NASA’s longshot bet on a revolutionary rocket may be about to pay off

Re:NASA’s longshot bet on a revolutionary rocket may be abou... by ethbro on Wednesday February 22, 2017 @12:50PM (#13707441)

(IAN a physics major) But to elaborate per my understanding, there are 2 important variables you're playing with in rocket engines:

- Specific impulse / Isp (aka how much thrust you get for a given amount of propellant)

- Maximum Thrust

Due to the rocket equation [1], adding more propellant increases the weight of your vehicle, making it harder to move, requiring more propellant, etc etc. So being efficient with your propellant is very good.

That said, given a requirement against a gravity well (e.g. a planet), there's usually a minimum total thrust required for a given maneuver to be successful (low total thrust = maneuver takes longer = more time for gravity to pull you = more propellant required).

Thus far, we generally have two types of engines. (1) High maximum thrust, lower Isp (chemical rockets) & (2) high Isp, low maximum thrust (ion/electric engines). The two are currently very far apart [2, sort by Specific Impulse decreasing, then look at the Thrust column].

As examples (Isp Vacuum / Thrust Vacuum): NEXT ion thruster 4,100s/0.236N @ 6.9 kW, VASIMR 5,000s/5.7N @ 200 kW, Space Shuttle SRBs 268s/14MN.

The hope with VASIMR is that it provides a middle ground where high Isp is available with enough total thrust to actually be useful for something other than slow orbital adjustments. An example of "something useful" would generally be anything beyond Earth orbit that covers large distances, e.g. flying to Mars.

[1] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

[2] https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_e...

[ Parent | on HN ]

Re:NASA’s longshot bet on a revolutionary rocket may be abou... by binarycoffee on Wednesday February 22, 2017 @01:19PM (#13707702)

This is correct but incomplete: Vasimr may have a higher power density (be it power per thruster mass or per thruster volume) than ion thrusters but I am pretty sure it has lower power density than a Hall thruster, while being also much more complex.

The main (and not yet verified) claim of Vasimr compared to existing and proven tech like Hall thrusters is the ability to greatly vary the specific impulse.

[ Parent | on HN ]

Re:NASA’s longshot bet on a revolutionary rocket may be abou... by jandrese on Wednesday February 22, 2017 @04:30PM (#13709329)

That's what the article claims. That you can run in a less efficient mode that produces more total thrust when you're climbing out of a gravity well, then switch to the lower thrust higher efficiency mode for the long haul.

[ Parent | on HN ]

Re:NASA’s longshot bet on a revolutionary rocket may be abou... by ethbro on Thursday February 23, 2017 @10:26AM (#13714589)

I thought one of the issues with Hall thrusters was that, in engineering-practical terms, getting enough total thrust out of them was impossible. Or would they scale effectively?

[ Parent | on HN ]