K^2

RocketStar has made discussion on this forum a couple of times, but with some of their older projects. This one's spicier. I just randomly came across an article effectively repeating RocketStar's press release, and I kind of wanted to take this one apart a little bit. To get the tl;dr out of the way, the claim is that they are boosting their 1U pulsed plasma drive with fusion to give it an extra kick "for free". How much of a kick isn't really specified. There is no point of going over an article, so lets just go to the source with RocketStar's Press Release. Note that they are referencing real experiments that have confirmed the reaction and talk about the upcoming flights of their current FireStar drive, but don't say much about the fusion results beyond detection of gamma and alpha radiation. Which is cool in itself, but doesn't say much for the practical side of things. They do specifically clarify in the press release that the "fusion boosted" really means "fusion-triggered-fission-boosted," which is something that needs to be taken apart a bit. Specifically, the claim is that by introducing boron to the water, which is used as main propellant, they are getting boron-proton fusion, which results in an unstable state of carbon, which decays to alpha particles. The release does not specify the isotopes used, but boron does come in 10B and 11B, with the latter being more abundant and more interesting of the two. The process 11B + p -> 12C is of interest here. As we all, know, 12C is stable, however, there is a rather well known Hoyle State of carbon which isn't. The natural way in which Hoyle State comes up is when stars burn helium in their cores. The process involves two 4He nuclei coming together to form a highly unstable 8Be nucleus, which has just enough time to collide with a third 4He nucleus to form the Hoyle State of 12C. The latter is highly unstable, and is almost like the 3 alpha particles bouncing around in a common strong force potential than a real nucleus. Consequently, most of the time it decays back to three alpha particles, and the star has to try again. Occasionally, however, the 12C** state emits a gamma, decays to a lower energy 12C* state and finally to ground state 12C. The Hoyle State has 7.7MeV above carbon's ground state, which is a lot. The full proton-proton cycle is about 24MeV. So we're talking a good fraction of typical nuclear energies. On paper, this works. The total binding energy of 11B is 76.21MeV and ground state 12C is 92.15MeV. That gives 11B + p plenty of energy to form Hoyle State, so long as protons are hot enough to overcome the repulsion barrier. It is much lower for proton-proton or even D-T fusion, since 11B has the charge much more distributed through the larger nucleus, but I don't really have any numbers to attach to it at this time. I'll try to find either something from the cited SBIR experiments or other publications at a later point to try and get a back-of-the-envelope estimate for how much fusion/fission we are actually expecting in a chamber of a plasma drive. What we have that's probably pretty reliable are the specs of the current generation FireStar M 1.4, which does not involve any fusion/fission processes. Just a conventional pulsed plasma propulsion unit, utilizing water, ionizing arc, and an accelerator cavity. Since this is a commercial product, and validation experiments are apparently available under the NDA, I have no reason to suspect them from being far off. Cited directly from RocketStar's website. The ISP cited is consistent with the total impulse with 250g of propellant and implies mean exhaust velocity of about 71km/s. If the protons are accelerated by the field of the same strength, they can be potentially traveling about twice as fast, resulting in effective plasma temperatures of up to low mega-Kelvins on collisions. That feels like ballpark reasonable for what it'd take for 11B + p fusion to take place. But again, I don't have good estimates for that at this time. On the net, interesting. I want to see the numbers attached to the claimed efficiency boost. Also, the fact that they're calling it a fusion rocket when it's really more of a fission rocket feels a little scummy, but maybe they were trying to not scare people with neutron radiation associated with fission? This is, if real, an aneutronic process, so it would be reasonably safe for the applications for which this is meant. Either way, a real nuclear-boosted drive that's commercially available is really exciting, even if the performance benefits are going to be marginal. Finally, even the base model, without fusion/fission is a really nifty example of how far the commercial plasma propulsion has gone. A 1U propulsion unit that can provide a 3U sat with 4.6km/s of delta-V? Yeah, that's not bad. With the right planning, you can take your 3U from LEO all the way to Mars utilizing a boost from Luna. Neat.

