Then there are questions along the lines of, would it actually work? It seems likely that any problems that would stop it from working could be solved eventually, but we won't know we've solved them until someone makes a full scale article and tests it. Then if we haven't solved them the damn thing crashes spectacularly.
Take the shock plates for instance. The whole idea of Orion being a practical thing comes down to the remarkable fact that there's reason to think a huge steel plate can sit next to a multi-kiloton nuclear explosion and not be vaporized, or rather not even partially vaporized, or melted. But that is apparently related to something on the surface ablating--a layer of oil say--taking the hit for the metal as it were. So not only do we need to fire nuclear charges in just the right place and time, every half second or so, we need to spray the surface of this plate with oil just the right thickness, during the short interval between charges? Or the "solid" shock plate needs to sweat oil somehow, to have zillions of micro passages though which oil seeps at the right rate, passages that are not gradual mashed flat gradually? This plate, in addition to its metal not being ablated away, also is not going to develop metal fatigue, it is not going to develop flaws and suddenly split in half in the middle of a thousand hammer blows to put it into orbit? And either the pulse units will always, without fail, detonate at just the right time, in just the right place, or else we know the plate will not suffer in the least from an off center charge, it won't get stressed or bent? All of this is 100 percent guaranteed, before we build the first model for launch? Or, we have means of full scale testing the plate, for longer, more pulses than an actual launch will take?
Well, sure we do! We set up a static test rig, and set off more bombs than we would for a real launch right here at sea level in the middle of Nevada or someplace like that! Good thing we've decided that worrying about fallout is for wimps, eh? Because such testing is going to generate a lot more of it than an actual launch would. And yet, how can we be confident in the mechanical elements, unless we test them with realistically scaled blasts, that have associated with them the same sorts of radiation we'd be worried could wear the plate and shock absorber and bomb dispenser units in operation? So even if keeping up a blast test schedule with HE would be possible, on full scale, we would not want to substitute it because we need to test everything with neutron and gamma fluxes just as they would be in a real launch.
And even so, the mechanism of transmission of a nuclear chain reaction's energy to a momentum pulse on a plate is different at sea level than way up past the stratosphere. Here on the surface, the bomb makes a fireball which makes a shock wave in air, and it is that striking the plate that gives it a push--meaning we moderate the explosion and engage the mass of the atmosphere to transmit the energy in useful form. But in operation, the plate is vibrating dynamically, and the spacecraft it is pushing is accelerating upward into ever thinner air, air that is also depleted because of the previous blast and the shock wave pattern of the craft cutting through it--how can we simulate these conditions on full scale realistically here on Earth's surface? At the other end of the boost phase, the spacecraft is essentially in vacuum, and the only way the energy of a pulse unit is transmitted usefully to the plate is to include some extra mass in the pulse units that is blasted and bounces off the plate. We can't test those sorts of units down here, the atmosphere is in the way.
The really neat thing about Orion, just ahead of the fact that we can get high thrust with high ISP at the same time for interplanetary trips, is that it enables the instant laughing of thousands, or hundreds of thousands, of tons into orbit using materials that don't mass ten times or more what goes up but in the ballpark of what gets to orbit--or indeed less! We think of a vehicle as something that you put a small fraction of the overall mass of the thing into as fuel, and it goes somewhere useful, and we don't stop and wonder at what point have we poured more gasoline by mass into the tanks than the automobile itself weights. Well, with airplanes we reach that point after just a few full range flights, typically, and I suppose if I did the math I'd find a car's mass is consumed in burnt fuel a few months after it is first purchased. But anyway, during those several flights or several months, quite a lot of "revenue ton miles" get delivered. When we use chemical fuel rockets, on the other hand, we labor to "pile fuel on top of fuel to put a grapefruit into orbit" as I recall one favorable reviewer for Orion being quoted in John McPhee's Curve of Binding Energy. The Shuttle for instance, STS, was, if you look at it the right way, actually more efficient than the Saturn V was, for both put about the same amount of useful material into low Earth Orbit--the Saturn V Lunar Apollo stack would orbit a 45+ ton stack of lunar manned vehicles that would proceed to the Moon, and to boost it there a 15 ton dry upper stage that retained some 60 tons of propellant for that job (after burning up some 40 finalizing the parking orbit first.) This is about 120, maybe 130 tons. And lo, a space shuttle Orbiter also massed about 120 tons on reaching low orbit. (This being no coincidence; Shuttle was evolved from Saturn V tech and designed to meet specs similar to it because these were the stresses and masses the Apollo legacy Saturn V launching pads could handle). But a Saturn V all up on the launch pad massed between 2800 and 2900 tons, whereas every STS ever launched massed within ten tons of exactly 2050 tons! As you can see, then, in terms of putting up X number of tons into a 100 nautical mile high parking orbit, the later tech was more efficient, just as one might hope. (Then unfortunately, the way the better tech was configured, only a sixth or less of the Orbiter mass would be available as payload, the rest being a reusable Space Winnebago. In other threads recently I've shared my notions as to how this diseconomy in terms of cargo mass to orbit per launch might have been addressed--hell, might still be addressed today!--and still get reuse of the expensive but high-performing SSMEs.) But when we compare even a Shuttle reconfigured so that standard 2050 tons can return the SSMEs and still deliver not 15 or 20 but 60 or 80 tons to orbit, to an Orion, which if it could be scaled down to a mere 2000 metric tons, would still deliver at least 500, maybe twice that, as useful payload even factoring its fixed structure which has to be massive, and consider that the Orion architecture is presumably more useful and efficient on much larger scales, on 20,000 or even 200,000 or perhaps even million ton scales, the whole chemical launch industry seems pathetic and silly.
