6 ms·

Maybe not but it's pretty significant. From 350 to 900 takes the payload mass fraction to LEO from 6% to 34%, so SSTO would be practical (though I wouldn't expe

Maybe not but it's pretty significant. From 350 to 900 takes the payload mass fraction to LEO from 6% to 34%, so SSTO would be practical (though I wouldn't expect to see this for launch). Or, with a 10% mass fraction, the delta-v goes from 7.9 km/s to 20 km/s. Convenient calculator: http://www.quantumg.net/rocketeq.html http://www.quantumg.net/rocketeq.html

greesil 6y ago
Or just use a bunch of Hall thrusters, and not lob fissile material into Earth's atmosphere.
- ashtonkem 6y ago
  The problem is that Hall-effect thrusters provide basically no thrust. Most Hall-effect thrusters provide millinewtons of thrust, with the best lab model producing 5.4N and the planned AEPS motor producing a theoretical max of 2.356N. A single Merlin 1D motor produces 690kN of thrust at sea level. You would need over 290,000 AEPS engines to match one Merlin 1D engine. At 40kW per engine, this would use 11.7 gigawatts to run, so you’d also have to add in the weight for a dozen or so nuclear power plants. Keep in mind that the falcon 9 uses nine Merlin 1D motors for the first stage.
  - CydeWeys 6y ago
    > You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.
    - ashtonkem 6y ago
      If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
      - CydeWeys 6y ago
        And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array. For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.
    - rbanffy 6y ago
      > So it can't even lift itself off the pad, let alone anything else. This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point. Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.
      - CydeWeys 6y ago
        > This is only a problem for take off from large bodies. Yes, that's what we're talking about here. > Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. Yes, that's what specific impulse measures (which is what most of the larger conversation has been about). As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
        rbanffy 6y ago
        Separate fuel tanks would demand the design to be more mission-specific. Ideally we want reusable spacecraft that can do multiple roles.
  - greesil 6y ago
    We're talking about orbit raising, not launch, which the nuclear thermal can't do. I can't believe I've been downvoted this much when I probably know more about electric propulsion and orbital dynamics than anyone in this thread.
- greesil 6y ago
  Also, downvoted for not wanting to accidentally create radioactive fallout? Do the cost benefit analysis when there's plenty of other technologies for moving mass around off planet and to other planets.
CydeWeys 6y ago
This isn't taking into effect the added mass of the engine itself, though. You'd need heavy radiation shielding, heavy shielding around the reactor itself so that it wouldn't be blown to bits in the event of a launch explosion, etc. Plus, hydrogen is the least dense propellant so your fuel tank needs to be much bigger than e.g. the RP-1/LOX fuel tanks used by the Falcon 9, Saturn V first stage, etc. You end up in a much worst place than even hydrolox, because at least in hydrolox the liquid oxygen fuel tank at least can be relatively small. With NERVA it's only hydrogen. So, once you take into account these factors (rather than looking at the raw efficiency of just the engine alone), it ends up not being as much of an obvious win.
- arethuza 6y ago
  The reactor isn't going to be running during launch - so it's not going to be highly radioactive. Also I'd expect the reactor to be fairly small, dense and robust - I doubt if it would be "blown to bit" in the event of a problem with the rest of the launcher. e.g. During the 1980 Damascus Titan missile accident the missile exploded underground in a bunker and the warhead was thrown a fair distance but was recovered relatively intact: https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_explosion https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_ex... And that was something that was intended to explode!
  - CydeWeys 6y ago
    It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. It needs to be able to survive any kind of catastrophic detonation/break-up/crash landing you can think of, because the alternative is spreading a large amount of radioactive material directly into the atmosphere/onto the Earth's surface. For example, the Challenger orbiter itself was fine, but when the SRB blew up it took up the orbiter with it. So if there'd been any nuclear materials in the orbiter, even if they weren't used at launch, they still would've needed serious shielding. And it doesn't matter if the consequences of said radioactive material release aren't actually that bad in the grand scheme of things (like Fukushima looks not to have been) -- what matters is that the public reaction to such an event would preclude the possibility of ever launching it again.
jaggederest 6y ago
Yes. The difference is definitely a sea change. To put it in concrete terms, a nuclear-powered rocket with sufficient thrust could be single-stage to mars and back for the same propellant budget and mass fraction of a first stage chemical engine which doesn't even get to orbit.
- CydeWeys 6y ago
  A nuclear thermal engine is not remotely a drop-in replacement for a chemical engine. There are substantial other changes required to the rest of the rocket that add lots of mass and expense, that work to substantially negate a lot of the benefits.
  - jaggederest 6y ago
    That's true from a political perspective but it's definitely not true from an engineering perspective. Got enough energy to hoist almost anything you like into orbit if you choose your thrust and scale appropriately. As to what happens to the ground underneath, that's a different problem :)
    - valuearb 6y ago
      No, extra mass is needed to make them operational, not politically acceptable. First is engine mass. (All numbers from memory) A SpaceX Raptor weighs around 4,000 lbs, and generates over 400,000 lbs of thrust (100x). NERVA weighed around 40,000 lbs and only generated 50,000 lbs of thrust (1.2x). A fully fueled orbital Starship in would weigh around 2.5M with about 25,000 lbs of engine, 150,000 lbs of ship and 2.3M lbs of fuel generating 1.2M lbs of thrust out of three vacuum Raptors. For a similar level of thrust from NERVA would add nearly 1M lbs of dry mass. Newer designs can cut that engine mass, but at most in half. And your nuclear ship could get by with less thrust and just burn the engine longer. But that brings two new questions Where does all the heat go? NTR send almost all of the heat out the back, but not all. Running that engine for hours on end in the vacuum of space (Instead of the very conductive earth NERVA was tested on) is going to heat stress your engine and everything it’s connected to. Do you need to add massive radiators and insulation to run the engines longer? How do you take off on Mars? If you have a lot less thrust, you will only be able to lift a lot less fuel and cargo. And you can’t land or take off on Earth. Unlike Starship NTRs may only be usable in between planets, requiring carrying heavy and expensive landers. Then there is propellant. A 900 ISP is only achievable with hydrogen (H2), using Methane or water or any other feasible propellant drops ISP below 600, making overall system performance far worse than burning methane in Starship Raptors. OK, let’s use Hydrogen then. Well unfortunately Hydrogen is a shitty propellent in every other measure. It doesn’t compress well so yet requires larger, heavier tanks. Also H2 is a tiny slippery molecule, forcing you to freeze it near absolute zero in order to keep it from leaking, which makes those tanks even heavier. And guess what? It still leaks. Will you have enough fuel left to fire a retro burn to match orbits with Mars? Better leave with at least 30% more than you need. Now, you probably don’t want to be killed by your engine on the way to Mars, so let’s add some radiation shielding between the crew and the NTR engines, or put the on a long spar sticking out behind the ship. Both add even more mass. A NTR ship will likely require at least 2-3x more dead mass than a chemical rocket ship, so the question becomes, what’s the point? Why jump through political hoops when Starship VP can already fly crew to Mars in only 90 days and deposit over hundred tons of cargo to the surface of Mars, and cheaply?

