Re: starship-design: Pellet track

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

Hello Isaac,

>>If the pellets are slow moving, then what is the use of pellets?
>>Catching pellets like this, will not give a significant energy advantage
>>over taking the pellets with you (=attached to the starship) from the start.
>
>The advantages are:
>
>1. You only have to accelerate the unfueled ship.

But you have to do more trouble to add momentum from the pellets.
The higher their relative velocity, the more energy is needed to add an
equal amount of momentum. (dE=0.5*dp*v)
In a self-fueled ship you don't have this problem, the mass that is on board
has zero relative velocity and thus momentum can be added much easier.

My guess it that both effects cancel out.
As long as you use the same amount of mass as for a self-fueled ship, a
pellet track design needs just as much energy!

However if you use more mass, and thus add less momentum per unit of weight,
then you can indeed save energy.

Steve VanDevender and I have a similar dilemma. If one has unlimited amounts
of energy then using high exhaust velocities (ie. adding lots of momentum to
small amounts of mass) is the best solution. However in reality we usually
have lots of mass and limited amounts of energy. In these cases it is
usually best to add  little energy to huge amounts of mass. 

Since most designs take the mass with them, their mass is limited and their
energy is limited, so one has to find a way somewhere in the middle.

What I didn't think of during all these years of starship discussion, is
that the pellet track can have quite large amounts of mass. That would mean
that much less energy is needed.
On the other hand you'd have quite unlimited amounts of fusion energy too.
The only disadvantage is that you've to start making the track years in
advance, depending on the lenght of the acceleration track and the velocity
of the pellets.

>2. You don't have to bend over backwards trying to ignite fusion (since
>   you're using the pellet's kinetic energy).  It's actually _easier_
>   to implement a fusion ramjet than it is to implement a onboard
>   fusion drive.

Why is it easier?

>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.

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?

>4. The effort involved may be spread over a long period of time.
>   For the acceleration track, this might not be much of an issue,
>   because all the time spent setting up the acceleration track
>   is simply delaying the completion of the mission that much
>   longer.  For the deceleration track, this is a critical bonus
>   because you can manufacture and send fuel packet drones over
>   a period of years while the starship is progressing toward
>   the target system.

Ah, this is new as far as I know. The pellets catch up with the starship
instead of vice versa.
That indeed decreases the track preparation time.

>>Still you have to launch the pellets many many years in advance.
>
>No, I assume that all resources prior to launch were devoted to
>the acceleration track and the starship itself.

Yes, now I understand. I assumed the pellets would have a slower velocity
than the ships cruising speed.


Timothy