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Re: [sfconsim-l] Propulsion Question

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  • Nyrath the nearly wise
    ... Well, your deltaV is dependant on two things and two things only, the velocity of the exhaust, and the ratio of the ship s mass full vs dry. Keep in mind
    Message 1 of 1 , Aug 11, 2000
      Donald McLean wrote:
      > So is this idea at all feasible, and how would I go about
      > determining the technical characteristics of the drive,
      > such as delta V given loaded and unloaded mass of the ship?

      Well, your deltaV is dependant on two things and two things
      only, the velocity of the exhaust, and the ratio of the ship's
      mass full vs dry.

      Keep in mind though that since you are using this as
      a surface to orbit propulsion system, you'll need to
      figure thrust as well. While the ship is close to
      the planet, it will have a "tax" imposed upon it
      by the planet's gravity.

      Basically, if the propulsion doesn't provide more
      acceleration than the deceleration provided by the
      planet, the rocket isn't going anywhere.

      Anyway:

      Lambda = Mf / Me
      where:
      Lambda = ship's "mass ratio"
      Mf = ship's mass when fully loaded with reaction mass
      Me = ship's mass "empty" (i.e,. with empty reaction mass tanks)
      Some solid fuel rockets have a mass ratio of 20 to 60,
      but that's pretty extreme. Liquid chemical rockets
      generally don't go over a mass ratio of 12.
      For a multi stage rocket, the mass ratio is the
      product of each stage's mass ratio (which is why
      NASA uses so many multistage rockets).
      For a merchant ship, you wouldn't want a mass ratio
      over, say, 4.

      total deltaV = Ve * ln[Lambda]
      where:
      total deltaV = the total amount of velocity change the ship is capable of
      Ve = exhaust velocity

      Naturally, whatever units you use for Ve will be the same ones
      used for deltaV (typically kilometers per second)

      Thrust = mDot * Ve
      where:
      Ve = exhaust velocity in kilometers per second
      mDot = propellant mass flow in kilograms per second
      Thrust = Thrust in kilo Newtons

      Acceleration is a pain:
      Accel = Thrust / Mc
      where:
      Mc = Ship's mass at this particular instant in time
      Accel = acceleration in kilometers per second per second

      Its a pain because, of course, the ships mass changes constantly
      as it expends reaction mass.

      At 100% efficiency, the best you are going to get out
      of fusion is 6.3 x 10^14 J/kg (joules per kilogram of fusion fuel)
      For details about figuring the exhaust velocity,
      refer to Mr. Robinson's post.
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