Loading ...
Sorry, an error occurred while loading the content.
 

Re: Physics for Poli Sci majors, excluding Chris W of course.

Expand Messages
  • Nyrath the nearly wise
    ... Agreed! There are ways of getting instantaneous acceleration, but they often rely on other bits of info that are not readily available. Ai = (Mdot * g *
    Message 1 of 6 , Aug 9, 1999
      > From: Steven Bonneville <bonnevil@...>
      >
      > ehenry@... (Eric Henry) wrote:
      > > i have come to the conclusion that I don't get it.
      > [...]
      > > How do I apply Isp, or Vexhaust, to determine how many Gs of acceleration my
      > > ship can do?
      >
      > You don't. Isp is a measure of *fuel efficiency*; how many seconds of
      > one Newton thrust you get per Newton of fuel burned. To determine how
      > many Gs of acceleration your ship can do, you need to know how massive
      > it is (which changes, as you use fuel) and how much thrust your engine
      > provides. [Incidentally, this means rockets can accelerate harder
      > once they've burned some fuel and vehicle mass decreases.]
      > For example, there are ion engines with high fuel efficiency (Isp) but
      > low thrust, and solid rockets with low fuel efficiency but high thrust.

      Agreed! There are ways of getting "instantaneous" acceleration,
      but they often rely on other bits of info that are not
      readily available.


      Ai = (Mdot * g * Isp) / Mc
      where:
      Ai = "instanteneous" accelleration in m/sec^2
      Mdot = propellant mass flow in kg/sec (very hard to look this up)
      g = one gravity of acceleration = 9.81 m/sec^2
      Isp = propulsion system's specific impulse in seconds
      Mc = ship's current mass at this instant in time
      (which will change as propellant is expended)

      F = Mdot * g * Isp
      where:
      F = thrust in Newtons or kg mt/sec

      In other words, Ai = F / Mc

      In case you are interested, Ve = g * Isp, where Ve = velocity of
      exhaust. So F = Mdot * Ve

      The MU that Mr. Watt was using appears to be based more on
      deltaV instead of thrust. Thus your confusion.

      What is often more useful (if travel time is of secondary consideration)
      is a propulsion system's delta-V. This is the total amount of
      change in velocity the drive can inflict on the ship, if all the
      propellant is expended. It is measured in meters per second (m/sec).
      (You'll see this in novels like ANTARES RISING, where the ensign
      tells the captain that the ship has 200 m/sec deltaV left in the
      propellant tanks.)

      At NASA, they measure energy requirements for various space missions
      in terms of the delta-V required.

      deltaV = g * Isp * 1n[Lambda]
      where:
      deltaV = velocity change in m/sec
      g = one gravity of acceleration = 9.81 m/sec^2
      Isp = propulsion system's specific impulse in seconds
      1n[x] = take the natural logarithm of x
      Lambda = ship's current "mass ratio"

      Lambda = Mt / Me
      where:
      Mt = ship's total mass (including current load of propellant)
      Me = ship's mass without propellant
      (Mass ratios tend to be from 2 to 10. Ships with a mass ratio
      of 10 are huge flimsy tinfoil soap bubbles holding titanic
      amounts of propellant)

      Note that the deltaV equation does not include a time component.
      Thus it cannot tell the difference between a low thrust ion drive
      that will take years to get to the target, and a high thrust
      chemical rocket.

      You may find the equations below to be useful. Or maybe not.

      * WARNING * The below equations assume a constant acceleration,
      which is not true for a ship expending mass (for instance,
      propellant). Ai = F/Mc so as the ship's mass goes down, the acceleration
      goes up.
      ============================================
      When you have two out of three of average velocity (Va) in m/sec,
      change in distance (S) in meters or time (T) in seconds
      Va = S / T
      S = Va * T
      T = S / Va
      ============================================
      When you have two out of three of acceleration (A) in m/sec^2,
      change in velocity (V) in m/sec or time (T) in seconds
      A = V / T
      V = A * T
      T = V / A
      ============================================
      When you have two out of three of change in distance (S) in meters,
      acceleration (A) in m/sec^2, or time (T) in seconds
      plus Initial Velocity (Vi) Note: if deaccelerating, acceleration A is negative
      S = (Vi * T) + ((A * (T^2)) / 2)
      A = (S - (Vi * T)) / ((T^2) / 2)
      T = (sqrt[(Vi^2) + (2 * A * S)] - Vi) / A
      If Vi = 0 then
      S = (A * (T^2)) / 2
      A = (2 * S) / (T^2)
      T = sqrt[(2 * S) / A]
      ============================================
      When you have two out of three of change in distance (S),
      acceleration (A), or final velocity (Vf)
      plus Initial Velocity (Vi) Note: if Vf < Vi, then A will be negative
      (deacceleration)
      S = (Vf^2 - Vi^2) / (2 * A)
      A = (Vf^2 - Vi^2) / (2 * S)
      Vf = sqrt[Vi^2 + (2 * A * S)]
      If Vi = 0 then
      S = (Vf^2) / (2 * A)
      A = (Vf^2) / (2 * A)
      Vf = sqrt[2 * A * S]
      ============================================
      If the ship constantly accelerates to the midpoint, then
      deaccelerates to arrive with zero velocity at the
      destination:
      T = 2 * sqrt[S / A]
      S = (A * (T^2)) / 4
      A = (4 * S) / (T^2)
    • ehenry@xxxxxxxxx.xxxxxxxxxxxxxxx)
      Did I mention i m a Poli Sci major? :) So, to sum up, Are most / all of the near future technology rockets incapable of escaping earth? However, a good gas
      Message 2 of 6 , Aug 10, 1999
        Did I mention i'm a Poli Sci major? :)

