It seems to me that thermonuclear energy might be adapted to space propulsion uses before it could provide comical civilian electric power. For one thing there will be a considerable amount of practical engineering to advance from the first demonstration fusion reactor to an economical utility application. Space systems tend to use emerging technologies. Plus fusion would provide the level of specific impulse that rival system couldn’t easily compete.
Fusion also would avoid the obvious political problems that an Orion-type system has, i.e. the nuclear test ban treaty for one. Also, it seems that the most me some political interests must oppose or fear fusion when you see Congress responding to success with budget cuts. If it were done as part of an interplanetary space program those concerns would be sort circuited. Later the technology could be spin-off to earthy applications.
I suppose a spacecraft could be built that uses somekind of particle beam, or laser implosion system to do a partial fusion burn--and use an onboard fission reactor to provide enough auxiliary power to sustain the fusion side. In effect this would be a fusion-augmented, pulsed ion drive of some kind. However, I would argue from an engineering standpoint, that the amount of fusion power that one would achieve would probably be measurable, but not enough to measurably generate more thrust than if the fission supplied power were shunted to a VASIMR type thruster system.
The VASIMR system would be simpler, more reliable, and more efficient in this operating mode.
Significant fusion propulsion can only be achieved if there is significant fusion power production. Otherwise, it makes more sense to simply use the power generated by the fission plant for propulsion.
Incidently, there is an interesting concept called the Mini-Mag Orion which basically creates a mini-nuclear pulse propulsion system where small fission-fusion "capsules" are imploded using a Zeta-Pinch mechanism. It is interesting because the system uses a fission reactor as an auxiliary power unit, and the amount of fission-fusion thrust is significant. It bridges the gap between low thrust-high efficiency systems such as ion and VASIMR thrusters, while producing a lower magnitude of thrust than the old Orion Nuclear Pulse concept. I'm not sure how feasible such a system is, but it is interesting. There is an informative site provided by the company that originated the concept: Andrews Space.
Dear Members What are you thinking about antimatter initiated fusion pulse for a Orion-Type derrivate? E.g. Nano-antimatter boms incorporated in a mini fusion bomb,where the antimatter (nano bomb)ignitor will be inserted in the fusion unit before it will jumped out to rear pusher plate. This system could theroretical start von Earth without secundary radiation. Kindley Regards:
Well, here the main problem is containment and then manufacture of antimatter. Since it would take literally billions of kilowatt-hours of energy to generate sufficient antimatter to intiate fusion for a modest jaunt to the outer planets and back, I really don't see the performance advantage in trying to obtain antimatter. It's better to just use the power source--probably a nuclear fission reactor for auxiliary power.
Antimatter will become important if and when humanity becomes serious about travelling to the stars. There the energy density requirements for such a mission may become important enough to drive the technology to development. Right now, I can't imagine safely storing more than just a few hundred micrograms of the stuff--and that wouldn't initiate much fusion at all (atleast not enough to propell much of anything!)
This site sure has some well-informed people on it! There is a lot to address.
It will depend a lot of time frame. I’m certainly not saying that fusion will be the next system to be deployed after the current chemical rockets. Nuclear thermal is probably the closest advanced system to realization. Project NERVA was well advanced when it was cancelled during a period of spectacular political error. Systems based on these technologies could give specific impulses in the range of 850 sec to 1000 sec. After this nuclear electric systems such as ion or plasma (such as VASMIR) should come next offering very high specific impulse at the sacrifice of thrust. We also could do classic nuclear pulse but I think this will not fly politically.
The fusion options are later in time and require more research. Inertial confinement fusion could mimic the Orion concept without requiring the scale or violating the test ban treaty. It should have a high specific impulse than a true Orion given all of the bomb components that would have be repeated for each charge. This approach will require some other power source such as nuclear reactor (fission) to power the laser that detonate the pellets just as GoogleNaut states. Also because the explosions are reactively small compared to the kiloton range bombs planned for Orion magnetic fields can be used to direct the resulting plasma thrust rather than a giant pusher plate. I can recall discussing this idea in general term’s way back in college. But, it has been develop on a far more serious level by scientists and engineers. A good source on this is a collection of papers edited by Terry Kammash titled Fusion Energy in Space Propulsion. He also goes into the idea of using anti-matter to generate muon-catalyzed fusion. I tend to agree with GoogleNaut that anti-matter is impractical.
The other concept for thermonuclear propulsion is magnetic confinement. The Kammash book analyzes a concept of using a version of the classic "mirror machine" that would let the plasma leak out of one end to provided an ultra-high specific impulse thrust. One of the papers presents and analysis of two concepts: D-T fusion and D-He3 fusion both reactors having Q=1. The first has the mass of a navy cruiser and the other the mass of four aircraft carriers. And them would need a power source to drive them. As presented they are impractical. Plus one requires thousand of kilograms of tritium and the similar amounts of He3 basically fuel not to be had. I think these concepts are still worth consideration since if made practical they are close the ultimate allowed by the known laws of physics. One needs to get the Q>3, the mass down considerably, and a different fuel, i.e. pure deuterium.
