3 ms·

VASIMR is one of the few test articles with flight hours of this category (or maybe a sibling category), and when you factor into account the thruster and the p

by mapt 2y ago

VASIMR is one of the few test articles with flight hours of this category (or maybe a sibling category), and when you factor into account the thruster and the power supply, its thrust to weight ratio for the VX-200 prototype was unworkably low. Not "too low for launch", I mean "too low for a steady burn of months to attain meaningful progress towards interplanetary or orbital objectives". You have to simultaneously solve for power to weight ratio issues on solar panels, power supply, and the actual thruster. The best solutions right now for most purposes are relatively low Isp (low propellant efficiency) Hall thrusters. In space, heat dissipation is remarkably harder than it is on Earth, so getting to, for example, 97% thermal efficiency rather than 94% would be a big deal in terms of the mass of the heat dissipation system, and I think this and reducing the mass of the actual electromagnet is the advance here, if there is any.

api 2y ago
One of the biggest things that always bothers me in sci-fi, even harder sci-fi like The Expanse, is the missing heat sinks. All those ships would need massive radiators or they’d melt. The starship in Avatar got that right. Rest of the film is comic book level and derivative but the ship is good.
- DennisP 2y ago
  It's worse than you think. Here's a really interesting article on what it would take to get Epstein drive performance: https://toughsf.blogspot.com/2019/10/the-expanses-epstein-drive.html https://toughsf.blogspot.com/2019/10/the-expanses-epstein-dr... This thing would produce more power than all of civilization does today. No radiator could handle it. They use laser fusion, exploding D-He3 fuel pellets well behind the ship, with a giant magnetic nozzle. A tungsten heat shield protects the ship, and reaches an equilibrium temperature without active cooling.
- XorNot 2y ago
  In the Expanse though The Drive is always on, which means the ship is tossing propellant mass (efficiently) out the back of it - which means it can dissipate most of it's heat that way.
  - mapt 2y ago
    The Drive in the Expanse is _nonsensically_ powerful; Turning it on at mass fractions that seem compatible with their ship designs would be akin to a nuclear exchange for anything within thousands of kilometers of the rear end. Talking about thermal and propulsive efficiency for a rocket is a little tricky because there are multiple useful metrics, but in this context "waste heat" is anything that doesn't get tossed out the back.
deleted 2y ago
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by mapt 2y ago

api 2y ago

One of the biggest things that always bothers me in sci-fi, even harder sci-fi like The Expanse, is the missing heat sinks. All those ships would need massive radiators or they’d melt. The starship in Avatar got that right. Rest of the film is comic book level and derivative but the ship is good.

DennisP 2y ago
It's worse than you think. Here's a really interesting article on what it would take to get Epstein drive performance: https://toughsf.blogspot.com/2019/10/the-expanses-epstein-drive.html https://toughsf.blogspot.com/2019/10/the-expanses-epstein-dr... This thing would produce more power than all of civilization does today. No radiator could handle it. They use laser fusion, exploding D-He3 fuel pellets well behind the ship, with a giant magnetic nozzle. A tungsten heat shield protects the ship, and reaches an equilibrium temperature without active cooling.
XorNot 2y ago
In the Expanse though The Drive is always on, which means the ship is tossing propellant mass (efficiently) out the back of it - which means it can dissipate most of it's heat that way.
- mapt 2y ago
  The Drive in the Expanse is _nonsensically_ powerful; Turning it on at mass fractions that seem compatible with their ship designs would be akin to a nuclear exchange for anything within thousands of kilometers of the rear end. Talking about thermal and propulsive efficiency for a rocket is a little tricky because there are multiple useful metrics, but in this context "waste heat" is anything that doesn't get tossed out the back.

deleted 2y ago

[deleted]