4 ms·

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 nozz

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 than a conventional rocket? There still needs to b some mass ejected obviously. Is it that nuclear heat is able to make it go faster and provide more reactive force than burning it? What does the analysis look like?

rich_sasha 3y ago
The propellant is important for its bulk (some mass to shoot out the back) and it's energy. In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit. In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides the bulk. This is way more efficient. But such rockets likely cannot achieve orbit on their own because the whole set up is too heavy.
- morkalork 3y ago
  Would it be possible to skim the upper atmosphere while in orbit around a planet and top-up the gas reserves?
  - MPSimmons 3y ago
    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.
    - dgoldstein0 3y ago
      Also you'd be fighting conservation of momentum - if the material you gather isn't going your speed and direction, you lose some of your speed and direction to pick it up. Given how fast anything has to go to reach orbit (about 17000mph for low Earth orbit) and escape velocity is about 41% higher, that speed difference would be a major problem. 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/Drag_(physics) https://en.wikipedia.org/wiki/Density_of_air https://en.wikipedia.org/wiki/Density_of_air
  - adastra22 3y ago
    Yes, there are designs for doing exactly this with outer gas giant atmospheres.
- morcheeba 3y ago
  Good explanation! For completeness: In an Ion jet, the energy comes from (usually) solar power and the gaseous fuel provides the bulk. These are even weaker because of the limited instantaneous power that can be generated to feed it, but they are very efficient in how much thrust they can provide per propellent weight. Some versions (like arcjet) are a combination chemical rocket/Ion jet.
- rgmerk 3y ago
  Are you sure about that? Some of the nuclear thermal rockets tested way back in the 1960s were very, very powerful. That said, it’s hard to imagine that they’ll ever be used for launches from the Earth’s surface. Mars, maybe.
  - rich_sasha 3y ago
    Hmm, maybe. Actually not sure where I got this from, I could very well be wrong. Either way I'm no expert.
hutzlibu 3y ago
"Is it that nuclear heat is able to make it go faster and provide more reactive force than burning it?" Yes, that is mostly it. Just compare the explosion from some hydrogen with a nuclear bomb. It is all about energy for mass in space, because all the fuel you have, you have to bring up into space, which needs more fuel, so need more fuel to bring that fuel up ... nuclear could help with that mass ratio a lot. And I like the concept in theory - as long as none of them explodes halfway up to space.
p1mrx 3y ago
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.
- dgoldstein0 3y ago
  Wouldn't we care more about the momentum of the exhaust rather than the mass? I'm not quite following why lighter particles are better
  - p1mrx 3y ago
    Assume that total propellant mass is a constant (limited by the launch vehicle) and temperature is a constant (because melting the rocket would be bad.) With constant mass, you increase the total momentum by increasing the exhaust velocity. With constant temperature, you increase the exhaust velocity by making the molecules smaller.
ridgeguy 3y ago
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 have an exhaust of H2O (assume it's running at stochiometric H2/O2 ratio to simplify things), or an average molecular weight of 18. For everything else being equal (temperature, pressure, etc.), the H2 Isp would be sqrt(18/2), about 3x the H2/O2 Isp. That's mainly why rocketeers would like nuclear engines, more oomf from a given mass of propellant. It's also (conceptually) simpler because you only need to handle one propellant vs. two or more for conventional liquid fuel combustion. It's more complex, of course. If the reactor is really hot, it can partially dissociate H2 into atomic hydrogen, lowering the exhaust molecular weight still more. But dissociation is an energetically expensive process, so there's bound to be a tradeoff between energy consumed by dissociation vs. increased Isp. I've no clue how that would work out.
- lazide 3y ago
  The real limiting issue tends to be materials in the reactor - at some point, every known material is a liquid or a gas and the reactor stops being an ongoing concern. 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.
  - ridgeguy 3y ago
    Yeah, the materials issues dominate design basics. They've looked at HfC and HfCN as nuclear engine materials (m.p. ~3950°C), pretty exotic. I feel like if designers would drop their effort to wring out the last increment of Isp from nuke engines and reduce temperatures to ~2000°C, it would give faster development. There are many materials that can live in 2000° hydrogen for >100K hours. There's an old Rand study that showed H2 would give an Isp >1100 at 1 atm chamber pressure (!), temp ~2000°C, exhaust pressure 10e-4 atm. Seems worth exploring.
physicles 3y ago
A rocket functions by ejecting mass out the back at high speed. By conservation of momentum, the rocket then accelerates. You want to optimize for velocity gain (also known as delta-v) per unit mass of propellant. So the question is, should you eject something heavier at lower speed, or something lighter at high speed? Which is more efficient? If you write out the equation for a rocket ejecting some propellant at some velocity, apply conservation of momentum, and solve for the ratio of delta-v to propellant mass (just a bit of high school physics), it turns out that a higher exhaust velocity makes a more efficient rocket. As a sibling post points out, if you compare water vapor with hydrogen at the same temperature, hydrogen will have the higher velocity. Chemical rockets usually produce water and/or carbon dioxide, which are bigger molecules and therefore less efficient propellants than hydrogen. So if you have a way to heat hydrogen to the same kinds of temperatures you get in rocket exhaust — say, in a nuclear reactor — that’ll make a more efficient rocket. There are of course considerations other than pure efficiency. Our most efficient rockets, ion engines, are indeed crazy efficient, but their thrust is measured in milli-newtons or newtons. They can’t be used to lift things off earth. They also use propellants like krypton and xenon, presumably because they’re easier to store than hydrogen.