Dear Members I´m agree Orion Type are political sensible.Maybe it will gets a exclusive release von time to time when it´s reliable and proof by UNO or competent authority? Also the launch location and the -manner is very importend.A subscribted 4000ton type started from Antarktika vertical without a transfer orbit to it´s destination could be accepteable.Antarctica is nearly unlived(the penguins living only in near of the coasts).The launching-place should be near in the center of Antarctica and in a circle of 500km should be permanently evacuated for safety reason.This project could only becomes true under whole world authority only. The second question is the propellant (mini fission- or fusion-bombs we need tausands of them)In this relation it´s very importand to save them for terrorism and geneally misuse!An international gremium could do this equivalent to Civil Nuclear Watch. In this case at the first time nuclear bombs will use as civil power scorce,not as weapon! Maybe international nuclear bombs ban must be armended? The third question is if we start with solid booster rockets or jackass-manner. Booster rocket start could prevent primary and secundary radiation when we need fission type boms.Jackass is my opinion will denied (altrough the pulse are "only".1kilotons )becaue the very hot plasma could be harmful to atmosphere and invironment generally.So we can´t affort us negative publics! So I told about vertical rising of the spaceship,the same at landing,whereby it will be use aerodynamic breaking and finally liquid rocket-engines for the final landing at the lauching -place. Best Regards for you all:
I think the original Orion 4000 ton design with kiloton range bombs is a brillant use of circa 1960 technology to achieve high specific impulse/high thrust density propulsion. If we had pursued a more radical space program post-Apollo is sort of space craft might have been actually built. Also, if we had to confront an immeadiate threat such as an small asteroid threatening Earth we might use an Orion type ship bombs and all to save the world.
As a side item I believe that instead of trying to blow up and asteroid with a giant bomb what would make more sense would be to divert it. One could use a version of nuclear pulse propulsion in that you would take a very large number of nuclear charges to the asteroid and bury them to some optimium depth. Then detonate them in sequence using the ejecta from the explosions as propellant to generate a thrust to alter the asteroid course and cause it to miss Earth.
However, by the time we might get a real space program to build true interplantary spacecraft (beyond initial Mars missions) I think that technology will have passed Orion by. I believe that Ted Taylor had calculated that bombs of about 5 kilotons are required to make effiecient use of fissionable material that drives use to such are vechicles. Which laser fusion one can detonate small pellets and achieve energy releases very much smaller and not even have deal with material dangerous in the wrong hands.
A really good idea to use Orion like a fire brigade against comets & co to destroy them or drive them into sun or let it crashed on rear side of the moon. I have red in John´s article fission bombs must have 1-5 kilotons!?-I thinking it was calculated 0,1kilotons means 100tons TNT equivalent.My "freehand"calculation deals with Saturn 5 parameters weight and fuel consume: 2900tons by 15tons fuel/second in relation of Orion 4000tons one 100ton bomb/second is relative close together.Because orion´s pulse-manner isn´t so efficient like a closed combustion chamber is. Maybe pellet technique will advance,like mentioned of yours could be resulting a closed rocket-like propusion system without pusherplate and hydraulic accellerator compensator systems,could came thrue-? Maybe some of the momentane seperate concepts(Orion/Vasmir/Nerva/Gascore...) will be combinated to get the optimum results. Always happy landings: Martin
I just want to clear up one point. One can make bombs smaller than 1 kiloton but only with an inefficient use of nuclear materials or by making them heavier by using a lot more high explosives to accomplish implosion. A book that gives the a lot of information on Orion is "Project Orion" by George Dyson. What is so implessive is what is possible with 1960 technology.
The use of bombs for propulsion is not a very efficient use of energy. The laser fusion approach is the natural successor to the Orion system.
I´m agree,with fission bombs we are fixed in a specially energy range,like you told about.Mikro fusion bombs,laser or antimatter ignigted could bring successful efficient performance to fly.Fission bombs are a very rusticale way in our time.When we collect all the projects within last 50years and combine with modern results of science(ITER,SDI-results,laser and quanten-mechanic,nano-technique,etc.),we´ll get a really spaceship! My opinion is not if we can do it ,but only when and how it will be done! Sine three decades is spaceflight frozen at the late sixties becaue lack of a good and reliable powerscource/propulsion. My policy in this manner: 1.) We need a propusion system could start from earth directly. Therefore we need a high performance density to get a accelleration greater 1g. Could be solved by micro-fusion units in max 1000kg units supported with injection of destillated water to get a high mass/second. 2.)At space we need a very high specific impuse,therefore water-injection stops,with the further positive result accelleation becomes a lower value. 3.)The landing of other planets/moons and the Earth could go at the same manner by injecting water to increase thrust and "Vortriebswirkungsgrad"!Sorry I can´t translate this in Englisch,means the relation of exhaust velocity and cruise speed.When exhaust and speed are came closing together the value becomes greather.That´s why rotocrafts have big rotors and not for jet engines arround. 4.) Public,trade and political engagement is urgend required to make it thrue. 5)And money,depend of point 4. Kindley Regards:
This is a good idea. However it will be more difficult to make fusion propulsion systems that work inside the atmosphere. I might work with the inertial confinment fusion (ICF) systems. The water augmentation gives momentum to the propellant flow. There will be some issues of radioactivity in anycase.
