shynung

I'd imagine there would be balance problems. Imagine holding a wide object while walking on a fence.

The Soyuz rocket family are transported to its launch pad in Baikonur by train, fully assembled. The central core was tapered to make room for the side boosters, so as to fit the train's maximum load dimensions.

For that purpose, only a single yo-weight is used. The asymmetric yo-weight release would induce the spent booster to tumble, provided it was positioned correctly.

AFAIK, in mining, explosives are used to break a chunk of ground into small-enough pieces that the diggers can scoop up into the trucks. So, use case for a nuke mining bomb would be to blow open the side of a mountain, to access the ore within.

Is it possible to make a megaton-class nuclear bomb that doesn't leave radioactive fallout? Say, for mining purposes.

The projectile is already mostly plasma after it goes through the frontmost plate. Slowing down small, high-velocity fragments are what aerogels are good for. Aerogels also have very low density, so putting on several meters thick of it would not give a significant mass penalty.

Best to fill the space between the thin plate (Whipple shield, I presume) and the main armor with some sort of aerogel. It adds additional mechanical resistance to the rapidly-disintegrating projectile/debris, giving it a chance that it may never reach the main armor at all, stuck inside the aerogel. Also, it's a good idea to stick a layer of fibrous armor (like aramid/Kevlar) just behind the main armor, to catch any spalls. Also a good idea is to angle the armor, to increase its effective thickness (a projectile impacting armor at an angle has to go through more armor compared to one impacting straight). Also, angling the armor gives the projectile a chance to bounce off instead of digging into the armor.

Is this level of maneuverability even possible? Say, with drones or missiles?

It is, it's called an air-augmented rocket. Still uses internally-stored fuel and oxidizer, but takes extra propellant from the surrounding atmosphere. It won't approach turbojet levels of efficiency, but pretty close to LH2-fed nuclear thermal rocket.

I'm thinking of a gas-core fission reactor (think nuclear lightbulb) with the fusion fuel snuck into the fuel loop.

Is it possible to build a hybrid fission-fusion reactor design? If so, how would it perform against standard fission reactor designs?

I read that this trick was put to use in the SR-71 spyplane. Some of the outer skin, in particular the chines along the side of the fuselage, are cooled by the incoming charge of fuel.

AFAIK, on a rocket engine, regenerative cooling was done to keep the nozzle and chamber from melting despite the vigorous reaction happening inside them.

So, like regenerative cooling, but being used to generate heat instead of cooling the chamber/nozzle. I don't think putting a turbine past the HX would do much good. That would sap propellant line pressure, and it's needed for thrust. Much simpler to plug a generator at the turbopump shaft instead.

Well, yeah. NSWR is still sketchy as it is. Mostly because there's no one ballsy enough to test it out. Project Orion nuclear pulse rocket at least had small-scale test models using conventional explosives. Well, OP seems to be concerned primarily on ISP, and not much else. FFRE is at the higher end of ISP performance, so that's what I pointed out. Of course, OP has found out about Slough's magneto-inertial fusion rocket independently. So that's that.

@PB666 Again, if you go with nuclear electric, a good portion of the reactor's output goes out the radiators. In a NTR, almost 100% of that output goes into the propellant. NTRs have dismal ISP because they can't get hot enough - at least, the solid core rockets are. Open-cycle gas core rockets, along with nuclear salt water rockets, get pretty good ISP because they let their cores get hot enough to turn into plasma. Also, perhaps you might want to take a look at fission fragment rockets. These are a class of NTRs that use their own reaction product as propellant. One Robert Werka figures that a first generation design can get 1.7%c exhaust velocity (~520 000 sec ISP). A refinement of this design involves plugging a LH2 injection system to boost thrust at the cost of specific impulse, creating a so-called afterburner system. ISP drops to 32 000 sec, but thrust increases massively (~4.5 kN, compared to vanilla FFRE's 43 N).

NTRs pretty much gobble any propellant without too much trouble. Favorite propellants of Children of a Dead Earth warships, besides hydrogen, are methane, decane, water, ammonia, and hydrogen deuteride (H-D). The last one have better ISP than regular hydrogen (H2), for reasons which escape me.

Actually, SCNTR gets a better ISP by ejecting lower-molecular-mass exhaust than their chemical counterparts. For the same chamber conditions (temperature, pressure), a gas of low molecular mass will achieve higher velocity post- expansion through a de Laval nozzle compared to that of a high molecular mass. So, in theory, you can boost the SCNTR ISP even further by using atomic hydrogen (single-H) as opposed to molecular hydrogen (H2), provided that you can convince the hydrogen atoms to not recombine before they leave the nozzle.

NTR, both fission and fusion types, are more energy efficient than nuclear electric propulsion systems, because their energy transfer efficiency is much better. Long story short, NTR converts heat->velocity, while NEP converts heat->electricity->velocity; that extra conversion step brings with it a huge energy-efficiency hit. The only thing holding NTR mass-efficiency (ISP) low is that their cores often can't get too hot, putting a limit on chamber temperature, so to speak. This is why NTR is often envisioned using LH2 propellant, because their low atomic mass means for the same input thermal energy, they carry more momentum, thus higher ISP. Of course, if mass efficiency (ISP) is all that matters, and energy conversion loss doesn't get into the equation, then by all means, go with NEP systems. The acceleration afforded would be pathetic, sure, and all those radiators would eat into the payload mass, but if propellant is scarce, it's the best. Also, the ship now has a power reactor by default, which can be used to power a laser or do other things; a NTR-propelled ship would need a separate reactor, if their NTR isn't a bimodal-type. TL;DR - In NTR, nearly all of the energy the reactor outputs goes into accelerating propellant, while NEP throws away much of the reactor power through the radiators. On the other hand, NEP often gets better ISP than NTR because electrostatic/electromagnetic does a better job of chucking propellant as fast as possible than a de Laval nozzle, so that's that.

I'm guessing this might be a terrestrial nuclear power plant. That long start-up/shutdown time might be to thermal inertia to the system as a whole - reactor, coolant, the works. AFAIK if enough neutrons are absorbed to stop further chain reaction, the remaining fuel goes to natural decay not too long afterwards.

Was about to point this out. Yes, gas-core rockets can reach 4 digit ISP, and they also eject used nuclear fuel along with propellant. Of course, that was spent nuclear fuel that could have been reprocessed to recover the unfissioned U235/freshly-made Pu239. It's possible to use a closed-cycle gas-core rocket, but that means taking a hit on ISP, and needing to build a more complicated rocket.

~80-100 km detection range, let's say. Not exactly backstab-range, but definitely a rude surprise.

The ion propulsion system on the diagram was there to provide ullage, trajectory corrections maneuver, and other low-load situations. NTR is used to cross the Van Allen belt quickly from LEO, and for Mars capture burn. This is done to reduce mass needing to be launched to LEO. More on the technical paper here.

IDK, 800-900 seconds of ISP on LH2 typically claimed on NTR performance is pretty good for me. Especially since chemical systems struggle to get over 400 seconds without using 'exotic' propellants (i.e. ones that do horrendous things to humans - fluorine and their ilk). Even when using water as propellant, 400 seconds is still possible.

This is a fission-based system. I can't find a detailed fusion power reactor design anywhere - the ones I do find are propulsion systems, not power generation systems. To be fair, it's not that difficult. Get the radiators hot enough, and they'll spew the heat away. The problem is engineering the radiator.