shynung

Converting heat to energy requires a cold spot to dump the heat later (i.e. radiators). Efficiency depends on temperature difference between the hot and cold spots. In case of solar thermal absorption from a black panel, I think it wouldn't have a very high efficiency, but I need more data to be sure. Best design for a solar thermal collector would be a parabolic mercury boiler, rather than a black panel. As for solar sails, it generates very little thrust. Ion engines are more powerful in most cases. On the other hand, it's free as long as the sails stay intact, so that's that.

@Nibb31 I like to think that operating an Orion-drive spacecraft as opposed to a chemical-powered spacecraft is like operating a freight train as opposed to a truck. Yes, it's theoretically more efficient and powerful, but aside from the enormous capital needed, things like radiation and fallout, Orion-proof infrastructure, among other problems, are something that has to be taken care of first. And I understand that it wouldn't be cheap; anything nuclear-related never was. In the end, you have a point. Not exactly a good idea to potentially harm the biosphere by irradiating them every time we send a bird up there. Especially when the ones we sent up are only GEO satellites and space probes. Maybe we'll find a use for these technologies once we found ourselves in need of transporting massive freight across the planets. Even then, I wouldn't be surprised if some other new propulsion tech got to it first.

Actually, you've got it the other way around. I think we have the technology. NERVA, Orion, what have you. Research on them has gone pretty far, and the blueprints exists. A few decades of R&D, and we could have got the first prototypes flying. What I think the problem is that nuclear-something is politically unacceptable for some reason. And I think it's easier to convince the public to accept these propulsion technologies, rather than inventing some other new technology to suit their whims.

It may seem like that at first, but a controllable nuclear pulse propulsion is possible. Choosing rate of pulse unit deployment, pulse unit energy yield, pusher plate angle, and more. In short, there are lots of things we can do with it other than turning it on or off. That's the only known way of getting there. One that actually works, might I add. Well, as Nibb31 explains, the mothership idea has been pretty much mainstream these days. Solar panels doesn't work on all EM spectrums, as far as I knew, so painting them black wouldn't help. It'd just make the panels overheat faster. Also, solar panels get less effective the farther the ship is from the sun. One would need a generator of some sort. We have the solution that solves all the requirements above, except the last one. I'm tempted to conclude that convincing the powers that be to accept the solution would be easier than developing the propulsion tech that covers everything.

@RenegadeRad <- This is how you call the attention of a certain person. Yes, the Orion design would experience thousands of Gs every detonation. That's why the design has a 'pusher plate' supporting the spaceship frame with several shock absorbers. Direction of flight is to the right. Nuclear pulse units (bombs) are stored in the magazines, fired through the hole in the pusher plate, and detonated a certain distance away. Nuclear detonation shockwave hits pusher plate, propels spaceship. As for 'braking', it's done like every other realistic spaceship: turn it around and thrust. You are likely correct that most future spaceships would probably be in a 'mothership-lander' configuration, rather than a single one-ship-does-all vehicle. Part of this is because rockets designed only for orbit-to-orbit voyages never need to worry about thrust/weight ratio, so they can be designed to have lower mass ratios (less propellant, more payload) by using a low-thrust, high-specific-impulse propulsion system. Landers, on the other hand, have to land and take off from celestial objects, so would need to carry higher-thrust propulsion systems, sacrificing specific impulse for thrust/weight ratio. This is acceptable, though, since they will spend most of their time being carried around. The other part is that the rocket equation is unforgiving. In space, every gram counts. This gets into play in the Apollo moon landing missions. Someone at NASA figured that separating the lander from the main spaceship would save propellant mass, because the lander only needs to carry enough propellant to get only itself on the surface, and back up to orbit. Landing the entire Apollo spaceship on the moon would take prohibitive amounts of propellant, enough to design a larger, entirely new booster rocket. As a result, I would argue that the Apollo CSM-LM spaceship stack would constitute a mothership-lander combination.

There was a reason the Orion didn't make it to full scale production: Nuclear Test Ban Treaty. Put short, no nuclear explosions allowed in space. That pretty much closed the curtain on the endeavor. And unfortunately, NSWR is still on paper. Orion used to have small-scale prototypes using chemical explosives, but that's it.

I'll go right over your head, and bring you the Nuclear Salt Water Rocket 12.900 kN at 6.728 seconds, provides continuous thrust rather than Orion's pulses, and it works much like a hypergolic rocket. Sure, the radiation's still a problem, but if designed correctly, this will get you anywhere fast.

Do you play KSP often? If so, try this mod. It's pretty far-fetched, but mostly-realistic implementation of engine designs that could serve your purposes. If you prefer something closer to today's technology, this pack of mods could suit your tastes. Within a few hours of KSP with either of those mods, you should get an idea of what's possible in spacecraft technology.

