Sketchy Orions

Originally published at: Sketchy Orions | The Associated Worlds

Hello, and welcome once again to Truly Awful Whiteboard Pictures season, as I present to you the basic workings of a nuclear-pulse Orion drive, Empire-style. Or at least Empire-of-the-Spaceflight-Initiative-Era style. Mostly so that I can refer to it later and y’all’ll know what I’m talking about.

Behold ye first (and note that I’m not really getting into explaining the Orion drive itself at this point: for that see here, which should give you an idea of what I’m riffing on) an exceptionally crude and simplified illustration of the mechanics of the drive itself:

The way this works as part of each firing cycle is approximately this:

  1. A pulse unit, taken from the magazines, is fed by the loader into the breech/spinner and magnetically suspended there. The “spinner” spins it up for the purpose of spin-stabilization, as later in the process, it will be vitally important that the coaxial firing laser be able to lay a beam on the lens at one end of the soda-can-sized pulse unit.
  2. The irises (both the primary one built into the pusher plate, and the secondary one at the end of the coilgun, snap open to let the pulse unit pass.
  3. The coilgun throws the pulse unit out and down towards firing position.
  4. As the pulse unit reaches firing position, the firing laser delivers an ignition pulse to it (right down the now-empty bore of the coilgun). The pulse unit does its prompt critical thing.
  5. Immediately afterwards, before the plasma from the pulse unit detonation can reach the pusher plate, the primary and secondary irises snap closed to protect the firing mechanism from the detonation.
  6. And repeat for the next cycle.

As for the pulse units themselves, they look like this. Very roughly, as I’m eliminating all the complexity that goes into making them Happy Fun Shaped Charges as well as streamlined, spin-stabilized projectiles. Conceptual only:

The upper one is the old-style fission-based unit. Basically, it’s a laser-triggered fission bomb. The firing laser hits the beamcatcher lens, which splits the laser impulse into many beamlets proceeding along optical fibers, which are set up so that the laser impulselets reach the various explosive lenses surrounding the pit at the same moment. Implosion, prompt criticality, and fission pulse proceed to happen.

The lower one is the new hotness fusion-based unit. It’s a laser-triggered pure fusion bomb. In this case, the beamcatcher lens focuses the considerably more powerful laser impulse onto a parabolic mirror and needed secondary mirrors (not shown), for the purpose of delivering the laser impulse to all sides of a fusible pellet at the same time, even as the mirrors themselves are busy vaporizing. Fusible pellet hits inertially-confined criticality, and fusion pulse proceeds to happen.

The important thing to note here is that as much of the complexity as possible has been offloaded from the pulse units to the drive, inasmuch as you might want to knock a quick thousand pulse units out in the field with a relatively crude shipboard machine shop, which is not something you would expect to be able to build the drive.

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A little detail that comes to mind: If i was building this, I would not like that iris. It’s expected to close in a timeframe of miliseconds, and a service life of thousands of cycles, under harsh conditions, and it’s mission critical, and using near-future tech. My inner engineer sweats.

If I was making this, i’d explore a few possibilities. Can the laser be fired offcenter via a mirror that rotates into/out of place? Can the laser fire through a pane of smart glass that is darkened at the speed of electricity? Can I route the bomb through a chute that dampens the plasma wave a-la a suppressor on a gun, forgoing any moving parts? Can the bomb be thrown around the blast shield and maneuvered into place via electromagnets? These are possibilities that come to mind after a minute or two of thought.

This is a bit of a nitpicky detail, but those are the best kind of detail.

I would concur. This iris seems like an easy failure point. Perhaps it could be thrown from off-center, but you could also do something where, by the time of detonation, the launcher is occluded by the blast plate? Or would that not work?
One would probably want there to be a dogleg in there as well, in any event.

Concept for later in history: the virtual Orion drive. Who needs a physical pusher plate when you can just project one (ablate that!), and as is physical the momentum is then transferred to the field generator, which then acts as the piston in a massive hydraulic momentum-transfer buffer.

Some thoughts:

That’s why there are two of them. And in the event that one fails closed, explosive bolts.

Magnets, probably. (Don’t want to add physical wear and tear to your other problems.) Of course, that depends on whether you can reliably use magnetic fields to keep the iris in place when the nuke goes off, which depends on numbers I haven’t run yet.

…ooh, I wonder if electroactive shape-memory composites would work?

