shynung

Amateur solid rockets use potassium nitrate (KNO3) as the oxidizer in their rocket candy mix, but the specific impulse is poor. Going up, there are ammonium nitrate and ammonium perchlorate used in larger solid rockets. Should you decide to use ammonium nitrate, take great care; it is a component in an ANFO explosive. For fuel, I recommend to use gaseous propane (or other light hydrocarbons) for its ubiquity and specific impulse. If a liquid fuel is required, in theory, flipping the propane tank upside down should yield liquid propane; the gaseous propane would float to the bottom (top?) part of the tank and pressurize it. I'd be really careful with that setup, though.

I've read from another thread that FTL travel enables time travel as well. Suppose one tried to use an FTL-capable spacecraft to travel back to the past; what kind of trajectory would he use? How much would the craft's maximum FTL speed affect the trajectory for the same destination?

How many mods do you have? It might be a memory problem.

Well, I think SpaceX may have been responsible for the beginning of cheap space launchers era. True, the prices they offered wasn't very dramatic, but it's enough to turn heads. Though, at the moment, I'm presuming that any complicated space operations (say, asteroid mining) will require a lot of mass in orbit. Whether upcoming affordable heavy launchers (like Falcon Heavy) will make such things profitable, I haven't enough data to have neither an idea nor opinion. Regarding cubesats, I think it has more to do with what a cubesat can do compared to a more conventional satellite. If a single cubesat can be used as a GEO comsat, then a reduction in launch prices, in the form of smaller rockets, would have a significant effect. But alas, they are only little boxes. As for now, there are only so much equipment that we can stuff into them.

Not necessarily. The demand for space access is inelastic. Those who need it really need it, those who don't rarely cares. Before continuing this line of discussion, I'd advise reading this old thread first. Otherwise, feel free to add anything else not covered there.

Nitroglycerine!? That's a highly-explosive material used in early dynamite sticks. I wonder if the engineers who built this thing has lost a few fingers during his careers.

The idea itself is pretty solid on paper. Technological stuff can be researched with enough funding. After all, the Space Race back then 'til the 70s was one heck of a technological advance achieved in a rather short period. Problem is, REL right now has very little in the way of funding, not to mention infrastructure and expertise. If it was Airbus or Boeing that made the offer, the situation might have been different.

It's not simply because it went to Mars, is it? What really happened?

I'm willing to bet the designer of this one has taken a long look at Whackjob's stuff and decided that he liked it.

If you haven't noticed, near that line is: They are actively trying to conserve the fuel elements here. The reason there was still some that's bled off by the reactor is from improper design. From the NASA archived document you linked: Their goal is maintaining integrity of the fuel elements. That means no leaks, no fuel element loss. The NERVA project didn't completely achieve this one bit. If that still worries you, the same engine that leaks almost 7 kg of fuel element and fission products in an hour would have exhausted several tons of propellant in the same length of time. It's not negligible, but not enough to be of significant concern. So what? NERVA project documents already stated that integrity of fuel element is to be maintained. A finished, service-ready engine would have eliminated the fuel loss down to negligible amounts. The NERVA project ended before this was achieved. Also, consider the use-case for these engines. I'm not talking what missions it would serve in, but on where the engine was likely to be used. Would you think an NTR would be fired while close to the ground, enough for the 'radioactive' exhaust to have any visible effects? Most NTR proposal I've seen was for either upper stage work, or for deep-space operations. In the former case, the exhaust would have dispersed wide enough to have little effect on the environment; in the latter, it wouldn't matter where it will end up anyway. (By the same reasons, engines developed for space work in the 60s were occasionally designed to burn chlorine trifluoride; if burned with hydrogen, the exhaust would have been HF and HCl, both being poisonous and corrosive.) Why bother fussing around about how dangerous the exhaust is?

Assuming an intact fuel containment, no, there shouldn't be any fission products in the exhaust.

If you want to name names anyway, at least make sure you get it right: I said it. First, D/T concentrations in a typical NTR nozzle is completely harmless to anyone or anything in the vicinity. By itself, deuterium has no radioactivity. 1 in every approximately 6000 hyrogen atoms has an extra neutron (deuterium) anyway, and has been shown to have little effect to life in general. Tritium is regularly used in self-illuminating equipment, and has been used for years without any major health complications arising from its use. Second, an NTR's moderator is not designed to be bled away in normal operations. Typically, they are coated with a heat-resistant metal to keep it from being carried away by the propellant. Third, unless the propellant mixture has xenon in it, there will be absolutely no Xenon isotopes in the exhaust. Even if xenon is used as a secondary coolant, the passages will be different, and there's no chance that the xenon goes out of the nozzle. If you still do not agree with me, do yourself a favor and link a reliable source that proves me wrong. Otherwise, don't bother continuing this discussion.

