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Re: starship-design: Motion of sail driven by constant-power beam -- Excellentwork Rex!
At 6:47 AM 9/12/96, Kevin \"Tex\" Houston wrote:
>DotarSojat@aol.com wrote:
>>
>> Hi all
>[Snip]
>> Kevin has written, on 9/4 to the Group,
>>
>> >How about a mission which has a constant beam power, the
>> >acceleration would drop off toward the turnaround point. In
>> >this case, the crew would start off with earth-like gravity,
>> >and towards the middle of the trip, the gravity would be more
>> >lunar-like. ... The advantage would be simplified beaming
>> >requirements, and the disadvantage would be a slightly longer
>> >flight time.
>>
[aggresive sniping]
>
>So, let me see if I understand this. For a trip to Tau Ceti (12 Lt-yrs
>away),
>(interpolating from above 6 is almost halfway in between 5.3 and 6.6)
>
>The ship accelerates for 4.25 years crew time to turnaround. It
>achieves
>a final velocity of .933 C and the acceleration felt by the crew is .186
>Earth's transmitters have been on for 1.63 years (as measured by earth)
>and turnaround would be achieved at 7.618 years earth time.
>
>Doubling these numbers (where appropriate) gives for a one-way trip:
>
>15 earth years for travel.
>8.5 crew years for travel.
>3.5 earth years of transmitter time.
>
>With 10 years for in-system study (and return maser construction),
>
>This gives a crew time of 27 years for the entire mission. and an earth
>time of 40 years. This is not too bad. The crew would be able to see
>earth again, the time differential upon return is not so horrendous that
>all your family would be dead. your twin would be "only" 13 years
>older than you. I think this is even favorable in terms of "political
>return" A politician who voted for this would expect to see results
>in about 30 years (travel time plus transmission time back to earth)
>Long enough to be forgetable if it goes bad, short enough to hope to
>be alive to see it if it works.
Nit, politicians generally arn't interested in effects past 4 years. For
example. Social security is expected to melt down in 20 years, but the
problem was considered 'solved' when that number was upped from 10 years.
On the other hane a 27 year service life is short enough to be reasonable
for realistic ships systems, and the professional lives of the crew.
>>
>> These results seem to confirm Kevin's intuitive estimates regard-
>
>Can I say "I told you so"? Aw, come on be a sport. ;-)
>
>> ing acceleration levels. They also substantiate Steve's conclu-
>> sion that the time of emission is limited (even for a constant-
>> output emitter); from the table above, the duration of emission
>> of the radiation accelerating a sail half way to a destination
>> more than 16 lt-yr away (the last entry) is only about a year
>> and three quarters.
>>
>> The deceleration phase (here assumed without justification to
>> be a mirror image of the acceleration phase) needs to be
>> addressed in a separate discussion. Timothy has already put
>> a lot of thought into it.
>
>I thought Tim's treatsy on the subject of deceleration with a beamed
>poser source showed that it will work. That is to say that the ship
>receives ennough power via the antenna to power an ion engine with
>a fairly decent mass ratio.
>
>=============begin included text =============================
>Calculations for the deceleration phase of the MARS design.
>by Timothy van der Linden (T.L.G.vanderLinden@student.utwente.nl)
>Last modified April 14th, 1996
>[derivation snipped]
>
>Vstart Vexh optimal Fuel:ship-ratio Energy per kg of ship (in Joules)
> 0.1 0.062 5.36 7.45E14 7.73E14
> 0.2 0.121 5.84 3.25E15 3.50E15
> 0.3 0.180 6.40 8.11E15 9.08E15
> 0.4 0.240 7.06 1.64E16 1.91E16
> 0.5 0.300 7.87 2.99E16 3.64E16
> 0.6 0.364 8.91 5.23E16 6.69E16
> 0.7 0.433 10.38 9.21E16 1.25E17
> 0.8 0.512 12.72 1.73E17 2.53E17
> 0.9 0.615 17.75 4.04E17 6.62E17
> 0.99 0.803 52.00 3.12E18 7.02E17
> 0.9996 0.906 238.81 2.91E19 9.26E19
>
>Note that the power of the maser-beam is NOT constant during the
>decelerating
>phase, it is supposed that it decreases while the ship gets lighter
>(because it repulses mass).