I've generally thought that some form a chemical propulsion will be need to get to orbit but perhaps I'm being too conservative. Then the advance propulsion system would be orbit to orbit.
Various schemes involving combinations of chemical and nuclear propulsion systems have been explored, some quite extensively--a few have been explored on other threads on this web board. One of the more interesting proposals regarding a 4000 ton Orion Nuclear Pulse vehicle, was to 'simply' straddle the retracted pusher plate and forward vehicle aeroshell fairing with a ring of 12 five segment space shuttle solid rocket boosters. Currently four segment SRB's as used by the space shuttle develop about 2.69 million pounds of thrust each. It seems to me that I've read that the 5 segment motors are closer to 2.9 to 3 million pounds each. Assuming 3 million pounds, with a gross take off weight of approximately 1.5 million pounds, means an almost 2 to 1 thrust to weight ratio at takeoff. So each motor can in effect lift something close to 1.4 million pounds. Times 12 gives 8400 tons---what this means is that such an arrangement could lift a 4000 ton Orion at nearly 2 g's right off the pad. Or a larger Orion, say a 6000 ton version, and toss it quite nicely to 25-30 kilometers altitute even before the nuclear pulses start!
It's an interesting idea.
One of the problems facing an Orion vehicle will be an effective reaction control system. It was shown on a previous thread that even modest corrective menuvers will eat propellant reaction mass like mad! The bigger and heavier the vehicle, then the more RCS propellant is needed for a given total 'velocity increment.' This relationship is linear--double vehicle mass, double RCS propellant loading. For space shuttle-like station keeping and menuvering, a good place to start in answering this question because of the extensive flight history already logged--it can be shown that almost 15-20% of the gross vehicle orbital mass will be RCS propellant. For the Space Shuttle this is about 30 to 40 thousand pounds of propellant. For a 4000 ton Orion, almost 800 tons will be chemical RCS propellant (assuming similar propellant combinations, such as hypergolic nitrogen tetroxide/monomethyl hydrazine used by the shuttle which achieves about 230 sec of specific impulse.) Going to a high performance liquid hydrogen and oxygen RCS system with 400 seconds of specific impulse will reduce this propellant loading by 1-230/400 = 0.425 or 42.5%. A RCS propellant loading of 460 tons would still be needed!
For an Orion vehicle, effective RCS design is critical to the success of the vehicle. It is startling to realize that even with all the power of pulsed atomic fission to lift incredible payloads--the basic rocket equations still -violently- assert themselves by eating up nearly half of the available payload with RCS propellant anyways!
[ "GRAVITY--not just some obscure principle of physics--it's the LAW!" scribbled in the grout of a bathroom stall of the library building at Humboldt State University. Author unkown. ]
Personally I think that there is a better way. Although I grin in wonder when I think of the brute force nature of Orion, I am humbled by the reality of physics. I have no doubt Orion could fly--if you toss a nuclear bomb underneath a 30 m diameter plate of steel a half meter thick, you better believe that thing's gonna move!--but a compromise must be made between vehicle thrust and the capabilities of control. While an Orion is underway on pulse-drive, control could be maintained by using large gyroscopes, and 'leaning' (controlling hydraulic actuators to actively shift vehicle mass around the center of gravity--I've really gotta patent this!) using these techiniques reduces or even elminates the need for RCS propellant expenditures. But fine trajectory shaping, attitude control, reundevues meneuvers all require RCS and this is where Orion hurts. This is where the concept of the MiniMag Orion vehicle really shines.
A MiniMag Orion is smaller and lighter than the full Orion concept as originally concieved. It does not generate nearly the same thrust, but it achieves similar exhaust velocities. Whereas the original Orion would have Jet Powers on the order of a few tens of terawatts (that is trillions of watts--they're exploding nuclear devices afterall!) the MiniMag Orion concept would achieve a more "modest" 300-400 Gigawatts. Mini-nuclear pulses of the equivalent of 1-10 tons of TNT equivalent (0.001 to 0.010 kilotons) with pulse repetion rates of about 1-2 per second could achieve good thrust with a fairly smooth ride. That and using magnetic fields instead of a mechanical pusher plate makes for significantly improved ride. The pulse units are imploded electromagnetically by small spherical aluminum shells in a Zeta-Pinch scheme--the imploding shells strike a pusher-tamper which then strikes a central ball of something ultimately fissile--which could be Pu-239, U-235, or more excitingly other isotopes not typically associated with the nuclear weapons industry may be used: certain isotopes of curium, cerium, and americium have been proposed as potential 'fuels' for a MiniMag Orion.