rich_sasha 3y ago

The propellant is important for its bulk (some mass to shoot out the back) and it's energy. In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit. In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides the bulk. This is way more efficient. But such rockets likely cannot achieve orbit on their own because the whole set up is too heavy.

morkalork 3y ago
Would it be possible to skim the upper atmosphere while in orbit around a planet and top-up the gas reserves?
- MPSimmons 3y ago
  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.
  - dgoldstein0 3y ago
    Also you'd be fighting conservation of momentum - if the material you gather isn't going your speed and direction, you lose some of your speed and direction to pick it up. Given how fast anything has to go to reach orbit (about 17000mph for low Earth orbit) and escape velocity is about 41% higher, that speed difference would be a major problem. 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/Drag_(physics) https://en.wikipedia.org/wiki/Density_of_air https://en.wikipedia.org/wiki/Density_of_air
- adastra22 3y ago
  Yes, there are designs for doing exactly this with outer gas giant atmospheres.
morcheeba 3y ago
Good explanation! For completeness: In an Ion jet, the energy comes from (usually) solar power and the gaseous fuel provides the bulk. These are even weaker because of the limited instantaneous power that can be generated to feed it, but they are very efficient in how much thrust they can provide per propellent weight. Some versions (like arcjet) are a combination chemical rocket/Ion jet.
rgmerk 3y ago
Are you sure about that? Some of the nuclear thermal rockets tested way back in the 1960s were very, very powerful. That said, it’s hard to imagine that they’ll ever be used for launches from the Earth’s surface. Mars, maybe.
- rich_sasha 3y ago
  Hmm, maybe. Actually not sure where I got this from, I could very well be wrong. Either way I'm no expert.

hutzlibu 3y ago

"Is it that nuclear heat is able to make it go faster and provide more reactive force than burning it?" Yes, that is mostly it. Just compare the explosion from some hydrogen with a nuclear bomb. It is all about energy for mass in space, because all the fuel you have, you have to bring up into space, which needs more fuel, so need more fuel to bring that fuel up ... nuclear could help with that mass ratio a lot. And I like the concept in theory - as long as none of them explodes halfway up to space.

p1mrx 3y ago

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.

dgoldstein0 3y ago
Wouldn't we care more about the momentum of the exhaust rather than the mass? I'm not quite following why lighter particles are better
- p1mrx 3y ago
  Assume that total propellant mass is a constant (limited by the launch vehicle) and temperature is a constant (because melting the rocket would be bad.) With constant mass, you increase the total momentum by increasing the exhaust velocity. With constant temperature, you increase the exhaust velocity by making the molecules smaller.

ridgeguy 3y ago

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 have an exhaust of H2O (assume it's running at stochiometric H2/O2 ratio to simplify things), or an average molecular weight of 18. For everything else being equal (temperature, pressure, etc.), the H2 Isp would be sqrt(18/2), about 3x the H2/O2 Isp. That's mainly why rocketeers would like nuclear engines, more oomf from a given mass of propellant. It's also (conceptually) simpler because you only need to handle one propellant vs. two or more for conventional liquid fuel combustion. It's more complex, of course. If the reactor is really hot, it can partially dissociate H2 into atomic hydrogen, lowering the exhaust molecular weight still more. But dissociation is an energetically expensive process, so there's bound to be a tradeoff between energy consumed by dissociation vs. increased Isp. I've no clue how that would work out.

lazide 3y ago
The real limiting issue tends to be materials in the reactor - at some point, every known material is a liquid or a gas and the reactor stops being an ongoing concern. 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.
- ridgeguy 3y ago
  Yeah, the materials issues dominate design basics. They've looked at HfC and HfCN as nuclear engine materials (m.p. ~3950°C), pretty exotic. I feel like if designers would drop their effort to wring out the last increment of Isp from nuke engines and reduce temperatures to ~2000°C, it would give faster development. There are many materials that can live in 2000° hydrogen for >100K hours. There's an old Rand study that showed H2 would give an Isp >1100 at 1 atm chamber pressure (!), temp ~2000°C, exhaust pressure 10e-4 atm. Seems worth exploring.

physicles 3y ago

A rocket functions by ejecting mass out the back at high speed. By conservation of momentum, the rocket then accelerates. You want to optimize for velocity gain (also known as delta-v) per unit mass of propellant. So the question is, should you eject something heavier at lower speed, or something lighter at high speed? Which is more efficient? If you write out the equation for a rocket ejecting some propellant at some velocity, apply conservation of momentum, and solve for the ratio of delta-v to propellant mass (just a bit of high school physics), it turns out that a higher exhaust velocity makes a more efficient rocket. As a sibling post points out, if you compare water vapor with hydrogen at the same temperature, hydrogen will have the higher velocity. Chemical rockets usually produce water and/or carbon dioxide, which are bigger molecules and therefore less efficient propellants than hydrogen. So if you have a way to heat hydrogen to the same kinds of temperatures you get in rocket exhaust — say, in a nuclear reactor — that’ll make a more efficient rocket. There are of course considerations other than pure efficiency. Our most efficient rockets, ion engines, are indeed crazy efficient, but their thrust is measured in milli-newtons or newtons. They can’t be used to lift things off earth. They also use propellants like krypton and xenon, presumably because they’re easier to store than hydrogen.