I have a doubt regarding rocket engines. Now which is the most responsible factor for propulsion using a rocket engine? Thrust / Specific Impulse?

Now, I need a comparison b/w three engines:

  1. MPDT1 [Magneto Plasma Dynamic Thruster]
  2. MPDT2
  3. VASIMR [Variable Specific Impulse Magnetoplasma Rocket]

Body Weight [On Earth] = 50k Ton

Launched from Earth, I want this body to reach space and escape the Earth's Gravitational Field

MPDT1 Specifications:

  • Propellant: Hydrogen
  • Thrust = 88,500 mN
  • Specific Impulse = 3,500 s
  • Power Required = 3,750 kW

MPDT2 Specifications:

  • Propellant: Hydrogen
  • Thrust = 26,300 mN
  • Specific Impulse = 4,900 s
  • Power Required = 1,500 kW

VASIMR Specifications:

  • Propellant: Argon
  • Thrust = 5,000 mN
  • Specific Impulse = 3,000-12,000 s
  • Power Required = 200 kW

Source for all this data: Wikipedia- Ion Thruster


1 Answer 1


For the initial ascent from Earth into space, thrust is most important. Once in orbit and free of the atmosphere, specific impulse becomes more important, though thrust will still be a factor.

Electric propulsion engines of the types you're interested in, however, make an extreme trade-off of minimal thrust in favor of very high specific impulse. They are very fuel efficient, but they don't have anywhere near enough thrust to lift their own weight against Earth's gravity, let alone a useful payload.

You simply can't get off the ground with any of those engines.

  • $\begingroup$ Even in deep space thrusters of very low thrust (like FEEP) are hardly useful for propulsion because it takes ages for them to provide any considerable delta-V. OTOH they are good for ultra-precise attitude/position control. $\endgroup$
    – SF.
    Feb 10, 2016 at 15:34
  • $\begingroup$ Might add the specific masses that they can actually lift from the surface: ~9.02 kg, ~2.68 kg, and ~0.51 kg respectively. Assuming metric tons, that's more than three orders of magnitude less than their actual mass. $\endgroup$ Feb 10, 2016 at 19:07

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