Orion Rising (RAR!)

[Zor]

As we should all know by know by now in the Fifties there was a US Government plan for called project Orion which was for the development of a spacecraft that used a series of smallscale nuclear detonations against a steel plate to launch a craft into orbit. Saving on Fuel consumtion. The Plug was pulled on the project with the Partial Test Ban Treaty in 1963.

Now here is the question. What if there was a clause in the PTBT that allowed for nuclear devices to be used for constructive causes (or something on that line) which allowed for NASA to contine work on the Orion Spacecraft. Would have Orions be practical/cost effective or would it be cheaper to build giant chemical rockets? Could project Orion have lead to earlier developments in mans push into space?

Zor

[Deathstalker]

Environmentalist effectively shut down new nuclear power plant construction because "safety" concerns. You really think they aren't going to say something when A bombs are used to hurl something into space, no matter how small a yield the A bomb needs to be?

NASA won't get past "So we are thinking of using nuclear explosions to lift spacecraft into space. Any problem with that?"

Assuming things progress in the '60s before the environmental movement gets going, it might have been feasible to use Orion craft to lift material into orbit or send it to the moon. I would think that conventional rockects would still be used for sending people, for saftey reasons. Conventional rockets are of course not completely safe, but there are a lot more options for surviving a mishap than with an Orion craft.

[CaptainChewbacca]

I once wrote a short story with this premise. In the end, Orions are deemed too hazardous, but by that time there's already something of an infrastructure in space for heavy industry, so space-planes serve to get cargo TO the orions, which now only use nuclear pulse in outer-space.

I also had "Battlecruisers" as a 'next step' up from nuclear missile subs. The ultimate second-strike weapon.

[Raesene]

I once wrote a short story with this premise. In the end, Orions are deemed too hazardous, but by that time there's already something of an infrastructure in space for heavy industry, so space-planes serve to get cargo TO the orions, which now only use nuclear pulse in outer-space.

I also had "Battlecruisers" as a 'next step' up from nuclear missile subs. The ultimate second-strike weapon.

I jope you post it, sounds like a good story.

[SirNitram]

This again?

A system that requires 2000 nuclear devices, when the US arsenal of such never got that much higher than 20,000.

Most proposals require craft tens of thousands of tons in weight. In short, you need to make a naval warship that's largely vertical, can withstand the strains of launch, and then you still have to invest in CRV's. Oh, and you can't re-crew the things without developing chemical rockets.

It's a fairly daft idea, all told, but those problems are always always always ignored.

[MagnusTheReD]

This again?

A system that requires 2000 nuclear devices, when the US arsenal of such never got that much higher than 20,000.

Most proposals require craft tens of thousands of tons in weight. In short, you need to make a naval warship that's largely vertical, can withstand the strains of launch, and then you still have to invest in CRV's. Oh, and you can't re-crew the things without developing chemical rockets.

It's a fairly daft idea, all told, but those problems are always always always ignored.

The idea of using nuclear devices for takeoff, is pretty laughable, I see, but what about using them to gain acceleration when already in space?
Can't we just assemble the huge ship in orbit piece after piece, like with this international station, or the Russian "Mir"?
It would sure be much easier to do it this way, and you won't need to make the ship tolerant for launch from the surface, since it's already in orbit?

[Darth Raptor]

Nuclear rockets, both fusion and fission, are a mainstay of most realistic deep spacecraft designs I've seen. Unless you want to take years to cross the solar system, that's really your only choice (barring wonky stuff like ion drives, solar/laser sails etc.).

[Batman]

Using fission or fusion power to energize the reaction mass? Yeah. Project Orion? No.
Okay, feel free to laugh at me if Wikipedia is hopelessly wrong about this (or if I am about how nukes work in space), but based on their article on the Orion project, isn't it based on making use of a plasma/pressure wave that doesn't exist in a vacuum? I mean other than the mass of the bomb what's there for it to turn into plasma in the first place?
Or am I once more missing something blindingly obvious?

[SirNitram]

The idea of using nuclear devices for takeoff, is pretty laughable, I see, but what about using them to gain acceleration when already in space?
Can't we just assemble the huge ship in orbit piece after piece, like with this international station, or the Russian "Mir"?
It would sure be much easier to do it this way, and you won't need to make the ship tolerant for launch from the surface, since it's already in orbit?