Momentum is mass times velocity. If momentum is increased, either by increasing the projectile velocity or mass, recoil will be increased as well. This applies whenever the projectile isn't self-propelling (i.e. rockets), including electrically-propelled projectile. That means, for the same projectile kinetic energy, recoil forces will be identical, no matter if it was a high-velocity low-mass projectile or a low-velocity high-mass projectile, whether the weapon is chemically (gunpowder) or electrically (rail/coilgun) powered. Unless we start using rocket guns, those recoil would still be there. RT-20 gets 47 kilojoules. I cannot find any data on the RT-20's penetration performance, but the Wiki page claims it can go through any modern APC's armor at up to 800 meters. True. I couldn't find data on maneuverability capabilities of current anti-tank missiles, so I have no clue on its effectiveness as a CIWS at the moment. Though, putting a sensitive warhead (i.e. explosive) on a high-velocity projectile fired from a short barrel (compared to, say, a 155mm howitzer) would subject the warhead to tremendous G-forces. Keeping the warhead stable enough to not be set off while still in the barrel/on the rails, while still sensitive enough to be set off on impact, would be tricky, to say the least.

The IWS 2000's tungsten dart massed 20 grams, according to this. That works out to 21 kilojoules. Far outclassed by the RT-20, but still serviceable for an anti-material rifle. It's comparable to the M82 Barrett rifle. That would be a CIWS. Single-projectile weapons aren't very good at targeting things that can change direction very quickly, like incoming missiles. The railgun would need to fire multiple projectiles, shotgun-style.

Why the infantry, though? Why not bolt it onto a pickup truck with an extra alternator and capacitor banks? Railgun technical FTW. Seriously, though, the current infantry stocks are content with their current weapon sizes. Any slugthrower weapon whose projectile's kinetic energy is too high would literally be quite a pain to shoot, due to the enormous recoil forces. This is the Croatian RT-20 anti-material rifle. Firing 20mm Hispano rounds, it launches a 130 gram projectile at 850 m/s, carrying almost 47 kilojoules of kinetic energy, or the equivalent kinetic energy of a 1 ton car moving at 34 km/h. Due to this, it has an oversized muzzle brake and a counterpressure tube (exhaust just over the bolt), which take some gases from the combusting propellant and releases it above the shooter's shoulder. Despite all this, and the weapon itself massing about 20 kg (!), it still generates much more recoil than a .50 caliber rifle of a simlar role. If a railgun could launch a projectile with a similar kinetic energy, etiher through mass or velocity of the projectile, the weapon or shooter must have some way to hold themselves against the recoil generated by the weapon. In my opinion, this would mean either a static tripod weapon, or a soldier wearing a powered exoskeleton.

Well, it does have the main guns of a battleship bolted on to it.

What you meant probably is the Active Denial System.

Inflatables would be much more valuable if we have a good ISRU capability. An inflatable water tank can carry more water than a fixed tank of the same weight, so if there's a source of water up there, launching an ISRU+inflatable tank combo would be a smarter move than an ISRU+fixed tank combo.

An inactive volcano at that.

I must mention that rail vehicles have limits to the physical size of the things they can transport. Most Russian rockets, including Proton and Soyuz, are generally designed around the biggest propellant tanks that the trains transporting them can carry, among other things, because their launch pad was in the middle of a continent. US and ESA rockets don't have that requirement because their launch pads are close to large bodies of water, so they can transport rocket parts by barge or ship. Also, trains are very bad at climbing hills. Building a launch pad at the top of a mountain would necessitate the construction of a very powerful transport system in an entirely new class of its own.

Ah, the cruise missiles. I was thinking AIM-120 air-to-air missiles. Those still use solids.

The missile would need a good battery, no? If the power's cut, it stops burning.

And that extra Isp demands extra mass for plumbing, power generation, and other subsystems. It's not without its drawbacks. Besides, kicker stages for satellites don't have to expend that much deltaV anyway. There, that's one niche this propellant can serve in. Granted, not the only one serving it (there are competing propellants), but good enough for this task.

@sgt_flyer Wait, the AP/HTPB is single-burn? No restarts? Dang.

For a cubesat thruster, that's actually a respectable performance. Also, that engine lists AP/HTPB as propellant, so presumably what DSSP is developing is some sort of catalyst that works only when zapped. If this is achievable with other AP/HTPB solid rockets, then we can use it for almost anything.

This is also useful for upper stage kick motors, like Payload Assist Modules, since their throttling capabilities means being able to put the payload satellite into a more precise target orbit than a traditional solid PAM. Also, it's denser than a monopropellant, and doesn't have to carry stuff like plumbing or heaters, so more mass can be dedicated to propellant to offset the specific impulse, which can potentially impart more impulse than a comparable liquid upper stage.

sgt_flyer explained it better than me in the post above.

@YNM What you're looking for, if you want to gimbal it, would be a set of exhaust vanes, like this old V2. They're less effective than a gimballed nozzle assembly, but much simpler.

@fredinno That, I don't know much, unfortunately. I do think using it as SRB propellant would be great, though. Especially the upper stages.