Illustration for article titled Physics / Rocket Science Question: Space Travel Time

So I have a problem I’m trying to solve for about a day or two that’s been bugging me quite a bit. And I was hoping you sciencey people who helped me out last time I had a problem might be able to help me again. Essentially, I’m trying to create a chart of various different types of rockets for my own personal use, listing such details as exhaust velocity, thrust, specific impulse, etc. Well, I’ve figured most of that out but one detail’s eluding me: how fast they’re actually likely to be for a given amount of fuel expended.


I’m sure that if you’re a physics major this is actually fairly simple. But the truth of the matter is it’s outside of my area of expertise and everything I’ve tried to read on the subject on my own has just left me more befuddled. I’ve searched Atomic Rocket, but while they’ve proven an excellent resource in determining details like exhaust velocity, fuel mass, etc. I haven’t been able to figure out precisely what I’m looking for.

So in the hopes that some of you might be better suited at figuring this out, here’s what I have so far, based on some models from Atomic Rocket:

  1. Liquid chemical rocket: For this I’m using the so-called Kuck mosquito as a basis. This means an exhaust velocity of 4,400 m/s, an effective thrust of 220,000 N, a specific impulse of 450 s, and the mass ratio is 3.5
  2. Nuclear pulse propulsion: For this I’m using the ACMF ICAN-II as a basis. Exhaust velocity is 132,000 m/s, thrust is 180,000 N, specific impulse is 13,500 s, and the mass ratio is 2.05
  3. Fission thermal rocket: For this I’m using the Widmer Mars mission proposal as a basis. Exhaust velocity is 8,000 m/s, thrust is 580,000 N, specific impulse is 815 s, and the mass ratio is 2.7
  4. Fusion rocket: For this I’m using this real-life revision of the Discovery’s design from 2001: A Space Odyssey as a basis. Exhaust velocity is 347,000 m/s, thrust is 18,000 N, specific impulse is 35,4000 s, and the mass ratio is 1.9
  5. Electrostatic drive: For this I’m using the “umbrella ship” as a basis. Exhaust velocity is 80,400 m/s, thrust is 490 N, specific impulse is 8,200 s, and the mass ratio is 2.01
  6. Electrothermal drive: For this I’m using the Wakefield E-Beam design as a basis. Exhaust velocity is 19,600 m/s, thrust is 4,600 N, specific impulse is 2,000 s, and the mass ratio is actually unstated (not sure what to make of that)
  7. Electromagnetic sail: For this I’m using a photon heliogyro sail as a basis. Since it has no propellant the only relevant statistic is its thrust, which is 140 N
  8. Mass driver: For this I’m using Atomic Rocket’s example of a mass driver engine as a basis. Exhaust velocity is 9,800 m/s, thrust is 10,400 N, specific impulse is 1,000 s, and mass ratio is again unstated

Specifically, I’m trying to figure out travel times (on average) from Earth to Mars and Earth to Neptune. If one of you can provide me with the figures that’d be great but what I’m actually hoping is someone can show me how to figure out the answer myself. Any help at all would be greatly appreciated. I also apologize in advance if I’ve left out any critical information or my question is confusing. Thanks.

EDIT : If anyone else has anything else they’d like to throw in feel free to do so, but I think djublonskopf has me covered. Thank you for all your help.

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