Anyways, the MiniMag Orion would perhaps mass a thousand to two thousand tons, so the associated RCS propellant loading is much less. It's possible that a much more efficient system based on large gyroscopes for attitude control with momentum 'bleed' and translation accomplished by using a network of onboard VASIMR thrusters for RCS. Since the minimag oreon vehicle must carry an onboard nuclear reactor for auxiliary power anyway (this gives the energy to start the main drive) then a VASIMR system does not seem out of the question. A much smaller RCS propellant loading is achieved possibly in the 50 ton range or less--do able. A MiniMag Orion ought to make the voyage to Mars in a fast and efficient manner--and could do so while carrying a substantial payload such as a couple of Logistics Supply Landers, and Crew Transport Vehicles.
Here's the hitch: MiniMag Orion would probably have to be assembled in orbit. It could not develop the thrust necessary to lift itself off the surface of the Earth. This necessitates a lot of low Earth Orbit infrastructure--such as a space station and did I mention a heavy lift launch vehicle, and a safe, reliable crew transportation system? The only other option is to use a very, very large lofting stage: like a cluster of 3 or 4 Saturn 5 rockets to bump the vehicle up to almost 100 km. Enough momentum must be imparted to give the vehicle a chance to accumulate velocity at its modest acceleration of 2-4 tenth's of a g. Anyways, its going to take some serious effort, but it can be done. We just need to decide as a society and as a world if we want to do it.
There is another reason why fusion could come first for propulsion.
With a power plant one is trying to "Generate" large amounts of power. This means either operating in "Ignition" mode (self-sustaining fusion reaction) or in an "Energy amplifier" mode with a Q factor in excess of 15 or so.
Propulsion is diferent. a fusion technology capable of acting as an "Energy amplifier" with much lower q factors, whilst inadiquate for a power station could be very usefull for propulsuion.
For instance, consider a nuclear electric/fusion system where instead of using the electricity in an "electric" drive (ion drive) the electricity is used to power a fusion reaction with q>1 (Q factors of less than 1 may still be usefull) The fusion reaction would be effectivly a way of converting the electrical power into a high ISP drive with a thrust much higher than for an ION drive alone.
Propulsion is diferent. a fusion technology capable of acting as an "Energy amplifier" with much lower q factors, whilst inadiquate for a power station could be very usefull for propulsuion.
I'm not convinced about that one -- to me a low q factor implies very heavy, low thrust-to-weight propulsion.
The various types of plasma heaters (particularly the particle injection types) are often fairly low efficiency devices, providing a relatively small amount of heating, compared to the electricity they consume. Plus they are big, heavy devices that don't lend themselves easily to light-weight construction.
Suppose your fusion reactor provides 300 megawatts of plasma heating. This consumes something like 450 megawatts of electricity.
That's machinery that will weigh several hundred tons.
The tokamak might put out, say, 2,500 megawatts of thermal power.
Because light-weight energy conversion systems are typically not nearly as efficient as ground-based power plants (~20% versus ~35% to 55%), about 2,250 megawatts of the reactor's output will be used to produce the 450 MW required by the plasma heaters.
That leaves 250 megawatts for propulsion -- which is peanuts for such a massive system. What you get is a "Battlestar Galactica" size spaceship, with a micro-gee acceleration.
Dusty has made a good point. There has been some research based even on the idea of Q<1 systems with a fission reactor as the driving energy source for the fusion system. The fusion system is used to turn the electrical energy generated by the fission system into ultra-high specific impulse plasma thrust.
10k has a good point too. Current technology would lead to very massive systems. However, I'm reminded of a study done around 1945 (by Vannevar Bush?) that showed a moon rocket totally unworkable. 24 years later Apollo 11 land on the Moon. We had made great progress from the technology of the V-2 to the Saturn V. We have made very little propulsion technogy progress since. I the NASA budget was so all concerned with flying things and a little more focus on advancing techology we might make so breakthroughs that we dramatically reduce the mass of these systems. Of course they could use a little more budget too.
Part what I was thinking when I started this thread was that even if ITER, etc are techically successful they still might not be economically competative for sometime (just like coal can be use to make gasoline but isn't competative although it might be soon). But space applications are expensive by definition and so fusion might be used there first.
Yes, that’s pretty much what I am getting at, imagine something like a VASMIR, but instead of simply heating hydrogen as propellant we heat D-He3 sufficient for low Q fusion to take place. While this will not increase the “Power” much it will increase the temperature quite a lot and therefore the ISP.
A Q of 5, whilst ineffective for a terrestrial power plant would be pretty good for a spaceship engine and though a Q<1 would only (I guess) generate thrust similar to a conventional VASMIR it might do so with considerably better ISP.
As for commercial fusion. I doubt if it will "Ever" be "Economic" as long as significant ammounts of fossil fuels remain available to be consumed. But the most expensive KWhr is the one that isnt there when you want it, If we follow the "Free Market" route the chances are that serious fusion development work will not be untertaken untill "After" the crisis has hit. Trying to develop and roll out a complex demanding technology such as fusion in an enviroment of chronic resource shortage and rolling power cuts will be hard if not impossible.
