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Re: starship-design: We need to get on the same (pellet) track first

Timothy van der Linden wrote:
>Hello Isaac,

>In order not to loose track of the situation, I left out the rest of you
>last letter. I will come back to the rest of that letter as soon as we agree
>about the following.

>>>>3. The fuel requirements for a given cruise velocity go up roughly
>>>>   linearly, as opposed to exponentially for a fusion drive.

>>>With "fusion drive" I assume you mean a design that takes all its fuel
>with it.
>>>You have to specify what you mean with fuel: mass or energy
>>>The energy (not mass) requirements for a selffueled design do not increase
>>>exponentially, but with a 3th power.

>>Actually, the energy _does_ increase exponentially.  The amount of
>>propellant required increases exponentially, so the amount of energy
>>required also does (because the amount of energy used goes up
>>linearly with the amount of propellant used).  This is well established
>>in the rocket equation.

>That is only so if you keep the exhaust velocity constant.
>You can also keep the mass ratio constant and increase the exhaust velocity.

No you can't.  Like it or not, there's a top exhaust velocity available
just barely acceptable if we're using fusion power.

If we assume antimatter production is economical, then almost
certainly an antimatter powered rocket of some sort is the best
stardrive.  There's almost no point in talking about fusion powered

>I should note that anything above 0.3c doesn't apply to pure fusion designs,
>but only for designs that use a more energy rich fuel.

Which means antimatter.  (Or something even more bizzare, like a black

>>>I'm not so sure that a pellet track uses linearly more fuel if it increases
>>>its cruising velocity. My guess is that it will be 3th power too.
>>>Can you show/explain me why you think it will increase linearly?

>>Because you do not have to carry the propellant with you.  The
>>amount of thrust you get is still roughly proportional to the
>>amount of propellant used, although you are right that it will
>>get more difficult with speed.  Due to the way the drive works,
>>this increase shouldn't be much (mostly because it's not very
>>efficient at low speeds--the amount of momentum added is limited
>>by the magnetic fields).

>You say that I'm right that it will get more difficult with speed. But then
>you say that is neglectable due to your design.
>This discussion is not about design inefficiencies. It is about the
>elementary physics that are involved. Once we agree about that, we can
>discuss the flaws of specific designs.

You simply can't ignore the realities of particular methods of
propulsion.  For instance, you can't ignore the fact that without
anti-matter power, maximum exhaust velocity is _the_ limiting
factor in traditional interstellar rocket designs.

Sometimes energy efficiency is the limiting factor (e.g. antimatter
rocket or intrasolar fusion rocket).  Sometimes acceleration
capability is the limiting factor (e.g. solar sails).  Sometimes
exhaust velocity is the limiting factor (e.g. fission rockets or
interstellar fusion rockets).  Sometimes its a combination (e.g.
laser sail).

For instance, it's really pointless in discussing the potential
energy efficiency of launching something solid at relativistic
velocities via an electromagnetic mass driver because you'd
never be able to build one long enough.

>The trust or force you get is not proportional to the amount of mass used:
>To accelerate a fast moving mass needs quadratically more energy than
>accelerating a slow moving mass.

This assumes you're using a method of accelerating the mass which is
limited by energy input.  This ramjet is _not_ limited by how much
energy it inputs into the mass stream, because it isn't providing
the energy (the pellets themselves provide the fusing energy).

It's limited by the strength of its magnetic fields.  A ramjet can
get more powerful the faster it goes.

>>However, assuming a perfectly ideal accelerator track scheme,
>>the increase will be with the square of the velocity.  In the
>>ideal situation, the same amount of energy is imparted to each
>>incoming pellet, so the amount of thrust you get from a pellet
>>is inversely proportional to its relative velocity.  This will
>>blow up track mass requirements as the square of the desired
>>cruise velocity.

>No, in an ideal track you will add as little energy as possible to each unit
>of mass.

You will add the _same_ amount of energy per pellet.

>That will give you the most momentum per unit of energy.
>An ideal(=minimum energy requirement) track therefore has unlimited amounts
>of mass available. 

Umm...it takes some energy to create the track in the first place.
The amount may vary, but I think a good assumption is that the
cost is roughly proportional to the mass of the track.

Thus, even if we use your minimum energy requirement (which is invalid
in this case), the ideal track will be some finite mass.
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi