a multi-engine approach, using slight cant for supersonic retro plume stability. This article uses a more realistic ballistic coefficient than the design presented in an earlier article.  Run with the corrected 1956-vintage model.

Some closely-related discussions are given in two very recent articles:

12-31-12 Mars Landing Options – this one discusses the choices to be made (and their effects on mission design) of in-situ return propellant manufacture versus bringing the ascent propellant with you, and of landing one vehicle at any given site versus landing several close enough together to interact effectively.  Too close is also a bad outcome.

12-31-12 On Long-Term Sustainable Interplanetary Travel – this one discusses the merits of choosing a solid-core nuclear propulsion design revised to use water instead of hydrogen as the propellant fluid. Future developments that might prove beneficial are also explored. 


13 comments:

  1. When you say 'solid core' do you mean something like NERVA rather than something like Timberwind (particle bed reactor)?

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    1. jstults:

      Thanks for the comments. I do appreciate hearing from readers.

      I think any design is a candidate for investigation (NERVA, DUMBO, and TIMBERWIND). Of those, NERVA was essentially flight ready and very well-tested. The other two were not.

      So I think the NERVA is the first that should fly. But, all three(and more besides) should be investigated and tested. Anything performing better should replace NERVA.

      The program should be continuously funded, and should yield a series of improved designs replacing earlier ones. I do not rule out gas core designs in this process, both open-cycle and nuclear lightbulb approaches.

      Finally, different applications might optimize out with different design approaches. You just need a safe and long-term inexpensive place to test them, and I'd consider doing it on the moon.

      If I can figure out how to do it, I'll sign the petition for which you furnished the link.

      GW

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  2. Gary,

    Please do not make the mistake Of thinking that by replacing a uranium based system by a thorium one, you won't have shielding issues. Since NERVA was uranium based, it is clear that any system that would need to be implemented would need to use that Same fuel.

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    1. Oh, I know a thorium system needs shielding. Just maybe not quite as much. Still significant, though.

      GW

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  3. Gary,

    Both uranium and thorium cycles produce the same fission products which are really the ones we shield against (on top of the neutron flux and its interaction with the reactor). A scrubbing mechanism for NERVA would be required the same way for a Thorium based rocket.

    Igor.
    PS: I love your blog entries.

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    1. Hi Igor and thanks ...
      After shutdown, the retained solid core of a uranium reactor always contains plutonium derived from the U-238 content. I don't think there's any plutonium in the thorium cycle. Plutonium is the wrst radiological threat I know about. But, as I said, I'm no nuclear expert.
      GW

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    2. Gary,

      Don't believe everything you read in the news. Plutonium per se is not the "worst radiological threat" you think it is. The radiological issues are mostly from fission products with half life near 30 years. Pu239 has a half life of 24000 years and therefore will be much much less of a concern as opposed to Cesium and Iodine fission products. From Wikipedia on toxicity of Pu (read till the end (http://en.wikipedia.org/wiki/Plutonium#Toxicity ):

      "...Several populations of people who have been exposed to plutonium dust (e.g. people living down-wind of Nevada test sites, Hiroshima survivors, nuclear facility workers, and "terminally ill" patients injected with Pu in 1945–46 to study Pu metabolism) have been carefully followed and analyzed. These studies generally do not show especially high plutonium toxicity or plutonium-induced cancer results.[91] "There were about 25 workers from Los Alamos National Laboratory who inhaled a considerable amount of plutonium dust during 1940s; according to the hot-particle theory, each of them has a 99.5% chance of being dead from lung cancer by now, but there has not been a single lung cancer among them."[97][98]..."

      Thorium or U cycle yield the dame fission products and need to be shielded in the same manner.

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  4. Gary! Long time no-"see"! After the old NM forums closed down I had so many issues, (still do actually) I haven't ever gotten back on. Coupled with my computer dying which lost me the majority of my "favs" (including yours :) ) I dropped down to only keeping tabs on the NASAspaceflight.com forums.