        So, to sum up, Are most / all of the near future technology rockets
        incapable of escaping earth? However, a good "gas mileage" in the form of
        specific impulse (Isp) is the greater issue, assuming these rockets can
        first be lifted / assembled to orbit via some other means.

        Are solid fuel rockets the only ones that can escape earth?

        My understanding is that liquid fuel rockets are capable of a constant
        acceleration even as the mass of the rocket declines as fuel is spent. Are
        we capable of fine control of acceleration or it gross control only.

        Any spreadsheets out there. Will trade Hipparcos data formatted for
        Nyrath's stereostar.


        -----Original Message-----
        Subject: Re: [sfconsim-l] Physics for Poli Sci majors, excluding Chris W of
        course.
      • Steven Bonneville
        ... Nope. I m not sure that solid rockets were even used for manned flights until Shuttle. Saturn V used liquid hydrogen/liquid oxygen for the upper stages,
        Message 3 of 6 , Aug 10, 1999
          > From: ehenry@... (Eric Henry)
          >
          > Did I mention i'm a Poli Sci major? :)
          >
          > So, to sum up, Are most / all of the near future technology rockets
          > incapable of escaping earth? However, a good "gas mileage" in the form of
          > specific impulse (Isp) is the greater issue, assuming these rockets can
          > first be lifted / assembled to orbit via some other means.
          >
          > Are solid fuel rockets the only ones that can escape earth?

          Nope. I'm not sure that solid rockets were even used for manned flights
          until Shuttle. Saturn V used liquid hydrogen/liquid oxygen for the upper
          stages, and kerosene/liquid oxygen in the first stage.

          > My understanding is that liquid fuel rockets are capable of a constant
          > acceleration even as the mass of the rocket declines as fuel is spent. Are
          > we capable of fine control of acceleration or it gross control only.

          The SSME (Space Shuttle Main Engine) rockets on the orbiter use liquid
          hydrogen and can be throttled. The older F-1 rockets on the first stage
          of a Saturn V burned kerosene and were fixed-thrust. The disadvantage
          of a fixed-thrust rocket is that you can't throttle it back as you burn
          fuel, so you accelerate harder, and harder.... The Shuttle doesn't hit
          the same peak acceleration as Saturn V, which means more people meet the
          physical requirements to fly on the Shuttle.

          The SRBs are a slightly different story. Once ignited you don't have
          any control over them; they just burn until they go out. However, they
          are manufactured to reduce thrust after they've been burning for a while,
          so that the spacecraft isn't put through too much stress. So in a sense
          they are fixed-thrust, but you can design them to have different amounts
          of pre-selected thrust during the burn.

          -- Steve Bonneville
        • Christopher Weuve
          On Tue, Aug 10, 1999 at 10:54:19 AM, Steven Bonneville ... Note that some solid fuel rockets have exhaust ports which can be opened to
          Message 4 of 6 , Aug 10, 1999
            On Tue, Aug 10, 1999 at 10:54:19 AM, Steven Bonneville <bonnevil@...>
            wrote:

            > The SRBs are a slightly different story. Once ignited you don't have any
            > control over them; they just burn until they go out. However, they are
            > manufactured to reduce thrust after they've been burning for a while, so
            > that the spacecraft isn't put through too much stress. So in a sense
            > they are fixed-thrust, but you can design them to have different amounts
            > of pre-selected thrust during the burn.


            Note that some solid fuel rockets have exhaust ports which can be opened to
            effectively cut their thrust, by letting the gases escape the chamber in many
            directions rather than simply through the rear. I do not believe they are
            designed to be throttled, but I suppose they could be.

            One of the things to keep in mind about solid rockets is that they don't burn
            from the middle, but rather from the inside out. The burning reaction is a
            function of surface area, so many of these rockets have hollow spaces that are
            shaped like stars rather than simple cylinders. A cylinder would have a
            greater surface area as the fuel burned, thereby increasing thrust; the star
            shape keeps the surface area approximately the same.

            FYI.

            chrisw
          • ehenry@xxxxxxxxx.xxxxxxxxxxxxxxx)
            what is the inverse square law and how does it apply to directed energy weapons fire? Is that the method Traveller uses to determine damage?
            Message 5 of 6 , Aug 18, 1999
              what is the inverse square law and how does it apply to directed energy
              weapons fire?

              Is that the method Traveller uses to determine damage?
            Your message has been successfully submitted and would be delivered to recipients shortly.