shynung

For a start, there are things such as bimodal nuclear reactors. This is the type of reactor that both propels a ship by being a NTR, while also providing power through a secondary coolant loop. The reactor isn't completely turned off, merely kept warm, to provide power to the ship.

If the reactor is powered down, it doesn't emit as much deadly radiation as it does under full power. If a NTR is powered down post-burn, it can be left still hot - the heat will radiate on its own, given enough time - without the crew needing to worry about deadly radiation, as long as the reactor components can withstand the temperature. Should a heat engine (thermocouple) be plugged in at this state, the reactor would get cold fast, because while heat flows through the heat engine, the reactor isn't producing additional heat to compensate. Then, once the reactor temperature is the same as the radiator's, the heat engine stops generating power, and the reactor stops cooling. Also, when you wrote 'fuel', you might meant 'propellant' - that's the stuff spewing out of the rocket nozzle. 'Fuel' in the context of nuclear thermal rocket would be the slug of U235 in the reactor itself.

Sure, but in space, there are multiple kinds of efficiencies, and their requirements often clash. Power plant efficiency (in terms of how many electric W comes out from a given thermal W) relies on high heat flux. That means a low radiator temperature. Cool-running radiators radiate heat less efficiently than hotter radiators, which means a lower mass efficiency (more radiators for a given thermal W input on the generator). In space, every gram counts (thank Tsiolkovsky for that), so for a given reactor temperature, allowing less efficiency in power generation by increasing radiator temperature would enable the power generation system to get away with less radiator mass.

The simple solution to this is to keep radiator temperature lower than reactor temperature. This way, heat gradient will naturally move heat to the radiators. Sticking a heat engine (closed-cycle gas turbine, thermoelectric, or anything) in between the reactor and radiator heat loop will make it generate energy. Here's an example design of a fission thermoelectric reactor, from Children of a Dead Earth: On this design, reactor temperature is kept no higher than 1688 K, radiator is kept at 1200 K. Heat gradient flows to the radiator, not the other way around. Two coolant loops connect the heat engine to the reactor (left loop in diagram) and the radiators (right loop). The heat engine produces 14.6 MW from the heat flux, reactor coolant pump eats ~500 kW, radiator coolant eats another ~600 kW, net power production is 13.5 MW. Coolant loop does not involve phase change (not boiling/condensing water/steam), to keep design simple and reliable.

In space, conduction and evaporation are infeasible - there are no outside substances (like ocean or atmosphere) to conduct or convect heat to. Radiation is the only way. The classic way to increase radiation efficiency is to heat up the radiating component, hence the glowing radiator. And, should this be not enough, additional radiating surfaces can be used, which means bigger radiators or more radiators, often both.

Not on this particular example. The radiators glow because they are hot - this is to increase heat emission efficiency, reducing needed radiator mass. The actual reactor is buried deep inside the ship.

Even in space, you'll need the equivalents of cooling towers, since almost all of the power extraction methods described in this thread works on heat flux (temperature difference) - they need a 'hot end' and a 'cool end'. On earth, the cooling towers provide the 'cool end', but in space, radiators are used instead.

When commercial tokamak fusion reactors are ready, I think it's likely to use MHD generators for power extraction.

As MatterBeam said, it uses evaporative/phase-change cooling with liquid hydrogen to minimize IR signature. The absorbent coating is there to minimize visibility from active Radar/Lidar sensors. Also, perhaps 'stealth' is not a completely appropriate word for the hydrogen steamer design. 'Low observable' would fit much better.

It's possible to compensate the lower TWR by increasing coilgun muzzle velocity. It'd be the ion-drive equivalent for stealth ships. Can also double as defensive weapon/CIWS, so I'd say it's useful to have.

Current-tech railguns being researched by US Navy can achieve muzzle velocities in the range of 2-3 km/s. That's close to storable liquid rockets and solid rockets.

This was kind of an interesting take on hard scifi take on the stealth ship concept, and I'd like to continue the topic a little further. Assuming it is actually possible, what would be an ideal expansion ratio for a 3000K thermal rocket to expel the exhaust at 22K? Also, is there any other way to make a high-performance propulsion system that does not expel hot exhaust? First thing that came to my mind was a coilgun/magnetic accelerator using 22K iron pellets as propellant, but would that be enough?

Yes, we can. This is the principle behind mass drivers - throw a bucket of dirt really fast with a railgun/coilgun setup, and the setup would go in the opposite direction of where the barrel was pointing.

Going all the way on that idea: just dump some chlorine trifluoride in their general direction. It'll eat through most materials, and thoroughly corrode what is left behind.

I suggest giving chargeable engines a short shutdown delay, so that whenever the engine is at 0% throttle, it waits for a short time before actually shutting down. 'Residual charge', if you will.

It's in the Issues section.

For some context, the small fission reactor (or oversized RTG) is meant to power a manned rover, something like NASA Space Exploration Vehicle.

How small can a functional nuclear power generator be built, in order to use them as a vehicle's power source? We know that nuclear-powered ships and submarines exist, but can we scale the reactors down even more, while keeping a usable power-to-weight ratio?

As much as I wanted that to happen, I don't mean to ask a lot from you. Just gauging how far are you willing to go into the rabbit hole, so to speak.

@Nertea Do you have any plans to revive Project Orion? You know, ol' Boom-Boom?

Project Orion. The one and only torch drive we have ever designed so far.

Adding to this, missiles are much smaller than orbital rockets (and has to be, to fit into existing launchers or missile mounts, and these have maximum mass and dimensions limits). Squeezing ever more performance out of the same vehicle dimension means a high-efficiency engine. In the past, this is achieved by high-performance chemical propellants (which sometimes do horrible things to the human anatomy), but now, with advancements in materials and CAD technology, airbreathing turbojets or air-augmented solid rockets are the order of the day. Though, those kinds of motors are typically reserved for anti-ship or cruise missiles, which flies for long distances at relatively low altitudes. Common air-to-air missiles such as the AIM-9 and AIM-120 still use solid motors.

Hi again Nert. So much questions this time, I hope you don't mind too much. - Thanks to CRP, orbital scans using the stock scanner antenna reveals surface availability of rare resources such as Helium-3 on planets and moons, such as the Mun. Do you plan to make these surface resources available through the mod? If so, would you rather make a specialized 'mining drill' part, or stick a MM config on the stock drill? (if the latter, I may be able to kludge together said config, should the need arises.) - Current fusion reactor implementation renders them capable of using either D-D or D-He3 reactions, while fusion engines only using D-He3. Do you plan to keep it that way, or are there plans to enable fusion engines to use D-D reactions at some other cost (say, specific impulse drop)? - Are there plans for: - antimatter power reactors? - antimatter factory part? - ship-to-ship energy transfer system? - Last, but not least, what's the current progress on the remaining FFT parts and elements yet to be released?

Was about to post a picture of the Tu-95, but @mrfox ninja'd me. Oh well, here it is. @Northstar1989 The basic idea is sound, but the biggest hurdle is battery capacity - current state-of-the-art battery tech can carry nowhere as much energy to a fuel tank of comparable size and mass. It's possible to sidestep the problem by using an onboard fuel cell or generator, but then someone would point out that it's much more straightforward to use the fuel these devices use to power a conventional turboprop/turbofan engine.

Did you mean 'no, never', or 'no, not right now'? Also, regarding that: are you planning to make some resources (e.g. metallic hydrogen) available only from Kerbin, or to make all resources used in this mod able to be procured elsewhere (ISRU)?