All very fine and good indeed, very attractive. Believe me with all my qualms about it I've still lusted after it.
But
1) there must surely be some fallout release with each launch. I've read the sites of enthusiasts, many of whom are incredibly arrogant in their absolute certainty that this would have worked fine and that only a bloc of weak-willed and wrong liberals stood in the way of a nuclear space paradise, a Utopia usually achieved by nuking the hell out of Russia somewhere along the way, because that would obviously be a better world--in some mentalities apparently anyway. Among the many certainties these people have, they sneer at the fallout. First of all, they say, there won't be any fallout at all, because the vast majority of pulses will be set up way up in the sky and fallout is something they define as what happens when material from the surface gets drawn up into the fireball and gets transmuted there. Not to worry, fallout only happens with ground bursts and the first few pulses we start with, will be above a treated launch surface in the desert, which is already glassed and won't allow fallout--as they define it!--to be produced. So stop whining about fallout, there isn't any, you tree-hugging wimp!
But this is false, if we define fallout more sensibly as "damaging isotopes produced in the process." So-called fusion bombs, if these can even be made small enough to be the appropriate sized charges for an Orion launch, as we have them in modern weapons engineering, are not really primarily fusion devices at all. They are fission bombs that use an intermediate fusion reaction to release a flood of energetic neutrons to trigger a tertiary fission reaction that supplies most (by which I mean, 90 percent or more) of the total blast energy. This is, for one thing, because the lowest-threshold energy fusion reaction that is the one all the bombs are designed to use, deuterium-tritium fusion, must release 80 percent of their net produced energy in the form of these neutrons, due to the fundamental mass balance of the products. We can and have made fusion bombs that omit the third-stage fission, they are called "neutron bombs," and their blast yield per unit mass going in is much much lower than the jacketed normal bombs; they can only be justified as weapons if we want to use the neutrons as killing agents instead of as triggers for the big blast yield. As pulse units for Orion they would be very foolish because the neutrons are useless for generating momentum; all they would accomplish is, per unit of thrust produced, greatly raising the prompt radiation hazard (and transmuting some air) and damage to the pulse plate. It would make more sense to just make the primary fission explosion that is needed to trigger the fusion, leave the fusible material out, and have a simpler, and actually cleaner per useful unit of blast power, pure fission bomb. Now if we want a great big blast, or if we can make a fission-fusion-fission pulse unit small enough to be right-sized for Orion, using the fusion phase as it is generally used in bombs can make sense.
But have no illusions. To get a big nuclear blast, you are always basically using fission, not fusion. And fission reactions by their nature produce a broad spectrum of daughter isotopes, many of which are radioactive. You can't make them "clean." Each pulse, if we could assume near 100 percent efficiency in triggering fission in the uranium, plutonium or perhaps in some designs maybe thorium, would convert all that fissionable metal into daughter isotopes averaging half the atomic mass of the parent isotopes. And a large fraction of these must be isotopes that are known to have pretty nasty effects in the ecosystem and in human beings, and being dispersed into the atmosphere as they must be, they will surely appear in these places eventually. That's what I call fallout, and if the fission process is highly efficient, that's what every bomb blast makes.
Now actually fission bombs are often not so efficient, which by the way is not great news for an Orion program, since you'd be inputting more mass of non-cheap fissionable metals and getting less yield from them. Parent isotope materials that don't get fissioned in the chain reaction will simply be released as they are as so much vaporized meta, that will cool down and also filter into the ecosystem. U-235 is not such great stuff to be breathing and eating with one's meals. Plutonium, I gather, is much much worse, for reasons of its detailed chemical reactions.
