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

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

Isaac,

>>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.
>
>Untrue, because at such low cruise velocities it's doubtful a fusion
>ramjet design will operate at all (without shoving the pellets at it
>at some high velocity, costing extra energy).  However, a chemical
>pellet ramjet design could work.  The energy required is no longer a
>useful method of comparison because the costs of that energy depends
>upon the energy source.

Sigh, there won't be much room for comparison then.

>[about the rocket equation being used with various exhaust/destination
>velocities]
>>>>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...
>
>Not exactly.

I suppose a black hole as stardrive does comply with the rocket equation?

>>>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.
>
>Unfortunately, the only criteria you discuss, energy consumption,
>isn't always the driving factor.  In fact, it rarely is directly
>significant, because the cost per energy unit varies so greatly
>from one source to another (cost and availability are the true
>important factors).

I assumed energy was an important part of the cost. But indeed for fusion
designs that may not be true.

>>>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.
>
>If you'd like, I can rehash my calculations on the potential muzzle
>velocities and other limitations of mass drivers.  The overall
>conclusions are actually pertinent to this discussion, since EM mass
>drivers would be used to fire pellets from the fuel packets in my
>stardrive proposal.

Any numbers with calculations are welcome. What I'm most interested in, is
how much mass you will need (and thus how much effort one has to do to put
it in a pellet track).

>>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.
>
>Yes, and me disagreeing with the basic assumptions in your steps
>isn't helping either.  In particular, the basic assumption that
>you can just look at any exhaust velocity you feel like is one
>I just can't agree with.

You can look at anything you like, I didn't suggest that when you look at
something that it should become a reality.
I suggested that looking at a whole range, instead of a single design will
give you a clearer overview about what the actual energy advantages over the
whole range are.
Sure there are many other things than energy use alone, but one has to
address them all.

>When it comes to exhaust velocity, the harshest fact of rocket design
>is that the limiting factor is _availability_.

True, but I wonder, is a ramjet of the size that we need physically
possible? It may be so inefficient that the power losses melt it away the
instant that it starts to produce 1E16 Watt of power.
The same may be true for other designs, I just don't know. For that matter,
I don't know that exactly what the availability and cost of the several
designs will be.

Timothy