Re: David Drake - Leary and the RCN


William George Ferguson wrote:
On 25 Jun 2006 19:07:22 -0700, "Jordan" <JSBassior2001@xxxxxxxxx> wrote:


Wayne Throop wrote:
: "Jordan" <JSBassior2001@xxxxxxxxx>
: Here's an extremely plausible (in terms of known physics) scenario.

OK. But how many SF works are there that use this scenario?

I don't know if any have used that _exact_ scenario, but plenty have
used sublight or NAFAL ramscoops as fictional starships, and talked
about the communications difficulties of such operations.

Keep in mind the two major limiting factors, even theoretically, of the
Bussard Ramjet. One, you have to attain a velocity of at least 0.04C for
the ramjet to work, and two, resistence of the medium (space isn't Really
empty, just mostly so) limits the top speed. Depending on what numbers
you use for the density of interstellar dust, I've seen it projected as
low as 0.15C, and as high as 0.42C. In any case, you aren't even going to
go fast enough with one to gain any great personal relativistic
advantages.

I wasn't assuming any such relativistic advantages. I'm also not
assuming that the only means of acceleration and deceleration is the
use of scooped hydrogen fuel; I mentioned antimatter, and what I did
not mention but always consider in this context is the possibility of
stationary launch / powerbeaming facilities in the base system. If I
was writing this as a story I would get very specific on these details;
since I'm merely talking about signal time lags they aren't that
relevant to the main issue.

Also, if you're using that, you're using a reaction engine for
acceleration. If you're using a reaction engine, then you can't
accelerate for any real length of time at greater than 1G, not if you want
to have a functional human crew afterwards. By 'real length of time', I
mean days, weeks, and months. The length of time a human body can take
accelerations signifcantly greater than 1G is measured in seconds,
minutes, and hours.

You actually don't need to accelerate any more rapidly than 1G to
attain considerable fractions of C. Indeed, you can do it accelerating
more slowly. The length of the voyage is primarily determined by
_maximum_ speed (which is in turn primarily based on the ship's ability
to shield itself against the impact of the interstellar medium against
its prow) rather than by acceleration capability.

I know this sounds weird, because for _interplanetary_ voyages it's the
other way around -- there is no effective maximum speed and sustained
acceleration capacity is very important -- but that is because
interplanetary distances are relatively short compared to interstellar
ones. Here's another weird thing -- STL starships have to be
streamlined to some extent. The reason why is that if they are not the
difficulty of shielding them against the interstellar medium increases
greatly.

In some ways, STL starships return to the model of oceanic vessels!

Let's optimistically say the ramjet's top speed (the speed at which the
stuff striking the ramscoop in front is as compressed as the stuff you're
pushing out the back) is 0.42C. So first we accelerate our ship up to C
fractional.

Now, the fastest we've been able to make a manmade spaceship go, without
using the solar whip, is less than 40,000 mph. The two Helios solar
orbiters both have exceeded 150,000 mph at their closest approach to the
sun, using the solar whip. 0.01C is better than 6.6 million mph. To
light the ramjet, we've got to reach at least 0.04C (around 27 million
mph). We aren't going to do that burning reaction mass, nuh uh,no way.

Lets say we use solar sails and push them with giant lasers to accelerate
the ship, until we get up to 0.04C, then light the torch, and away we go
toward Alpha Centauri. Since compared to 0.04C, any initial speed
relative to the Earth (or anything else in the solar system) is
effectively zero, it's a fairly simple calc to how long it would take for
the sail/laser-pusher bit to reach 0.04C at 1G acceleration, around 14
days. Once we light the torch, we'll still want to keep the acceleration
at around 1G, so our kidneys and liver don't fail, our hearts don't have
to be replaced and our circulatory system doesn't break down.

Oh no, this isn't necessary, and for two reasons:

1) You can simulate gravity by centrifugal force, and this is a heck of
a lot easier than running the drive at 1G during the whole voyage.
Assuming a big starship (and manned starships probably _will_ be big)
the radius should be high enough to have centrifugal gravity at least
over _part_ of the ship.

2) It is highly improbable that we will have attained the ability to
construct ships capable of flying at 0.04 to 0.40 C without first
having constructed and operated a lot of slower spacecraft, and in the
course of all these extensive interplanetary space operations, we will
have solved the medical problems. Several times over.

At 1G, it
will take us a little over half a year to reach 0.42C, at which point,
we'll be around 0.2 light years out. We can run at that speed for over
halfway, because at fractional C, the resistance of the medium means that
we don't just keep coasting at the same speed without propulsion. In
fact, the ramscoop, without the torch lit, would make a fairly good brake
or drag chute until our speed's down to the point where the stuff hitting
the back of the ramscoop is going more or less the same speed (relative to
us) as the stuff hitting the front of the ramscoop. That will probably be
something under 0.01C.

This is actually one possible use of the electromagnetic field that
I've considered -- as a deceleration device. Depending on the engine
design, it may also be possible to run the torch backwards -- in other
words, without flipping the ship end over end (a dangerous maneuver at
high fractions of C), to have the photon rocket fire out from the front
end.

By the way, if you run the engine at a level which would normally
produce 1G acceleration but fails to actually change the vector owing
to the resistance of the interstellar medium, there is no acceleration
and consequently no acceleration simulated gravity. This is another
reason why it makes more sense, if gravity is still medically
necessary, to spin part of the ship.

The big fun will be on the first trip, where we have to arrange our own
powered braking, to get our speed down to the point where we can safely
interact with anything larger than dust particles at the other end. Once
we've made the first trip, we can mount laser pushers at the destination.

Which of course you can't if the target system is enemy-held, because
any lasers in that system will not be trying to do anything as benign
as decelerate you!

You could also send a mirror ahead of the starship, unfold it in the
target system, and then bounce the beam back for the deceleration
maneuver. This is tricky though and requires a long wait for the
mirror to arrive at the correct point: you could lose the ship _in
flight_ if you try to hurry this and anything goes wrong!

Using all this tech, which we don't have, but which is certainly within
current standard theory, a round trip to Alpha Centauri and back would
take over 20 years.

Well yes, but it's only 10 one way. And I'm assuming that the people
doing this are effectively immortal.

We really do need to figure out a reactionless drive.

Well, it would be nice.

- Jordan

.

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