Eh, you say that, and it's not wrong, but there are a couple of counterpoints to consider that make things spicy. First, there's modern BVR combat, where aircraft need to get into their weapons range, hopefully undetected, launch, risking revealing themselves, and bolt. That last part is crucial, because firing the weapons is when you're most likely to revel yourself, and if the enemy gets even an approximate fix on you, they can fire their stand-off munitions and let them acquire once they're a lot closer to you. And per the g-factors point, your odds of outmaneuvering a missile that got a firm lock on you are not good. Of course, you have a variety of coutnermeasures, but by far best option is to simply not be in range of the missile heading towards you. You're firing your weapon in the direction you're heading, extending its range, and the enemy is, hopefully, not as lucky, and you can put some extra distance by flying fast in the opposite direction. If you're in a 160km range of a modern AIM-120D that will streak towards its target at mach 4, if the enemy returns fire with a similar weapon, and you've been running at mach 2 the whole time, you'd be around 240km away by the time it catches up to you, which means it probably won't. The risk you're running is any aircraft or ground launchers that are concealed from you and are closer than the intended target. If that concealed enemy's missile is still in that 160km / mach 4 ballpark, you need to be more than 105km from a potential counter-launch to be confident you'll outrun it. And that means you have a 55km margin of advantage. Not bad. The problem is apparent once you start doing the math on the turning around and bolting part. At mach 2, pulling 5g the whole time, your turning radius is about 10km, and doing 180 takes about a minute and a half. By the time you finish your turn, you're basically the same distance from whatever could have answered, and their mach4 missile has advanced by 30km. Now, it's 30km out of its max range, but you have less time to run. The net effect is that it costs you about a third of that, 10km, in your margin. It's "only" 20%, and you still take it, because you still have a 45km advantage, but oh, how you wish you could turn faster. And maybe you do, take the risk, and pull the stick a little harder. After all, you could take a sustained turn at 6-7g in training, and even push into 8-9g in bursts. But is that something you can do in combat conditions? Sometimes pilots find out that they can't the hard way. Anything we could do to push that limit further out, is going to give your pilots an enormous edge and safety margin in modern air combat. So that brings us to the second point. Drones. A drone operator is more likely to stay cool in combat, performing as well as they have in training, the drone can in theory take higher g load, and losing it isn't as bad as losing a plane and its pilot. It can still be expensive, but now you can play with the balance of cost-to-build vs survivability with both fewer moral conflicts and without running a risk of burning through your pilots too fast if the conflict gets hot. And we're seeing examples of all of this in practice, ranging from drones meant to perform the same functions as a jet fighter, to lighter drones performing CAS missions or serving as loitering munitions. And it sounds like we should just dump the piloted aircraft and go with drones for everything, but we're already seeing major limitations in real combat conditions. Communications in the battlefield can be spotty. We see a lot of effective uses of electronic countermeasures, particularly effective against long range drones. We've seen everything from unreliable contractors cutting coms, to jammers ranging from ECM trucks to portable devices. A lot of this stuff has been largely theoretical until now, and the battlefield evolves really rapidly, but we're already seeing ECM measures around Moscow, for example, being very effective against long range drones that have no problems striking targets at that range in other parts of Russia. We're also seeing a bullseye painted on any control centers. They become priority targets for enemy strikes. If all of your trained pilots are sitting in one truck, even if they're far away from the combat, they are far easier to hit than if they're spread between multiple aircraft moving in the sky. If you have a reliable satellite link, maybe you can put your pilots far enough away, but even that depends on the nature of the conflict, and in practice, you often have to balance that against response times. As a result, the long range and marine drones are being controlled from central locations, but CAS and loitering munition drones are usually operated with the infantry or artillery units they're attached to, and so they're set up with their command, making them juicy targets for artillery and even ballistic missile strikes. We're still shifting more and more focus towards drones, but I don't think we're going to replace piloted aircraft any time soon, and so the ability to pull high g turns to get out of harm's way is still going to be very much desired. Anything we can do to improve that, people are going to be trying. It really would be lying down for a plane, as its lift can push much harder than its thrust. But that's kind of part of the problem. For a turn, you want to lie down. Lets say we fix the problem of seeing from that position with displays. Engines on afterburner are still capable of more than 2gs. That's basically a guaranteed brown-out if you're flying feet first. Well, what if you fly head first? Now you have all sorts of safety issues while landing... And overall, just the fact that you might be accelerating or decelerating at a few g while also performing a high g turn means the angle has to adjust. Putting a full tilt bed in the plane sounds like a nightmare. Where I can see this working is space. Main rocket pushes you forward, and that's the thrust you get. It does give you that "standing" position. And we see it recognized in sci-fi. The example I'd use is Starfuries in Babylon 5. Whoever designed these, put a lot of thought into it. They have four main thrusters with a smaller reverse attached to each. They are placed in X-configuration on "wings" (really just structural pilons), with more span horizontally than vertically. Why? Because just like with linear acceleration, rotation generates g forces on your body, and you can be spun around your vertical axis a lot faster than around any horizontal axis. So if you need to make a rapid turn, that's the direction you're going to go. That's where the realism in the show ends, as you're seeing these things engage in dogfights using projectile weapons. That is a lot more cinematic, of course, but we'd still be dealing with BVR combat in space, if it ever becomes a thing, and you'd have all of the considerations from the first part of this post coming up. The difference is that now you don't have air to play against. You can go as fast as you want, but you better have fuel left to slow down afterwards. And that's the same fuel you'll be burning doing any sharp turns. So now it's really a game of how fast you want to be trying to get out of a missile's range, and how hard you're prepared to maneuver once it catches up to you. How much is it keeping in reserves while "playing dead" coasting towards you? I hope we'll never find out. As much as wars suck, wars in space are going to suck more. All the worst of unpredictability of air combat, fear of being marooned on a sub, and being on the wrong end of a bombing raid rolled into one. And all of it goes not only for soldiers, but for any civilians in the crossfire.

