Back in the days when I was studying Old Icelandic (this was a long time ago, well before Centauri Dreams), I took a bus out of Reykjavik for the short journey to Þingvellir, where the Icelandic parliament was established in the 10th Century. It was an unusually sunny day but that afternoon the storms rolled in, and just before sunset I remember looking out from the small hotel where I was staying to a rainbow that had formed over the lava-ridden landscape. It inevitably brought to mind Bifröst, the multi-colored bridge that in Norse mythology connected our world with Asgard, where the gods lived. The idea may have been inspired by the Milky Way.
In the world of rocketry, a new Bifröst has emerged, one designed to link the nuclear rocket technologies that were brought to a high level of development in the NERVA program with our present-day propulsion needs. For despite a serious interest that resulted in a total of $1.4 billion in research and the testing of a nuclear engine, NERVA (Nuclear Engine for Rocket Vehicle Application) was cancelled at the end of 1972. The work that went into the concept dated back to studies at Los Alamos starting in 1952 and extended through the 1950s with Project Rover.
The design in question is a nuclear thermal rocket (NTR), which uses nuclear fission instead of chemical combustion to heat a hydrogen propellant, making for both high exhaust velocity and high thrust. As Tabitha Smith notes in an article on Project Bifrost, Rover/NERVA technologies were once regarded as a natural follow-on to the Apollo missions, cited by President Kennedy in the same speech in which he challenged the United States to land a man on the Moon and bring him home by the end of the decade. In fact, some believed NERVA could get us to Mars before 1980. Its advantages were clear: If used as an upper stage for the Saturn rocket (Saturn S-N), the nuclear technology would have allowed payloads as large as 340,000 pounds to reach low Earth orbit, up from the 280,000 pounds that the all-chemical Saturn V could achieve.
Image: NERVA nuclear rocket under test. (Smithsonian Institution Photo No. 75-13750).
Anti-nuclear sentiment was part of NERVA’s downfall, but so too was the post-Apollo retreat from space and the expenditures it involved. It was probably NERVA’s link with a possible Mars mission that caused many politicians to put on the brakes, unwilling to see NASA commit itself to an even more intractable and expensive goal than the original Moon missions. Whatever the case, nuclear propulsion has been in the doldrums ever since, which is why Project Bifrost has sprung into existence. The abovementioned Tabitha Smith is research lead for Bifrost as well as being chief strategic officer for Washington DC-based General Propulsion Sciences. Along with colleague Brad Appel at GPS, Smith initiated the project in collaboration with Icarus Interstellar.
Appel sees nuclear technologies as a major step toward next-generation space travel, drivers for the manned mission to Mars we have been anticipating since the dawn of the space era and the concept studies of Wernher von Braun. Smith quotes Appel on the advantages of a nuclear thermal rocket:
“…imagine you are planning a road trip from New York to Los Angeles and back. Except, there are no gas stations along the way — you need to pack all of the fuel along with you. Using a chemical rocket to send humans to Mars would be like making the road trip in a cement truck. You might barely make it, but it would be one enormous, inefficient, and expensive voyage. Using an NTR, however, would be more akin to taking a Prius. It’ll make it there comfortably, and it can go a lot further too.”
Project Bifrost makes sense, given that while commercial space companies like SpaceX are moving to become cargo carriers to LEO, there is little work within the US government to further develop concepts like NERVA. Smith was recently in Moscow to pursue the idea of international collaboration in nuclear thermal rocketry, invited there as part of President Dmitry Medvedev’s initiatives to spur entrepreneurship and international collaborations in Russia. Whether or not the journey bears fruit, General Propulsion Sciences and Icarus Interstellar intend to bring NTR technologies up to date. A nuclear alternative to chemical methods would spur renewed interest in a Mars mission once thought to be all but inevitable before the end of the 20th Century.
Probably dumb question – but how does a nuclear rocket sit with international non-proliferation treaties and treaties on the peaceful use of space? Might such a rocket simply be illegal?
Someone here will know the answer, I’m sure.
“and the concept studies of Wernher von Braun. Smith quotes him on the advantages of a nuclear thermal rocket:”
“…imagine you are planning a road trip . . . taking a Prius. ”
Prius?
Your Wernher von Braun quote referencing a Prius is completely fabricated! Seriously, I was thrown until I realized you were using the pronoun “him” to refer to Appel way on the other side of your paragraph.