Congrats, now you must pay the energy cost of lifting this massive assembly(ANd to get worthwhile acceleration for cost you must have an absolutely massive assembly) in chemical rockets.

Stick to Nuclear Thermal or Nuclear Electric Rockets.

[Winston Blake]

Using fission or fusion power to energize the reaction mass? Yeah. Project Orion? No.
Okay, feel free to laugh at me if Wikipedia is hopelessly wrong about this (or if I am about how nukes work in space), but based on their article on the Orion project, isn't it based on making use of a plasma/pressure wave that doesn't exist in a vacuum? I mean other than the mass of the bomb what's there for it to turn into plasma in the first place?
Or am I once more missing something blindingly obvious?

IIRC the bombs have a conical mass of some kind of plastic IIRC that absorbs the X-rays that are channeled into it and gets converted into a huge cigar-shaped high-velocity blast of plasma.

IIRC the main problem with Orion wasn't technical - it could have been done like they say - but there was simply no real reason to go to all that trouble. Exploring the moons of Saturn may sound like a great idea scientifically, but while landing on the Moon had the whole 'my ICBM is bigger than yours' thing, Orion had nothing to offer for its cost. Once again, sociopolitical stuff boondoggles technical feasibility (and there really isn't anything wrong with that).

[Jadeite]

If I remember right, one estimate for deaths due to launch side effects would be around three people a year. Vending machines kill more people per year than that.

[Atlan]

If I remember right, one estimate for deaths due to launch side effects would be around three people a year. Vending machines kill more people per year than that.

Sounds about right. Also, people need to remember that although this sounds crazy now, Orion was proposed in an era when above ground nuclear tests were commomplace. IIRC the total fallout of a Orion launch was calculated to be roughly equivalent to one one megaton sized nuclear test. And there were dozens of those back then.

[SirNitram]

If I remember right, one estimate for deaths due to launch side effects would be around three people a year. Vending machines kill more people per year than that.

Sounds about right. Also, people need to remember that although this sounds crazy now, Orion was proposed in an era when above ground nuclear tests were commomplace. IIRC the total fallout of a Orion launch was calculated to be roughly equivalent to one one megaton sized nuclear test. And there were dozens of those back then.

Let me see. Was this the study by the people who were researching it? You know what one assumption they used was? Fission-free fusion bombs. Guess what wasn't availiable then?

[SirNitram]

Fission-free fusion bombs. Guess what wasn't availiable then?

No, Project Orion was researching pure fission systems. Fusion discussions didn't start happening until the 1970's, long after Orion was canned.

The only estimates for enviromental effects of Orion presumed the fallout from fission-free fusion nukes. Honestly, you'd think people would be able to work out that detonating 2,000 nuclear warheads would produce a noticable amount of fallout.

[Atlan]

Fission-free fusion bombs. Guess what wasn't availiable then?

No, Project Orion was researching pure fission systems. Fusion discussions didn't start happening until the 1970's, long after Orion was canned.

The only estimates for enviromental effects of Orion presumed the fallout from fission-free fusion nukes. Honestly, you'd think people would be able to work out that detonating 2,000 nuclear warheads would produce a noticable amount of fallout.

You're kinda forgetting that most of those nukes wouldn't be going off in the atmosphere, or even close to earth, and the researchers in the fifties definatively assumed fission devices for all of their calculations.
Numbers for a orbital insertion were more long the lines of one or two hundred bombs.

[Beowulf]

Also, there isn't much fallout from an airburst. It's only ground bursts that cause a bunch of fallout, because only ground bursts have several thousand tons of material moved into the atmosphere via the fireball. An airburst that doesn't touch the ground at any point will only have the material of the bomb itself as fall out, and that material can generally be chosen to be non neutron activatable. So you don't end up with much from the bomb itself.

[SirNitram]

I will concede they'll be less fallout.. The majority of irradiated material will be the bomb materials, and the pusher plate. Though things like soaking the entire hemisphere's orbital area in X-rays is not gonna do good things for, say, electronics. There's so many damn things wrong with this it's hard to keep them straight, but that's my fault.