D-He3 fusion requires nearly an order of magnitude higher temperature than D-T, and we haven't even succeeded in making that practical on earth (with Olympic-pool-size cryostats housing enormous Tokamak toroids), much less any sort of light weight aerospace versions. Also, a D-He3 fusion reactor will produce a lot of D-D reactions -- could be more than the D-He3 reactions, or could be less, depending on the temperature. But in any event, it will be a large fraction. And at the multi-thousand megawatt power levels we're talking about, this will result in an enormous neutron flux, that will be both damaging to the equipment and difficult to shield against (more dead mass).
I got interested in this (well the current phase anyway) back around 1997 when I noticed that D-T fusion would give 80 percent of its energy in neutrons. While this can work in two ways for commercial electric power: 1) you can use the neutrons to breed more tritium than the plant uses; 2) this interaction with neutrons transforms their kinetic energy into heat to produce steam that drives a turbine. This would not really be what you would want in space propulsion. I then some how can across the idea of D-He3 perhaps from the lunar He3 advocates. The point is that D + He3 --> He4 + p was a reaction in which most of the energy came off in charged particles.
From this I came up with the turbojet analogy for a fusion propulsion system. It would be linear "leaky mirror" system in which a thrust would come out the rear after the fusion reaction. But the thrust would pass through an MHD generator taking part of its energy to produce electric power to run the magnetic confinement system. The Q would have to be great enough to run itself on just part of the plasma flow the rest would be very high specific impluse thrust. Also, in my idealized version no neutrons either.
It sounded good but first how feasible was D-He3 fusion compared with D-T which is still a major challenge? How rare is He3? And, how do we start this thing up (probably a less problem)? At the time I hadn't read Terry Kammesh's work and my limited searches for information turned up little on D-He3 fusion other than the lunar He3 stuff. But, it was clear that He3 is very rare. Year later I found that these idea or at least major part of them had been around for years and the papers in the Kammash book has showed that D-He3 reaquires almost 10 times the temperature (at equal densities) as D-T. In my days of no data I had hoped for only twice based on just the difference in Coulomb repulsion but as I was aware quantum effects could be and obviously are dominate for D-He3. In addition at the very high temperatures of D-He3 you also get a lot of D-D fusion leading to neutrons and a lot of energy in electromagnetic radiation, i.e. Bremsstrahlung and synchrotron.
I went back to thinking about D-T. This started of with the idea of breeding enough tritium and letting it decay for its 12.5 years to get He3 or this would just have to wait until commercial fusion power was widespead and we could get surplus tritium fromt there. To finally the idea of having a space propulsion system that produced its own tritium in flight from Li6. All which seem to have some big issues. But given that the He3 process operates at temperatures where one gets a lot of D-D reactions, I have come to the conclusion that D-D is the way to go. This reaction occurs in the high range just like D-He3 and produces abut 3.5 Mev per reaction. But it also produces T and He3 as its first generation reaction "ash" which the fuel we need. So the D-D is running at Q<1 but the whole process is Q>1. About 60 percent of the energy comes of in charges particles, He4 and p. This process is called cat D-D (for catalytic D-D). The good news is that the fuel is readily availible and not radioactive.
(I see that 10k has already addressed some of this while I was writing. He is right that there are many challenges to get to the magnetic confined thermonuclear space systems. They really are the ultimate within the known laws of physics.)
Well, essentially what it means to do D-He3 fusion is that you need to heat the plasma to about a billion degrees kelvin, instead of 'merely' 100 million kelvin for D-T fusion. What does this mean? This means that the average bulk kinetic energy of particles in this plasma will possess 100 times the average kinetic energy of particles in the fusing D-T plasma. So what!?
Well, here's the rub.
Bremsstrahlung losses are much, much higher for D-He3 reactions than for D-T. According to a Wikipedia article on nuclear fusion at:
Bremsstrahlung losses are about 26 times higher for a D-He3 reaction at the same conditions of density than for a D-T reaction. It scales non linearly with temperature (it is a complicated function of which one variable is T^1/2) but essentially since we humans can't make a plasma dense enough to be opaque to bremsstrahlung radiation (maybe in nuclear weapons!) then unlike in the cores of stars, bremsstralung radiation cools the plasma. Infact, even if we could make a billion degree plasma that reaches ignition temperatures for D-He3, bremsstrahlung radiation would quickly 'quench' the reaction and fusion could no longer take place. The only way around this is to increase the bulk density to the point that the bremsstrahlung radiation (soft gammas) cannot escape. The magnitude of density is many thousands of times the density of normal matter--which occurs only in the cores of stars, or in the cores of exploding H-bombs. But these conditions will NEVER exist in the core of a Tokamak. Ergo, fusion of D-He3 will probably never be practical in a Tokamak device. Sorry--wish it weren't so, but physics is physics.