Very good to hear this.
Since NERVA, there was a classified program Timberwind that developed the Isp to beyond 1000 (Saturn Vs got about 250). The Starship Symposium talk with Geoff Landis and Stan Borowski of NASA Glenn detailed ongoing theoretical engineering work there, too.
Lofting a NTR into low orbit on a chem rocket, flying fuel in drone rockets, then starting an NTR from cold condition avoids most of the anti-nuclear complaints (not that they’re rational). Polls show the public isn’t that skittish about nuclear rockets, too.
Are we talking NTR to orbit, or NTR in space only, or both? NTR to space is not going to happen because of the potential contamination due to an accident, and I expect the opposition to even mildly radioactive exhausts.
NTR in space is a different matter, but there is still going to have to be some very good explanation concerning avoiding an accident delivering the NTR vehicle or components to space where they can be safely used.
If people can get all bent out of shape with RTG power units, they are hardly going to be mute over systems many orders of magnitude larger and more powerful.
All the figures I have ever seen show Isp’s for NTR at no more than 3x traditional chemical engines. This doesn’t seem like much of a performance benefit to me, given the risks.
There are many other potential propulsion systems for different missions, many of which offer potentially very high performance without the associate nuclear risks.
We could pre-position nuclear fuel in LEO with the safest possible launcher. Then construct a modular Mars colonial ship piece by piece. Assemble and fuel it at ISS. The reactor will serve the colonists for years. Let DnD do it secretly. (Maybe they already have). Mars lands need to be surveyed and sold to investors. Just like Dakota and Alberta were 1oo yrs ago.
Re my statement:
I’ve changed the text to avoid the confusion noted by comments above. It was Appel, not von Braun, who made the Prius statement.
I wonder who has a sophisticated space and nuclear program, lots of money, resources, and humanpower, and a big barren wasteland to test and launch nuclear rockets in?
I include Orion in this mix.
From NY to LA and back is nearly 5600 miles. The max range on a Prius without refueling is perhaps a tenth of that. Couldn’t find a better analogy?
if this is just talk i am willing to have fun to, if this is serious then the party’s involved should attempt to sign a space act agreement with NASA.
A space act agreement might be a great way to do in end round of DOE and a space act agreement might be an implicit agreement by the government that it is indeed legal to do ground testing of such an engine. The NRC would have to approve.
The figures seem strange here. I have seen 127,000 kg listed elsewhere as the maximum delivery to LEO of a Saturn V configuration, include fuel to be used up upon eventual trans-lunar injection. 340,000 lb is not three times this figure, at least certainly not if you’re talking of pounds as a unit of mass not weight, and if those are of the avoirdupois type.
@ Greg Benford
Lofting a NTR into low orbit on a chem rocket, flying fuel in drone rockets, then starting an NTR from cold condition avoids most of the anti-nuclear complaints (not that they’re rational).
Please elaborate on how you suggest lofting the nuclear engine in a fail safe way.
What happens in the event of a failure? Are you assuming that there can be protection against rupture, or are you assuming rupture isn’t important?
I would agree on Orion . Russia has plenty of nuclear weapons left as well why not go for 8 percent light speed?
I hope Tabitha Smith brings this up
It makes sense Russia Energy Inc should expand its Aerospace Division.
The current Chairman (Putin) and CEO(Medvedev) have some restive shareholders
Not to get all meta, but geez, Brett. First of all, it’s not Paul’s analogy. Second, the idea isn’t to analyze the actual feasibility of driving a Prius from NY to LA without refueling, but comparing the ease of that trip to in a Prius vs. a concrete mixer. Third, even if you were going to over-analyze the analogy, the better comparison to space flight would be the Prius, plus a full cargo load of fuel taken with you.
To add to Alex’s point, I wonder if cost is an issue as well? Any ideas the cost comparison between a single NTR be as opposed to three chemical rocket launches? And is that 3x number based on the models from 50 years ago? Presumably there may be more efficient design possibilities today.
You could easily pack the required ~100 extra gallons in a Prius, with the back seats down. I like the analogy….
WvB: “Except, there are no gas stations along the way — you need to pack all of the fuel along with you.”
There’s your mistake right there.
NERVA was tested in open air in Nevada, releasing very little radioactivity, which dispersed to undetectable concentrations. Most people don’t know there’s low level radiation everywhere, especially near granite.
