I understand that they have an interesting method of using microwaves to produce plasma from water. They claim to have a "high" thrust and "high" specific impulse. Their Isp numbers are published, but apparently not thrust. Has there been anything published on that end? More about the company here: homepage, but it doesn't look like much in the way of technical specs

  • $\begingroup$ Yes, sorry. Question has been edited $\endgroup$ – Michael Stachowsky Nov 23 '20 at 20:47
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    $\begingroup$ I also haven't seen many on their homepage, but the "high thrust and high isp" claim needs some evidence. All engines which have flown are either high thrust or high isp. I believe VASIMR can achieve both, but they can't generate enough power without onboard nuclear power... What can be claimed without evidence, can be dismissed without evidence. $\endgroup$ – ChrisR Nov 23 '20 at 21:28
  • $\begingroup$ Please could you put the published Isp number into your question? I can't see any sign of it on their web page. The limits to what can be achieved for Isp and Thrust at the same time are fundamentally connected to how much power is available. If you can set out in your question the Isp and also the max electrical power available in any of their platform options (assume all power available to the client for the mission phase is used by the propulsion system in the deployment phase) then I could provide an answer to the max thrust in principle. $\endgroup$ – Puffin Nov 24 '20 at 11:09
  • $\begingroup$ @ChrisR it's not a particularly extraordinary claim, it's just a trade of energy efficiency vs. propellant efficiency. For a given flow of mass, thrust is proportional to the exhaust velocity, power to the square of the exhaust velocity. Since it's just an electrothermal thruster, they can in principle increase thrust at the cost of specific impulse by just pumping more water through the thruster. $\endgroup$ – Christopher James Huff Nov 24 '20 at 17:05
  • $\begingroup$ @ChristopherJamesHuff if I read you correctly, we're on the same page: one can increase thrust by decreasing isp, and vice-versa. $\endgroup$ – ChrisR Nov 24 '20 at 17:16

Probably, partial answer. Momentus has been published (slide #58) value of the thrust for its Vigoride Extended vehicle of 0.8 N (800 millinewtons) with 650 seconds Isp and total impulse capability of 2.000.000 N-s (for maximum "wet" mass of 700 kg).

Picture from the above mentioned source: enter image description here

Aslo the thrust values can be obtained from the relation between the deltaV and the total mass for known Isp and total mass (slide #53, the same source):
enter image description here


Just to add to Peter Nazarenko's good answer on the thrust as the power requirement was also discussed in comments:

The electrical power required to feed the beam power of the thruster is P= T V/(eta 2) where eta is a combined efficiency factor comprising thermal losses from the thruster (20% - 50%) and electrical losses (maybe 5% - 10%).

Thus the absolute minimum, artificially setting eta to 1, Isp = 650s and Thrust = 0.8 thus electrical power = beam power = 650 * 9.81 * 0.8/2 = 2,550W.


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