[Beowulf]

Well, you just screwed up again. The pusher plate should remain entirely intact after use. The secret? coating the plate with a hydrocarbon. The hydrocarbon ablates off leaving the plate intact, except possibly for neutron activation. In one of the nuclear tests, a pair of graphite covered steel spheres were placed within the fireball radius (IIRC) of the initiation. Said spheres were later recovered completely intact.

I will conceed that it will end up giving an entire hemisphere some dose of ionizing radiation.

[SirNitram]

Well, you just screwed up again. The pusher plate should remain entirely intact after use. The secret? coating the plate with a hydrocarbon. The hydrocarbon ablates off leaving the plate intact, except possibly for neutron activation. In one of the nuclear tests, a pair of graphite covered steel spheres were placed within the fireball radius (IIRC) of the initiation. Said spheres were later recovered completely intact.

This I did not know. And, y'know, knowing is half the batte means I lost that one. Hardcore.

I will conceed that it will end up giving an entire hemisphere some dose of ionizing radiation.

A bad thing if you like satellites. Then we're back to the problem of the fact you can't conceivably land a ten-thousand ton spaceship, so you need a CRV and a chemical-powered ship to get crew there.

[drachefly]

The spheres were placed quite close to the device in question, IIRC.

The 'cover it in oil' approach was decided on after a series of appropriately-scaled tests using a conventional explosive.

Not saying it's a good idea... maybe if the world was sufficiently at peace that no one would be worried by the idea of having a couple hundred small nuclear devices going into space.

[SirNitram]

The spheres were placed quite close to the device in question, IIRC.

The 'cover it in oil' approach was decided on after a series of appropriately-scaled tests using a conventional explosive.

Not saying it's a good idea... maybe if the world was sufficiently at peace that no one would be worried by the idea of having a couple hundred small nuclear devices going into space.

Methinks you don't get it.

1) Failure during launch does not cause the launch vehicle to be lost. It causes multiple nuclear detonations around the launch zone.

2) The vehicle must be 10,000 tons or it's not worth the cost.

3) Any bombs detonating above the atmosphere expose satellites with LOS to X-rays. X-rays and electronics bad.

4) The ship can't land, and if it's in a decaying orbit, it's gonna hurt something like a motherfucker.

[drachefly]

1 is bullshit. Nuclear devices are not like TNT. If you whack it hard, it will never go off, not when you whack it, and not later, even if you want it to. Fragments are another story, but that's not what you describe.

2... okay, and?

3 Space is big. Earth orbit is tiny. If the acceptable distance is not more than a few dozen earth-moon orbital radii, that hardly adds to travel time on the main outer legs.
As for takeoff, it may be possible to get your upwards acceleration done before you're far enough out of the atmosphere for your X-rays to to fry the satellites. I don't know whether this is feasible.
Keep in mind it's not at all too much to ask that the propellant units have stuff placed around them to enhance absorbtion of such radiation. In the upwards direction, that stuff will be called 'the pusher plate'. In other directions, it would be something dropped for that purpose.

4 So don't let it get into orbits low enough that they'd decay.

I don't see Orions being ideal for launch, I agree on that.

With a space elevator for the heavy lifting and initial push away from our geosynch satellites, I haven't seen anything to suggest they wouldn't be competitive for interplanetary destinations.

[drachefly]

Note: that means I think it would not have yielded much earlier; but in an alternate history like we have going around here, it could come up in the moderately near future.

[Beowulf]

Also to 3:

And? it's not as if there isn't any X-rays out in space already. Satelites are hardened against radiation already, because they don't work otherwise. Also, it's not as if you can't launch quite a few satelites on 10k tons payload.

[Sikon]

A good introduction to Orion is here.

Orion concepts could be divided into multiple categories. The first category is Orion ground launch, going from earth to orbit and beyond. The ship mass varied depending upon the design. The one mentioned in the preceding article is a 10,000-ton vehicle, most of its mass reaching orbit, utilizing 0.1-kiloton pulse units initially, flying straight up out of the atmosphere to minimize radioactive contamination, increasing yield as appropriate until 20-kiloton nukes were going off in space. Several hundred pulse units would be involved in going from earth to orbit or earth to escape velocity.