The myth of widespread opposition to use of nukes in space is ancient. Polls don’t show it. The small demonstration against the Cassini launch was paid for by a self-promoter. The true reactor the Russians let deorbit fell in Canada and had such a small effect it was never even found.
The treaty regulating uses of nukes in space has clauses allowing propulsion and power use if guidelines get followed. The treaty itself, like nearly all others, has a one year exit clause. Treaties don’t establish laws, either. What court would you go to?
Gregory Benford said on January 27, 2012 at 1:47:
“The myth of widespread opposition to use of nukes in space is ancient. Polls don’t show it. The small demonstration against the Cassini launch was paid for by a self-promoter. The true reactor the Russians let deorbit fell in Canada and had such a small effect it was never even found.”
The Mars rover Curiousity is carrying an RTG and unlike with Cassini, I did not see or hear anything against it. Maybe this due to people having other things to focus on and worry about in this economy, or maybe it has something to do with the fact that some of the big environmentalist leaders were starting to change their flocks’ views on nuclear power as a cleaner alternative to fossil fuels. At least that is what was underway before the earthquake in Japan that wrecked those old-style nuclear power plants.
As for the Russian satellite reactor, are you referring to Cosmos 954, which craashed upon Canada in 1978. They recovered about 12 pieces of the satellite, two of which were radioactive.
For manned flight I would think Thrust to Weight ratio is much more important than Isp, not that it still isn’t important. In your example, a high Isp is like the Prius going 10mph and taking years to get you from Ny to LA.
@ljk
The Mars rover Curiousity is carrying an RTG and unlike with Cassini, I did not see or hear anything against it.
From which statement we are to conclude that a full fledged nuclear reactor is as innocuous as a low power RTG?
@ Greg Benford
From your comment, I take it that you are OK with exposed fissile material falling to earth. A US launch from Florida or the EU from French Guiana would result in the reactor falling into the ocean and sinking after a launch failure. But reentry, like Kosmos 954 could result in pieces falling on populated areas. Just how sanguine would you be if pieces fell over LA, or NY? We already know that commercial nuclear reactors are only feasible economically with government caps on liability, else the insurance costs would make the delivered power uneconomic. The insurance costs of launching an NTR could make the project a non-starter. Environmentalists may or may not be irrational, but insurance companies with serious skin in the game? If the Space Shuttle had nuclear engines, we would have had a nasty mess to clean up over Texas when Columbia disintegrated.
Source the fissile material elsewhere (the moon?) and deliver it to the waiting spacecraft, that is a whole different game.
Fission rockets are sort of limiting, there are no good sources of uranium known beyond earth, and if there are there is no industrial base to refine and even isotope enrich this… so all flights have to originate from earth based uranium 235 ( which is less radioactive and much safer than than Pu). in addition, radioactivity from use of the rocket is a problem for the passengers, and these highly radioactive components are hard to engineer ( the booster has to have a very high burn to fuel ratio. The component just “rot” under the heavy neutron/gamma ray bombardment.
by contrast the solar powered ion drives used by Dawn are apractical or near term solution, and we can wait for fusion ( or godot) .
Just a question, if we only ever have fission and chemical rockets will we explore the stars? I doubt it. Using a chain of small fission bombs to push a ship has about zero chance of getting funded and sounds like a sort of desperate measure. Solar sails , anti-matter, fusion or exotic matter or physics are needed for the stars, and fission rockets are not needed for the solar system.
sorry for sounding harsh. It is an opinion. Fission as a source of electrical power for colonies, well that is different. Ship U235 for use on mars, that might be useful until the infrastructure is built for more permanent solutions. Safe nuclear reactors that are efficient are not light, even nuclear power subs depend on the vast heat sink of the oceans. we sould need material form in place to build and fill the heat exchange system, though the core and key reactor parts can be supplied from earth. It turns out there is plenty of water on mars. ( imagine saying that with certainty just 10 years ago!)
kittlej said on January 27, 2012 at 15:21:
“Just a question, if we only ever have fission and chemical rockets will we explore the stars? I doubt it. Using a chain of small fission bombs to push a ship has about zero chance of getting funded and sounds like a sort of desperate measure.”
Orion may not find funding in America, but there are other spacefaring nations with nuclear capabilities who may not have that issue. Plus, the very fact that the United States would hobble itself over such a propulsion method could be a strong incentive to those other nations to go ahead with the project.