greesil 6y ago

Or just use a bunch of Hall thrusters, and not lob fissile material into Earth's atmosphere.

ashtonkem 6y ago
The problem is that Hall-effect thrusters provide basically no thrust. Most Hall-effect thrusters provide millinewtons of thrust, with the best lab model producing 5.4N and the planned AEPS motor producing a theoretical max of 2.356N. A single Merlin 1D motor produces 690kN of thrust at sea level. You would need over 290,000 AEPS engines to match one Merlin 1D engine. At 40kW per engine, this would use 11.7 gigawatts to run, so you’d also have to add in the weight for a dozen or so nuclear power plants. Keep in mind that the falcon 9 uses nine Merlin 1D motors for the first stage.
- CydeWeys 6y ago
  > You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.
  - ashtonkem 6y ago
    If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
    - CydeWeys 6y ago
      And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array. For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.
  - rbanffy 6y ago
    > So it can't even lift itself off the pad, let alone anything else. This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point. Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.
    - CydeWeys 6y ago
      > This is only a problem for take off from large bodies. Yes, that's what we're talking about here. > Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. Yes, that's what specific impulse measures (which is what most of the larger conversation has been about). As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
      - rbanffy 6y ago
        Separate fuel tanks would demand the design to be more mission-specific. Ideally we want reusable spacecraft that can do multiple roles.
- greesil 6y ago
  We're talking about orbit raising, not launch, which the nuclear thermal can't do. I can't believe I've been downvoted this much when I probably know more about electric propulsion and orbital dynamics than anyone in this thread.
greesil 6y ago
Also, downvoted for not wanting to accidentally create radioactive fallout? Do the cost benefit analysis when there's plenty of other technologies for moving mass around off planet and to other planets.