The public radiation exposure from a ground-launched Orion wouldn't be nearly as much as some might intuitively assume, but it would be somewhat undesirable. For nuclear devices detonated close enough to earth for the bulk of the plasma from the explosion to end up within the atmosphere, there can be some public exposure from dispersion of the 0.06 kilograms of fission products produced per kiloton of total yield. By proportionality with the 189000 kilotons of fission yield in historical above-ground tests, estimated exposure would be < ~ 37 man-Sv per kiloton.* Local fallout for a detonation close to the ground would temporarily have high radiation, but the craft is assumed to be launched far away from populated areas, as was done with nuclear tests. For some idea of the amount of a man-Sv, total natural radiation exposure of 2.4 mSv per person annually worldwide is ~ 160,000,000 man-Sv per decade.

* Such is from proportionality with the UNSCEAR estimating cumulative exposure from nuclear weapons tests both historically and from the past radioisotopes up through the year 2200, giving an approximate upper limit on man-Sv per kiloton.

Since a 1 GW coal power plant causes ~ 49 man-Sv per decade of public radiation exposure, an Orion ground launch with X kilotons of its total detonated at low enough altitude would cause as much radiation exposure as ~ (X * 0.76) coal power plants do in a decade, though this isn't entirely a fair comparison since the smoke of the coal power plant has much more chemical carcinogens and pollutants than its trace concentrated natural radioisotopes alone. Still, I don't support coal power plants either when there are economical alternatives, and there have long been alternatives to ground-launched Orion for developing inexpensive space access.

It is tempting to suggest ground-launched Orion would have resulted in much more progress in space. The first launch alone could launch much more mass of equipment than in NASA's whole total from 1958 to 2007, and history would seem less likely to have had economics quite like a 500-ton spacestation costing $100+ billion (the ISS of today). However, even aside from the issue of public radiation exposure, there were alternatives for low cost, with a greater, more fundamental limiting factor being lack of a need for heavy space launch as viewed by the government.

While ground-launched Orion ships could have been proportionally inexpensive compared to launch vehicles like the Space Shuttle of today costing $26000 per kilogram of payload, that would also be true for chemical launch vehicles designed for minimum cost. And those would be more likely to lead to use by private industry later. For example, there was a period in the 1960s when some thought that the government might launch a large mass of hardware for major space activities beyond Apollo while using rockets designed for minimum cost as a priority. A number of companies came up with concepts for reduced launch cost. Converted to 1993 dollars, here are some example cost figures:

$936/kg Boeing booster concept
$767/kg McDonnell Douglas booster concept
$474/kg Martin Marietta booster concept

The above are described in chapter 9 of an excellent publication, LEO On The Cheap by Lt. Col. London, which is in PDF files online.

Based on Truax's observations comparing the Thor and Athena for how little rocket costs scaled with size, if unnecessary extra complexity is avoided, the Sea Dragon was far bigger than the ones mentioned before. It would have launched a 600-ton payload to LEO with an estimated cost of $32 million to $340 million per launch, corresponding to only $59 to $620 per kilogram, depending upon factors like launch rates. Although "NASA Marshall gave the Aerojet designs to TRW for evaluation" and "TRW fully confirmed Aerojet's costs and engineering," this occurred when Congress was cutting back NASA. In their view, there was no justification to have a rocket capable of launching such large payloads.

In regard to the political unpopularity of spending government funds to develop minimum cost rockets, LTC London wrote this observation:

The idea of using simple unmanned boosters with steel tanks and pressure-fed engines was not technically or operationally exciting to the aerospace community at large, and it did not seem to hold the promise of billions of government dollars for development and for thousands of aerospace jobs. Further, it did not engender within the American people or their political representatives a grand vision of the future (like the Space Shuttle did), and it was far afield of NASA’s charter to advance aerospace technology. Consequently, initiatives to develop a minimum-cost launch system were quietly halted.

One may add that the Space Shuttle was designed to be a "jack of all trades" to fulfill a bunch of different roles: launching cargo, theoretically safely launching astronauts, having the extra mass of wings for a flexibility in landing sites desired by the military, and more. It was a glamorous spaceplane with the "high-tech" extreme complexity and challenging design that fascinates many academics and the public, and it was aimed at satisfying the requests of everyone from the Air Force to NASA. Meanwhile, the MCD proposals could only be appreciated properly by quantitative comparison.