That actually sounds right. The inside of an h-bomb sitting on a missile inside its silo is only slightly warmer than the room temperature, and there is some heating of the rocket's skin on the way up. On the other hand, during the detonation, the heat evaporates tens to hundreds of meters of rock in an instant, so I'm not entirely sure what sort of shielding we'd be talking about. The thermal radiation would be kicking out x-rays at 10keV and up, which strip nuclei bare, causing any and all matter to disassociate into a rapidly expanding ball of superheated plasma. Anyone who thinks you can alleviate that with some thermal tiles is beyond help. They have been failed by society and should be consigned to whatever is the educational equivalent of hospice care.

MD-88

I think we'd need to attach some numbers to it, and it's not trivial. The first question I have is how fast the capsule can hit the ground in order to stop without burring itself far below the surface, because there's a limit to that. Both from the type of surface you hit and from the material properties of the capsule. The second question I'd have is if we do find a sweet spot for the speed at which the capsule buries itself most of the way in, but leaves an exit on the surface, how big of a crater is that going to dig? Which, again, might depend on the type of soil you're hitting. Even without putting numbers to it, I think it's pretty clear that the method is far from universal. Hitting a swamp might be comparable to hitting water, and hitting hard rock is unlikely to give you any significant penetration without going far beyond what any theoretical material you could use for capsule can withstand. (We're not even talking about the pilot here.) But if we kind of ignore the fact that it's a game where you need to be able to plunk the exit point anywhere, and go with a more realistic, "Yeah, we're going to be picky about the landing site," something like sand or soil might be in the right ballpark. That said, the entry velocity will be moderate, far below orbital, meaning we will need to rely on another method of braking. It could still be entirely passive, such as aerobraking, or rely on the retrorockets of some sort. Either way, having a number for velocity on impact might give us an idea of whether it gives you AA evasion benefits or not. I'll try to do a quick scan through literature, because the naive thoughts I had on how to estimate the impact speed for given penetration depths are not giving me anything reasonable. There might be good models for soil as loose particles that should be appropriate here.

