Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B New rocket engine may enable trips to Mars

  1. Oct 19, 2016 #1

    wolram

    User Avatar
    Gold Member

  2. jcsd
  3. Oct 19, 2016 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Uh, well...
    11,000 seconds of thrust sound impressive, but one should not forget that higher Isp needs more power per thrust. In this case, 50 kW/N at unrealistic 100% efficiency. Changing the velocity by 5 km/s in a month needs 100 W/kg. That is a huge amount of power.

    It also does not work for launches or landings because its thrust is way too low. Its application is limited to Earth orbit -> Mars transfer and Mars orbit -> Earth transfer. Orbital insertions can be done with heat shields, but those increase the spacecraft mass. Launches need chemical rockets.

    It does not really help to have Magnesium as potential fuel - both xenon and magnesium are so cheap that their costs are negligible anyway, and you have to launch the material, as collecting space junk is impractical, at least in the near future.
     
  4. Oct 24, 2016 #3
    A spacecraft capable of one gravity of continuous thrust would be ideal for manned-missions, but any form of continuous propulsion, no matter how small, would be an immense improvement over our current methodology of merely coasting. Coasting we are looking at a minimum of 260 days to make a journey from Earth to Mars and back. With one gravity of continuous thrust we could make the same round-trip journey in under 5 days. That could mean the difference between life and death for the astronauts.
     
  5. Oct 24, 2016 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    No. Put a typical ion thruster (3 kW, 0.1 N) on a manned rocket (> 10 tons), and you get 25m/s velocity change per 1 month. Completely useless.

    Coasting is the fastest way to reach a target if your velocity change (delta_v) capability is the limit. Continuous acceleration needs larger velocity changes for the same trip duration - its only positive side is the longer timespan for the acceleration, which allows to use power more efficiently.

    1 g continuous acceleration for a trip to Mars needs insane amounts of energy and power densities - even with fusion you would need a significant fraction of the total mass as fuel and propellant. There is no way to make such a trip without fusion or a huge amount of fissionable material.
     
  6. Oct 25, 2016 #5
    Last edited: Oct 25, 2016
  7. Oct 27, 2016 #6
    Solar-powered thrusters other than solar sails make no sense. You would be capturing light, which has the highest theoretically possible specific impulse already. If you then used that light for anything other than just emitting it opposite to your desired thrust direction, you'd be decreasing your efficiency.
     
  8. Oct 27, 2016 #7
    I wouldn't go quite that far. I agree that solar sails are the most efficient from the standpoint of conversion of photon energy to thrust. You don't need to carry reaction mass and the on-board energy requirements are minimal. However there are issues regarding the available range of thrust vectors that limit maneuverability. NASA has been studying solar sailing technology since the early 60's and have flown a number of missions that employ it. the earliest, as far as I know, is the Venus Mariner of the mid 60's which used solar sails to augment attitude control. I worked on that project and am quite familiar with the pros and cons. Sometimes effectiveness is more attractive than efficiency. I am currently consulting with a company that is researching a propulsion system based on positron annihilation. There are several promising approaches including a positron powered VASIMR engine and another that involves direct conversion of anti matter reactions to thrust.

    Here are some interesting links.
    https://kdp.amazon.com/self-publishing/reports
    http://www.smithsonianmag.com/scien...space-mission-to-an-exoplanet-3948923/?no-ist
    https://prezi.com/47subao-0uaq/solar-sails/

    And on antimatter propulsion

    https://en.wikipedia.org/wiki/Antimatter_rocket
    http://thefutureofthings.com/3031-new-antimatter-engine-design/

    There is lots of info out there.
     
  9. Oct 27, 2016 #8

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Light has the worst possible thrust to power ratio. Capturing some of the energy to emit propellant at high speeds allows much larger speed changes over relevant mission times. Dawn changed its velocity by more than 10 km/s over the last years with solar-powered ion thrusters. At the same time, solar radiation contributed a few m/s velocity change, 3 orders of magnitude less.
    In addition, light pressure is limited in the possible directions of thrust.
     
  10. Oct 27, 2016 #9
    But I never claimed that solar sails are the most efficient thing ever. Just the most efficient thing that's powered by light. I.e. capturing light and turning it to electricity to power some other engine is just plain stupid.
    Now if you have antimatter or a nuke or something, I have nothing against that.
    [Edit: mfb shows that reality does not match my thinking, so apparently I'm the one being plain stupid. I'll need some time for counseling and for figuring out the math of that, in the meantime just ignore me]
     
    Last edited: Oct 27, 2016
  11. Oct 27, 2016 #10
    It's actually a question of which technology is best suited for the mission. The celestial mechanics problems of solar sails are quite different from those faced by reaction mass systems (Newtonian rockets). There are some missions that are best served by light sails, interstellar for example. But there others that require thrust vector management that sails do not provide. It's best to keep your "bag of tricks" as full as possible.
     
  12. Nov 5, 2016 #11
    Is it possible to make a solar engine by using solar radiation to expand a compressible fluid in one tank and release it down a long tube into another tank that is chilled in the shade. Once equalized the tanks would be rotate end for end and the process is repeated?
     
    Last edited: Nov 5, 2016
  13. Nov 5, 2016 #12

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Where is the point of that? If you want to produce electricity, photovoltaics is way more efficient than a detour over thermal expansion. It also works without moving parts which makes it much more reliable.
     
  14. Jan 19, 2017 #13

    Al_

    User Avatar

    "suggested that magnesium, used commonly as a light and strong alloy for space materials and prevalent in space junk orbiting the Earth, could be re-used to fuel the engine"

    It's not about going to Mars. This engine could be used in a satellite that captures space junk.
     
  15. Jan 19, 2017 #14

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    How large will the metal separation and refinement plant in orbit be?
     
  16. Jan 24, 2017 #15

    Al_

    User Avatar

    upload_2017-1-24_12-56-56.png
     
  17. Jan 24, 2017 #16

    Al_

    User Avatar

    How does that work?
    Is it related to dusty plasma fission fragment rockets?
    https://en.wikipedia.org/wiki/Fission-fragment_rocket
    If you could save the mass of the Neutron Moderator, the thrust to mass ratio goes right up!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: New rocket engine may enable trips to Mars
  1. New Mars Photos (Replies: 10)

  2. Asteroid may strike Mars (Replies: 20)

Loading...