A method of obtaining far less launch cost per kilogram than even a low cost rocket is to build a terrestrial mass driver, as my past posts discussed, but this is getting off-topic.

In the context of Orion, the observation from the above is that the U.S. government as a whole has never had much interest in launching large payloads to space or creating space infrastructure, caring little about launch costs. There are plenty of exceptions among individuals, such as the authors at NASA of the space settlement webpage linked to within my signature, or those developing the 1975 summer study proposal for a large spacestation and solar power satellites after a couple hundred thousand tons launched from earth. But multi-billion-dollar projects don't happen without Congressional support.

The ground-launched Orion concept was initially pursued in 1958-1959, eventually obtaining $1 million annual funding, including a flight test of a small model with chemical explosives. But that was losing political momentum, as implied in the article:

The Air Force felt that Orion had no value as a weapon, and NASA had made a strategic decision in 1959 that the civilian space program would be non-nuclear for the near future (33). NASA was and is a very publicity-conscious organization, and it is hard to overcome the negative perception of atomic devices of any kind on the part of the public. In addition, NASA was filled with engineers who had spent their careers building ever-larger chemical rockets and either did not understand or were openly opposed to nuclear flight. In this situation the Orion workers were truly outsiders. A crisis came in late 1959, when ARPA decided it could no longer support Orion on national-security grounds.

There was a temporary resurgence of Orion with a different variant. Instead of a large Orion vehicle taking off from the ground, the concept became to launch a miniature Orion vehicle with a pusher plate 10 meters in diameter after it was put into space by a Saturn V. Such could reach Mars and other destinations with far less travel time and deliver far more payload than a conventional craft of equal size. A General Atomics summary report is here.

With nuclear pulses beginning only far out in space, the preceding would avoid the public radiation exposure of terrestrial launch. Although some within NASA like Von Braun loved the potential of the project, as a whole NASA was concerned about public relations. The nuclear test ban treaty of 1963 weakened the project, and it ended in 1964. If the project had continued, it would have been interesting, with much potential for missions to Mars and perhaps more.

The cost of the nuclear bombs used wouldn't have been more than on the order of $0.1 billion per mission, a small part of total cost. The original Orion would have used simpler fission devices, and the early Martin design was estimated to cost $10,000 to $40,000 per unit. Assuming that is in 1958 dollars, that is up to $300,000 per unit in today's dollars. For several hundred units, the cost is then ~ $100 million.

In fact, the launch cost of the Saturn V, $2.6 billion in today's dollars, would have been a far greater component of mission cost.

The estimate for pulse unit cost was not unreasonable. In 1984, more complex thermonuclear warheads for "560 ground-launched cruise missiles were expected to cost $630 million," $1.1 million each. Total spending related to nuclear weapons by the U.S. has been large over the decades, but most relates to delivery systems like ICBMs and to development cost. The actual marginal production cost to the U.S. weapons infrastructure of a basic nuclear device is not much; indeed, the cost of plutonium made in reactors is not more than a few thousand dollars per kilogram.

Although the space-launched miniature Orion vehicle concept might have revolutionized space exploration, there would still tend to be limits on its overall effect, even if Congress did fund some interplanetary missions with it. Consider what happened with Apollo. Putting a man on the Moon fascinated many. However, in the months and years after success was obtained, the continuing interest of the general public dropped.

There were enormous post-Apollo cutbacks in NASA funding, partially due to the Vietnam war but also due to politicians viewing the situation primarily as just "mission accomplished." The last Saturn V rocket was launched in 1973, putting up the 80-ton Skylab spacestation in a single launch for what would be several billion dollars in today's dollars, and that was in some regards the end of an era, transitioning later to the Shuttle and eventually the International Space Station.

Although ideally a scenario with Orion interplanetary missions would lead to much more in space afterwards, that is actually rather uncertain. The primary motivation for public interest in space after Apollo seems to have been interest in Mars; even interest in the return to the Moon plan today is strengthened in large part by it being seen as a stepping-stone to Mars. Like many other people, I would love to see a Mars mission. However, from the standpoint of answering the opening post, it is logical to consider what would might happen after the public and Congress saw "mission accomplished" again.

