71–80 of 187 posts
The reason communications with the Voyager 1/2 twin probes have lasted for like 45 years: Voyager 2 is equipped with three Multihundred-Watt radioisotope thermoelectric generators (MHW RTG). Each RTG includes 24 pressed plutonium oxide spheres, and provided enough heat to generate approximately 157 W of electrical power at launch.
Note that RTGs don't provide propulsion.
The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space. Can anyone give a eli5 explanation of what makes this better…
It's the high temperature and the lower exhaust molecular weight. Specific impulse (Isp) is how much force you get from each mass unit of propellant you exhaust. So an engine with Isp = 300 gives 300lbs force for every pound of exhaust gas mass. IIRC, Isp scales as the inverse square root of exhaust molecular weight. So a pure (molecular) hydrogen exhaust would have a molecular weight of 2. A pure H2/O2 engine would…
Usually reactors run cooler by several orders of magnitude, as this is referred to as a ‘meltdown’ and people get snippy about the releases of radiation and expensive cleanup crews, etc.
Space is more forgiving and has fewer HOA types, so they can go closer to the limits - but it’s still the same underlying issue.
Earlier quoted context omitted.
Peace time = smart phone incrementalism, social media, ad tech, Bitcoin, NFTs, WeWork, low interest rate malinvestment, Twitter/Tumblr pessimism Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism I know most of this is happenstance and that peace time doesn't itself cause lackluster development, but it is interesting an…
Broad optimism? Science hype? Are we living in the same country?
Finally! This is one of the few fission uses I'm genuinely excited about. Nuclear-thermal just makes so much sense for in-space propulsion of large vessels compared to chemical rockets.
US seems to be in a hardware/science renaissance, now that there is a new Cold War enemy, and manufacturing and research are being brought back to US.
Finally! This is one of the few fission uses I'm genuinely excited about. Nuclear-thermal just makes so much sense for in-space propulsion of large vessels compared to chemical rockets.
I think the obvious concern would be failure scenarios. There will always be failures in space technology. As space expands exponentially (as it is likely to do so in the future), failures will become relatively regular. So the obvious question would be what would happen if a nuclear rocket, with reaction in process, crashed to Earth. Or even if it broke up in the atmosphere.
Or it could go the way of the airline industry where failures become less regular as the industry develops ways of systematically reducing them.
Also, nuclear rockets should probably be launched to earth orbit in pieces, separately, so that if any one launch fails it won’t create a nuclear accident in the biosphere. Then assemble and power up the nuclear rocket as far away from earth as possible.
Earlier quoted context omitted.
From across the pond it feels like American millennials and Gen-Z have already declared their country to be a failed state. How much can a nation progress if its youth is pessimistic about everything? It doesn't matter whether the pessimism is based on actual problems (healthcare, housing prices etc.) or learned through terminally online reddit/twitter consumption.
As the folk-wisdom goes "Twitter isn't real life". The ones who have that opinion are the terminally online.
> Wernher von Braun, the German engineer who defected to the United States after World War II, recognized the potential of nuclear thermal propulsion even before his Saturn V rocket landed humans on the Moon with chemical propulsion > German engineer Why does von Braun get such a pass from his Nazi past? Because he was useful?
Partly it's because his Operation Paperclip dossier was conveniently misplaced before it could be declassified.
Who knows what was in there? Not historians, that's who...
Source: https://www.archives.gov/iwg/declassified-records/rg-330-def...
Earlier quoted context omitted.
Would it be possible to skim the upper atmosphere while in orbit around a planet and top-up the gas reserves?
Almost definitely not. If there's enough material to collect in a realistically short timeframe, there's enough drag to bring the craft back into the atmosphere.
For atmospheric flight the closest thing is a jet engine which is "air breathing", i.e. requires air to run and works in part by sucking air in, using some of the Oxygen in it for combustion, and shoving the extra air out the back. This gives them much higher efficiency than chemical rocket engines, as they don't have to carry their own oxidizer or the mass to eject for propulsion - but only works because the air is dense enough where they operate. Which is not true for space flight. Commercial jets typically fly a bit under the speed of sound in air which is 767mph. Of course supersonic air craft exist - I think mach 5 is achievable by some military jets - but that's still a fraction of orbital velocity. Anyhow running in the atmosphere means jets don't "scoop up air" in any sense but rather just use it immediately - so any lost momentum can be immediately countered by the engine.
Another direction to think about it - a simple model for air resistance says that air resistance increases with the square of your velocity (and with the density of air, which exponentially decays with height though also depends on temperature. I think the height wins out though. Not sure if the equation works at extremely high altitude. But certainly this favors using air at lower altitudes and probably also lower speeds
https://en.wikipedia.org/wiki/Drag_(physics) https://en.wikipedia.org/wiki/Density_of_air
The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space. Can anyone give a eli5 explanation of what makes this better…
The temperature of a gas is proportional to the average kinetic energy of its particles. So if you compare (e.g.) water vapor to hydrogen at the same temperature, the hydrogen molecules are moving faster. A hydrogen+oxygen chemical rocket propels H₂O, while a nuclear thermal rocket can use the lightest propellant (H₂) because the heat comes from elsewhere. Smaller molecule, faster exhaust, better rocket.
Earlier quoted context omitted.
Peace time = smart phone incrementalism, social media, ad tech, Bitcoin, NFTs, WeWork, low interest rate malinvestment, Twitter/Tumblr pessimism Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism I know most of this is happenstance and that peace time doesn't itself cause lackluster development, but it is interesting an…
> Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism Next time the cold war cools down, we need to invent something to replace it.