    Now that I've re-found your site I see I have a lot of reading to do :)

    Point:
    "I am also a very big fan of revising the nuclear rocket engine to use water instead of liquid hydrogen as the propellant fluid. I think the better heat-absorbing characteristics of the water may allow higher reactor power levels, thus offsetting in part the loss of specific impulse due to the higher molecular weight. This has never been implemented and tested. The water provides a better radiation shield, and logistically is far easier to handle and store in space. Further, it seems to be very widely available as native ice, at many interesting destinations in the solar system, including Mars"

    Water and solid core NTRs do NOT mix well. Point of fact the water tends to disassociate in the reactor causing numerous issues (very hot oxygen in against your core matrix, VERY bad) and then recombine in the throat/nozzle in an uncontrolled manner which is also an issue.

    Despite the "short-comings" LH2 remains the best propellant for an NTR followed by Ammonia.

    In regards to Thorium as a NTR fuel, the main issue with it is that it isn't self-fissiling and needs a neutron source to transmute to U-238.

    Igor: Thorium cycle reactors do NOT produce the "same" fission products as a "standard" U235 cycle system. They do NOT produce plutonium which is why they were dropped in favor of U235 reactors for commercial power plants. The majority of the research was directed at U235 fission in order to produce Plutonium for weapons. In order to "benefit" from the government research into reactor design and construction commercial reactors had to use the same design.

    Since they can't initiate or sustain fission without an external neutron source thorium reactors tend to be much less radioactive when they are "off" than the standard U235 based reactor.

    Gary; Speaking of an "external source" of neutron for the thorium cycle have you heard of the "Hybrid Thorium" or "Hybrid Fusion/Fission" reactor?

    The idea is to use a Gas-Dynamic Mirror pulsed fusion system to generate the neutrons needed to transmute the thorium in a specially designed "blanket" around the fusion "plug" and initiate fission. The advantages are that the pulsed plasma fusion process is very much easier to initiate than steady-state fusion. It has a much lower power input and maintenance needs (no where near "break-even" on its own and it doesn't need to be) and it avoids the need for high-temperature systems and fluorine that the standard "Liquid" thorium reactor designs use.

    Info here:
    http://www.transpowerusa.com/wordpress/clean-energy/hybrid-nuclear-reactors/

    http://www.transpowerusa.com/wordpress/clean-energy/hybrid-nuclear-reactors/technology-overview/

    What I find REALLY interesting is that Boeing is partnered with this group both as advisers on commercial applications but also specifically to work on aerospace applications. Which are addressed here:
    http://www.transpowerusa.com/wordpress/space-transportation/

    Specifically they are looking at a hybrid thorium powered aerospace plane with some pretty wild "destinations" in mind:
    (Check the chart at the bottom of this page)
    http://www.transpowerusa.com/wordpress/space-transportation/commercial-space-plane/operating-goals/

    I'd be interested in your "take" on this when/if you have the time :)
    (Oh they have a website specifically for the spaceplane here: http://www.myspaceplane.com/)

    Randy

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    1. Hey Randy! Glad to see you again!

      Have you found the new NewMars forums yet? They lost some of the stuff, but there's been a lot since the crash.

      It'll take some time, but I'll check out the links you sent. Nope, I never heard of that hybrid thorium fusion (??!!) thing before. Glad somebody's looking, though.

      GW

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    2. Randy,

      In nuclear engineering fission products refer to " atomic fragments left after a large atomic nucleus fissions." (see wikipedia, http://en.wikipedia.org/wiki/Nuclear_fission_product ). The other elements which you seem to refer to (Pu) are actinides and we don't care as much about them because some of them will naturally be part of future nuclear fission reactions of the reactor core which themselves will yield fission products.

      When shielding a reactor core, we shield mostly against those fission products since a few of them have half lives in the neighborhood of 30 years (not good).

      Thorium may not produce plutonium but plutonium is not your main concern when it comes shielding which was the initial issue Gary was hinting on.

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  5. Oh yes and I forgot THE "go-to-site" in the inter-web:
    Atomic Rockets of the Space Patrol!
    http://www.projectrho.com/public_html/rocket/index.php

    (Seriously is IS a VERY serious site dealing with advanced propulsion and real "realism" in science fiction :) )

    Randy

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    1. Yeah! I've been to Atomic Rocket. Pretty good site, especially the technical stuff where the nomograms are.

      GW

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