5 ms·

> Thermal rocket is going to loose to well designed electric one. Depends on your metric. For a given tech level, you'll get higher thrust/weight out of a Nucl

> Thermal rocket is going to loose to well designed electric one. Depends on your metric. For a given tech level, you'll get higher thrust/weight out of a Nuclear Thermal Rocket than you would from a Nuclear Electric Rocket, even if the specific impulse is lower. And 'very small velocities' here is still double the ISP of hydrolox, so not exactly shabby...

lmilcin 6y ago
I assume neither design is going to lift off Earth's surface. Most likely any nuclear engine is going to be lifted cold and disassembled and we'll packaged in case of mishap. Since you are going to have months to run the engine, thrust to weight is less important than ISP. Small accelerations do wonders if you can run them continuously.
- valuearb 6y ago
  Thrust to weight still will matter. NTR and NERs still have to drag all that dead mass of engine, radiators and shielding wherever they go.
  - m4rtink 6y ago
    Not to forget NTR dumping most of the heat during the burn kinda by definition while NER has to radiate lots of heat continuously to power the engine. Also without high trust you can't really make use of the Oberth effect: https://en.m.wikipedia.org/wiki/Oberth_effect https://en.m.wikipedia.org/wiki/Oberth_effect
    - lmilcin 6y ago
      Oberth effect is for long term missions where you have time to swing by other planets or moons just to get some free velocity. This is fantastic, but costs huge amount of mission time. The nuclear engine is about getting so much delta v that you can cut the crap and power directly to your destination.
      - laverya 6y ago
        Oberth effect is useful for every reasonable trip - since every reasonable trip either originates or ends deep within a gravity well. (And being in orbit deep within a gravity well implies being at high velocity) It's why you make an escape burn at periapsis and not apoapsis.
      - valuearb 6y ago
        Not really. An 85 ton Starship fueled in LEO can use Oberth effect, get to Mars just as fast, and land directly on Mars. Your deep space NTR has engines that weigh way at least ten times more, while providing only a fraction of the thrust, and also has to push not only hundreds of tons of radiators, shielding and heavier tanks, but also a lander since NTR doesn’t have the thrust to climb out of any significant gravity well. And also is going to have substantial propellant evaporation by the time it reaches Mars since it can only achieve that ISP with hydrogen.
- laverya 6y ago
  Small accelerations over long periods are great! But when you're using electric propulsion, how small starts to become an issue. As far as I'm aware ion thrusters have TWRs in the 1/1000 range - NERVA's were in the ~1 range. This means you're taking a 3000 second burn and replacing it with a 3,000,000 second burn - add in efficiency losses and things start getting interesting. (assuming a constant mass fraction devoted to engines, and that the electric propulsion TWR includes power generation and cooling)
  - lmilcin 6y ago
    Ion thrusters = technical problem. Thermal rocket = fundamentally limited by rocket equation. With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do. Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.
    - laverya 6y ago
      Ion engines are still limited by power density - higher ISPs take quadratically more power input at reasonable ISPs. (As in, at non-relativistic velocities) Thermal rockets have a lower upper bound, but come with higher thrusts. Everything is tradeoffs, in the end. And why on earth would the kind of newtonial engine mean that you wouldn't be limited by the rocket equation? The only escape that is to have reaction mass outside of your reference frame somehow. (picking up fuel from interstellar space, having thrust beamed to you via laser, some form of reactionless drive...)
    - Zardoz84 6y ago
      No body are talking about plasma rockets (VASIMIR). Elctric rockets that have variable ISP on demand (low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust). Also, VASIMR does not use electrodes so not erosion problem.
- numpad0 6y ago
  Build on the Moon and launch with a mass driver. Dock with a shuttle on HEO. Easy!
rocqua 6y ago
From what I read, electric engines have some pretty big limitations. - Very low thrust makes it hard to use the oberth effect - Low thrust to weight ratio makes the actual wet/dry mass ratio harder to get down - You are not just thrust limited, but also thermal limited. Many other rockets expel a lot of the heat they generate through the exhaust. Electrical engines do not, which means needing to get rid of that heat in other ways.

lmilcin 6y ago

I assume neither design is going to lift off Earth's surface. Most likely any nuclear engine is going to be lifted cold and disassembled and we'll packaged in case of mishap. Since you are going to have months to run the engine, thrust to weight is less important than ISP. Small accelerations do wonders if you can run them continuously.

valuearb 6y ago
Thrust to weight still will matter. NTR and NERs still have to drag all that dead mass of engine, radiators and shielding wherever they go.
- m4rtink 6y ago
  Not to forget NTR dumping most of the heat during the burn kinda by definition while NER has to radiate lots of heat continuously to power the engine. Also without high trust you can't really make use of the Oberth effect: https://en.m.wikipedia.org/wiki/Oberth_effect https://en.m.wikipedia.org/wiki/Oberth_effect
  - lmilcin 6y ago
    Oberth effect is for long term missions where you have time to swing by other planets or moons just to get some free velocity. This is fantastic, but costs huge amount of mission time. The nuclear engine is about getting so much delta v that you can cut the crap and power directly to your destination.
    - laverya 6y ago
      Oberth effect is useful for every reasonable trip - since every reasonable trip either originates or ends deep within a gravity well. (And being in orbit deep within a gravity well implies being at high velocity) It's why you make an escape burn at periapsis and not apoapsis.
    - valuearb 6y ago
      Not really. An 85 ton Starship fueled in LEO can use Oberth effect, get to Mars just as fast, and land directly on Mars. Your deep space NTR has engines that weigh way at least ten times more, while providing only a fraction of the thrust, and also has to push not only hundreds of tons of radiators, shielding and heavier tanks, but also a lander since NTR doesn’t have the thrust to climb out of any significant gravity well. And also is going to have substantial propellant evaporation by the time it reaches Mars since it can only achieve that ISP with hydrogen.
laverya 6y ago
Small accelerations over long periods are great! But when you're using electric propulsion, how small starts to become an issue. As far as I'm aware ion thrusters have TWRs in the 1/1000 range - NERVA's were in the ~1 range. This means you're taking a 3000 second burn and replacing it with a 3,000,000 second burn - add in efficiency losses and things start getting interesting. (assuming a constant mass fraction devoted to engines, and that the electric propulsion TWR includes power generation and cooling)
- lmilcin 6y ago
  Ion thrusters = technical problem. Thermal rocket = fundamentally limited by rocket equation. With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do. Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.
  - laverya 6y ago
    Ion engines are still limited by power density - higher ISPs take quadratically more power input at reasonable ISPs. (As in, at non-relativistic velocities) Thermal rockets have a lower upper bound, but come with higher thrusts. Everything is tradeoffs, in the end. And why on earth would the kind of newtonial engine mean that you wouldn't be limited by the rocket equation? The only escape that is to have reaction mass outside of your reference frame somehow. (picking up fuel from interstellar space, having thrust beamed to you via laser, some form of reactionless drive...)
  - Zardoz84 6y ago
    No body are talking about plasma rockets (VASIMIR). Elctric rockets that have variable ISP on demand (low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust). Also, VASIMR does not use electrodes so not erosion problem.
numpad0 6y ago
Build on the Moon and launch with a mass driver. Dock with a shuttle on HEO. Easy!

rocqua 6y ago

From what I read, electric engines have some pretty big limitations. - Very low thrust makes it hard to use the oberth effect - Low thrust to weight ratio makes the actual wet/dry mass ratio harder to get down - You are not just thrust limited, but also thermal limited. Many other rockets expel a lot of the heat they generate through the exhaust. Electrical engines do not, which means needing to get rid of that heat in other ways.