I think I’ve worked it out. The phrase “the nuclear technology would have allowed payloads as large as 340,000 pounds to reach low Earth orbit” must be a printing error. Saturn V would allow a maximum mass of 45,000 kg to reach escape velocity. Given the uncertainty in the Saturn configuration utilised this is about a third the above figure, so Paul probably meant escape not LEO when he compared rocket efficiencies.
Standard nuclear fuel is uranium enriched so that u-235 is ~3%, instead of the naturally occurring <1% . It still mostly u-238, bound in ceramic pellets. This doesn't really become hazardous until it been in a reactor for awhile, as the neutron bombardment coverts some of the uranium into various isotopes of plutonium, and the decay products build up. So launching virgin nuclear fuel into space really doesn't constitute an additional hazard.
In a few months, we should know how the Polywell research has turned out, and the Lawrenceville Plasma Physics experiment is still coming along. We may get fusion powered propulsion yet.
Thomas Eddison knew it . If you want to do something really difficult that noboddy has ever done before , Try Everything . Until nuclear-fision rockets has been thoroughly explored and developed , nobody can know where this might lead us . In order to make a systematic design contest (for the best avaiable propultion systems) , you have to expore all leads far enough to know how they can compete whith each other across a wide range of conditions . And dont make such a feeble -harted fuss about the prospect of an increase of 0.0000% in your chance of cancer . The insurance companies wont .
Rob Henry writes:
Actually, I was pulling the information from this Los Alamos site:
http://www.lanl.gov/science/NSS/issue1_2011/story4full.shtml
The quote:
But I’ve evidently made an error in the comparison between nuclear and all-chemical Saturn configurations somewhere along the line and will fix that in the text. The 127,000 kg figure looks to be correct (roughly 280,000 lbs.) for chemical Saturn to LEO.
kittlej:
The important characteristic for propulsion fuel is its energy density. It can be expressed as the ideal exhaust velocity for that fuel: the velocity that the waste products can attain if all the liberated energy is used to accelerate them. For fission, this exhaust velocity is around 0.1 c, for fusion not much more. For antimatter it is c.
You can see, then, that fission is all the way up there in terms of usefulness for interstellar travel. It’s main advantages, ease of obtaining, storing, and burning the fuel, are matched by no other approach. It is also the only one that we could do now, without requiring a technology breakthrough.
However, NERVA style engines are not going to cut it, with exhaust velocities many orders of magnitude to low. You need either inertial confinement (the small bombs you mention), or a fission fragment rocket. Solar sails and power beaming are pretty hopeless. Exotic physics, well, we’ll talk about it after it happens. Don’t hold your breath.
You could sleep with the fuel from an even moderately enriched reactor design before it is started up. It is after criticality that fission reactors become dangerously radioacive. A reactor assembled orbit before startup would be safer than an RTG 1/100th the size (unless you plan on using plutonium for fuel). Obviously this would be a motor that would never land on earth again.
For launch to LEO the nuclear Saturn used an NTR Upper-Stage which would only activate above a certain altitude. If it misfired and fell back into the ocean, the unactivated core would be as hazardous as unused fuel elements – unhealthy due to uranium toxicity, but not a radiological risk. But the other point, often neglected in these discussions, is just how rugged an NTR core is – handling thermal conditions HOTTER than re-entry heating during normal operations and strong enough to withstand multi-bar pressures at 3,000 K, thus much higher pressures at normal temperatures. In otherwords a reactor core is going to hit the ground intact and not disperse its contents to the four winds. Even more importantly a core using oxide fuel elements is chemically unreactive and not about to enter the biosphere.
“It’s main advantages, ease of obtaining, storing, and burning the fuel, are matched by no other approach. ”
I would actually disagree with this, in the context of interstellar travel, though not interplanetary. Achieving really high exhaust velocities with fission require that the fissile element be most of the fuel, and building a craft which can carry a large fraction of it’s weight in pure fissile isotopes without achieving premature criticality in the fuel is not a straightforward thing. Very challenging, to say the least.
For interplanetary travel, OTOH, you really want to use the fissile material to heat a larger fraction of light elements, which eases greatly the problem of keeping your U235 from prematurely disassembling your craft.
And so, this is the arena in which I think I’ve heard interest expressed about using Thorium in lieu of (or as a way to consume) U-235, U-238 and or Plutonium. I wonder how it might be used, either for ground-based or space-based sourcing strategies. Anyone know if it would help/hinder the whole “launch fail-safely” issue?
