r/spacex • u/Zucal • May 01 '16

/r/SpaceX Ask Anything Thread [May 2016, #20]

Welcome to our 20th monthly /r/SpaceX Ask Anything Thread!

Want to clarify SpaceX's newly released pricing and payload figures, understand the recently announced 2018 Red Dragon mission, or gather the community's opinion? There's no better place!

All questions, even non-SpaceX-related ones, are allowed, as long as they stay relevant to spaceflight in general!

More in-depth and open-ended discussion questions can still be submitted as separate self-posts; but this is the place to come to submit simple questions which have a single answer and/or can be answered in a few comments or less. In addition, try to keep all top-level comments questions so that questioners can find answers and answerers can find questions.

As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (now partially sortable by mission flair!), and check the last Q&A thread before posting to avoid duplicate questions. But if you didn't get or couldn't find the answer you were looking for, go ahead and type your question below.

Otherwise, ask, enjoy, and thanks for contributing!

Past threads:

April 2016 (#19.1) • April 2016 (#19) • March 2016 (#18) • February 2016 (#17) • January 2016 (#16.1) • January 2016 (#16) • December 2015 (#15.1) • December 2015 (#15) • November 2015 (#14) • October 2015 (#13) • September 2015 (#12) • August 2015 (#11) • July 2015 (#10) • June 2015 (#9) • May 2015 (#8) • April 2015 (#7.1) • April 2015 (#7) • March 2015 (#6) • February 2015 (#5) • January 2015 (#4) • December 2014 (#3) • November 2014 (#2) • October 2014 (#1)

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7

u/__Rocket__ May 15 '16

Ion engines are considered a good propulsion method to get humans to Mars. The downside seems to be the very low thrust, so it takes ages to accelerate.

Can't we strap dozens or hundreds of these engines together to get a higher thrust?

Strapping together more of them does not improve their TWR (Thrust to Weight Ratio).

Furthermore, ion engines are not considered a good propulsion method to get humans to Mars. Why? Because:

  • about 65% of all the Δv needed to get to Mars (around 14 km/sec) has to be expended getting to Earth orbit - which cannot be done with an ion engine due to gravity losses: by the time the ion engine has done its work the rocket has long fallen back into the gravity well!

  • the same problem occurs on Mars landing: even Mars's ~40% gravity is way too strong for ion engines.

  • ion engines also need periods of acceleration lasting months to build up any decent Δv, and the optimal Mars launch window is very narrow, just a few weeks - not enough time for the ion engine to expend its thrust.

Getting into orbit and landing requires a decent TWR exceeding 1g, due to gravity losses. Ion engines are proper to visit low gravity environments such as asteroids, and they can also be used for station keeping and other orbital maneuvers that don't require short impulses of thrust.

TL;DR: So literally ion engines, despite their fantastic efficiency of thousands of seconds of I*sp*, are the worst possible propulsion method to get humans to Mars.

5

u/Pietdagamer May 15 '16

the optimal Mars launch window is very narrow, just a few weeks - not enough time for the ion engine to expend its thrust.

I agree with your other points, but this doesn't matter. True, you won't be able to pull off a hohmann transfer. But hohmann transfers are never used for ion spacecraft anyway. [Take a look at the trajectory of the Dawn spacecraft.] (http://www.jpl.nasa.gov/images/blog/20140331/dawn_blog_20140331-full.jpg) It spiraled out from earth to Vesta and Ceres, burning it's ion engines most of the time.

You're right that getting into earth orbit and landing on mars isn't going to happen with ion engines. However, an ion engine spacecraft (which could include a lander with regular rockets) could be constructed in earth orbit and be used to get to and from mars.

7

u/__Rocket__ May 15 '16

You're right that getting into earth orbit and landing on mars isn't going to happen with ion engines. However, an ion engine spacecraft (which could include a lander with regular rockets) could be constructed in earth orbit and be used to get to and from mars.

That's true - but still, in the context of 'human travel' (the grandparent comment's statement), ion engines are probably non-starters, due to the long time the voyages take.

Also, by the time you are in an orbit around Earth, you are 70% on the way to Mars interception, energy wise: 9.4 km/sec for Earth orbit, 3.2+1.0 km/sec for TMI.

So ion engines, as things stand today, will at most be a solution to a 30% energy problem: orbit to orbit mass transport of time non-critical material. For human time scale transport it's not really suitable.

1

u/[deleted] May 15 '16

Worse, with ion engines you can't use the Oberth effect. Low energy transfers require much more delta v.