So, if we could achieve ignition in a plasma, then a D-T reaction is probably the only realistic option available. Since most of the energy is carried by energetic neutrons, then perhaps a way could be concieved to take advantage of that. I know some have proposed using antimatter to create a 'fission sail' where supposedly antimatter can cause fission in uranium nuclei. Well, depleted uranium (mostly U-238 which is fairly cheap) is fissionable by the energetic neutrons produced by D-T fusion and has already been demonstrated quite a few times in the explosions of fission-fusion-fission hydrogen bombs--the high yield tests in the South Pacific during the early 1950's to 1960's. If it can work in bomb tests, I don't see why it can't work in another kind of test. Still, the amount of thrust generated by a U-238 fission product sail would be pretty low. I'm not sure if such a complicated system would ultimately be worth it.
I understand your point Googlenaut. By the way thanks for pointing out the Wikipedia article. It contains some additional data over what I already had. I had pointed out the Bremsstrahlung and synchrotron radiation issues which are very problematic for the whole concept.
One of my main points is that if you do somehow get into this ultrahigh temperature regime it seems to me that cat D-D makes more sense that D-He3 since you don't have to mine the fuel on the moon and deutrium is readily available. Second these fusion propulsion sysems aren't really tokamaks but are linear mirror systems. The example in the Kammash book has a particle density of 4.4 X 10^17 per cm^3 which is hardly of solar density. The Bremsstrahlung and synchrotron radiation is 65% of the total fusion power (very high). By the way this was for D-He3.
I think these systems are a long way off. But, I don't completely give up on them as they could with the required advances be the ultimate systems (within known physics and for that matter within likely physics). It seems to me that the inertial confinement approach will be realized in space before any of this. The specific impluse will be much lower but far beyond at lot of other possibilities.
On your comments on the neutron sail. I liked that one. What about using a neutron reflector like Beryllium?
I don't really see how a neutron reflector will help all that much. Even assuming perfect reflection (an impossibility) then the best you could do was double the neutron flux on surface of the sail. And believe me, a full up operating fusion reactor burning D-T at ignition conditions will produce an aewful lot of neutrons. Infact, for some of the fusion propulsion schemes explored at length by the British Interplanetary Society (Project Daedelus) the generated neutron flux could be so high that sublimation (evaporation) of support structures could become an issue, let alone magnet shielding.
What about using a neutron reflector like Beryllium?
There seems to be a slight misconception here, concerning neutron reflectors.
There is nothing particularly unique about Be that would make it all that much better than other materials.
Basically, a good neutron reflector is any material that is fairly dense, but doesn't absorb neutrons.
Even then, to reflect a high fraction of incident neutrons requires on the order of a foot of thickness -- not something you would want to use in designing a spacecraft "sail" that is miles across....
In fact, the large mass that typically makes up neutron reflectors (which surround a volume where the neutron source, such as a reactor, is located), makes Be undesirable for that application, due simply to its high cost. Graphite performs comparably well, and is very much cheaper.
For certain small-scale applications Be does have the advantage of being a small neutron source as well as a reflector (via the n, 2n reaction), but a reflector made up of HEU or Pu will do that even better, and in a thinner wall (due to the much higher density).
When I red all statements of this topic I can see two different ways to use fision/fusion power. First is the permanent linear propulsion policy and second the cycle propulsion with explosive units.
To the first manner we have a very high velocity of particles but very low thrust and weight plus very bad energy-efficience. Further only a gascore-derivate cold give a suitable perforance-/weight-ratio as a APU or directly for heating up reaction-mass like hydrogen etc. In each case it could´nt start from Earth due the bad weight/thrust ratio.Maybe reaction mass is heated by microvaves/laser/magnetic fields or other manners it could be assists by chemical boosters by launching from Earth.If the exhaust mass radioactive then the ship must be in orbit first.
I´ll prefer the 2nd manner because it´s more reliable,easyer,closer to recent technique,better weight/performance ratio and cheaper! Also we can affort to build it moore solid and give more comfort for astronauts convinience.My opinnion is it´t more safe than all outers because no dangerous hydrogen,no extreme lightweight-calculations.It´s the momentan only way to build a really spaceship which can be launch from Earth and returning as whole unit! I would be recomend to develop the mentioned MINI MAG Orion with mini fusion bombs(0.01to 0,1ktons) who ignited with just generated antimatter-flashes or x-ray laser.the power source could be an onboard gascore or a pebble-bed derivate or later a fusion reactor like ITER or DEMO. Laser or on an other mentioned way,s above could give "clean"explovives without secundary radiation. During start sequence thrugh atmosphere to get more mass flow demineralisired water in 50-100 kg cans which contans the small 0,01ktons units could give the wanted high mass "low"velocity ratio and a "softer transit"to ambient atmosphere. I think with 2 of this units per second for the first 100 seconds after lift off Orion is in 100-120 km over surface could crossover to "pure" 0,05 or 0,1 bombs.Reaction mass is now the pusherplate ablative coating,which can/should hold 10.000 shots and replaced then. Maybe the pusherplate is part of active flight control like a woobbleplate in a axial pisten hydraulic pump like mentioned by colleauge Google Naut. Enough for today. Best Regards:
Gimballing the pusher plate is possible, however the momentum dampers will definitely be non-optimal, as they cannot dissipate off-axis momentum. Off axis momentum could be damped by by using a whole network of off axis dampers--it's difficult to describe without an image. But imagine that every node on the pusher plate connects to two adjacent hydraulic dampers, and similarly the bottom of the Orion superstructure has nodes which connects to two others. Each pair of hydraulic dampers could thus form triangular networks--structuraly strong and with variable rigidity, they could dissipate off axis momentum. This could be done, but the basic problem becomes torque. The torque imparted on the superstructure, bending moments and shearing loads--and then there is the problem of passengers--people have to ride this thing so lateral (turning) impulses have to be not just survivable but at the very least, they must be only marginally uncomfortable. It is extremely difficult to control forces measured in tens of thousands of tons per impulse--it is likely that our crew would be quickly shaken to death...the magnitude of these 'turning impulses' will be magnified by the length of the structure. I'm afraid that with the length of the super structure being what it is, that the poor occupants in the crew cabin will be at the business end of a 'cracking whip.'