Enough to provide basic information about the engines themselves. If nothing goes wrong, NTRs emit only hydrogen (or whatever they used as propellant) out the nozzle. One can add many complexities to the engine itself, and this would still hold. Since when did rocket scientists ever cared much about environmental concerns? Just before the NERVA project, they fired rockets burning anything from nitric acid, diborane, liquid fluorine, chlorine trifluoride, and even liquid mercury in some cases. These beasts are far more unpleasant than a typical NTR in terms of what goes out of the nozzle, not to mention the fuels themselves being horrifically corrosive, explosive, or a combination of both. You can read more about them here. Oh, and the anti-nuclear sentiment is still with us. I'm not making it up; just trying to get building permit for a civilian nuclear power station is exceedingly hard these days. It's just that whenever someone says 'nuclear' stuff, all that came into the commoner's mind would be Hiroshima-Nagasaki bombs, the Chernobyl/Fukushima accidents, and stories of people getting hurt by the radiation. Advantage against what? What are you comparing the NTR here against? If chemicals, the advantage is clear: higher specific impulse, which means much lower propellant requirements, which means lower launch mass, and therefore launch cost. If electric engines, the advantage is travel time. Why would anyone want to take a decade-long trip to Mars when a chemical/NTR equivalent could do it in a year? Life support mass gets heavier the longer you the mission is, so that'll effectively eat into the final scientific payload.

There's a nuclear engine concept called a Bimodal NTR. Instead of letting the reactor cool down completely on engine shutdown, it runs a small heat engine-radiator complex through secondary cooling passages to power the spacecraft. This way, the reactor never completely shuts down, and can be fired up much faster than a conventional NTR when a maneuver node is coming up. Also, electric propulsion system, while have greater specific impulse (more than 4-digit seconds are typical), their thrust is abysmally small (about a few to a few hundred Newtons, compared to tens of Kilonewtons when using nuclear thermal), so travel time will take a significant hit. For a comparison, the SMART-1 probe launched by ESA in 2003, powered by a xenon Hall thruster, took a year just to reach the earth-moon L1 point, while NASA's Lunar Reconnaissance Orbiter, using a common chemical thruster, took about 4 1/2 days. EDIT: You want basic designs? Here they are. You're right, NERVA designs won't play a significant role; they're from the 70s. Whatever future designs that would have flown would have been much safer, easier to manage, and has much better performance, and possibly at a cheaper overall cost. The only significant barrier to nuclear thermal rocket developments are political. Nothing more.

What, deuterium? Tritium? They're pretty harmless in the quantities produced during a NTR firing test. The propellant molecules only spends a few microseconds near the actual nuclear fuel itself, so the chances of any creature in the vicinity ingesting enough of it to visibly harm them is pretty much negligible. The only thing dangerous about it is that it's very hot (some 2000 K) and is travelling at 8 km/sec. That alone would be the only thing harming anything near it.

They won't be able to say that if the rocket exhaust goes into a flame hood of some sort. This was used when fluorine rockets were a thing back in the 70s. NTRs are rather benign compared to these beasts.

Ah, I see. I thought we were talking about technical problems. The secret is to not let the public know either the engine tests or facts about the engine itself, the latter with more priority. How to do that, I'll leave it as an open question.

Extraplanetary Launchpads. The ore is supposed to be processed using a smelter, to be made into 'rocketParts' in flight. Ask in the EPL thread for more details.

Why would it be a problem? Unless the reactor's propellant passages have leaks in it, nothing radioactive should come out of the exhaust; just pure hot hydrogen in gaseous form. There'd be a big flame in atmospheric conditions, but people would expect that in rocket engine tests anyway.

The only viable use-case for NTR in the near future is a manned Mars mission. That isn't currently popular with the public at large, which is why we're not seeing the hardware being developed.

True, but contra-rotating single-shaft helicopter rotors have been around for some time. Theoretically, it should work, but the margin of safety is very thin. What would happen if the blades on adjacent differently-rotating turbines flex and slap against each other?

There's a turbocompressor right behind the precooler. At takeoff, this will generate a low-pressure region between it and the precooler, sucking in the immediate atmosphere as a result. Though, I think we should be concerned more about REL's current condition. They don't have any factories able to mass-produce these kinds of engines, nor have they announced who will make the engines for them.

It has the lowest molecular mass of all the hydrocarbons. For a given chamber temperature and pressure, less molecular mass means more specific impulse. I just wish someone was brave enough to burn it with FLOX 70.

Cost. Long story short, it's much cheaper to make one-use missiles than reusable ones. Consider this analogy. You have a 500kg TNT warhead which you intend to send to a target across the Pacific ocean. You have 2 choices: send the warhead via a one-use cruise missile, or via a reusable UAV. The cruise missile will only need enough fuel to get there, an engine powerful enough to lift them, and an airframe sturdy enough to hold them all in one piece while on the trip. With an UAV, you'd need all of the above, plus fuel to get back, plus fuel to carry that fuel (courtesy of the rocket equation), landing gears and other recovery mechanisms, a more powerful engine to lift all that, and a sturdier airframe to hold all that as well. You can probably tell which option is more expensive. Even shotgun-style shrapnel kinetic warheads can be detected. What only mattered about the detection is how early. A typical tank-fired HESH round (the squished plastic explosive) is quite effective against solid armors, especially concrete. Tomorrow's combat spacecraft may not be made out of concrete, but they'd have a very bad day if hit by one of these.

Well, it's a 'science report' for the uninformed public. Surely they'd have to replace some technical jargon with something easier to understand? That said, I'd wait for a more comprehensive follow-up study before closing the case.