How is that different for motion between the Earth and the Moon? Or between a starship and a star it's traveling to? How do you measure relative velocities across empty space? You can't put a little turbine out and measure how fast the "space" moves past you. That's the most fundamental principle of relativity. Mathematically, you fill the space in between with probe particles that are infinitesimally close to each other, and adding up all the relative velocities across the chain. (Because remember, motion is relative so if the chain is "static" relative to you, it might be moving relative to me.) If you do that between distant galaxies, you can't tell the difference between the two galaxies moving apart or the space in between expanding. In practice, you send a beam of light instead. Or rather, you just wait for it to happen naturally, because it happened billions of light years away and billions of years ago... But regardless, with light, you again run into the same problem. Whether the object far away is receding or the space in between is expanding, you'll get the same result. Motion due to expansion is true relative motion, and it is true relative motion at the superluminal speeds for the galaxies. What if I told you that if you write it in the framework of GR, properly accounting for frames of reference, effects of driving a vehicle across the planet, and saying that the vehicle's wheels cause the planet to spin underneath with vehicle staying put are identical? It's just a mathematical perspective. We happen to use math that explains this type of motion through space-time geometry, because that's the easiest thing for us to put into formulas. There is an equivalent formulation of it under which you just "drive the Jeep". That is, fly the ship at FTL speeds. It's just the kind of math that makes the epicycles of a Geocentric model look quite reasonable in comparison. There are other problems with the Alcubierre Drive, of course. I actually missed the mark a bit on saying the behavior has to be different in curved sapce-time. I had time to revise my understanding in the years since... It's a lot worse. The Alcubierre Drive just straight up doesn't work if it carries any non-zero mass inside of it. Even in flat space-time. The negative energy of the bubble walls has to cancel the mass energy of the ship, or the bubble starts radiating gravity waves. So how do you avoid angular momentum conservation problems when warping around a star system? Super easy, barely an inconvenience. You make sure that the total mass that gets transported is zero. Where you get that negative mass at origin and what you do with it at the destination is left as an exercise to the reader. *Her. I refuse to corroborate on whether this has anything to do with the effects of FTL or time travel.

There are entire galaxies, observed and measured to be receding from us at the speeds exceeding the speed of light. It's not about the boundary. Stuff in the universe is moving relative to other stuff in this universe at superluminal speeds. This isn't some hypothetical on a napkin. It's a firmly established cosmological fact. Causality is a mathematical statement. There are concrete theorems and several notable conjectures that are yet to be proven within a framework. If you don't understand it, all it signals is limits of your education on the subject. Cool. I was doing research in particle physics. That is, I specifically worked with the concept of matter propagating at energies where space-time metric is the ruling factor, and understanding time ordering is crucial in getting correct results that match experimental data. You can dream all you want. The measurement precision on QM and GR set the expected scale limitations on where these break down. Classical mechanics was breaking down at sizes much larger than an atom and masses smaller than these of a planet. We could work at these scales and exploit these violations. The QM holds for many orders of magnitude below the scale of any known particle, and at masses exceeding these of galaxies. Humanity isn't going to reach these numbers. Even if there is a higher order theory that is more correct, we aren't going to see the difference, because it'd take several times the energy of the known universe to do anything that does. So unless you want to believe in magic fairies that break down the space-time barrier for us as some sort of a favor, we are going to have to work within the confines of the theory that we have at least until we make crossing the universe as easy as sending a GPS satellite to orbit. Because that's a prerequisite for getting to these scales. The best we can hope for is understanding the theory we have a lot better and make full use of it. We aren't tapping into a fraction of the near-magical bull crap that QM and GR actually say are possible. Which already covers FTL, wormholes, teleportation, time travel, computational capacity that borders on omniscience, and more. It just has to follow the rules that have already been firmly established. You can't flap your arms and fly despite the fact that we've figured out jetpacks. No matter how much you want to imagine it.

I was trying to get an actual mention in, but your name wasn't showing up for some reason. Anyways, glad to know you saw it eventually. I'm the one who posted the original electric rotor ascent on Jool video, and I was super excited that somebody made a working mission out of it.

