6 ms·

From https://momentus.space/development/ https://momentus.space/development/ it looks like they are using a microwave generator with water to generate a plasma

by y04nn 6y ago

From https://momentus.space/development/ https://momentus.space/development/ it looks like they are using a microwave generator with water to generate a plasma thruster. The technical name seem to be microwave electro-thermal thruster (MET). There is more info on the thurster in the abstract here: https://ieeexplore.ieee.org/abstract/document/1420619 https://ieeexplore.ieee.org/abstract/document/1420619

DavidSJ 6y ago
So in a sense it’s a steam engine powering a space ship.
- hnuser123456 6y ago
  Wonder what an effective steam rocket in lower atmosphere would look like.
  - tobmlt 6y ago
    SSME’s? Or does it count if the water started off in separate pieces?
- TheAdamAndChe 6y ago
  Steam is a gas, meaning the thrust is a function of the pressure. Plasma is made of ions, which can be significantly accelerated by electromagnets, meaning the thrust depends on the acceleration caused by those magnets, and can be a lot higher than steam. (Technically, the thrust generated is more complicated than just pressure, heavily affected by temperature too, but I digress)
walrus01 6y ago
How does the specific impulse compare to the current state of the art hall effect / ion thrusters that use xenon as a fuel?
- Rebelgecko 6y ago
  Probably worse. Raw steam has an ISP of around 100s. There was an experimental ion thruster from U Michigan a few years ago that got that up to nearly 700s. Some of the more efficient ion thrusters are like 20,000s (although they're operating at a different scale so idk if that's an apples to apples comparison) Edit- I looked at the Momentus paper linked above and apparently their ISP is 800s
  - walrus01 6y ago
    It's my understanding that current state of the art very low kN, but high ISP efficiency hall effect/ion grid thrusters can achieve 6000-8000 specific impulse without an absolutely ridiculous amount of electricity. There are designs that have been ground tested but not flown which could achieve 15000-20000 ISP but would also require immense photovoltaic arrays, or a small nuclear reactor. Once in orbit unless you need to achieve a lot of delta-v in a relatively short period of time, I'm not sure why an engine/fuel combo with only 800 ISP would be a logical choice. It's certainly a whole lot more efficient than a bipropellant chemical rocket engine, which maxes out around 385-400 ISP. Although it's never flown I believe the theoretical efficiency of a NERVA would be around 800.

by y04nn 6y ago

DavidSJ 6y ago

So in a sense it’s a steam engine powering a space ship.

hnuser123456 6y ago
Wonder what an effective steam rocket in lower atmosphere would look like.
- tobmlt 6y ago
  SSME’s? Or does it count if the water started off in separate pieces?
TheAdamAndChe 6y ago
Steam is a gas, meaning the thrust is a function of the pressure. Plasma is made of ions, which can be significantly accelerated by electromagnets, meaning the thrust depends on the acceleration caused by those magnets, and can be a lot higher than steam. (Technically, the thrust generated is more complicated than just pressure, heavily affected by temperature too, but I digress)

walrus01 6y ago

How does the specific impulse compare to the current state of the art hall effect / ion thrusters that use xenon as a fuel?

Rebelgecko 6y ago
Probably worse. Raw steam has an ISP of around 100s. There was an experimental ion thruster from U Michigan a few years ago that got that up to nearly 700s. Some of the more efficient ion thrusters are like 20,000s (although they're operating at a different scale so idk if that's an apples to apples comparison) Edit- I looked at the Momentus paper linked above and apparently their ISP is 800s
- walrus01 6y ago
  It's my understanding that current state of the art very low kN, but high ISP efficiency hall effect/ion grid thrusters can achieve 6000-8000 specific impulse without an absolutely ridiculous amount of electricity. There are designs that have been ground tested but not flown which could achieve 15000-20000 ISP but would also require immense photovoltaic arrays, or a small nuclear reactor. Once in orbit unless you need to achieve a lot of delta-v in a relatively short period of time, I'm not sure why an engine/fuel combo with only 800 ISP would be a logical choice. It's certainly a whole lot more efficient than a bipropellant chemical rocket engine, which maxes out around 385-400 ISP. Although it's never flown I believe the theoretical efficiency of a NERVA would be around 800.