>======================End included text==============================
>
>By repluses mass, I think Tim means the beam tends to accelerate the
>ship more than the engine can compensate. this was supposed to be a
>minimum energy solution, perhaps a constant energy beam will raise the
>costs. But turning down a power beam is very easy. (think dimmer
>switch)
>Also Fuel:ship ratio is Reaction Mass:Dry Mass ratio I think.
This is a key point, and I'm not sure we ever decided one way or the other.
But obviously if you can't stop a MARS configuration, the rest is
irrelavant.
>For a top speed of .9333 C, we are looking at a mass ratio of "only"
>30.4 (interpolated). this seems to me to much better than any other
>mission discussed so far.
Certainly beats the Explorer or Fuel/Sail mass ratios, probably come close
to a anti-matter too. But of course you have to figure out how to make it
work. Thats a lot of power to transmit, and a lot for a ship to swallow
and process.
>> (Note: This exercise may turn out to be purely academic because
>> the inverse-square effects, without unforeseeable advances in
>> focusing abilities, would be much larger.)
>
>I think we can make some advances here using :
>
>1) Coherent energy (masers instead of microwaves)
>2) Large focusing elements in outer solar system (ala Robert Foreward)
>3) Phased array transmitters to simulate large arperature ~ 400km
>
>With proper microwave optics (moptics? :) yeah, Moptics!) I think we
>can make the beam nearly non-divergent. Not totally, but enough to
>make this mission possible. Remember, Tim's derivation requires a
>diminished beam during deceleration. Inverse square is too much
>diminished certainly, but at least it's the right direction for a
>change.
Agreed. Acceptable optics should be doable, thou you still would need to
'pad' by boosting the transmisionm power level up a couple orders of
magnitude. After all, most of the beam (and power) won't hit the sail.
>I think this mission profile is looking more and more doable. the high
>energy costs were (mostly) due to:
>
>1) The extremely large ship 1 E 09 Kg when I calculated that number.
>2) The desire to accelerate at 1G the entire time.
>3) The foolish notion of increasing the exhaust velocity to save mass.
>
>#1 can be changed any time we wish. We just have to settle on a mass
>that seems reasonable (although in this group, the very word is open to
>debate)
We really do neeed to get a handel on what the ship would need to carry and
how big it will need to be. Still I dobt you could scale down the ship by
a factor of 10 from my Explorer numbers (about 500,000 tons) without
seriously cripliing an mission potential.
>#2 has been shown by Rex and others for the folly it is. A gradual
>decline into lunar gravity, and a gradual climb back out will certainly be
>doable for the astronauts. no clumsy swiveling sections, no artificial gravity
>generators (although if we had those, we could do this mission easily ;)
I don't follow this? Low grav is still bad for you, and spending a decade
parked in orbit in zero-g is REAL bad for you. Given that the centrafuges
don't have a serois weigh problems (well possibly shielding) I think we'ld
have to carry them.
>#3 Was shown by Tim to be foolish. According to the above chart, the
>optimum Vexh is .685 (interpolated)
>
>A beamed mission is safer than anti-matter (and easier); it is faster
>and less costly than a fusion fuel mission (excepting perhaps the hybrid
>mission Kelly has proposed). It only requires one major advance (self
>replicating machines to make the solar collectors.) Other than that,
>the technology is all present day.
We're a very long way from self replicating machines or systems. So I
wouldn't jump over that so quickly.
It is definatly faster then a fusion system. Even Fuel/Sail couldn't get
past .4C without rediculus deceleration fuel masses.
>--
>Kevin "Tex" Houston http://umn.edu/~hous0042/index.html
Kelly
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