If the post-Apollo experience is any guide, not all but a substantial segment of the general public would have reduced continuing interest after the initial Mars manned landing and after the first few months of astronauts on Mars. The result would likely be major cut-backs in NASA afterwards. NASA's funding as a percentage of federal budget never has recovered from what happened after the last major goal was successfully achieved.

The situation for private industry is by default unchanged in this scenario from what happened historically. There are still extreme launch costs from earth to orbit, and space hardware still costs many times its mass in gold.

Of course, from my perspective, such a drop in interest in space would all be wrong. But the average person's interest in space is more about a Mars mission than about reducing launch costs, building massive space infrastructure, or expecting future mankind to one day live primarily in space rather than on earth.

Such is why in the thread on colonization of the solar system I suggested that such would most likely occur only in a manner indirectly initiated by private space tourism. Unlike the government, that doesn't depend upon the bulk of the population funding something they don't think is directly benefiting them. Rather, suborbital tourism today and orbital tourism later gets money from individuals receiving direct, obvious benefit if they want to personally go into space. If that happens, one day some significant amount of the world's economic output could start being used on space development, and even a very small portion would be a lot, as implied in my discussion in the recent "Global Warming: You're God" thread in off-topic.



Although a little bit of a separate topic, one of the most interesting aspects of nuclear pulse propulsion is what it could mean for future technology. Consider the general aspects for a possible future space civilization:

While a propulsion system can involve either internal or external reaction of propellant, exceptional performance can be obtained without exceeding materials limits through external detonations, e.g. nuclear pulse propulsion.

Survival of materials is determined by the amount of mechanical energy and net thermal energy actually absorbed, as opposed to the temperature of the environment in itself. As an analogy, the amount of heat transfer is the difference between pain and injury from putting one's hand in boiling water at 100 degrees Celsius versus only mild discomfort from briefly passing one's hand through air coming out of a newly opened oven at a temperature just as high of 100 degrees Celsius or even substantially more.

As another illustration of the general idea of heat transfer mattering more than surrounding temperature in itself, in chemical-fueled rocket engines of today, the combustion chamber temperature can be up to 4000 K, well above even the melting temperature of its metal construction, but a typical regeneratively-cooled engine survives because channels carrying cold fuel enroute to the chamber keep the metal at much less temperature, actually typically at just a few hundred degrees, due to counteracting heat transfer into the metal with rapid heat transfer out, preventing net heat transfer and thus preventing further temperature rise.

But the sustained heat transfer in that internal-reaction system amounts to much each second and minute, despite the exhaust temperature and specific impulse performance of such a chemical-rocket engine being very low compared to high performance nuclear pulsed propulsion concepts. When the objective is to handle far greater exhaust temperature, up to millions of degrees, the goal can become to much reduce the magnitude of heat transfer, which typically can best be done by reducing the duration of exposure to a very tiny proportion of total time, e.g. through an external pulsed system.

The article gave an analogy regarding pulsed power of how the momentary temperature of gases within a car engine is far above the melting point of its metals, during the moment of combustion. That article also mentions a test in which steel graphite-covered spheres suspended 30ft from a nuclear detonation at Eniwetok had a little ablation but survived. Very little of the energy of the momentary nuclear blast was absorbed by them as heat, not enough to melt them, with heat transfer having occurred over a very limited period of time.

When the coating of a pusher plate is exposed to the low-density plasma of a semi-distant nuclear detonation in space for a brief moment of time, the amount of actual heat transfer is relatively limited compared to the energy of the blast, because the duration of contact is so short. There is a relatively high ratio of desired mechanical impulse compared to undesirable heat transfer and ablation. And the possibility of using a magnetic nozzle may prevent most of the plasma pulse from physically contacting engine material at all, reducing ablation.

Depending upon the technological level, there are multiple options for nuclear pulse propulsion, some fission like the original Orion, some fusion like IC fusion, and some a combination, such as antiproton catalyzed microfission/fusion or the thermonuclear bombs utilized by some later Orion concepts.