CydeWeys 6y ago

This isn't taking into effect the added mass of the engine itself, though. You'd need heavy radiation shielding, heavy shielding around the reactor itself so that it wouldn't be blown to bits in the event of a launch explosion, etc. Plus, hydrogen is the least dense propellant so your fuel tank needs to be much bigger than e.g. the RP-1/LOX fuel tanks used by the Falcon 9, Saturn V first stage, etc. You end up in a much worst place than even hydrolox, because at least in hydrolox the liquid oxygen fuel tank at least can be relatively small. With NERVA it's only hydrogen. So, once you take into account these factors (rather than looking at the raw efficiency of just the engine alone), it ends up not being as much of an obvious win.

arethuza 6y ago
The reactor isn't going to be running during launch - so it's not going to be highly radioactive. Also I'd expect the reactor to be fairly small, dense and robust - I doubt if it would be "blown to bit" in the event of a problem with the rest of the launcher. e.g. During the 1980 Damascus Titan missile accident the missile exploded underground in a bunker and the warhead was thrown a fair distance but was recovered relatively intact: https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_explosion https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_ex... And that was something that was intended to explode!
- CydeWeys 6y ago
  It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. It needs to be able to survive any kind of catastrophic detonation/break-up/crash landing you can think of, because the alternative is spreading a large amount of radioactive material directly into the atmosphere/onto the Earth's surface. For example, the Challenger orbiter itself was fine, but when the SRB blew up it took up the orbiter with it. So if there'd been any nuclear materials in the orbiter, even if they weren't used at launch, they still would've needed serious shielding. And it doesn't matter if the consequences of said radioactive material release aren't actually that bad in the grand scheme of things (like Fukushima looks not to have been) -- what matters is that the public reaction to such an event would preclude the possibility of ever launching it again.

jaggederest 6y ago

Yes. The difference is definitely a sea change. To put it in concrete terms, a nuclear-powered rocket with sufficient thrust could be single-stage to mars and back for the same propellant budget and mass fraction of a first stage chemical engine which doesn't even get to orbit.

CydeWeys 6y ago
A nuclear thermal engine is not remotely a drop-in replacement for a chemical engine. There are substantial other changes required to the rest of the rocket that add lots of mass and expense, that work to substantially negate a lot of the benefits.
- jaggederest 6y ago
  That's true from a political perspective but it's definitely not true from an engineering perspective. Got enough energy to hoist almost anything you like into orbit if you choose your thrust and scale appropriately. As to what happens to the ground underneath, that's a different problem :)
  - valuearb 6y ago
    No, extra mass is needed to make them operational, not politically acceptable. First is engine mass. (All numbers from memory) A SpaceX Raptor weighs around 4,000 lbs, and generates over 400,000 lbs of thrust (100x). NERVA weighed around 40,000 lbs and only generated 50,000 lbs of thrust (1.2x). A fully fueled orbital Starship in would weigh around 2.5M with about 25,000 lbs of engine, 150,000 lbs of ship and 2.3M lbs of fuel generating 1.2M lbs of thrust out of three vacuum Raptors. For a similar level of thrust from NERVA would add nearly 1M lbs of dry mass. Newer designs can cut that engine mass, but at most in half. And your nuclear ship could get by with less thrust and just burn the engine longer. But that brings two new questions Where does all the heat go? NTR send almost all of the heat out the back, but not all. Running that engine for hours on end in the vacuum of space (Instead of the very conductive earth NERVA was tested on) is going to heat stress your engine and everything it’s connected to. Do you need to add massive radiators and insulation to run the engines longer? How do you take off on Mars? If you have a lot less thrust, you will only be able to lift a lot less fuel and cargo. And you can’t land or take off on Earth. Unlike Starship NTRs may only be usable in between planets, requiring carrying heavy and expensive landers. Then there is propellant. A 900 ISP is only achievable with hydrogen (H2), using Methane or water or any other feasible propellant drops ISP below 600, making overall system performance far worse than burning methane in Starship Raptors. OK, let’s use Hydrogen then. Well unfortunately Hydrogen is a shitty propellent in every other measure. It doesn’t compress well so yet requires larger, heavier tanks. Also H2 is a tiny slippery molecule, forcing you to freeze it near absolute zero in order to keep it from leaking, which makes those tanks even heavier. And guess what? It still leaks. Will you have enough fuel left to fire a retro burn to match orbits with Mars? Better leave with at least 30% more than you need. Now, you probably don’t want to be killed by your engine on the way to Mars, so let’s add some radiation shielding between the crew and the NTR engines, or put the on a long spar sticking out behind the ship. Both add even more mass. A NTR ship will likely require at least 2-3x more dead mass than a chemical rocket ship, so the question becomes, what’s the point? Why jump through political hoops when Starship VP can already fly crew to Mars in only 90 days and deposit over hundred tons of cargo to the surface of Mars, and cheaply?