This is one reason why I don't think gimballing the pusher plate is the answer.
Instead I proposed that a rather simple and straightforward way can be found to steer the Orion during its high acceleration boost phase by using a system to mechanically shift the center of gravity of the whole vehicle. How can this be done? Simply by seperating the upper part of the structure that houses the crew, crew life support modules, mission modules, cargo and payload from the aft propulsion section containing the propellant (pulse unit) magazines, momentum conditioners, and the pusher plate. By seperating the two halves of the ship by a large, flat thrust bearing, and then 'simply' using a relatively small network of perpendicular hydraulic rams, it becomes quite easy to shift the upper part relative to the lower part. In this way the center of gravity shifts in amount proportional to the ratio of the two section masses multiplied by the actual displacement. In this way, Orion can 'lean' into a turn precisely the same why a motorcycle rider leans to turn a bike. It is much more precise to control and is a lot easier on the passengers. It is smooth and more continous despite the discrete nature of the impulses.
Martin has the right idea. It will be interesting if the fusion concepts work well inside the atmosphere. The idea trading specific impluse for more thrust by using water is similar in principle (not exact method of course) to VASIMR's use of increase propellant flow to increase thrust.
I really doubt the linear/mirror approach would work inside the atmosphere. Laser fusion of pellets might work however. There might still be enviromental objections that would be two hard to overcome. But the idea should be studied.
In general it seems to me these systems would be orbit to orbit. We would use chemical propelled vehicles, i.e. shuttles and/or booster rockets to achieve orbit. Also, a chemical propelled landing craft would be needed at the destination.
A good question is just how close are we to having the technology to do the mini-mag or ICF approach assuming the use of lasers to detonate the pellets?
just how close are we to having the technology to do the mini-mag or ICF approach assuming the use of lasers to detonate the pellets?
Just look at the size of NIF (the National Ignition Facility).
Its not something we're going to be putting into orbit any time soon.
But a clever combination using slightly larger fission-fusion pellets, in bootstrap mode, and perhaps with a small amount of antiproton booster shots, might possibly be made small enough to launch into orbit in a reasonable number of pieces....
I accept, for many technical reasons, that I am talking Sci-Fi here rather than current tech IE an extrapolation, if not an extreme extrapolation, of known technology (as opposed to “Impossible” Sci-Fantasy such as “Stargates”, FTL "Hyperdrives" etc). However Sci-Fi has frequently been a good predictor of future developments!
IM(H)O NEP is unlikely to ever be of interest for anything more than space "craft" or "Probes". The amount of electrical energy required to produce even a miserable 1000Lbs of thrust. (Less than a tenth than that of the Apollo LEM’s descent engine) for a high ISP electric drive would be sufficient to run a sizeable city! Forget the "energy source" there is simply no way that this amount of “electricity” could be handled within the confines of a space ship using any known technology (Think in terms of Generators, Cables, transformers, switchgear etc)
To produce as much thrust as a single Apollo F1 engine would take more electricity than is generated on the entire planet!
A high thrust, high ISP, electric drive is simply never going to happen. Not ever! Erm EVER! (Not without "Sci-Fantacy anyway :) )
To get high(ish) thrust with high ISP requires the use of nuclear energy more or less directly and not via some other process such as the generation and use of electricity
This means one of three things.