The fission one could maybe do the latter; the fusion one - I don’t think you’re going to find smart glass that has a decent service life after a fusion-initiation pulse passes through it. Anyway, the problem here isn’t protecting the laser alone, otherwise putting a shutter on it would have been a simpler solution. You don’t want the plasma front backwashing into your coilgun, or worse, up through the breech into the magazines, either.

Actually firing the laser at the pulse unit off-center, rather than just routing it from an offcenter laser to its axial firing position, breaks the simplicity the design was trying to achieve. For the fission unit, you either have to lose spin-stabilization or else have a worse timing problem trying to lay a beam on an off-axis window in the pulse unit at the right point in its spin. For the fusion unit, it uses laser-inertial confinement ignition, so you’re back to having a whole circular array of lasers peeking over the sides of the pusher plate. Either way, you’re eating complexity.

Possibly, although it would significantly increase the length of the firing assembly…

Given the choice between a couple of redundant irises and a complex electromagnetic bomb-juggling apparatus installed on the wrong side of the shielding pusher plate - well, you tell me where the bigger chance of failure is?

(And which one results in a loose pulse unit going off the map?)

That exists in the feed from the magazines. Coilguns, like other guns, don’t work so well when you bend the barrel.

Ah, i’ve got a better idea! You know how some Orion concepts use the idea of spraying the plate with silicone oil before every blast, to reduce thermal effects? Do that in place of a iris.

Have a chamber that efficiently captures and recirculates a film of opaque silicone oil after the charge leaves the accelerator. It won’t affect the charge, but it will stop everything except the rads from the blast. Not sure what to do about the laser, though.

Ah, good point. I overlooked how the entire thing being in a line greatly simplifies the aiming process. Even if you managed to, for instance, give the charge a off-center spin and fire the laser off center, you still introduce a major timing difficulty. Hmm.

I’m not so much worried about redundancy, but rather getting it to work at all. To me it sounds like trying to make a rocket bell, but it’s also a complex moving part. Subject to ablation, the sputtering and depositing of it’s own metal into itself and everything nearby, frequent high impulses, high temperatures, vacuum conditions, ionizing radiation, etc.

I don’t know that it’s wholly infeasible. Stuff like vectored jet engines exist. But those constraints still sound harsh.

(… This still feels like a bit of a nitpicky detail. But I always find fiction is better for those little clever details.)

I’m surprised that having mastered high-gain fusion and having a presumably maturely developed coilgun as the pulse unit launcher, they didn’t just fully tilt into the MAGNETS!! camp and fashion a magnetic nozzle. Avoids many of the problems with plasma blowback while still allowing entry for pulse units. The only concern then is the thermal X-rays, which smart glass and/or baffled mirrors can feasibly manage.

Indeed if you want to get really really torchy IRL, the only way I can see of making it work without square mile size radiators is a sort of M2P2 system where the pulse unit when energized connects itself to the pusher nozzle via a tube of self-contained plasma, so you can standoff the pulse unit at a great distance and minimize the solid angle subtended by the spacecraft against those pesky X-rays.

It’s a case of not all progress happening at once. (And do bear in mind that the first iteration of these was designed to use fission pulse units.)

Specifically, it’s substantially easier to build a laser-triggered fusion bomb than an inertial-confinement pulsed fusion reactor.

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Hrm. In other thoughts, how about extending the nuclear shaped charge concept a little more. If we can do a cone, surely we can do a hollow cone?

You can indeed generate all sorts of whacky shapes on the charge if you vary the pellet propellant geometry, but I would think the main concern when getting torchy is all the uncharged EM and particle radiation flying off the pellet, that you can’t just intercept with more mass because you want to keep that specific energy high enough for brachy-class specific impulse. This will simply spread out as a sphere, and assuming they haven’t invented orichalcium before they made torch drives, the only way to work around it would be increasing standoff distance.

This can actually already be done by selecting low-Z materials for the pulse-unit propellant material, which results in a very narrow jet; the problem there is now your pulse units must be bona fide spacecraft in order to aim themselves at the pusher plate with milliradian accuracy, which then drives propellant costs through the roof. An M2P2 system would reduce the need for accurate pointing - the 2 inflated magnetospheres (ship and pulse unit) simply create a tube for the plasma to flow, and you get a hot lance of plasma that guides itself to the magnetic pusher plate that is quite tolerant of pellet misalignment, which also means a cheaper pellet.

Just to remind where this started:

We’re not building fast torchships, here. We’re building second-/third-generation heavy-lifters.

Orichalcium was mined from Precursor trash long before it was creatable. Of course, that made it all kinds of incredibly rare and expensive, and as such not to be used in anything even slightly disposable, but such is life.

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