A 1970 Soviet thermonuclear bomb design had 100 kilotons fusion yield initiated by heating and compression from the energy of only 0.3 kilotons fission yield, corresponding to 420 TJ of fusion triggered by 1.3 TJ of fission energy release. If technology eventually permits igniting fusionable fuel capsules with energy delivered by a pulse of external beams substituting for the fission energy release, the capsule gain or ratio of fusion energy release to driver energy delivered can be as high as 1500. Such would also not be subject to the minimum critical mass scaling limits of fission triggers. As a random example, 0.1 kilotons of fusion yield could be initiated by 0.3 GJ of delivered driver energy, able to be stored in a few hundred tons or less of capacitors even with current technology, although neither such a capacitor bank nor equipment delivering that energy to a small fuel capsule within the necessary timeframe is available so far.

The original Orion was based on the existing technological base of nuclear bombs for the pulse units, and studies indicated obtainable specific impulse if high Isp was a priority could be 10000 to 20000+ sec. As a random illustration, a spacecraft with 10000 sec Isp could obtain 68 km/s delta-v after expending as much propellant as half of its initial mass. For perspective, maximum chemical-fueled Isp is about 450 sec (aside from some exotics of doubtful practicality), and a chemical spacecraft with fuel as 90% of initial mass obtains only 10 km/s velocity. As another comparison, ion engines powered by solar cells are limited by the electrical power, and such tends to be vastly less than power obtainable by nuclear detonations, limiting thrust, acceleration, and practical delta-v. For still another comparison, nuclear rocket concepts with an internal nuclear reactor heating propellant are limited to an estimated 800 to 1000 sec for a solid core like NERVA, or possibly up to 3000 to 5000 sec for a hypothetical gas core design. The value of external pulse propulsion is apparent in context.

There are various variants of nuclear pulse propulsion, such as a technique to bypass the usual critical mass scaling limit of a fission trigger by having a driver with external magnetic compression. It also has a magnetic nozzle, which increases efficiency and can reduce ablation compared to just having a pusher plate. That recent Mini-Mag Orion concept obtains 13700 sec Isp and 0.3g to 0.64g acceleration in a baseline concept with plutonium, although it could be upwards of 20000 sec Isp if designed for higher specific impulse as a priority over thrust. Such is workable for relatively small craft, and it avoids having to build and use actual nuclear bombs for pulse units, with implosion by the external system and not chemical explosives. Also, the ability to utilize relatively small pulses reduces the mass of shock absorbers needed, as there would no longer be as large, infrequent shocks.

Freeman Dyson's 1968 nuclear pulse propulsion ablative starship concept using pulse units equivalent to thermonuclear bombs had an effective specific impulse of ~ 467,000 sec Isp.

Some commentary relevant to far future possibilities for the hypothetical fusion variant of nuclear pulse propulsion is within a study here:

The application of ICF to rocket propulsion began in the early 1970's. Balcomb, et al. at the Los Alamos National Laboratory (LANL) proposed a laser-fusion concept that retained the idea of acquiring acceleration through particles striking a pusher plate. The pusher-plate idea originated in Project Orion, in which nuclear explosions were detonated behind a massive plate attached to the spacecraft through a pneumatic spring system. Then Hyde, Wood, and Nuckolls at the Lawrence Livermore National Laboratory (LLNL) proposed the use of laser fusion with a magnet to redirect the charged-particle debris from the fusion microexplosions, so that the debris would exit the rear of a thrust chamber and provide thrust. In their concept, the debris never touched any vehicle structure, and was hence never forced to thermalize with other materials. Avoidance of thermalization permitted specific impulses in the range from 10^5 to 10^6 seconds, which were much larger than the 10^4 obtainable with the LANL concept.

Even with future technology, there would be a trade-off between specific impulse and thrust, as the one relatively plausible method to have high specific impulse without an excessive heat load from the power involved is to have thrust low. The LLNL paper describes hypothetical pulse fusion technology, and it shows at the end how an exhaust velocity as high as 14400 km/s might one day be attainable, with obtainable acceleration estimated as dropping to 0.0014g at that 1.5 million sec Isp. Such low thrust at high Isp is a reason such wouldn't be utilized in a high Isp variant for interplanetary missions, but interstellar missions would take many years anyway, giving time for slow acceleration. Of course, the technology for the preceding is not available now.

Still, one day the intellectual descendents of Project Orion might spread humanity to the stars, a good legacy for a canceled project.