“building a craft which can carry a large fraction of it’s weight in pure fissile isotopes without achieving premature criticality in the fuel is not a straightforward thing. Very challenging, to say the least.”
Is it really so difficult? There’s a great deal of available, er, _space_. Just store it in a very dispersed form: a very large but very thin sheet, for example. Since there’s no gravity and the needed accelerations are very small, the structure can be very weak and still work. You can’t launch it from earth, but any economical starship has to have its propellant made from a low escape velocity object anyway.
Brett: You are right, I did not consider fuel criticality. It is a problem, but probably not as bad as trying to contain He3 for a long time. Either a spatial arrangement that is non-critical, as Nick suggests, or a breeder type fuel cycle that relies on a non-critical precursor as the primary fuel. Perhaps U238, or Thorium, as Ed suggests.
thorium does have some distinct advantages as a power source in orbit or on a colony. and yes U235 is not all THAT radioactive prior to fission, so launching uranium or thorium supplies makes sense. . – So I could see this begin used to support the electricity needs of a mars colony. If there is thorium on the moon ( and hopefully mars) we could practically expand its use to the outer solar system. but using it as as a source of heat for a thermal nuclear rocket that may be difficult. Beyond that, I have not seen any practical proposals for using the fission directly in propulsion other than having kiloton explosions and uncontrolled release of the radioactive products.
I do not think Thorium can be used directly to construct a practical nuclear explosive device. ( this actually makes if a great fuel for civilian uses) . if “emerging” economies were really serious about development of nuclear power plants and not nuclear bombs, they would spend the effort making a thorium reactor functional and not on the expensive ( cold war era) technology of isotopic enrichment.
Some comments on all this…
U235 has low radioactivity, and is present in the environment anyway, so its radioactivity is not itself a hazard in launch accidents. The concerns would be accidental criticality or U235 falling into the wrong hands.
If I am not mistaken, nuclear rockets use highly enriched uranium, and are fast reactors. There is no mass budget for large amounts of moderator. The poster who said the uranium would be enriched to 3% would not be correct.
Mining uranium in space for use in nuclear reactors in space will not make sense for a very long time, because of the need for enrichment. The mass of the enrichment facility (and the mining equipment) will be much greater than the mass of highly enriched material they could produce — so it would be much cheaper to just ship the fabricated fuel elements (or entire rocket engines) up from Earth.
It’s not really obvious that nuclear thermal rockets buy you all that much, even for going to Mars. The killer is the mass of the engine and associated shielding.
If high specific impulse does turn out to be useful, there are ways to increase the Isp of a nuclear thermal stage beyond that of the original NERVA. The basic idea is to gradually heat/expand the hydrogen to generate electricity, before finally running the hydrogen through the reactor core proper for explusion out the nozzle. In the nozzle, the electricity would be used to arc-heat the hydrogen beyond the thermal limts of the fuel elements. No thermal waste heat radiators would be needed for this scheme.
Paul D very sensible post. however if you are using electicity to heat the hydrogen in sort of an “afterburner” you might as well simplify the design and use the nuclear pile to make elecricity and then have a separate hydrogen plasma rocket less worry about design of high temperature nuclear components and radioactivity leak issues . With the separate plasma system there would be little or no radioactive release. the only issue is how to efficiently make electricity without a large heat sink. If only we had a way to convert fission power directly to current instead of having to use a glorified heat engine.
RTGs with far “scarier” plutonium are launched in re-entry containers designed to survive intact on launch explosion or orbit re-entry. A NERVA in geocentric orbit could receive a shipment of uranium fuel encased in such a re-entry canister.
Now that Pluto is no longer a planet, shouldn’t we re-name this element? :)
kittlej: the Isp of the system would be maximized if all the reaction mass is expelled at the same velocity. Having a slow stream and a fast stream would reduce performance.
As an aside: nuclear rocket engines are interesting in that, unlike chemical rocket engines, the limit on performance is set by entropy rather than energy.
In a chemical rocket, there is a fixed quantity of energy available per mass of propellant.
