4 ms·

No, because thrust per unit power drops as exhaust velocity increases. You’ll be increasing your power supply and thruster mass far more than any propellant you

No, because thrust per unit power drops as exhaust velocity increases. You’ll be increasing your power supply and thruster mass far more than any propellant you save. It’s also energy inefficient to use too high of an exhaust velocity. The mass-optimal exhaust velocity is approximately equal to the square root of (twice the thrusting time times overall power-and-propulsion system specific power times electrical efficiency). Read more here: https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-522-space-propulsion-spring-2015/lecture-notes/MIT16_522S15_Lecture3-4.pdf https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-... Assuming no dry mass (spherical cow!), energy optimal Exhaust velocity is equal to the total elapsed mission velocity at any point. If you can’t adjust exhaust velocity, it’s proportional to approximately to 60-65% of the total mission delta-v (off the top of my head). More discussion of where this energy-optimal exhaust velocity comes from: https://www.youtube.com/watch?v=ogKKjpQvfuM https://www.youtube.com/watch?v=ogKKjpQvfuM

btilly 5y ago
Indeed, one of the memorable stories from https://www.amazon.com/Ignition-Informal-History-Liquid-Propellants-ebook/dp/B076838QS2 https://www.amazon.com/Ignition-Informal-History-Liquid-Prop... was when the author wrote a joke paper analyzing the optimal amount of mercury (!) to mix into rocket fuel for maximal thrust. It takes away from the power, but since mercury is heavy it slows the exhaust, and therefore can increase thrust. To the author's surprise, the general evaluating the crazy ideas that the lab came up with failed to recognize that it was meant as a joke, and ordered that it actually be tested. So some unfortunate test site had to actually set up the test, and verify the theory. Luckily the military came to their senses about then and never actually built real rockets using the principle. (For those who are confused, mercury may be heavy, but is a nasty contaminant. Organic compounds including mercury have a disturbing tendency to be neural toxins. See https://en.wikipedia.org/wiki/Karen_Wetterhahn https://en.wikipedia.org/wiki/Karen_Wetterhahn for a famous example of how little is needed to be lethal. You really don't want to be spewing it over the countryside in a flaming ball.)
- nkoren 5y ago
  Stories like this make Charlie Stross' "A Tall Tale" seem that much more believable: https://www.tor.com/2012/07/20/a-tall-tail/ https://www.tor.com/2012/07/20/a-tall-tail/
jcims 5y ago
Thank you so much for those links, I’ve casually been following space travel for some time and this is the first time I’ve heard about optimal exhaust velocity. I’m about 20 minutes into that presentation and it’s great!

btilly 5y ago

Indeed, one of the memorable stories from https://www.amazon.com/Ignition-Informal-History-Liquid-Propellants-ebook/dp/B076838QS2 https://www.amazon.com/Ignition-Informal-History-Liquid-Prop... was when the author wrote a joke paper analyzing the optimal amount of mercury (!) to mix into rocket fuel for maximal thrust. It takes away from the power, but since mercury is heavy it slows the exhaust, and therefore can increase thrust. To the author's surprise, the general evaluating the crazy ideas that the lab came up with failed to recognize that it was meant as a joke, and ordered that it actually be tested. So some unfortunate test site had to actually set up the test, and verify the theory. Luckily the military came to their senses about then and never actually built real rockets using the principle. (For those who are confused, mercury may be heavy, but is a nasty contaminant. Organic compounds including mercury have a disturbing tendency to be neural toxins. See https://en.wikipedia.org/wiki/Karen_Wetterhahn https://en.wikipedia.org/wiki/Karen_Wetterhahn for a famous example of how little is needed to be lethal. You really don't want to be spewing it over the countryside in a flaming ball.)

nkoren 5y ago
Stories like this make Charlie Stross' "A Tall Tale" seem that much more believable: https://www.tor.com/2012/07/20/a-tall-tail/ https://www.tor.com/2012/07/20/a-tall-tail/

jcims 5y ago

Thank you so much for those links, I’ve casually been following space travel for some time and this is the first time I’ve heard about optimal exhaust velocity. I’m about 20 minutes into that presentation and it’s great!