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

[TLG.van.der.Linden@tip.nl (Timothy van der Linden)]

Hello Isaac,

>>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.

Then we agree. All that I should have added is "for low cruise velocities
with a fusion design":
For low cruise velocities one needs just as much energy when using the same
amount of mass for a ramjet design as for a self-fueled-fusion design.

>>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
>hole.)

Or a partial beamed design...


>>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.

I wasn't discussing the realities of a particular method, but of a
self-fueled design in general. Now that we cleared things up, I guess we can
indeed go to the particular cases.

>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.

True, but before you can discard a method, you have to determine what the
limits are. I've found that there are few numbers available about the
designs we are talking about. To avoid needless calculation I approach the
designs in a general way. Then after having looked at the results, I'll
discard a particular case.
I guess I was put a little bit off balance by your cooked-&-ready approach,
just as you probably were by my step-by-step approach.

>>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).

True, the pellets not used as exhaust mass are then more or less used as
batteries.


>>>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.

OK.

>>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.

Yes, I was exaggarating a bit. Clearly an infinite amount of mass wouldn't
be practical, since you had to pass trough it.

Timothy