In the nuclear rocket, by contrast, there is potentially an arbitrary amount of energy available, but each unit of energy comes with a certain amount of entropy (how much depends on the temperature of the fuel elements). All this entropy must be carried away somehow. If it all goes into the propellant, the entropy of the propellant stream at the exit plane of the nozzle sets an upper bound on how much energy can have been used. This also means nuclear rocket engines should be designed to have as little thermodynamic irreversibility in them as possible (if ones goal is to maximize Isp).
paul d. getting rid of heat entrophy- hence we are back to heat radiators.. they could be relatively small if you are not in a hurry to dump heat quickly. this would argue for a low acceleration/ high ISP system . plasma or ionic drives…
one idea is to use the local mass in space but it is exceedingly low density and collecting and accelerating is is good idea but I am not sure how much realistic work has been done on the idea.
Alex Tolley wrote:
Here’s what getting 3x the Isp can mean, for the same mission:
1x: 35% ship and payload
Over seven times the useful mass seems like much to me.
bchurch wrote:
No such luck, the limiting factor is how hot you can run the reactor before it melts.
How about the Dandridge Cole/Helios Nuclear Pulse Engine:
http://up-ship.com/blog/?p=5353
Think of it as an Orion under control.
jkittle: I was trying to avoid radiators, since they have terrible power density. The scheme I described would not require them.
Please pardon me as I am an absolute ignorant in physics, but I have a question:
Eniac says that “for fission, the exhaust velocity is around 0.1 c, for fusion not much more”. From Wikipedia it seems that a fast neutron such as from a fission reaction has a speed of 14,000 km/s. Then how could an exhaust velocity of 0.1 c be achieved with fission? Thanks for the answer.
Jean-Pierre, a simple calculation involving the mass fraction converted to energy during fission shows that the fission fragments which carry most of the energy are expelled at about 0.1 c. Specifically, we equate the potential energy converted (0.007*mc^2) to the final kinetic energy (0.5 m v^2) and obtain v/c = sqrt(0.014) ~ 0.12. Actually, there is some loss due to the neutrons and assymmetric fragment mass distribution, but it certainly is in the ballpark of 0.1 c. See http://en.wikipedia.org/wiki/Fission-fragment_rocket on how this might be achieved.
Thanks Eniac for the answer.
The WP article you point at, covers a technology which is much more advanced than the thermal or “chain of small fission bombs” that KittleJ discussed.
In your answer to my message you discuss about an unspecified mechanism that can convert perfectly mass in energy, this mechanism is probably impossible to achieve with a fission mechanism. I also don’t understand what you mean by “we equate the potential energy converted … to the final kinetic energy…” and what is 0.007.
In my message what I tried to tell was that as the quickest heavy elements from a fission explosion were the “fast neutrons”, it was likely that more classical ejectas such as thermal neutrons (much slower), heavy ions or debris (much, much more slower), would not permit the 0.1 C speed.
In the Clark/Sheldon article that is summarized in WP, the high-speed fragments are used directly as rocket exhaust material, which is similar to the best case I had in mind.
My understanding of the C/S article (which is poor) is that in the original (Chapline) design, charged particles are used as exhaust material because a magnetic field is involved for collimating the flow out of the reactor. This Chapline design seems an unlikely mix between an atomic reactor and the fragile mechanism of a Winchester hard disk.
From my layman point of view, the Clark/Sheldon looks more realistic.
The Clark/Sheldon proposal seems to me to also involving charged particles, in fact electrons and ions are mentioned and later atomic number between 4 and 140 are indicated in table 2.
Their speed is indicated to be between 3% and 5% of C. Other sources sometimes tell about 2% light speed for fission products. Probably neutrons do not play any role for propulsion in this proposal. Those speeds are consistent with the claim of a perfect case of 5% of light speed.
Probably there is a lost of speed for the heavy particles that started their travel backward or sideward through the dust cloud before they were deflected by the magnetic field toward the exhaust axle (~ 5/6 of the total), but that doesn’t matter.
So to summarize the Clark/Sheldon article comforts me in the idea that “classical” fission based propulsion mechanisms like thermal reactors or “chain of tiny bombs” can’t be used at all for interstellar travel. The Clark/Sheldon design is much more effective and realistic than most other fission proposals but as they write it is fine for the Solar system exploration and even the Oort cloud might be too far.
I am continuing my exploration of the Clark/Sheldon article (http://www.rbsp.info/rbs/RbS/PDF/aiaa05.pdf). I think that the estimated quantity of fuel is overly optimist.
The power requirements are estimated by C/S by assuming an exhaust speed of 0.05 C, then assuming a mass ratio of 3% to incorporate in a rocket equation to estimate the maximum speed.