"Discrete" on the timescale of the age of the Solar System. These are still very slow migrations from one stable location to another. Take two massive planets around a star and consider their mean interaction in the constant of motion coordinates for the central potential. When the planets are far out of resonance, the energy and momentum flow between the two are very slow. The orbits will drift, but very, very slowly. As the two approach the resonance, the transfer rate becomes much higher, with some sort of an equilibrium at the exact resonance. For just two planets, all that really means is that they'll drift slowly and then almost "snap" to resonant orbits like a pair of magnets when they get close to resonance. Again, on the "age of the star system" kind of time scale. As you start adding more objects to the system, interactions get more complex. There aren't just drifts, but also precessions. Several planets might be happily spinning in their own planes for ages and ages as the planes of their individual orbit slowly precess, until the two planes align, and suddenly, these two planets are strongly interacting with each other or with some 3rd body, causing their orbits to start changing rather rapidly on the cosmic scale. Point is, there are a lot of quaistable arrangements that become unstable once some of the parameters of the system happen to align in a certain way, then they become highly unstable, and start shifting until a new quasistable arrangement is achieved. Truly dynamically stable systems are exceptionally rare. There's currently only one system I'm aware of (HD 110067) that is suspected to have all of its known planets nearly co-planar and in simple resonances with each other, therefore, being exceptionally stable. Pretty much everything else we've found has some combo-breakers in the system that are orbiting out of the main plane, with high eccentricity, or way out of resonance, meaning they'll throw a wrench into the stability at some point in the future. Solar System is unusually messy, based on what we've seen so far in other star systems, but not incredibly so. The current arrangement is stable enough, and there is no expectation of drastic shifts for the near future, but in the system's past, we've had a lot of rearrangements that would come in bursts of activity for the aforementioned reasons.

The 3D image isn't meant to represent the plumes. It represents regions that have different wave-propagation properties than the rest of the mantle, which happen to correspond to some, but not necessarily all plumes. That sums it up nicely.

Yeah, I'm going to just say that nozzle efficiencies quoted in that section are impossible due to black body radiation losses at the temperatures in the detonation region that this design implies. Estimates in spoiler. Point is, if you were to build a NSWR to that section's spec, almost all of the energy from the detonation region would be escaping as X-rays, drastically reducing the thermal energy that can be converted into the exhaust velocity by the nozzle. It's still a cool concept, if you dial it down a bit. The previous section building up a more realistic proposal for 6,730s version seems a lot more plausible. Again, that 4th power in temperature does a lot of damage to rockets that convert thermal energy into propulsion, but only on the high end. There's a lot of room to grow beyond what we can squeeze out of chemical rockets. But if we want to get these 6-figure ISP engines, we have to have a different way of accelerating the exhaust. It has to be some sort of an electromagnetic drive.

You're overreacting. No way the update release will go that badly.

Do you have a quote on that? Six-digit ISP would put the core temperature in a 100MK ballpark, which cools basically instantly by emission of hard gamma radiation. I'm wondering how that's addressed.

I don't see a way for Intercept to make interstellar drives anything but exceptionally useful for in-system travel. It's been stated directly that the interstellar distances are meant to be realistic. I don't think we still got any confirmation how that correlates to the 1/10th scale of the game and potential relativistic limits of the real world, but we're still talking distances in billions of km under the most generous interpretation. It takes 1 year accelerating at 1g to reach light speed. I know we're all going to have warp set to absolute max the game allows when traveling between the stars, but there are inherent limits to how much warp that can be, and if it takes hours of real time, that just simply won't do for a game. Interstellar ships have to accelerate at a pretty good clip to make the game playable. Maybe not quite 1g, but a sensible fraction of it even when fully loaded with fuel and cargo. So what we learn from all of the above is that the interstellar drives are going to be very efficient, having ISP several orders of magnitude above anything we had in the original game or have access to now. And that to be pushing all that fuel at a reasonable acceleration, these engines will have to come with very good TWR. Far better than ions, likely better than the NTR engines have been, and possibly approaching TWR of the chemical engines in the game, depending on just how big the interstellar gaps are. There might be ships you want to build that are just too small to make use of an interstellar drive in the end game, or ships that have to go in the atmosphere. So I don't mean to say that there will be no uses for other engines. But I cannot think of any sensible barrier Intercept can put in the game that will allow interstellar gameplay to be fun and not make interstellar drives hands down the absolute top choice for in-system hauling.

At some scale, you do have to start coming up with something different. What are you going to do against a mass driver strike on a colony? Turn off the Sun and release a fake Mars? I can already write a Horizons query that will tell me where any given crater on Mars is going to be at any specific point in time for the next century, and there is no reason to believe that protection of fixed assets is going to become less relevant.