Aha, but what if we could trigger fusion reactions--not the tritium-deuterium reactions that are so messy in producing mainly energetic neutrons, but some aneutronic reaction like say Helium-3 in pairs? Well, without qualifying it to be those higher threshold reactions, which are harder to trigger by far, the OTL Orion gang (Freeman Dyson, Ted Taylor, and others) did indeed believe and for this purpose, hope, that alternate ways of triggering rapid and powerful pulses of pure fusion reactions would indeed be on the table and on the shelf within a decade or so. But this is not happening. Not quite yet anyway, not enough to achieve break-even, although a lot of workers claim they are just this close to that point and will get there any year now. They are still working with De-Tri, and with the neutrons absorbing 80 percent of the output. And all the schemes to get these isotopes to fuse without the benefit of a huge fission reaction slamming them together with a huge concentrated surge of energy involve big machines imparting huge concentrated surges of energy on itty bitty packets of fusible material, and hoping to get a big enough tiny fraction of them to fuse, big enough that the useful yield is enough to power the input and sustain the reaction somehow. As it happens, there is a particular line of research, being funded by NASA as a deep-space propulsion system, that involves triggering the fusion by kinetically crushing the material in a much more massive pulse of lithium, a kind of momentum contained and triggered approach. And the lithium is said to be enough to absorb the neutrons, and thus convert their energy into useful heat (and also avoid damaging the surrounding necessary equipment). I have my own notions how this particular reaction might be accelerated in rate enough to perhaps produce a fusion powered rocket of high thrust, in the ISP range of 2000-3000, that might be so high in thrust that despite the generating equipment massing a ten tons or so a rocket with payload in the ballpark of 10 or more tons using this engine might be launched from Earth into orbit. That is not at all the intention of the team developing it, and I might find my notion for accelerating the pulse rate is completely impractical. What we'd get is a lithium rocket, and we would have some fallout as I define it--tritium.
Meanwhile in the sense Ted Taylor and others hoped for, non-fission-triggered fusion never did emerge, and now Taylor said later--Thank God it did not. Because nuclear proliferation is bad enough now, but if there were ways of triggering pulses of fusion in the bomb-blast ranges, we can presume it would be ten-100 times worse.
The higher energy (not in yield, but in threshold to overcome before they go forward) reactions that don't produce neutrons, or not so many of them are that much more difficult to trigger than the neutronic ones. If someone can make a clever electromagnetic gizmo to trigger a fusion reaction, we can be pretty sure it would mainly make neutrons. Perhaps we can jacket them in lithium and thus absorb them into useful thermal form.
Meanwhile the charm of Orion, such as it is, is that we can in theory make it with known, off the shelf tech. This means that whatever sort of exotic nuclear alchemy might be happening in some lab today, and might be put on the shelf tomorrow, for now we must focus on nuclear energy as we know it, which is to say we use the bombs we've got. And those will produce a lot of fallout, make no mistake!
2) Ah, you wimp, the boosters say--once the Orion is up above most of the atmosphere, the bomb products will all be flowing at more than escape velocity and will be blasted away from Earth completely! Stop crying about it!
Well, yes, the point of Orion is to get reactions that are energetic. But this also means, especially blasting them out into a vacuum, that they are charged particles too. Earth has a magnetic field, and we are releasing these high-energy ions inside this field. A fair number of them are going to get trapped in the Van Allen Belts, and others will simply collide with the upper atmosphere and rejoin us that way. In the Van Allen belts, those ions eventually come down at the poles. A good part of it winds up right back here anyway.
And the same applies to interplanetary launches from low Earth orbit too. It does not matter that we are blasting above the atmosphere; again many, maybe all the isotopes will get whipped around and dumped right back into our air.
Orion is dirty, if it works.
We don't know that all the mechanisms we need to work together will do so flawlessly. If they don't, maybe we have a little bobble in the launch sequence but the mission goes on Ok. Or because the glitch does damage something crucial breaks down--the plate starts to crack, or the charge delivery system goes out of whack. And then what we have in progress after that is a crash, a crash delivering all the remaining pulse units right back to Earth's surface. Maybe they vaporize, and we get all that uranium or worse plutonium is mixed into the air. Maybe they are contained nicely and fall to Earth intact. Which is to say, a prize for whichever broker to terrorists can get their hands on them first. Meanwhile, we have a total mission failure.
Even if the components all test well on the ground, we need lots of test flights to verify they work right put together and operated in graduations from sea level to vacuum. None of this work has been completed. It is not reasonable to be certain it works without these tests. And we know if it does work, it will make something of a mess. Against that of course we can put huge tonnage into orbit.
But there are other possible way to get megatons and gigaton of payloads into orbit, that don't contaminate the air with Uranium or worse.