In turn this maximum speeds permits to evaluate the energy required for a given trip, then the power is deduced.
All the numbers used until that step are reasonable, but now the authors tell that the fission fuel is taken as equal to the mass ejected which was calculated by the means of the rocket equation.
This is a strange reasoning as it suppose the nuclear reactions are nearly perfect, 46% of nuclear fuel must be converted in exhaust material at 0.05 C. In reality it doesn’t work that way, nuclear reactions are imperfect and less than 5% of the fuel is useful.
I don’t know if it’s true but I feel than the efficiency should be even lower as at 0.05 C the fuel stays around 10?9 seconds in the reactor core. Fission is only possible if there is a high density of neutrons in the fuel, if the nuclear reactor moves at nearly the same speed than the neutrons, the efficiency should drop in my view.
Anyway if we use the burnup numbers (http://en.wikipedia.org/wiki/Burnup) for CANDU
(gw * days ) / (10 * M) = 7 (Candu)
M = (208 * days) / 70 (in metric mass tons)
7592000 / 70 = 108457
So around one million metric tons (weight) of fuel is necessary to keep the rate of exhaust material.
Indeed there are more efficient fuels, and the S/C proposal is several times more efficient than a classical nuclear reactor:
(gw * days ) / (10 * M) = 60 (REP UOX)
M = (208 * days) / 600 (in metric mass tons)
7592000 / 600 = 12653
Let be optimistic and say as in the article that the S/C reactor efficiency is 2 times of a traditional reactor: It still needs more than 100,000 metric tons of fuel, where the article tells of 240 tons.
Not only this is a good fraction of the total of nuclear fuel on Earth but it renders all the calculations obsolete as it was based on a payload of 3% of the total rocket mass. This fuel mass in turn needs more power, etc…
Nevertheless my feeling is that there are other possibilities still unexplored. What we need to go to the nearest star is a process roughly 1000 times more efficient than the one described by Clark and Sheldon article. Even fusion can’t provide such numbers or perhaps only barely. And there are many other technological difficulties that need to be overcome, for example the fact that a nuclear core is full of fast neutrons being one of them, the fact that the proposal must rest on wide operational margins to stay functional each of the 36,500 days of the 100 years travel. The fact that it must be possible to repair the engine safely, etc.. The great thing that Clarke and Sheldon show in their article is that perhaps it’s not as much a theoretical physic challenge than an enthusiastic technological challenge!
No, I assumed that 0.7% of the mass is converted to energy, which is far from perfect and is what I understand fission reactions yield.
The former is a formulation of energy conservation, the latter is the 0.7% from above. The energy produced in a fission reaction is 0.007 mc^2, where m is the mass of the fissioning nucleus. Assuming the energy is converted completely into kinetic energy of fission fragments, their combined kinetic energy is 0.5 mv^2, where m is the combined mass of the fragments (nearly equal to the mass of the original nucleus), and v is the velocity with which they are expelled. We assume here that all fragments have the same velocity, which is not true, but should be good enough in first order. Perhaps that explains the difference between my 12% and the S/C 5%.
You make a good point in that real reactors burn only a small percentage of their fuel. This must be changed for a fission fragment rocket. We must be able to fission most of the fuel and expel most of the fragments at their original velocity. It is a tall order, for sure, but still much easier than doing the same for fusion.
Looks like my numbers were not correct. This reference
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c4
gives numbers of 0.375% and 0.1% for the mass defects of fusion and fission, respectively. So, more correctly, v/c = sqrt(0.002) ~ 4.5%, which is very much in line with the S/C estimate. Fusion would give a little less than double that.
It came to me that the requirement of expelling the fission fragments also helps increase the burn-up fraction. As I understand, the reason for small burn-up fractions in current reactors is neutron poisoning by some of the fragments which remain in the core. So, if it can be arranged that the fragments are expelled as reaction mass, as is necessary, increased burn-up may come naturally as a consequence. Of course, the fuel must also be pure, e.g. 100% enriched U235, or whatever other fissionable nucleus is found appropriate. Pure fuel also helps the burn-up fraction.
One more comment on fusion vs. fission: 2:1 is already not that great an advantage, but it gets worse: Fusion reactions usually have one large and one small fragment, which hurts propulsion efficiency substantially. Even worse, in the most feasible reactions one of the fragments is a neutron which cannot be utilized at all.