Re: Spaceship with constant acceleration

On Jan 5, 11:34 pm, Erik Max Francis <m...@xxxxxxxxxxx> wrote:

One weird thing is in a way, Issac is implicitly assuming constant
_thrust_ (not acceleration) drives, because he's taking into account the
increased acceleration you get at the start of your trip by bringing
less fuel.  So in a way he's not really talking about
constant-acceleration drives, if you see what I mean.

If acceleration does not depend on the mass of the ship, then
you can use your "constant-acceleration" drive to accelerate
planets or stars around. Heck, why stop there? You could accelerate
galaxies around.

Although, speaking of moving planets around--that would actually
be a case which calls for continuous thrust with no coast phase.
I noted before that if propellant is extremely expensive, then you
would want to have a mass ratio as near 1:1 as possible (meaning
you have as little propellant consumption as possible).
Alternatively,
you'd want a near 1:1 mass ratio if the payload had an extremely
small value per kilogram. A planet would qualify. Even a wonderful
planet like Earth is almost entirely dead weight.

One thing I thing would be useful, and it seems to be a common problem,
is that people glibly talk about efficiency or optimization without
stating what it is exactly that they're really trying to optimize.

This is why I give an explicit list of the factors which are
improved by using a drift leg...sorry, I really should have
been using the term "coast phase".

 Of
the drive parameters, some are fixed (say, exhaust speed), some are
tweakable (say, thrust in some circumstances, though there may be
boundaries) and some you're trying to optimize for (say, power or energy
input or time or deltavee).  It would be instructive to get more
concrete about this so that we can construct functions and find the
maxima, to get better ideas on 1. under what circumstances this
optimization is useful and 2. how much of a benefit you get for what
change -- i.e., given a set of parameters, what is the optimum cruise
phase length?

I've been making the general assumption that given a
particular mission plan, increasing payload or decreasing
cost is always a good thing (while keeping the trip time
the same).

Isaac Kuo
.

Relevant Pages

  • Re: Spaceship with constant acceleration
    ... _thrust_ (not acceleration) drives, because he's taking into account the ... constant-acceleration drives, if you see what I mean. ... conjunction with the discussion about constant acceleration ... It's not clear to me that a chosen optimization to have a cruise phase for the former leads to a cruise phase for the latter, since you can now get more bang for your buck if you burn your fuel later rather than sooner. ...
    (rec.arts.sf.science)
  • Re: Spaceship with constant acceleration
    ... constant acceleration or constant thrust drives. ... Higher exhaust velocity is better when you're operating ...
    (rec.arts.sf.science)
  • Re: "Impossible" EM space drive?
    ... any device that can provide a constant thrust for a fixed electrical input *must* be an over-unity device in free space irrespective of any relativistic effects. ... unles you provide extra energy. ... And it would seem from the analysis presented that you can produce low thrust in free space, and get some motion, as long as you keep either the thrust or the velocity small. ... What seems to happen is that the acceleration is limited, so even though you can have significant thrust, you accelerate the device away from the EM field trapped inside. ...
    (sci.electronics.design)
  • Re: Spaceship with constant acceleration
    ... constant acceleration, not thrust. ... engine has a maximum performance of 10 Terawatts. ... acceleration over 1AU if your dry mass is greater than 31 tons. ... Well, if you know your mass ratio and V_e, you know your delta-V, ...
    (rec.arts.sf.science)
  • Re: "Impossible" EM space drive?
    ... any device that can provide a constant thrust for a fixed electrical input *must* be an over-unity device in free space irrespective of any relativistic effects. ... Actually no, a device can produce constant thrust without using energy, ... And it would seem from the analysis presented that you can produce low thrust in free space, and get some motion, as long as you keep either the thrust or the velocity small. ... What seems to happen is that the acceleration is limited, so even though you can have significant thrust, you accelerate the device away from the EM field trapped inside. ...
    (sci.electronics.design)