1) NTR! Either solid or gas core. Solid core being “solid” gives Reasonable thrust and a modest ISP improvement over chem-but nothing really exiting, since it cannot exceed the melting temperature of the reactor components. Its Do-able now (in fact, it has been done, but not as yet applied to an actual mission) Gas core NTR allows (in theory) the possibility of much higher operating temperatures which could give much higher ISP but there is a limit as to how much power can be handled “within” an engine before it melts simply from the radiant energy emanating from the reactor core. I’m not sure what that limit may actually be, but once you reach the power ceiling, higher temperatures, while leading to higher ISP will result in reduced thrust (just like the gearing in a VASMIR) this comes under the heading of “Law of diminished returns” which means that high ISP, technologically demanding, GCNR systems may simply not be worth the effort when perfectly workable low tech (Relatively speaking) alternatives are available (See #3 J )
2) Some sort of “Contained” or “semi contained” nuclear reaction where the reaction products are used directly to provide thrust, either on their own or mixed with an additional propellant. NSWR is one example using a “continuous” fission reaction to provide high thrust and High ISP. Another is Fiss-frag which would give very high ISP but relatively low thrust. Other examples include mini-mag Orion, the “Rapid fire” NPR fusion drive proposed in project Daedulus. And the “Low Q Fusion VASMIR” I suggested earlier (I also had an idea, mentioned elsewhere, for a fusion VASMIR that used aspects of the Daedulus drive) Most, if not all, of these options are NOT do-able now!
3) Good ole Orion. Poor efficiency but almost unlimited power. Basically you can have as much thrust as you want but much of the energy is wasted. This is also do-able now.
So after having gone round the propulsion system hypotheticals loop yet again ISTM that we come back to what we have always known all along.
For all practical purposes, Solid core NTR is the way to go for smaller shorter range space vessels (eg Lunar ferry) with Orion propulsion for larger longer range interplanetary space ships. (Having said that, NASA was interested in GCNR engines back in the sixties. Exactly What configuration they were considering though I know not!)
Orion is not by any means Beautiful or Elegant. On the contrary, it is rather crude, grossly inefficient, and positively agricultural But, My word, The performance!
It is SO Russian that I am AMAZED the Russians didn’t do it.
Indeed, is the sort of thing I could have seen Verne writing about and Brunel building!
The main question to my mind is how to build re-useable NTR’s and Orions. Can our NTR lunar ferry be simply refuelled and sent off on another trip, how many Start-stop cycles will the engine be capable of? How many total hours operation before it needs to be re-built/re-cored/re-placed?
How many pulses could an Orion pusher withstand? Could it be built capable of withstanding many voyages or is 2-5000 or so pulses be the limit?
Think about this, nuclear marine engines would have been just peachy during the era of the Liners (1860-1960) But coal and steam engines worked just fine!
Advanced exotic propulsion systems for building a solar system wide civilisation would also be just peachy, but farting our way (as someone put it the other day) around the solar system with Orions would also be just fine too.
If we pulled a finger out we could get men to Pluto before new horizons got there.
What a shame the US got to the moon first! If the Russians had beaten them to it I am sure we would have seen the Saturn boosted Orion on a Mars trip a few years later
Good points, Dusty. Orion isn't without its problems though--there is still the delicate problem of RCS propellants for fine rotational and translational control. And there is the issue of landing--which I wouldn't want to even try with Orion. As far as I'm concerned once it is in space--it always must be in space. Let it carry the landing craft with it, but don't try to land with the whole thing.
VASIMR and other NEP are intermediate value propulsion systems. As you point out, because of the power conversion necessary, this creates severe limitations upon the amount of thrust that can be efficiently generated. NTR doesn't have this problem really, because the energy conversion (heating) is done directly within the power generation system (reactor core.) Thus thermodynamically this is just about the most efficient system there is--but limitations on specific impulse (ultimately only about twice as good as chemical) means that the overal performance jump over chemical isn't spectacular either. But it's better for higher acceleration, lower delta-v burns.
Orion is stupendoussly (almost ridiculously!) ineffecient, but because the amount of power available (here I mean power as in "real" kinetic power of the plasma 'jet') is so astronomical, the doubled edged sword of high exhaust velocity and high thrust is indeed satisfied. A good baseline for measuring the amount of power generated by a rocket vehicle is to multiply the thrust in newtons by the exhaust velocity in meters per second. This gives an estimate of the actual jet power. A more exact figure is found by mutiplying 1/2 of the mass flow rate by the exhaust velocity in meters per second--this gives actual jet power. Most good sized chemical rocket engines will return powers of a few gigawatts. A good sized NTR will be perhaps 2.5 Gigawatts--about the same as a Space Shuttle Main Engine. A typical NEP for a JIMO mission might be around 500 KW to 1MW. An advanced VASIMR might be anywhere from 1 MW to 100 MW. MiniMag Orion is a more astounding 100-200 GW!
So how much could good ol' Orion do? Utilizing 1 kiloton pulse units, exploding 1 per second achieves anywhere from 100-200 Gigawatts of jet power (actual energy production will approach 430 GW) A much larger Orion with 5kt pulse units will be in the neighborhood of 1 TW (1 Trillion Watts!) Energy to burn, literally!
A moderately sized vessel (very large by today's standards) utilizing some kind of fusion power plant that produces moderately good accelerations will still require hundreds of gigawatts of jet power. Possibly even as high as tens of Terawatts if the vessel were large, acceleration high, and jet plasma very hot. This is an astounding amount of power--and we haven't even looked at interstellar ships yet.
